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Chapter 1: What is EarthScience?

Nature of Science

Lesson Objectives

Explain the importance of asking questions.State the steps of the scientific method.Describe the three major types of scientificmodels.Use appropriate safety precautions inside andoutside the science laboratory.

Introduction

Think of your favorite science fiction movie.What is it about? Maybe it’s about spaceshipsgoing to distant planets, or people being cloned in

laboratories, or undersea civilizations, or robotsthat walk among us. These entertaining imaginingsare make-believe fantasies, that’s why they’recalled science “fiction.” They are not real. Butwhy are they called “science” fiction?

The answer is that science uses a disciplinedprocess to answer questions. In science,“disciplined” does not mean well-behaved. Itmeans following orderly steps in order to come upwith the best answers. Science involves observing,wondering, categorizing, communicating,calculating, analyzing, and much more. In order toconvert creativity into reality, we need science. Inorder to travel beyond where anyone has gonebefore, we need science. In order to understand theworld, make sense of it, and conserve it, we needscience. In order to confirm our best guesses aboutthe universe and the things in it, we need science.Science fiction stories extend and expand on all theideas of science and technology in creative ways.

Asking Questions

Why is the sky blue?How tall will this tree grow?Why does the wind blow so hard?Will it be cold tonight?How many stars are out there?Are there planets like Earth that orbit about

some of those stars?How did this rock get holes in it?Why are some rocks sharp and jagged, while

others are round?You probably ask yourself a thousand

questions a day, many of which you never askanyone else. For many of the questions you do ask,you never even get an answer. But your brainkeeps churning with questions and curiosity. Wecan’t help but want to know.

The list of questions above are some of thesame questions that scientists ask. Science hasdeveloped over centuries and centuries, and ourability to measure the tiniest trait has increasedimmensely. So although there is no wrong question,there are questions that lend themselves more tothe scientific process than others. In other words,

some questions can be investigated using thescientific method while others rely on pure faith oropinion.

Scientific Methods

The scientific method is not a list ofinstructions but a series of steps that help toinvestigate a question. By using the scientificmethod, we can have greater confidence in how weevaluate that question. Sometimes, the order of thesteps in the scientific method can change, becausemore questions arise from observations or data thatwe collect. The basic sequence followed in thescientific method is illustrated in Figure below.

Figure 1.1 The Scientific Method.

Question

The scientific method almost always beginswith a question that helps to focus theinvestigation. What are we studying? What do wewant to know? What is the problem we want tosolve? The best questions for scientificinvestigation are specific as opposed to general,they imply what factors may be observed or

manipulated.Example: A farmer has heard of a farming

method called “no-till farming.” In this method,certain techniques in planting and fertilizingeliminate the need for tilling (or plowing) the land.Will no-till farming reduce the erosion of thefarmland (Figure below)?

Figure 1.2 Soil Erosion

Research

Before we go any further, it is important to find

out what is already known about the topic. You canresearch a topic by looking up books andmagazines in the library, searching on the Internet,and even talking to people who are experts in thearea. By learning about your topic, you’ll be ableto make thoughtful predictions. Your experimentaldesign might be influenced by what you haveresearched. Or you might even find that yourquestion has been researched thoroughly. Althoughrepeating experiments is valid and important inscience, you may choose to introduce new ideasinto your investigation, or you may change yourinitial question.

Example: The farmer decides to research thetopic of no-till farming (Figure below). She findssources on the Internet, at the library, and at thelocal farming supply store that discuss what type offertilizer might be used and what the best spacingfor her crop would be. She even finds out that no-till farming can be a way to reduce carbon dioxideemissions into the atmosphere, which helps in thefight against global warming.

Figure 1.3 The farmer would need to research no-till

farming methods.

Hypothesis

Now that you have researched the topic, youcan make an educated guess or explanation to thequestion. This is your hypothesis. The besthypothesis is directly related to the question and istestable, so that you can do experiments todetermine whether your hypothesis is correct.

Example: The farmer has researched herquestion and developed the following hypothesis:

No-till farming will decrease the soil loss on

hills of similar steepness as compared to thetraditional farming technique because there will beless disturbance to the soil.

A hypothesis can be either proved ordisproved by testing. If a hypothesis is repeatedlytested and proven to be true, then scientists will nolonger call it a hypothesis.

Experiment

Not all questions can be tested byexperimentation. However, many questions presentus with ways to test them that give us the clearestconclusions. When we design experiments, weselect the factor that will be manipulated orchanged. This is the independent variable. Wewill also choose all of the factors that must remainthe same. These are the experimental controls.Finally, we will choose the factor that we aremeasuring, as we change the independent variable.This is the dependent variable. We might say thatthe dependent variable “depends” on the

independent variable. How much soil is erodeddepends on the type of farming technique that wechoose.

Figure 1.4 A farmer takes careful measurements in the

field.Example: The farmer will conduct an

experiment on two separate hills with similarslopes or steepnesses (Figure above). On one hill,he will use a traditional farming technique whichincludes plowing to stir up the nutrients in the soil.On the other hill, he will use a no-till technique byspacing plants further apart and using specializedequipment that plants the plants without tilling. Hewill give both sets of plants identical amounts of

water and fertilizer.In this case, the independent variable is the

farming technique—either traditional or no-till—because that is what is being manipulated. In orderto be able to compare the two hills, they must havethe same slope and the same amount of fertilizerand water. If one had a different slope, then itcould be the angle that affects the erosion, not thefarming technique, for example. These are thecontrols. Finally, the dependent variable is theamount of erosion because the farmer will measurethe erosion to analyze its relatedness to the farmingtechnique.

Data and Experimental Error

Data can be collected in many different waysdepending on what we are interested in finding out.Scientists use electron microscopes to explore theuniverse of tiny objects and telescopes to ventureinto the universe itself. Scientists routinely travelto the bottom of the ocean in research submersiblesto make observations and collect samples. Probes

are used to make observations in places that aretoo dangerous or too impractical for scientists toventure. Probes have explored the Titanic as it layon the bottom of the ocean and to other planets inour solar system. Data from the probes travelsthrough cables or through space to a computerwhere it can be manipulated by scientists. Ofcourse, many scientists work in a laboratory andperform experiments and analyses on a bench top.

During an experiment, we may make manymeasurements. These measurements are ourobservations that will be carefully recorded in anorganized manner. This data is often computerizedand kept in a spreadsheet that can be in the form ofcharts or tables that are clearly labeled, so that wewon’t forget what each number represents. “Data”refers to the list of measurements that we havecollected. We may make written descriptions ofour observations but often, the most useful data isnumerical. Even data that is difficult to measurewith a number is sometimes representednumerically. For example, we may makeobservations about cleanliness on a scale from one

to ten, where ten is very clean and one is verydirty. Statistical analyses also allow us to makemore effective use of the data by allowing us toshow relationships between different categories ofdata. Statistics can make sense of the variability(spread) in a data set. By graphing data, we canvisually understand the relationships between data.Besides graphs, data can be displayed as charts ordrawings so that other people who are interestedcan see the relationships easily.

As in just about every human endeavor, errorsare unavoidable. In an experiment, systematicerrors are inherent in the experimental setup so thatthe numbers are always skewed in one direction oranother. For example, a scale may always measureone-half ounce high. Like many systematic errors,the scale can be recalibrated or the error can beeasily corrected. Random errors occur because nomeasurement can be made exactly precisely. Forexample, a stopwatch may be stopped too soon ortoo late. This type of error is reduced if manymeasurements are taken and then averaged.Sometimes a result is inconsistent with the results

from other samples. If enough tests have beendone, the inconsistent data point can be thrown outsince likely a mistake was made in that experiment.The remaining results can be averaged.

Not all data is quantified, however. Ourwritten descriptions are qualitative data, data thatdescribes the situation observed. In any case, datais used to help us draw logical conclusions.

Conclusions

After you have summarized the results of theexperiments and presented the data as graphs,tables and diagrams, you can try to draw aconclusion from the experiments. You must gatherall your evidence and background information.Then using logic you need to try formulate anexplanation for your data. What is the answer tothe question based on the results of the experiment?A conclusion should include comments about thehypothesis. Was the hypothesis supported or not?Some experiments have clear, undeniable resultsthat completely support the hypothesis. Others do

not support the hypothesis. However, allexperiments contribute to our wealth ofknowledge. Even experiments that do not supportthe hypothesis may teach us new information thatwe can learn from. In the world of science,hypotheses are rarely proved to one hundredpercent certainty. More often than not, experimentslead to even more questions and more possibleways of considering the same idea.

Example: After a full year of running herexperiment, the farmer finds 2.2 times as mucherosion on the traditionally farmed hill as on theno-till hill. She intends to use no-till methods offarming from now on and to continue researchingother factors that may affect erosion. The farmeralso notices that plants in the no-till plots are tallerand the soil moisture seems higher. She decides torepeat the experiment and measure soil moisture,plant growth, and total water needed to irrigate ineach kind of farming.

Theory

If a topic is of interest to scientists, manyscientists will conduct experiments and makeobservations, which they will publish in scientificjournals. Over time the evidence will mount in,for, or against the hypothesis being tested. If ahypothesis explains all the data and no datacontradicts the hypothesis, the hypothesis becomesa theory. A theory is supported by manyobservations and there are no majorinconsistencies. A theory is also used to predictbehavior. Although a theory can be overthrown ifconflicting data is discovered, the longer a theoryhas been in existence the more data it probably hasto back it up and the less likely it will be provenwrong. A theory is a model of reality that issimpler than the phenomenon itself.

The common usage of the word theory is verydifferent from the scientific usage; e.g. I have atheory as to why Joe likes Sue more than Kay. Theword hypothesis would be more correct in mostcases.

Scientific Models

Many scientists use models to understand andexplain ideas. Models are representations ofobjects or systems. Simpler than the real lifesystem, models may be manipulated and adjustedfar more easily. Models can help scientists tounderstand, analyze and make predictions aboutsystems that would be impossible without them.

Models are extremely useful but they havemany limitations. Simple models often look at onlya single characteristic and not at the myriadconditions other aspects of a system. Since thescientists who construct a model often do notentirely understand the system they are modeling,the model may not accurately represent reality.Models are very difficult to test. One way to test amodel is to use as its starting point a time in thepast and then have the model predict the present. Amodel that can successfully predict the present ismore likely to be accurate when predicting thefuture.

Many models are created on computersbecause only computers can handle and manipulatesuch enormous amounts of data. For example,

climate models are very useful for trying todetermine what types of changes we can expect asthe composition of the atmosphere changes. Areasonably accurate climate model would beimpossible on anything other than the mostpowerful computers.

There are three types of models and each typeis useful in certain ways.

Physical Models

Physical models are physical representationsof whatever subject is being studied. These modelsmay be simplified by leaving out certain realcomponents, but will contain the importantelements. Model cars and toy dinosaurs areexamples of physical models. Drawings and mapsare also physical models. They allow us to see andfeel and move them, so that we can compare themto one another and illustrate certain features.

We can use a drawing to model the layers ofthe Earth (Figure below). This type of model isuseful in understanding the composition of the

Earth, the relative temperatures within the Earth,and the changing densities of the Earth beneath thesurface. Yet there are many differences between acut-away model of the Earth and the real thing.First of all, the size is much different. It is difficultto understand the size of the Earth by looking at asimple drawing. You can’t get a good idea of themovement of substances beneath the surface bylooking at a drawing that does not move. Themodel is very useful but has its shortcomings.

Figure 1.5 The Earths Center.

Conceptual Models

A conceptual model is not a physical model,but rather a mental explanation that ties togethermany ideas to attempt to explain something. Aconceptual model tries to combine knowledge andmust incorporate new knowledge that may changeit as knowledge is acquired. The origin of themoon, for example, is explained by some as aMars sized planet that hit the Earth and formed agreat cloud of debris and gas (Figure 1.6). Thisdebris and gas eventually formed a singlespherical body called the Moon. This is a usefulmodel of an event that probably occurred billionsof years ago. It incorporates many ideas about thecraters and volcanoes on the Moon, and thesimilarity of some elements on both the moon andthe Earth. Not all data may fit this model, however,and there may be much information that we simplydon’t know. Some people think that the Moon wasinitially an asteroid out in space which wascaptured in orbit by the gravity of the Earth. Thismay be a competing conceptual model which hasits own arguments and weaknesses. As withphysical models, all conceptual models have

limitations.

Figure 1.6 A collision showing a meteor striking the

Earth.

Mathematical Models

A third type of model is the mathematicalmodel. These models are created through a greatdeal of consideration and analysis of data. Amathematical model is an equation or formula thattakes many factors or variables into account. Thesemodels may help predict complex events liketornadoes and climate change. In order to predict

climate change, for example, a mathematical modelmay take into account factors such as temperaturereadings, ice density, snow fall, and humidity.These data may be plugged into equations to give aprediction. As with other models, not all factorscan be accounted for, so that the mathematicalmodel may not work perfectly. This may yieldfalse alarms or prediction failures. No model iswithout its limitations.

Models are a useful tool in science. Theyallow us to efficiently demonstrate ideas andcreate hypotheses. They give us visual orconceptual manners for thinking about things. Theyallow us to make predictions and conductexperiments without all of the difficulties of real-life objects. Could you imagine trying to explain aplant cell by only using a real plant cell or trying topredict the next alignment of planets by onlylooking at them? In general, models havelimitations that should be taken into considerationbefore any prediction is believed or anyconclusion seen as fact.

Safety in Science

Accidents happen from time to time ineveryday life. Since science involves an adventureinto the unknown, it is natural that accidents canhappen. Therefore, we must be careful and useproper equipment to prevent as many accidents aspossible (Figure below). We must also be sure totreat any injury or accident appropriately.

Figure 1.7 Safety Symbols: A. Corrosive , B. Oxidizing

Agent, C. Toxic, D. High Voltage.

Inside the Science Laboratory

If you work in the science lab, you may comeacross dangerous materials or situations. Sharpobjects, chemicals, heat, and electricity are allused at times in earth science laboratories. Withproper protection and precautions, almost allaccidents can be prevented and sufferingminimized (Figure below). Below is a list ofsafety guidelines that you should follow whendoing labs:

Follow directions at all times.Although working in the science lab can befun, it is not a play area. Be sure to obey anysafety guidelines given in lab instructions orby the lab supervisor.Use only the quantities of materials directed.Check with your teacher before you dosomething different than what’s described onthe lab procedure.

Tie back long hair. Wear closed shoes withflat heels and shirts with no hanging sleeves,hoods, or drawstrings.Use gloves, goggles, or safety aprons wheninstructed to do so.Use extreme care with any sharp or pointedobjects like scalpels, knives, or broken glass.Never eat or drink anything in the science lab,even if you brought it there yourself. Tabletops and counters could have dangeroussubstances on them.Keep your work area neat and clean. Be sureto properly clean and maintain materials liketest tubes and beakers. Leftover substancescould interact with other substances in futureexperiments. A messy work area leaves moreopportunities for spills and breakage.Be careful when you reach. Flames or heatplates could be beneath your arms or longhair could get burned.Use electrical appliances and burners asinstructed.Know how to use an eye wash station, fire

blanket, fire extinguisher, or first aid kit.Alert the lab supervisor in the case thatanything out of the ordinary occurs. Anaccident report may be required if someone ishurt and the lab supervisor must know if anymaterials are damaged or discarded.

Figure 1.8 Safety Equipment in the Laboratory.

Outside the Laboratory

Many earth science investigations areconducted outside of the science laboratory(Figure below). Of course, the same precautionsmust be taken with lab-like materials but we musttake additional considerations into mind. For anyscientific endeavor outside or at home:

Be sure to wear appropriate clothing. Hikinginto a canyon requires boots, long pants, andprotection from the sun, for example.Bring sufficient supplies like food and water,even for a short trip. Dehydration can occurrapidly.Have appropriate first aid available.Be sure to let others know where you aregoing, what you will be doing, and when youwill be returning. Be sure to take a map withyou if you don’t know the area and you mayleave a copy of the map with someone athome.Be sure you have access to emergencyservices and some way to communicate. Keepin mind that not all places have coverage for

cellular phones.Finally, be sure that you are accompanied bya person familiar with the area to which youare traveling or familiar with the type ofinvestigation that you are going to do.

Figure 1.9 Outdoor Excursions.

Review Questions

1. Write a list of five questions about the worldaround you that you find interesting.

2. A scientist was studying the effects of oilcontamination on ocean seaweed. Hebelieved that oil runoff from storm drainswould keep seaweed from growing normally.He had two large aquarium tanks of equalsize. He monitored the dissolved oxygen inthe water to keep it equal as well as thewater’s temperature. He introduced somemotor oil into one tank but not in the other. Hethen measured the growth of seaweed plantsin each tank. In the tank with no oil, theaverage growth was 2.57cm. The averagegrowth of the seaweed in the tank with oilwas 2.37cm. Based on this experiment,answer the following questions:

1. What was the question that the scientiststarted with?

2. What was his hypothesis?3. Identify the independent variable, the

dependent variable, and the experimentalcontrol(s).

4. What did the data show?3. Explain three types of scientific models. What

is one benefit of each and one disadvantage ofeach?

4. Identify or design five of your own safetysymbols, based on your knowledge of safetyprocedures in a science laboratory.

5. Design your own experiment based on one ofyour questions from question 1 above. Includethe question, hypothesis, independent anddependent variables, and safety precautions.You may want to work with your teacher or agroup.

Vocabulary

conceptual modelAn abstract, mental representation ofsomething using thoughts and ideas instead ofphysical objects.

controlFactors that are kept the same in anexperiment, in order to focus just on theindependent and dependent variables.

dependent variableThe variable in an experiment that you aremeasuring as you change the independentvariable. It "depends" on the independentvariable.

hypothesisA good working explanation for a problemthat can be tested.

independent variableThe variable (or thing) in an experiment thatis controlled and changed by the researcher.

mathematical modelA set of mathematical equations and numbersthat simulates a natural system being modeled.

physical modelA representation of something using objects.

theoryA hypothesis that has been repeatedly testedand proven to be true.

Points to Consider

What parts of the Earth do you think are mostimportant and should be better studied?What type of model have you had experiencewith? What did you learn from it?What situations are both necessary anddangerous for scientists to study? Whatprecautions do you think they should use whenthey study them?If you could go anywhere, where would it be?What safety equipment or precautions would

you take?

Earth Science and ItsBranches

Lesson Objectives

Define and describe Earth Science as ageneral field with many branches.Identify the field of geology as a branch ofEarth Science that deals with the solid part ofthe Earth.Describe the field of oceanography as abranch of Earth Science that has severalsubdivisions that deal with the variousaspects of the ocean.Define the field of meteorology as a branch ofEarth Science that deals with the atmosphere.Understand that astronomy is an extension ofEarth Science that examines other parts of thesolar system and universe.List some of the other branches of EarthScience, and how they relate to the study of

the Earth.

Overview of Earth Science

Earth is the mighty planet upon which we alllive. Only recently have humans begun tounderstand the complexity of this planet. In fact, itwas only a few hundred years ago that wediscovered that Earth was just a tiny part of anenormous galaxy, which in turn is a small part ofan even greater universe. Earth Science deals withany and all aspects of the Earth. Our Earth hasmolten lava, icy mountain peaks, steep canyons andtowering waterfalls. Earth scientists study theatmosphere high above us as well as the planet'score far beneath us. Earth scientists study parts ofthe Earth as big as continents and as small as thetiniest atom. In all its wonder, Earth scientists seekto understand the beautiful sphere on which wethrive (Figure below).

Figure 1.10 Earth as seen from Apollo 17.Because the Earth is so large and science is so

complex, Earth scientists specialize in studyingjust a small aspect of our Earth. Since all of thebranches are connected together, specialists worktogether to answer complicated questions. Let’slook at some important branches of Earth Science.

Geology

Geology is the study of the solid matter thatmakes up Earth. Anything that is solid, like rocks,minerals, mountains, and canyons is part ofgeology. Geologists study the way that theseobjects formed, their composition, how theyinteract with one another, how they erode, and howhumans can use them. Geology has so manybranches that most geologists become specialistsin one area. For example, a mineralogist studiesthe composition and structure of minerals such ashalite (rock salt), quartz, calcite, and magnetite(Figure below).

Figure 1.11 Mineralogists focus on all kinds of minerals.

Figure 1.12 Seismographs are used to measure earthquakes

and pinpoint their origins.A volcanologist braves the high temperatures

and molten lava of volcanoes. Seismologists studyearthquakes and the forces of the Earth that createthem. Seismologists monitor earthquakesworldwide to help protect people and propertyfrom harm (Figure above). Scientists interested in

fossils are paleontologists, while scientists whocompare other planets’ geologies to that of theEarth are called planetary geologists. There aregeologists who only study the Moon. Somegeologists look for petroleum, others arespecialists on soil. Geochronologists study howold rocks are and determine how different rocklayers formed. There are so many specialties ingeology that there is probably an expert in almostanything you can think of related to the Earth(Figure below).

Figure 1.13 Geology is the study of the solid Earth and its

processes.

Oceanology

Oceanology is the study of everything in theocean environment. More than 70% of the Earth’ssurface is covered with water. Most of that wateris found in the oceans. Recent technology hasallowed us to go to the deepest parts of the ocean,yet much of the ocean remains truly unexplored.Some people call the ocean the last frontier. But itis a frontier already deeply influence by humanactivity. As the human population gets ever bigger,we are affecting the ocean in many ways.Populations of fish and other marine species haveplummeted because of overfishing; contaminantsare polluting the waters, and global warmingcaused by greenhouse gases is melting the thick icecaps. As ocean waters warm, the water expandsand, along with the melting ice caps, causes sealevels to rise.

Climatologists help us understand the climateand how it will change in the future in response toglobal warming. Oceanographers study the vast

seas and help us to understand all that happens inthe water world. As with geology, there are manybranches of oceanography. Physical oceanographyis the study of the processes in the ocean itself, likewaves and ocean currents (Figure above). Marinegeology uses geology to study ocean earthquakes,mountains, and trenches. Chemical oceanographystudies the natural elements in ocean water andpollutants.

Figure 1.14 Physical oceanography studies things like

currents and waves.

Climatology and Meteorology

Meteorologists don’t study meteors — theystudy the atmosphere! Perhaps this branch of EarthScience is strangely named but it is very importantto living creatures like humans. Meteorologyincludes the study of weather patterns, clouds,hurricanes, and tornadoes. Using moderntechnology like radars and satellites,meteorologists work to predict or forecast theweather. Because of more accurate forecastingtechniques, meteorologists can help us to preparefor major storms, as well as help us know whenwe should go on picnics.

Climatologists and other atmospheric scientistsstudy the whole atmosphere, which is a thin layerof gas that surrounds the Earth. Most of it is withinabout 10 - 11 kilometers of the Earth’s surface.Earth’s atmosphere is denser than Mars's thinatmosphere, where the average temperature is -63oC, and not as thick as the dense atmosphere onVenus, where carbon dioxide in the atmospheremakes it hot and sulfuric acid rains in the upperatmosphere. The atmosphere on Earth is just denseenough to even out differences in temperature from

the equator to the poles, and contains enoughoxygen for animals to breathe.

Over the last several decades, climatologistsstudying the gases in our atmosphere have foundthat humans are putting a dangerous amount ofcarbon dioxide into the air by burning fossil fuels(Figure below). Normally, the atmospherecontains only small amounts of carbon dioxide, andtoo much of it makes it trap heat from the sun,causing the Earth to heat up, an effect we callglobal warming. Climatologists can help us betterunderstand the climate and how it may change inthe future in response to different amounts ofgreenhouse gases and other factors (Figurebelow).

Figure 1.15 Carbon dioxide released into the atmosphere is

causing global warming.

Figure 1.16 When hurricanes are accurately forecast by

meteorologists, many lives can be saved.

Astronomy

Astronomers have proven that our Earth andsolar system are not the only set of planets in theuniverse. By 2007, over a hundred planets outsideour solar system had been discovered. Although noone can be sure how many there are, astronomersestimate that there are billions of other planets. Inaddition, the universe contains black holes, othergalaxies, asteroids, comets, and nebula. As big asEarth seems to us, the entire universe is vastlygreater. Our Earth is an infinitesimally small partof our universe.

Astronomers use resources on the Earth tostudy physical things beyond the Earth. They use avariety of instruments like optical telescopes andradio telescopes to see things far beyond what thehuman eye can see. Spacecraft travel great

distances in space to send us information onfaraway places, while telescopes in orbit observeastronomical bodies from the darkness of space(Figure below).

Figure 1.17 The Hubble Space Telescope.Astronomers ask a wide variety of questions.

Astronomers could study how an object or energyoutside of Earth could affect us. An impact from anasteroid could have terrible effects for life onEarth. Strong bursts of energy from the sun, calledsolar flares, can knock out a power grid or disturbradio, television or cell phone communications.But astronomers ask bigger questions too. How

was the universe created? Are there other planetson which we might live? Are there resources thatwe could use? Is there other life out there?Astronomy also relies on Earth Science, whenscientists compare what we know about life onEarth to the chances of finding life beyond thisplanet.

Other Branches of Earth Science

Geology, oceanography, and meteorologyrepresent a large part of Earth science, whileastronomy represents science beyond Earth.However, there are still many smaller branches ofscience that deal with the Earth or interact greatlywith Earth sciences. Most branches of science areconnected with other branches of science in someway or another. A biologist who studies monkeysin rainforests must be concerned with the watercycle that brings the rain to the rainforests. Shemust understand the organic chemistry of the foodthe monkeys eat, as well as the behavior between

the monkeys. She might examine the soil in whichthe trees of the rainforest grow. She must evenunderstand the economy of the rainforest tounderstand reasons for its destruction. This is justone example of how all branches of science areconnected.

Below are examples of a few branches ofscience that are directly related to Earth science.Environmental scientists study the ways thathumans interact with the Earth and the effects ofthat interaction. We hope to find better ways ofsustaining the environment. Biogeography is abranch of science that investigates changes inpopulations of organisms in relation to place overtime. These scientists attempt to explain the causesof species’ movement in history. Ecologists focuson ecosystems, the complex relationship of all lifeforms and the environment in a given place (Figurebelow). They try to predict the chain reactions thatcould occur when one part of the ecosystem isdisrupted.

Figure 1.18 In a marine ecosystem, coral, fish, and other

sea life depend on each other for survival.As opposed to an oceanographer, a limnologist

studies inland waters like rivers and lakes. Ahydrogeologist focuses on underground waterfound between soil and rock particles, whileglaciologists study glaciers and ice.

None of these scientific endeavors would bepossible without geographers who explore thefeatures of the surface and work withcartographers, who make maps. Stratigraphy isanother area of Earth science which examineslayers of rock beneath the surface (Figure below).

This helps us to understand the geological historyof the Earth. There is a branch of science for everyinterest and each is related to the others.

Figure 1.19 Folded strata are layers in the rock that have

bent over time. Stratigraphy attempts to explainthese layers and the geologic history of the area.

Review Questions

1. What are three major branches of Earthscience?

2. What branch of science deals with stars &galaxies beyond the Earth?

3. List important functions of Earth scientists.4. What do you think is the focus of a

meteorologist?5. A meteorologist studies the atmosphere. This

includes weather and climate changes as wellas global warming

6. An ecologist notices that an important coralreef is dying off. She believes that it has to dowith some pollution from a local electricplant. What type of scientist might help heranalyze the water for contamination?

7. Design an experiment that you could conductin any branch of Earth science. Identify theindependent variable and dependent variable.What safety precautions would you have totake?

Vocabulary

astronomersScientists who study the universe, galaxiesand stars.

geologyThe study of the rocks, processes and historyof Earth.

meteorologyStudy of the atmosphere, weather and storms.

oceanologyStudy of the ocean realm in all its aspects.

Points to Consider

Why is Earth science so important?Which branch of Earth science would youmost like to explore?What is the biggest problem that we facetoday? Which Earth scientists may help us tosolve the problem?What other branches of science or society arerelated to and necessary for Earth science?

Chapter 2: StudyingEarth’s Surface

Introduction to Earth’sSurface

Lesson Objectives

Distinguish between location and direction.Describe topography.Identify various landforms and brieflydescribe how they came about.

Location

Wherever you are on Earth’s surface, inorder to describe your location, you need somepoint of reference. Right now you are probablyreading this chapter at your computer. But where is

your computer? It may set up in a certain place oryou may be on a laptop computer, which meansyou can change where you are. In order to describeyour location, you could name other items aroundyou to give a more exact position of yourcomputer. Or you could measure the distance anddirection that you are from a reference point. Forexample, you may be sitting in a chair that is onemeter to the right of the door. This statementprovides more precise information for someone tolocate your position within the room.

Similarly, when studying the Earth’ssurface, Earth scientists must be able to pinpointany feature that they observe and be able to tellother scientists where this feature is on theEarth’s surface. Earth scientists have a systemto describe the location of any feature. To describeyour location to a friend when you are trying to gettogether, you could do what we did withdescribing the location of the computer in theroom. You would give her a reference point, adistance from the reference point, and a direction,such as, “I am at the corner of Maple Street and

Main Street, about two blocks north of yourapartment." Another way is to locate the feature ona coordinate system, using latitude and longitude.Lines of latitude and longitude form a grid thatmeasures distance from a reference point. You willlearn about this type of grid when we discuss mapslater in this chapter.

Direction

If you are at a laptop, you can change yourlocation. When an object is moving, it is notenough to describe its location; we also need toknow direction. Direction is important fordescribing moving objects. For example, a windblows a storm over your school. Where is thatstorm coming from? Where is it going? The mostcommon way to describe direction in relation tothe Earth’s surface is by using a compass. Thecompass is a device with a floating needle that is asmall magnet (Figure below). The needle alignsitself with the Earth’s magnetic field, so that

the compass needle points to magnetic north. Onceyou find north, you can then describe any otherdirection, such as east, south, west, etc., on acompass rose (Figure below).

Figure 2.1 A compass is a device that is used to determine

direction. The needle points to the Earths magneticnorth pole.

Figure 2.2 A compass rose shows the various directions,

such as North (N), East (E), South (S), West (W)and various combinations.

A compass needle aligns to the Earth’smagnetic North Pole, not the Earth’sgeographic North Pole or true north. Thegeographic North Pole is the top of the imaginaryaxis upon which the Earth’s rotates, much likethe spindle of a spinning top. The magnetic NorthPole shifts in location over time. Depending onwhere you live, you can correct for this difference

when you use a map and a compass (Figurebelow).

Figure 2.3 Earths magnetic north pole is about 11 degrees

offset from its geographic north pole on the axis ofrotation.

When you study maps later, you will see thatcertain types of maps have a double compass roseto make the corrections between magnetic northand true north. An example of this type is a nauticalchart that sailors and boaters use to chart their

positions at sea or offshore (Figure below).

Figure 2.4 Nautical maps include a double compass rose

that shows both magnetic directions (inner circle)and geographic compass directions (outer circle).

Topography

As you know, the surface of the Earth is notflat. Some places are high and some places arelow. For example, mountain ranges like the SierraNevada in California or the Andes mountains in

South America are high above the surroundingareas. We can describe the topography of a regionby measuring the height or depth of that featurerelative to sea level (Figure below). You mightmeasure your height relative to your best friend orclassmate. When your class lines up, some kidsmake high “mountains†and others are morelike small hills!

Figure 2.5 Topographical map of the Earth showing North

America and South America.What scientists call relief or terrain includes

all the major features or landforms of a region. Atopographic map of an area shows the differencesin height or elevation for mountains, craters,valleys, and rivers. For example, Figure belowshows the San Francisco Mountain area in northernArizona as well as some nearby lava flows andcraters. We will talk about some differentlandforms in the next section.

Figure 2.6 This image was made from data of the Landsat

satellite and shows the topography of the SanFrancisco Mountain and surrounding areas innorthern Arizona. You can see the differences inelevation of the mountain and surrounding lava

flows.

Landforms

If you look at the Earth’s surface and takeaway the water in the oceans (Figure below), youwill see that the surface has two distinctivefeatures, continents and the ocean basins. Thecontinents are large land areas extending fromhigh elevations to sea level. The ocean basinsextend from the edges of the continents down steepslopes to the ocean floor and into deep trenches.

Figure 2.7 This image shows examples of some of the

main features found on the ocean floor, as well astheir above-water continuations. The red areas arehigh elevations (mountains). Yellow and greenareas are lower elevations and blue areas are thelowest on the ocean floor. (i.e., the image below is

a slice through a relief map of world.Both the continents and the ocean floor have

many features with different elevations. Someareas of the continents are high. These are themountains we have already talked about. Even onthe ocean floor there are mountains! Let’sdiscuss each.

Continents

Continents are relatively old (billions of years)compared to the ocean basins (millions of years).Because the continents have been around forbillions of years, a lot has happened to them! Ascontinents move over the Earth’s surface,mountains are formed when continents collide.Once a mountain has formed, it gradually wearsdown by weathering and erosion. Every continenthas mountain ranges with high elevations (Figurebelow). Some mountains formed a very long timeago and others are still forming today:

Young mountains (< 100 million years) –

Mountains of the Western United States(Rocky Mountains, Sierra Nevada,Cascades), Mountains around the edge of thePacific Ocean, Andes Mountains (SouthAmerica), Alps (Europe), HimalayanMountains (Asia)Old mountains (> 100 million years) –Appalachian Mountains (Eastern UnitedStates), Ural Mountains (Russia).

Mountains can be formed when the Earth’scrust pushes up, as two continents collide, like theAppalachian Mountains in the eastern UnitedStates and the Himalayas in Asia. Mountains canalso be formed by a long chain of volcanoes at theedge of a continent, like the Andes Mountains inSouth America.

Figure 2.8 Features of continents include mountain ranges,

plateaus, and plains.Over millions of years, mountains are worn

down by rivers and streams to form high flat areascalled plateaus or lower lying plains. Interiorplains are in the middle of continents while coastalplains are on the edge of a continent, where itmeets the ocean.

Figure 2.9 Summary of major landforms on continents and

features of coastlines.As rivers and streams flow across continents,

they cut away at rock, forming river valleys(Figure above). The bits and pieces of rockcarried by rivers are deposited where rivers meetthe oceans. These can form deltas, like the

Mississippi River delta and barrier islands, likePadre Island in Texas. Our rivers bring sand to theshore which forms our beaches.

Ocean Basins

The ocean basins begin where the ocean meetsthe land. The names for the parts of the oceannearest to the shore still have the word“continental†attached to them because thecontinents form the edge of the ocean. Thecontinental margin is the part of the ocean basinthat begins at the coastline and goes down to theocean floor. It starts with the continental shelf,which is a part of the continent that is underwatertoday. The continental shelf usually goes out about100 – 200 kilometers and is about 100-200meters deep, which is a very shallow area of theocean (Figure below).

Figure 2.10 Diagram of the continental shelf and slope of

the southeastern United States leading down to theocean floor.

From the edge of the continental shelf, thecontinental slope is the hill that forms the edge ofthe continent. As we travel down the continentalslope, before we get all the way to the ocean floor,there is often a large pile of sediments broughtfrom rivers, which forms the continental rise. Thecontinental rise ends at the ocean floor, which iscalled the abyssal plain.

Figure 2.11 A chain of seamounts is located off the coast of

New England (left) and oceanographers mappedone of these seamounts called Bear Seamount ingreat detail (right).

The ocean floor itself is not totally flat. Smallhills rise above the thick layers of mud that coverthe ocean floor. In many areas, small underseavolcanoes, called seamounts (Figure above) risemore than 1000 m above the seafloor. Besidesseamounts, there are long, very tall (about 2 km)mountain ranges that form along the middle parts ofall the oceans. They are connected in huge ridgesystems called mid-ocean ridges (Figure below).The mid-ocean ridges are formed from volcaniceruptions, when molten rock from inside the Earthbreaks through the crust, flows out as lava and

forms the mountains.

Figure 2.12 Map of the mid-ocean ridge system (yellow-

green) in the Earths oceans.The deepest places of the ocean are the ocean

trenches. There are many trenches in theworld’s oceans, especially around the edge ofthe Pacific Ocean. The Mariana Trench, which islocated east of Guam in the Pacific Ocean, is thedeepest place in the ocean, about 11 kilometersdeep (Figure below). To compare the deepestplace in the ocean with the highest place on land,Mount Everest is less than 9 kilometers tall. Inthese trenches, the ocean floor sinks deep insidethe Earth. The ocean floor gets constantly recycled.

New ocean floor is made at the mid-ocean ridgesand older parts are destroyed at the trenches. Thisrecycling is why the ocean basins are so muchyounger than the continents.

Figure 2.13 This map shows the location of the Mariana

Trench in the Pacific Ocean.The Earth’s surface is constantly changing

over long periods of time. For example, newmountains get formed by volcanic activity or upliftof the crust. Existing mountains and continentallandforms get worn away by erosion. Rivers andstreams cut into the continents and create valleys,plains, and deltas. Underneath the oceans, newcrust forms at the mid-ocean ridges, while oldcrust gets destroyed at the trenches. Wave activityerodes the tops of some seamounts and volcanicactivity creates new ones. You will explore theways that the Earth’s surface changes as youproceed through this book.

Lesson Summary

Earth scientists must be able to describe theexact positions or locations of features on theEarth’s surface.Positions often include distances anddirections. To determine direction, you canuse a compass, which has a tiny magneticneedle that points toward the Earth’s

magnetic North Pole. Once you have foundnorth, you can find east, west and south, usingyour compass for reference.Topography describes how the Earth’ssurface varies in elevation. Mountains formthe highest areas. Valleys and trenches formthe lowest areas. Both continents and oceanbasins have mountains and mountain ranges.They each also have plateaus, plains, andvalleys or trenches.Mountains form as continents collide and asvolcanoes erupt. Mountains are worn away bywind and water. The earth’s surface isconstantly changing due to these creative anddestructive processes.

Review Questions

1. What information might you need to describethe location of a feature on the Earth’ssurface?

2. Why would you need to know direction if an

object is moving?3. Explain how new ocean floor is created and

also how ocean crust is destroyed. Why arethe ocean basins younger than the continents?

4. Why do nautical charts have two compassroses on them

5. What landforms are the highest on thecontinents?

6. Explain what landforms on the continents arecreated by erosion from wind and water.How does erosion create a landform?

7. What is topography?

Further Reading/SupplementalLinks

http://www.cerritos.edu/earth-science/tutor/landform_identification. htmhttp://www.enotes.com/earth-science/landformshttp://oceanexplorer.noaa.gov/explorations/04etta/welcome

Vocabulary

abyssal plainThe very flat, deep ocean floor.

barrier islandA long, narrow island parallel to the shore.

beachesAreas along the shore where sand or gravelaccumulates.

compassHand-held device with a magnetic needleused to find magnetic north.

compass roseFigure on a map or nautical chart fordisplaying locations of north, south, east andwest.

continentLand mass above sea level.

continental marginSubmerged, outer edge of the continent.

continental riseGently sloping accumulation of sediments thatforms where the continental slope meets theocean floor.

continental shelfVery gently sloping portion of the continentcovered by the ocean.

continental slopeSloping, underwater edge of the continent.

deltaOften triangular shaped deposit of sediment atthe mouth of a river.

elevationHeight of a feature measured relative to sealevel.

mid-ocean ridge

A large, continuous mountain range found inthe middle of an ocean basin; marks adivergent plate boundary.

ocean basinsAreas covered by ocean water.

plainsLow lying continental areas, can be inland orcoastal.

plateausFlat lying, level elevated areas.

reliefDifference in height of landforms in a region.

river valleysAreas formed as water erodes the landscape,often 'V' shaped.

seamountUnderwater, volcanic mountain more than1000 meters tall.

topographyChanges in elevation for a given region.

Points to Consider

A volcano creates a new landform in Mexico.As the earth scientist assigned to study thisfeature, explain how you would describe itsposition in a report or scientificcommunication?Suppose you wanted to draw a map to showall the changes in elevation around the areawhere you live. How might you show lowareas and high areas? What would you do ifyou wanted this map to show these changes asif you were flying above your home?Why do you think continents are higher areason Earth than our ocean basins?

Modeling Earth’sSurfaceLesson Objectives

Describe what information a map can convey.Identify some major types of map projectionsand discuss the advantages and disadvantagesof each.Discuss the advantages and disadvantages ofusing a globe.

Maps as Models

Imagine you are going on a road trip. Perhapsyou are going on vacation. How do you knowwhere to go? Most likely, you will use a map.Maps are pictures of specific parts of theEarth’s surface. There are many types of maps.Each map gives us different information. Let’slook at a road map, which is the probably the most

common map that you use (Figure below).

Figure 2.14 shows a road map of the state of Florida. What

information can you get from this map?Look for the legend on the top left side of the

map. It explains how this map records differentfeatures. You can see the following:

The boundaries of the state show its shape.Black dots represent the cities. Each city isnamed. The size of the dot represents thepopulation of the city.Red and brown lines show major roads thatconnect the cities.Blue lines show rivers. Their names are

written in blue.Blue areas show lakes and other waterways -the Gulf of Mexico, Biscayne Bay, and LakeOkeechobee. Names for bodies of water arealso written in blue.A line or scale of miles shows the distancerepresented on the map – an inch orcentimeter on the map represents a certainamount of distance (miles or kilometers).The legend explains other features andsymbols on the map.Although this map does not have a compassrose, north is at the top of the map.

You can use this map to find your way aroundFlorida and get from one place to another alongroadways.

There are many other types of maps besidesroad maps. Some examples include:

Topographic maps show detailed elevationsof landscapes on the map.Relief maps show elevations of areas, but

usually on a larger scale. Relief maps mightshow landforms on a global scale rather thana local area.Satellite view maps show terrains andvegetation – forests, deserts, andmountains.Climate maps show average temperatures andrainfall.Precipitation maps show the amount ofrainfall in different areas.Weather maps show storms, air masses, andfronts.Radar maps also show storms and rainfall.Geologic maps detail the types and locationsof rocks found in an area.Political or geographic maps show theoutlines and borders of states and/orcountries.

These are but a few types of maps that variousearth scientists might use. You can easily carry amap around in your pocket or bag. Maps are easyto use because they are flat or two-dimensional.

However, the world is three-dimensional. So, howdo map makers represent a three-dimensionalworld on flat paper? Let’s see.

Map Projections

The Earth is a three-dimensional ball orsphere. In a small area, the Earth looks flat, so it isnot hard to make accurate maps of a small place.When map makers want to map the Earth on flatpaper, they use projections. Have you ever tried toflatten out the skin of a peeled orange? Or have youever tried to gift wrap a soccer ball to give to afriend as a present? Wrapping a round object withflat paper is difficult. A projection is a way torepresent the Earth’s curved surface on flatpaper (Figure below).

Figure 2.15 A map projection translates Earth's curved

surface onto two dimensions.There are many types of projections. Each uses

a different way to change three-dimensions intotwo-dimensions.

There are two basic methods that the mapmaker uses in projections:

The map maker “slices†the sphere insome way and unfolds it to make a flat map,like flattening out an orange peel.The map maker can look at the sphere from acertain point and then translate this view ontoa flat paper.

Let’s look at a few commonly used

projections.

Mercator Projection

In 1569, Gerardus Mercator (1512-1594)(Figure below) figured out a way to make a flatmap of our round world, called a Mercatorprojection (Figure below). Imagine wrapping ourround, ball shaped Earth with a big, flat piece ofpaper to make a tube or a cylinder. The cylinderwill touch the Earth at the equator, the imaginaryline running horizontally around the middle of theEarth, but the poles will be further away from thecylinder. If you could shine a light from the insideof your model Earth out to the cylinder, the imageyou would project onto the paper would be aMercator projection. Your map would be just rightat the equator, but the shapes and sizes ofcontinents would get more stretched out for areasnear the poles. Early sailors and navigators foundthe Mercator map useful because most explorers atthat time traveled to settlements that were located

near the equator. Many world maps still useMercator projection today.

Figure 2.16 Gerardus Mercator developed a map

projection used often today, known as the Mercatorprojection.

Figure 2.17 A Mercator projection translates the curved

surface of Earth onto a cylinder.The Mercator projection best describes the

shapes and sizes of countries within 15 degreesnorth or south of the equator. For example, if youlook at Greenland on a globe, you see it is arelatively small country near the North Pole. Yeton a Mercator projection, Greenland looks almostas big the United States. Greenland’s shapeand size are greatly increased, while the UnitedStates is represented closer to its true dimensions.In a Mercator projection, all compass directionsare straight lines, which makes it a good type ofmap for navigation. The top of the map is north, thebottom is south, the left side is west and the right

side is east. However, because it is a flat map of acurved surface, a straight line on the map is not theshortest distance between the two points itconnects.

Figure 2.18 A conic map projection wraps the Earth with a

cone shape rather than a cylinder.Instead of a cylinder, you might try wrapping

the flat paper into a cone. Conic map projectionsuse a cone shape to better represent regionsequally (Figure above). This type of map doesbest at showing the area where the cone shapetouches the globe, which would be along a line oflatitude, like the equator. Maybe you don’t liketrying to wrap a flat piece of paper around a roundobject at all. In this case, you could put a flat piece

of paper right on the area that you want to map.This type of map is called a gnomonic mapprojection (Figure below). The paper only touchesthe Earth at one point, but it will do a good jobshowing sizes and shapes of countries near thatpoint. The poles are often mapped this way, but itworks for any area that you chose.

Figure 2.19 A gnomonic projection places a flat piece of

paper on a point somewhere on Earth and projectsan image from that point.

Robinson Projection

In 1963, Arthur Robinson made a map thatlooks better in terms of shapes and sizes. Hetranslated coordinates onto the map instead ofusing mathematical formulas. He did this so thatregions on the map would look right. This map is

shaped like an ellipse (oval shape) rather than arectangle (Figure below).

Figure 2.20 A Robinson projection uses mathematical

formulas to best represent the true shapes and sizesof areas on Earth.

Robinson’s map shows less distortion nearthe poles and keeps shapes and sizes of continentsclose to their true dimensions, especially within 45degrees of the equator. The distances along theequator and lines parallel to it are true, but thescales along each line of latitude are different. In1988, the National Geographic Society adoptedRobinson’s projection for all of its worldmaps. Whatever map projection is used, maps aredesigned to help us find places and to be able toget from one place to another. So how do you findyour location on a map? Let’s look.

Map Coordinates

Most maps use a grid or coordinate system tofind your location. This grid system is sometimescalled a geographic coordinate system. The systemdefines your location by two numbers, latitude andlongitude. Both numbers are angles that you makebetween your location, the center of the Earth, anda reference line (Figure below).

Figure 2.21 Lines of latitude start with the equator. Lines of

longitude begin at the prime meridian.Lines of latitude circle around the Earth. The

equator is a line of latitude right in the middle ofthe Earth, which is the same distance from both the

North and South Pole. In a grid, your latitude tellsyou how far you are north or south of the equator.Lines of longitude are circles that go around theEarth from pole to pole, like the sections of anorange. Lines of longitude start at the PrimeMeridian, which is a circle that runs north to southand passes through Greenwich, England. Longitudetells you how far east or west you are from thePrime Meridian. You can remember latitude andlongitude by doing jumping jacks. When yourhands are above your head and your feet aretogether, say longitude (your body is long!), thenwhen you put your arms out to the sidehorizontally, say latitude (your head and armsmake a cross, like the “t†is latitude). Whileyou are jumping, your arms are going the same wayas each of these grid lines; horizontal for latitudeand vertical for longitude.

Figure 2.22 Lines of latitude and longitude form convenient

reference points on a map.If you know the latitude and longitude for a

particular place, you can find it on a map. Simplyplace one finger on the latitude on the vertical axisof the map. Place your other finger on the longitudealong the horizontal axis of the map. Move yourfingers along the latitude and longitude lines untilthey meet. For example, if the place you want tofind is at 30oN and 90oW, place your right fingeralong 30oN at the right of the map (Figure above).Place your left finger along the bottom at 90oW.Move them along the lines until they meet. Yourlocation should be near New Orleans, Louisianaalong the Gulf coast of the United States. Also, ifyou know where you are on a map, you can reverse

the process to find your latitude and longitude.One other type of coordinate system that you

can use to go from one place to another is a polarcoordinate system. Here your location is markedby an angle and distance from some referencepoint. The angle is usually the angle between yourlocation, the reference point, and a line pointingnorth. The other number is a distance in meters orkilometers. To find your location or move fromplace to place, you need a map, a compass, andsome way to measure your distance, such as arange finder. Suppose you need to go from yourlocation to a marker that is 20oE and 500 m fromyour current position. You must do the following:

Use the compass and compass rose on themap to orient your map with North.Use the compass to find which direction is20oE.Walk 500 meters in that direction to reachyour destination.

Figure 2.23 A topographic map like one that you might use

for the sport of orienteering.Polar coordinates are used most often in a

sport called orienteering. Here, you use a compassand a map to find your way through a course acrosswilderness terrain (Figure above). You moveacross the terrain to various checkpoints along thecourse. You win by completing the course to thefinish line in the fastest time.

Globe

A globe is the best way to make a map of thewhole Earth, because the Earth is a sphere and sois a globe. Because both the Earth and a globehave curved surfaces, sizes and shapes of countriesare not distorted and distances are true to scale.(Figure below).

Figure 2.24 A globe is the most accurate way to represent

Earth's curved surface.Globes usually have a geographic coordinate

system and a scale on them. The shortest distancebetween two points on a globe is the length of thearc (portion of a circle) that connects them.Despite their accuracy, globes are difficult to makeand carry around. They also cannot be enlarged toshow the details of any particular area. GoogleEarth is a neat site to download to your computer.This is a link that you can follow to get there:earth.google.com/download-earth.html. The mapson this site allow you to zoom in or out, look fromabove, tilt your image and lots more.

Lesson Summary

Maps and globes are models of theEarth’s surface. There are many ways toproject the three-dimensional surface of theEarth on to a flat map. Each type of map hassome advantages as well as disadvantages.Most maps use a geographic coordinatesystem to help you find your location usinglatitude and longitude.

Globes are the most accurate representations,because they are round like the Earth, but theycannot be carried around easily. Globes alsocannot show the details of the Earth’ssurface that maps can.

Review Questions

1. Which of the following gives you the mostaccurate representations of distances andshapes on the Earth’s surface?

1. Mercator projection map2. Robinson projection map3. Globe

2. Explain the difference between latitude andlongitude?

Figure 2.25

World map with geographic coordinatesystem

3. Use (Figure above). In what country are youlocated, if your coordinates are 60oN and120oW?

4. Which map projection is most useful fornavigation, especially near the equator?Explain

5. In many cases, maps are more useful than aglobe. Why?

6. Which of the following map projections givesyou the least distortion around the poles?

1. Mercator projection map2. Robinson projection map3. Conic projection

Further Reading / SupplementalLinks

http://erg.usgs.gov/isb/pubs/MapProjections/projections.htmlhttp://www.msnucleus.org/membership/html/jh/earth/mapstype/index.htm

http://erg.usgs.gov/isb/pubs/booklets/usgsmaps/usgsmaps.htmlhttp://www.mywonderfulworld.org/toolsforadventure/usingmaps/index.html, explorers.htmlhttp://maps.google.com/mapshttp://www.nationalatlas.gov/http://www.nationalatlas.gov/articles/mapping/a_projections.htmlhttp://www.nationalatlas.gov/articles/mapping/a_latlonghtmlhttp://www.fao.org/docrep/003/T0390E/T0390E04.htmhttp://en.wikipedia.org/

Vocabulary

conic mapA map projection made by projecting Earth'sthree dimensional surface onto a conewrapped around an area of the Earth.

coordinate systemNumbers in a grid that locate a particular

point.

gnomonic mapA map projection made by projecting onto aflat paper from just one spot on the Earth.

latitudeAn imaginary horizontal line drawn aroundthe Earth parallel to the equator, which is 0o

latitude.

longitudeAn imaginary vertical line drawn on theEarth, from pole to pole; the Prime Meridianis 0o longitude.

mapA two dimensional representation of Earth'ssurface.

Mercator projectionA map projection created by Mercator using acylinder wrapped around the Earth.

projectionA way to represent a three dimensionalsurface in two dimensions.

Points to Consider

Imagine you are a pilot and must fly fromNew York to Paris. Use a globe and a worldmap to do the following:

Plot your course from New York toParis on a globe. Make it the shortestdistance possible.Measure the distance by using the scale,a ruler, and a string.Draw the course from the globe on aworld map.Draw a line on the map connecting NewYork and Paris.

How does the course on the globe comparewith the line on the map? Which is theshortest distance? Write a brief paragraphdescribing the differences and explain why

they are different.Would you choose a map that used a Mercatorprojection if you were going to exploreAntarctica? Explain why this would not be agood choice. What other type of map wouldbe better?Maps use a scale, which means a certaindistance on the map equals a larger distanceon Earth. Why are maps drawn to scale? Whatwould be some problems you would havewith a map that did not use a scale?

Topographic Maps

Lesson Objectives

Describe a topographic map.Explain what information a topographic mapcontains.Explain how to read and interpret atopographic map.Explain how various earth scientists usetopographic maps to study the Earth.

What is a Topographic Map?

Mapping is a crucial part of earth science.Topographic maps represent the locations ofmajor geological features. Topographic maps use aspecial type of line, called a contour line, to showdifferent elevations on a map. Contour lines aredrawn on a topographic map to show the locationof hills, mountains and valleys. When you use a

regular road map, you can see where the roads go,but a road map doesn’t tell you why a roadstops or bends.A topographic map will show youthat the road bends to go around a hill or stopsbecause that is the top of a mountain. Let’slook at topographic maps.

Look at this view of the Swamp Canyon Trailin Bryce Canyon National Park, Utah (Figurebelow). You can see the rugged canyon walls andvalley below. The terrain clearly has many steepcliffs. There are high and low points between thecliffs.

Figure 2.26 View of Swamp Canyon in Bryce Canyon

National Park, looking southeast from Swamp

Canyon Trail overlook.Now look at the corresponding section of the

Visitor’s map (Figure below). You can see agreen line which is the main road. The black dottedlines are trails. You see some markers forcampsites, a picnic area, and a shuttle bus stop.But nothing on the map shows the height of theterrain. Where are the hills and valleys located?How high are the canyon walls? Which way willstreams or rivers flow?

Figure 2.27 Portion of Bryce Canyon National Park road

map showing Swamp Canyon Loop.You need a special type of map to represent the

elevations in an area. This type of map is called atopographic map (Figure below).

Figure 2.28 Topographic map of Swamp Canyon Trail

portion of Bryce Canyon National Park.What makes a topographic map different from

other maps? Contour lines help show variouselevations.

Contour Lines and Intervals

Contour lines connect all the points on the mapthat have the same elevation. Let’s take acloser look at this (Figure above).

Each contour line represents a specificelevation and connects all the places that areat the same elevation. Every fifth contour lineis bolded. The bold contour lines are labeledwith numerical elevations.The contour lines run next to each other andNEVER cross one another. That would meanone place had two different elevations, whichcannot happen.Two contour lines next to one another areseparated by a constant difference inelevation (e.g. 20 ft or 100 ft.). Thisdifference between contour lines is called thecontour interval. You can calculate thecontour interval. The legend on the map willalso tell you the contour interval.

Take the difference in elevation between2 bold lines.Divide that difference by the number of

contour lines between them.

If the difference between two bold lines is 100feet and there are five lines between them, what isthe contour interval? If you answered 20 feet, thenyou are correct (100 ft/5 = 20 ft)

Interpreting Contour Maps

How does a topographic map tell you about theterrain? Well, in reading a topographic map,consider the following principles:

1. Contour lines can indicate the slope of theland. Closely-spaced contour lines indicate a steepslope, because elevation changes quickly in asmall area. In contrast, broadly spaced contourlines indicate a shallow slope. Contour lines thatseem to touch indicate a very steep or vertical rise,like a cliff or canyon wall. So, contour lines showthe three-dimensional shape of the land. Forexample, on this topographic map of Stowe,Vermont (Figure below), you will see a steep hill

rising just to the right of the city of Stowe. You cantell this because the contour lines there are closelyspaced. Using the contour lines, you can see thatthe hill has a sharp rise of about 200 ft and then theslope becomes less steep as you proceed right.

Figure 2.29 Portion of a USGS topographic map of Stowe,

VT. In this map, you can see how the spacings ofthe contour lines indicate a steep hill just to theright of the city of Stowe in the right half. The hillbecomes less steep as you proceed right.

2 . Concentric circles indicate a hill. Figure

below shows another side of the topographic mapof Stowe, Vermont. When contour lines formclosed loops all together in the same area, this is ahill. The smallest loops are the higher elevationsand the larger loops are downhill. If you look atthe map, you can see Cady Hill in the lower leftand another, and another smaller hill in the upperright.

Figure 2.30 Portion of a USGS topographic map of Stowe,

VT. In this map, you can see Cady Hill (elevation1122 ft) indicated by concentric circles in the

lower left portion of the map and another hill(elevation ~ 960 ft) in the upper right portion of themap.

3 . Hatched concentric circles indicate adepression. The hatch marks are short,perpendicular lines inside the circle. Theinnermost hatched circle would represent thedeepest part of the depression, while the outerhatched circles represent higher elevations (Figurebelow).

Figure 2.31 On a contour map, a circle with inward hatches

indicates a depression.4. V-shaped portions of contour lines indicate

stream valleys. Here the V- shape of the contourlines “point†uphill. The channel of the streampasses through the point of the V and the open end

of the V represents the downstream portion. Thus,the V points upstream. A blue line will indicate thestream if water is actually running through thevalley; otherwise, the V patterns will indicatewhich way water will flow In Figure below, youcan see examples of V-shaped markings. Try tofind the direction a stream would flow.

Figure 2.32 Illustrations of 3-dimensional ground

configurations (top) and correspondingtopographic map (bottom). Note the V-shaped

markings on the topographic maps correspond todrainage channels. Also, the closely-spacedcontour lines denote the rapid rising cliff face onthe left side.

5. Like other maps, topographic maps have ascale on them to tell you the horizontal distance.The horizontal scale helps to calculate the slope ofthe land (vertical height/horizontal distance).Common scales used in United States GeologicalService (USGS) maps include the following:

1:24,000 scale – 1 inch = 2000 ft1:100,000 scale – 1 inch = 1.6 miles1:250,000 scale – 1 inch = 4 miles

So, the contour lines, their spacing intervals,circles, and V-shapes allow a topographic map toconvert 3-dimensional information into a 2-dimensional representation on a piece of paper.The topographic map gives us an idea of the shapeof the land.

Information from a Topographic

Map

As we mentioned above, topographic mapsshow the shape of the land. You can determineinformation about the slope and determine whichway streams will flow. We’ll examine each ofthese.

How Do Earth Scientists UseTopographic Maps?

Earth scientists use topographic maps for manythings:

Describing and locating surface features,especially geologic features.Determining the slope of the Earth’ssurface.Determining the direction of flow for surfacewater, ground water, and mudslides.

Hikers, campers, and even soldiers use

topographic maps to locate their positions in thefield. Civil engineers use topographic maps todetermine where roads, tunnels, and bridgesshould go. Land use planners and architects alsouse topographic maps when planning developmentprojects like housing projects, shopping malls, androads.

Oceanographers use a type of topographic mapcalled a bathymetric map (Figure below). In abathymetric map, the contour lines represent depthfrom the surface. Therefore, high numbers aredeeper depths and low numbers are shallowdepths. Bathymetric maps are made from depthsoundings or sonar data. Bathymetric maps helpoceanographers visualize the bottoms of lakes,bays, and the ocean. This information also helpsboaters to navigate safely.

Figure 2.33 Bathymetric map of Bear lake, UT.

Geologic Maps

A geologic map shows the geological featuresof a region. Rock units are shown in a color

identified in a key. On the map of Yosemite, forexample, volcanic rocks are brown, the TuolumneIntrusive Suite is peach and the metamorphosedsedimentary rocks are green. Structural features,for example folds and faults, are also shown on ageologic map. The area around Mt. Dana on theeast central side of the map has fault lines.

On a large scale geologic map, colorsrepresent geological provinces.

Lesson Summary

Topographic maps are two-dimensionalrepresentations of the three-dimensionalsurface features of a given area. Topographicmaps have contour lines which connect pointsof identical elevation above sea level.Contour lines run next to each other andadjacent contour lines are separated by aconstant difference in elevation, usually notedon the map. Topographic maps have ahorizontal scale to indicate horizontaldistances. Topographic maps help users see

the how the land changes in elevation.Many people use topographic maps to locatesurface features in a given area, to find theirway through a particular area, and todetermine the direction of water flow in agiven area.Oceanographers use a special type oftopographic map called a bathymetric map,which shows the bottom of any given body ofwater.Geologic maps display rock units andgeologic features of a region of any size. Asmall scale map displays individual rockunits while a large scale map shows geologicprovinces.

Review Questions

1. On a topographic map, you see contour linesforming closed loops that all lie in the samearea. Which of the following features wouldthis indicate?

a stream channela hilltopdepressiona cliff

2. Describe the pattern on a topographic mapthat would indicate a stream valley. How youwould determine the direction of water flow?

3. On a topographic map, five contour lines arevery close together in one area. The contourinterval is 100 ft. What feature does thatindicate? How high is this feature?

4. On a topographic map, describe how you cantell a steep slope from a shallow slope?

5. On a topographic map, a river is showncrossing from Point A in the northwest toPoint B in the southeast. Point A is on acontour line of 800 ft and Point B is on acontour line of 900 ft. In which direction doesthe river flow? What information would helpyou figure this out?

6. On a topographic map, six contour lines spana horizontal distance of 0.5 inches. Thehorizontal scale is 1 inch equals 2000 ft. How

far apart are the first and sixth lines?7. On a geologic map of the Grand Canyon, a

rock unit called the Kaibab Limestone takesup the entire surface of the region. Downsome steep topographic lines is a very thinrock unit called the Toroweap Formation andjust in from that is another thin unit, theCoconino sandstone. Describe how thesethree rock units sit relative to each other.

Further Reading / SupplementalLinks

http://interactive2.usgs.gov/learningweb/teachers/mapsshow_lesson4.htmhttp://erg.usgs.gov/isb/pubs/booklets/symbols/topomapsymbols.pdfhttp://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/manuals/instructor_manual/how_to/topographic_profile.htmlhttp://raider.muc.edu/~mcnaugma/Topographic%20Maps/topomapindexpage.htmhttp://www.globalsecurity.org/military/library/policy/army/fm/3-25-26/ch10.htmhttp://www.map-reading.com/intro.php

Vocabulary

bathymetric mapA special type of topographic map used byoceanographers that show depth of areasunderwater.

contour intervalThe constant difference in elevation betweentwo contour lines on a topographic map.

contour linesLines drawn on a topographic map to showelevation; these lines connect all the placesthat are the same elevation.

topographic mapA special type of map that show elevations ofdifferent geologic features of a region.

Points to Consider

Imagine that you are a civil engineer.Describe how you might use a topographicmap to build a road, bridge, or tunnel through

the area such as that shown in Figure 2.30.Would you want your road to go up and downor remain as flat as possible? What areaswould need a bridge in order to cross themeasily? Can you find a place where a tunnelwould be helpful?

If you wanted to participate in orienteering,would it be better to have a topographic mapor a regular road map? How would atopographic map help you?

If you were the captain of a very large boat,what type of map would you want to have tokeep your boat traveling safely?

Using Satellites andComputers

Lesson Objectives

Describe various types of satellite images andthe information that each provides.Explain how a Global Positioning System(GPS) works.Explain how computers can be used to makemaps.

Satellite Images

If you look at the surface of the Earth from youryard or street, you can only see a short distance. Ifyou climb a tree or go to the top floor of yourapartment building, you can see further. If you flewover your neighborhood in a plane, you could seestill further. Finally, if you orbited the Earth, you

would be able to see a very large area of the Earth.This is the idea behind satellites. To see things ona large scale, you need to get the highest view.

Figure 2.34 (left) Track of hurricane that hit Galveston,

Texas on Sept. 8, 1900. (right) Galveston in theaftermath.

Let’s look at an example. One of thedeadliest hurricanes in United States history hitGalveston, Texas in 1900. The storm was firstspotted at sea on Monday, Aug 27, 1900. It was atropical storm when it hit Cuba on Sept. 3rd. BySept. 8th, it had intensified to a hurricane over theGulf of Mexico. It came ashore at Galveston(Figure above). There was not advanced warningor tracking at the time. Over 8000 people lost theirlives.

Figure 2.35 Satellite view shows four hurricanes in the

Atlantic Ocean on Sept. 26, 1998.Today, we have satellites with many different

types of instruments that orbit the Earth. With thesesatellites, we can see hurricanes (Figure above).Weather forecasters can follow hurricanes as theymove from far out in the oceans to shore. Weatherforecasters can warn people who live along thecoasts. Their advanced warning gives people timeto prepare for the storm, which helps save lives.

Figure 2.36 Satellite in a geostationary orbit.Satellites orbit high above the Earth in several

ways. One of the most useful ways is called thegeostationary orbit (Figure above).

The satellite orbits at a distance of 36,000 km.It takes 24 hours to complete one orbit. Since thesatellite and the Earth both complete one rotationin 24 hours, the satellite appears to “hang†inthe sky over the same spot. In this orbit, thesatellite stays over one area of the Earth’ssurface. Weather satellites use this type of orbit toobserve changing weather conditions.Communications satellites, like satellite TV, alsouse this type of orbit.

Figure 2.37 Satellite in a polar orbit.Another useful orbit is the polar orbit (Figure

above). The satellite orbits at a distance of severalhundred kilometers. It makes one complete orbitaround the Earth from the North Pole to the SouthPole about every 90 minutes. In this same amountof time, the Earth rotates slightly underneath thesatellite. In less than a day, the satellite can see theentire surface of the Earth. Some weather satellitesuse a polar orbit to get a picture of how theweather is changing globally. Also some satellitesthat observe the lands and oceans use a polar orbit.

Figure 2.38 NASAs fleet of satellites to study the Earth.The National Aeronautics and Space

Administration (NASA) has launched a fleet ofsatellites to study the Earth (Figure above). Thesatellites are operated by several governmentagencies, including NASA, the NationalOceanographic and Atmospheric Administration(NOAA) and the United States Geological Survey(USGS). By using different types of scientificinstruments, satellites make many kinds ofmeasurements of the Earth.

Some satellites measure the temperatures ofthe land and oceans.

Some record amounts of gases in theatmosphere such as water vapor and carbondioxide.Some measure their height above the oceansvery precisely.

From this information, they can get an idea ofthe sea surface below.

Some measure the ability of the surface toreflect various colors of light. Thisinformation tells us about plant life.

Some examples of the images from these typesof satellites are shown in Figure below).

Figure 2.39 Various satellite images: A water vapor in

atmosphere, B ocean surface temperatures, Cglobal vegetation.

Global Positioning System

Previously, we talked about your position onEarth. In order to locate your position on a map,you must know your latitude and your longitude.But you need several instruments to measurelatitude and longitude. What if you could do thesame thing with only one instrument? Satellites canalso help you locate your position on theEarth’s surface (Figure below).

Figure 2.40 There are 24 satellites in the US Global

Positioning System.By 1993, the United States military had

launched 24 satellites to help soldiers locate theirpositions on battlefields. This system of satelliteswas called the Global Positioning System (GPS).Later, the United States government allowed thepublic to use this system. Here’s how it works.

Figure 2.41 (A) You need a GPS receiver to use the GPS

system. (B) It takes signals from 4 GPS satellitesto find your location precisely on the surface

You must have a GPS receiver to use thesystem (Figure A above). You can buy many ofthese in stores. The GPS receiver detects radiosignals from nearby GPS satellites. There areprecise clocks on each satellite and in the receiver.The receiver measures the time for radio signalsfrom satellite to reach it. The receiver uses thetime and the speed of radio signals to calculate thedistance between the receiver and the satellite. Thereceiver does this with at least four differentsatellites to locate its position on the Earth’ssurface (Figure B above). GPS receivers are nowbeing built into many items, such as cell phonesand cars.

Computer-Generated Maps

Prior to the late 20th and early 21st centuries,

map-makers sent people out in the field todetermine the boundaries and locations for variousfeatures for maps. State or county borders wereused to mark geological features. Today, people inthe field use GPS receivers to mark the locationsof features. Map-makers also use various satelliteimages and computers to draw maps. Computersare able to break apart the fine details of a satelliteimage, store the pieces of information, and putthem back together to make a map. In someinstances, computers can make 3-D images of themap and even animate them. For example,scientists used computers and satellite images fromMars to create a 3-D image of a large Martianvalley called Valles Marineris (Figure below).The image makes you feel as if you are on thesurface of Mars and looking into the valley.

Figure 2.42 This three-dimensional image of a large valley

on Mars was made from satellite images andcomputers.

When you link any type of information to ageographical location, you can put togetherincredibly useful maps and images. Theinformation could be numbers of people living inan area, types of plants or soil, locations ofgroundwater or levels of rainfall for an area. Aslong as you can link the information to a positionwith a GPS receiver, you can store it in a computerfor later processing and map-making. This type ofmapping is called a Geographic InformationSystem (GIS). Geologists can use GIS to make

maps of natural resources. City leaders might linkthese resources to where people live and help planthe growth of cities or communities. Other types ofdata can be linked by GIS. For example, Figurebelow shows a map of the counties where farmershave made insurance claims for crop damage in2008.

Figure 2.43 Map of insurance filings for crop damage in

2008.Computers have improved how maps are

made. They have also increased the amount ofinformation that can be displayed. During the 21st

century, computers will be used more and more inmapping.

Lesson Summary

Satellites give a larger view of theEarth’s surface from high above. Theymake many types of measurements for earthscientists.A group of specialized satellites calledGlobal Positioning Satellites help people topinpoint their location.Location information, satellite views, andother information can be linked together inGeographical Information Systems (GIS).GIS are powerful tools that earth scientistsand others can use to study the Earth and itsresources.

Review Questions

1. Which type of satellite can be used topinpoint your location on Earth?

weather satellitecommunications satellite

global positioning satelliteclimate satellite

2. Explain the difference betweengeosynchronous orbits and polar orbits?

3. Describe how GPS satellites can find yourlocation on Earth?

4. What is a Geographical Information Systemor GIS?

5. If you want to map the entire Earth’ssurface from orbit, which type of orbit wouldyou use?

6. Explain how weather satellites could track atropical storm from its beginnings?

Further Reading / SupplementalLinks

Nous, A., “Satellite Imaging,†The ScienceTeacher, Dec. 1998. Available on the Web at:

pdf.http://www3.nsta.org/main/news/pdf/tst9812_46.

pdf.

"Isaac’s Storm": Available on the Web at:

http://www.randomhouse.com/features/isaacsstorm/.

USGS, Geographic Information Systems

http://erg.usgs.gov/isb/pubs/gis_poster/

GIS.com, What is GIS?

html http://www.gis.com/whatisgis/index.html

USGS Topographical Mapping

http://erg.usgs.gov/isb/pubs/booklets/topo/topo.htmlhttp://www.ccpo.odu.edu/SEES/index.htmlhttp://earthobservatory.nasa.gov/http://www.noaa.gov/satellites.htmlhttp://landsat.usgs.gov/http://school.discoveryeducation.com/lessonplans/programshttp://oceanexplorer.noaa.gov/explorations/07mexico/background/navigation/navigation.html

http://www.geoplane.com/gps. htmlhttp://oceanservice.noaa.gov/education/kits/geodesy/geo09_gps.html

Vocabulary

Geographic Information System (GIS)An information system that links data to aparticular location.

geostationary orbitA type of orbit that allows a satellite to stayin above one location on Earth's surface.

polar orbit satelliteOrbit that moves over Earth's north and southpoles as Earth rotates underneath.

Points to Consider

Imagine that you are tracking a hurricaneacross the Atlantic Ocean. What information

would you need to follow its path? Whatsatellite images might be most useful?Research and explain how the NationalWeather Service tracks and monitorshurricanes.If you had to do a report on the naturalresources for a particular state, what type ofmap would help you find the mostinformation?What are some ways that people use GlobalPositioning Systems? What problems areeasier to solve using GPS?

Chapter 3: Earth's Minerals

What are Minerals?

Lesson Objectives

Describe the characteristics that all mineralsshare.Summarize the structure of minerals.Identify the groups in which minerals areclassified.

What are Minerals?

You use objects that are made from mineralsevery day, even if you do not realize it. You areactually eating a mineral when you eat food thatcontains salt. You are drinking from a containermade from a mineral when you drink from a glass.You might even wear silver jewelry. The shiny

metal silver, the white grains of salt, and clearglass may not seem to have much in common, butthey are all made from minerals (Figure below).Silver is a mineral. Table salt is the mineral halite.Glass is produced from the mineral quartz.Scientists have identified more than 4,000 mineralsin Earth’s crust. Some minerals are found in verylarge amounts, but most minerals are found insmall amounts. If minerals can be so different fromeach other, what makes a mineral a mineral?

Figure 3.1 Silver is used to make sterling silver jewelry.

Table salt is the mineral halite. Glass is producedfrom the mineral quartz.

A mineral is a crystalline solid formed throughnatural processes. A mineral can be an element or

a compound, but it has a specific chemicalcomposition and physical properties that aredifferent from those of other minerals. Silver,tungsten, halite, and quartz are all examples ofminerals. Each one has a different chemicalcomposition, as well as different physicalproperties such as crystalline structure, hardness,density, flammability, and color. For example,silver is shiny and salt is white.

Natural Processes

Minerals are made by natural processes. Anatural process occurs in or on the Earth. Onecommon natural process that forms minerals is thecrystallization of magma. Some natural processesshape Earth’s features, while others includevolcanic activity and the movement of tectonicplates. Rocks and minerals are formed insedimentary layers of sand and mud and in thefolding of those layers deep in the Earth, wherethey are exposed to high pressures andtemperatures. A technician might make a gemstone

in the laboratory, but this would have been createdsynthetically, not by natural processes.

Inorganic Substances

A mineral is an inorganic substance, whichusually means it was not made by living organisms.Organic substances are all the carbon-basedcompounds made by living creatures, includingproteins, carbohydrates, and oils. This definitionincludes fossil fuels such as coal and oil, whichwere originally made by living organisms millionsof years ago. Everything else is consideredinorganic. In a few exceptional cases, livingorganisms produce inorganic materials, such as thecalcium carbonate shells of marine organisms.

Crystalline Solids

Minerals are crystalline solids. Therefore,natural inorganic substances that are liquids are notminerals. For example, liquid water is inorganic,

but it is not a mineral because it is a liquid. Evensome solids may not be crystalline. A crystal is asolid in which the atoms are arranged in a regular,repeating pattern. Figure below shows how theatoms are arranged in table salt (halite). Table saltcontains the ions sodium and chloride. Notice howthe atoms are arranged in an orderly way. Also,notice that pattern continues in all threedimensions.

Figure 3.2 As you can see from this model, sodium ions

bond with chloride ions in a certain way to form

halite crystals. The green balls represent thechloride ions and the purple balls represent thesodium ions.

The pattern of atoms in different samples of thesame mineral is the same. Think about all of thegrains of salt that are in a salt shaker. The atomsare arranged in the same way in every piece ofsalt.

Chemical Composition

All minerals have a specific chemicalcomposition. Minerals are either pure elements orchemical compounds. An element is a substance inwhich all of the atoms have the same number ofprotons. (Protons are the positive particles in thecenter of every atom, the nucleus.) You cannotchange an element into another element bychemical means because the number of protonsdoes not change. Silver, sodium, silicon, andoxygen are a few of the elements found in minerals.A few minerals are made of only one kind ofelement. The mineral silver is a pure element

because it is made up of only silver atoms.Most minerals, such as halite and quartz, are

made up of chemical compounds. A chemicalcompound is a substance in which the atoms oftwo or more elements bond together. The elementsin a chemical compound are in a certain ratio.Solid water (ice) is probably one of the simplestcompounds that you know. As you can see inFigure below, a molecule of the compound wateris made of two hydrogen atoms and one oxygenatom. All water molecules have a ratio of twohydrogen atoms to one oxygen atom. In ice, all thewater molecules are arranged in a definite, orderlypattern.

Figure 3.3

As this model of a water molecule shows, allwater molecules have two hydrogen atoms (shownin gray) bonded to one oxygen molecule (Shown inred).

Minerals that are not pure elements are madeof compounds. For example, the mineral quartz ismade of the compound silicon dioxide, or SiO2.This compound has one atom of the element siliconfor every two atoms of the element oxygen. When amineral has a different chemical formula, it is adifferent mineral. For example, the mineralhematite has two iron atoms for every threeoxygen atoms, while the mineral magnetite hasthree iron atoms for every four oxygen atoms.Many minerals contain more complex chemicalcompounds that are made of several elements.However, even the elements in more complicatedcompounds occur in certain ratios.

Structure of Minerals

The crystal structure of a mineral affects the

mineral’s physical properties. Imagine you havethree samples of halite. Each sample was found ina different country. They are all different sizes andshapes. They may have even been formed bydifferent geologic processes. Will the samples allhave the same crystal structure? Yes! All halite hasthe same chemical composition and the samecrystal structure, despite physical differences.

Crystals in Minerals

The shape of the crystals of a mineral isdetermined by the way the atoms are arranged.When crystals grow large, you can see how thearrangement of atoms influences the shape. Noticehow the large halite crystal in Figure above hassquare shapes. This shape is the result of thepattern of sodium and chlorine atoms in crystal.Now, compare the crystal in Figure above with thegrains of salt magnified under a microscope shownin Figure below. These small crystals have similarshapes to the large crystal. You can see that the

shapes of the crystals are made up of squares. Youcan try this at home. If you sprinkle salt into yourhand and look carefully at each grain of salt, youwill see that it is perfect little cube.

Figure 3.4 When you look at grains of table salt under a

microscope, you can see that the crystals are madeof square shapes.

Large crystals only form when they have roomto grow. Often, crystals are very small. Even if youcannot see the individual crystals in a mineralsample, the atoms are still ordered in a regular,repeating pattern. This pattern can be used to help

identify an unknown mineral sample. A trainedscientist may be able to determine the crystalstructure by the shape of a large crystal. If theycannot figure out the crystal structure by looking atthe mineral, scientists use an instrument that uses Xrays to find out how the atoms are arranged in amineral sample.

A mineral has both a characteristic chemicalcomposition and a characteristic crystal structure.Sometimes, minerals have the same chemicalcomposition, but different crystal structures. Whatdo you know about diamond and graphite?Diamonds are valued as gems for jewelry. Theyare also very hard. Graphite is used as pencil leadand has a slippery feel. Compare the diamond withthe pencil lead in Figure below. Diamond andgraphite are both made of only carbon, but they arenot the same mineral. The crystal structure ofdiamond differs from the crystal structure ofgraphite. The carbon atoms in graphite bond toform layers. The bonds between each layer areweak, so the sheets can slip past each other. Thecarbon atoms in diamonds bond together in all

three directions to form a strong network. As aresult, the properties of diamond differ from theproperties of graphite.

Figure 3.5 Even though they are both made of carbon,

diamonds and graphite have differentcharacteristics.

Groups of Minerals

Imagine you were in charge of organizing morethan 100 minerals for an exhibit at a museum nearyou. You want the people who visit your exhibit tolearn as much as possible about the minerals theysee. How would you group the minerals together inyour exhibit? Mineralogists are scientists whostudy minerals. They use a system that dividesminerals into groups based on chemicalcomposition and structure. Even though there are

over 4,000 minerals, most minerals fit into one ofeight mineral groups. Minerals with similar crystalstructures are grouped together.

Silicate Minerals

Silicate minerals make up over 90 percent ofEarth's crust. When you think of the Earth's crust,you may think of the people, animals or trees thatlive on the Earth's surface. Yet living organismsare made of organic matter and there is only asmall amount of organic matter in Earth's crust.About 1,000 silicate minerals have been identified,making the silicate minerals the largest mineralgroup.

Silicates are minerals that contain siliconatoms bonded to oxygen atoms. The basic buildingblock for all silicate minerals is called atetrahedron, where one silicon atom is bonded to 4oxygen atoms (Figure below). Silicate mineralsalso often contain other elements, such as calcium,iron, and magnesium.

Figure 3.6 One silicon atom bonds to four oxygen atoms to

form the building block of silicate minerals.Notice that the silicon and oxygen form a shape

like a pyramid; this is the tetrahedron. Thepyramid-shaped building blocks can combinetogether in numerous ways. The silicate mineralgroup is divided into six smaller groups, which aredetermined by the way the silicon-oxygen buildingblocks join together. The pyramids can standalone, form into connected circles called rings,link into single and double chains, form large flatsheets of pyramids or join in three dimensions.

Feldspar and quartz are the two most commonsilicate minerals. Beryl is a silicate mineral, whichforms rings from the tetrahedra. The gemstoneemerald is a type of beryl that is green because ofchemical impurities. Biotite is a mica, which isanother silicate mineral that can be broken apartinto thin, flexible sheets. Compare the beryl andthe biotite shown in Figure below.

Figure 3.7 Beryl and biotite are both silicate minerals.

Native Elements

Native elements are minerals that contain onlyatoms of one type of element. The elements are notcombined with other elements. In nature mostelements are combined with other elements to formchemical compounds. So, the native elementsmineral group contains a relatively small number

of minerals. Some of the minerals in this group arerare and valuable. Gold, silver, sulfur, anddiamond are examples of native elements.

Carbonates

From the name “carbonate,” what would youguess carbonate minerals contain? If you guessedcarbon, you would be right! More specifically, allcarbonates contain one carbon atom bonded tothree oxygen atoms. Carbonates may include otherelements, such as calcium, iron, and copper.

Carbonate minerals are often found in areaswhere ancient seas once covered the land. Somecarbonate minerals are very common. Calcite isone such mineral. Calcite contains calcium,carbon, and oxygen. Have you ever been in alimestone cave or seen a marble tile? Calcite is inboth limestone and marble. Azurite and malachiteare also carbonate minerals, but they containcopper instead of calcium. They are not ascommon as calcite, as you can see in Figurebelow, they are very colorful.

Figure 3.8 Azurite is a deep blue carbonate mineral.

Malachite is an opaque green carbonate mineral.

Halides

Halide minerals are salts that can form whensalt water evaporates. This mineral class includesmore than just table salt. It includes minerals thatcontain the elements fluorine, chlorine, bromine, oriodine. These elements combine with metalelements. Halite is a halide mineral that containsthe elements chlorine and sodium. Fluorite isanother type of halide that contains fluorine andcalcium. Fluorite can be found in many colors. Ifyou shine an ultraviolet light on some samples offluorite, they will glow!

Oxides

Earth’s crust contains a lot of oxygen, whichcombines with many other elements. Oxides areminerals that contain one or two metal elementscombined with oxygen. Oxides are different fromsilicates because oxides do not contain silicon.Many important metals are found as oxides. Forexample, hematite and magnetite are both oxidesthat contain iron. Hematite (Fe2O3) has a ratio oftwo iron atoms to three oxygen atoms. Magnetite(Fe3O4) has a ratio of three iron atoms to fouroxygen atoms. You might have noticed that thew o r d magnetite contains the word magnet.Magnetite is a magnetic mineral.

Figure 3.9

Phosphates

Phosphates have a tetrahedron building blockthat is similar to that of the silicates. But, insteadof silicon, phosphates have an atom of phosphorus,arsenic, or vanadium bonded to oxygen. Althoughthere are many minerals in this group, most of theminerals are rare. The chemical composition ofthese minerals tends to be more complex than someof the other mineral groups. Turquoise is aphosphate mineral that contains copper, aluminum,and phosphorus. It is rare and is used to make

jewelry.

Figure 3.10

Sulfates

Sulfate minerals contain sulfur atoms bonded tooxygen atoms. Like halides, they can form inplaces where salt water evaporates. Manyminerals belong in the sulfate group, but there areonly a few common sulfate minerals. Gypsum is acommon sulfate mineral that contains calcium,sulfate, and water. Gypsum is found in various

forms. For example, it can be pink and look like ithas flower petals. However, it can also grow intovery large white crystals. Gypsum crystals that are11 meters long have been found—that is about aslong as a school bus! Gypsum also forms the whitesands of White Sands National Monument in NewMexico, shown in Figure below.

Figure 3.11 The white gypsum sands at White Sands

National Monument look like snow.

Sulfides

Sulfides contain metal elements combined withsulfur. Unlike sulfates, sulfides do not containoxygen. Pyrite, a common sulfide mineral, contains

iron combined with sulfur. Pyrite is also known asfool’s gold. Gold miners have mistaken pyrite forgold because the two minerals look so similar.

Lesson Summary

For a substance to be a mineral, it must be anaturally occurring, inorganic, crystallinesolid that has a characteristic chemicalcomposition and crystal structure.The atoms in minerals are arranged in regular,repeating patterns that can be used to identifya mineral.Minerals are divided into groups based ontheir chemical composition.

Review Questions

1. What is a crystal?2. Which elements do all silicate minerals

contain?3. Obsidian is a glass that formed when lava

cools so quickly that the atoms to not have achance to arrange themselves in crystals. Isobsidian a crystal? Explain your reasoning.

4. What are the eight major mineral groups?5. One mineral sample has a ratio of two iron

atoms to three oxygen atoms. Another samplehas a ratio of three iron atoms to four oxygenatoms. Explain whether the mineral samplesare made of the same chemical compound.

6. How does the native elements mineral groupdiffer from all of the other mineral groups?

7. You take a trip to the natural history museumwith your friend. During your visit your friendsees two minerals that are similar in color.One mineral contains the elements zinc,carbon, and oxygen. The other mineralcontains the elements zinc, silicon, oxygen,and hydrogen. Your friend tells you that theminerals are in the same mineral group.Would you agree with your friend? Explainyour reasoning.

Further Reading / SupplementalLinks

http://www.minsocam.org/MSA/K12/uses/uses.htmlhttp://en.wikipedia.org/wiki/Glasshttp://www.science.uwaterloo.ca/~cchieh/cact/applychem/inorganic.htmlhttp://chemed.chem.purdue.edu/genchem/topicreview/bp/ch10/carbon.phphttp://www.usgs.gov/science/science.php?term=451http://www.minsocam.org/msa/ima/ima98(04).pdf;nhttp://wrgis.wr.usgs.gov/docs/parks/misc/glossarymn.htmlhttp://geology.csupomona.edu/alert/mineral/minerals.htmhttp://en.wikibooks.org/wiki/Regents_Earth_Science_(High_School)#Rocks_and_Mineralshttp://www.minsocam.org/MSA/K12/crystals/crystal.htmlhttp://www.minsocam.org/MSA/K12/groups/groups.htmlhttp://webmineral.com/danaclass.shtmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/geophys/mineral.html#c1http://mineral.galleries.com/minerals/silicate/class.htmhttp://library.eb.com/eb/article-9067760http://www.minsocam.org/MSA/K12/groups/silicates.htmlhttp://geology.wr.usgs.gov/parks/rxmin/mineral.html#biotite

http://www.minsocam.org/MSA/K12/groups/natives.htmlhttp://www.minsocam.org/MSA/K12/crystals/crystal.htmlhttp://mineral.galleries.com/minerals/silicate/class.htmhttp://news.nationalgeographic.com/news/2007/04/photogalleries/giant-crystals-cave/photo3.html

Vocabulary

chemical compoundA substance in which the atoms of two ormore elements bond together.

crystalA solid in which all the atoms are arranged ina regular, repeating pattern.

elementA substance in all of the atoms have the samenumber of protons.

mineralA naturally occurring, inorganic, crystallinesolid with a characteristic chemical

composition.

mineralogistA scientist who studies minerals.

silicatesMinerals that contain silicon atoms bonded tooxygen atoms.

Points to Consider

Scientists can make diamonds. The diamondsthey make are called synthetic diamonds.Explain whether or not you think syntheticdiamonds are minerals.Artists used to grind up the mineral azurite tomake colorful pigments for paints. Is thepowdered azurite still crystalline?What is one way you could tell the differencebetween two different minerals?

Identification of MineralsSuppose you bought a new shirt with the money

you saved from your allowance. How would youdescribe your shirt when you are talking to yourbest friend on the phone? You might describe thecolor, the way the fabric feels, and the length of thesleeves. These are all physical properties of yourshirt. If you did a good job describing your shirt,your friend would recognize the shirt when youwear it. Minerals also have physical propertiesthat are used to identify them.

Lesson Objectives

Explain how minerals are identified.Describe how color, luster, and streak areused to identify minerals.Summarize specific gravity.Explain how the hardness of a mineral ismeasured.

Describe the properties of cleavage andfracture.Identify additional properties that can be usedto identify some minerals.

How are Minerals Identified?

Imagine you were given a mineral samplesimilar to the one shown in Figure below. Howwould you try to identify your mineral? If you werea mineralogist, you would use certain properties toidentify the mineral. Mineralogists are scientistswho study minerals. You can observe someproperties by looking at the mineral. For example,you can see that the mineral in Figure below is thecolor of gold and is shiny. But, you cannot see allmineral properties. You need to do simple tests todetermine some properties, such as how hard themineral is. You can use a mineral’s properties todetermine its identity because the properties aredetermined by the chemical composition andcrystal structure, or the way that the atoms are

arranged.

Figure 3.12 You can use the properties of a mineral to

identify it.

Color, Streak, and Luster

Diamonds have many valuable properties, butone of the reasons they are used in jewelry isbecause they are sparkly. Turquoise is anothermineral that is used to make jewelry. However,turquoise is prized for its striking greenish-bluecolor. Even minerals that are not used to makejewelry often have interesting appearances.

Specific terms are used to describe the appearanceof minerals.

Color

Color is probably the easiest property toobserve. Unfortunately, you can rarely identify amineral only by its color. Sometimes differentminerals are the same color. Take another look atFigure above. The mineral is a gold color, so youmight think that it is gold. The mineral is actuallypyrite, or "fool's gold," which is made of iron andsulfide. It contains no gold atoms.

Often, the same mineral comes in differentcolors. Figure below shows two samples of quartz—one is colorless (although on a purplebackground) and one is purple. The purple color ofthe quartz comes from a tiny amount of iron in thecrystal. The iron in quartz is a chemical impuritybecause it is not normally found in quartz. Manyminerals are colored by chemical impurities. Otherfactors, such as weathering, can also affect a

mineral’s color. Weathering affects the surface of amineral. Because color alone is unreliable,geologists identify minerals by several traits.

Figure 3.13 Even though these mineral samples are not the

same color, they are both quartz. Amethyst isquartz that is purple. The white quartz on the leftappears slightly purple only because it is on apurple background.

Streak

Streak is the color of the powder of a mineral.To do a streak test, you scrape the mineral acrossan unglazed porcelain plate. The plate is harderthan many minerals, causing the minerals to leave astreak of powder on the plate. The color of thestreak often differs from the color of the largermineral sample, as Figure below shows. If you dida streak test on the yellow-gold pyrite, you would

see a blackish streak. This blackish streak tells youthat the mineral is not gold because gold has agold-colored streak.

Figure 3.14 You rub a mineral across an unglazed

porcelain plate to determine the streak. Thehematite shown here has a red-brown streak.

Streak is a more reliable property than thecolor of the mineral sample. The color of a mineralmay vary, but its streak does not vary. Also,different minerals may be the same color, but theymay have a different color streak. For example,samples of hematite and galena can both be dark

gray, but hematite has a red streak and galena has agray streak.

Luster

Luster describes the way light reflects off ofthe surface of the mineral. You might describediamonds as sparkly or pyrite as shiny, butmineralogists have special terms to describe theluster of a mineral. They first divide minerals intometallic and non-metallic luster. Minerals likepyrite that are opaque and shiny have a metallicluster. Minerals with a non-metallic luster do notlook like metals. There are many types of non-metallic luster, six of which are described in theTable below.

Minerals with Non-Metallic LusterNon-MetallicLuster Appearance

Adamantine SparklyEarthy Dull, clay-like

Pearly Pearl-likeResinous Like resins, such as tree sapSilky Soft-looking with long fibersVitreous Glassy

(Source: http://en.wikipedia.org/wiki/Mineral,License: GNU-FDL)

Can you match the minerals in Figure belowwith the correct luster from Table (above withoutlooking at the caption?

Figure 3.15 Diamond has an adamantine luster. Quartz is

not sparkly like a diamond is. It has a vitreous, orglassy, luster. Sulfur reflects less light than quartz,so it has a resinous luster.

Density

You are going to visit a friend. You fill one

backpack with books so you can study later. Youstuff your pillow into another backpack that is thesame size. Which backpack will be easier tocarry? Even though the backpacks are the samesize, the bag that contains your books is going to bemuch heavier. It has a greater density than thebackpack with your pillow.

Density describes how much matter is in acertain amount of space. Substances that have morematter packed into a given space have higherdensities. The water in a drinking glass has thesame density as the water in a bathtub orswimming pool. All substances have characteristicdensities, which does not depend on how much ofa substance you have.

Mass is a measure of the amount of matter in anobject. The amount of space an object takes up isdescribed by its volume. So, density of an objectdepends on its mass and its volume. Density can becalculated using the following equation.

Samples that are the same size, but havedifferent densities, will have different masses.

Gold has a density of about 19 g/cm3. Pyrite has adensity of only about 5 g/cm3. Quartz is even lessdense than pyrite and has a density of 2.7 g/cm3. Ifyou picked up a piece of pyrite and a piece ofquartz that were the same size, the pyrite wouldseem almost twice as heavy as the quartz.

Hardness

Hardness is a mineral’s ability to resist beingscratched. Minerals that are not easily scratchedare hard. You test the hardness of a mineral byscratching its surface with a mineral of a knownhardness. Mineralogists use Mohs Scale, shown inTable below, as a reference for mineral hardness.The scale lists common minerals in order of theirrelative hardness. You can use the minerals in thescale to test the hardness of an unknown mineral.

As you can see, diamond is a 10 on MohsScale. Diamond is the hardest mineral, whichmeans that no other mineral can scratch a diamond.Quartz is a 7, so it can be scratched by topaz,

corundum, and diamond. Quartz will scratchminerals, such as fluorite, that have a lowernumber on the scale. Suppose you tested a piece ofpure gold for hardness. Calcite would scratch thegold, but gypsum would not because gypsum is a 2and calcite is a 3. That would mean gold isbetween the hardness of gypsum and calcite, or 2.5on the scale. A hardness of 2.5 means that gold is arelatively soft mineral. It is only about as hard asyour fingernail.

Mohs ScaleHardness Mineral1 Talc2 Gypsum3 Calcite4 Fluorite5 Apatite6 Orthoclase feldspar7 Quartz8 Topaz

9 Corundum10 Diamond

(Source:http://en.wikipedia.org/wiki/Mohs_scale, Adaptedby: Rebecca Calhoun, License: Public Domain)

Cleavage and Fracture

Minerals break apart in characteristic ways.Remember that all minerals are crystalline, whichmeans that the atoms in a mineral are arranged in arepeating pattern. The pattern of atoms in a mineraldetermines how a mineral will break. When youbreak a mineral, you break chemical bonds.Because of the way the atoms are arranged, somebonds are weaker than other bonds. A mineral ismore likely to break where the bonds between theatoms are weaker.

Cleavage is the tendency of a mineral to breakalong certain planes to make smooth surfaces.Minerals with different crystal structures willcleave in different ways, as Figure below shows.

Halite tends to form cubes with smooth surfaces,mica tends to form sheets, and fluorite can formoctahedrons.

Figure 3.16 Minerals with different crystal structures have

a tendency to break along certain planes.Minerals can form various shapes like the

polygons, shown in Figure below, when they arebroken along their cleavage planes. The cleavageplanes are important for people who cutgemstones, such as diamonds and emeralds. Theplanes determine how the crystals can be cut tomake smooth surfaces.

Figure 3.17 Cubes have six sides that are all the same size

square. All of the angles in a cube are equal to 90.Rhombohedra also have six sides, but the sides arediamond-shaped. Octahedra have eight sides thatare all shaped like triangles.

Fracture describes how a mineral breakswhen it is not broken along a cleavage plane. Allminerals break but fracture describes a break whenthe resulting surface is not smooth and flat. Youcan learn about a mineral from the way it fractures.Jagged edges are usually formed when metalsbreak. If a mineral splinters like wood it may befibrous. Some minerals, such as quartz, formsmooth curved surfaces when they fracture. Amineral that broke forming a smooth, curvedsurface is shown in Figure below.

Figure 3.18 This mineral formed a smooth, curved surface

when it fractured

Other Identifying Characteristics

Minerals have some other properties that canbe used to identify them. For example, a mineral’scrystal structure can be used to help identify themineral. Sometimes, a trained mineralogist can tellthe crystal structure just by looking at the shape ofmineral. In other cases, the crystals in the mineralare too small to see and a mineralogist will use aspecial instrument that uses X rays to find out the

crystal structure.Some unusual and interesting properties can be

used to identify certain minerals. Some of theseproperties are listed in the Table below. Althoughthese properties are rare, several minerals havethem. An example of a mineral that has eachproperty is also listed in the Table below.

Property Description Exampleof Mineral

Fluorescence Mineral glows underultraviolet light Fluorite

Magnetism Mineral is attracted to amagnet Magnetite

Radioactivity

Mineral gives offradiation that can bemeasured with Geigercounter

Uraninite

ReactivityBubbles form whenmineral is exposed to aweak acid

Calcite

Some minerals have aSulfur

Smell Some minerals have adistinctive smell

(smellslike rotteneggs)

(Source: Adapted by: Rebecca Calhoun,License: CC-BY-SA)

Lesson Summary

You can identify a mineral by its appearanceand other properties.The color and luster describe the appearanceof a mineral, and streak describes the color ofthe powdered mineral.A mineral has a characteristic density.Mohs hardness scale is used to compare thehardness of minerals.The way a mineral cleaves or fracturesdepends on the crystal structure of themineral.Some minerals have special properties thatcan be used to help identify the mineral.

Review Questions

1. Which properties of a mineral describe theway it breaks apart?

2. A mineral looks dry and chalky. Why sort ofluster does it have?

3. What causes a mineral to have the propertiesthat it has?

4. You are trying to identify a mineral sample.Apatite scratches the surface of the mineral.Which mineral would you use next to text themineral’s hardness—fluorite or feldspar?Explain your reasoning.

5. Why is streak more reliable than color whenidentifying a mineral?

6. You have two mineral samples that are aboutthe size of a golf ball. Mineral A has a densityof 5 g/cm3. Mineral B is twice as dense asMineral A. What is the density of Mineral B?

7. Why do some minerals cleave along certainplanes?

Further Reading / SupplementalLinks

http://en.wikibooks.org/wiki/Regents_Earth_Science_(High_School)#Properties_of_Mineralshttp://geology.csupomona.edu/alert/mineral/color.htmhttp://mineral.galleries.com/minerals/property/http://www.mindat.org/min-198.htmlhttp://geology.csupomona.edu/alert/mineral/streak.htmhttp://www.minsocam.org/MSA/collectors_corner/id/http://www.minerals.net/glossary/terms/r/resinous.htmhttp://mathworld.wolfram.com/Octahedron.htmlhttp://en.wikipedia.org/

Vocabulary

cleavageThe tendency of a mineral to break alongcertain planes to make smooth surfaces.

densityHow much matter is in a certain amount ofspace; mass divided by volume.

fractureThe way a mineral breaks when it is notbroken along a cleavage plane.

hardnessThe ability to resist scratching.

lusterThe way light reflects off of the surface of themineral.

mineralogistA scientist who study minerals.

streakThe color of the powder of a mineral.

Points to Consider

Some minerals are colored because theycontain chemical impurities. How did theimpurities get into the mineral?What two properties of a mineral sample

would you have to measure to calculate itsdensity?

Formation of MineralsMinerals are all around you. They are used to

make your house, your computer, even the buttonson your jeans. But, where do minerals come from?There are many types of minerals, and they do notall form in the same way. Some minerals formwhen salt water on Earth’s surface evaporates.Others form from water mixtures that are seepingthrough rocks far below your feet. Still others formwhen mixtures of really hot molten rock cool.

Lesson Objectives

Describe how melted rock produces minerals.Explain how minerals form from solutions.

Formation from Magma and Lava

You are on vacation at the beach. You takeyour flip-flops off to go swimming because it is

one of the hottest days of the summer. The sand isso hot it hurts your feet, so you have to run to thewater. Imagine if it were hot enough for the sand tomelt. Some minerals start out in liquids that arethat hot.

There are places inside Earth where rock willmelt. Melted rock inside the Earth is also calledmolten rock, or magma. Magma is a moltenmixture of substances that can be hotter than1,000oC. Magma moves up through Earth’s crust,but it does not always reach the surface. Whenmagma erupts onto Earth’s surface, it is known aslava. As lava flows from volcanoes it starts tocool, as Figure below shows. Minerals form whenmagma and lava cool.

Figure 3.19

Lava is melted rock that erupts onto Earthssurface.

Rocks from Magma

Magma cools slowly as it rises towardsEarth’s surface. It can take thousands to millions ofyears to become solid when it is trapped insideEarth. As the magma cools, solid rocks form.Rocks are mixtures of minerals. Granite, shown inthe Figure below, is a common rock that formswhen magma cools. Granite contains the mineralsquartz, plagioclase feldspar, and potassiumfeldspar. The different colored speckles in thegranite are the crystals of the different minerals.The mineral crystals are large enough to seebecause the magma cools slowly, which gives thecrystals time to grow.

Figure 3.20 Granite is a type of rock that forms from

magma. It contains the minerals quartz (clear),plagioclase feldspar (shiny white), potassiumfeldspar (pink), and other minerals.

The magma mixture changes over time asdifferent minerals crystallize out of the magma. Avery small amount of water is mixed in with themagma. The last part of the magma to solidifycontains more water than the magma that firstformed rocks. It also contains rare chemicalelements. The minerals formed from this type of

magma are often valuable because they haveconcentrations of rare chemical elements. Whenmagma cools very slowly, very large crystals cangrow. These mineral deposits are good sources ofcrystals that are used to make jewelry. Forexample, magma can form large topaz crystals.

Minerals from Lava

Lava is on the Earth's surface so it coolsquickly compared to magma in Earth. As a result,rocks form quickly and mineral crystals are verysmall. Rhyolite is one type of rock that is formedwhen lava cools. It contains similar minerals togranite. However, as you can see in Figure below,the mineral crystals are much smaller than thecrystals in the granite shown in Figure above.Sometimes, lava cools so fast that crystals cannotform at all, forming a black glass called obsidian.Because obsidian is not crystalline, it is not amineral.

Figure 3.21 Rhyolite rocks contain minerals that are similar

to granite, but the crystal size is much smaller.

Formation from Solutions

Minerals also form when minerals are mixed inwater. Most water on Earth, like the water in theoceans, contains minerals. The minerals are mixedevenly throughout the water to make a solution.The mineral particles in water are so small thatthey will not come out when you filter the water.

But, there are ways to get the minerals in water toform solid mineral deposits.

Minerals from Salt Water

Tap water and bottled water contain smallamounts of dissolved minerals. For minerals tocrystallize, the water needs to contain a largeamount of dissolved minerals. Seawater and thewater in some lakes, such as Mono Lake inCalifornia or Utah's Great Salt Lake. are saltyenough for minerals to "precipitate out" as solids.

When water evaporates, it leaves behind asolid "precipitate" of minerals, which do notevaporate, as the Figure below shows. After thewater evaporates, the amount of mineral left is thesame as was in the water.

Figure 3.22 when the water in glass A evaporates, the

dissolved mineral particles are left behind.Water can only hold a certain amount of

dissolved minerals and salts. When the amount istoo great to stay dissolved in the water, theparticles come together to form mineral solids andsink to the bottom. Salt (halite) easily precipitatesout of water, as does calcite, as the Figure belowshows.

Figure 3.23 The limestone towers are made mostly of

calcite deposited in the salty and alkaline water ofMono Lake, in California. These rocks formed

under water when calcium-rich spring water at thebottom of the lake bubbled up into the alkalinelake, forming theses calcite "tufa" towers. If thelake level drops, the tufa towers appear ininteresting formations.

Minerals from Hot UndergroundWater

Cooling magma is not the only source forunderground mineral formations. When magmaheats nearby underground water, the heated watermoves through cracks below Earth’s surface.

Hot water can hold more dissolved particlesthan cold water. The hot, salty solution reacts withthe rocks around it and picks up more dissolvedparticles. As it flows through open spaces in rocks,it deposits solid minerals. The mineral depositsthat form when a mineral fills cracks in rocks arecalled veins. Figure below shows white quartzveins. When the minerals are deposited in openspaces, large crystals can form. These special

rocks are called geodes. Figure below shows ageode that was formed when amethyst crystalsgrew in an open space in a rock.

Figure 3.24 Quartz veins formed in this rock.

Figure 3.25 An amethyst geode that formed when large

crystals grew in open spaces inside the rock.

Lesson Summary

Mineral crystals that form when magma coolsare usually larger than crystals that form whenlava cools.Minerals are deposited from salty watersolutions on Earth’s surface and underground.

Review Questions

1. How does magma differ from lava?2. What are two differences between granite and

rhyolite?3. What happens to the mineral particles in salt

water when the water evaporates?4. Explain how mineral veins form.

Further Reading / SupplementalLinks

Hydrothermal Mineral Deposit. (2007). In

Encyclopedia Britannica. Retrieved 14, 2007,from Encyclopedia Britannica Online LibraryEdition. Available on the Web at:http://library.eb.com/eb/article-9041735.Mineral Deposit. (2007). In EncyclopediaBritannica. Retrieved 14, 2007, fromEncyclopedia Britannica Online LibraryEdition. Available on the Web at:http://library.eb.com/eb/article-82171.http://socrates.berkeley.edu/~eps2/wisc/Lect3.htmlhttp://en.wikipedia.org/wiki/Mineralogy#Formation_environmentshttp://volcanoes.usgs.gov/Products/Pglossary/magma.htmlhttp://www.minsocam.org/MSA/K12/rkcycle/igneousrks.htmlhttp://geology.wr.usgs.gov/parks/rxmin/rock.htmlhttp://volcanoes.usgs.gov/Products/Pglossahttp://www.monolake.org/naturalhistory/chem.htmhttp://www.monolake.org/naturalhistory/tufa.htmhttp://www.sdnhm.org/kids/minerals/grow-crystal.htmlhttp://ut.water.usgs.gov/greatsaltlake/index.htmlhttp://en.wikipedia.org/

Vocabulary

lavaMolten rock that has reached the Earth'ssurface.

magmaMolten rock deep inside the Earth.

rocksMixtures of minerals.

Points to Consider

When most minerals form, they combine withother minerals to form rocks. How can theseminerals be used?The same mineral can be formed by differentprocesses. How can the way a mineral formsaffect how the mineral is used?

Mining and Using MineralsWhen you buy a roll of aluminum foil or some

baby powder, do you think about how the productswere made? Probably not. We take many everydayitems that are made from minerals for granted. But,before the products can be put on store shelves,minerals have to be removed from the ground andmade into the materials we need. A mineraldeposit that contains enough minerals to be minedfor profit is called an ore. Ores are rocks thatcontain concentrations of valuable minerals. Thebauxite shown in the Figure below is a rock thatcontains minerals that are used to make aluminum.

Lesson Objectives

Explain how minerals are mined.Describe how metals are made from mineralores.Summarize the ways in which gemstones are

used.Identify some useful minerals.

Figure 3.26 Aluminum is made from the minerals in rocks

known as bauxite.

Finding and Mining Minerals

Geologists need to find the ore deposits thatare hidden underground. Different geologicprocesses concentrate mineral resources. Theystudy geologic formations searching for areas thatare likely to have ore deposits. They test the

physical and chemical properties of soil and rocks.For example, they might test rocks to see if therocks are magnetic or contain certain chemicalelements. Then, geologists make maps of theirfindings to locate possible ore deposits. Today,satellites do some of the work for geologists.Satellites can make maps of large areas morequickly than geologists on the ground can.

After a mineral deposit is found, geologistsdetermine how big it is. They also calculate howmuch of the valuable minerals they think they willget from mining the deposit. The minerals will onlybe mined if it is profitable. If is profitable to minethe ore, they decide the way it should be mined.The two main methods of mining are surfacemining and underground mining.

Surface Mining

Surface mining is used to obtain mineral oresthat are close to Earth’s surface. The soil androcks over the ore are removed by blasting.Typically, the remaining ore is drilled or blasted

so that large machines can fill trucks with thebroken rocks. The trucks take the rocks to factorieswhere the ore will be separated from the rest of therock. Surface mining includes open-pit mining,quarrying, and strip mining.

As the name suggests, open-pit mining createsa big pit from which the ore is mined. Figurebelow shows an open-pit diamond mine in Russia.The size of the pit grows until it is no longerprofitable to mine the remaining ore. Strip minesare similar to pit mines, but the ore is removed inlarge strips. A quarry is a type of open-pit minethat produces rocks and minerals that are used tomake buildings.

Figure 3.27 This diamond mine is more than 600 m deep.Placers are valuable minerals that have

collected in stream gravels, either modern riversor ancient riverbeds. California’s nickname, theGolden State, can be traced back to the discoveryof placer gold in 1848. The gold that attractedwould-be miners from around the world weatheredout of a hard rock, travelled downstream and thensettled in a deposit of alluvium. The goldoriginated in the metamorphic belt in the westernSierra Nevada, which also contains deposits ofcopper, lead zinc, silver, chromite and othervaluable minerals. Currently, California has activemines for gold and silver, and also for non-metalminerals like sand and gravel, which are used forconstruction.

Underground Mining

Underground mining is used for ores that aredeep in Earth’s surface. For deep ore deposits, itcan be too expensive to remove all of the rocks

above the ore. Underground mines can be verydeep. The deepest gold mine in South Africa ismore than 3,700 m deep (that is more than 2miles)! There are various methods of undergroundmining. These methods are more expensive thansurface mining because tunnels are made in therock so that miners and equipment can get to theore. Underground mining is dangerous work. Freshair and lights must also be brought in to the tunnelsfor the miners. Miners breathe in lots of particlesand dust while they are underground. The ore isdrilled, blasted, or cut away from the surroundingrock and taken out of the tunnels. Sometimes thereare explosions and sometimes mines collapse asore is being drilled or blasted.

Mining and the Environment

Mining provides people with many resourcesthey need, but care needs to be taken to reduce theenvironmental impact of mining. After the miningis finished, the area around the mine is supposed tobe restored to its natural state. This process of

restoring the natural area is called reclamation.Native plants are planted. Pit mines may berefilled or reshaped so that they can becomenatural areas again. They may also be allowed tofill with water and become lakes. They may alsobe turned into landfills. Underground mines may besealed off or left open as homes for bats.

Mining can cause pollution. Chemicalsreleased from mining can contaminate nearbywater sources. Figure below shows water that iscontaminated from a nearby mine. The UnitedStates government has standards that mines mustfollow to protect water quality. It is also importantto use mineral resources wisely. It takes millionsof years for new mineral deposits to form in Earth,so they are nonrenewable resources.

Figure 3.28 Scientists test water that has been contaminated

by a mine.

Making Metals from Minerals

We rely on metals, such as aluminum, copper,

iron, and gold. Look around the room. How manyobjects have metal parts? Remember to includeanything that uses electricity. Metals are used inthe tiny parts inside your computer and on theoutside of large building, such as the one shown inthe Figure below. Whether the metal makes thealuminum can that you drink out of or the copperwires in your computer, it started out as an ore. Butthe ore’s journey to becoming a useable metal isonly just beginning when the ore leaves the mine.

Figure 3.29 The De Young Museum in San Francisco is

covered in copper panels.Mining produces a mixture of rocks that

contain ore and other rocks that do not contain ore.So, the ore must be separated from unwanted

rocks. Then, the minerals need to be separated outof the ore. The work is still not done once themineral is separated from the unwanted materials(Figure below).

Most minerals are not pure metals, butchemical compounds that contain metals and otherelements. The minerals must go through chemicalreactions to make pure metals. In order for thereactions to happen, chemicals must be added toores that have been melted. High temperatures areneeded to melt ores. Think about the ways you usealuminum foil in your home. It is put into hot ovensand over flames on a grill, but it does not melt.Making aluminum requires a lot of energy.Temperatures greater than 900oC are needed tomake pure aluminum. Then, a huge amount ofelectricity is needed to separate the aluminum fromother elements to produce pure aluminum. If yourecycle just 40 aluminum cans, you will save theenergy in one gallon of gasoline. We use over 80billion cans each year. If all of these cans wererecycled, we would save the energy in 2 billiongallons of gasoline!

Figure 3.30 When the water in glass A evaporates, the

dissolved mineral particles are left behind.

Gemstones and Their Uses

Some minerals are valuable because they are

beautiful. Jade has been used for thousands ofyears in China. Native Americans have beendecorating items with turquoise since ancienttimes. Minerals like jade, turquoise, diamonds, andemeralds are gemstones. A gemstone, or gem, is amaterial that is cut and polished to use in jewelry.Many gemstones, such as those shown in theFigure below, are minerals.

Figure 3.31 Gemstones come in many colors.In addition to being beautiful, gemstones are

rare and do not break or scratch easily. Generally,rarer gems are more valuable. Other factors, suchas how popular the gem is, its size and the way itis cut can also affect its value.

Most gemstones are not used exactly as theyare found in nature. Gems are usually cut andpolished. Figure below shows an uncut piece ofruby and a ruby that has been cut and polished. Theway a mineral splits along a surface or cleavesdetermines how it can be cut to produce smoothsurfaces. Notice that the cut and polished rubyseems more sparkly. Gems appear to be sparklybecause light bounces back when it hits them.Some light passes through some gems, such asrubies and diamond. These gems are cut so that themost amount of light possible bounces back. Lightdoes not pass through gemstones that are opaque,such as turquoise. So, these gems are not cut in thesame way as diamonds and rubies.

Figure 3.32 Ruby is cut and polished to make the gemstone

sparkle(Left)Ruby Crystal(Right)Cut Ruby.Gemstones are known for their use in jewelry,

but they do have other uses. Most diamonds areactually not used as gemstones. Diamonds are usedto cut and polish other materials, such as glass andmetals, because they are so hard. The mineralcorundum, of which ruby and sapphire arevarieties, is used in products like sandpaper.Synthetic rubies and sapphires are also used inlasers.

Other Useful Minerals

Metals and gemstones are often shiny, so they

catch your eye. Many minerals that we useeveryday are not so noticeable. For example, thebuildings on your block could not have builtwithout minerals. The walls in your home mightuse the mineral gypsum for the sheetrock. The glassin your windows is made from sand, which ismostly the mineral quartz. Talc was oncecommonly used to make baby powder. The mineralhalite is mined for rock salt. Diamond is used as agemstone but is commonly used in drill bits andsaw blades to improve their cutting ability. Copperis used in electrical wiring and the ore bauxite isthe source for the aluminum in your soda can.

Lesson Summary

Geologists use many methods to find mineraldeposits that will be profitable to mine.Ores that are close to the surface are minedby surface mining methods. Ores that are deepin Earth are mined using undergroundmethods.

Metals ores must be melted to make metals.Many gems are cut and polished to increasetheir beauty.Minerals are used in a variety of ways.

Review Questions

1. What type of mining would be used to extractan ore that is close to the Earth's surface?

2. Describe some methods used in surfacemining.

3. What are some disadvantages of undergroundmining?

4. What are some ways an area can reclaimed,or returned to its natural state, after beingmined?

5. What steps are taken to extract a pure metalfrom an ore?

6. What makes a gemstone valuable?

Further Reading / SupplementalLinks

Dave Frank, John Galloway, and KenAssmus. U.S. Geological Survey. 2005.Available on the Web at:http://pubs.usgs.gov/gip/2005/17/."Mining." Encyclopædia Britannica. 2007.Encyclopædia Britannica Online LibraryEdition. Available on the Web at:http://library.eb.com/eb/article-81268.http://geology.wr.usgs.gov/parks/misc/glossaryo.htmlhttp://www.mii.org/Minerals/photoal.htmlhttp://minerals.usgs.gov/minerals/pubs/commodity/bauxite/http://www.amnh.org/exhibitions/diamonds/mining.htmlhttp://www.aluminum.org/Content/NavigationMenu/News_and_Stats/CansForHabitat_org/Aluminum_Recycling/Fun_with_Recycling/Top_10_Reasons_to_Recycle/Top_10_Reasons_to_Recycle.htmhttp://www.alcoa.com/australia/en/pdf/Community/Smelting%20student%20notes.pdfhttp://www.eia.doe.gov/kids/energyfacts/saving/recycling/solidwaste/metals.htmlhttp://www.famsf.org/deyoung/about/subpage.asp?subpagekey=46http://socrates.berkeley.edu/~eps2/wisc/Lect2.htmlhttp://howtobuyagemstone.gia.edu/00_enter.htmlhttp://www.sdnhm.org/kids/minerals/games/search/index.htmlhttp://www.eia.doe.gov/kids/energyfacts/saving/recycling/solidwaste/metals.htmlhttp://www.mnh.si.edu/exhibits/si-gems/index.htmln

http://en.wikipedia.org/

Vocabulary

gemstoneAny material that is cut and polished to use injewelry.

oreA mineral deposit that contains enoughminerals to be mined for profit.

Points to Consider

Are all mineral deposits ores?An open-pit diamond mine may one day beturned into an underground mine. Why wouldthis happen?Diamonds are not necessarily the rarest gem.Why do people value diamonds more thanmost other gems?

Chapter 4: Rocks

Types of Rocks

Lesson Objectives

Define rock and describe what rocks aremade of.Know how rocks are classified anddescribed.Explain how each of the three main rock typesare formed.Describe the rock cycle.

Introduction

Have you ever heard something described as“rock solid?” We usually use the phrase todescribe something that does not and cannotchange. It also means something is absolutely sure

and will not fail or go wrong somehow. If we say aplan is rock solid that means the plan is a sure bet—it will not change and it will not go wrong.When you see pictures like this rocky mountainsidein Costa Rica, it is easy to get the feeling that rocksneither change nor move, but instead always staythe same (Figure below).

Figure 4.1 A rocky mountainside from Costa Rica.

The Rock Cycle

The truth is, however, that rocks do change. Allrocks on Earth change as a result of natural

processes that take place all the time. Thesechanges usually happen very slowly. They mayeven happen below Earth’s surface so that we donot notice the changes. The physical and chemicalproperties of rocks are constantly changing in anatural, never-ending cycle called the rock cycle.The rock cycle describes how each of the maintypes of rocks is formed, and explains how rockschange within the cycle. This lesson will discussthe characteristics of rocks, how rocks areclassified, and details of the rock cycle. Thefollowing three lessons of this chapter will discussthe three main types of rocks in more detail.

A rock is a naturally-formed, nonliving Earthmaterial. Rocks are made of collections of mineralgrains that are held together in a firm, solid mass(Figure below). The individual mineral grains thatmake up a rock may be so tiny that you can onlysee them with a microscope, or they may be as bigas your fingernail. A rock may be made of grainsof all one mineral type, or it may be made of amixture of different minerals. Most rocks containmore than one mineral. Each rock has a unique set

of minerals that make it up, and rocks are usuallyidentified by the minerals observed in them. Sincedifferent minerals form under differentenvironmental conditions, the minerals in a rockcontain clues about the conditions, liketemperature, that were present when the rockformed.

Figure 4.2 This rock contains several different minerals,

as shown by the different colors and textures found

in the rock.Rocks can also be described by their texture,

which is a description of the size, shape, andarrangement of mineral grains. Rocks may be smallpebbles less than a centimeter, or, they may bemassive boulders that are meters wide (Figurebelow). Smaller rocks form when larger rocks arebroken apart and worn down.

Figure 4.3 This massive boulder is an example of how

large rocks can be. It is in Colorado Springs,Colorado.

Three Main Categories of Rocks

Rocks are classified according to how theywere formed. The three main kinds of rocks are:

1. Igneous Rocks - form when magma (moltenrock inside the Earth) or lava (molten rock that haserupted onto the surface of Earth) cools either at orbelow Earth’s surface (Figure below).

Figure 4.4 This flowing lava is an example of molten

mineral material. It will harden into an igneousrock.

2 . Sedimentary Rocks - form by thecompaction of sediments, like gravel, sand, silt orclay (Figure below). Sediments may includefragments of other rocks that have been worn downinto small pieces, materials made by a livingorganism or organic materials, or chemical

precipitates, which are the solid materials leftbehind after a liquid evaporates. For example, if aglass of salt water is left in the sun, the water willeventually evaporate, but salt crystals will remainbehind as precipitates in the bottom of the glass.

Figure 4.5 This sandstone is an example of a sedimentary

rock. It formed when many small pieces of sandwere cemented together to form a rock.

3. Metamorphic Rocks - form when an existingrock (of any type) is changed by heat or pressurewithin the Earth, so that the minerals undergo somekind of change (Figure below).

Figure 4.6 This quartzite is an example of a metamorphic

rock. It formed when sandstone was changed byheat and pressure within the Earth.

Rocks can be changed from one type toanother, and the rock cycle describes how thishappens. Figure below shows the rock cycle, andhow the three main rock types are related to eachother. The arrows within the circle show how onetype of rock may change to rock of another type.For example, igneous rock may break down intosmall pieces of sediment and become sedimentaryrock, or it may be buried within the Earth andbecome metamorphic rock, or it may change backto molten material and re-cool into a new igneousrock.

Figure 4.7 The Rock Cycle.

Processes of the Rock Cycle

Any type of rock can undergo changes andbecome any new type of rock. Several processesare involved in the rock cycle that make thispossible. The key processes of the rock cycle arecrystallization, erosion and sedimentation, andmetamorphism. Let’s take a closer look at each ofthese:

Crystallization. Crystallization occurs when

molten material hardens into a rock. An existingrock may be buried deep within the earth, melt intomagma and then crystallize into an igneous rock.The rock may then be brought to Earth’s surface bynatural movements of the Earth. Crystallization canoccur either underground when magma cools, or onthe earth’s surface when lava hardens.

Erosion and Sedimentation. Pieces of rock atEarth’s surface are constantly worn down intosmaller and smaller pieces. The impacts of runningwater, gravity, ice, plants, and animals all act towear down rocks over time. The small fragmentsof rock produced are called sediments. Runningwater and wind transport these sediments from oneplace to another. They are eventually deposited, ordropped somewhere. This process is callederosion and sedimentation. The accumulatedsediment may become compacted and cementedtogether into a sedimentary rock. This wholeprocess of eroding rocks, transporting anddepositing them, and then forming a sedimentaryrock can take hundreds or thousands of years.

Metamorphism. Sometimes an existing rock is

exposed to extreme heat and pressure deep withinthe Earth. Metamorphism happens if the rock doesnot completely melt but still changes as a result ofthe extreme heat and pressure. A metamorphic rockmay have a new mineral composition and/ortexture.

An interactive rock cycle diagram can be foundh e r e : RockCycle(http://www.classzone.com/books/earth_science/terc/content/investigations/es0602/es0602page02.cfm?chapter_no=investigation)

Note that the rock cycle really has no beginningand no end: therefore, it’s a never-ending cycle.The concept of the rock cycle was first developedby James Hutton, an eighteenth century scientistoften called the “father of geology” (Figurebelow). Hutton spoke of the cyclic nature of rockformation and other geologic processes and saidthat they have “no [sign] of a beginning, and noprospect of an end.” The processes involved in therock cycle take place over hundreds or eventhousands of years, and so in our lifetime, rocksappear to be fairly “rock solid” and unchanging.However, a study of the rock cycle shows us that

change is always taking place. The next threelessons of this chapter will discuss each type ofrock in more detail.

Figure 4.8 James Hutton is considered the Father of

Geology.

Lesson Summary

There are three main types of rocks; igneous,sedimentary and metamorphic.Crystallization, erosion and sedimentationand metamorphism transform one type of rockinto another type of rock or change sedimentsinto rock.The rock cycle describes the transformationsof one type of rock to another.

Review Questions

1. Describe the difference between a rock and amineral.

2. Why can the minerals in a rock be a clueabout how the rock formed?

3. What is the difference between magma andlava?

4. What are the 3 main types of rocks and howdoes each form?

5. Describe how an igneous rock can change to ametamorphic rock.

6. Explain how sediments form.

7. In which rock type do you think fossils, whichare the remains of past living organisms, aremost often found?

8. Suppose that the interior of the Earth was nolonger hot, but all other processes on Earthcontinued unchanged. How would this affectthe distribution of rocks formed on Earth?

Vocabulary

chemical compositionDescription of the elements or compoundsthat make up a substance and how thoseelements are arranged in the substance.

depositedPut down or dropped by water or wind ontothe ground.

lavaMolten rock at the surface of Earth.

magma

Molten rock below Earth’s surface.

mineralNaturally-occurring solid that has a definitecrystal structure.

moltenSomething that is melted.

organicHaving to do with living things.

precipitatesSolid substance that separates out of a liquid;a solid substance that was once dissolved in aliquid and gets left behind when the liquidevaporates.

sedimentsSmall particles of soil or rock deposited bywind or water.

Points to Consider

What processes on Earth are involved informing rocks?Stone tools were important to early humans.Do you think rocks are still important tomodern humans today?

Igneous Rocks

Lesson Objectives

Describe how igneous rocks are formed.Describe the properties of some commontypes of igneous rocks.Relate some common uses of igneous rocks.

Introduction

This lesson will discuss igneous rocks, howthey form, how they are classified, and some oftheir common uses. Igneous rocks may or may notbe found naturally where you live, but chances arethat you have seen materials made from igneousrocks. One of the most common igneous rocks isgranite (Figure below). Granite is usedextensively in building materials and makingstatues. Perhaps you have used a pumice stone tosmooth your skin or to do jobs around the house.

Pumice is another example of an igneous rock(Figure below). Pumice is used to make stone-washed denim jeans! Pumice stones are put intogiant washing machines with newly-manufacturedjeans and tumbled around to give jeans thatdistinctive “stone-washed” look. You alsoprobably use igneous rock when you brush yourteeth every morning. Ground up pumice stone issometimes added to toothpaste to act as anabrasive material that scrubs your teeth clean.

Figure 4.9 Granite is an igneous rock used commonly in

statues and building materials.

Figure 4.10 Pumice is a light igneous rock used for

abrasive materials.

Crystallization

Igneous rocks form when molten material coolsand hardens. They may form either below or aboveEarth’s surface. They make up most of the rocks onEarth. Most igneous rock is buried below thesurface and covered with sedimentary rock, and sowe do not often see just how much igneous rockthere is on Earth. In some places, however, large

areas of igneous rocks can be seen at Earth’ssurface. Figure below shows a landscape inCalifornia’s Sierra Nevada that consists entirely ofgranite, an igneous rock.

Figure 4.11 This landscape high in Californias Sierra

Nevada is completely made up of granite exposedat Earths surface.

Igneous rocks are called intrusive when theycool and solidify beneath the surface. Because theyform within the Earth, cooling can proceed slowly,as discussed in the chapter "Earth's Minerals."

Because such slow cooling allows time for largecrystals to form, intrusive igneous rock hasrelatively large mineral crystals that are easy tosee. Granite is the most common intrusive igneousrock (Figure below).

Figure 4.12 A close-up of a granite sample. Notice the

black and white portions. Each color represents adifferent mineral in the rock. You can easily seethe mineral crystals that make up this intrusiveigneous rock.

Igneous rocks are called extrusive when theyform above the surface. They solidify after moltenmaterial pours out onto the surface through an

opening such as a volcano (Figure below).Extrusive igneous rocks cool much more rapidlythan intrusive rocks. They have smaller crystals,since the rapid cooling time does not allow timefor large crystals to form. Some extrusive igneousrocks cool so rapidly that crystals do not developat all. These form a glass, such as obsidian (Figurebelow). Others, such as pumice, contain holeswhere gas bubbles were trapped when the materialwas still hot and molten. The holes make pumiceso light that it actually floats in water. The mostcommon extrusive igneous rock is basalt, a rockthat is especially common below the oceans(Figure below).

Figure 4.13

Extrusive igneous rocks form after lava coolsabove the surface. The lava spills out from theEarth at a volcano.

Figure 4.14 Obsidian is an extrusive igneous rock. It cools

so rapidly that crystals do not form, and it has aglassy texture.

Figure 4.15 These are examples of basalt below the South

Pacific Ocean. Basalt is an extrusive igneous rockcommon below the oceans, though it is also foundin some places on continents.

Composition

Igneous rocks are classified according to howand where they formed (in other words, if they’reintrusive or extrusive) and their mineralcomposition (describing the minerals they contain).The mineral compositions of igneous rocks areusually described as being felsic, intermediate,mafic, or ultramafic (as examples, see Figure

below and Figure below). Felsic rocks are madeof light-colored, low-density minerals such asquartz and feldspar. Mafic rocks are made of dark-colored, higher-density minerals such as olivineand pyroxene. Intermediate rocks havecompositions between felsic and mafic. Ultramaficrocks contain more than 90% mafic minerals andhave very few light, felsic minerals in them. Tablebelow shows some common igneous rocksclassified by mode of occurrence and mineralcomposition.

Common Igneous RocksMode ofOccurrence

MineralCompositionFelsic Intermediate Mafic Ultramafic

Extrusive Rhyolite Andesite Basalt KomatiiteIntrusive Granite Diorite Gabbro Peridotite

Figure 4.16 Rhyolite is an example of an extrusive felsic

rock. Notice its light color and small crystals.

Figure 4.17 This is a close-up photograph of peridotite, an

extrusive ultramafic igneous rock. The greenmineral is olivine.

The rocks listed in the table above are the mostcommon igneous rocks, but there are actually morethan 700 different types of igneous rocks. Graniteis perhaps the most useful one to humans. We usegranite in many building materials and in art. Asdiscussed in the introduction to this lesson, pumiceis commonly used for abrasives. Peridotite issometimes mined for peridot, a type of gemstoneused in jewelry. Diorite is extremely hard and iscommonly used for art. It was used extensively byancient civilizations for vases and other decorativeart work (Figure below).

Figure 4.18 This vase was made by ancient Egyptians

about 3,600 BC. It is made of the igneous rockdiorite.

Lesson Summary

Igneous rocks form either when they cool veryslowly deep within the Earth or when magmacools rapidly at the Earth's surface.

Composition of the magma will determine theminerals that will crystallize forming differenttypes of igneous rocks.

Review Questions

1. What is the difference between an intrusiveand an extrusive igneous rock?

2. Why do extrusive igneous rocks usually havesmaller crystals than intrusive igneous rocks?

3. How are igneous rocks classified?4. Describe two ways granite is different from

basalt.5. List three common uses of igneous rocks.6. Occasionally, igneous rocks will contain both

large crystals and tiny mineral crystals.Propose a way that both these size crystalsmight have formed in the rock.

7. Why is the ocean floor more likely to haveextrusive rocks than intrusive rocks?

Vocabulary

outcropAn exposure of rock that can be seen on thesurface of Earth.

solidifyTo harden after cooling.

intrusiveA type of igneous rock that forms inside theEarth.

extrusiveA type of igneous rock that forms aboveEarth’s surface.

felsicA type of igneous rock composition that ismade mostly of light minerals such as quartzand feldspar.

maficA type of igneous rock composition that ismade mostly of dense, dark minerals likeolivine and pyroxene.

intermediateA type of igneous rock composition that is inbetween felsic and mafic.

ultramaficA type of igneous rock that contains more than90% mafic minerals.

Points to Consider

Do you think igneous rocks could form whereyou live?Would all igneous rocks with the samecomposition have the same name? Explainwhy they might not.Could an igneous rock cool at two differentrates? What would the crystals in such a rocklook like?

Sedimentary Rocks

Lesson Objectives

Describe how sedimentary rocks are formed.Describe the properties of some commonsedimentary rocks.Relate some common uses of sedimentaryrocks.

Introduction

Figure 4.19 The White House of the United States of

America is made of a sedimentary rock calledsandstone.

You probably recognize the Figure above asthe White House, the official home and workplaceof the President of the United States of America.Do you know why the White House is white? Itscolor has a lot to do with the stone materials thatwere used to construct it.

Construction for the White House began in1792, and most of the work was carried out bypeople who had only recently come to the newlyformed country of America. Its outside walls aremade of a type of sedimentary rock calledsandstone. The sandstone that was used toconstruct the White House is very porous, whichmeans that rainwater can easily penetrate thesandstone. This made the White House susceptibleto water damage in its early days of construction.To stop the water damage, workers had to coverthe sandstone in a mixture of salt, rice, and glue,giving the White House its distinct white color.

Sediments

In this lesson, you will learn about sedimentaryrocks like sandstone, how they form, how they areclassified, and how people often use sedimentaryrocks.

Recall from Lesson 4.1 that sedimentary rocksare formed by the compaction of sediments.Sediments may include:

fragments of other rocks that have been worndown into small pieces, like sandorganic materials, or in other words, theremains of once-living organisms,or chemical precipitates, which are materialsthat get left behind after the water evaporatesfrom a solution.

Most sediments settle out of water (Figurebelow). For example, running water in riverscarries huge amounts of sediments. The riverdumps these sediments along its banks and at theend of its course. When sediments settle out of

water, they form horizontal layers. One layer at atime is put down. Each new layer forms on top ofthe layers that were already there. Thus, each layerin a sedimentary rock is younger than the layerunder it and older than the layer over it. When thesediments harden, the layers are preserved. Inlarge outcrops of sedimentary rocks, you can oftensee layers that show the position and order inwhich the original sediment layers were deposited.Scientists can figure out the relative ages of layersby knowing that older ones are on the bottom andyounger ones are on top.

Figure 4.20 Most sediments settle out of running water,

such as in this river.There are many different types of environments

where sedimentary rocks form. Some places whereyou can see large deposits of sediments todayinclude a beach and a desert. Sediments are alsocontinuously depositing at the bottom of the oceanand in lakes, ponds, rivers, marshes and swamps.Avalanches produce large unsorted piles ofsediment. The environment where the sedimentsare deposited determines the type of sedimentaryrock that will form there.

Sedimentary Rock Formation

Sediments accumulate and over time may behardened into rock. Lithification is the hardeningof layers of loose sediment into rock (Figurebelow). Lithification is made up of two processes:cementation and compaction. Cementation occurswhen substances crystallize or fill in the spacesbetween the loose particles of sediment. Thesecementing substances come from the water that

moves through the sediments. Sediments may alsobe hardened into rocks through compaction. Thisoccurs when sediments are squeezed together bythe weight of layers on top of them. Sedimentaryrocks made of cemented, non-organic sedimentsare called clastic rocks. Those that form fromorganic remains are called bioclastic rocks, andsedimentary rocks formed by the hardening ofchemical precipitates are called chemicalsedimentary rocks. Table below shows somecommon types of sedimentary rocks and the typesof sediments that make them up.

Figure 4.21 This cliff is made of a sedimentary rock called

sandstone. The bands of white and red representdifferent layers of sediment. The layers ofsediments were preserved during lithification.

Common Sedimentary Rocks

Picture Rock Name Type ofSedimentary Rock

ConglomerateClastic (fragmentsof non-organicsediments)

Breccia Clastic

Sandstone Clastic

Siltstone Clastic

Shale Clastic

Rock Salt Chemicalprecipitate

Rock Gypsum Chemicalprecipitate

Dolostone Chemicalprecipitate

Limestone

Bioclastic(sediments fromorganic materials,or plant or animalremains)

Coal Organic

Note from the pictures in the table that clasticsedimentary rocks vary in the size of theirsediments. Both conglomerate and breccia aremade of individual stones that have been cementedtogether. In conglomerate, the stones are rounded;in breccia, the stones are angular around the edges.Sandstone is made of smaller, mostly sand-sized

particles cemented together. Siltstone is mademostly of silt, particles that are smaller than sandbut larger than clay. Shales have the smallest grainsize, being made mostly of clay-sized particles andhardened mud.

Lesson Summary

Weathering and erosion produce sediments.Once these sediments are deposited, they canbecome sedimentary rocks.Sediments must be compacted and cementedto make sedimentary rock. This process iscalled lithification.

Review Questions

1. What are three things that the sediments insedimentary rocks may be made of?

2. If you see a sedimentary rock outcrop and redlayers of sand are on top of pale layers ofsand, what do you know for sure about the

ages of the two layers?3. Why do sedimentary rocks have layers of

different colors sometimes?4. Describe the two processes necessary for

sediments to harden into rock.5. What type of sedimentary rock is coal?6. Think back to the story at the start of the

lesson about why the White House originallywas white. Why do you think sandstonewould be a particularly porous rock?

Vocabulary

crystalSolid substance that has a regular geometricarrangement.

outcropLarge rock formation at the surface of theEarth.

fossilSomething that is left behind by a once-living

organism, such as bones or footprints.

organicMade from materials that were once livingthings.

precipitateThe solid materials left behind after a liquidevaporates.

compactionOccurs when sediments are hardened bybeing squeezed together by the weight oflayers on top of them.

cementationOccurs when substances harden crystallize inthe spaces between loose sediments.

Points to Consider

If you were interested in learning aboutEarth's history, which type of rocks would

give you the most information?Could a younger layer of sedimentary rockever be found under an older layer? How doyou think this could happen?Could a sedimentary rock form only bycompaction from intense pressure?

Metamorphic Rocks

Lesson Objectives

Describe how metamorphic rocks are formed.Describe the properties of some commonmetamorphic rocks.Relate some common uses of metamorphicrocks.

Introduction

In this lesson you will learn about metamorphicrocks, how they form, and some of their commonuses. Figure below shows a large outcrop ofmetamorphic rocks. Notice the platy layers that runfrom left to right within the rock. It looks as thoughyou could easily break off layers from the frontsurface of the outcrop. This layering is a result ofthe process of metamorphism. Metamorphism isthe changing of rocks by heat and pressure. During

this process, rocks change either physically and/orchemically. They change so much that they becomean entirely new rock.

Figure 4.22 The platy layers in this large outcrop of

metamorphic rock show the effects of pressure onrocks during metamorphism.

Metamorphism

Metamorphic rocks start off as igneous,sedimentary, or other metamorphic rocks. Theserocks are changed when heat or pressure alters theexisting rock’s physical or chemical make up. Oneways rocks may change during metamorphism is by

rearrangement of their mineral crystals. When heatand pressure change the environment of a rock, thecrystals may respond by rearranging their structure.They will form new minerals that are more stablein the new environment. Extreme pressure mayalso lead to the formation of foliation, or flatlayers in rocks that form as the rocks are squeezedby pressure. Foliation normally forms whenpressure was exerted on a rock from one direction.If pressure is exerted from all directions, then therock usually does not show foliation.

There are two main types of metamorphism:

1. Contact metamorphism—occurs when magmacontacts a rock, changing it by extreme heat(Figure below).

2. Regional metamorphism—occurs when greatmasses of rock change over a wide area dueto pressure deep within the earth or throughextreme pressure from rock layers on top ofit.

Figure 4.23 This diagram shows hot magma within the

earth contacting various rock layers. This is anexample of contact metamorphism.

It is important to note that metamorphism doesnot cause complete melting of the initial rock. Itonly causes changes to a rock by heat or pressure.The rearrangement of the mineral crystals is themost common way that we notice these changes.Table below shows some common metamorphicrocks and the original rocks that they come from.

Common Metamorphic Rocks

Picture Rock NameType ofMetamorphicRock

Comments

Slate Foliated Metamorphismof shale

Phyllite Foliated

Metamorphismof slate, butunder greaterheat andpressure thanslate

Schist Foliated

Often derivedfrommetamorphismof claystone orshale;metamorphosedunder moreheat andpressure thanphylliteMetamorphismof variousdifferent rocks,

Gneiss Foliated under extremeconditions ofheat andpressure

Hornfels Non-foliated

Contactmetamorphismof variousdifferent rocktypes

Quartzite Non-foliated Metamorphismof sandstone

Marble Non-foliated Metamorphismof limestone

Metaconglomerate Non-foliatedMetamorphismofconglomerate

Hornfels, with its alternating bands of dark andlight crystals is a good example of how minerals

rearrange themselves during metamorphism. In thiscase, the minerals separated by density andbecame banded. Gneiss forms by regionalmetamorphism from both high temperature andpressure.

Quartzite and marble are the most commonlyused metamorphic rocks. They are frequentlychosen for building materials and artwork. Marbleis used for statues and decorative items like vases(Figure below). Ground up marble is also acomponent of toothpaste, plastics, and paper.Quartzite is very hard and is often crushed andused in building railroad tracks (Figure below).Schist and slate are sometimes used as buildingand landscape materials.

Figure 4.24 Marble is used for decorative items and in art.

Figure 4.25 Crushed quartzite is sometimes placed under

railroad tracks because it is very hard and durable.

Lesson Summary

Metamorphic rocks form when heat andpressure transform an existing rock into a newrock.Contact metamorphism occurs when hotmagma transforms rock that it contacts.Regional metamorphism transforms largeareas of existing rocks under the tremendousheat and pressure created by tectonic forces.

Review Questions

1. Why do the minerals in a rock sometimesrearrange themselves when exposed to heat orpressure?

2. What is foliation in metamorphic rocks?3. Describe the different conditions that lead to

foliated versus non-foliated metamorphicrocks.

4. List and describe the two main types ofmetamorphism.

5. How can metamorphic rocks be a clue to howthey were formed?

6. Suppose a phyllite sample wasmetamorphosed again. How might it lookdifferent after this second round ofmetamorphism.

Vocabulary

stableSteady and not likely to change significantly

any more.

contact metamorphismResults from temperature increases when abody of magma contacts a cooler existingrock.

regional metamorphismOccurs when great masses of rock changeover a wide area due to pressure.

foliationProperty of some metamorphic rocks in whichflat layers are formed; seen as evidence ofsqueezing by pressure.

Points to Consider

What type of plate boundary would producethe most intense metamorphism of rock?Do you think new minerals could form whenan existing rock is metamorphosed?

Chapter 5: Earth's Energy

Energy Resources

Lesson Objectives

Compare ways in which energy is changedfrom one form to another.Discuss what happens when we burn a fuel.Describe the difference between renewableand nonrenewable resources, and classifydifferent energy resources as renewable ornonrenewable.

Introduction

Did you know that everything you do takesenergy? Even while you are sitting still, your bodyis using energy to breathe, to keep your bloodcirculating, and to control many different

processes. But it’s not just you. Everything thatmoves or changes in any way—from plants toanimals to machines—needs energy. Have youever wondered where all of this energy comesfrom?

The Need for Energy

Figure 5.1 Electrical transmission towers like the one

shown in this picture help deliver the electricitythat you use for energy every day.

Energy can be defined as the ability to move orchange matter. Every living thing needs energy to

live and grow. Your body gets its energy fromfood, but that is only a small part of the energy youuse every day. Cooking your food takes energy,and so does keeping it cold in the refrigerator orthe freezer. The same is true for heating or coolingyour home. Whether you are turning on a light inthe kitchen or riding in a car to school, you areusing energy all day long. And because billions ofpeople all around the world use energy, there is ahuge need for resources to provide all of thisenergy. Why do we need so much energy? Themain reason is that almost everything that happenson Earth involves energy. Most of the time whensomething happens, energy is changing forms.

Even though energy does change form, the totalamount of energy always stays the same. The Lawof Conservation of Energy says that energy cannotbe created or destroyed. Scientists discovered thislaw by noticing that any time they observed energychanging from one form to another, they couldmeasure that the overall amount of energy did notchange.

For an example of how energy changes from

one form to another, think about what goes on whenyou kick a soccer ball. Your body gets its energyfrom food. When your body breaks down the foodyou eat, it stores the energy from the food in a formcalled chemical energy. But some of this storedenergy has to be released to make your leg musclesmove. When this happens, the energy that isreleased changes from chemical energy to anotherform, called kinetic energy. Kinetic energy is theenergy of anything in motion. Your muscles moveyour leg, your foot kicks the ball, and the ball gainskinetic energy by being kicked. So you can think ofthe action of kicking the ball as a story of energymoving and changing forms. The same is true foranything that happens involving movement orchange. Potential energy is energy that is stored.Potential energy has the potential to do work or thepotential to be converted into other forms ofenergy. An example of potential energy might bethe ball you kicked, if it ended up at the top of ahill.

Energy, Fuel, and Heat

As you have learned, energy is the ability tomove or change matter. To put it another way, youcould say that energy is the ability to do work. Butwhat makes energy available whenever you needit? If you have ever accidentally unplugged a lampwhile you were using it, you have seen that thelamp does not have a supply of energy to keepitself lit. When the lamp is plugged into an outlet, ithas the source of energy it needs—electricity. Theelectricity comes from a power plant, and thepower plant has to have energy to produce thiselectricity (Figure above). The energy to make theelectricity comes from a fuel, which stores theenergy and releases it when it is needed. A fuel isany material that can release energy in a chemicalchange. The food you eat acts as a fuel for yourbody. You probably hear the word fuel most oftenwhen someone refers to its use in transportation.Gasoline and diesel fuel are two fuels that providethe energy for most cars, trucks, and buses. But

many different kinds of fuel are used to meet thewide variety of needs for energy storage.

For a fuel to be useful, its energy must bereleased in a way that can be controlled.Controlling the release of the energy makes itpossible for the energy to be used to do work.When a fuel is used for its energy, the fuel isusually burned, and most of the energy is releasedas heat. The heat can be used to do work. For anexample of how the energy in a fuel is releasedmostly as heat, think about what happens whensomeone starts a fire in a fireplace. First, theperson strikes a match and uses it to set some smalltwigs on fire. After the twigs have burned for awhile, they get hot enough to make some largersticks burn. The fire keeps getting hotter, and soonit is hot enough to burn whole logs.

You might think at first that the heat comesfrom starting the fire. After all, someone struck amatch to start the fire, and then the fire just spread,right? But if you think about this fire in terms ofenergy, almost all of the heat comes from theenergy that has been stored in the wood. In otherwords, the wood is the fuel for the fire. There is areason why it is easy to be confused about thesource of the fire’s heat. The reason is that someenergy has to be put into starting the fire beforeany energy can come out of the fire. At first, thereis energy stored in the head of the match aschemical energy. When someone strikes a match,this chemical energy is released as heat.

The lit match gives off enough heat to set thetwigs on fire. This heat is enough energy to start

changing the chemical energy in the wood (and theoxygen in the air, which the wood needs in order toburn) into heat. What happens is that the heat fromthe match breaks chemical bonds in the twigs.When these bonds break, the atoms in the twigs arefree to move around and form new bonds. Whenthe atoms form new bonds, they release more heat.This heat causes more and more of the wood tochange its stored chemical energy into heat. So,what started as a fairly small amount of heat fromthe match turns into a much, much larger amount ofheat from the wood. The same thing is true for anyfuel. We have to add some energy to the fuel to getit started. But once the fuel starts burning, it keepschanging its chemical energy into heat. As long asthe conditions are right, the fuel will keep turningits energy into heat until the fuel is all gone.

Types of Energy Resources

Energy resources can be put into twocategories—either renewable or nonrenewable.

Resources that are nonrenewable are used fasterthan they can be replaced (Figure below). Otherresources that are called renewable will never runout. In most cases, these resources are replaced asquickly as they are used.

In a way, the difference between nonrenewableand renewable resources is like the differencebetween ordinary batteries and rechargeable ones.If you have a flashlight at home that uses ordinarybatteries, and you accidentally leave the flashlighton all night long, you will need to buy newbatteries once the ones in the flashlight have died.The energy in the ordinary batteries isnonrenewable. But if the flashlight hasrechargeable batteries, you can put them in abattery charger and use them in the flashlight again.In this way, the energy in the rechargeable batteriesis “renewable.”

Figure 5.2 Like other fossil fuels, this piece of anthracite

coal is a nonrenewable energy resource.Fossil fuels are the most common example of

nonrenewable energy resources. Renewableenergy resources include solar, water, and windpower. If you traced the energy in all of theseresources back to its origin, you would find thatalmost all energy resources—not just solar energy—come from the sun. Fossil fuels are made of theremains of plants and animals that stored the sun'senergy millions of years ago. These plants andanimals got all of their energy from the sun, eitherdirectly or indirectly. The sun heats some areasmore than others, which causes wind. The sun'senergy drives the water cycle, which moves water

over the surface of the Earth. Both wind and waterpower can be used as renewable resources.

Types of Nonrenewable ResourcesFossil fuels, which include coal, oil, and

natural gas are nonrenewable resources. Millionsof years ago, plants used energy from the sun toform sugars, carbohydrates, and other energy-richcarbon compounds that were later transformed intocoal, oil, or natural gas. The solar energy stored inthese fuels is a rich source of energy, but whilefossil fuels took millions of years to form, we areusing them up in a matter of decades and will soonrun out. Fossil fuels are nonrenewable resources.The burning of fossil fuels also releases largeamounts of the greenhouse gas carbon dioxide.

Types of Renewable Resources

Renewable energy resources include solar,water, wind, biomass, and geothermal power.These resources can usually be replaced at thesame rate that we use them. Scientists know thatthe sun will continue to shine for billions of years

and we can use the energy from the sun as long aswe have a sun. Water flows from high places tolower ones and wind blows from areas of highpressure to areas of low pressure. We can use theflow of wind and water to generate power and wecan count on wind and water to continue to flow.Some examples of biomass energy are burningsomething like wood or changing grains intobiofuels. We can plant new trees or crops toreplace the ones we use. Geothermal energy useswater in the rocks that has been heated by magma.The magma will heat more water in the rocks aswe take hot water out.

Even renewable resources can come withproblems, though. We could cut down too manytrees or we might need grains to be used for foodrather than biofuels. Some renewable resourceshave been too expensive to be widely used orcause some types of environmental problems. Asthe technology improves and more people userenewable energy, the prices may come down.And, as we use up fossil fuels, they will becomemore expensive. At some point, even if renewable

energy is expensive, nonrenewable energy will beeven more expensive. Ultimately, we will have touse renewable sources (and conserve).

Important Things to Consider About EnergyResources

With both renewable and nonrenewableresources, there are at least two important things toconsider. One is that we have to have a practicalway to turn the resource into a useful form ofenergy. The other is that we have to consider whathappens when we turn the resource into energy.

For example, if we get much less energy fromburning a fuel than we put into making it, then thatfuel is probably not a practical energy resource.On the other hand, if another fuel gives us largeamounts of energy but also creates large amountsof pollution, that fuel also may not be the bestchoice for an energy resource.

Lesson Summary

According to the law of conservation ofenergy, energy is neither created or destroyed.Renewable resources can be replaced at therate they are being used.Nonrenewable resources are available inlimited amounts or are being used faster thanthey can be replaced.

Interdisciplinary Connection

Health: Read the nutrition labels on some foodpackages in your kitchen or at the grocery store.How is the energy in food measured? What aresome foods that provide the most energy perserving? Are the foods that contain the most energythe most healthful foods?

Review Questions

1. What is needed by anything that moves orchanges in any way

2. What is the original source of most of our

energy?3. When your body breaks down the food you

eat, in what form does it store the energy fromthe food?

4. When we burn a fuel, what is released thatallows work to be done?

5. For biomass, coal, natural gas, oil andgeothermal energy, identify each energyresource as renewable or nonrenewable.Explain your reasoning.

6. What factors are important in judging howhelpful an energy resource is to us?

7. Is a rechargeable battery a renewable sourceof energy? Explain.

Further Reading / SupplementalLinks

Kydes, Andy, "Primary Energy."Encyclopedia of Earth, 2006. Available onthe Web at:http://www.eoearth.org/article/Primary_energy

http://www.earthportal.org/

Vocabulary

chemical energyEnergy that is stored in the connectionsbetween atoms in a chemical substance.

energyThe ability to move or change matter.

fuelMaterial that can release energy in a chemicalchange.

kinetic energyThe energy that an object in motion hasbecause of its motion.

law of conservation of energyLaw stating that energy cannot be created ordestroyed.

potential energyEnergy stored within a physical system.

Points to Consider

How long do fossil fuels take to form?Are all fossil fuels nonrenewable resources?Do all fossil fuels affect the environmentequally?

Nonrenewable EnergyResources

Lesson Objectives

Describe the natural processes that formeddifferent fossil fuels.Describe different fossil fuels, and understandwhy they are nonrenewable resources.Explain how fossil fuels are turned into usefulforms of energy.Understand that when we burn a fossil fuel,most of its energy is released as heat.Describe how the use of fossil fuels affectsthe environment.Describe how a nuclear power plantproduces energy.

Introduction

Have you ever seen dinosaur fossils at amuseum? If so, you may have read about how thedinosaur bones turned into fossils. The sameprocesses that formed these fossils also formedsome of our most important energy resources.These resources are called fossil fuels. Fossilfuels provide a very high quality energy, butbecause of our demand for energy, we are using upthese resources much faster than they formed.

Formation of Fossil Fuels

As you might guess from their name, fossilfuels are made from fossils. Fossil fuels comefrom materials that began forming about 500million years ago. As plants and animals died,their remains settled on the ground and at thebottom of bodies of water. Over time, theseremains formed layer after layer. Eventually, all ofthese layers were buried deep enough that theywere under an enormous mass of earth. The weightof the earth pressing down on these layers created

intense heat and pressure.After millions of years of heat and pressure,

the material in these layers turned into chemicalscalled hydrocarbons, which are compounds ofcarbon and hydrogen. The hydrocarbons in theselayers are what we call fossil fuels. Thehydrocarbons could be solid, liquid, or gaseous.The solid form is what we know as coal. Theliquid form is petroleum, or crude oil. We call thegaseous hydrocarbons natural gas.

You may be surprised to learn that anything thatused to be alive could change enough to becomesomething so different, such as coal or oil. There isenough heat and pressure deep below the earth’ssurface even to create diamonds, which are thehardest natural material in the world.

Like fossil fuels, diamond is made of carbon.In fact, diamond is a type of pure carbon, so it doesnot contain the hydrogen that fossil fuels do. Whatdetermines whether the remains of living thingsdeep in the earth turn into coal, oil, natural gas,diamond, or something else? All of these materialsform under high heat and pressure, but the

conditions are different for each material.

Coal

Coal is the solid fossil fuel that forms fromdead plants that settled at the bottom of swampsmillions of years ago. The water and mud in theswamps affected how the remains of plants brokedown as they were compressed. The water andmud in the swamp keep oxygen away from theplant material. When plants are buried withoutoxygen, the organic material can be preserved orfossilized. Then, other material, such as sand andclay, settles on top of the decaying plants andsqueezes out the water and some other substances.Over time, the pressure removes most of thematerial other than carbon, and the carbon-containing material forms a layer of rock that weknow as coal.

Coal is black or brownish-black inappearance. Coal is a rock that burns easily. Mostforms of coal are sedimentary rock. But the

hardest type of coal, anthracite, is a metamorphicrock, because it is exposed to higher temperatureand pressure as it forms. Coal is mostly carbon,but some other elements can be found in coal,including sulfur.

Around the world, coal is the largest source ofenergy for electricity. The United States is rich incoal, which is used for electricity. California oncehad a number of small coal mines but the state nolonger produces coal.

A common way of turning coal into a usefulform to make electricity starts with crushing thecoal into powder. Then, a power plant burns thepowder in a furnace that has a boiler. Like otherfuels, coal releases most of its energy as heat whenit burns. The heat that the burning coal releases inthe furnace is enough to boil the water in theboiler, making steam. The power plant uses thissteam to spin turbines, and the spinning turbinesmake generators turn to create electricity.

For people to use coal as an energy source,they need to get it out of the ground. The process ofremoving coal from the ground is known as coal

mining. Coal mining can take place underground orat the surface. The process of coal mining,especially surface mining, affects the environment.Surface mining exposes minerals from undergroundto air and water from the surface. These mineralscontain the chemical element sulfur, and sulfurmixes with air and water to make sulfuric acid,which is a highly corrosive chemical. The sulfuricacid gets into nearby streams and can kill fish,plants, and animals that live in or near the water.The process of burning coal causes other problemsfor the environment. A little later, we will look atthese other pollution problems, when we exploreproblems with fossil fuels in general.

Oil

Figure 5.3 Refineries like this one separate crude oil into

many useful fuels and other chemicals.Oil is a thick liquid that is usually dark brown

or black in appearance. It is found mostly informations of porous rock in the upper layers of theEarth's crust. Oil is currently the single largestsource of energy in the world. How does oil form?The process of making oil is similar in many waysto the process of making coal. The main differenceis in the size of the living things—the organisms—whose remains turn into these fossil fuels. Theorganisms that die and became the material formaking oil are much smaller than the plants thatturned into coal. These organisms are calledplankton and algae. When the plankton and algae

die, their remains settle to the bottom of the sea.There, they were buried away from oxygen, just asthe plants did in the process of becoming coal. Aslayers of sediment pile on top of these decayingorganisms, heat and pressure increase. Over aperiod of millions of years, the heat and pressureturn the material into liquid oil.

The United States produces oil, although onlyabout one-quarter as much as it uses. The main oilproducing regions are the Gulf of Mexico, Texas,Alaska, and California. Most of California’s oilfields are in the southern San Joaquin Valley.Compression from when the region was aconvergent plate boundary produced a set ofanticlines that are parallel to the San AndreasFault. Oil collects in permeable sediments that arecapped by an impermeable cap rock. Oil is alsopumped on and off the southern California coast.

Oil as it comes out of the ground is calledcrude oil. Crude oil is a mixture of many differenthydrocarbons. Oil refining is used to separate thecompounds in this mixture from one another(Figure above). We can separate crude oil intoseveral useful fuels because each hydrocarboncompound in crude oil boils at a differenttemperature. An oil refinery heats the crude oilenough to boil the mixture of compounds. Specialequipment in the refinery separates thesecompounds from one another as they boil.

Most of the compounds that come out of therefining process are fuels. The rest make up waxes,plastics, fertilizers, and other products. The fuelsthat come from crude oil, including gasoline,

diesel, and heating oil, are rich sources of energythat can be easily transported. Because of this,fuels from oil provide about 90% of the energyused for transportation around the world.

We get gasoline from refining oil. Like oil,gasoline is most commonly used for transportationbecause it is a concentrated form of energy that iseasily carried. Let's consider how gasoline powersa car. Like other fuels you have learned about,gasoline burns and releases most of its energy asheat. When it burns, the gasoline turns into carbondioxide gas and water vapor. The heat makes thesegases expand, like the heated air that fills a hot-airballoon. The expanding gases create enough forceto move pistons inside an engine, and the enginemakes enough power to move the car.

When a resource like gasoline is concentratedin energy; it contains a large amount of energy forits weight. This is important because the more anobject weighs, the more energy it takes to movethat object. If we could only get a little energy froma certain amount of gasoline, a car would have tocarry more of it to be able to travel very far. But

carrying more gasoline would make the carheavier, so moving the car would take even moreenergy. So a resource with highly concentratedenergy is a practical fuel to power cars and otherforms of transportation.

Unfortunately, using gasoline to powerautomobiles also affects the environment. Theexhaust fumes from burning gasoline include gasesthat cause many different types of pollution,including smog and ground-level ozone. Theseforms of pollution cause air-quality problems forcities where large numbers of people drive everyday. Burning gasoline also produces carbondioxide, which is a cause of global warming.

Natural Gas

Natural gas is a fuel that is a mixture ofmethane and several other chemical compounds. Itis often found along with coal or oil inunderground deposits. The conditions that createnatural gas are similar to those that create oil. In

both cases, small organisms called plankton andalgae die and settle to the bottom of the sea. In bothcases, the remains of these organisms decaywithout oxygen being present. The difference isthat natural gas forms at higher temperatures thanoil does.

The largest natural gas reserves in the UnitedStates is found are in the Rocky Mountain states,Texas and the Gulf of Mexico region. Californiaalso has natural gas, mostly in the northernSacramento Valley and the Sacramento Delta. Inthat region, a sediment filled trough formed asidean ancient convergent margin. Organic materialburied in the sediments hardened to become ashale formation that is the source of the gas.

Because it is a mixture of different chemicals,natural gas must be processed before it can be usedas a fuel. Some of the chemicals in unprocessednatural gas are poisonous to humans. Other parts,such as water, make the gas a less useful fuel. Theprocessing removes almost everything but methanefrom natural gas. At this point, the gas is ready tobe delivered and used.

Natural gas, often known simply as gas, isdelivered to homes for uses such as cooking andheating. Many ranges and ovens use natural gas asa fuel, and gas-powered furnaces, boilers, waterheaters, and clothes dryers are also common.

Natural gas is a major source of energy forpowering gas turbines and steam turbines to makeelectricity. When it is used in this way, natural gasworks similarly to the way coal does in producingenergy for electricity. Like coal and other fuels,natural gas releases most of its energy as heatwhen it burns. The power plant is able to use thisheat, either in the form of hot gases or steam fromheated water, to spin turbines. The spinningturbines turn generators, and the generators createelectricity.

Processing and using natural gas does havesome harmful effects on the environment. Naturalgas does burn cleaner than other fossil fuels,meaning that it causes less air pollution. It alsoproduces less carbon dioxide than the other fossilfuels for the same amount of energy.

Problems with Fossil Fuels

Although they are rich sources of energy, fossilfuels do present many problems. Because thesefuels are nonrenewable resources, their supplieswill eventually run out. Safety can be a problem,too, because these fuels burn so easily. Forexample, a natural gas leak in a building or anunderground pipe can lead to a deadly explosion.

Using fossil fuels affects the environment in avariety of ways. There are impacts to theenvironment when we extract these resources.There are problems that arise because we arerunning out of supplies of these resources. Burningthese fuels can cause air pollution and burningthem releases carbon dioxide, which is a majorfactor in global warming (Figure below).

Figure 5.4 Coal power plants like this one release large

amounts of steam and smoke into the air.Many of the problems with fossil fuels are

worse for coal than for oil or natural gas. Coalcontains less energy for the amount of carbon itcontains than oil or gas. As a result, burning coalreleases more carbon dioxide than burning eitheroil or gas (for the same energy). And yet coal is themost common fossil fuel and so we continue toburn large amounts of it. Coal is the biggestcontributor to global warming.

Another problem with coal is that it usuallycontains sulfur. When coal burns, the sulfur goesinto the air as sulfur dioxide. Sulfur dioxide is the

main cause of acid rain, which can be deadly toplants, animals, and whole ecosystems. Burningcoal also puts other polluting chemicals and alarge number of small solid "particulates" into theair. These particles are dangerous to people,especially those who have an illness, like asthma,that makes breathing hard for them.

Nuclear Energy

When scientists learned how to split thenucleus of an atom, they released a huge amount ofenergy. Scientists and engineers have learned tocontrol this release of energy. The controlledrelease of this energy is called nuclear energy.Nuclear power plants use uranium that has beenprocessed and concentrated in fuel rods (Figurebelow). The uranium atoms are split apart whenthey are hit by other extremely tiny particles. Theseextremely tiny particles need to be controlled orthey would cause a dangerous explosion.

Nuclear power plants use the energy they

produce to heat water. Once the water is heated,the process is a lot like what happens in a coalpower plant. The hot water or steam causes aturbine to spin. When the turbine spins, it makes agenerator turn, which in turn produces electricity.

Figure 5.5 Nuclear power plants like this one provide

France with almost 80% of its electricity.Many countries around the world use nuclear

energy as a source of electricity. For example,France gets about 80% of its electricity fromnuclear energy. In the United States, a little lessthan 20% of electricity comes from nuclear energy.

Nuclear energy does not pollute the air. In fact,

a nuclear power plant releases nothing but steaminto the air. But nuclear energy does create otherenvironmental problems. The process of splittingatoms creates a dangerous by-product calledradioactive waste. The radioactive wastesproduced by nuclear power plants remaindangerous for thousands or hundreds of thousandsof years. So far, concerns about this waste havekept nuclear energy from being a larger source ofenergy in this country. Scientists and engineers arelooking for ways to keep this waste safely awayfrom people.

Lesson Summary

Coal, oil and natural gas are all fossil fuelsformed from the remains of once livingorganisms.Coal is our largest source of energy forproducing electricity.Mining and using coal produce manyenvironmental impacts, including carbon

dioxide emissions and acid rain.Oil and natural gas are important sources ofenergy for many types of vehicles and uses inour homes and industry.Nuclear energy is produced by splittingatoms. It also produces radioactive wastesthat are very dangerous for many years.Fossil fuels are nonrenewable sources ofenergy that produce environmental damage.

Interdisciplinary Connection

Social Studies: Find a map that shows thelocation of oil refineries in the United States.Which states have the most refineries?

Review Questions

1. How does a fossil fuel form?2. The hardest type of coal is called anthracite.

Why is anthracite harder than other kinds ofcoal?

3. What product of nuclear energy has causedconcerns about the use of this resource?

4. What is one important fuel that comes out ofthe oil refining process?

5. Which chemical element exposed in surfacecoal mining can cause environmentalproblems in nearby bodies of water?

6. Waxes can be made from the processing ofwhich fossil fuel?

7. Why does natural gas need to be processedbefore we can use it as a fuel?

8. What are some problems with using coal? butnot for using gasoline?

9. What characteristic of gasoline is mostimportant in making it a useful fuel fortransportation? Explain.

10. Does nuclear energy cause air pollution?Explain.

Further Reading / SupplementalLinks

Perry, Mildred, "Coal." Encyclopedia ofEarth, 2007. Available on the Web athttp://www.eoearth.org/article/Coalhttp://www.earthportal.org/

Vocabulary

corrosiveAble to cause chemical changes to asubstance that weaken or destroy thesubstance.

hydrocarbonA chemical compound that contains onlycarbon and hydrogen.

metamorphicA type of rock that forms when existing rockis exposed to high temperature and pressure.

nuclear energyEnergy that is released from the nucleus of anatom when it is changed into another atom.

sedimentaryA type of rock that forms from layers ofsediment under high pressure.

Points to Consider

1. How are renewable sources of energydifferent from nonrenewable sources ofenergy?

2. Are all renewable energy sources equallypractical?

3. Are all renewable energy sources equallygood for the environment?

Renewable EnergyResources

Lesson Objectives

Describe different renewable resources, andunderstand why they are renewable.Understand that the sun is the source of mostof Earth’s energy.Describe how energy is carried from oneplace to another as heat and by movingobjects.Understand how conduction, convection, andradiation transfer energy as heat whenrenewable energy sources are used.Understand that some renewable energysources cost less than others and some causeless pollution than others.Explain how renewable energy resources areturned into useful forms of energy.Describe how the use of different renewable

energy resources affects the environment.

Introduction

What if we could have all of the energy weneeded and never run out of it? What if we coulduse this energy without polluting the air and water?In the future, renewable sources of energy may beable to provide all of the energy we need. Some ofthese resources can give us “clean” energy thatcauses little or no pollution.

Plenty of clean energy is available for us touse. The largest amount of energy to reach Earth’ssurface is from solar radiation. Each year is 174petawatts (1.74 x 1017 W) of energy from the sunenter the Earth’s atmosphere. Because the planet’sinterior is hot, heat flows outward from theinterior, providing about 23 terawatts (2.3 x 1013W) of energy per year. By contrast, the total worldpower consumption is around 16 terawatts (1.6 x1013W) per year. So solar or geothermal energyalone could provide all of the energy needed for

people if it could be harnessed.

Solar Energy

Figure 5.6 Solar panels like these can turn the suns energy

into electricity to provide power to homes.When you think of the sun, you probably think

of two things—light and heat. The sun is Earth’smain source of energy, and light and heat are twodifferent kinds of energy that the sun makes. Thesun makes this energy when one element, calledhydrogen, changes into another element, calledhelium. Changing hydrogen into helium releases

huge amounts of energy. The energy travels to theEarth mostly as visible light. The light carries theenergy through the empty space between the sunand the Earth in a process called radiation. Wecan use this light from the sun as an energyresource called solar energy (Figure above).

Solar energy is a resource that has been usedon a small scale for hundreds of years. Its use on alarger scale is just starting to ramp up and peopleincrease production of renewable energy sources.One focus of solar power development in theUnited States is the desert southwest. Solar powerplants are in the works for southeastern California,near the California-Nevada border.

Solar energy is used to heat homes, to heatwater, and to make electricity. Solar energy can beused to heat the water in your pool or to heat tilefloors in your home. In recent years, scientists andengineers have found new ways to get more andmore energy from this resource (Figure below).Because there are many different uses for solarenergy, there are also many different ways ofturning the sun’s energy into useful forms. One of

the most common ways is by using solar cells.Solar cells are devices that can turn sunlightdirectly into electricity. You may have seen solarpanels on roof tops. Lots of solar cells make up anindividual solar panel.

Solar power plants turn sunlight into electricityusing a large group of mirrors to focus sunlight onone place, called a receiver (Figure below). Whena liquid, such as oil or water flows through thisreceiver, the focused sunlight heats the liquid to ahigh temperature. Then, this heated liquid transfersits heat by conduction. In conduction, energymoves between two objects that are in contact withone another. The object that is at a highertemperature transfers energy as heat to the objectthat is at a lower temperature. For example, whenyou heat a pot of water on a stove top, conductioncauses energy to move from the pot to its metalhandle, and the handle gets very hot. In the case ofthe solar power plant, the energy conducted by theheated liquid is used to make electricity.

Solar energy has many benefits. It does notproduce any pollution. Also, there is plenty of it

available. In fact, the amount of energy that reachesEarth from the sun every day is many times morethan all of the energy we use. For this reason, weconsider solar energy a renewable form of energy.For as long as sunlight continues to warm theEarth, we will never run out of this resource. Oneproblem with solar energy is that it cannot be usedat night, unless a special battery stores extra energyduring the day for use at night. The technology formost uses of solar energy is still expensive. Untilthis technology becomes more affordable, mostpeople will prefer to get their energy from othersources. As you learned earlier, most of theEarth’s energy comes from the sun. Otherrenewable resources also come from the sunoriginally. You will be learning about theseresources later in this lesson.

Figure 5.7 This experimental car is one example of the

many uses that engineers have found for solarenergy.

Figure 5.8 This solar power plant uses mirrors to focus

sunlight on the tower in the center. The sunlightheats a liquid inside the tower to a very hightemperature, producing energy to make electricity.

Water Power

Figure 5.9 Hydroelectric dams like this one use the power

of moving water to create electricity.Earlier in this lesson, you learned that energy

can travel in the form of light and heat, just as itdoes when it travels from the sun to the Earth.Now, you will learn about one way in whichenergy can travel in the form of a moving object. Inthis case, the moving object is water (Figureabove). Water power uses the energy of water inmotion to make electricity. It is the most widelyused form of renewable energy in the world, and itprovides almost one fifth of the world’s electricity.

In most power plants that use water power, a

dam holds water back from where it wouldnormally flow. Instead, the water is allowed toflow into a large turbine. Because the water ismoving, it has energy of motion, called kineticenergy. The energy of this moving water makes theturbine spin. The turbine is connected to agenerator, which makes electricity.

Many of the streams in the United States wherewater flows down a slope have probably beendeveloped for hydroelectric power. This is amajor source of California’s electricity, about 14.5percent of the total. Most of California’s nearly400 hydropower plants are located in the easternmountain ranges where large streams descenddown a steep grade.

One big benefit of water power is that it doesnot burn a fuel. This benefit gives water power anadvantage over most other energy resources in howit affects the environment. Because water powerdoes not burn a fuel, it causes less pollution thanmany other kinds of energy. Another benefit ofwater power is that, like the other resources youare learning about in this lesson, it is a renewable

resource. We use energy from the water’smovement, but we are not using up the water itself.Water keeps flowing into our rivers and lakes, sowherever we can build plants to use it, water willbe available as a source of energy. The energy ofwaves and tides can also be used to produce waterpower.

Water power does have its problems, though.When a large dam is built, this creates a reservoir,changing the ecosystem upstream. Large riverecosystems are inundated, killing all the plants andanimals. The dams and turbines also change thedownstream environment for fish and other livingthings. Dams also slow the release of silt, so thatdownstream deltas retreat and seaside citiesbecome dangerously exposed to storms and risingsea levels. Tidal power stations may need to closeoff a narrow bay or estuary. Wave powerapplications have to be able to withstand coastalstorms and the corrosion of seawater.

Wind Power

Figure 5.10 Wind turbines like the ones shown above turn

wind into electricity without creating pollution.As you learned earlier, the sun provides plenty

of energy to the Earth. The energy from the sun alsocreates wind (Figure above). Learning about whatcauses wind will help you understand that energycan move as heat, not just by radiation andconduction, but also by convection. Wind happenswhen the sun heats some parts of the Earthdifferently. For example, sunlight hits the equatormuch more directly than it hits the North and SouthPoles. Hot air rises and cooler air moves in, sowhen the air near the equator is heated much more

than the air near the poles, the air begins to movecarrying heat through the air in a process calledconvection. This movement of air is wind.

Wind power uses moving air as a source ofenergy. Some examples of wind power have beenaround for a long time. Windmills have been usedto grind grain and pump water for hundreds ofyears. Ships with sails have depended on wind foreven longer. Wind can be used to generateelectricity, too. Like the moving water that createswater power, the moving air can make a turbinespin to make electricity.

To help you understand how moving air can beused to make electricity, you could think back towhat you have learned about energy of motion,called kinetic energy. Any form of matter that ismoving has kinetic energy. Even though you cannotsee air, it is matter, because it takes up space andhas mass. So, when wind makes the air move, thisair has kinetic energy. When the moving air hits theblades of a turbine, it makes those blades move,and the turbine spins. The spinning of the turbinecreates electricity.

Wind power has many advantages. It is cleanenergy, meaning that it does not cause pollution orrelease carbon dioxide. Also, wind is plentifulalmost everywhere. One problem with windenergy is that the wind does not blow all of thetime. One solution is to find efficient ways to storeenergy for later use. Until then, another energysource needs to be available when the wind is notblowing. Lastly, windmills are expensive andwear out quickly. For the amount of energy theygenerate, windmills are more expensive than someother forms of renewable energy.

California was an early adopter of windpower. Windmills are found in mountain passes,especially where cooler Pacific ocean air issucked across the passes and into the warmerinland valleys. Large fields of windmills can beseen at Altamont pass in the eastern San FranciscoBay Area, San Gorgonio Pass east of Los Angeles,and Tehachapi Pass at the southern end of the SanJoaquin Valley.

Biomass

Another renewable source of energy isbiomass. Biomass is the material that comes fromplants and animals that were recently living.Biomass also includes the waste that plants andanimals produce. People can use biomass directlyfor heating. For example, many people burn woodin fireplaces or in wood-burning stoves.

Besides burning biomass directly for heating,people can process biomass to make fuel. Thisprocessing makes what is called biofuel. Biofuel isa fairly new type of energy that is becoming morepopular. People can use fuels from biomass inmany of the same ways that they use fossil fuels.For example, some mechanics have made changesto car, truck, and bus engines to allow them to usea fuel called biodiesel. Other engines can run onpure vegetable oil or even recycled vegetable oil.

If we use fuels made from biomass, we can cutdown on the amount of fossil fuel that we use.Because living plants take carbon dioxide out of

the air, growing plants for biofuel can mean thatwe will put less of this gas into the air overall.This could help us do something about the problemof global warming.

Geothermal Energy

Geothermal energy is a source of energy thatcomes from heat deep below the surface of theEarth. This heat produces hot water and steamfrom rocks that are heated by magma. Power plantsthat use this type of energy get to the heat bydrilling wells into these rocks. The hot water orsteam comes up through these wells. Then, the hotwater or steam makes a turbine spin to makeelectricity. Because the hot water or steam can beused directly to make a turbine spin, geothermalenergy is a resource that can be used withoutprocessing. The fact that it does not need to beprocessed makes geothermal energy different frommost other energy resources. Geothermal energy isclean and safe. It is renewable, too, because the

power plant can pump the hot water back into theunderground pool. There, the water can pick upheat to make more steam.

This source of energy is an excellent resourcein some parts of the world. For example, Iceland isa country that gets about one fourth of its electricityfrom geothermal sources. In the United States,California leads all states in producing geothermalenergy. Geothermal energy in California isconcentrated in a few areas in the northern part ofthe state. The largest geothermal power plant in thestate is in the Geysers Geothermal Resource Areain Napa and Sonoma Counties. The source of heatis thought to be a large magma chamber lyingbeneath the area, a part of the Pacific Ring of Fire.Many parts of the world do not have undergroundsources of heat that are close enough to the surfacefor building geothermal power plants.

Lesson Summary

Solar energy, water power, wind power,biomass energy and geothermal energy arerenewable energy sources.Solar energy can be used either by passivelystoring and holding the sun's heat, convertingit to electricity or concentrating it.There are many ways to use the energy ofmoving water including hydroelectric dams.Wind power uses the energy of moving air toturn turbines.Biomass energy uses renewable materialslike wood or grains to produce energy.

Geothermal energy uses heat from deepwithin the earth to heat homes or producesteam that turns turbines.

Review Questions

1. If you turn on the burner on a gas stove undera pan of cold water, energy moves from theburner to the pan of water. What is this energycalled? How does this energy move?

2. What are some ways that we can use solarpower?

3. If you burn wood in a fireplace, which type ofenergy resource are you using?

4. Which form of energy is an important factor inmaking electricity from water power?

5. When the air moves around as wind, it carriesheat from warmer areas to cooler areas. Whatis this movement of heat called?

6. Most of the energy that travels from the sun tothe Earth arrives in the form of visible light.What is this movement of energy called?

7. Explain how mirrors can be useful in somesolar energy plants.

8. Explain how wind power uses kinetic energy.

Further Reading / SupplementalLinks

Cleveland, Cutler, "Energy Transitions Pastand Future." Encyclopedia of Earth, 2007.Available on the Web at:http://www.eoearth.org/article/Energy_transitions_past_and_futurehttp://www.earthportal.org/

Vocabulary

conductionThe process in which energy moves throughmatter as heat, moving from an area of highertemperature to an area of lower temperature.

convectionThe movement of heat in an air current from a

warmer space to a cooler space.

radiationThe movement of energy through empty space.

Points to Consider

What areas do you think would be best forusing solar energy?What causes the high temperatures deepinside the Earth that make geothermal energypossible?Do you think your town or city could usewind or water power?

Chapter 6: Plate Tectonics

Inside Earth

Lesson Objectives

Compare and describe each of Earth's layers.Compare some of the ways geologists learnabout Earth’s interior.Define lithosphere, oceanic and continentalcrust.Describe how heat moves, particularly howconvection takes place in the mantle.Compare the two parts of the core anddescribe why they are different from eachother.

Introduction

Plate tectonics is the unifying theory of

geology. This important theory explains whyEarth's geography has changed through time andcontinues to change today. It explains why someplaces are prone to earthquakes and some are not;why some regions have deadly volcanic eruptions,some have mild ones, and some have none at all;and why mountain ranges are located where theyare. Plate tectonic motions affect Earth’s rockcycle, climate, and the evolution of life. Platetectonic theory was developed through the effortsof many scientists during the twentieth century.

Before you can learn about plate tectonics, youneed to know something about the layers that arefound inside Earth. From outside to inside, theplanet is divided into crust, mantle, and core. Oftengeologists talk about the lithosphere, which is thecrust and the uppermost mantle. The lithosphere isbrittle--it is easily cracked or broken--whereas themantle beneath it behaves plastically; it can bend.Geologists must use ingenious methods, such astracking the properties of earthquake waves, tolearn about the interior of our planet.

Exploring Earth’s Interior

Earth is composed of several layers. On theoutside is the relatively cold, brittle crust. Belowthe crust is the hot, convecting mantle. At thecenter is the dense, metallic inner core. How doscientists know this? Rocks yield clues, butgeologists can only see the outermost rocky layer.Rarely, a rock or mineral, like a diamond, maycome to the surface from deeper down in the crustor the mantle. Mostly, though, Earth scientists mustuse other clues to figure out what lies beneath theplanet’s surface.

One way scientists learn about Earth’s interioris by looking at seismic waves (Figure below).Seismic waves travel outward in all directionsfrom where the ground breaks at an earthquake.There are several types of seismic waves, eachwith different properties. Each type of wavemoves at different speeds though different types ofmaterial and the waves bend when they travel fromone type of material to another. Some types of

waves do not travel through liquids or gases andsome do. So scientists can track how seismicwaves behave as they travel through Earth and canuse the information to understand what makes upthe planet’s interior. Much more about earthquakesand seismic waves will be presented in theEarthquakes lesson.

Figure 6.1 Different types of seismic waves bend or even

disappear as they travel encounter the differentproperties of the layers that make up Earthsinterior. Letters describe the path of an individualP wave or S wave.

Scientists also learn about Earth’s interior

from rocks from outer space. Meteorites are theremains of the material that the early solar systemformed from. Some iron and nickel meteorites arethought to be very similar to Earth's core (Figurebelow). For this reason they give scientists cluesas to the core’s makeup and density. An ironmeteorite is the closest thing to a sample of thecore that scientists can hold in their hands!

Figure 6.2 An iron meteorite, which is thought to be

representative of the Earths core.

Crust and Lithosphere

Of course, scientists know the most aboutEarth’s outermost layer and less and less aboutlayers that are found deeper in the planet’s interior(Figure below). Earth’s outer surface is its crust; athin, brittle outer shell made of rock. Geologistscall the outermost, brittle, mechanical layer thelithosphere. The difference between crust andlithosphere is that lithosphere includes theuppermost mantle, which is also brittle.

Figure 6.3 A cross section of Earth showing the following

layers: (1) crust (2) mantle (3a) outer core (3b)inner core (4) lithosphere (5) asthenosphere (6)outer core (7) inner core. The lithosphere is madeof the crust plus the uppermost part of the mantle.The asthenosphere is directly under the lithosphere

and is part of the upper mantle.The crust is the very thin, outermost physical

layer of the Earth. The crust varies tremendously;from thinner areas under the oceans to much thickerareas that make mountains. Just by looking aroundand thinking of the places you’ve been or seenphotos of, you can guess that the crust is not all thesame. Geologists make an important distinctionbetween two very different types of crust: oceaniccrust and continental crust. Each type has its owndistinctive physical and chemical properties. Thisis one of the reasons that there are ocean basinsand continents.

Oceanic crust is relatively thin, between 5 to12 kilometers thick (3 - 8 miles). This crust ismade of basalt lavas that erupt onto the seafloor.Beneath the basalt is gabbro, an igneous intrusiverock that comes from basalt magma but that coolsmore slowly and develops larger crystals. Thebasalt and gabbro of the oceanic crust are dense(3.0 g/cm3) when compared to the average of therocks that make up the continents. Sediments covermuch of the oceanic crust, primarily rock dust and

the shells of microscopic sea creatures, calledplankton. Near shore, the seafloor is thick withsediments that come off the continents in rivers andon wind currents.

Continental crust is much thicker than oceaniccrust, around 35 kilometers (22 miles) thick onaverage. Continental crust is made up of manydifferent rocks of all three major types: igneous,metamorphic, and sedimentary. The averagecomposition of continental crust is about that ofgranite. Granite is much less dense (2.7 g/cm3)than the basalt and gabbro of the oceanic crust.Because it is thick and has relatively low density,continental crust rises higher above the mantle thanoceanic crust, which sinks into the mantle to formbasins. When filled with water these basins formthe planet’s oceans.

Since it is a combination of the crust anduppermost mantle, lithosphere is thicker than crust.Oceanic lithosphere is about 100 kilometers (62miles) thick. Continental lithosphere is about 250kilometers (155 miles) thick.

Mantle

Beneath the crust, lies the mantle. Like thecrust, mantle is made of rock. Evidence fromseismic waves and meteorites let scientists knowthat the mantle is made of iron- and magnesium-rich silicate minerals that are part of the rockperidotite. These types of ultramafic rocks arerarely found at Earth's surface. One very importantfeature of the mantle is that it is extremely hot. Thisis mainly due to heat rising from the core. Throughthe process of conduction, heat flows fromwarmer objects to cooler objects until all are thesame temperature. Knowing the ways that heatflows is important for understanding how themantle behaves.

Heat can flow in two ways within the Earth. Ifthe material is solid, heat flows by conduction, andheat is transferred through the rapid collisionamong atoms. If a material is fluid and able tomove—that is, it is a gas, liquid, or a solid that canmove (like toothpaste)—heat can also flow by

convection. In convection, currents form so thatwarm material rises and cool material sinks. Thissets up a convection cell (Figure below).

Figure 6.4 In a convection cell, warm material rises and

cool material sinks. In mantle convection, the heatsource is the core.

Convection occurs when a pot of water isheated on a stove. The stove heats the bottom layerof the water, which makes it less dense than thewater above it, so the warmer bottom water rises.

Since the layer of water on the top of the pot is notnear the heat source, it is relatively cool. As aresult, it is denser than the water beneath it and soit sinks. Within the pot, convection cells becomewell established as long as there is more heat atthe bottom of the pot than on the top.

Convection cells are also found in the mantle(Figure below). Mantle material is heated by thecore and so it rises upwards. When it reaches thesurface of the Earth, it moves horizontally. As thematerial moves away from the core’s heat, it cools.Eventually the mantle material at the top of theconvection cell becomes cool and dense enoughthat it sinks back down into the deeper mantle.When it reaches the bottom of the mantle, it travelshorizontally just above the core. Then it reachesthe location where warm mantle material is rising,and the mantle convection cell is complete. Therelationship between mantle convection and platetectonics will be discussed in the final section ofthis chapter.

Figure 6.5 Diagram of convection within Earth's mantle.

Core

At the planet’s center lies a dense metalliccore. Scientists know that the core is metal for tworeasons: The first is that some meteorites aremetallic and they are thought to be representativeof the core. The second is that the density ofEarth's surface layers is much less than the overalldensity of the planet. We can calculate Earth'sdensity using our planet’s rotation. If the surfacelayers are less dense than the average for theplanet, then the interior must be denser than theaverage. Calculations indicate that the core isabout 85% iron metal with nickel metal making up

much of the rest. These proportions agree withthose seen in metallic meteorites. Seismic wavesindicate that the outer core must be liquid and theinner core must be solid.

If Earth's core were not metallic, the planetwould not have a magnetic field. Metal conductselectricity, but rock—which makes up the mantleand crust—does not. The best conductors aremetals that can move, so scientists assume that themagnetic field is due to convection in the liquidouter core. These convection currents form in theouter core because the base of the outer core isheated by the even hotter inner core.

Lesson Summary

The Earth is made of three layers: the crust,mantle and core.The brittle crust and uppermost mantle aretogether called the lithosphere.Beneath the lithosphere, the mantle is solidrock that can flow, or behave plastically.

The hot core warms the base of the mantle,which cause mantle convection.Mantle convection is very important to thetheory of plate tectonics.

Review Questions

1. List two ways that scientists learn about whatmakes up the planet’s interior.

2. What types of rock make up the oceanic crust?3. What types of rock make up the continental

crust?4. List two reasons that scientists know that the

outer core is liquid.5. Describe the properties of each of these parts

of the Earth’s interior: lithosphere, mantle,and core. What are they made of? How hotare they? What are a few of their physicalproperties?

6. Suppose that Earth’s interior contains a largeamount of lead. Based on your priorknowledge, how dense is lead? Would the

lead be more likely to be found in the crust,mantle, or core?

7. When you put your hand near a pan above apan filled with boiling water, does your handwarm up because of convection orconduction? If you touch the pan, does yourhand warm up because of convection orconduction? Based on your answers, whichtype of heat transfer moves heat more easilyand efficiently?

Vocabulary

conductionThe process in which energy moves throughmatter as heat by direct contact, moving froman area of higher temperature to an area oflower temperature.

convectionThe transport of heat by movement.

convection cell

A circular pattern of warm material rising andcool material sinking.

continental crustThe crust that makes up the continents.

coreThe dense metallic center of the Earth. Theouter core is liquid and the inner core issolid.

crustThe rocky outer layer of the Earth’s surface.The two types of crust are continental andoceanic.

lithosphereThe layer of solid, brittle rock that makes upthe Earth’s surface. The lithosphere iscomposed of the crust and the uppermostmantle.

mantleThe middle layer of the Earth, between the

crust and the core. The mantle is made of hotrock that circulates by convection.

meteoriteFragments of planetary bodies such as moons,planets, asteroids and comets that strikeEarth.

oceanic crustThe crust that underlies the oceans.

plate tectonicsThe theory that the Earth’s surface is dividedinto lithospheric plates that move on theplanet’s surface. The driving force behindplate tectonics is mantle convection.

seismic wavesAlso called earthquake waves. Seismicwaves give scientists information on Earth’sinterior.

Points to Consider

The oceanic crust is thinner and denser thancontinental crust. All crust sits atop themantle. What might our planet be like if thiswere not true.If sediments fall onto the seafloor over time,what can sediment thickness tell scientistsabout the age of the seafloor in differentregions?How might convection cells in the mantleaffect the movement of plates of lithosphereon the planet’s surface?

Continental Drift

Lesson Objectives

Be able to explain the continental drifthypothesis.Describe the evidence Wegener used tosupport his continental drift idea.Describe how the north magnetic poleappeared to move, and how that is evidencefor continental drift.

Introduction

An important piece of plate tectonic theory isthe continental drift idea. This was developed inthe early part of the 20th century, mostly by a singlescientist, Alfred Wegener. His hypothesis statesthat continents move around on Earth’s surface andthat they were once joined together as a singlesupercontinent (Figure below). Wegener’s idea

eventually helped to form the theory of platetectonics, but while Wegener was alive, scientistsdid not believe that the continents could move.

The Continental Drift Idea

Find a map of the continents and cut each oneout. Better yet, use a map where the edges of thecontinents show the continental shelf. In this case,your continent puzzle piece includes all of thecontinental crust for that continent and reflects thetrue size and shape of the continent. Can you fit thepieces together? The easiest link is between theeastern Americas and western Africa and Europe,but the rest can fit together too!

Figure 6.6 The continents fit together like pieces of a

puzzle. This is how they looked 250 million yearsago.

Alfred Wegener, an early 20 th century Germanmeteorologist believed that the continents could fittogether. He proposed that the continents were notstationary but that they had moved during theplanet’s history. He suggested that at one time, allof the continents had been united into a single

supercontinent. He named the supercontinentPangaea, meaning entire earth in ancient Greek.Wegener further suggested that Pangaea broke uplong ago and that the continents then moved to theircurrent positions. He called his hypothesiscontinental drift.

Evidence for Continental Drift

Besides the fit of the continents, Wegener andhis supporters collected a great deal of evidencefor the continental drift hypothesis. Wegener foundthat this evidence was best explained if thecontinents had at one time been joined together.

Wegener discovered that identical rocks couldbe found on both sides of the Atlantic Ocean.These rocks were the same type and the same age.Wegener understood that the rocks had formedside-by-side and that the land has since movedapart. Wegener also matched up mountain rangesthat had the same rock types, structures, and ages,but that are now on opposite sides of the Atlantic

Ocean. The Appalachians of the eastern UnitedStates and Canada, for example, are just likemountain ranges in eastern Greenland, Ireland,Great Britain, and Norway. Wegener concludedthat they formed as a single mountain range thatwas separated as the continents drifted.

Wegener also found evidence from ancientfossils (Figure below). He found fossils of thesame species of extinct plants and animals in rocksof the same age, but on continents that are nowwidely separated. Wegener suggested that thecontinents could not have been in their currentpositions because the organisms would not havebeen able to travel across the oceans. For example,fossils of the seed fern Glossopteris are foundacross all of the southern continents. But theplants’ seeds were too heavy to be carried acrossthe ocean by wind. Mesosaurus fossils are foundin South America and South Africa, but the reptileonly could only swim in fresh water. Cynognathusand Lystrosaurus were reptiles that lived on land.Both of these animals were unable to swim, letalone swim across wide seas! Their fossils have

been found across South America, Africa, Indiaand Antarctica. Wegener proposed that theorganisms had lived side by side, but that the landshad moved apart after they were dead andfossilized.

Figure 6.7 Wegener used fossil evidence to support his

continental drift hypothesis. The fossils of theseorganisms are found on lands that are now farapart. Wegener suggested that when the organismswere alive, the lands were joined and theorganisms were living side-by-side.

Wegener also looked at evidence from ancientglaciers. Large glaciers are most commonly found

in frigid climates, usually in the far northern andsouthern latitudes. Using the distribution ofgrooves and rock deposits left by ancient glacierson many different continents, Wegener traced theglaciers back to where they must have started. Hediscovered that if the continents were in theircurrent positions, the glaciers would have formedin the middle of the ocean very close to theequator. Wegener knew that this was impossible!However, if the continents had moved, the glacierswould have been centered over the southern landmass much closer to the South Pole.

Wegener also found evidence for hishypothesis from warm climate zones. Coral reefsand the swamps that lead to the formation of coalare now found only in tropical and subtropicalenvironments. But Wegener discovered ancientcoal seams and coral reefs in parts of thecontinents that were much too cold today. Thecoral reef fossils and coal had drifted to newlocations since the coal and coral formed.

Although Wegener’s evidence was sound, mostgeologists at the time rejected his hypothesis of

continental drift. These scientists argued that therewas no way to explain how solid continents couldplow through solid oceanic crust. At the time,scientists did not understand how solid materialcould move. Wegener’s idea was nearly forgottenuntil technological advances presented puzzlingnew information and gave scientists the tools todevelop a mechanism for Wegener’s driftingcontinents.

Magnetic Polarity Evidence

The puzzling new evidence came from studyingEarth's magnetic field and how it has changed. Ifyou have ever been hiking or camping, you mayhave used a compass to help you find your way. Acompass uses the Earth's magnetic field to locatethe magnetic North Pole. Earth’s magnetic field islike a bar magnet with the ends of the bar stickingout at each pole (Figure below). Currently, thefield’s north and south magnetic poles are verynear to the Earth’s north and south geographic

poles.Some iron-bearing minerals, like tiny

magnetite crystals in igneous rocks, point to thenorth magnetic pole as they crystallize frommagma. These little magnets record both thestrength and direction of the Earth's magnetic field.The direction is known as the field’s magneticpolarity. In the 1950's, scientists began usingmagnetometers to look at the magnetic propertiesof rocks in many locations.

Figure 6.8 Earths magnetic field is like a magnet with its

north pole near the geographic north pole and thesouth pole near the geographic south pole.

Geologists noted that magnetite crystals infresh volcanic rocks pointed to the currentmagnetic north pole. This happened no matterwhere the rocks were located, whether they wereon different continents or in different locations onthe same continent. But for older volcanic rocks,this was not true. Rocks that were the same age andwere located on the same continent pointed to thesame point, but that point was not the current northmagnetic pole. Moving back in time, rocks on thesame continent that were the same age pointed atthe same point. But these rocks did not point to thesame point as the rocks of different ages or thecurrent magnetic pole. In other words, although themagnetite crystals were pointing to the magneticnorth pole, the location of the pole seemed towander. For example, 400 millon year old lavaflows in North America indicated that the northmagnetic pole was located in the western PacificOcean, but 250 million year old lava flowsindicated a pole in Asia, and 100 million year oldlava flows had a pole in northern Asia. Scientistswere amazed to find that the north magnetic pole

changed location through time (Figure below)!

Figure 6.9 The magnetic north pole appears to move

around with time. This diagram shows where thepole was 80 million years before present (mybp),then 60, 40, 20 and now. Since we know that thepole does not move, this path is called apparentpolar wander.

There were three possible explanations for thispuzzling phenomenon: (1) the continent remainedfixed and the north magnetic pole moved (2) thenorth magnetic pole stood still and the continent

moved (3) both the continent and the north polemoved.

The situation got stranger when scientistslooked at where magnetite crystals pointed forrocks of the same age but on different continents.They found these rocks pointed to differentmagnetic north poles! For example, 400 millionyears ago the European north pole was differentfrom the North American north pole at that sametime. At 250 million years, the north poles werealso different for the two continents. The scientistsagain looked at the three possible explanations. Ifthe correct explanation was that the continents hadremained fixed while the north magnetic polemoved, then there had to be two separate northpoles. Since there is only one north pole today,they decided that the best explanation had toinvolve only one north magnetic pole. This meantthat the second explanation must be correct, that thenorth magnetic pole had remained fixed but that thecontinents had moved.

To test this, geologists fitted the continentstogether as Wegener had done. They discovered

that there had indeed been only one magnetic northpole but that the continents had drifted. Theyrenamed the phenomenon of the magnetic pole thatseemed to move but actually did not apparentpolar wander. This evidence for continental driftgave geologists renewed interest in understandinghow continents could move about on the planet’ssurface. And we know that the magnetic polewanders, too, so the correct explanation was thatboth the continents and the magnetic poles move(Figure below).

Figure 6.10 The left side image shows the apparent north

pole locations for two different continents, Europeand North America, if the continents were alwaysin their current locations. When continental drift istaken into account, the two paths merge into one

since there is only one magnetic north pole.

Lesson Summary

In the early part of the 20th century, scientistsbegan to put together evidence that thecontinents could move around on Earth’ssurface.The evidence for continental drift includedthe fit of the continents; the distribution ofancient fossils, rocks, and mountain ranges;and the locations of ancient climatic zones.Although the evidence was extremely strong,scientists could not think of a mechanism thatcould drive solid continents to move aroundon the solid earth and most rejected the idea.Continental drift would resurface after WorldWar II when a mechanism was discovered.

Review Questions

1. Why can paper cutouts of the continents

including the continental margins be piecedtogether to form a single whole?

2. How can the locations where ancient fossilsare found be used as evidence for continentaldrift?

3. To show that mountain ranges on oppositesides of the Atlantic formed as two parts ofthe same range and were once joined, whatwould you look for?

4. What are the three possible explanations forapparent polar wander when the rocks are allon one continent? If the rocks are on morethan one continent, which explanation is theonly one likely to be true and why?

5. In the face of so much evidence in support ofcontinental drift, how could scientists rejectthe idea?

6. Look at a world map. Besides the coast ofwest Africa and eastern South America, whatare some other regions of the world that lookas they could be closely fit together?

Further Reading / SupplementalLinks

http://www.youtube.com/watch?v=gO2qYMsNHGkhttp://www.exploratorium.edu/origins/antarctica/ideas/gondwana2.html

Vocabulary

apparent polar wanderThe path on the globe showing where themagnetic pole appeared to move over time.

continental driftThe hypothesis developed in the early 20thcentury that states that the continents moveabout on the surface.

magnetiteA magnetic mineral that takes on the polarityof the Earth’s magnetic field at the time itforms.

magnetic fieldThe region around a magnet that is susceptibleto the magnetic force. Earth’s magnetic fieldis like a magnet.

magnetic polarityThe direction of the Earth’s magnetic field,north is normal or south is reversed.

magnetometerAn instrument that measures the magneticfield intensity.

Points to Consider

Why is continental drift referred to as ahypothesis (or idea) and not a theory?Why was Wegener’s continental drift idearejected by the scientific community and whyis it accepted today?Explain how each of these phenomena can beused as evidence for continental drift:

Seafloor Spreading

Lesson Objectives

List the main features of the seafloor: mid-ocean ridges, deep sea trenches, and abyssalplains.Describe what seafloor magnetism tellsscientists about the seafloor.Describe the process of seafloor spreading.

Introduction

Perhaps surprisingly, it was World War II thatgave scientists the tools to find the mechanism forcontinental drift that had eluded Wegener and hiscolleagues. Scientists used maps and other datagathered during the war to develop the seafloorspreading hypothesis. This hypothesis tracesoceanic crust from its origin at a mid-ocean ridgeto its destruction at a deep sea trench. Scientists

realized that seafloor spreading could be themechanism for continental drift that they had beenlooking for.

Seafloor Bathymetry

During the war, battleships and submarinescarried echo sounders to locate enemy submarines(Figure below). Echo sounders produce soundwaves that travel outward in all directions, bounceoff the nearest object, and then return to the ship.The round-trip time of the sound wave is thenrecorded. By knowing the speed of sound inseawater, scientists can calculate the distance tothe object that the sound wave hit. During the war,the sound wave rarely encountered an enemysubmarine, and so most of the sound wavesricocheted off the ocean bottom.

Figure 6.11 A ship sends out sound waves to create a

picture of the seafloor below it. The echo sounderpictured has many beams and as a result it createsa three dimensional map of the seafloor beneath theship. Early echo sounders had only a single beamand created a line of depth measurements.

After the war, scientists pieced together thebottom depths to produce a a map of the seafloor.This is known as a bathymetric map and issimilar to a topographic map of the land surface.While a bathymetric map measures the distance ofthe seafloor below sea level, a topographic mapgives the elevation of the land surface above sealevel. Bathymetric maps reveal the features of the

ocean floor as if the water were taken away.The bathymetric maps that were produced at

this time were astonishing! Most people hadthought that the ocean floor was completely flat butthe maps showed something completely different.As we know now, majestic mountain ranges extendin a line through the deep oceans. Amazingly, themountain ranges are connected as if they were theseams on a baseball. These mountain ranges arenamed mid-ocean ridges. The mid-ocean ridgesand the areas around them rise up high above thedeep seafloor (Figure below).

Figure 6.12 A modern map of the eastern Pacific and

Atlantic Oceans. Darker blue indicates deeperseas. A mid-ocean ridge can be seen runningthrough the center of the Atlantic Ocean. Deep seatrenches are found along the west coast of Centraland South America and in the mid-Atlantic east ofthe southern tip of South America. Isolatedmountains and flat featureless regions can also bespotted.

Another astonishing feature is the deep sea

trenches that are found at the edges of continentalmargins or in the sea near chains of activevolcanoes. Trenches are the deepest places onEarth. The deepest trench is the Marianas Trenchin the southwestern Pacific Ocean, which plungesabout 11 kilometers 35,840 feet (35,840 feet)beneath sea level. Near the trenches, the seaflooris also especially deep.

Besides these dramatic features, there are lotsof flat areas, called abyssal plains, just as thescientists had predicted. But many of these plainsare dotted with volcanic mountains. Thesemountains are both large and small, pointy and flat-topped, by themselves as well as in a line. Whenthey first observed the maps, the amazingdifferences made scientists wonder what hadformed these features.

Seafloor Magnetism

In the previous lesson, you learned thatmagnetometers used on land were important in

recognizing apparent polar wander.Magnetometers were also important inunderstanding the magnetic polarity of rocks in thedeep sea. During WWII, magnetometers that wereattached to ships to search for submarinesdiscovered a lot about the magnetic properties ofthe seafloor.

In fact, using magnetometers, scientistsdiscovered an astonishing feature of Earth’smagnetic field. Sometimes, no one really knowswhy, the magnetic poles switch positions. Northbecomes south and south becomes north! When thenorth and south poles are aligned as they are now,geologists say the polarity is normal. When theyare in the opposite position, they say that thepolarity is reversed.

Scientists were surprised to discover that thenormal and reversed magnetic polarity of seafloorbasalts creates a pattern of magnetic stripes! Thereis one long stripe with normal polarity, next to onelong stripe with reversed polarity and so on acrossthe ocean bottom. Another amazing feature is thatthe stripes are form mirror images on either side of

the mid-ocean ridges. The ridge crest is of normalpolarity and there are two stripes of reversedpolarity of roughly equal width on each side of theridge. Further distant are roughly equal stripes ofnormal polarity, beyond that, roughly equal stripesof reversed polarity, and so on. The magneticpolarity maps also show that the magnetic stripesend abruptly at the edges of continents, which aresometimes lined by a deep sea trench (Figurebelow).

Figure 6.13 Scientists found that magnetic polarity in the

seafloor was normal at mid-ocean ridges butreversed in symmetrical patterns away from the

ridge center. This normal and reversed patterncontinues across the seafloor.

The scientists used geologic dating techniquesto find the ages of the rocks that were found withthe different magnetic polarities. It turns out thatthe rocks of normal polarity are located along theaxis of the mid-ocean ridges and these are theyoungest rocks on the seafloor. The ages of therocks increases equally and symmetrically on bothsides of the ridge.

Scientists also discovered that there arevirtually no sediments on the seafloor at the axis,but the sediment layer increases in thickness inboth directions away from the ridge axis. This wasadditional evidence that the youngest rocks are onthe ridge axis and that the rocks are older withdistance away from the ridge (Figure below). Thescientists were surprised to find that oldestseafloor is less than 180 million years old whilethe oldest continental crust is around 4 billionyears old. They realized that some process wascausing seafloor to be created and destroyed in arelatively short time.

Figure 6.14 Seafloor is youngest near the mid-ocean ridges

and gets progressively older with distance from theridge. Orange areas show the youngest seafloor.The oldest seafloor is near the edges of continentsor deep sea trenches.

The scientists also discovered that the seafloorwas thinner at the ridge axis and grew thicker asthe crust became older. This is because over time,additional magma cools to form rock. The addedsediments also increase the thickness of the oldercrust.

The Seafloor Spreading Hypothesis

Scientists brought all of these observationstogether in the early 1960s to create the seafloorspreading hypothesis. They suggested that hotmantle material rises up toward the surface at mid-ocean ridges. This hot material is buoyant andcauses the ridge to rise, which is one reason thatmid-ocean ridges are higher than the rest of theseafloor.

The hot magma at the ridge erupts as lava thatforms new seafloor. When the lava cools, itsmagnetite crystals take on the current magneticpolarity. The polarity is locked in when the lavasolidifies and the magnetite crystals are trapped inposition. Reversals show up as magnetic stripes onopposite sides of the ridge axis. As more lavaerupts, it pushes the seafloor that is at the ridgehorizontally away from ridge axis. This continuesas the formation of new seafloor forces olderseafloor to move horizontally away from the ridgeaxis.

The magnetic stripes continue across theseafloor. If the oceanic crust butts up against acontinent, it pushes that continent away from the

ridge axis as well. If the oceanic crust reaches adeep sea trench, it will sink into it and be lost intothe mantle. In either case, the oldest crust iscoldest and lies deepest in the ocean.

It is the creation and destruction of oceaniccrust, then, that is the mechanism for Wegener’sdrifting continents. Rather than drifting across theoceans, the continents ride on a conveyor belt ofoceanic crust that takes them around the planet’ssurface.

One of the fundamental lines of evidence forcontinental drift is the way the coastlines ofcontinents on both sides of the Atlantic Ocean fittogether. So let’s look at how seafloor spreadingmoves continents in the Atlantic by looking moreclosely at figure 3 above. New oceanic crust isforming at the mid-ocean ridge that runs through thecenter of the Atlantic Ocean basins, which iscalled the Mid-Atlantic Ridge. Stripes of differentmagnetic polarity are found on opposite sides ofthe Mid-Atlantic Ridge. These stripes go all theway to the continents, which lie on opposite sidesof the Atlantic. So new seafloor forming at the

Mid-Atlantic Ridge is causing the Americas andEurasia to move in opposite directions!

Lesson Summary

Using technologies developed to fight WorldWar II, scientists were able to gather data thatallowed them to recognize that seafloorspreading is the mechanism for Wegener’sdrifting continents.Bathymetric maps revealed high mountainranges and deep trenches.Magnetic polarity stripes give clues as toseafloor ages and the importance of mid-ocean ridges in the creation of oceanic crust.Seafloor spreading processes create newoceanic crust at mid-ocean ridges and destroyolder crust at deep sea trenches.

Review Questions

1. Describe how sound waves are used to

develop a map of the features of the seafloor.2. Why has no ocean crust been located that is

older than about 180 million years when theoldest continental crust is about 4 billionyears old?

3. Describe the major features of mid-oceanridges, deep sea trenches, and abyssal plainsand their relative ages.

4. Describe continents move across the oceanbasins as if they are on a conveyor belt ratherthan as if they are drifting, as we Wegner’soriginal idea.

5. Explain why the following scenario isimpossible: Oceanic crust is not destroyed atoceanic trenches, but new crust is still createdat mid-ocean ridges.

6. If you were a paleontologist who studiesfossils of very ancient life forms, wherewould be the best place to look for very oldfossils: on land or in the oceans?

7. Imagine that Earth’s magnetic field was fixedin place and the polarity didn't reverse. Whateffect would this have on our observations of

seafloor basalts?

Further Reading / SupplementalLinks

http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_seafloorspreading.html

Vocabulary

abyssal plainsVery flat areas that make up most of the oceanfloor.

bathymetric mapA map of the seafloor created from themeasurement of water depths.

echo sounderA device that uses sound waves to measurethe depth to the seafloor.

mid-ocean ridge

The location on the seafloor where magmaupwells and new seafloor forms. Mid-oceanridges are the dominant feature of divergentplate boundaries found in the oceans.

seafloor spreadingThe mechanism for moving continents. Theformation of new seafloor at spreading ridgespushes lithospheric plates on the Earth’ssurface.

trenchA deep hole in the seafloor where subductiontakes place. Trenches are the deepest placeson Earth.

Points to Consider

How were the technologies that weredeveloped to fight World War II used byscientists for the development of the seafloorspreading hypothesis?In what two ways did magnetic data lead

scientists to understand more aboutcontinental drift and plate tectonics?How does seafloor spreading provide amechanism for continental drift?The features of the Atlantic Ocean basin aredescribed in terms of seafloor spreading andcontinental drift. Now look at the features ofthe North Pacific Ocean basin and explainthem in those terms as well.

Theory of Plate Tectonics

Lesson Objectives

Describe what a plate is and how scientistscan recognize its edges.Explain how mantle convection moveslithospheric plates.Describe the three types of plate boundariesand whether they are prone to earthquakes andvolcanoes.Describe how plate tectonics processes leadto changes in Earth’s surface features.

Introduction

Wegener’s continental drift hypothesis had agreat deal of evidence in its favor but it waslargely abandoned because there was no plausibleexplanation for how the continents could drift. Inthe meantime, scientists developed explanations to

explain the locations of fossils on widely differentcontinents (land bridges) and the similarity of rocksequences across oceans (geosynclines), whichwere becoming more and more cumbersome. Whenseafloor spreading came along, scientistsrecognized that the mechanism to explain driftingcontinents had been found. Like the scientists didbefore us, we are now ready to merge the ideas ofcontinental drift and seafloor spreading into a newall-encompassing idea: the theory of platetectonics.

Earth’s Tectonic Plates

Now you know that seafloor and continentsmove around on Earth’s surface. But what is it thatis actually moving? In other words, what is the“plate” in plate tectonics? This question was alsoanswered due to war, in this case the Cold War.

Although seismographs had been around fordecades, during the 1950s and especially in theearly 1960s, scientists set up seismograph

networks to see if enemy nations were testingatomic bombs. Seismographs record seismicwaves. Modern seismographs are sensitive enoughto detect nuclear explosions. While watching forenemy atom bomb tests, the seismographs werealso recording all of the earthquakes that weretaking place around the planet. These seismicrecords could be used to locate an earthquake’sepicenter, the point on Earth’s surface directlyabove the place where the earthquake occurs.Earthquakes are associated with large cracks in theground, known as faults. Rocks on opposite sidesof a fault move in opposite directions.

Earthquakes are not spread evenly around theplanet, but are found mostly in certain regions. Inthe oceans, earthquakes are found along mid-oceanridges and in and around deep sea trenches.Earthquakes are extremely common all around thePacific Ocean basin and often occur nearvolcanoes. The intensity of earthquakes andvolcanic eruptions around the Pacific led scientiststo name this region the Pacific Ring of Fire(Figure below). Earthquakes are also common in

the world’s highest mountains, the HimalayaMountains of Asia, and across the Mediterraneanregion.

Figure 6.15 The bold pink swatch outlines the volcanoes

and active earthquake areas found around thePacific Ocean basin, which is called the PacificRing of Fire.

Scientists noticed that the earthquakeepicenters were located along the mid-oceanridges, trenches and large faults that mark theedges of large slabs of Earth’s lithosphere (Figurebelow). They named these large slabs oflithosphere plates. The movements of the plates

were then termed plate tectonics. A single platecan be made of all oceanic lithosphere or allcontinental lithosphere, but nearly all plates aremade of a combination of both.

Figure 6.16 A map of earthquake epicenters shows that

earthquakes are found primarily in lines that run upthe edges of some continents, through the centers ofsome oceans, and in patches in some land areas.

The lithosphere is divided into a dozen majorand several minor plates. The plates’ edges can bedrawn by the connecting the dots that areearthquakes epicenters. Scientists have named eachof the plates and have determined the direction thateach is moving (Figure below). Plates movearound the Earth’s surface at a rate of a few

centimeters a year, about the same rate fingernailsgrow.

Figure 6.17 The lithospheric plates and their names. The

arrows show whether the plates are moving apart,moving together, or sliding past each other.

How Plates Move

We know that seafloor spreading moves thelithospheric plates around on Earth’s surface butwhat drives seafloor spreading? The answer is inlesson one of this chapter: mantle convection. Atthis point it would help to think of a convection

cell as a rectangle or oval (Figure below). Eachside of the rectangle is a limb of the cell. Theconvection cell is located in the mantle. The baseis deep in the mantle and the top is near the crust.There is a limb of mantle material moving on oneside of the rectangle, one limb moving horizontallyacross the top of the rectangle, one limb movingdownward on the other side of the rectangle, andthe final limb moving horizontally to where thematerial begins to move upward again.

Now picture two convection cells side-by-sidein the mantle. The rising limbs of material from thetwo adjacent cells reach the base of the crust at themid-ocean ridge. Some of the hot magma melts andcreates new ocean crust. This seafloor moves offthe axis of the mid-ocean ridge in both directionswhen still newer seafloor erupts. The oceanicplate moves outward due to the eruption of newoceanic crust at the mid-ocean ridge.

Figure 6.18 Convection in the mantle is the driving force of

plate tectonics. Hot material rises at mid-oceanridges and sinks at deep sea trenches, which keepsthe plates moving along the Earths surface.

Beneath the moving crust is the laterallymoving top limb of the mantle convection cells.Each convection cell is moving seafloor awayfrom the ridge in opposite directions. Thishorizontal mantle flow moves with the crust acrossthe ocean basin and away from the ridge. As thematerial moves horizontally, the seafloor thickensand both the new crust and the mantle beneath itcool. Where the limbs of the convection cellsplunge down into the deeper mantle, oceanic crustis dragged into the mantle as well. This takes placeat the deep sea trenches. As the crust dives into the

mantle its weight drags along the rest of the plateand pulls it downward. The last limbs of theconvection cells flow along the core. The materialis heated and so is ready to rise again when itreaches the rising limb of the convection cell. Asyou can see, each convection cell is found beneatha different lithospheric plate and is responsible forthe movement of that plate.

Plate Boundaries

Back at the planet’s surface, the edges wheretwo plates meet are known as plate boundaries.Most geologic activity, including volcanoes,earthquakes, and mountain building, takes place atplate boundaries where two enormous pieces ofsolid lithosphere interact.

Think about two cars moving around a parkinglot. In what three ways can those cars moverelative to each other? They can move away fromeach other, they can move toward each other, orthey can slide past each other. These three types of

relative motion also define the three types of plateboundaries:

Divergent plate boundaries: the two platesmove away from each other.Convergent plate boundaries: the two platesmove towards each other.Transform plate boundaries: the two platesslip past each other.

What happens at plate boundaries depends onwhich direction the two plates are moving relativeto each other. It also depends on whether thelithosphere on the two sides of the plate boundaryis oceanic crust, continental crust, or one piece ofeach type. The type of plate boundary and the typeof crust found on each side of the boundarydetermines what sort of geologic activity will befound there: earthquakes, volcanoes, or mountainbuilding.

Divergent Plate Boundaries

Plates move apart, or diverge, at mid-oceanridges where seafloor spreading forms newoceanic lithosphere. At these mid-ocean ridges,lava rises, erupts, and cools. Magma cools moreslowly beneath the lava mostly forming the igneousintrusive rock gabbro. The entire ridge system,then, is igneous. Earthquakes are also common atmid-ocean ridges since the movement of magmaand oceanic crust result in crustal shaking.Although the vast majority of mid-ocean ridges arelocated deep below the sea, we can see where theMid-Atlantic Ridge surfaces at the volcanic islandof Iceland (Figure below).

Figure 6.19

The Leif the Lucky Bridge straddles the Mid-Atlantic ridge separating the North American andEurasian plates on Iceland.

Although it is uncommon, a divergent plateboundary can also occur within a continent. This iscalled continental rifting (Figure below). Magmarises beneath the continent, causing it to thin,break, and ultimately split up. As the continentalcrust breaks apart, oceanic crust erupts in the void.This is how the Atlantic Ocean formed whenPangaea broke up. The East African Rift iscurrently splitting eastern Africa away from theAfrican continent.

Figure 6.20 The Arabian, Indian, and African plates are

rifting apart, forming the Great Rift Valley inAfrica. The Dead Sea fills the rift with seawater.

Convergent Plate Boundaries

What happens when two plates convergedepends on the types of crust that are colliding.Convergence can take place between two slabs ofcontinental lithosphere, two slabs of oceanic

lithosphere, or between one continental and oneoceanic slab. Most often, when two plates collide,one or both are destroyed.

When oceanic crust converges with continentalcrust, the denser oceanic plate plunges beneath thecontinental plate. This process occurs at theoceanic trenches and is called subduction (Figurebelow). The entire region is known as asubduction zone. Subduction zones have a lot ofintense earthquakes and volcanic eruptions. Thesubducting plate causes melting in the mantle. Themagma rises and erupts, creating volcanoes. Thesevolcanoes are found in a line above the subductingplate. The volcanoes are known as a continentalarc. The movement of crust and magma causesearthquakes. The Andes Mountains, which line thewestern edge of South America, are a continentalarc. The volcanoes are the result of the Nazca platesubducting beneath the South American plate(Figure below).

Figure 6.21 Subduction of an oceanic plate beneath a

continental plate forms a line of volcanoes knownas a continental arc and causes earthquakes.

Figure 6.22 This satellite image shows the trench lining the

western margin of South America where the Nazcaplate is subducting beneath the South Americanplate. The resulting Andes Mountains line westernSouth America and are seen as brown and reduplands in this image.

The volcanoes of northeastern California—Lassen Peak, Mount Shasta, and Medicine Lakevolcano—along with the rest of the CascadeMountains of the Pacific Northwest, are the resultof subduction of the Juan de Fuca plate beneath theNorth American plate (Figure below). Mount St.Helens, which erupted explosively on May 18,1980, is the most famous and currently the mostactive of the Cascades volcanoes.

Figure 6.23 The Cascade Mountains of the Pacific

Northwest are formed by the subduction of the Juande Fuca plate beneath the North American plate.The Juan de Fuca plate forms near the shoreline atthe Juan de Fuca ridge.

Sometimes the magma does not rise all the waythrough the continental crust beneath a volcanic

arc. This usually happens if the magma is rich insilica. These viscous magmas form large areas ofintrusive igneous rock, called batholiths, whichmay someday be uplifted to form a mountain range.The Sierra Nevada batholith cooled beneath avolcanic arc roughly 200 million years ago(Figure below). Similar batholiths are likelyforming beneath the Andes and Cascades today.

Figure 6.24 The granite batholith of the Sierra Nevada

Mountain range is well exposed here at MountWhitney, the highest mountain in the range at14,505 feet (4,421 meters) and the second highestmountain in North America.

When two oceanic plates converge, the older,

denser plate will sink beneath the other plate andplunge into the mantle. As the plate is pusheddeeper into the mantle, it melts, which formsmagma. As the magma rises it forms volcanoes in aline known as an island arc, which is a line ofvolcanic islands (Figure below).

Figure 6.25 A convergent plate boundary subduction zone

between two plates of oceanic lithosphere. Meltingof the subducting plate causes volcanic activity andearthquakes.

The Japanese, Indonesian, and Philippineislands are examples of island arc volcanoes. Thevolcanic islands are set off from the mainland in anarc shape as seen in this satellite image of Japan(Figure below).

Figure 6.26 Japan is an island arc composed of volcanoes

off the Asian mainland, as seen in this satelliteimage.

When two continental plates collide, they aretoo thick to subduct. Just like if you put your handson two sides of a sheet of paper and bring yourhands together, the material has nowhere to go butup (Figure below)! Some of the world’s largestmountains ranges are created at continent-continentconvergent plate boundaries. In these locations, thecrust is too thick for magma to penetrate so thereare no volcanoes, but there may be magma.Metamorphic rocks are common due to the stress

the continental crust experiences. As you mightthink, with enormous slabs of crust smashingtogether, continent-continent collisions bring onnumerous earthquakes.

Figure 6.27 When two plates of continental crust collide,

the material pushes upward forming a highmountain range. The remnants of subductedoceanic crust remain beneath the continentalconvergence zone.

The world’s highest mountains, the Himalayas,are being created by a collision between the Indianand Eurasian plates (Figure below). TheAppalachian Mountains are the remnants of a largemountain range that was created when NorthAmerica rammed into Eurasia about 250 million

years ago.

Figure 6.28 The Himalaya Mountains are the result of the

collision of the Indian Plate with the EurasianPlate, seen in this photo from the InternationalSpace Station. The high peak in the center isworlds tallest mountain, Mount Everest (8,848meters; 29,035 feet).

Transform Plate Boundaries

Transform plate boundaries are seen astransform faults. At these earthquake faults, twoplates move past each other in opposite directions.Where transform faults bisect continents, there are

massive earthquakes. The world’s most notorioustransform fault is the 1,300 kilometer (800 mile)long San Andreas Fault in California (Figurebelow). This is where the Pacific and NorthAmerican plates grind past each other, sometimeswith disastrous consequences.

Figure 6.29

At the San Andreas Fault in California, thePacific Plate is sliding northeast relative to theNorth American plate, which is moving southwest.At the northern end of the picture, the transformboundary turns into a subduction zone.

California is very geologically active. Atransform plate boundary creates the San AndreasFault. A convergent plate boundary between anoceanic plate and a continental plate creates theCascades volcanoes. Just offshore, the Juan deFuca ridge is subducting beneath the NorthAmerican plate at a divergent plate boundary.

Earth’s Changing Surface

Geologists now know that Wegener was rightwhen he said that the continents had once beenjoined into the supercontinent Pangaea and are nowmoving apart. Most of the geologic activity that wesee on the planet today is due to the interactions ofthe moving plates. Where plates come apart at adivergent boundary, there is volcanic activity and

small earthquakes. If the plates meet at aconvergent boundary, and at least one is oceanic,there is a chain of volcanoes and manyearthquakes. If both plates at a convergentboundary are continental, mountain ranges grow. Ifthe plates meet at a transform boundary, there is atransform fault. These faults do not have volcanicactivity but they have massive earthquakes.

If you look at a map showing the locations ofvolcanoes and earthquakes in North America, youwill see that the plate boundaries are now alongthe western edge. This geologically active areamakes up part of the Pacific Ring of Fire.California, with its volcanoes and earthquakes, isan important part of this region. The eastern edgeof North America is currently mostly quiet,although mountain ranges line the area. If there isno plate boundary there today, where did thosemountains come from?

Remember that Wegener used the similarity ofthe mountains in eastern North America, on thewest side of the Atlantic, and the mountains inGreat Britain, on the eastern side of the Atlantic, as

evidence for his continental drift hypothesis. Thesemountains were formed at a convergent plateboundary as the continents that made up Pangaeacame together. So about 200 million years agothese mountains were similar to the Himalayatoday (Figure below)!

Figure 6.30 The Appalachian Mountains of eastern North

America were probably once as high as theHimalaya, but they have aged since the breakup ofPangaea.

Before the continents collided they wereseparated by an ocean, just as the continentsrimming the Pacific are now. That ocean crust hadto subduct beneath the continents just as the

oceanic crust around the Pacific is being subductedtoday. Subduction along the eastern margin ofNorth America produced continental arcvolcanoes. Ancient lava from those volcanoes canbe found in the region.

Currently, Earth’s most geologically activearea is around the Pacific. The Pacific is shrinkingat the same time the Atlantic is growing. Buthundreds of millions of years ago, that wasreversed: the Atlantic was shrinking as the Pacificwas growing. What we’ve just identified is acycle, known as the supercontinent cycle, whichis responsible for most of the geologic features thatwe see and many more that are long gone.Scientists think that the creation and breakup of asupercontinent takes place about every 500 millionyears.

Intraplate Activity

While it is true that most geological activitytakes place along plate boundaries, some is found

away from the edges of plates. This is known asintraplate activity. The most common intraplatevolcanoes are above hotspots that lie beneathoceanic plates. Hotspot volcanoes arise becauseplumes of hot material that come from deep in themantle rise through the overlying mantle and crust.When the magma reaches the plate above, it erupts,forming a volcano. Since the hotspot is stable,when the oceanic plate moves over it, and it eruptsagain, another volcano is created in line with thefirst. With time, there is a line of volcanoes; theyoungest is directly above the hot spot and theoldest is furthest away. Recent research suggeststhat hotspots are not as stable as scientists oncethought, but some larger ones still appear to be.

The Hawaiian Islands are a beautiful exampleof a chain of hotspot volcanoes. Kilauea volcanoon the south side of the Big Island of Hawaii liesabove the Hawaiian hot spot. The Big Island is onthe southeastern end of the Hawaiian chain. MaunaLoa volcano, to the northwest, is older thanKilauea and is still erupting, but at a lower rate.Hawaii is the youngest island in the chain. As you

follow the chain to the west, the islands getprogressively older because they are further fromthe hotspot (Figure below).

Figure 6.31 This view of the Hawaiian islands showing the

youngest islands in the southeast and the oldest inthe northwest. Kilauea volcano, which makes upthe southeastern side of the Big Island ofHawaiian, is located above the Hawaiian hotspot.

The chain continues into the EmperorSeamounts, which are so old they no longer reachabove sea level. The oldest of the Emperor

seamounts is about to subduct into the Aleutiantrench off of Alaska; no one knows how manyolder volcanoes have already subducted. It’sobvious from looking at the Emperor seamountsthat the Pacific plate took a large turn. Radiometricdating has shown that turn to have taken placeabout 43 million years ago (Figure below). TheHawaii hotspot may also have been movingsouthward during this time. Still, geologists canuse some hotspot chains to tell not only thedirection but the speed a plate is moving.

Figure 6.32

The Hawaii-Emperor chain creates a largeangular gash across the Pacific basin in thissatellite image. The bend in the chain is due to achange in the direction of motion of the Pacificplate 43 million years ago.

Hot spots are also found under the continentalcrust, although it is more difficult for the magma tomake it through the thick crust and there are feweruptions. One exception is Yellowstone, whichcreates the activity at the Yellowstone hotspot. Inthe past, the hotspot produced enormous volcaniceruptions, but now its activity is best seen in theregion’s famous geysers.

Lesson Summary

Driven by mantle convection, the plates oflithosphere move around Earth’s surface.New oceanic crust forms at the ridge andpushes the older seafloor away from the ridgehorizontally.Plates interact at three different types of plate

boundaries, divergent, convergent andtransform fault boundaries, which are wheremost of the Earth’s geologic activity takesplace.These processes acting over long periods oftime are responsible for the geographicfeatures we see.

Review Questions

1. What are the three types of plate boundaries?For each type, what sort of geologic activitydo you find?

2. As a working geologist, you come across alandscape with a massive fault zone thatproduces lots of large earthquakes, but has novolcanoes. What type of plate boundary haveyou come across? What are the movements ofplates relative to each other at type ofboundary? Where would you find a plateboundary of this type in California?

3. You continue on your geologic tour to a

location where there is a chain of volcanoeson land, but not too far inland from the edgeof the continent. The region experiencesfrequent large earthquakes. What type of plateboundary have you come across? What typesof plates are involved? Where would you finda plate boundary of this type in California?

4. What is the driving force behind themovement of lithospheric plates on theEarth’s surface? About how fast do the platesmove?

5. How does the theory of plate tectonicsexplain the locations of volcanoes,earthquakes and mountain belts on Earth?

6. Thinking about the different types of plateboundaries, explain why continental crust ismuch thicker than oceanic crust.

7. Why are there few (if any) volcanoes alongtransform plate boundaries?

Vocabulary

batholithAn enormous body of granitic rock that isformed from a large number of plutons.

continental arcA line of volcanoes sitting on a continentalplate and aligned above a subducting oceanicplate near a deep sea trench.

continental riftingA divergent plate boundary that forms in themiddle of a continent.

convergent plate boundaryA location where two lithospheric platescome together.

divergent plate boundaryA location where two lithospheric platesspread apart.

epicenterThe point on the Earth’s surface directlyabove an earthquake’s focus, which is the

place where the ground breaks.

faultA fracture along which there has beenmovement of rock on one or both sides.

intraplate activityGeologic activity such as volcanic eruptionsand earthquakes that takes place away fromplate boundaries.

island arcA line of volcanoes sitting on an oceanicplate above a subducting oceanic plate near adeep sea trench.

plateA slab of the earth’s lithosphere that canmove around on the planet’s surface.

plate boundaryA location where two plates come together.

The driving force behind plate tectonics is

mantle convection.plate tectonics

The theory that the Earth’s surface is dividedinto lithospheric plates that move on theplanet’s surface.

plutonA relatively small body of igneous intrusiverock.

subductionThe sinking of one lithospheric plate beneathanother.

subduction zoneThe area where two lithospheric plates cometogether and one sinks beneath the other.

supercontinent cycleThe cycle in which the continents join intoone supercontinent on one side of the planetand then break apart.

transform fault

An earthquake fault where relative motion issliding past.

transform plate boundaryThe type of plate boundary where two platesslide past one another.

Points to Consider

On the map in Figure 3 above, the arrowsshow the directions that the plates are going.The Atlantic has a mid-ocean ridge, whereseafloor spreading is taking place. ThePacific ocean has many deep sea trenches,where subduction is taking place. What is thefuture of the Atlantic plate? What is the futureof the Pacific plate?Using your hands and words, explain tosomeone how plate tectonics works. Be sureyou describe how continents drift and howseafloor spreading provides a mechanism forcontinental movement.

Now that you know about plate tectonics,where do you think would be a safe place tolive if you wanted to avoid volcanic eruptionsand earthquakes?

Chapter 7: Earthquakes

Stress in the Earth’s Crust

Lesson Objectives

List the different types of stresses that causedifferent types of deformation.Compare the different types of folds and theconditions under which they form.Compare fractures and faults and define howthey are related to earthquakes.Compare how mountains form and at whattypes of plate boundaries.

Introduction

When you think about enormous plates oflithosphere traveling around on the planet’ssurface, you can probably imagine that the process

is not smooth. Most geological activity takes placewhere two plates meet, at plate boundaries. In theEarthquakes chapter, you will learn that nearly allearthquakes, volcanic eruptions, and mountainbuilding occur at plate boundaries. In this chapter,you will learn more about the geological activitythat occurs because of plate tectonics, specificallymountain building and earthquakes.

When plates are pushed or pulled, the rock issubjected to stress. Stress can cause a rock tochange shape or to break. When a rock bendswithout breaking, it folds. When the rock breaks, itfractures. Mountain building and earthquakes aresome of the responses rocks have to stress.

Causes and Types of Stress

Stress is the force applied to an object. Ingeology, stress is the force per unit area that isplaced on a rock. There are four types of stressesthat act on materials.

A deeply buried rock is pushed down by theweight of all the material above it. Since therock is trapped in a single spot, it is as if therock is being pushed in from all sides. Thispushing causes the rock to becomecompressed, but it cannot deform becausethere is no place for it to move. This is calledconfining stress.Compression is the stress that squeezes rockstogether. Compression causes rocks to fold orfracture (break)(Figure below). When carsdriving around a parking lot collide,compression causes the cars to crumple.Compression is the most common stress atconvergent plate boundaries.

Figure 7.1 Stress caused these rocks to fracture.

Rocks that are being pulled apart are undertension (also called extension). Tensioncauses rocks to lengthen or break apart.Tension is the major type of stress found atdivergent plate boundaries.When forces act parallel to each other but inopposite directions, the stress is called shear(Figure below). Shear stress causes twoplanes of material to slide past each other.This is the most common stress found attransform plate boundaries.

Figure 7.2 Rocks showing dextral shear. Note how the

white quartz vein has been elongated by shear.If the amount of stress on a rock is greater than

the rock’s internal strength, the rock bendselastically. This type of change is called elasticbecause when the stress is eliminated the rock goesback to its original shape, like a squeezed rubber

ball. If more stress is applied to the rock, it willeventually bend plastically. In this instance, therock bends, but does not return to its original shapewhen the stress is removed. If the stress continues,the rock will fracture; that is, it breaks. When amaterial changes shape, it has undergonedeformation. Deformed rocks are common ingeologically active areas (Figure below).

Figure 7.3 When stress is applied to a material, it initially

deforms elastically. With more stress, the materialdeforms plastically and when the materials strengthis exceeded, it fractures. The amount of stress thatcan be applied before the material transitions tothe next type of deformation depends on the

material and the conditions where it is located.What a rock does in response to stress depends

on many factors: the rock type; the conditions therock is under, primarily the surroundingtemperature and pressure; the length of time therock is under stress; and the type of stress. It seemsdifficult to imagine that rocks would not justsimply break when exposed to stress. At theEarth's surface, rocks usually break quite quicklyonce stress is applied. But deeper in the crust,where temperatures and pressures are higher,rocks are more likely to deform plastically.Sudden stress, like a hit with a hammer, is morelikely to make a rock break. Stress applied overtime, often leads to plastic deformation.

Geologic Structures

Sedimentary rocks are often found in layers.This is most magnificently displayed at the GrandCanyon, where the rock layers are exposed like alayer cake (Figure below). Each layer is made of

sediments that were deposited (laid down) in aparticular environment, perhaps a lake bed,shallow offshore region, or a sand dune. Sedimentsare deposited horizontally. the lowest layers arethe oldest and the highest layers are the youngest.Some volcanic rocks, like ash falls, resemblesedimentary rocks because they are laid downhorizontally as well.

Figure 7.4 The layered rocks of the Grand Canyon from

the rim.It’s important to remember that sediments are

deposited horizontally when thinking aboutgeologic structures. This is because you can tracethe deformation the rock has experienced by seeinghow it differs from its original horizontal, oldest-on-bottom, position (Figure below). Geologicstructures are the folds, joints and faults that arecaused by stresses.

Figure 7.5 Geologic column of the Grand Canyon. The

sedimentary rocks of groups 3 through 6 weredeposited horizontally and remain horizontal.Group 2 rocks were deposited horizontally buthave been tilted. Group 1 rocks are notsedimentary. The rock layers at the bottom of the

stack are the oldest while the ones at the top arethe youngest.

Folds

When rocks experiencing compressive stressdeform plastically, the rocks crumple into folds.Folds are just bends in the rock. You can easilymake folds by placing your hands on oppositeedges of a piece of cloth and pushing the clothtogether. In layered sedimentary rocks, you cantrace the folding of the layers with your eyes(Figure below).

Figure 7.6

Snow accentuates the fold exposed in theserocks in Provo Canyon, Utah.

Once rocks are folded, they do not return totheir original shape. If the rocks experience morestress, they may undergo more folding, or evenfracture. Folds often occur in groups.

There are three types of folds: monoclines,anticlines, and synclines. A monocline is a simplebend in the rock layers so that they are no longerhorizontal but are inclined (Figure below). In amonocline, the oldest rocks are at the bottom andthe youngest are at the top. In the Grand Canyongeologic column, the rocks in group 2 have beenfolded into a monocline.

Figure 7.7 A monocline can be spotted in the photo taken

at Colorado National Monument where the rocksplunge toward the ground.

An anticline is a fold that arches upward. Therocks dip away from the center of the fold (Figurebelow).

Figure 7.8 Diagram of an anticline. An anticline is a

convex upward fold.The oldest rocks are found at the center of an

anticline and the youngest ones are draped overthem at the top of the structure (Figure below).

Figure 7.9 An anticline exposed in a road cut in New

Jersey.When rocks arch upward to form a circular

structure, that structure is called a dome. If the topof the dome is eroded off, the oldest rocks will beexposed at the center.

A syncline is a fold that bends downward. Therocks curve down to a center (Figure below).

Figure 7.10 A syncline in Rainbow Basin in California.In a syncline, the youngest rocks are at the

center and the oldest at the outsides (Figurebelow).

Figure 7.11

Diagram of a syncline. A syncline is a concavedownward fold.

When rocks bend downward in a circularstructure, that structure is called a basin. If therocks are exposed, the youngest rocks will be atthe center. Basins can be enormous. For example,the Michigan Basin is centered on the state ofMichigan, but extends into four other states and aCanadian province (Figure below).

Figure 7.12 Geologic map of the Michigan Basin.Folds are sometimes, but not always the cause

of geographic features such as hills or valleys.Geologic processes that work at the surface, likeerosion, are important in creating geographicfeatures as well.

Faults

A rock under enough stress will fracture, orbreak. When there is a block of rock still standingon either side of a fracture line, as shown inFigure below, the fracture is called a joint. Oneexample of how joints form is when confiningstress is removed from an underlying granite.

Figure 7.13 Granite rocks in Joshua Tree National Park

showing horizontal and vertical jointing. Overmillions of years, wind and water have brokendown the granite, enlarging the joints and makingthe pattern of jointing more obvious.

If the blocks of rock on one or both sides of afracture move, the fracture is called a fault(Figure below). Earthquakes happen when thereare sudden motions along faults. When rocks breakand move suddenly, the energy released causes anearthquake. Faults may occur at the Earth’s surfaceor deeper in the crust. Faults are found alone or inclusters, creating a fault zone.

Figure 7.14

Faults are easy to recognize as they cut acrossbedded rocks.

Slip is the distance rocks move along a fault.Slip is said to be relative, because there is usuallyno way to know whether both sides moved or onlyone. The only thing we can say for sure, is that oneblock of rock moved passed the other. Faults lie atan angle to the horizontal surface of the Earth. Thatangle is called the fault’s dip. The dip defineswhich of two basic types a fault is. If the fault’sdip is inclined relative to the horizontal, the fault isa dip-slip fault. Slip can be up or down the faultplane.

In the following images, you are looking at thefault straight on, as if you are standing on a roadand the fault is exposed in the road cut. Thehanging wall is the rock that overlies the fault,while the footwall is beneath the fault. You canremember which part is the hanging wall andwhich is the footwall by imagining you are walkingalong a fault. The hanging wall is above you andthe footwall is where your feet would be. Minersoften extract mineral resources along faults. They

used to hang their lanterns above their heads. Thatis why these layers were called the hanging wall.

In normal faults, the hanging wall drops downrelative to the footwall. Normal faults are causedby tensional stress that pulls the crust apart,causing the hanging wall to slide down relative tothe footwall. When compression squeezes the crustinto a smaller space, the hanging wall pushes uprelative to the footwall. This creates a reversefault (Figure below).

Figure 7.15 The two types of dip-slip faults. In normal

faults the hanging wall drops down relative to thefootwall. In reverse faults, the footwall dropsdown relative to the hanging wall.

A type of reverse fault is called a thrust fault.

At a thrust fault, the fault plane angle is nearlyhorizontal and rocks can slip many miles alongthrust faults (Figure below).

Figure 7.16 At Chief Mountain in Montana, stresses that

raised up the Rocky Mountains caused a block ofancient Precambrian crust to be thrust more than 80kilometers (50 miles) over much youngerCretaceous rocks. The result is that the upper rocksat the Lewis Overthrust are more than 1 billionyears older than the lower rocks.

Normal faults can be huge. They can beresponsible for uplifting mountain ranges inregions experiencing tensional stress (Figure

below).

Figure 7.17 The Teton Range in Wyoming rose up along a

normal fault.

Strike-Slip

A strike-slip fault is a dip-slip fault where thedip of the fault plane is vertical. Strike-slip faultsresult from shear stresses. If you stand with onefoot on one side and one foot on the other side of astrike-slip fault, the block on one side will bemoving toward you and the block on the other sidewill be moving away from you. If the block moving

toward you is the block that your right foot is on,the fault is known as a right-lateral strike-slipfault. If the block moving toward you is the oneyour left foot is on, the fault is a left-lateral strike-slip fault (Figure below).

Figure 7.18 The two types of strike-slip faults.The world’s most famous strike-slip fault is the

San Andreas Fault in California, which is a right-lateral strike slip fault (Figure below). Becausethe San Andreas is a plate boundary, it is alsocalled a transform fault. People sometimes say thatCalifornia will fall into the ocean someday, butthis is a joke. The portion west of the San AndreasFault is moving northeastward and someday LosAngeles will be a suburb of San Francisco, but theland west of the San Andreas Fault is a solid piece

of continental crust that will not disappear entirely.

Figure 7.19 The San Andreas is a transform fault separating

the Pacific from North American Plates. The faultcreates a scar on the land as it moves across theCarizzo Plains in eastern San Luis Obispo County,California.

A fault may have broken and moved only once,but most faults are active repeatedly. There aretwo reasons for this. One is that plate tectonicprocesses continue in the same locations. The otheris that a fault is a zone of weakness in the crust,and it is easier for movement to take place alongan existing fault than for a new fault to be createdin solid crust.

Stress and Mountain Building

Mountains can stand alone or in ranges thatformed at a similar time and in a similar way.Many processes can create mountains. Althoughmost mountains form along plate boundaries, someresult from intraplate activity. For example,volcanoes build upwards at hotspots within thePacific Plate.

Most of the world’s largest mountains resultfrom compression at convergent plate boundaries.The largest mountains arise when two continentalplates smash together. Continental lithosphere istoo buoyant to get pushed down into the mantle orsubduct, so when the plates smash together, thecrust crumples upwards, causing uplift. Thestresses cause folds, reverse faults, and thrustfaults, all of which allow the crust to grow thickerand rise upwards.

The world’s highest mountain range, theHimalayas, is growing from the collision betweenthe Indian and the Eurasian plates. About 80

million years ago, the Indian plate was separatedfrom the Eurasian plate by an ocean (Figurebelow). As the Indian plate moved northward, asubduction zone formed beneath Eurasia. Theseafloor was subducted and caused the formationof a set of continental arc volcanoes. When theoceanic lithosphere was completely subducted,about 40 million years ago, the Indian plate beganto collide with the Eurasian plate. Some of theIndian plate was thrust beneath Asia and some ofAsia was thrust onto India. Rock also folded,which thickened the crust and formed themountains. In places, the old seafloor that wasbetween the two slabs of continental crust havebeen thrust over the Asian continent and are foundhigh in the Himalayas (Figure below).

Figure 7.20 The India plate collides with the Eurasian plate

to form the worlds largest mountain range, the

Himalayas.

Figure 7.21 The Himalayas are the worlds highest mountain

range.Subduction of oceanic lithosphere at

convergent plate boundaries also builds mountainranges. The Andes Mountains are a chain ofcontinental arc volcanoes that build up as theNazca plate subducts beneath the South Americanplate (Figure below).

Figure 7.22 The Andes Mountains have arisen due to the

convergence of two lithospheric plates.Rifting at a divergent plate boundary can also

build mountain ranges. When crust is pulled apart,it occupies more area. The crust breaks into blocksthat slide up and down along the normal faults thatseparate them. The result is alternating mountainranges and valleys, known as a basin-and-range(Figure below). The state of Nevada is the centerof a classic basin and range province (Figurebelow).

Figure 7.23 Where the crust is being pulled apart by

tension, it breaks apart along normal faults. Someblocks are uplifted to form ranges, known ashorsts, and some are down-dropped to formbasins, known as grabens.

Figure 7.24 In this photo taken in the Basin and Range

province, the photographer is standing at EmigrantPass in the Nopah Range, and looking across abasin to the Kingston Range beyond.

Lesson Summary

Stress is the force applied to a rock, whichmay cause deformation. The three main typesof stress go along with the three types of plateboundaries: compression is common atconvergent boundaries, tension at divergentboundaries, and shear at transformboundaries.

Where rocks deform plastically, they tend tofold. Brittle deformation brings aboutfractures and faults. The two main types offaults are dip-slip and strike-slip.In dip-slip faults, the angle of the fault planeis inclined to the horizontal, in strike-slipfaults the fault plane is perpendicular to thehorizontal.The world’s largest mountains grow atconvergent plate boundaries, primarily bythrust faulting and folding.

Review Questions

1. Why don’t rocks deform under confiningstress?

2. What type of stress is compression and atwhat type of plate boundary is this found?

3. What type of stress is tension and at what typeof plate boundary is it found?

4. What type of stress is shear and at what typeof plate boundary is it found?

5. What is the difference between plastic andelastic strain?

6. A Under what conditions is a rock morelikely to deform plastically than to break?

7. You are a geologist walking around in thefield, when you spot a monocline. Youinspect the fossils in each layer of the rockand you discover that the oldest rocks are atthe top and the youngest at the bottom. Howdo you explain how the rocks came to be thisway?

8. Describe an anticline and name the age orderof rocks.

9. Describe a syncline and name the age order ofrocks.

10. What is the difference between a dome and abasin and what is the age order of rocks ineach?

11. Name one similarity and one differencebetween a fracture and a fault?

12. What are the two types of dip-slip faults andhow are they different from each other?

13. California is plagued by earthquakes along

the San Andreas Fault zone. Why are there somany earthquakes and why are they sosevere?

14. Volcanoes are mountains that form in twoways. Describe these two ways and how theyare associated with a plate boundary.

15. Describe the plate tectonics processes andassociated stresses that have led to theformation of the Himalayas, the world’slargest mountain range?

Vocabulary

anticlineA fold that arches upward, in which the olderrocks are in the center and the younger rocksare at the outside.

basinA block of rock that has slipped downwardbetween two normal faults.

compression

Stresses that push toward each other. Thiscauses a decrease in the space a rock takesup.

confining stressThe stress due to the weight of material abovea buried object. Confining stress reducesvolume but causes no deformation.

deformationThe change of shape that a rock undergoeswhen it has been altered by stresses. Alsocalled strain.

dip-slip faultA fault in which the dip of the fault plane isinclined relative to the horizontal.

domeA circular anticline. A dome has the oldestrocks in the center and the youngest on theoutside.

elastic strain

Strain that alters the shape of a rock but that isnot permanent. The rock goes back to itsoriginal shape when the stress is removed.

fault zoneA network of related faults.

foldA bend in a set of rocks caused bycompression. Folds are easiest to see insedimentary or volcanic rocks that weredeposited horizontally.

footwallThe block of rock that is beneath a dip-slipfault.

fractureA break in rock caused by stresses. Theremay or may not be movement of the rock oneither or both sides of a fracture.

hanging wall

The block of rock that is above a dip-slipfault.

jointA break in rock caused by stresses alongwhich there is no movement.

monoclineA bend in a set of rocks that causes them to beinclined relative to the horizontal.

normal faultA dip-slip fault in which the hanging walldrops down relative to the footwall.

plastic strainStrain that causes deformation in which therock deforms but does not return to itsoriginal shape when the strain is removed.

reverse faultA dip-slip fault in which the hanging wallpushes up relative to the footwall.

shearStresses that pushed past each other inopposite directions.

slipThe distance rocks move along a fault.

strainDeformation in a rock that is due to a stressthat exceeds the rock’s internal strength.

stressForce per unit area in a rock.

strike-slip faultA fault in which the dip of the fault plane isvertical.

synclineA fold in rocks that bends downward, inwhich the youngest rocks are at the center.

tensionStresses that pull material in opposite

directions so that it is pulled apart.

thrust faultA reverse fault in which the dip of the faultplane is nearly horizontal.

upliftThe upward rise of rock material.

Points to Consider

Think about stresses in the ocean basins.Where in the ocean basis do you think youwould find the features that indicate tensionalstresses? Where would you find the featuresthat indicate compressional stresses?Earthquakes are primarily the result of platetectonic motions. List the three types of plateboundaries and what you think the stresses arethat would cause earthquakes there.Which type of plate boundary do you think hasthe most dangerous earthquakes? How doearthquakes cause the greatest damage?

Nature of Earthquakes

Lesson Objectives

Be able to identify an earthquake focus and itsepicenter.Identify earthquake zones and what makessome regions prone to earthquakes.Compare the characteristics of the differenttypes of seismic waves.Describe how tsunamis are caused byearthquakes, particularly using the 2004Boxing Day Tsunami as an example.

Introduction

A n earthquake is sudden ground movementcaused by the sudden release of energy stored inrocks. The earthquake happens when so muchstress builds up in the rocks that the rocks rupture.An earthquake’s energy is transmitted by seismic

waves. Each year there are more than 150,000earthquakes strong enough to be felt by people and900,000 recorded by seismometers.

Causes of Earthquakes

Almost all earthquakes occur at plateboundaries. All three boundary types—divergent,convergent and transform—are prone to earthquakeactivity. Plate tectonics causes the lithosphericplates to move. As you might imagine, having giantslabs of lithosphere moving about on a sphericalshape is not smooth. When stresses build, they firstcause the rocks to bend elastically. If the stressespersist, energy continues to build in the rocks.When the stresses are greater than the internalstrength of the rocks, the rocks snap. Although theyreturn to their original shape, the stresses cause therocks to move to a new position. This movementreleases the energy that was stored in the rocks,which creates an earthquake. During an earthquakethe rocks usually move several centimeters or

maybe as much as a few meters. This descriptionof how earthquakes occur is called elasticrebound theory (Figure below).

Figure 7.25 Elastic rebound theory. Stresses build on both

sides of a fault, causing the rocks to deformplastically (Time 2). When the stresses become toogreat, the rocks return to their original shape but

they move (Time 3). This motion releases theenergy that creates an earthquake.

The point where the rock ruptures is usuallybelow the Earth’s surface. The point of rupture iscalled the earthquake’s focus. The focus of anearthquake can be shallow - less than 70kilometers (45 miles), intermediate - 70 to 300kilometers (45 to 200 miles), or deep - greater than300 kilometers (200 miles). About 75% ofearthquakes have a focus in the top 10 to 15kilometers (6 to 9 miles) of the crust. Shallowearthquakes cause the most damage because thefocus is near the Earth's surface where people live.

Just above the focus on the land surface, is theearthquake’s epicenter (Figure below). It is theepicenter of an earthquake that is reported byscientists and the media. The epicenter of the 1906San Francisco earthquake, for example, wasoffshore, 1.5-3 kilometers (1-2 miles) west ofGolden Gate Park.

Figure 7.26 A vertical cross section through the crust

shows an earthquakes focus below ground and itsepicenter at the ground surface.

Earthquake Zones

Some locations are prone to earthquakes andsome are not. Nearly 95% of all earthquakes takeplace along one of the three types of plateboundaries. Scientists use the location ofearthquake epicenters to draw the boundaries ofthe plates because earthquakes frequently occur

along plate boundaries (Figure below).

Figure 7.27 Earthquake epicenters can be used to outline

the edges of the lithospheric plates. Mostearthquakes occur around the Pacific Ocean basinsand in the Mediterranean-Asiatic belt.

The region of the planet with the mostearthquakes is the area around the Pacific Ocean.About 80% of all earthquakes strike this area. Thisregion is called the Pacific Ring of Fire becausemost volcanic eruptions occur there as well. ThePacific Ocean is surrounded by convergent andtransform plate boundaries (Figure below).

Figure 7.28 The Pacific Ring of Fire is the most

geologically active region of the world.Convergent plate boundaries cause earthquakesand volcanic eruptions all around the PacificOcean basin. There are also transform plateboundaries along the San Andreas Fault inCalifornia and the Alpine Fault in New Zealand.

About 15% of all earthquakes take place in theMediterranean-Asiatic belt. This is whereconvergent plate boundaries are shrinking theMediterranean Sea and causing the Himalayas togrow. The remaining 5% of earthquakes arescattered around the other plate boundaries with afew occurring in the middle of a plate, away from

plate boundaries.All three types of plate boundaries have

earthquakes. Enormous and deadly earthquakesoccur at transform plate boundaries. Because theslabs of lithosphere slide past each other withoutmoving up or down, transform faults have shallowfocus earthquakes. The most notorious earthquakefault in North America is the San Andreas Faultthat runs through California. The 1,300 kilometer(800 mile) long fault is the transform boundarybetween the northeastward-moving Pacific plateand the southwestward-moving North Americanplate. The San Andreas is a right-lateral strike-slipfault (Figure below).

Figure 7.29 The San Andreas Fault runs up western

California. It is the transform plate boundarybetween the northwestward moving Pacific Plateand the southeastern moving North American Plate.

The largest earthquake on the San AndreasFault in historic times occurred in 1906 in San

Francisco (Figure below). This earthquake likelymeasured magnitude 7.8, which is a very largeearthquake. The earthquake and the subsequent fireis still the most costly natural disaster in Californiahistory. An estimated 3,000 people died and about28,000 buildings were lost, mostly in the fire.

Figure 7.30 Damage after the 1906 San Francisco

Earthquake and fireIn 1989, the Loma Prieta earthquake struck

near Santa Cruz, California. The magnitude 7.1quake resulted in 63 deaths, 3,756 injuries and leftmore than 12,000 people homeless. The propertydamage was estimated at about $6 billion. In 1994,an earthquake on a blind thrust fault struck near

Los Angeles, California in the neighborhood ofNorthridge. It registered 6.7 on the momentmagnitude scale. Seventy two people died, 12,000more were injured and damage was estimated at$12.5 billion.

There are many other faults spreading off theSan Andreas, which together with the main faultproduce around 10,000 earthquakes a year (Figurebelow). While most of those earthquakes cannoteven be felt by people nearby, occasionally one ismassive. In the San Francisco Bay Area, theHayward Fault was the site of a magnitude 7.0earthquake in 1868.

Figure 7.31 The San Andreas Fault Zone in the San

Francisco Bay Area. The San Andreas Fault isseen running through San Francisco and MarinCounty, east of Point Reyes. The Hayward Faultlies across the East Bay and runs through Berkeley.Many other small faults are shown in the map aswell.

Convergent plate boundaries also producemassive and deadly earthquakes. Earthquakes markthe motions of subducting lithosphere as it plungesthrough the mantle. The earthquakes can be

shallow, intermediate or deep focus. Convergentplate boundaries produce earthquakes all aroundthe Pacific Ocean basin.

The Philippine plate and the Pacific platesubduct beneath Japan creating a chain ofvolcanoes and as many as 1,500 annualearthquakes. The great Kanto earthquake of 1923 isthought to have killed 140,000 people, many in thesubsequent fire. In Yokohama, 90% of houses weredamaged or destroyed and 60% of Tokyo’spopulation became homeless. In the Great Hanshin(Kobe) Earthquake of 1995, 6,434 people died(Figure below).

Figure 7.32 Destruction in Kobe, Japan, from the 1995

Great Hanshin Earthquake.Subduction is also taking place along the

Cascades Mountains in the Pacific Northwest aspart of the Pacific Ring of Fire. The Juan de Fucaplate is plunging beneath the North American plateand forming volcanoes that extend south intonorthern California. The Cascades volcanoes areactive and include Mount Saint Helens, which hada large eruption in 1980. Mount Lassen, MountShasta, and Medicine Lake volcano in northeasternCalifornia are the three southernmost volcanoes inthe Cascades chain.

Yet the Cascadia subduction zone is one of theworld’s quietest subduction zones, with relativelyfew earthquakes. Though they don't happen often,they are extremely powerful when they hit. The lastmajor earthquake on the Juan de Fuca occurred in1700, with a magnitude estimated at between 8.7and 9.2. The geologic history of the area revealsthat major earthquakes occur here about every 300to 600 years. Since it has now been more than 300years since the last earthquake in the area, thePacific Northwest is at risk from a potentiallymassive earthquake that could strike any time.

The thrust faulting and folding that result from

the convergence of continental plates createsmassive earthquakes. The region in and around theHimalaya, for example, is the site of manyearthquakes. The 2001 Gujarat, India earthquake isresponsible for about 20,000 deaths with manymore people injured or made homeless.

Earthquakes also occur at divergent plateboundaries. At mid-ocean ridges, theseearthquakes tend to be small because the plates areyoung and hot. The earthquakes are shallowbecause the new plates are thin. Since divergentplate boundaries in the oceans are usually far fromland, they have little effect on peoples’ lives. Onland, where continents are rifting apart,earthquakes are larger and stronger.

About 5% of earthquakes take place within aplate; that is, away from plate boundaries. A largeintraplate earthquake occurred in 1812 when amagnitude 7.5 earthquake struck near New Madrid,Missouri. The earthquake was strongly felt overaround 50,000 square miles, and altered the courseof the Mississippi River. Because very few peoplelived here at the time, only 20 people died.

However many more people live here today andthe New Madrid Seismic Zone continues to beactive (Figure below). A similar earthquake todaywould undoubtedly kill many people and cause agreat deal of property damage. Intraplateearthquakes are caused by stresses due to platemotions acting in solid slabs of lithosphere.

Figure 7.33 The location of the New Madrid Seismic Zone

is well within the North American plate far fromthe nearest plate boundary. This figure shows thataround 4,000 earthquakes have occurred in the

region since 1974.

Seismic Waves

Energy is transmitted in waves. Every wavehas a high point called a crest and a low pointcalled a trough. The height of a wave from thecenter line to its crest is its amplitude. Thedistance between waves from crest to crest (ortrough to trough) is its wavelength (Figurebelow).

Figure 7.34 Features of a set of waves.The energy from earthquakes (and also from

explosions) travels in waves called seismicwaves. Other types of waves transmit other typesof energy; for example, sound waves transmit achild’s laughter and other sounds. The study ofseismic waves is known as seismology.Seismologists use seismic waves to learn aboutearthquakes and also about the Earth’s interior.

Seismic waves move outward in all directionsaway from their source. There are two major typesof seismic waves. Body waves travel through thesolid body of the Earth from the earthquake’s focusthroughout the Earth’s interior and to the surface.Surface waves just travel along the groundsurface. The different types of seismic wavestravel at different speeds in different materials. Allseismic waves travel through rock, but not alltravel through liquid or gas. In an earthquake, bodywaves are responsible for sharp jolts. Surfacewaves are responsible for rolling motions. Surfacewaves do most of the damage in an earthquake.

Body Waves

There are two types of body waves – primarywaves (P-waves) and secondary waves (Swaves). These waves travel through the Earth’sinterior. P-waves are the fastest at about 6 to 7kilometers (about 4 miles) per second. They arenamed primary waves because they are the firstwaves to reach a seismometer. S-waves areslower and so are the second waves to reach aseismometer. Body waves move at differentspeeds depending on the type of material they arepassing through.

P-waves are longitudinal waves. They movematerial forward and backward in the samedirection that they are traveling. This motionresembles a spring squeezing and unsqueezing. Thematerial returns to its original size and shape afterthe P-wave goes by. For this reason, P-waves arenot the most damaging earthquake waves. P wavescan travel through solids, liquids and gases.

S-waves are transverse waves, that move upand down. Their oscillations are perpendicular tothe direction the wave is traveling. In a rock, thismotion produces shear stresses. S-waves are about

half as fast as P-waves, traveling at about 3.5 km(2 miles) per second. S-waves can only movethrough solids because liquids and gases have noshear strength.

Figure 7.35 The top figure shows how body waves,

including P-waves and S-waves, move through a

grid. The bottom figure shows how surface wavesmove. The two types of surface waves are Lovewaves and Rayleigh waves.

Surface Waves

Surface waves travel along the groundoutward from an earthquake’s epicenter. Surfacewaves are the slowest of all seismic waves,traveling at 2.5 km (1.5 miles) per second. Thereare two types of surface waves. Love waves moveside-to-side much like a snake. Rayleigh wavesmove in rolls, like ocean swells (Figure above).These waves cause objects to fall and rise, whileswaying back and forth. These motions causedamage to rigid structures during an earthquake.

Tsunami

Earthquakes can cause deadly ocean wavescalled tsunami, although tsunami can be caused byany shock to ocean water, including a meteorite

impact, landslide, or a nuclear explosion. Whenocean water is displaced by the sharp jolt of anundersea earthquake, the seismic energy forms aset of waves. The waves travel through the seaentirely unnoticed since they have low amplitudesand long wavelengths. When these waves comeonto shore, they can grow to enormous heights andcause tremendous destruction and loss of life.Fortunately, few undersea earthquakes generatetsunamis.

The Boxing Day Tsunami of December 26,2004 was by far the deadliest of all time. Thetsunami was caused by the 2004 Indian OceanEarthquake, also called the Great Sumatra-Andaman earthquake (Figure below). Thisearthquake, with a magnitude of 9.2, was thesecond largest earthquake ever recorded. Theenergy that reached the planet’s surface was 1,502times the amount released by the atomic bombdropped on Hiroshima, but the total amount ofenergy released was estimated at 550 million timesHiroshima.

The Indian Ocean Earthquake struck 160

kilometers (100 miles) off of Sumatra, Indonesia.In this region the Indian plate is subducting beneaththe Burma plate. Slip along the earthquake faultwas an incredible 15 meters (50 feet), about two-thirds of that in a horizontal direction and one-thirdin a vertical direction. The fault ruptured overabout 1,600 kilometers (1,000 miles). Faultingwent on for up to 10 minutes, the longest durationever witnessed.

The extreme movement of the crust displacedtrillions of tons of water. Water displacementoccurred along the entire length of the rupture. Thismeans that tsunami waves formed along a greatdistance, which increased the area that the killerwaves traveled to. Several tsunami were created,with about 30 minutes between the peaks of eachone.

The water traveled rapidly across the IndianOcean outward from the fault. As is typical fortsunami, the waves were not noticeable in openwater. Satellites measured the height of the wavesacross the sea at just 50 centimeters (20"). Thefirst wave hit the northern regions of Sumatra in

about 15 minutes. At its worst, the waves rose toaround to 10 meters (33 feet) in height. Within 1.5to 2 hours, waves were striking Sri Lanka and theeastern coast of India. Thailand was battered twohours after the earthquake. Somalia was hit sevenhours after the earthquake. The size of the wavesdecreased with distance from the earthquake sothat the waves in Sri Lanka, Thailand, and Somaliawere relatively small, about 4 meters (13 feet) inheight.

Figure 7.36 The location of the 2004 Indian Ocean

earthquake and the countries that were mostaffected by the tsunami.

Like other waves, a tsunami wave has a crestand a trough. What people see when the tsunamihits the beach depends on whether the crest or thetrough hits first. In some locations, the trough of thewave hit the beach first. When this happens, wateris sucked out to sea and the seafloor just offshorefrom the beach is exposed. Curiosity is often fatalin this instance, since people who go out to thebeach to see the unusual sight are drowned whenthe wave crest hits.

One amazing story was that of Tilly Smith, a 10year old British girl who was visiting MaikhaoBeach in Thailand with her parents. About twoweeks before the earthquake, Tilly had learnedabout tsunamis in school. She knew that thereceding water and the frothy bubbles at the seasurface indicated an approaching tsunami. As thetrough of the tsunami wave hit the beach, shepointed these features out to her parents. They toldother tourists and the staff at their hotel and thebeach was evacuated. No one on Maikhao Beachdied and Tilly is credited with saving nearly 100people.

On other beaches, people were not so lucky. Inall, the tsunami struck eight countries, withIndonesia, Sri Lanka, India and Thailand thehardest hit (Figure below). About 230,000 peopledied, with fatalities even as far away as SouthAfrica, nearly ,000 kilometers (5,000 miles) fromthe earthquake epicenter. More than 1.2 millionpeople lost their homes and many more lost theirways of making a living. For example, fishermenlost their boats, and business people lost theirrestaurants and shops. Many marine animals werewashed inland, including dolphins, turtles, andsharks.

Figure 7.37 The Boxing Day tsunami strikes a beach in

Thailand.Only a few scientists had thought that a

massive tsunami would strike the Indian Ocean sono warning system had been in place. Tsunami aremuch more common in the Pacific due to theenormous number of subduction zones that line thePacific basin, and communities around the Pacifichave had a tsunami warning system in operationsince 1948 (Figure below). As a result of the 2004tsunami, an Indian Ocean warning system was putinto operation in June 2006.

Figure 7.38 A sign indicating a tsunami hazard zone in

California.Warning systems are of limited use. They base

their warnings on the location of earthquakeswithin an ocean basin. Unfortunately, communitiesthat are very close to the earthquake do not receivethe warning in time to move inland or uphill sincethe wave hits too fast. Still evacuation of low-lyingareas could save many people in a large tsunamithat is further from the earthquake.

Lesson Summary

During an earthquake, the ground shakes asstored up energy is released from rocks.Nearly all earthquakes occur at plate tectonicboundaries and all types of plate boundarieshave earthquakes.The Pacific Ocean basin and theMediterranean-Asiatic belt are the twogeographic regions most likely to experiencequakes. The seismic waves that do the mostdamage are surface waves, which only travelalong the surface of the ground.Body waves travel through the planet and

arrive at seismograms before surface waves.Tsunamis are deadly ocean waves that can becaused by undersea earthquakes.

Review Questions

1. What is an earthquake’s focus? What is itsepicenter?

2. Why do most earthquakes take place alongplate boundaries?

3. Using elastic rebound theory, describe whattriggers an earthquake.

4. Use plate tectonics theory to describe why arethere far more earthquakes around the PacificOcean than anywhere else on Earth?

5. Since intraplate earthquakes are not near plateboundaries, give a general idea of what youthink might cause them.

6. Do the largest earthquakes cause the mostdeaths and the most damage to property?

7. California is famous for earthquakes along theSan Andreas Fault zone but there is another

type of plate boundary where largeearthquakes occur. What type of plateboundary is it and where the earthquakeslikely to occur?

8. Using what you know about plate tectonicsand elastic rebound theory, describe what istaking place in the Cascades Mountains of thePacific Northwest, including northernCalifornia. What is likely to occur in thefuture? Include earthquakes and tsunamis.

9. What type of faulting is found where twoslabs of continental lithosphere areconverging? Explain what this would looklike on a diagram of the faults and the rockson either side.

10. What are the characteristics of body waves?What are the two types?

11. What materials can P-waves travel throughand how fast are they? Describe a P-wave’smotion.

12. What materials can S-waves travel throughand how fast are they? Describe an S-wave’smotion.

13. How are surface waves different from bodywaves? In general, which type of waves ismore damaging in an earthquake?

14. What did Tilly Smith notice on the beach inThailand that caused the adults around her toevacuate the beach before the enormoustsunami hit in 2004? How were these signsevidence of a tsunami?

Further Reading / SupplementalLinks

http://earthquake.usgs.gov/

http://www.exploratorium.edu/faultline/index.html

Vocabulary

amplitudeThe height of a wave from a center line to thetop of the crest (or to the bottom of the trough.

body wavesA type of seismic wave that travels throughthe body of a planet. The two types areprimary waves and secondary waves.

crestThe highest point of a wave.

earthquakeGround shaking caused by the release ofenergy stored in rocks.

elastic rebound theoryThe theory of how earthquakes are generated.Elastic rebound theory states that stressescause strain to build up in rocks until they canno longer bend elastically and they break,causing an earthquake.

epicenterThe point on the earth’s surface that liesabove an earthquake’s focus.

focus

The point where rocks rupture during anearthquake.

Love wavesThese surface waves have a side-to-sidemotion, much like a slithering snake.

primary waves (P-waves)P-waves are body waves that are the first toarrive at a seismometer because they are thefastest. P-waves are longitudinal waves thattravel through solids, liquids, and gases.

Rayleigh wavesThese surface waves have a rolling motion.

secondary waves (S-waves)S-waves are body waves that are the secondto arrive at a seismometer. S-waves aretransverse waves that can only move throughsolids.

seismic wave

Seismic waves transport the energy releasedduring an earthquake. The two main types arebody waves and surface waves.

seismologyThe study of seismic waves includingearthquakes and the earth’s interior.

surface wavesSurface waves are seismic waves that travelalong the ground surface. The two types areLove waves and Rayleigh waves. Surfacewaves do the most damage after anearthquake.

troughThe lowest point of a wave.

tsunamiA deadly set of waves that are ordinarilycaused by an undersea earthquakes or anothershock in which large amounts of seawater aredisplaced. Tsunamis rise high on a beach and

can travel far inland, causing death anddestruction as they go.

wavelengthThe distance from crest to crest or trough totrough between two waves.

Points to Consider

The last time there was a large earthquake onthe Hayward Fault in the San Francisco Bayarea of California was in 1868. Use elasticrebound theory to describe what may behappening along the Hayward Fault today andwhat will likely happen in the future.Why is California so prone to earthquakes?How could coastal California be damaged bya tsunami? Where would the earthquakeoccur? How could such a tsunami bepredicted?

Measuring and PredictingEarthquakes

Lesson Objectives

List the different types of seismic waves, theirdifferent properties and describe howseismologists can use them to learn aboutearthquakes and the Earth’s interior.Describe how to find an earthquake epicenter.Describe the different earthquake magnitudescales and what the numbers for momentmagnitude mean.Describe how earthquakes are predicted andwhy the field of earthquake prediction has hadlittle success.

Introduction

Seismograms tell seismologists how strong an

earthquake is and how far away it is. At least threeseismograms must be used to calculate where theepicenter is located. Over the past century,scientists have developed several ways ofmeasuring earthquake intensity. The currentlyaccepted method is the moment magnitude scale,which measures the total amount of energyreleased by the earthquake. At this time,seismologists have not found a reliable method forpredicting earthquakes.

Measuring Magnitude

A seismometer is a machine that recordsseismic waves. In the past, all seismometers wereseismographs because they produced a graph-likerepresentation of the seismic waves they received.The paper record is called a seismogram. Modernseismometers record ground motions usingelectronic motions detectors. The data are thenkept digitally on a computer.

Seismographs have a pen suspended from a

stationary frame, while a drum of paper rotatesbeneath it. The pen is weighted so that it issuspended and not attached to the ground. Thedrum is attached to the ground. As the earth shakesin an earthquake, the pen remains stationary but thedrum moves beneath it. This creates the squigglylines that make up a seismogram (Figure below).

Figure 7.39 A seismograph that had recorded an earthquake

is beginning to record another earthquake, likely anaftershock.

Seismograms contain information on howstrong an earthquake was, how long it lasted, andhow far away it was. The wiggly lines that are

produced in a seismogram clearly show thedifferent arrival times of P- and S-waves (Figurebelow). As with words on a page, the seismogramrecord goes from left to right. First, there is a flatline, where there was no ground shaking. The firstwaves to be recorded by the seismogram are P-waves since they are the fastest. S-waves come innext and are usually larger than P-waves. Thesurface waves arrive just after the S-waves. If theearthquake has a shallow focus, the surface waveswill be the largest ones recorded.

Figure 7.40 These seismograms show the arrival of P-

waves and S-waves. The surface waves arrive justafter the S-waves and are difficult to distinguish.

Time is indicated on the horizontal portion (or x-axis) of the graph.

If a seismogram has recorded P-waves andsurface waves, but not S-waves, the seismographwas on the other side of the planet from theearthquake. Scientists know that the earth’s outercore is liquid because S-waves cannot travelthrough liquid. The liquid outer core creates an S-wave shadow zone on the opposite side of theplanet from the earthquake’s focus where no S-waves reach. The amplitude (height) of the wavescan be used to determine the magnitude of theearthquake. How magnitude is calculated will bediscussed in a later section.

Finding the Epicenter

A single seismogram can tell a seismologisthow far away the earthquake was but it does notprovide the seismologist with enough informationto locate the exact epicenter. For that, theseismologist needs at least three seismograms.

Determining distance to an earthquake epicenterdepends on the fact that different seismic wavestravel at different speeds. P-waves always arriveat a seismometer first, but the amount of time ittakes for the S-waves to arrive after the P-waveindicates distance to the epicenter. If the epicenteris near the seismometer, the P-waves, S-waves andsurface waves will all arrive in rapid succession.If the epicenter is further away, the S-waves willlag further behind. In other words, the longer it isbetween the arrival of the P-wave and S-wavefrom an earthquake, the farther the epicenter isfrom the seismometer.

After many years of study, geologists know thespeed at which the different types of waves travelthrough various earth materials. Based on thedifference in the arrival times of the first P waveand the first S wave, seismologists determine thedistance between the epicenter and a seismometer.Once the distance to the epicenter is known,scientists can identify each point that is thatdistance away. Let’s say that they know that anearthquake’s epicenter is 50 kilometers from

Kansas City. When each point that is that distanceaway from Kansas City is marked, the marks createa circle. This circle can be drawn with a compass.

To locate the earthquake epicenter,seismologists must have data from at least threeseismometers. A circle drawn at the correctdistance to the epicenter from a secondseismometer will intercept the first circle in twoplaces. A third circle showing the distance to theepicenter from a third seismometer will interceptthe other two circles at a single point. This point isthe earthquake epicenter (Figure below). Whilethis method was extremely useful for locatingepicenters for decades, the technique has beenreplaced by digital calculations.

Figure 7.41 Circles are drawn with radii representing the

distance from each seismic station to theearthquake's epicenter. The intersection of thesethree circles is the earthquake's epicenter.

Earthquake Intensity

People have always tried to quantify the size ofand damage done by earthquakes. Early in the 20th

century, earthquakes could only be described interms of what nearby residents felt and the damagethat was done to nearby structures. This was calledthe Mercalli Intensity Scale and was developedin 1902 by the Italian seismologist GiuseppeMercalli. The Mercalli Scale is sometimes usedtoday in conjunction with the more modernintensity scales described below.

Table below shows an abbreviated descriptionof the twelve Mercalli intensity levels:Number Description

Not felt except by a very few under

I especially favorable conditions.

II Felt only by a few persons at rest,especially on upper floors of buildings.

III

Felt noticeably by people indoors,especially on upper floors of buildings.Many people don't know it's anearthquake. Standing automobiles mayrock lightly. Vibrations similar to thepassing of a truck. Duration estimated.

IV

Felt indoors by many, outdoors by fewduring the day. At night, some awakened.Dishes, windows, doors disturbed; wallsmake cracking sound. Sensation likeheavy truck striking building. Standingcars rocked noticeably.

V

Felt by nearly everyone; manyawakened. Some dishes, windowsbroken. Unstable objects overturned.Pendulum clocks may stop.

VIFelt by all, many frightened. Some heavyfurniture moved; a little fallen plaster.Damage slight.

VII

Damage negligible in buildings of gooddesign and construction; slight tomoderate in well-built ordinarystructures; considerable damage inpoorly built or designed structures; somechimneys broken.

VIII

Damage slight in specially designedstructures; considerable damage inordinary substantial buildings withpartial collapse. Damage great in poorlybuilt structures. Fallen chimneys, factorystacks, columns, monuments, walls.Heavy furniture overturned.

IX

Damage considerable in speciallydesigned structures; well-designed framestructures thrown out of plumb. Damagegreat in substantial buildings, withpartial collapse. Buildings shifted offfoundations.

X

Some well-built wooden structuresdestroyed; most masonry and framestructures destroyed with foundations.

Rails bent.

XI Few if any structures still standing.Bridges destroyed. Rails bent greatly.

XII Damage total. Lines of sight and leveldistorted. Objects thrown into air.

There were many problems with the Mercalliscale. What people feel and see in an earthquake isaffected by how far they are from the earthquake’sfocus, the type of rock that lies beneath them, theconstruction type of the nearby buildings, and manyother factors. Different observers will alsoperceive the experience differently. For example,one might exaggerate while the other downplaysthe damage done. With the Mercalli scale,comparisons between earthquakes are difficult tomake.

To address these problems, in 1935 CharlesRichter developed his Richter magnitude scale.The Richter scale measures the magnitude of thelargest jolt of energy released in an earthquake.Because Richter's scale is logarithmic, theamplitude of the largest wave increases 10 times

from one integer to the next. For example, theamplitude of the largest seismic wave of amagnitude 5 quake is 10 times that of a magnitude4 quake and 100 times that of a magnitude 3 quake.One integer increase in magnitude roughlycorrelates with a 30-fold increase in the amount ofenergy released. A difference of two integers onthe Richter scale equals a 1,000-fold increase inreleased energy.

Seismologists recognize that the Richter scalehas limitations, since it measures the height of thegreatest earthquake wave. A single sharp jolt willmeasure higher on the Richter scale than a verylong intense earthquake that releases more energy.In other words, earthquakes that release moreenergy are likely to do more damage than those thatare short, but have a larger single jolt. Using theRichter scale, a high magnitude may notnecessarily reflect the amount of damage caused.

The moment magnitude scale is the currentmethod of measuring earthquake magnitudes. Thismethod measures the total energy released by anearthquake and so more accurately reflects its

magnitude. Moment magnitude is calculated fromthe area of the fault that is ruptured and thedistance the earth moved along the fault. Like theRichter scale, the moment magnitude scale islogarithmic. An increase in one integer means that30 times more energy was released, while twointegers means that 1,000 times the energy wasreleased released. The Richter and momentmagnitude scales often give very similarmeasurements.

In a single year, more than 900,000earthquakes are recorded. 150,000 of them arestrong enough to be felt. About 18 per year aremajor, with a Richter magnitude of 7.0 to 7.9. Eachyear, on average, one earthquake with a magnitudeof 8 to 8.9 strikes. Remember that many of theseearthquakes occur deep in the crust and out in theoceans and do not cause much or any damage onland.

Earthquakes with a magnitude in the 9 rangeare rare. The United States Geological Survey listssix such earthquakes on the moment magnitudescale in historic times (see Figure below and

Table below). All but one of them, the GreatIndian Ocean Earthquake of 2004, occurredsomewhere around the Pacific Ring of Fire.

Figure 7.42 The 1964 Good Friday Earthquake centered in

Prince William Sound, Alaska released the secondmost amount of energy of any earthquake inrecorded history.Location Year MagnitudeChile 1960 9.5Prince William Sound, Alaska 1964 9.2Great Indian Ocean Earthquake 2004 9.1Kamchatka, Alaska 1952 9.0

Africa, Peru (now Chile) 1868 9.0Cascadia Subduction Zone 1700 9.0

(Data from: United States Geological Survey)

Earthquake Prediction

To be valuable, an earthquake prediction mustbe accurate. A good predication would anticipatethe date, location, and magnitude of the earthquake.The prediction would need to be accurate so thatauthorities could convince people to evacuate. Anunnecessary evacuation would be very expensiveand would decrease the credibility of authoritieswho might need to evacuate the region at a latertime. Unfortunately, accurate predictions like theseare not likely to be common for a long time.

The easiest thing to predict is where anearthquake will occur (Figure below). Becausenearly all earthquakes take place at plateboundaries, and because earthquakes tend tohappen where they’ve occurred before, scientistsknow which locations are likely to have

earthquakes. This information is useful tocommunities because those that are earthquake-prone can prepare for the event. For example,these communities can implement building codes tomake structures earthquake safe. The added workand expense can be avoided in areas that are not atrisk.

Figure 7.43

The probabilities of earthquakes striking alongvarious faults in the San Francisco area between2003 (when the work was done) and 2032.

Predicting when an earthquake will occur ismuch more difficult. Scientists can get a generalidea by looking at the historical and geologicalrecords of earthquakes in an area. If stress on afault builds up at the same rate over time, thenearthquakes should occur at regular intervals.While this is true, there is a large margin of errorin these predictions. Using this method, scientistscannot even be accurate to within a few years, andevacuation is not practical.

Seismologists have also used the seismic gaptheory for long-term earthquake prediction. In thistheory, scientists assume that, on average, all of therocks on the same side of a fault move at the samerate. For example, they say that rocks on the NorthAmerican plate side of the San Andreas Fault inCalifornia move at the same speed over time.While this may be true, the frequency andmagnitude of earthquakes along the fault is not thesame: there are more quakes in the northern and

southern sections, but a relatively inactive zone inthe center.

Seismologists attempted to use the seismic gaptheory to predict an earthquake in a seismic gap.Around Parkfield, California earthquakes occurregularly: an earthquake of magnitude 6.0 or higheroccurs about every 22 years. Using thisinformation, seismologists predicted that amagnitude 6 or greater earthquake would strike theregion in 1993. In the mid-1980s, seismologistswith the United States Geological Survey set up anenormous number of instruments along theParkfield section of the San Andreas to monitor theexpected earthquake. While they were right that anearthquake was due in that segment of the fault,they were quite far off in predicting theearthquake’s timing. A magnitude 6.0 quake did notstrike Parkfield until 2004, 11 years late.

Scientists have recognized some indicators thatallow them to recognize that a large earthquake islikely. Large earthquakes are often preceded bysmall tremors, called foreshocks, that occurbetween a few seconds and a few weeks before a

major quake. However, many earthquakes are notpreceded by foreshocks and clusters of smallearthquakes are not necessarily followed by alarge earthquake.

Large earthquakes are also often preceded bythe tilting of the ground surface, which is caused bythe buildup of stress in the rocks. Seismologistsmeasure the ground tilt and use the changes topredict an impending earthquake. While thistechnique has been somewhat successful, it hasalso been a part of predictions of earthquakes thatnever came and has failed to predict some that did.Water levels in wells fluctuate as water movesinto or out of fractures before an earthquake. Thisinformation can also be used as a possible, butuncertain, predictor of large earthquakes.

The most successful earthquake prediction wason February 4, 1975. At the recommendation ofChinese seismologists, officials evacuated many ofthe residents of the Manchurian province ofLiaoning. Although the region was not prone toearthquakes, the seismologists made theirprediction because the area experienced about 400

small foreshocks over a few days. The night of theevacuation an earthquake of magnitude 7.3 struckthe town and only a few hundred people died. Anestimated 150,000 people may have been saved.However, a little more than a year later, Chineseseismologists failed to predict the Tangshanearthquake, which killed more than 250,000people. One month after that, Chinese officialsevacuated residents of the Guandong Province foran earthquake that never came.

There is value in predicting the arrival ofseismic waves from an earthquake that is alreadytaking place. Seismometers can detect P-waves afew seconds before more damaging S-waves andsurface waves arrive. Although a few seconds isnot much, a coordinated computerized system canuse that time to shut down gas mains and hightension electrical transmission lines, and initiateprotective measures in chemical plants, nuclearpower plants, mass transit systems, airports and onroadways.

Folklore tells of animals behaving erraticallyjust before an earthquake. Mostly these anecdotes

are told after the earthquake, when peopleremember back to the time before the shakingbegan. Memories are notoriously faulty. However,Chinese scientists actively study the behavior ofanimals before earthquakes to see if there issomething to the anecdotes.

One interesting tale involves the number ofanimals killed in the 2004 Boxing Day Tsunami,which appeared to be surprisingly low. Reportsabound suggesting that the animals had a “sixthsense” that warned them of the danger. In SriLanka’s Yala National Park, for example, about 60tourists and park employees drowned but few largeanimals. Three elephants were seen fleeing tohigher ground. On closer inspection, the elephantswith tracking collars appeared to have exhibitednormal movements for the day. If indeed animalssense danger from earthquakes or tsunamis,scientists do not know what it is they could besensing, but they would like to find out.

Lesson Summary

Seismologists use seismograms to determinehow strong an earthquake is, how far away itis, and how long it lasts.Epicenters can be calculated using thedifference in the arrival times of P-and S-waves from three seismograms.The intensity of an earthquake can bedetermined in many ways. The Mercalli Scaleidentifies the damage done and what peoplefeel, the Richter Scale measures the greatestamplitude of the earthquake, and the momentmagnitude scale measures the total energyreleased by an earthquake.Despite some successes, seismologists havenot come too far in their ability to predictearthquakes.

Review Questions

1. How can a seismograph measure groundshaking if all parts of it must be attached tothe ground?

2. On a seismogram, which waves arrive first,second, third and which arrive last?

3. What information is needed for seismologiststo calculate the distance that a seismic stationis from an earthquake’s epicenter?

4. If a seismogram records P-waves and surfacewaves but not S-waves, where was theearthquake epicenter located relative to theseismograph and why?

5. Like the Richter scale, the magnitude momentscale is logarithmic. What is the difference inthe amount of energy released by anearthquake that is a 7.2 versus an 8.2 inmagnitude? A 7.2 versus a 9.2?

6. Why do you need at least three seismographsto locate an earthquake epicenter?

7. While the Mercalli scale is still used formeasuring earthquake magnitude, why is it notthe only scale used? Where does it fall shortrelative to the Richter and Moment Magnitudescales?

8. Why is the moment magnitude scale thought tobe more useful than the Richter scale for

measuring earthquake magnitudes?9. What is the difference in energy released

between a 6 and a 7 on the Richter scale?How about a 6 and a 7 on the momentmagnitude scale?

10. How do seismologists use earthquakeforeshocks to predict earthquakes? Why areforeshocks not always an effective predictiontool?

11. What are the characteristics of a goodearthquake prediction? Why are these featuresneeded?

12. Why were Chinese seismologists sosuccessful at predicting the 1975 earthquakein the Manchurian province of Liaoning? Whydid they fail to predict the 1976 Tangshanearthquake and evacuate Guandong Provincea month later needlessly?

Further Reading / SupplementalLinks

http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/swf_earthquake_triangulation/p_activity_eqtriangulation.html

Vocabulary

mercalli intensity scaleThis scale measures the effects of anearthquakes seen on the land surface and feltby humans. It measures I-XII.

moment magnitude scaleThis is a logarithmic scale that measures thetotal energy released by an earthquake. Anincrease of one integer indicates a 30-foldincrease in energy released. An increase oftwo integers indicates a 1,000-fold increasein energy released.

Richter scaleThe Richter scale measures the largest joltproduced by an earthquake. It is a logarithmicscale.

seismogram

A seismogram is the printed record of seismicactivity produced by a seismometer.

seismographAn older type of seismometer in which a penthat was suspended and weighted wrote on adrum that moved with the ground.

seismometerA seismometer is a machine that measuresseismic waves and other ground motions.

Points to Consider

If you live in an earthquake prone area, howdo you feel about your home now that you’veread this section? Since earthquakes areunlikely to be predicted, what can you do tominimize the risk to you and your family? Ifyou do not live in an earthquake prone area,what would it take to get you to move to one?Also, what risks from natural disasters do youface where you live?

What do you think is the most promising set ofclues that scientists might some day be able touse to predict earthquakes?What good does information about possibleearthquake locations do for communities inthose earthquake-prone regions?

Staying Safe in Earthquakes

Lesson Objectives

Describe different types of earthquakedamage.Describe the features that make a structuremore earthquake safe.Describe the ways that a person and ahousehold can protect themselves inearthquake country.

Introduction

Earthquakes are rivaled only by hurricanes intheir ability to cause enormous amounts of damage.Earthquake damage comes not only from groundshaking, but also from the fires, landslides, andtsunamis that may result from the shaking. Thereare ways for communities to prepare forearthquakes by using earthquake-safe construction

techniques or retrofitting old structures.Individuals and households can take actions suchas securing heavy objects and preparing anemergency kit. Still, despite the best precautions, amassive earthquake can cause enormous numbersof fatalities and damage.

Damage from Earthquakes

Earthquakes kill people and damage property.There are a lot of falsehoods about howearthquakes do their damage and what sort ofdamage they do or can be expected to do. Theground shaking almost never kills or injurespeople; rarely, if ever, does the ground open upand swallow someone. Fatalities and injuriescaused by earthquakes are due to structures fallingon people. More damage is done and more peopleare killed by the fires that usually follow anearthquake than by the earthquake itself.

Damage to people and property depends on anearthquake’s magnitude, the distance of the

epicenter to population centers, and how long theground shakes. But human factors are importanttoo. The type of ground structures are built on is anenormous factor in the amount of damage done.Damage also depends on the quality of structures,including what materials are used.

The largest earthquakes are not necessarily thedeadliest. Only about 2,000 people died in the1960 Great Chilean earthquake, which was thelargest earthquake ever recorded at magnitude 9.5.The 1556 Shaanxi earthquake in China measured amagnitude 8.0, but is estimated to have killed about830,000 people. The Great Sumatra - Andamanearthquake of 2004 makes the list of the largestearthquakes and was also one of the deadliest.However, most of the 230,000 fatalities were dueto the tsunami that followed the earthquake, not theearthquake itself.

Damage during an earthquake depends on thetype of ground under the buildings. Solid bedrockvibrates much less than soft sediments. Buildingson bedrock will only fail if the earthquake isextremely violent. Soft ground, such as sand, silt,

or clay, settles when it is shaken and so buildingstilt or fall over, and pipelines, roadways, and otherstructures break. Sediments that are saturated withwater undergo liquefaction during an earthquakeand become like quicksand. Soil on a hillside thatis shaken loose can become a landslide, whichtakes houses downhill with it or buries structuresat the hill’s base.

In earthquake-prone areas, city planners spenda lot of time understanding which locations are themost vulnerable and trying to reduce the hazards.For example, in the San Francisco Bay Area,planners study maps that show how much shakingis expected in various areas for differentmagnitudes and locations of earthquakes (Figurebelow). Using this information can allow them tounderstand and prepare for the hazards. Forexample, when faced with two possible locationsfor a new hospital, planners must build on bedrockrather than silt and clay.

Figure 7.44 This map shows the amount of shaking on the

Modified Mercalli Intensity Scale that would beexpected for an earthquake of magnitude 7.1 on thenorthern portion of the Hayward Fault. Much of theland near the bay, where shaking is predicted to bemost violent is loose mud & soil, called fill. Thehills around the bay, which are mostly coloredgreen, are bedrock. The outline of the HaywardFault can be seen in black, since it is where themost violent shaking is predicted to be.

Mexico City provides an example of how softground can magnify earthquake damage. In 1985, amagnitude 8.1 earthquake struck about 350kilometers west of the city. The earthquake was

caused by subduction of the Cocos Plate beneaththe North American Plate. Mexican governmentrecords show that the earthquake killed at least9,000 people, injured 30,000 more, left 100,000people homeless, destroyed 416 buildings, andseriously damaged 3,000 other buildings. Thereason for so much destruction so far from theearthquake’s epicenter is that Mexico City is builton a drained lakebed. Beneath the capital city, theground is soft silt and clay in a basin made of solidrock. When the earthquake struck, seismic wavesbounced back-and-forth off the sides and bottom ofthe rock basin amplifying the shaking. In addition,the wet clay experienced liquefaction (Figurebelow). The buildings were not anchored tobedrock as they should have been and so theysettled into the muck, causing enormous damage.

Figure 7.45 Liquefaction of sediments in Mexico City

caused the collapse of many buildings in the 1985earthquake.

Water, sewer and electrical systems weredestroyed, resulting in fires. Acapulco, which wasmuch closer to the epicenter but built on bedrocksuffered little damage. To prevent Mexico Cityfrom being taken by surprise again, the governmentbuilt an alert system. The next time there is anearthquake in the subduction zone, a signal will beactivated and sirens will sound in the city. Thiswill give residents about one minute to prepare forthe inevitable earthquake. At the least, this isenough time for most people to get in a secure

location.The population density of a region is also

important to the number of casualties and theamount of damage. The 1964 Great AlaskaEarthquake, near Anchorage, was the largestearthquake ever recorded in North America andthe second largest globally, with a magnitude of9.2. The earthquake lasted for several minutes,resulted in slip of up to 11.5 meters (38 feet), andaffected an area of 100,000 square miles (250,000square km). Ground liquefaction caused landslides(Figure below).

Figure 7.46 A landslide in a neighborhood in Anchorage

Alaska after the 1964 Great Alaska earthquake.

Because the earthquake occurred at asubduction zone offshore, large tsunami (up to 70meters (20 feet)) were created. Despite theintensity of the earthquake, only 131 people died,mostly due to the tsunami and property damagewas relatively modest, at just over $300 million($1.8 billion in 2007 U.S. dollars). The reasonthere was such a small amount of damage for sucha large earthquake is that very few people lived inthe area at that time (Alaska had only been a statefor five years!). A similar earthquake today wouldcause immeasurably more casualties. The numberof people that an earthquake kills or injures isoften related to the time of day that it strikes andwhere it strikes. The most lethal earthquakes strikedensely populated cities when people are at workand school. Being at home in bed is usually safer.

Earthquake-Safe Structures

The way a building is built—its construction—is a large factor in what happens during an

earthquake. Building construction is the reasonmany more people died in the 1988 Armeniaearthquake than the 1989 Loma Prieta, Californiaearthquake. Although the Armenian earthquake wasonly slightly lower in magnitude, the mud housesthat are found throughout the area collapsed. Mostbuildings in California’s earthquake country aredesigned to be earthquake safe. However evenearthquake safe buildings can be damaged by alarge earthquake.

Engineers who design earthquake safebuildings must understand seismic waves and howthey affect different types of ground. Skyscrapersand other large structures built on soft ground mustbe anchored to bedrock, even if it is lies hundredsof meters below the ground surface.

The materials used to construct a structureaffect its ability to weather an earthquake. The typeof material that is best depends on the size of thebuilding. Small structures, like houses, do better ifthey are constructed of materials that bend andsway such as wood and steel rather than brick,stone, and adobe, which are brittle and will break.

Brittle materials are less likely to break if they arereinforced by steel or wood. Larger buildings mustsway, but not so much that they touch nearbybuildings. Counterweights and diagonal steelbeams are used to hold down sway. A completelydifferent approach for large buildings is to placethem on rollers so that they move with the groundbut do not collapse. Buildings may also be placedon layers of steel and rubber, which absorb theshock of the passing seismic waves. Structures thatfail usually do so because they are weak at theconnections, such as where the walls meet thefoundation. Earthquake safe buildings are wellconnected. In a multi-story building, the first storymust be supported or the structure may collapse(Figure below).

Figure 7.47 The first floor of this San Francisco building is

collapsing after the 1989 Loma Prieta earthquake.The building is being held up by the two walls thatare not in the photograph and the strength of theupper floors. The two walls that are in view arenot strong because they had doors in them.

Older structures can be retrofitted to be moreearthquake safe. Retrofitting includes adding steelor wood to reinforce a buildings structure and itsconnections (Figure below). Elevated freewaysand bridges can also be retrofitted so that they donot collapse. The goal of retrofitting is differentdepending on the type of structure being altered.Most structures are retrofit only to a strength that

protects human life. More important structures, likebridges, are made to survive intact, but may needextensive repair after the earthquake. Structuresthat need to be used in an emergency, likehospitals, are retrofit to higher standards so thatthey will need only superficial repairs after anearthquake. The highest level of protection is aretrofit that will allow a building to surviveunaffected. This is very expensive and is only donefor buildings that are of great historical or culturalsignificance.

Figure 7.48 Steel trusses were built in an x-pattern to

retrofit a dormitory at the University of California,Berkeley. The building is very near the HaywardFault.

Of course, one of the biggest problemsstemming from earthquakes is fire. Fires startbecause earthquakes rupture gas and electricallines. Breaks in water mains compound theproblem by making it difficult to fight those fires.One effective way of dealing with this is to zigzagpipes so that they bend and flex when the groundshakes. Straight pipes will break in a quake. In SanFrancisco, water and gas pipelines are separatedby valves so that areas can be isolated if onesegment breaks.

Since engineers know what sorts of structuresdo best in earthquakes, why aren’t all structures inearthquakes zones constructed for maximumsafety? Of course, the reason is cost. More sturdystructures are much more expensive to build. Sinceno one knows which structures will be exposed toa large earthquake during their effective lifetimes,communities must decide how safe to make theirbuildings. They must weigh how great the hazardis, what different building strategies will cost, andhow much risk they are willing to take. In poorcommunities, the choice may come down to

spending money on earthquake-safe buildings orfunding other priorities, such as a water sanitationproject. The choice often comes down toprotecting against a known risk versus unknownone; for example, many people in developingnations die each year from drinking and bathing inunclean water.

Protecting Yourself in anEarthquake

If you live in an earthquake zone, there aremany things you can do to protect yourself before,during and after an earthquake. The two goals areto make sure that the house and its contents are nota hazard and for the household to be ready to liveindependently for a few days until emergencyservices are available in full force.

Before the Earthquake:

Have an engineer evaluate your house forstructural integrity. Make sure the separate

pieces—floor, walls, roof and foundation—are all well attached to each other.Bracket or brace brick chimneys to the roof.Be sure that heavy objects are not stored inhigh places. Move them to low places so thatthey do not fall.Secure water heaters all around and at the topand bottom.Bolt heavy furniture onto walls with bolts,screws, or strap hinges.Replace halogen and incandescent light bulbswith fluorescent bulbs to lessen fire risk.Check to see that gas lines are made offlexible material so that they do not rupture.Any equipment that uses gas should be wellsecured.Everyone in the household should know howto shut off the gas line. A wrench should beplaced nearby for doing so.Prepare an earthquake kit with at least threedays supply of water and food. Include aradio and batteries.Place flashlights all over the house so that

there is always one available. Place one inthe glove box of your car.Keep several fire extinguishers around thehouse to fight the small fires that might breakout.Be sure to have a first aid kit. Everyone in thehousehold who is capable should know basicfirst aid and CPR.Plan in advance how you will evacuate yourproperty and where you will go. Do not planon driving as roadways will likely bedamaged.

During the Earthquake:

If you are in a building, drop to the ground,get beneath a sturdy table or desk, cover yourhead, and hold on.Stay away from windows and mirrors sinceglass can break and fall on you. Stay awayfrom large furniture that may fall on you.If the building is structurally unsound, getoutside as fast as possible. Run into an open

area away from buildings and power linesthat may fall on you.If you are in a car, stay in the car and stayaway from structures that might collapse likeoverpasses, bridges, or buildings.

After the Earthquake:

Be aware that aftershocks are likely.Avoid dangerous areas like hillsides that mayexperience a landslide.Turn off water and power to your home.Use your phone only if there is an emergency.Many people with urgent needs will be tryingto get through to emergency services.Be prepared to wait for help or instructions.Assist others as necessary.

Lesson Summary

A person standing in an open field in anearthquake will almost certainly be safe.Nearly all earthquake danger is from

buildings falling, roadways collapsing, orfrom the fires and tsunamis that come after theshaking stops.Communities can prepare for earthquakes byrequiring that buildings be earthquake safeand by educating citizens on how to preparefor an earthquake.Individuals and households can prepare intwo ways: by making sure that their house andits contents are not a hazard and by beingready to live independently for a few dayswhile emergency services regroup and get toall parts of the region.

Review Questions

1. What usually kills or injures people in anearthquake?

2. In two earthquakes of the same size, whatreasons are there that more people would bekilled in a location further from the epicenterthan in one nearer the epicenter?

3. Describe why Mexico City was so devastatedby the far away 8.1 earthquake that struck in1985. Why did Acapulco, which is locatedmuch closer to the quake site, fare so muchbetter?

4. What is liquefaction and how does it causedamage in an earthquake?

5. Pretend that you live in an old home in anearthquake-prone region. No work has everbeen done to prepare your home for anearthquake. What should you do to minimizethe harm that will come to yourself and yourhome?

6. What can an architect do to make a skyscraperearthquake safe?

7. Which types of buildings deserve the greatestprotection from earthquake hazards?

8. Using what you know about elastic strength,will a building better withstand an earthquakeif it is built absolutely solid or if it is able tosway? Why?

9. Why do wealthy communities (such as thosein California) tend to have greater earthquake

protection than poorer communities (such asthose in developing nations)?

10. What are the two goals of earthquakepreparation?

11. What should you include in an earthquake kit?12. Under what circumstances should you run

outside in an earthquake?

Vocabulary

liquefactionClay, silt, and sand saturated with waterbecome like quicksand, lose their strength andbehave more like a liquid than a solid.

Points to Consider

Many people think that in a large earthquakeCalifornia will fall into the ocean and thatArizona and Nevada will be beachfrontproperty. Why is this not true?If you were the mayor of a small city in an

earthquake-prone area, what would you liketo know before choosing the building site of anew hospital?How are decisions made for determining howmuch money to spend preparing people andstructures for earthquakes?

Chapter 8: Volcanoes

Volcanic Activity

Lesson Objectives

Explain how volcanoes form.Describe places where volcanoes occur.Describe what volcanic hot spots are andwhere they occur.

Introduction

Everybody has heard of volcanoes—the angryEarth spewing up its wrath, the sudden explosionof a distant mountain top, and lava that slithersominously toward villages. Volcanoes are fantasticdisplays of the power of the Earth. But whatactually is a volcano? How and where are theyformed? Why do some places have a history of

volcanic activity? Volcanoes explain a key pieceof the Earth’s geologic puzzle (Figure below).

Figure 8.1 Mount Merapi, Indonesia.

How Volcanoes Form

You have already learned about tectonicplates. Beneath the Earth’s surface, powerfulforces are at work. These forces move lithosphericplates and produce huge chambers of magma,molten rock beneath the Earth’s crust. Like waterthat bursts from a tiny hole in a water pipe, theliquid magma seeks cracks or fissures in theEarth’s crust through which it could flow. This is avolcano — an opening in the earth’s crust through

which magma or gases can erupt onto the surface.When molten rock escapes from beneath the

Earth’s surface, it changes from magma to lava,molten rock above the Earth’s surface. Because thetemperatures at the Earth’s surface are much lowerthan in the magma chambers, the lava does not takelong to solidify back into rock. As layer upon layerof lava solidifies, a mountain is formed (Figurebelow).

Figure 8.2 As you can see in Figure 2, magma begins in

the lava chamber and comes to the surface throughthe throat of the volcano. The volcano isconstructed layer by layer, as ash and lava

solidify, one upon the other. However, other typesof volcanoes exist, and will be discussed later inthis chapter.

Where Volcanoes Occur

Because volcanoes are vents for magma, itmakes sense that volcanoes would be formedabove underground magma chambers. If you recall,magma is molten rock that has been heated becauseof high temperatures and pressures beneath theEarth’s crust. This pressure mostly occurs wherethe tectonic plates meet and subduct. Look at themap of tectonic plates in Figure below.

Figure 8.3

Tectonic plates. Some of the largest tectonicplates meet along the coasts of Asia and NorthAmerica. Compare this map to the map in .

So, volcanic activity tends to occur alongsubduction plate boundaries, where one plateslides underneath another. The edges of the PacificPlate make up a long subduction boundary. Thereare a huge number of earthquakes along theseboundaries, because these are regions where theplates are colliding. For the same reason, themajority of the volcanic activity on the Earth alsooccurs along these convergent boundaries. This iscalled the Pacific Ring of Fire where over 75% ofthe world’s volcanoes are found. The CascadeRange of volcanoes runs through southwesternCanada and the Pacific Northwest of the UnitedStates. These volcanoes are the result ofsubduction of the Juan de Fuca plate beneath theNorth American plate. (Figure below).

Figure 8.4 The Pacific Ring of Fire is where the majority

of the volcanic activity on the Earth occurs.Of course, this is not the only area where

volcanoes occur. Beneath the ocean, there are alsodivergent boundaries, where tectonic plates arepulling away from each other. As the plates pullaway from each other, they create a deep canyon orfissure on the sea floor through which molten rockescapes. Mid-ocean ridges, like the Mid-Atlanticridge, form here as lava flows out through thefissure (Figure below). Submarine volcanoes canalso form on the ocean floor. At times thesevolcanoes can grow to create islands above thewater’s surface. This explains how Surtsey, asmall island near Iceland formed (Figure below).

Figure 8.5 The Mid-Atlantic Ridge is formed by divergent

tectonic plates.

Figure 8.6 A volcanic eruption in Iceland.

Volcanic Hot Spots

Although most volcanic activity on Earthoccurs at plate boundaries, there are somevolcanically active spots that are in the middle of atectonic plate. These areas are called hot spots.The islands of Hawaii formed over a hot spot andare not located on the Pacific Ring of Fire (Figurebelow). The Hawaiian islands are the exposed

peaks of a great chain of volcanoes that wereformed over millions of years. The islands arethought to lie directly above a column of hot rockcalled a mantle plume. Mantle plumes are more orless fixed in place and continuously bring magmaup form the mantle towards the crust. As thetectonic plates move above them, they leave a trailof volcanic activity, which forms island chains likeHawaii. Scientists believe there are about 50 hotspots on the Earth. Other hot spots includeYellowstone and the Galapagos Islands.

Figure 8.7 Hawaii is a volcanic hot spot. It was formed

by volcanic activity fed by mantle plumes.Don’t confuse hot spot volcanoes with islands

that are formed by plate tectonics like the Aleutian

island chain in Alaska. These long lines ofvolcanoes form as the edge of a subducted platemelts, producing magma which rises to the surfacealong the edge of the plate. These volcanicmountains will all be about the same age. Whenislands form over a hot spot, the youngest island isover the hot spot. As you move along the islandchain, each island further from the hot spot will beolder than the one before it.

Lesson Summary

Volcanoes form when magma reaches theEarth's surface.Volcanoes occur most often along plateboundaries.Convergent plate boundaries where oceaniccrust is subducted form many of the volcanoesfound on Earth.Divergent plate boundaries produce hugemountain ranges under water in every oceanbasin.

Volcanoes like those that form the islands ofHawaii, form over areas called hot spots.

Review Questions

1. What is the difference between magma andlava?

2. Explain how volcanoes are formed.3. Why are there volcanoes along the west coast

of the United States?4. Why do volcanoes occur where tectonic

plates pull apart or diverge?5. Explain how the Pacific Ring of Fire got its

name.6. What is a mantle plume?7. Suppose a new volcano suddenly formed in

the middle of the United States. How mightyou explain what caused this volcano?

8. Volcanoes have been found on Venus, Mars,and even Jupiter’s moon Io. What do youthink this indicates to planetary geologists?

Vocabulary

fissuresA long crack into the Earth’s surface, fromwhich lava erupts onto the surface.

hot spotA fixed region of hot magma that rises throughthe mantle and creates volcanoes on theEarth’s surface. A hot spot is above a mantleplume.

lavaMolten rock that has been erupted onto theEarth’s surface. Magma becomes lava once itemerges on the surface.

magmaMolten rock found under the Earth’s surface.

mantle plumeA column of very hot rock that rises upthrough the mantle. Mantle plumes will form

hot spots if they make it all the way up to thesurface of the Earth’s crust.

Points to Consider

When you look at the map of tectonic plates(Figure 3), what areas besides the PacificRing of Fire would you expect to havevolcanic activity?Some volcanoes are extinct; they are nolonger active and will probably never beagain. What do you think causes a volcano tobecome extinct?Hot spots are still poorly understood by earthscientists. Given your understanding of theEarth’s interior, why do you think it’s hard tostudy hot spots?

Volcanic Eruptions

Lesson Objectives

Explain how volcanoes erupt.Describe and compare the types of volcaniceruptions.Distinguish between different types of lavaand understand the difference between magmaand lava.Describe a method for predicting volcaniceruptions.

Introduction

In 1980, Mount St. Helens erupted in one of themost deadly and costly volcanic eruptions in theUnited States ever. The eruption was particularlydeadly since Mount St. Helens, one of the CascadeRange, is in a populated area between Portland,Oregon and Seattle, Washington. The eruption

killed 57 people, destroyed 250 homes, and sweptaway 47 bridges. The elevation of the volcanodropped by over 400 meters (1,300 feet) becauseof the immense explosion created by the eruption.Today Mt. St. Helens is still active (Figurebelow). The volcano now has a horseshoe-shapedcrater that is 1.5 km (nearly one mile) across.Within the crater, a new lava dome has formed.How did this eruption occur? Why aren’t allvolcanoes explosive like Mt. St. Helens? Why didso many people perish if we knew that it was goingto erupt? The study of volcanoes has manyquestions still unanswered. However, scientistshave studied volcanoes for many years and arepiecing together evidence that explains thesepowerful geologic phenomena.

Figure 8.8 Mount St. Helens, Washington, two years after

its eruption.

How Volcanoes Erupt

All volcanoes share the same basic features.The magma collects in magma chambers that canbe 160 kilometers (100 miles) beneath the surface.As the rock heats, it expands, which creates evenmore pressure. As a result, the magma seeks a wayout pushing toward the surface, the magma seepsthrough cracks in the Earth’s crust called vents.Eventually, the magma reaches the surface; when itcomes out, we call it an eruption. The word

eruption is used in other contexts, as well. Aneruption can be an outburst or explosion, a violentand sudden occurrence, like when a crowd eruptsin anger. But an eruption can also be a spreading ofsomething like a rash on your skin, gradual andrelatively calm. These two definitions are similarto the two kinds of eruptions that we see involcanoes.

Types of Eruptions

Every geological formation is unique. Theircomposition and construction depend on so manyfactors, that it would be impossible for twoformations to be exactly alike. In the same way,each volcano and its eruptions are unique.However, we tend to see two major kinds oferuptions. We talked about eruption to mean both aviolent explosion or a sort of silent spreading.These are the two types of volcanic eruptions thatwe see--explosive and non-explosive eruptions.When we think of volcanic eruptions, we often

think of huge clouds of volcanic ash ejected highinto the atmosphere and then thick rivers of redlava snaking down the mountainside. In reality,these two phenomena rarely occur in the samevolcano. Volcanic eruptions tend to be one or theother.

Explosive Eruptions

Figure 8.9 An explosive eruption from the Mayon

Volcano in the Philippines in 1984.Imagine the devastation and force caused by

the atom bomb dropped on Nagasaki at the end ofWorld War II in which over 40,000 people died.Now imagine an explosion 10,000 times as

powerful. Explosive volcanic eruptions can be thatpowerful (Figure below). As hot magma beneaththe surface interacts with water, gases accumulateand the magma pressure builds up. This pressuregrows and grows until these dissolved gases causeit to burst in an enormous explosion.

This great explosion takes with it the magmaand volcanic gases, which can shoot manykilometers into the sky and forms a mushroomcloud, similar to that formed by a nuclearexplosion (Figure below). The debris travels upinto the air at very high speeds and cools in theatmosphere to form solid particles calledpyroclasts. Some of these particles can stay in theatmosphere for years, which can disrupt weatherpatterns and affect the temperature of the Earth.The rest of the debris comes falling back to Earthwhere it rains down for kilometers and kilometersaround.

Figure 8.10 Explosive eruption of Mt. Redoubt in Alaska,

1989. This huge mushroom cloud reached 45,000feet and caught a Boeing 747 in its plume.

Sometimes secondary explosions occur that areeven greater than the first. Additionally, volcanicgases like water vapor, carbon dioxide, sulfurdioxide, hydrogen sulfide, and hydrogen chloridecan form poisonous and invisible clouds that roamabout the atmosphere. These gases contribute toenvironmental problems like acid rain and ozonedestruction, and can actually cool the Earth’satmosphere.

In the Cascade Range, the explosive eruptionof Mount St. Helens was preceded by the eruption

of Lassen Peak, one of the three CascadeVolcanoes in northern California. On May 22,1915, an explosive eruption sent a column of ashand gas 30,000 feet into the air and triggered ahigh-speed pyroclastic flow, which melted snowand created a lahar. Lassen continues to havegeothermal activity and could erupt explosivelyagain. Mt. Shasta erupts every 600 to 800 years.An eruption would most likely to create a largepyroclastic flow, and perhaps a lahar. However,the volcano could explode like Mt. Mazama, whichblew itself in an eruption about 42 times morepowerful than Mount St. Helens in 1980, to createCrater Lake.

Non-explosive Eruptions

Figure 8.11 In effusive eruptions, lava flows more readily,

producing rivers of molten rock.A second type of volcanic eruption is a non-

explosive or effusive eruption (Figures aboveand below). Because the composition of magma isdifferent in different volcanoes, the properties of

the lava are different. In effusive eruptions, lavaflows are relatively calm and do not explode out ofthe volcano. As a result, people generally have agreat deal of warning before lava reaches them, sonon-explosive eruptions are much less deadly.That does not keep them from being destructive,however. Even when we know that a lava flow isapproaching, there are few ways of stopping it,given the huge quantity and temperature of lava.

Figure 8.12 When lava flows readily, pressure does not

build up so great explosions do not occur.

Magma and Lava

Volcanoes wouldn’t be nearly as interestingwithout the great explosions they create and theglowing red rivers of lava. All igneous rock comesfrom magma or lava. The next time you go hikingnear a volcanic zone, you might try to identify thetypes of lava that the volcano erupted, based on thetypes of igneous rocks you find.

Magma

Deep beneath the Earth, magma forms as thefirst stage in creating a volcano. This occursbecause rock below the surface is subjected togreat amounts of pressure from gravity. The decayof radioactive materials generates additional heat.The substantial heat and pressure melt the rockbelow the surface to form a taffy-like substance.You may have seen a candle that has been left outin the hot sun too long. It becomes softer and morelike a liquid. As the molecules absorb heat, theybegin to slide past one another becoming morefluid. A similar process occurs with magma.However, different substances melt at different

temperatures. For that reason, the temperature atwhich rocks melt depends on the specific types ofrocks. The Earth’s crust and mantle are made ofmany substances so the temperature required tocreate magma varies. Most magmas are formedbetween 600oC and 1300oC (Figure below).

Figure 8.13 Cutaway of the Earth. The melting of rock in

the crust and upper mantle create magma.Melted rock or magma can be found in magma

chambers beneath the Earth. Since the magmachambers are so far beneath the Earth's surface, itis difficult for scientists to study them. Scientistsknow that magma chambers are created where the

heat and pressure are greatest. When tectonicplates collide and rub against each other, magma isformed there. That is how the Pacific Ring of Firewas created. We also know there are volcanoes faraway from plate boundaries, so we know there aremagma chambers in these areas as well. Magmachambers can be found where there are mantleplumes or hot spots.

Just how or why these hot spots are createdisn’t exactly known. However, because differentsubstances melt at different temperatures, thecreation of magma depends on what substancesmake it up—its composition. Just like the flavor ofa cake depends on the ingredients that you put in it,the behavior of magma and lava depends on itscomposition. Certain melted rocks act in certainways. So when the magma becomes lava, not alllava acts the same.

Lava

Once magma reaches the surface, it becomeslava. Consider different liquids that you might see

in your house—honey and a bottle of cola, forexample. You might agree that the two liquids aredifferent in many regards. They taste different,have different colors, have different gases in them,and they flow differently. In fact, honey is a liquidthat resists flowing, whereas cola flows easily.Honey has a higher viscosity than the cola; itresists flowing (Figure below). Cola has a lowviscosity because it flows easily. One of the majordifferences in different types of lava is theirviscosity.

Figure 8.14 Honey flows slowly; it is more viscous than

water.A highly viscous lava is one that doesn’t tend

to flow easily. It tends to stay in place. Lavas withhigh silica contents tend to be more viscous. Since

it is so resistant to moving, it clogs the vents in avolcano. The pressure becomes greater and greateruntil the volcano finally explodes. This type oflava is found in explosive eruptions. It also tendsto trap a lot of gas. When the gas is released, itmakes the eruption more explosive. Most of thislava is shot up into the air where it hardens andbecomes solid rock. This molten rock thatsolidifies in the air is known as pyroclasticmaterial. In an igneous rock like pumice, smallholes in the solid rock show where gas bubbleswere when the rock was still liquid lava.

Low-viscosity lava slides or flows downmountainsides. There is more than one type of low-viscosity lava. The differences between them comefrom the lavas’ different composition and differentspots where they come to the surface. The type ofigneous formations formed depends on which typeof lava it is. The three major categories are a’a,pahoehoe, and pillow lava.

A’a Lava

A’a lava is the more viscous of the non-explosive lavas (Figure below). This lava forms athick and brittle crust which is torn into rough andjagged pieces. The solidified surface is jagged andsharp. It can spread over large areas as the lavacontinues to flow underneath.

Figure 8.15 Aa lava flow.

Pāhoehoe Lava

Pāhoehoe lava is less viscous than a'a lava,and flows more readily. Its surface looks morewrinkly and smooth than the jagged a'a lava.Pāhoehoe lava flows in a series of lobes orrounded areas that form strange twisted shapes and

natural rock sculptures (Figure of Pāhoehoe lava).Pāhoehoe lava can also form lava tubes beneaththe ground (Figure of Thurston Lava Tube).

Figure 8.16 Phoehoe lava

Figure 8.17

The Thurston Lava Tube in Hawaii VolcanoesNational Park.

Pillow Lava

Pillow lava is lava that comes out fromvolcanic vents underwater (Figure below). Whenit comes out underwater, it cools down veryquickly and forms roughly spherical rocks thatresemble pillows, from which more lava leaks andcreates more pillows. Pillow lava is particularlycommon along underwater spreading centers.

Figure 8.18 Pillow lava.

Predicting Volcanic Eruptions

Volcanic eruptions can be devastating,particularly to the people who are closer tovolcanoes. As meteorologists attempt to predict, orforecast, hurricanes and tornados, so too dovolcanologists attempt to forecast volcaniceruptions. Although predicting volcanic eruptionsis far from perfect, many pieces of evidence canindicate that a volcano is about to erupt. Some ofthose factors are hard to measure, contributing tothe difficulty in predicting eruptions.

History of Volcanic Activities

One important factor in predicting eruptions isa volcano’s history. That is, we consider how longsince it has erupted and the time span between itsprevious eruptions. Volcanoes are categorized intothree subdivisions—active, dormant, and extinct.An active volcano is one that is currently eruptingor shows signs of erupting in the near future. A

dormant volcano no longer shows signs ofactivity, but has erupted in recent history (Figurebelow). Finally, an extinct volcano is one that hasnot erupted in recent history and will probably noterupt again in the future. Both active and dormantvolcanoes are heavily monitored because evendormant volcanoes could suddenly show signs ofactivity.

Figure 8.19 Vesuvius is a dormant volcano near the city of

Naples. Although it shows no current signs oferuption, it could one day become active.

Earthquakes

As magma beneath a volcano pushes upward, itshakes the ground and causes earthquakes.Although earthquakes probably occur every daynear a volcano, the quantity and size of theearthquakes increases before an eruption. In fact, avolcano that is about to erupt may produce acontinuous string of earthquakes, as magma movingunderground creates stress on the neighboringrocks. In order to measure these earthquakes,scientists use seismographs that record the lengthand strength of each earthquake.

Slope Deformation

All that magma and gas pushing upwards canmake the ground or the volcano’s slope begin toswell. Sometimes, ground swelling reveals hugechanges in the shape of a volcano. Most cases ofground deformation are subtle, though, and canonly be detected by tiltmeters, which areinstruments that measure the angle of the slope of avolcano. Additionally, ground swelling may causeincreased rock falls and landslides.

Gas Emissions

Oftentimes, gases are able to escape a volcanobefore magma reaches the surface in an eruption.So, scientists can measure gas output, or gasemissions, in vents on or around the volcano.Gases, like sulfur dioxide (SO2), carbon dioxide(CO2), hydrochloric acid (HCl) and even watervapor can be measured at the site or, in somecases, at a distance with satellites. The amounts ofgases and their ratios are calculated to help predicteruptions.

Remote Monitoring

As mentioned, some gases can be monitoredusing satellite technology (Figure below).Satellites are able to measure other factors, too,like temperature readings of particularly warmspots at a volcano site or areas where the volcanosurface is changing. As our technology continues toimprove, scientists are better able to detect

changes accurately and safely.

Figure 8.20 An Earth-Observation Satellite before launch.Although monitoring methods are getting better

and better, it is still difficult to predict a volcaniceruption with certainty. No scientist or governmentagency wants to be considered alarmist byannouncing that an eruption is going to occur andthen it really doesn’t. The cost and disruption tosociety of a large-scale evacuation would leavemany people displeased and the scientistsembarrassed. However, the possibility of saving

lives and property most certainly makes the pursuitof eruption prediction a worthy cause.

Lesson Summary

Volcanoes are produced when magma risestowards the Earth's surface because it is lessdense than the surrounding rock.Volcanic eruptions can be non-explosive orexplosive depending on the viscosity of themagma.Explosive type eruptions happen along theedges of continents and produce tremendousamounts of material ejected into the air.Non-explosive type eruptions mostly producevarious types of lava, such as a'a, pahoehoeand pillow lavas.Some signs that a volcano may soon eruptinclude earthquakes, surface bulging, gasesemitted as well as other changes that can bemonitored by scientists.

Review Questions

1. What are the two basic types of volcaniceruptions?

2. Several hundred years ago, a volcano eruptednear the city of Pompeii. Archaeologists havefound the remains of people embracing eachother, suffocated by ash and rock that coveredeverything. What type of eruption must havethis been?

3. 4. What is pyroclastic material?

5. Name three liquids that have low viscosityand three that have high viscosity.

6. What is the difference between a magmachamber and a mantle plume?

7. The boiling point of water is 100oC. Whymight water make an eruption moreexplosive?

8. What are three names for non-explosive lava?9. What factors are considered in predicting

volcanic eruptions?10. Why is predicting volcanoes so important?11. Given that astronomers are far away from the

subjects they study, what evidence might theylook for to determine the composition of aplanet on which a volcano is found?

Further Reading / SupplementalLinks

http://www.usgs.gov/http://www.learner.org/channel/courses/essential/earthspace/session4/closer1.htmlhttp://www.wikipedia.org/

Vocabulary

active volcanoA volcano that is currently erupting or justabout to erupt.

dormant volcanoA volcano that is not currently erupting, butthat has erupted in the recorded past.

effusive eruptionA relatively gentle, non-explosive volcaniceruption.

eruptionThe release of magma onto the Earth’ssurface. Usually an eruption is accompaniedby the release of gases as well.

explosive eruptionA volcanic eruption that releases largeamounts of gas, so that magma is violentlythrown up into the air.

extinct volcanoA volcano that has not erupted in recordedhistory, and is considered unlikely to eruptagain.

magma chamberA region within Earth surrounded by solidrock and containing magma.

pyroclastA rock made up of fragments of volcanic rockthrown into the air by volcanic eruptions.

viscosityThe “thickness” or “stickiness” of a liquid.The more viscous a liquid is, the harder itwill be for the liquid to flow.

Points to Consider

What types of evidence do you think wouldtell scientists whether an ancient volcaniceruption was explosive or non-explosive?

Are all volcanoes shaped like tall mountainswith a crater on the peak?What do you think is the origin of the namesA’a and Pāhoehoe?Earthquakes do not always indicate that avolcano is going to erupt. What factors aboutan earthquake might indicate a relationship toa volcanic eruption?

Types of Volcanoes

Lesson Objectives

Describe the basic shapes of volcanoes.Compare the features of volcanoes.Describe the stages in the formation ofvolcanoes.

Introduction

When most people think of volcanoes, theythink of a tall mountain with a crater on the top,maybe a little snow at the summit and some treesscattered around the base. There are manyvolcanoes like this, but volcanoes exist in manyother forms as well. Each type of volcano hascharacteristic features that distinguish it from othertypes. Volcanoes differ in appearance because ofthe composition of their magma and the processesthat originally created them.

Types of Volcanoes

The tall cone shape you usually think of whenyou think of a volcano describes a compositevolcano, one common form of volcanoes. Othertypes of volcanoes include the shield volcano, thecinder cone, and the supervolcano.

Composite Volcanoes

Figure 8.21 Mt. Fuji is a dormant composite volcano that is

the highest mountain in Japan.The picture above shows Mt. Fuji, a classic

example of the composite volcano (Figure above).This is the type of volcano many people think ofwhen they imagine volcanoes. Compositevolcanoes have broad bases and sides that getsteeper and steeper as you get closer to the top.These volcanoes frequently have a large crater atthe top created during its last eruption.

Composite volcanoes are also calledstratovolcanoes because of the alternating layers,or strata, of which they are made (Figure below).The magma that creates stratovolcanoes tends to bemore viscous, or thick. Viscous lava createsgreater pressure which, in turn, tends to createexplosive eruptions. In addition, the viscous lavacannot travel far down the sides of the volcanobefore it solidifies. This viscous lava thus createssteep sides on stratovolcanoes.

When a stratovolcano erupts, it ejects a greatdeal of pyroclastic material into the air, which thensettles back down on the Earth. After an initialexplosion, lava then flows from the volcanocreating a second layer of material. As these layerssolidify, they create alternating levels, or strata, of

material. Ash from the volcanic eruption is alsopresent between the lava layers along the edge ofthe volcano. Composite volcanoes are commonalong the Pacific Ring of Fire and other majortectonic plate boundaries where the presence ofwater in the magma chamber creates explosiveeruptions.

Figure 8.22 A composite or stratovolcano is created by

many levels of alternating materials.

Shield Volcanoes

Figure 8.23 The Mauna Loa Volcano in Hawaii is the

largest shield volcano on Earth.Shield volcanoes get their name from their

shape—a huge shield laid on its side. Figureabove shows the Mauna Loa Volcano. You can seethat shield volcanoes do not have the steepmountainous sides of composite volcanoes. Theyhave a very wide base and are much flatter on thetop than composite volcano. Although they are notsteep, they may be very large. The Mauna LoaVolcano has a diameter of over 112 kilometers (70miles) and forms a significant part of the island ofHawaii. The Mauna Kea Volcano, also in Hawaii,

is another shield volcano that is over tenkilometers (6 miles) high from its base below sealevel to its peak.

Shield volcanoes are more common atspreading centers or volcanic hot spots in themiddle of tectonic plates (Figure below). Themagma that creates shield volcanoes is lessviscous, so it flows much more easily. For thisreason, the eruptions of shield volcanoes are non-explosive. In addition, the less viscous lavaspreads out more, which makes shield volcanoesmuch larger and flatter than stratovolcanoes.Although shield volcanoes are built by many layersover time, the composition of the layers do notalternate between ash and lava, as they do instratovolcanoes.

Figure 8.24 A shield volcano is built by layers of more

fluid lava that spreads out over broad areas.

Cinder Cones

Cinder cones are both the most common type ofvolcano and also the smallest. The cinder coneresembles a composite volcano but on a muchsmaller scale. They rarely reach even 300 metersin height but have even steeper sides than acomposite volcano. They usually have a crater atthe summit. Cinder cones are composed of smallfragments of rock piled on top of one another.These volcanoes usually do not produce streams of

lava.In 1943, a farmer in Mexico witnessed the first

eruption of a cinder cone in his field (Figurebelow). Within a year, the cinder cone Paricutíngrew to 336 meters high. By 1952, it grew to apeak of 424 meters tall, and then stopped erupting.This rapid growth and single eruption cycle ischaracteristic of cinder cones. For this reason,cinder cones do not reach the sizes ofstratovolcanoes or shield volcanoes. Oftentimes,cinder cones appear near larger volcanoes, butthey also may be found away from all othervolcanoes, as was the case with Paricutín. Theexact composition of a cinder cone depends on thecomposition of the lava ejected from the Earth.

Figure 8.25 Paricutn erupting in 1943, when it first formed.

Cinder cones like this one rarely reach even 300meters high.

Supervolcanoes

Figure 8.26 The caldera at Santorini in Greece is so large

that its circular shape can only be seen by satellite.In certain areas of the world, huge calderas

have been found to be the remains of volcaniceruptions of enormous scale (Figure above). Thesecalderas are volcanic features that are formed bythe collapse of a huge amount of land due to thepowerful eruptions. Caldera comes from Latinword, meaning cauldron. Calderas are generallycircular shaped geographic formations like the

picture in figure 6. These are not singularmountains but entire geographical areas.Yellowstone National Park in Wyoming is anothercaldera that has blown about a hundred times in thelast 16 million years.

Supervolcanoes represent the most dangeroustype of volcano. An eruption from a supervolcanocould change life on Earth as we know it for manyyears. Supervolcanoes were not even accepted involcanology until this millennium. Manysupervolcano eruptions are thought to haveoccurred, the most recent in New Zealand less than2000 years ago. That explosion was thought tohave ejected about 100 cubic kilometers ofmaterial. A supervolcano eruption near what isnow Colorado was thought to have let loose over5,000 cubic kilometers of material millions ofyears ago. In comparison, the Mt. Saint Helenseruption ejected about 1 cubic kilometer ofmaterial.

The eruptions from supervolcanoes can be solarge that the ash ejected into the air blocks the Sunand lowers the temperature on the entire planet.

The lowered temperatures caused by theseeruptions is called a volcanic winter. Asupervolcano eruption at Lake Toba in northernSumatra may have annihilated about 60% of theworld’s human population about 75,000 years ago.One can only imagine how such a huge eruptionwould change the world in modern times.

The largest supervolcano in North America isYellowstone, which had three super eruptions at2.1 million, 1.3 million and 640,000 years ago,and much more recent smaller (but still enormous)eruptions. Long Valley caldera, south of MonoLake in California, is the second largestsupervolcano in North America, eruptingextremely hot and explosive rhyolite around700,000 years ago. An earthquake swarm in 1980alerted geologists to the possibility of anothereruption in the future, but the timing of such anevent is unknown.

Supervolcanoes are a fairly new idea so theexact cause of supervolcano eruptions is stilldebated. However, scientists believe that an entireand very large magma chamber erupts in a

catastrophic explosion. This enormous eruptioncreates a huge hole or caldera where the surfacearea collapses.

Lesson Summary

Composite cones, shield volcanoes, cindercones and supervolcanoes are some of thetypes of volcanoes formed.Composite cones are tall, cone shapedvolcanoes that produce explosive eruptions.Shield volcanoes form very large, gentlysloped volcanoes with a wide base.Cinder cones are the smallest volcaniclandform. They are formed from accumulationof many small fragments of ejected material.A caldera forms when an explosive eruptionleaves a large crater when the mountainblows apart.Supervolcanoes are tremendously devastatingtypes of volcanoes that could destroy largeareas when they erupt.

Review Questions

1. Rank the four types of volcanoes in orderfrom smallest to largest in diameter.

2. What factor is most important in determiningthe type of volcano formed in a given area?

3. Which type of volcano is most common?4. Why is it that pahoehoe and a’a lava are more

frequent in shield volcanoes than incomposite volcanoes?

5. Why do you think that cinder cones are short-lived?

6. If supervolcanoes are so big, why do youthink it took so long for scientists to discoverthem?

Vocabulary

calderaCircular-shaped geographic features formedfrom a massive eruption of an ancientvolcano, and the subsequent collapse of the

volcano back into the ground.

cinder coneA smaller volcano that grows rapidly but onlyerupts over a short period of the time. Cindercones are composed of small rock fragmentspiled on top of one another. They rarely aremore than 300 m in height.

composite volcanoA volcano with a broad base, steep sides, andoften a crater at the top. The volcano iscomposed of alternating layers of ash andlava flows. Also called a stratovolcano.

strataLayers of rock that are similar in compositionto one another.

supervolcanoA massive volcanic eruption that is rare butincredibly powerful. Thousands of cubickilometers of matter can be ejected, and the

dust and ash from their eruption can cool theworld’s climate for years.

Points to Consider

Composite volcanoes and volcanic conesusually have craters on the top. Why are thecraters not always circular, but sometimes“U” or horseshoe-shaped?A shield volcano is relatively flat, and acomposite volcano is relatively steep becauseof the type of magma that creates them. Whatprocess might create a volcano that is moresteep than a shield volcano but not as steep asa composite volcano?Some people have theorized that if a hugeasteroid hits the Earth, the results would becatastrophic. How might an asteroid impactand a supervolcano eruption be similar?

Volcanic Landforms andGeothermal Activity

Lesson Objectives

List and describe landforms created by lava.Explain how magma creates differentlandforms.Describe the processes that create hot springsand geysers.

Introduction

As you know, magma is molten rock foundbeneath the Earth’s surface. Sometimes, it appearsat the surface of the Earth as lava after movingthrough a volcano. At other times, magma does notcome to the surface, but stays underground. In bothcases, the magma eventually solidifies and theresulting rocks and formations are igneous. The

rocks that solidify beneath the ground are calledintrusive rocks, while those that solidify above thesurface are called extrusive rocks. Extrusiverocks are sometimes small rocks that you can holdin your hand. At other times, entire landforms arecreated when lava flows onto the surface. Intrusiverocks do not always remain hidden below thesurface. They can appear on the surface whenrocks that once covered them are eroded away,exposing the intrusive igneous rock. Hot springsand geysers are some more examples of surfacefeatures related to igneous rock.

Landforms from Lava

The most obvious landforms created by lavaare volcanoes. Volcanoes, of course, are theplaces where lava comes to the surface. Asalready discussed, volcanoes come in many forms,most commonly as cinder cones, stratovolcanoes,and shield volcanoes. However, lava can createother notable landforms, as described below.

Lava Domes

When lava is fairly viscous, it is thick andflows slowly. Although it might not be so viscousthat it causes an explosive eruption, it can create alarge sort of round “blob” or a lava dome.Because it is so thick, it does not flow far from thevent from which it came. In fact, lava flows oftenmake mounds right in the middle of craters at thetop of volcanoes (Figure below and Figure below).

Figure 8.27 Lava domes are large, round landforms created

by thick lava that does not travel far from the vent.

Figure 8.28 Sometimes lava domes are formed in the crater

of composite volcanoes. This lava dome is formingin the crater of Mt. St. Helens in Washington State.

Plateaus

A lava plateau is caused by a large amount ofless viscous lava that flows over a large area.When it solidifies, it creates a large, flat surface ofigneous rock. Some plateaus are huge, like theColumbia Plateau in Washington, Oregon, andIdaho that covers over 161,000 square kilometers(63,000 square miles).

Land Area

Another important land formation created bylava is islands. The Hawaiian Islands are formedfrom solidified lava from shield volcanoes thathave grown over the last 5 million years (Figurebelow). The land area grows as lava continues tosolidify on the coast or emerges from beneath thewater.

Figure 8.29 Solidified lava flows created the island of

Hawaii.

Landforms from Magma

Of course, not all magma reaches the surface.

Sometimes magma stays beneath the ground whereit solidifies. These formations are calledintrusions (Figure below). Because they formunderground, they only become land formations ifthey arrive at the surface of the Earth. This occursbecause of weathering, erosion and plate tectonics.In other words, when tectonic plates collide, theycreate mountain ranges where one plate is lifted ina subduction zone. This lifting (which occurs at arate of centimeters per year) and subsequenterosion can eventually uncover intrusive rockformations. As erosion removes the topmost rocksand soil, solidified magma is exposed in the sameform in which it cooled and solidified many yearsbefore.

Figure 8.30 Devils Tower in Wyoming is a huge rock

formation that was once magma that cooled withina volcano. It rises to nearly 400 meters from itsbase.

Hot Springs and Geysers

Beneath the surface of the Earth, water worksits way through porous rocks or soil. Most caves,for example, are results of water’s erosion of theground. At times, that water crosses paths withvolcanic activity. The same heat that melts rockinto lava heats the water beneath the surface. If thewater makes its way to the surface, it may emergeas either a hot spring or a geyser.

Hot Springs

When that water comes to the surface underregular pressure, it creates a hot spring (Figurebelow). A hot spring is a crack in the Earth through

which water reaches the surface, after being heatedbelow the ground. Many people disagree on theexact definition of a hot spring. However,everyone agrees that the water’s temperature ishigher than normal. The water in hot springs caneven reach temperatures in the hundreds of degreesCelsius beneath the surface. Most hot springs donot reach those great temperatures. In fact, manyhot springs are used by people as natural hot tubs.Many people believe that hot springs hold curativeproperties. Hot springs are found all over theworld, even in Antarctica!

Figure 8.31 Even some animals enjoy relaxing in natures

hot tubs.

Geysers

Like hot springs, geysers are created by waterthat is heated beneath the Earth’s surface. Whenwater is both superheated by magma and flowsthrough a narrow passageway underground, theenvironment becomes ideal for a geyser. Thenarrow passageway traps the heated waterunderground, where heat and pressure continue tobuild. Sooner or later, the pressure grows so greatthat the superheated water bursts out onto thesurface. This explosion is called a geyser. Thereare only a few areas in the world where theconditions are right for the formation of geysers.About 1,000 geysers exist worldwide and abouthalf of those are found in the United States ofAmerica. Perhaps the most famous geyser is OldFaithful, which erupts every 60 to 70 minutes in aplume of hot water nearly 60 meters in the air. It israre for a geyser to erupt regularly, which

contributes to Old Faithful's fame (Figure below).

Figure 8.32 Old Faithful Geyser in Yellowstone National

Park during an eruption.

Lesson Summary

Very thick lava that doesn't travel very far can

produce lava domes at or near the Earth'ssurface or even within a volcano.Lava plateaus form from large lava flows thatspread out over large areas.Many islands are formed from volcanoes.Magma can also cool and crystallize belowthe Earth's surface forming igneous intrusions.When magma heats groundwater, it can formhot springs and geysers.

Review Questions

1. What is the difference between intrusive andextrusive rocks?

2. What are four different landforms created bylava?

3. What is the major difference between hotsprings and geysers?

4. The geyser called Old Faithful has beenerupting for perhaps hundreds of years. Oneday, it could stop. Why might geyserscompletely stop erupting?

5. After earthquakes, hot springs sometimes stopbubbling, and new hot springs form. Whymight this be?

Vocabulary

intrusiveDescribing a volcanic rock that cooledunderground. Intrusive rocks contain largemineral crystals that are visible to the nakedeye.

extrusiveDescribing a volcanic rock that cooled at theearth’s surface. Extrusive rocks have smallcrystals, and may contain air bubbles.

lava domeA dome-shaped plug of viscous lava thatcools near the vent of a volcano.

intrusionA rock mass formed by magma solidifying

underground.

hot springA stream of hot water that flows out of theground continuously.

geyserA fountain of hot water and steam that shootsinto the air at either regular or randomintervals. The water in a geyser encounterssome sort of constriction underground, whichforces pressure to build up in the water,eventually causing the geyser to erupt.

Points to Consider

What might the Earth look like if there wereno tectonic plates? Can you think of anyplanets or satellites (moons) that may nothave tectonic plates? How is their surfacedifferent from that of the Earth?What kind of land formations have you seenthat may have been created by volcanic

activity? Were the formations made ofextrusive or intrusive rock?Water is not the only material that can beejected from geysers and hot springs.Consider the composition of the Earth’s crust.What other materials might be ejected fromgeysers and hot springs?

Chapter 9: Weathering andFormation of Soil

Weathering

Lesson Objectives

Define mechanical and chemical weathering.Discuss agents of weathering.Give examples of each type of weathering.

What is Weathering?

Weathering is the process that changes solidrock into sediments. Geologists use the wordsediment to describe all different sizes of rockparticles. Sediment includes really large pieces ofrock, like boulders or gravel, but it also includessand and much smaller particles, called silt and

clay. In the process of weathering, rock isdisintegrated and decomposed. Disintegration ofrock happens as rock is broken into pieces. Oncethe pieces are separated from the rocks, erosion isthe process that moves those pieces. Gravity is oneway that pieces of rock move, as broken pieces ofrock fall or tumble from high places to lower ones.Gravity causes large and small pieces to fall fromcliffs, as well as moving water in rivers andstreams from mountaintops to the ocean. Wind andglaciers also move pieces of rock from one placeto another. Wind moves sand sized and smallerpieces of rock through the air. Glaciers can moveall sizes of particles, from extremely largeboulders to the tiniest fragments.

Weathering happens at the Earth’s surface.When most rocks form, they are forming at veryhigh temperatures and pressures. This is a verydifferent environment than the low temperaturesand pressures at Earth’s surface. When rocks reachEarth’s surface, weathering causes them to changeform. The new form will include minerals that arestable at the low temperatures and pressures of

Earth’s surface. So while powerful forces onEarth, such as those resulting from plate tectonics,work to build huge mountains like the Himalayasor majestic volcanoes like Mt Fuji, the forces ofweathering gradually wear away rocks, changingonce tall mountains into hills and even plains. TheAppalachian Mountains along the east coast ofNorth America were once as tall as the Himalayas!So what happened?

No human being can watch for millions ofyears as mountains are slowly built, nor can wewatch as those same mountains gradually wearaway. However you probably have been able toride your bike or walk along a brand new sidewalkor road. What do you experience? The new road orsidewalk is smooth and even. If it was made well,there won’t be any cracks or bumps. Does thatsmooth surface stay that way? Certainly overmillions of years, it will completely disappear, butwe don’t have to wait that long. If you live in apart of the world that has cold winters, you mayonly have to wait one year to start seeing changes.We will talk next about what types of weathering

change that brand new, smooth and even sidewalkinto areas that are rough or cracked, chipped orbuckled (Figure below).

Figure 9.1 You can see the once smooth road surface has

cracks and fractures, plus a large pothole.

Mechanical Weathering

Mechanical weathering (also called physicalweathering) is the breaking of rock into smallerpieces. These smaller pieces will be just like thebigger rock, the pieces will just be smaller. Thatmeans the rock has been changed mechanically (orphysically) without changing its composition. The

smaller pieces will have the same minerals, in justthe same proportions as the original rock. Youcould actually use the expression, ‘A chip off theold block’ to describe mechanical weathering! Themain agents of mechanical weathering are water,wind, ice, and gravity. You will see how each ofthese works to break rock into smaller pieces.

There are two main ways that rocks can breakapart into smaller pieces. The way that is mostcommon in cold climates is called ice wedging.Ice wedging is the main form of mechanicalweathering in any climate that regularly cyclesabove and below the freezing point (Figurebelow). Some places where this happens includeEarth’s polar regions and mid latitudes. It alsohappens in the colder climates of higherelevations, like mountainous regions.

Figure 9.2 Water seeps into cracks and fractures in rock.

As it freezes, it expands which wedges the rockapart.

This is how it works. When water changesfrom a liquid into a solid (ice), it increases involume. This is a very unusual property. Mostsubstances contract (get smaller) as they changefrom a liquid to a solid, but water does just theopposite. You may have already experienced thisif you ever filled an ice cube tray all the way to thetop with water and then put it into the freezer. Theice cubes will be much larger than the amount ofwater you first put in. You may have also made themistake of putting your favorite soda into thefreezer to cool it down quickly. If you leave yourdrink in the freezer too long, it will expand somuch that it bends or pops the can. Ice wedginghappens for the same reason. Water works its wayinto cracks and fractures in rock, and then expandsas that water freezes. The ice takes up more spacethan the water did, which wedges the rock apart,physically breaking the rock into pieces. Icewedging breaks apart so much rock that you willfind large piles of broken rock at the base of a cliff

or mountain, as broken pieces separate and tumbledown its sides. Ice wedging will work quickly,breaking apart lots of rock in areas that go aboveand below the freezing point every night and day,and also in areas that cycle with the seasons.

Abrasion is another form of mechanicalweathering. Abrasion can happen anywhere. Allthat is needed is one rock bumping against anotherrock. Gravity can cause abrasion as a rock tumblesdown a mountainside or cliff. Moving watercauses abrasion as particles carried in the watercollide and bump against one another. Strongwinds can pick up pieces of sand and blastsurfaces with those sand grains. Finally, the ice inglaciers carries many bits and pieces of rock. Asthe glacier moves, pieces of rock embedded in theice scrape against the rocks below. Broken piecesof rock tumbling down a mountain stream or tossedabout by waves crashing onto the shore, willbecome smooth and rounded as abrasions smoothand round the sharp or jagged edges. If you haveever collected beach glass or cobbles from astream, you have benefited from the work of

abrasion.Scientists talk about a few other types of

mechanical weathering but ice wedging andabrasion are the two most important types. Withoutthese two types of mechanical weathering, verylittle rock would break apart and that would slowdown the rate of chemical weathering as well.Sometimes biological elements can do the work ofmechanical weathering. This could happen slowlyas a plant’s roots grow into a crack or fracture inrock and gradually grow larger, wedging open thecrack. Burrowing animals can also break apartrock as they dig for food or to make living spacesfor themselves. Today, of course, human beings doquite a bit of mechanical weathering, whenever wedig or blast into rock to build homes, roads,subways, or to quarry stone for construction orother uses.

Actually whenever there is mechanicalweathering, it increases the rate of chemicalweathering. This happens because as rock breaksinto smaller pieces, the surface area of the piecesincreases (Figure below). With more surfaces

exposed, there are more places for chemicalweathering to occur. Let’s say you wanted to makesome hot chocolate on a cold day. You can imaginehow hard it would be to get a big chunk ofchocolate to dissolve in your milk or hot water.Maybe you could make hot chocolate from somesmaller pieces like chocolate chips, but it is mucheasier to add a powder to your milk. This isbecause the smaller the pieces are, the moresurface area they have and the easier it is todissolve in the milk.

Figure 9.3 As rock breaks into smaller pieces, overall

surface area increases.

Salt weathering of building stone on the islandof Gozo, Malta.

Chemical Weathering

Another important type of weathering thathappens on the Earth’s surface is chemicalweathering. Chemical weathering is different thanmechanical weathering because with this type ofweathering, rock is changed, not just in size ofpieces, but changed in composition. This meansthat one type of mineral changes into a differentmineral. The reason chemical weathering happensis that most minerals form at high pressure or hightemperatures, deep within the Earth. When rocks

reach the Earth’s surface, they are now at very lowtemperatures and pressures. This is a very differentenvironment from the one in which they formed.The environment at Earth’s surface is so differentthat these minerals are no longer stable. That’swhere chemical weathering begins. Mineralsformed deep within the Earth must change tominerals that are stable at Earth’s surface.Chemical weathering is important because it startsthe process of changing solid rock into the soil weneed to grow food and for the plants we need forour clothing and medicine. The way that chemicalweathering works is through chemical reactionsthat cause changes in the rock.

There are many types of chemical weatheringbecause there are many agents of chemicalweathering. You probably remember thatmechanical weathering is caused by severalagents, such as water, wind, ice,and gravity. Well,water is also an agent of chemical weathering, sothat makes it a double agent! Two other importantagents of chemical weathering are carbon dioxideand oxygen. We will talk about each of these one at

a time.The minerals that make up most of the Earth’s

crust are called silicate minerals. These mineralsare mostly made of just eight elements; oxygen (O),silicon (Si), aluminum (Al), iron (Fe), magnesium(Mg), calcium (Ca), potassium (K) and sodium(Na). When chemical weathering occurs, theelements that make up the minerals react to formnew minerals. The minerals that form at the lowesttemperatures and pressures (closest to the situationat the Earth’s surface) are the most stable whileminerals that form from very hot magmas or at veryhigh pressures are the least stable. The elementssodium, calcium, potassium and magnesiumactually dissolve easily in water. Iron reacts withoxygen, which leaves atoms of silicon, oxygen andaluminum to combine to form new minerals, likeclay minerals.

Water is an amazing molecule. It has a verysimple chemical formula, H2O, which means it ismade of just two hydrogen atoms bonded to oneoxygen atom. Even though it is simple toremember, water is pretty remarkable in terms of

all the things it can do. Water is an excellentsolvent. The way that a water molecule joinstogether allows water to attract lots of otherelements, separate them from their compounds anddissolve them. Water is such a good solvent thatsome types of rock can actually completelydissolve in water. Other minerals change byadding water into their structure.

Hydrolysis is a chemical reaction between amineral and water. When this reaction takes place,water itself separates into ions. These ions grabonto other ions, dissolving them in water. As thedissolved elements are carried away, we say thatthese elements have been leached. Throughhydrolysis, a mineral like potassium feldspar ischanged into a clay mineral. Once clay mineralshave formed, they are stable at the Earth’s surface.

Carbon dioxide (CO2) combines with water asraindrops fall through the air in our atmosphere.This makes a weak acid, called carbonic acid.This happens so often that carbonic acid is a verycommon, weak acid found in nature. This acidworks to dissolve rock. It also slowly changes the

paint on a new car or eats away at sculptures andmonuments. The normal situation can be madeworse when we add pollutants to the air. Any timewe burn any fossil fuel, it adds nitrous oxide to theair. When we burn coal rich in sulfur, it adds sulfurdioxide to the air. As nitrous oxide and sulfurdioxide react with water, it forms nitric acid andsulfuric acid. These are the two main componentsof acid rain. Acid rain accelerates chemicalweathering.

Oxidation is the type of chemical reaction thathappens when oxygen reacts with elements at theEarth’s surface. Oxygen is very stronglychemically reactive. The type of oxidation that youare probably most familiar with produces rustwhen iron reacts with oxygen (Figure below).

Many minerals are rich in iron. They break downas the iron oxidizes, forming new compounds. Ironoxide produces the red color in soils. Chemicalweathering can also be contributed to by plants andanimals. As plant roots take in soluble ions asnutrients, certain elements are exchanged. Plantroots and bacterial decay use carbon dioxide in theprocess of respiration.

Figure 9.4 When iron rich minerals oxidize, they produce

the familiar red color found in rust.

Differential Weathering

Rates of weathering depend on several factors.Different types of rocks weather at different rates.Certain types of rock, like granite, are veryresistant to weathering. Igneous rocks tend toweather slowly because it is hard for water topenetrate them. Other types of rock, like limestoneand marble are easily weathered because theydissolve easily in weak acids. More resistantrocks remain at the surface and form ridges orhills. Devil’s Tower in Wyoming is an interestingexample of how different types of rock weather atdifferent rates (Figure below). As the softermaterials of the surrounding rocks were wornaway, the resistant center of the volcano remainedbehind. Different minerals also weather at differentrates. Some minerals completely dissolve in water.As less resistant minerals dissolve away, a rock’ssurface becomes pitted and rough. When a lessresistant mineral dissolves, more resistant mineralgrains are released from the rock.

Figure 9.5 Devils Tower is an amazing example of

differential weathering. All that remains of thevolcano today is this central plug of resistant lavathat forms the tower.

Most importantly, the climate of a regioninfluences weathering. Climate is determined bythe temperature of a region plus the amount ofrainfall it receives. As the amount of precipitationincreases, so does the rate of solution and thenumber of chemical reactions. In general, as theamount of rainfall increases, so does the degree ofweathering. Remember that water is an agent ofboth mechanical and chemical weathering, so whenwater is not available, the rate of weathering

slows tremendously. Two amazing examples ofpreservation include mummification and freezing.Both of these situations occur in the absence ofliquid water. Therefore a dry climate will producethe lowest rate of weathering, followed by a verycold climate, regardless of the amount of rainfall itreceives. The rates of highest weathering wouldoccur in a wet climate that is also warm or hot. Asthe temperature of a region increases, so does therate of chemical reactions. For each 10oC increasein average temperature, the rate of chemicalreactions doubles. The warmer a climate is, themore types of vegetation it will have and thegreater the rate of biological weathering. Thishappens because plants and bacteria grow andmultiply faster in warmer temperatures. If you wantan easy way to remember these examples, thinkabout where you would put your sandwich if youwant it to stay fresh for a while. How quickly doesit go bad in your lunch box? Where would you putfood from the grocery store if you wanted to save itfor a week or more?

Some resources are actually concentrated for

us by the actions of weathering. In tropicalclimates, intense chemical weathering carriesaway all soluble minerals, leaving behind just theleast soluble components. The aluminum oxide,bauxite forms this way and is our main source ofore for producing aluminum. The actions of movingwater can also concentrate heavier minerals, likegold. This process fueled the gold rush out west inNorth America in the 1800’s.

Lesson Summary

Mechanical weathering breaks existing rockinto smaller pieces without changing thecomposition of the rock.Ice wedging and abrasion are two importantprocesses of mechanical weathering.The main agents of mechanical weathering aremoving water, wind, glacial ice and gravity.Chemical weathering decomposes or breaksdown existing rock, forming new mineralsthat are stable at the Earth's surface.

Water, carbon dioxide and oxygen areimportant agents of chemical weathering.Different types of rocks weather at differentrates. More resistant types of rocks willremain longer.

Review Questions

1. Name two types of mechanical weathering.Explain how each works to break apart rock.

2. What are three agents of chemicalweathering? Give an example of each.

3. What type of climate would likely producethe greatest degree of weathering? Explain.

4. Would a smooth even surface weather fasterthan an uneven, broken surface?

5. What type of rocks would be best suited tomaking monuments?

Vocabulary

abrasion

A form of mechanical weathering that occurswhenever one rock hits another.

chemical weatheringThe form of weathering which decomposesrock; minerals that form at high temperaturesand pressures change to minerals that arestable at the Earth’s surface.

erosionThe transport of weathered materials bywater, wind, ice or gravity.

hydrolysisChemical reactions between minerals andwater in which hydrogen or hydroxide ionsreplace the cations in the mineral.

ice wedgingThe form of mechanical weathering thatoccurs as water expands as it freezes,wedging apart rock.

leaching

The process of removing dissolved mineralsas they are carried to lower layers in soil.

mechanical weatheringThe form of weathering which disintegratesrock; bigger pieces of rock are broken intosmaller pieces of the same composition as theoriginal rock.

oxidationA form of chemical weathering in whichoxygen reacts with elements; happens whenan atom or ion loses an electron.

sedimentsBits and pieces of weathered rock; the largestpieces would be gravel or pebbles, then sand,silt, and clay sized particles.

Points to Consider

What other types of surfaces are affected byweathering other than rock?

What might the surface of the Earth look likeif weathering did not occur on Earth?Do you think that you would be alive today ifwater did not dissolve elements?Would the same composition of rock weatherthe same way in three very different climates?

Soils

Lesson Objectives

Discuss why soil is an important resource.Describe how soil forms from existing rocks.Describe the different textures andcomponents of soil.Draw and describe a soil profile.Define the three climate related soils: apedalfer, pedocal and laterite soil.

Characteristics and Importance ofSoil

Thank goodness for mechanical and chemicalweathering, because without these forces workingto breakdown rock we would not have any soil onEarth. It is unlikely that humans would have beenable to live on Earth without soil! Your life and thelives of many organisms depend on soil. We get

wood, paper, cotton, medicines and even purewater from soil. So soil is a very importantresource. Even though it is actually only a very thinlayer on Earth’s surface over the solid rocksbelow, it is the place where our atmosphere,hydrosphere, biosphere and the rocks of the Earthmeet. Within our soil layer, reactions betweensolid rock, liquid water and air take place. It is amistake on our part to disregard this importantresource, yet we say things like “soiled” or “dirty”when we talk about ruining something. Ourprecious soil resource needs to be carefullymanaged and cared for. If we neglect or abuse thesoil we have, it will not remain the renewableresource that we have relied on throughout humanexistence.

We can think about soil as a living resource oran ecosystem all by itself. Within soil, there aremany elements. It is a complex mixture of differentmaterials. Some of them are inorganic, like theproducts of weathered rock, including pebbles,sand, silt and clay particles. There are also bits oforganic materials, formed from the partial

breakdown and decomposition of plants andanimals. In general, the pieces of rock andminerals make up about half of the soil, with theother half made of organic materials. In between,in the spaces of soil, there are thousands or evenmillions of living organisms. Those organismscould be earthworms, ants, bacteria, or fungi, aswell as many other types of organisms. In betweenthe solid pieces, there are tiny spaces filled withair and water. In some soils, the organic portioncould be entirely missing, such as desert sand. Atthe other extreme, a soil could be completelyorganic, such as the materials that make up peat ina bog or swamp. The organic materials arenecessary for a soil to be fertile. The organicportion provides the nutrients, like nitrogen,needed for strong plant growth. We will learnabout that organic portion in just a bit.

Soil Formation

How well soil forms and what type of soil

forms depends on several different factors. Someof these factors are the climate, the original rockthe soil formed from, the slope, the amount of timeand biological activity. Climate is ultimately themost important factor and will determine the typeof soil that forms in a particular region. Theclimate of a region is principally determined bytemperature and the amount of precipitation. Thisalso influences the type of vegetation that grows inthe region. We can identify different climates bythe types of plants that grow there. Depending onhow closely you look, we can divide land areasinto many different climate regions (Figurebelow).

Figure 9.6 Climate is the most important factor in

determining the type of soil that will form in aparticular area.

Given enough time, even different rock typeswill produce a similar climate related soil.Climate is such an important factor that even thesame type of rock in different climates willproduce a different climate related soil. This istrue because the rocks on Earth are predominantlycomposed of eight elements and as rock breaksdown, there will mostly be just these eightelements. Surely, if an element is not present in theoriginal rock, then it will also not be present in thesoil that forms from it. The amount of precipitationin an area is important because it influences therate of weathering. The more it rains, the morerainwater passes through the soil and the more itreacts chemically with the particles. Thosereactions are most efficient in the top layers of thesoil and become less effective as the watercontinues to percolate through lower layers of soil.The top layers of soil in contact with the freshestwater react most. Increased rainfall in a regionincreases the amount of rock that is dissolved as

well as the amount of material carried away bymoving water. As materials are carried away, newsurfaces are exposed and this also increases therate of weathering.

The temperature for a region is important too.The rate of chemical reactions increases withhigher temperatures. For every 10o C increase intemperature, the rate of chemical reactionsdoubles. Warmer regions also have morevegetation because plants and bacteria grow andmultiply faster. This means that in tropical regions,where temperatures and amounts of rainfall areconsistently high, thick soils form with no unstableminerals and therefore no nutrients. Conversely,arid regions produce thin soils, rich in unstableminerals. The rate of soil formation increases withgreater amounts of time. The longer the amount oftime that soil remains in a particular area, thegreater the degree of alteration.

The original rock is the source of the inorganicportion of the soil. Chemical reactions fromweathering break down the rock’s originalminerals into sand, silt and clays. A soil is called a

residual soil when it forms in place, with theunderlying rock breaking down to form the layersof soil that reside above it. Only about one third ofthe soils in the United States form this way. Therest of the soils form from materials that have beentransported in from somewhere else. These soilsare called transported soils. Glaciers bring bits ofrock from far away, depositing the materials theycarried as the ice of the glacier melts. Wind andrivers also transport materials from their places oforigin. These soils form from the loose particlesthat have been transported in to a new location anddeposited. For transported materials, the rate ofsoil formation is faster because the transportedmaterials have already been weathered. The closerthe materials are to their place of origin, thegreater the influence of the original materials. Thefurther those materials move from their origin, thegreater the degree of weathering and the influenceof the original materials becomes obscured.

Soils thicken as the amount of time availablefor weathering increases. The warmer thetemperatures, the more rainfall and greater amount

of time, the thicker the soils will become.Biological activity produces the organic materialand nutrients in soil. The partial decay of plantmaterial and animal remains also forms organicacids which in turn increase the rate of soilformation and the rate of weathering. The organicmaterial increases the ability of the soil to holdwater, create a soil’s structure and enhance itsfertility and ability to be cultivated.

The decayed remains of plant and animal lifeare called humus. Humus is an extremelyimportant part of the soil. It coats the mineralgrains, binding them together into clumps that thenhold the soil together. The humus in soil alsoincreases the porosity and water holding capacityof a soil. Humus helps to buffer rapid changes insoil acidity and helps the soil to hold its nutrients.Decomposing organisms in the soil breakdown thecomplex organic molecules of plant matter andanimal remains to form simpler inorganicmolecules that are soluble in water. Bacteria in thesoil change atmospheric nitrogen into nitrates.

One indicator of a soil’s fertility is its color.

Soils that are rich in nitrogen and contain a highpercentage of organic materials are usually blackor dark brown in color. Soils that are nitrogenpoor and low in organic material might be gray oryellow or even red in color.

Soil Texture and Composition

The inorganic portion of soil is made of manydifferent size particles. In addition to many particlesizes, there can be different proportions of eachparticle size. The combination of these two factorsdetermines some of the properties of the soil. Asoil will be very permeable, which means thatwater will flow through it easily, if the spacesbetween the inorganic particles are large enoughand are well connected. Soils that have lots of verysmall spaces tend to be water holding soils. Claysare an example of a type of soil that holds water.When clay is present in a soil, the soil is heavierand holds together more tightly. Sandy or siltysoils are considered ‘light’ soils because they are

permeable, water draining types of soils. When asoil contains a mixture of grain sizes, the soil iscalled a loam. When soil scientists want toprecisely determine the soil type, they measure thepercentage of sand, silt and clay and plot thisinformation on a triangular diagram, with each typeof soil at one corner (Figure below).

Figure 9.7 Soil texture triangular plot diagram.Soil scientists use a diagram like this to plot

the percentages of sand, silt & clay in a soil. Thesoil type can then be determined from the locationon the diagram. At the top, a soil would be a clay;at the left corner, it would be a sand and at the

right corner it would be a silt. Regions in thelower middle with less than 50% clay are calledloams.

Soil Horizons and Profiles

A residual soil forms from the underlyingbedrock. This happens over many years, asmechanical and chemical weathering slowlychange solid rock into soil. The more timeavailable, the greater the degree of alteration thatwill occur. Perhaps the first changes to bare rockwould be cracks or fractures due to mechanicalweathering from ice wedging. Then plants likelichens or grasses become established. As moreand more layers of material weather, the soildevelops soil horizons, as each layer becomesprogressively altered. The place where the greatestdegree of weathering occurs is the top layer. Eachsuccessive, lower layer is altered just a little bitless. This is because the first place where waterand air come in contact with the soil is at the top.

As water moves down through the layers, it is ableto do less work to change the soil. If you were ableto dig a deep hole into the ground, you could seeeach of the different layers of soil. All together,these are called a soil profile. Each horizon has itsown particular set of characteristics (Figurebelow).

Figure 9.8 Soil is an important resource. Each soil

horizon is distinctly visible in this photograph.In the simplest soil profile, a soil is considered

to have three horizons. The first horizon is the topsoil, which is called the A horizon. The topsoilwill usually be the darkest layer of the soil,

because this is the layer with the highestproportion of organic material. Remember thathumus forms from all the plant and animal debristhat falls to the ground. This includes branches andtwigs, acorns and pine needles as well as wastefrom animals and fungi. The top soil is the regionof most intense biological activity. Many livingorganisms live within this layer and plants stretchtheir roots down into this layer. In fact, plant rootsare very important to this layer because vegetationhelps to hold this layer of soil in place. The topsoil layer is usually a layer where minerals thatcan dissolve and very small particles like clay areabsent. This is because clay sized particles getcarried to lower layers as water seeps down intothe ground. Soluble minerals are missing becausethey readily dissolve in the fresh water that movesthrough this layer and are carried down to lowerlayers of the soil.

The next soil horizon is the subsoil, which iscalled the B horizon. This is the region wheresoluble minerals and clays accumulate. This layerwill be lighter brown in color and more water

holding than the top soil, due to the presence ofiron and clay minerals. There will be less organicmaterial in this layer. The next layer down, calledthe C horizon will be a layer of partially alteredbedrock. There will be some evidence ofweathering in this layer, but pieces of the originalrock can still be seen and it would be possible toidentify the original type of rock from which thissoil formed (Figure below).

Figure 9.9 A soil profile is the complete set of soil layers.

Each layer is called a horizon.Not all climate regions develop soils, and not

all regions develop the same horizons. Some areasdevelop as many as five or six distinct layers,while others develop only very thin soils orperhaps soil doesn’t form well at all.

Types of Soils

If we were to talk to soil scientists, you wouldlearn that there are thousands of types of soil. Soilscientists study each of the many differentcharacteristics of each soil and put them into veryspecific groups and have many different names forsoils. Let’s consider a much simpler model thatconsiders just three types of soil. This will helpyou to understand some of the basic ideas abouthow the particular climate of an area produces acertain type of soil, but there are many exceptionsto what we will learn right now.

Let’s consider the type of soil that would formin a region of the world where there are forests oftrees that lose their leaves each winter, calleddeciduous trees. In order for trees to grow here,there needs to be lots of rain, at least 65 cm ofrainfall per year. Wherever there are trees, there isenough rain to help them grow! The type of soilthat forms in a forested area is called a pedalferand this type of soil is common in many areas ofthe temperate, eastern part of the United States(Figure below). The word pedalfer comes fromsome of the elements that are commonly found inthe soil. The element aluminum has the chemicalsymbol Al and the element iron has the chemicalsymbol Fe. These two symbols are combined ‘-al’and ‘-fe’ to make the word ped –al –fe r. This typeof soil is usually a very fertile, dark brown orblack soil. It is rich in aluminum clays and ironoxides. Because it rains often in this type ofclimate, most of the soluble minerals dissolve andare carried away, leaving the less soluble claysand iron oxides behind.

Figure 9.10 A pedalfer is the dark, fertile type of soil that

will form in a forested region.Another type of climate related soil, called a

pedocal, forms in drier temperate areas wheregrasslands and brush are the usual types ofvegetation (Figure below). It rains less than 65 cmper year in these areas, so there is less chemicalweathering for these soils. With lower amounts ofrainfall, there is less water to dissolve awaysoluble minerals, so more soluble minerals arepresent here but fewer clay minerals are produced.With lower rainfall, there is also less vegetationhere, so the soils have lower amounts of organicmaterial, making them slightly less fertile types of

soils than a pedalfer. A pedo cal is named for thecalcite enriched layer that forms. Some waterbegins to move down through the soil layers, butbefore it gets very far, it begins to evaporate away.Soluble minerals, like calcium carbonate,concentrate in a layer that marks the lowest placethat water was able to reach before it evaporatedaway. This layer is called caliche.

Figure 9.11 A pedocal is the alkaline type of soil that forms

in grassland regionsA third type of soil called a laterite forms in

tropical areas, where rainfall is so intense that itliterally rains every day. The tropical rainforest isan example of this type of region (Figure below).

In these hot, wet tropical regions nutrient poorsoils form due to intense chemical weathering. Somuch weathering happens here that there ispractically no humus. All soluble minerals areremoved from the soil and all plant nutrients getleached or carried away. What are left behind arethe least soluble materials like aluminum and ironoxides. These soils are often red in color from theiron oxides. These soils bake as hard as a brick ifthey are set out in the sun to dry.

Figure 9.12 A laterite is the type of thick, nutrient poor soil

that forms in the rainforest.You can probably very quickly name many

climates that have not been mentioned here. Eachclimate will produce a distinctive soil that formsin the particular circumstances found there. Wherethere is less weathering, soils are thinner butsoluble minerals may be present. Where there isintense weathering, soils may be thick but nutrientpoor. In any case, soil development takes a verylong time. It may take hundreds or even thousandsof years to form a good fertile top soil. Soilscientists estimate that in the very best soil formingconditions, soil forms at a rate of about 1mm/year.In poor conditions it may take thousands of years!

Soil Conservation

Soil is only a renewable resource if wecarefully manage the ways in which we use soil.There are natural cycles of unfortunate events like

drought or insect plagues or outbreaks of diseasethat negatively impact ecosystems and also harmthe soil. But there are also many ways in whichhumans neglect or abuse this important resource.One harmful practice is removing the vegetationthat helps to hold soil in place. Sometimes justwalking or riding your bike over the same place,will kill the grass that normally grows there. Othertimes land is deliberately cleared to make way forsome other use. The ‘lost’ soils may be carriedaway by wind or running water. In many areas ofthe world, the rate of soil erosion is many timesgreater than the rate at which it is forming. Soilscan also be contaminated if too much saltaccumulates in the soil or where pollutants sinkinto the ground.

Figure 9.13 Organic material can be added to soil to help

increase its fertility.There are many ways that we can protect and

preserve our precious resources of soil. There aremany ways to help to keep soil in good condition.Adding organic material to the soil in the form ofplant or animal waste, like manure or compost,increases the fertility of the soil as well asimproving its ability to hold onto water and

nutrients (Figure above). Inorganic fertilizer canalso temporarily increase the fertility of a soil andmay be less expensive or time consuming, butwon’t provide the same long term improvements asorganic materials. Agricultural practices likerotating crops, alternating the types of cropsplanted in each row and planting nutrient richcover crops all help to keep soil more fertile as itis used season after season. Planting trees aswindbreaks, plowing along contours of the field orbuilding terraces into steeper slopes will all helpto hold soil in place (Figure below). No till orlow tillage farming helps to keep soil in place bydisturbing the ground as little as possible whenplanting.

Figure 9.14 Steep slopes can be terraced to make level

planting areas and decrease surface water runoffand erosion.

Lesson Summary

Soil is an important resource. Life on Earthcould not exist as it does today without soil.The type of soil that forms depends mostly onclimate but to a lesser extent on originalparent rock material.Soil texture and composition plus the amountof organic material in a soil determine a soil'squalities and fertility.Given enough time, existing rock willproduce layers within the soil, called a soilprofile.Ultimately, the climate of a particular regionwill produce a unique type of soil for thatclimate.

Review Questions

1. Describe at least two ways in which soil is aliving resource.

2. Name two factors that influence soilformation.

3. Which region of a soil profiles reacts themost?

4. Is the soil in your back yard most likely aresidual soil or a transported soil? Howcould you check?

5. Name several advantages to adding humus tothe soil.

6. What are three soil horizons? Describe thecharacteristics of each.

7. Name three climate related soils. Describethe climate and vegetation that occurs in thearea where each forms.

8. Where would you choose to buy land for afarm if you wanted fertile soil and did notwant to have to irrigate your crops?

Vocabulary

deciduous treesTrees that lose their leaves once a year.

humusThe partially decayed remains of plants andanimals; forms the organic portion of soil.

inorganicParts of the soil which do not come fromliving organisms; the rock and mineral portionof the soil.

lateriteNutrient poor, red, tropical soil which formsin a region with rainforest vegetation.

loamSoil texture which forms from a roughly equalcombination of sand, silt and clay.

organic

Generally considered to mean components ofthe soil which come from living organisms.

pedalferFertile, dark soil which forms in mid latitude,forested regions.

pedocalSlightly less fertile soils which forms indrier, grassland regions.

permeableDescribes a type of soil which allows waterto move through it easily.

residual soilSoil that forms from the bedrock upon whichit resides or lies.

soil horizonAn individual layer of a complete soilprofile; examples include A, B & C horizons.

soil profile

The entire set of soil layers or horizons for aparticular soil.

subsoilThe B horizon of a soil; the zone where ironoxides and clay minerals accumulate.

topsoilThe A horizon of a soil; most fertile layer ofsoil where humus, plant roots & livingorganisms are found.

transported soilA soil formed from weathered componentstransported by water, wind or ice to adifferent area.

Points to Consider

Why is soil such an important resource?Do you think a mature soil would form fasterfrom unaltered bedrock or from transportedmaterials?

If soil erosion is happening at a greater ratethan new soil can form, what will eventuallyhappen to the soil in that region?Do you think there are pollutants that couldnot easily be removed from soil?

Chapter 10: Erosion andDeposition

Water Erosion andDeposition

Lesson Objectives

Describe how surface rivers and streamsproduce erosion.Describe the types of deposits left behind byrivers and streams.Describe landforms that are produced asgroundwater flows.

Introduction

Rivers and streams complete the hydrologiccycle by returning precipitation that falls on land to

the oceans (Figure below). Ultimately, gravity isthe driving force, as water moves frommountainous regions to sea level. Some of thiswater moves over the surface and some movesthrough the ground as groundwater. As this waterflows it does the work of both erosion anddeposition. You will learn about the erosionaleffects and the deposits that form as a result of thismoving water.

Figure 10.1 As rivers and streams move towards the ocean,

they carry weathered materials.

Erosion and Deposition by Riversand Streams

Erosion from Runoff

As streams move over the ground, theytransport weathered materials. Streams continuallyerode material away from their banks, especiallyalong the outside curves of meanders. Some ofthese materials are carried in solution. Manyminerals are ionic compounds that dissolve easilyin water, so water moves these elements to the seaas part of the dissolved load that the streamcarries. As groundwater leaches through layers ofsoil and rock, minerals dissolve and are carriedaway. Groundwater contributes most of thedissolved components that streams carry. Once anelement has completely dissolved, it will likely becarried to the ocean, regardless of the velocity ofthe stream. In some circumstances, the streamwater could become saturated with dissolvedmaterials, in which case elements of thoseminerals might precipitate out of the water beforethey reach the ocean.

Another way that rivers and streams moveweathered materials is as the suspended load.

These are pieces of rock that are carried as solidsas the river flows. Unlike dissolved load, the sizeof the particle that can be carried as suspendedload is determined by the velocity of the stream.As a stream flows faster, it can carry larger andlarger particles. The larger the size particle thatcan be carried by a stream, the greater the stream’scompetence. If a stream has a steep slope orgradient, it will have a faster velocity, whichmeans it will be able to carry larger materials insuspension. At flood stage, rivers flow much fasterand do more erosion because the added waterincreases the stream’s velocity. Sand, silt and claysize particles generally make up the suspendedload for a stream (Figure below). As a streamslows down, either because the stream’s slopedecreases or because the stream overflows itsbanks and broadens its channel, the stream willdeposit the largest particles it has been carryingfirst.

Figure 10.2 Rivers carry sand, silt and clay as suspended

load. During flood stage, the suspended loadgreatly increases as stream velocity increases.

The last way that rivers and streams moveweathered materials is as bed load. This meansthat although the water in the stream is capable ofbumping and pushing these particles along, it is notable to pick them up and carry them continuously.Bed load is named for the fact that these particlesget nudged and rolled along the stream bed as thewater flows. Occasionally a larger size particlewill get knocked into the main part of the streamflow, but then it settles back down to the streambed because it is too heavy to remain suspended in

the water. This is called saltation, which we willlearn about later in this chapter with transport ofparticles by wind. Streams with high velocitiesand steep gradients do a great deal of downcuttinginto the stream bed, which is primarilyaccomplished by movement of particles that makeup the bed load. Particles that move along as thebed load of a stream do not move continuouslyalong, but rather in small steps or jumps withperiods of remaining stationary in between.

Stream and River Erosion – Stages of Streams

As a stream moves water from high elevations,like mountains, towards low elevations, like theocean, which is at sea level, the work of the streamchanges. At high elevations, streams are justbeginning streams that have small channels andsteep gradients. This means that the stream willhave a high velocity and will do lots of workeroding its stream bed. The higher the elevation,the farther the stream is from where it eventually

meets the sea. Base level is the term for where astream meets sea level or standing water, like alake or the ocean. Streams will work to downcuttheir stream beds until they reach base level.

As a stream moves out of high mountainousareas into lower areas closer to sea level, thestream is closer to its base level and does morework eroding the edges of its banks thandowncutting into its stream bed. At some point inmost streams, there are curves or bends in thestream channel called meanders (Figure below).The stream erodes material along its outer banksand deposits material along the inside curves of ameander as it flows to the ocean (Figure below).This causes these meanders to migrate laterallyover time. The erosion of the outside edge of thestream’s banks begins the work of carving afloodplain, which is a flat level area surroundingthe stream channel.

Figure 10.3 Here a stream can be seen actively eroding its

outer banks along a meander.

Figure 10.4 This stream has deposited larger materials like

gravel and pebbles along the inside curve of a

meander.

Stream and River Deposition

Once a stream nears the ocean, it is very closeto its base level and now deposits more materialsthan it erodes. As you just learned, one placewhere a river deposits material is along the insideedges of meanders. If you ever decide to pan forgold or look for artifacts from an older town orcivilization, you will sift through these deposits.Gold is one of the densest elements on Earth.Streams are lazy and never want to carry morematerials than absolutely necessary. It will dropoff the heaviest and largest particles first, that iswhy you might find gold in a stream deposit.Imagine that you had to carry all that you wouldneed for a week as you walked many kilometers.At first you might not mind the weight of what youare carrying at all, but as you get tired, you willlook to drop off the heaviest things you arecarrying first!

When a river floods or overflows its channel,

the area where the stream flows is suddenly muchbroader and shallower than it was when it was inits channel. This slows down the velocity of thestream’s flow and causes the stream to drop offmuch of its load. The farmers who use thefloodplain areas around the Nile River rely onthese deposits to supply nutrients to their fieldseach year as the river floods its banks. At floodstage, a river will also build natural levees as thelargest size particles build a higher area around theedges of the stream channel (Figure below).

Figure 10.5 After many floods, a stream builds natural

levees along its banks.When a river meets either standing water or

nearly flat lying ground, it will deposit its load. Ifthis happens in water, a river may form a delta.From its headwaters in the mountains, along ajourney of many kilometers, rivers carry theeroded materials that form their stream load.Suddenly the river slows down tremendously in

velocity, and drops the tremendous load ofsediments it has been carrying. Deltas arerelatively flat topped, often triangular shapeddeposits of sediments that form where a large rivermeets the ocean. The name delta comes from thecapital Greek letter delta, which is a triangle, eventhough not all deltas have this shape. A triangularshaped delta forms as the main stream channelsplits into many smaller distributaries. As thechannel shifts back and forth dropping offsediments and moving to a new channel location awide triangular deposit forms.

There are three types of beds that make up adelta (Figure below). The first particles to bedropped off are the coarsest sediments and theseform sloped layers called foreset beds that makeup the front edge of the delta. Further out intocalmer water, lighter, more fine grained sedimentsform thin, horizontal layers. These are calledbottomset beds. During floodstage, the whole deltacan be covered by finer sediments which willoverlie the existing delta. These are called topsetbeds. These form last and lie on top of the rest of

the delta.

Figure 10.6 The three types of beds that form the layers of a

delta.Not all large rivers form deltas as they meet

the ocean. Whether a delta forms depends on theaction of waves and tides. If the water is quietwater such as a gulf or shallow sea, a delta mayform. If the sediments are carried away, then nodelta will form. Sediments brought to the shore anddistributed along coastlines by longshore transportform our beaches and barrier islands.

If a river or stream suddenly reaches nearly flatground, like a broad flat valley or plain, an alluvialfan develops at the base of the slope (Figurebelow). An alluvial fan is a curved top, fan shapeddeposit of coarse sediments that drop off as thestream suddenly loses velocity. The fan spreadsout in a curve in the direction of the flat land as

many stream channels move across the curvedsurface of the alluvial fan, forming and unformingmany channels as sediments are deposited.Alluvial fans generally form in more arid regions.

Figure 10.7 This satellite photo of an alluvial fan in Iran

show the typical fan shape of these deposits. Themountains in this image are in the lower rightcorner of the photograph.

Groundwater Erosion andDeposition

Introduction

Not all water that falls on the land flowsthrough rivers and streams. When it rains, much ofthe water sinks into the ground and moves throughpore spaces in soil and cracks and fractures inrock. This water necessarily moves slowly, mostlyunder the influence of gravity. Yet groundwater isstill a strong erosional force, as this water worksto dissolve away solid rock. If you have everexplored a cave or seen a sinkhole, you have someexperience with the work of groundwater (Figurebelow).

Figure 10.8 Groundwater forms when water sinks into the

ground rather than forming rivers or streams.

Formation of Caves

As groundwater moves through spaces betweenmineral grains, it works to dissolve and carryaway different elements. Some types of mineralsare easily dissolved by groundwater. Rainwaterabsorbs carbon dioxide (CO2) as it falls throughthe air. The carbon dioxide combines with water toform carbonic acid. This naturally occurring weakacid readily dissolves many types of rock,including limestone. If you have ever watched anantacid tablet dissolve in water, you have seen anexample of just how quickly this type of rock iseroded away. Caves are one of nature’s mostspectacular demonstrations of erosion (Figurebelow). Working slowly over many years,groundwater dissolves and carries away elementsof once solid rock in solution. First it travels alongsmall cracks and fractures, gradually enlargingthem. In time, caverns many football fields longand as high as many meters tall can form.

Figure 10.9 Caves form where groundwater erodes away

rock.A sinkhole could form if the roof of an

underground cave collapses. Some sinkholes arelarge enough to swallow up a home or severalhomes in a neighborhood (Figure below). Asgroundwater dissolves away solid rock, it carries

those minerals in solution as it travels. Asgroundwater drips through openings, severalinteresting types of formations occur. Stalactitesare icicle like deposits of calcium carbonate whichform as layer on layer of calcite drips from theceiling, coating the ‘icicle’ (Figure below). Asmineral rich material drips to the floor of a cave,stalagmites form rounded deposits of calciumcarbonate on the floor of the cave. The wordstalactite has a ‘C,’ so you can remember it formsfrom the ceiling, while the ‘G’ in stalagmitereminds you it forms on the ground. If a stalactiteand stalagmite join together, they form a column.One of the wonders of visiting a cave is to witnessthe beauty of these amazing and strangelycaptivating structures. Caves also produce abeautiful type of rock, formed from calciumcarbonate called travertine. This happens whengroundwater saturated with calcium carbonatesuddenly precipitates out as the mineral calcite oraragonite. Mineral springs that produce travertinecan be hot springs or the water may just be warmor could even be cold (Figure below).

Figure 10.10 This sinkhole formed in Florida.

Figure 10.11 Stalactites form as calcium carbonate drips

from the ceiling of a cave, forming beautiful iciclelike formations.

Figure 10.12 Travertine is a beautiful form of limestone that

forms as calcium carbonate precipitates.When lots of calcium carbonate is carried by

groundwater, we call the water ‘hard.’ If the waterin your area is hard, it might be difficult to get soapto lather or make soapsuds. Hard water might alsohave a taste to it, perhaps one that some peopledon’t like as much as pure water. If your water is‘hard,’ you may treat your water with a filterbefore you drink it. Zeolites are minerals that helpto absorb ions from the water as it passes throughthe filter. When the water passes through the filter,it comes out tasting good!

Lesson Summary

Rivers and streams erode the land as theymove from higher elevations to the sea.Eroded materials can be carried in a river asdissolved load, suspended load or bed load.A river will deeply erode the land when it isfar from its base level, the elevation where itenters standing water like the ocean.As a river develops bends, called meanders,it forms a broad, flat area known as afloodplain.At the end of a stream, a delta or an alluvialfan might form where the river drops off muchof the load of sediments it carries.Caves form underground as groundwatergradually dissolves away rock.

Review Questions

1. What are the three kinds of load that make upthe particles a stream carries. Name and

define each type.2. What is a stream’s gradient? What effect does

it have on the work of a stream?3. Describe several erosional areas produced by

streams. Explain why erosion occurs here.4. What type of gradient or slope would a river

have when it is actively eroding its streambed? Explain.

5. When would a river form an alluvial fan andwhen will it form a delta? Describe thecharacteristics of each type of deposit.

6. What are two formations that form insidecaves?

7. What erosional feature formed bygroundwater could swallow up your house?

Vocabulary

alluvial fanCurved top, fan shaped deposit of coarsesediments that forms when a stream suddenlymeets flat ground.

base levelThe elevation at which a river meets standingwater; a stream cannot erode below thislevel.

bed loadThe largest particles moved by streams; moveby rolling or bumping along the stream bed.

competenceA measure of the largest particle a stream cancarry.

deltaA flat topped, triangular shaped deposit ofsediments that forms where a river meetsstanding water.

dissolved loadThe elements carried in solution by a stream.

distributariesSmaller branching channels that spread outover the surface of a delta.

floodplainBroad, flat lying plain surrounding a streamchannel; created by the stream.

gradientThe slope of a stream.

groundwaterWater that moves through pore spaces andfractures in soil and rock.

meanderA bend or curve in a stream channel.

natural leveesCoarse grained deposits of sediments thatbuild up along a stream’s banks as it floods.

saltationThe intermittent movement of bed loadparticles, as they are carried by the flow andthen settle back down.

sinkholeCircular hole in the ground that forms as theroof of a cave collapses.

stalactiteAn icicle like formation of calcium carbonatethat forms as saturated water drips from theceiling of a cave.

stalagmiteRounded, cone shaped formation of calciumcarbonate that forms in caves as water dripsonto the floor.

suspended loadSolid particles that are carried in the mainstream flow.

travertineBeautiful deposit of calcium carbonate thatforms around hot springs.

Points to Consider

Would a stream that begins at high elevationbe likely to do more erosion than a stream thatbegins at lower elevations?What differences would there be on theEarth’s surface without rivers and streams?Do you think a flash flood along a normallydry river valley would be a dangerous event?Do you think caves could form in yourneighborhood?

Wave Erosion andDeposition

Lesson Objectives

Describe how the action of waves producesdifferent shoreline features.Discuss how areas of quiet water producedeposits of sand and sediment.Discuss how some of the structures humansbuild to help defend against wave erosion.

Wave Action and Erosion

Have you ever been to visit a beach? Somebeaches have large, strong rolling waves that riseup and collapse as they crash into the shore. Allwaves are energy traveling through some type ofmaterial (Figure below). The waves that we aremost familiar with travel through water. Most of

these waves form from wind blowing over thewater; sometimes steady winds that blow andsometimes from a storm that forms over the water.The energy of waves does the work of erosionwhen a wave reaches the shore. When you find apiece of frosted glass along a beach, you havefound some evidence of the work of waves. Whatother evidence might you find?

Figure 10.13 Ocean waves are energy traveling through

water.As wind blows over the surface of the water, it

disturbs the water, producing the familiar shape ofa wave. You can see this shape in Figure below.

The highest part of a wave is called the wavecrest. The lowest part is called the wave trough.The vertical distance from the highest part of awave to the lowest is called the wave height. Thehorizontal distance between one wave crest andthe next crest, is called the wavelength. Threethings influence how big a wave might get. If thewind is very strong, and it blows steadily for along time over a long distance, the very largestwaves will form. The wind could be strong, but ifit gusts for just a short time, large waves won’tform. Bigger waves do more work of erosionwhich changes our shorelines. Each day that wavesbreak along the shore, they steadily erode away alittle bit of the shoreline. When one day, a reallybig storm like a hurricane arrives, it will do a lotof damage in just a very short time.

Figure 10.14 Each wave form has a wave crest, trough,

height and wavelength.As waves come into shore, they usually reach

the shore at some angle. This means one part of thewave reaches shallow water sooner than the partsof the wave that are further out. As a wave comesinto shore, the water ‘feels’ the bottom whichslows down the wave. So the shallower parts ofthe wave slow down more than the parts that arefurther from the shore. This makes the wave‘bend,’ which is called refraction. The way thatwaves bend as they come into shore eitherconcentrates wave energy or disperses it. In quietwater areas like bays, wave energy is dispersedand sand gets deposited. Areas like cliffs that stick

out into the water, are eroded away by the strongwave energy that concentrates its power on thecliff (Figure below).

Figure 10.15 Cliffs are eroded by wave action that

concentrates energy in these areas.Wave-cut cliffs form where waves cut into the

bottom part of the cliff, eroding away the soil androcks there. First the waves cut a notch into thebase of the cliff. If enough material is cut away, thecliff above can collapse into the water. Many yearsof this type of erosion can form a wave-cutplatform (Figure below).

Figure 10.16 This large wave cut platform was formed by

the cutting action of waves on the cliffs to the left.If waves erode a cliff from two sides, the

erosion produced can form an open area in the cliffcalled an arch (Figure below). If the materialabove the arch eventually erodes away, a piece oftall rock can remain in the water, which is called asea stack (Figure below).

Figure 10.17 A sea arch can form if waves erode from

opposite sides of a cliff.

Figure 10.18 A sea stack forms if the upper layers of rock

collapse, leaving an isolated pinnacle.

Wave Deposition

Rivers carry the sand that comes from erosionof mountains and land areas of the continents to theshore. Soil and rock are also eroded from cliffsand shorelines by waves. That material istransported by waves and deposited in quieterwater areas. As the waves come onto shore and

break, water and particles move along the shore.When lots of sand accumulates in one place, itforms a beach. Beaches can be made of mineralgrains, like quartz, but beaches can also be madeof pieces of shell or coral or even bits of brokenhardened lava (Figure below).

Figure 10.19 Quartz, rock fragments and shell make up the

sand along a beach.Waves continually move sand grains along the

shore. Smaller particles like silt and clay don’t getdeposited at the shore because the water here istoo turbulent. The work of waves moves sand fromthe beaches on shore to bars of sand offshore as the

seasons change. In the summer time, waves oflower energy bring sand up onto the beach andleave it there. That is good for the many peoplewho enjoy sitting on soft sand when they visit thebeach (Figure below). In the wintertime, wavesand storms of higher energy bring the sand from thebeach back offshore. If you visit your favoritebeach in the wintertime, you will find a steeper,rockier beach than the flat, sandy beach of summer.Some communities truck in sand to resupply sandto beaches. It is very important to study the energyof the waves and understand the types of sandparticles that normally make up the beach beforespending lots of money to do this. If the sand that istrucked in has pieces that are small enough to becarried away by the waves on that beach, the sandwill be gone in a very short time.

Figure 10.20 Sand deposits in quiet areas along a shoreline

to form a beach.Sand transported by the work of waves

breaking along the shore can form sand bars thatstretch across a bay or ridges of sand that extendaway from the shore, called spits. Sometimes theend of a spit hooks around towards the quieterwaters of the bay as waves refract, causing thesand to curve around in the shape of a hook.

When the land that forms the shore is relativelyflat and gently sloping, the shoreline may be linedwith long narrow islands called barrier islands(Figure below). Most barrier islands are just a

few kilometers wide and tens of kilometers long.Many famous beaches, like Miami Beach, arebarrier islands. In its natural state, a barrier islandacts as the first line of defense against storms likehurricanes.

Figure 10.21 These sand dunes are part of Padre Island off

the coast of Texas, which is a barrier island.Instead of keeping barrier islands natural, these

areas end up being some of the most built up,urbanized areas of our coastlines. That meansstorms, like hurricanes, damage houses andbusinesses rather than hitting soft, vegetated sandyareas. Some hurricanes have hit barrier islands sohard that they break right through the island,removing sand, houses and anything in the way.

Protecting Shorelines

Humans build several different types of

structures to try to slow down the regular work oferosion that waves produce and to help preventdamage to homes from large storms. One structurethat people build, called a groin, is a long narrowpile of rocks that extends out into the water, at rightangles to the shoreline (Figure below). The grointraps sand on one side of the groin, keeping thesand there, rather than allowing it to move alongthe coastline. This works well for the person whois on the upcurrent side of the groin, but it causesproblems for the people on the opposite,downcurrent side. Those people no longer havesand reaching the areas in front of their homes.What happens as a result is that people must buildanother groin to trap sand there. This means lots ofpeople build groins, but it is not a very goodanswer to the problem of wave erosion.

Figure 10.22 Groins are built perpendicular to the shoreline

to help keep sand from moving off the beach.Some other structures that people build include

breakwaters and sea walls (Figure below). Bothof these are built parallel to the shoreline. Abreakwater is built out away from the shore in thewater while a sea wall is built right along theshore. Breakwaters are built in bay areas to helpkeep boats safe from the energy of breaking waves.Sometimes enough sand deposits in these quietwater areas that people then need to work toremove the sand. Seawalls are built to protectbeach houses from waves during severe storms. Ifthe waves in a storm are very large, sometimes

they erode away the whole sea wall, leaving thearea unprotected entirely. People do not alwayswant to choose safe building practices, and insteadchoose to build a beach house right on the beach. Ifyou want your beach house to stay in good shapefor many years, it is smarter to build your houseaway from the shore.

Figure 10.23 Breakwaters are visible in this satellite image,

built parallel to the shoreline to protect areas fromstrong waves.

Lesson Summary

Waves in the ocean are what we see as

energy travels through the water.The energy of waves produces erosionalformations like cliffs, wave cut platforms, seaarches and sea stacks.When waves reach the shore, deposits likebeaches, spits and barrier islands form incertain areas.Groins, jetties, breakwaters and seawalls arestructures humans build to protect the shorefrom the erosion of breaking waves.

Review Questions

1. Name three structures that people build to tryto prevent wave erosion.

2. Name three natural landforms that areproduced by wave erosion.

3. What are the names of the parts of awaveform?

4. Describe the process that produces waverefraction.

5. If you were to visit a beach in a tropical area

with coral reefs, what would the beach therebe made of?

Vocabulary

archAn erosional landform that is produced whenwaves erode through a cliff.

barrier islandLong, narrow island, usually composed ofsand that serves as nature’s first line ofdefense against storms.

breakwaterStructure built in the water, parallel to theshore to protect boats or harbor areas fromstrong waves.

groinLong, narrow piles of stone or timbers builtperpendicular to the shore to trap sand.

sea stackIsolated tower of rock that forms when a seaarch collapses.

sea wallStructure built along the shore, parallel to theshore, to protect against strong waves.

spitLong, narrow bar of sand that forms as wavestransport sand along shore.

wave-cut platformFlat, level area formed by wave erosion aswaves undercut cliffs.

wave crestThe highest part of a waveform.

wave heightThe vertical distance from wave crest towave trough.

wave length

The horizontal distance from wave crest towave crest.

wave troughThe lowest part of a wave form.

Points to Consider

What situations would increase the rate oferosion by waves?If barrier islands are nature’s first line ofdefense against ocean storms, why do peoplebuild on them?Could a seawall ever increase the amount ofdamage done by waves?

Wind Erosion andDeposition

Lesson Objectives

Describe the ways particles are carried bywind.Discuss several ways that wind erosionchanges land surfaces.Describe how sand dunes form.Describe the type of deposits formed bywindborne silts and clays.

Introduction

Moving water does much of the work oferosion that shapes the land surface of our Earth.Wind also flows over the Earth’s surface,sometimes carrying particles long distances beforethey are deposited. Wind blows from areas of high

pressure to areas of lower pressure. The erosivepower of wind varies with the strength of thewinds that blow, but usually wind transportssmaller particles like silt and clay. Somewhatlarger particles may be bumped or rolled along bythe wind. Wind can carry particles across oceanbasins and to great heights within our atmosphere.Wind is a stronger erosional force in arid regionsthan humid areas for two reasons. In arid regions,temperatures change greatly from night to day,which produces wind. Even strong winds in humidareas are less effective erosional agents becausethe ground is wet, so soil particles are heavier andless likely to be removed or transported by wind.

Transport of Particles by Wind

Wind is able to transport the smallest particlesof sediment, like silt and clay, over great distancesand areas. Once these particles become mixed intothe air, wind can keep them suspended for hours ormaybe even days at a time. If nothing disturbs these

tiny particles, wind would have trouble pickingthem off the ground surface. This is because veryclose to the ground, there is very little motion dueto wind. Look behind a car or truck as it drivesover an unpaved road. You will see a big cloud ofdust that wasn’t there before the truck disturbed theground surface. Once these fine particles aredisturbed, wind easily picks them up anddistributes them.

Just as water carries different size particles invarious ways, wind also transports particles asboth bed load and suspended load. For wind, sandsized particles make up the bed load. These sandgrains are moved along by the wind in a bump, rolland jump kind of motion. First, a grain of sand getsknocked into the air. It is too heavy to have windcarry it for long in suspension, so it falls back tothe ground, possibly knocking another sand graininto the air as it hits the ground. This starts theprocess all over again. This process is calledsaltation, which comes from a Latin word meaning‘to leap’ (Figure below). The suspended load forwind will always be very small particles of silt

and clay, which are still able to be carriedsuspended in the air by wind.

Figure 10.24 Saltation moves sand size particles along the

desert floor or on sand dunes.Erosion by WindAs wind moves sand sized particles, they will

remain close to the ground, usually less than ameter from the ground even in the strongest winds.In a sandstorm, about a quarter of the particles aresand which moves as bed load. In arid regions, asandstorm actually moves much smaller particlesthan sand in the winds. Wind can carry these smallparticles high into the air and these particles caninfiltrate cracks around windows and doors in adust storm (Figure below).

Figure 10.25 A dust storm approaches Al Asad, Iraq.Sometimes these small particles are deposited

in areas relatively close to their original source,but often silts and clays have been carried halfwayacross a continent or from desert areas on onecontinent across an entire ocean basin. Wind ismore effective at erosion in arid regions becausein humid regions smaller particles are heldtogether by the moisture in the soil and by plantroots from the vegetation. Where it is dry, plantsdon’t grow as well, so both these factors increasethe ability of wind to transport particles, erodingthe landscape.

The process of smaller particles being

selectively transported by the wind is calleddeflation (Figure below). This means the groundsurface gets lower and rockier, as more and moresmall particles are blown away. Eventually, mostof the smaller particles will have been removedand the rockier surface left behind is called desertpavement. This surface is covered by pebbles andgravel sized particles that are not easily moved bywind. If no disturbance from vehicles or animalsdisrupts the surface, deflation will stop once thisrocky surface has formed.

Figure 10.26 This desert pavement formed in the Mojave

Desert as a result of deflation.

All the particles moved by wind, whether assuspended load or by saltation as bed load, do thework of abrasion. As one particle hits another,each grain erodes another. You may have seenworkers sandblasting the front of a building inorder to remove paint or dirt. In the naturalsituation, erosion by wind polishes surfaces ofrock. In the desert, rocks or boulders developdifferent polished flat surfaces as wind blows fromdifferent directions. These polished stones arecalled ventifacts (Figure below).

Figure 10.27 A polished stone called a ventifact, is

produced by abrasion from sand grains.

Exposed rocks in desert areas often develop adark brown to black coating called desert varnish.This coating forms on stable rock surfaces thatdon’t get much precipitation. The first part of theprocess is the transport of clay sized particles bywind which chemically react with other substancesat high temperatures. The coating is formed of ironand manganese oxides. Ancient people carved intothese darkened surfaces to make petroglyphs(Figure below).

Figure 10.28 These petroglyphs were carved into desert

varnish near Canyonlands National Park in Utah.

Deposition by Wind

When you think of a desert or perhaps even abeach, the image that comes to mind might includesand dunes (Figure below). In coastal regions,you will find sand dunes in the landward directionof the beach. Sand dunes form here as sand isblown from the shore inland. The sand dunes alonga beach are likely to be composed of individualgrains of the mineral quartz, unless the beach is ina tropical area. In humid regions, other mineralsbreak down readily to form clays, leaving behindonly the more resistant quartz. In the tropics, sanddunes may be composed of calcium carbonate. In adesert, the sand dunes may be composed of avariety of minerals. This is because a desertregion, by definition, has very little water. Thismeans that mostly mechanical weathering and verylittle chemical weathering occurs here. So desertsand dunes will include even unstable minerals.

Figure 10.29 This sand dune in Morocco shows secondary

sand ripples along its slip face.Just as water waves are very selective about

the size particles they carry and deposit, so willthe size of the sand grains in a dune be very

uniform. The sand dunes are formed of a particularsize particle which is too heavy for the wind totransport. This process is sometimes so selectivethat wind will transport and carry rounded grainsof sand, which roll easily, more readily thanangular grains.

The faster and stronger the wind, the moreparticles it can carry. As wind slows down, it willdrop off the heaviest particles first. This oftenhappens as wind moves over some type ofobstacle, such as a rock or an area of vegetation.As the wind moves up and over the obstacle, itincreases in speed, but as soon as it passes thearticle, wind speed decreases. That is why youwill often see deposits of sand on the downwindside of an obstacle. These deposits are the startingmaterial for formation of sand dunes. This is thefirst condition needed for dunes to form.

In order for sand dunes to form, two moreconditions must be met. The first of theseconditions is that there is an abundant supply ofsand. The last condition is that there are steadywinds. As steady winds blow over an ample

supply of sand, sand grains will bump and rollalong, moving by saltation up the gently sloping,upwind side of the dune. As a grain of sandreaches the crest of the dune, it cascades down thesteeper, downwind side of the dune, forming theslip face of the dune. The slip face is steepbecause it forms at the angle of repose for drysand, which is about 34o (Figure below).

Figure 10.30 Sand dunes have a gently sloping face in the

upwind direction. Downwind, a steeper slip faceforms.

So as wind erodes and transports sand grainsalong the gently sloped upwind side of a dune, itdeposits sand along the downwind slip face. Aseach new layer of sand falls down the slip face ofthe dune, cross beds are formed. Cross beds arenamed for the way each layer is formed at an angleto the ground. Some of the most beautifulsandstones are crossbedded sandstones (Figure

below). These sandstones preserve sandsoriginally deposited as sand dunes in desertsmillions of years ago.

Figure 10.31 These beautiful rocks are crossbedded

sandstones from the Canyons of the Escalante inUtah.

Sand is always moving up the gently slopedside of a dune, and depositing on the downwindside, which means that dunes themselves slowlymigrate in the downwind direction. This means thatover a period of years, sand dunes will move manymeters downwind. This is something that beachhouse owners need to consider if they live nearcoastal sand dunes. Once a sand dune becomesstabilized by vegetation, such as sea grasses, itsmigration will stop. Beach goers need to be carefulnot to disturb these grasses when they go to and

from the beach.

Reducing Wind Erosion – Types ofSand Dunes

There are several different forms that sanddunes will take. The differences are due to theamount of sand available to be moved, thecharacter and direction of the wind and the type ofground the sand is moving over. The most usualcrescent shaped dune is called a barchan dune(Figure below). This type of dune forms whenthere is an adequate amount of sand being movedby constant winds that blow from one directionover hard ground. The crescent shape will curve inthe direction the wind blows. In an area of constantwinds with an abundant supply of sand, barchandunes blend together into large scale sand ripplescalled transverse dunes. Many coastal dunes arethis type of dune. Desert areas that are completelysand covered can join many transverse dunestogether into a sand sea.

Figure 10.32 This crescent shaped dune forms from constant

winds moving sand over hard ground.Star shaped dunes form in areas of constantly

changing wind direction with enough sand fordunes to form. They are called stars becauseseveral ridges of sand all radiate out from a centralpoint. Linear dunes form when there isn’t muchsand and winds come together from differentdirections. This type of dune forms long straightlines of sand that line up parallel to the winddirection. Coastlines are places of steady windsand abundant supplies of sand. Parabolic dunesform where some type of vegetation at least partlycovers the sand. These coastal dunes form acurved shape that curves into the direction of the

wind. This probably occurs as the central portionof sand is blown out, leaving a U-shaped curve ofsand.

Loess

In many parts of the world, the finest grains ofwindblown silt and clay are deposited layer onlayer, covering whole regions with these tinyparticles (Figure below). Geologists call thesedeposits loess, which comes from the Germanword ‘loose.’ These deposits form downwind ofareas of glacial outwash or desert areas. There areextensive loess deposits in China where desertsare the original source for these fine grained,windborne particles. One unusual characteristic ofloess deposits is their ability to form nearlyvertical cliffs, without grains sliding or slumpingdown the face. In China, people once built homesdirectly into these deposits because they are easyto dig into and they keep their shape. Loessdeposits are also the source of wind transported

materials that make very fertile soils in manyregions of the world.

Figure 10.33 These vertical cliffs are formed from fine

grained, windblown silt and clay.Much of the fine grained mud that covers the

deepest parts of the ocean floor comes from siltsand clays brought there by winds from the land.These tiny particles are easily carried longdistances by wind. Once they are deposited on thewater surface, they settle ever so slowly to thedeep ocean floor, forming brown, greenish orreddish clays. Another source of windborneparticles is volcanic activity. Explosive volcanoes

eject volcanic ash and dust high into the air,sometimes reaching the stratosphere. Once thesefine grained particles are airborne, they can travelhundreds or thousands of kilometers. Regionsclosest to the volcano are the areas with thickestdeposits, but volcanic ash has even completelycircled the Earth in extremely violent eruptionslike Krakatau in 1883. Windborne volcanic ashcan produce spectacularly beautiful sunsets, aswell as decreasing worldwide temperatures, asash and dust block out incoming sun’s rays.

Lesson Summary

Wind can carry small particles like sand, siltand clay.Wind erosion produces sand blasting ofsurfaces and produces desert pavement,ventifacts and desert varnish.Sand dunes are some of the most commonwind born deposits, which come in manydifferent shapes and sizes.

Loess is a very fine grained, wind bornedeposit that is important to soil formation inmany regions.

Review Questions

1. Discuss suspended load and bed loadtransport by wind.

2. Describe how desert pavement forms.3. Discuss the factors necessary for sand dunes

to form.4. Name four types of sand dunes that form in

desert areas.5. Name one type of wind deposition.6. Why is wind erosion more important in arid

regions than humid areas?

Vocabulary

barchan duneCrescent shaped dune that forms in regions ofample sand, constant winds & hard ground.

bed loadThe portion of sand carried by wind as grainsroll, bump and jump along the ground surface.

deflationThe process of wind removing finer grains ofsilt and clay; causes the ground surface tosubside.

desert pavementRocky, pebbled surface created as finer siltsand clays are removed by wind.

desert varnishDark mineral coating that forms on stable,exposed rock surfaces as windborne clays aredeposited.

loessExtremely fine grained, windborne deposit ofsilts & clays; forms nearly vertical cliffs.

petroglyphsRock carvings formed by cutting into desert

varnish of exposed rock surfaces.

saltationMovement of sand sized particles by rolling,bumping and jumping along the groundsurface.

sand duneDeposit of sand formed in regions of abundantsand and constant winds.

slip faceSteeper, downwind side of a dune; regionwhere sand grains fall down from the crest ofthe dune.

suspended loadParticles of silt and clay carried in the air bythe energy of winds.

ventifactsPolished, faceted stones formed by abrasionof sand particles.

Points to Consider

Do you think strong hurricane winds along acoastline would produce wind relatederosion?What would be needed to convert a desertarea back to a productive region for farming?Do you think wind could sculpt exposedrocks? Explain how this might happen.

Glacial Erosion andDeposition

Lesson Objectives

Discuss the different erosional featuresformed by alpine glaciers.Describe the processes by which glacierschange the underlying rocks.Discuss the sorting and types of particlesdeposited by glaciers as they advance andrecede.Describe the landforms created by glacialdeposits.

Introduction

Today glaciers cover about 10% of the landsurface on Earth, but there have been times inEarth’s recent history when glaciers have covered

as much as 30% of the land surface. Around eightto six hundred million years ago, geologistsbelieve that almost all of the Earth was covered insnow and ice. So today, scientists do a kind ofdetective work to figure out where the ice oncewas. We can figure this out by observing the waysthe land has been eroded and by looking at thedeposits that have been left behind. It is possiblethat there once was ice on the land where you areliving right now. How can you find out? Let’s talkabout some of the features that scientists look for.

Formation and Movement ofGlaciers – Continental and ValleyGlaciers

Today, we have glaciers near Earth’s polesand at high altitudes in mountainous regions. Theice in a glacier erodes away the underlying rocks,just as rivers and streams shape the land they flowover. Like rivers and streams, glaciers tend to flowalong existing valleys, but while the thick ice of

glaciers is slowly moving over the land, it scoursaway the rocks below somewhat like a very slowand steady bulldozer. Especially up in themountains, rivers cut ‘V’ shaped valleys as runningwater cuts deep into the rock. As a glacier flowsthrough this same valley, it widens the valley andforms steeper sides to the valley walls, making a‘U’ shape valley instead (Figure below).

Figure 10.34 This valley in Glacier National Park shows the

characteristic U shape of a glacially carved valley.In mountainous areas, often many smaller

glaciers flow from higher elevations joining themain glacier as they move to lower places.Generally, these smaller glaciers carve shallower‘U’ shaped valleys than the main glacier. Abeautiful erosional feature, called a hangingvalley, forms where the smaller ‘U’ shaped valleymeets the deeper one of the main glacier. Riverwater cascades down the steep valley walls

forming breathtaking waterfalls (Figure below).

Figure 10.35 Bridalveil Falls waterfall flows today in the

hanging valley produced where a smaller glacierjoins the main glacier.

Glacial Erosion

The two main ways that glaciers erode theunderlying rock are abrasion and plucking. As thethick layer of ice pushes against the underlyingrock, it scrapes and polishes the rock surface. Asglaciers flow, they scratch the underlying bedrockwith all the rocky material they are carrying. Thesescratches make long, parallel grooves in thebedrock, called glacial striations, which show thedirection the glacier moved. Also as the glacierslowly moves over the rock, glacial meltwaterseeps into cracks and fractures of the underlyingrock. As the water freezes, it pushes pieces of rockout of the underlying rock surface. These pieces ofrock get plucked out and carried away by theflowing ice of the moving glacier (Figure below).

Figure 10.36 Iceberg Cirque in Glacier National Park was

carved by glaciers.There are several erosional features that form

as a glacier both scours the rock and pulls piecesaway. As rocks are pulled away from valley walls,a steep sided, bowl shaped depression forms at thetop of a mountain, called a cirque. The wordcomes from the French word for circle. Once theice melts away, a high altitude lake, called a tarnoften forms from meltwater trapped in the cirque.If several glaciers flow down in differentdirections from a central mountain peak, thesesteep walled depressions can leave behind an

angular, sharp sided peak called a horn. TheMatterhorn in Switzerland is the most famousexample of this type of erosion (Figure below).

Figure 10.37 The Matterhorn in Switzerland is a classic

example of a horn.When two glaciers move down opposite sides

of the mountain, the erosional landform that iscreated where they meet is a sharp edged, steepsided ridge, called an arête. Sometimes hikingtrails follow along these narrow ridges, providingdramatic views in all directions (Figure below).

Figure 10.38 When glaciers move down opposite sides of a

mountain, a sharp edged ridge forms between them.As glaciers flow down a mountainside, the ice

may also sculpt and shape the underlying bedrockas it flows. When a knob of bedrock is carved intoan asymmetrical hill, it is called a rochemoutonnée. In French, it means ‘sheep rock.’Perhaps the villagers below the mountain thoughtthese hills looked like sheep grazing in the valley.

A roche moutonnée has a gently sloping side in theuphill direction of ice flow, with a steep sidefacing the downslope direction (Figure below).

Figure 10.39 A roche moutone forms where glaciers smooth

the uphill side of the bedrock and pluck away rockfrom the downslope side.

Depositional Features of Glaciers

As glaciers flow over many years, all sorts ofdebris falls onto the glacier through mechanicalweathering of the valley walls. Glaciers are solidice, so unlike water, they can carry pieces of rockof any size. Glaciers move boulders as large as ahouse as easily as the smallest particles of sand

and silt. These pieces of rock are carried by theglacier for many kilometers and are only depositedas the ice melts. When you think of a glacier, youmay think of white ice and snow, but actuallyglaciers have lots of rocky bits all over them. Eachof these different deposits has its own name basedon where it forms, but as a group they are calledmoraines. A long pile of rocky material at the edgeof a glacier is called a lateral moraine and one inthe middle of the glacier is called a medialmoraine. Lateral moraines form at the edges of theglacier as material drops onto the glacier fromerosion of the valley walls. Medial moraines formwhere two glaciers join together. In this case, thelateral moraines from the edges of each glaciermeet in the middle to form the medial moraine(Figure below).

Figure 10.40 These long, dark lines on the Aletsch glacier in

Switzerland are examples of medial and lateralmoraines.

Wherever a glacier is located, it is alwaysslowly flowing downhill. Sometimes the rate atwhich it is flowing downhill is faster than the rateat which it melts. In this situation, you will see theglacier advancing down the valley, with more andmore ice with each successive year. More likelywhat you will see today if you get the chance tovisit a glacier, is that the glacier is retreating. Thismeans that is there is less ice in the glacier thisyear than there was the year before (Figure

below).

Figure 10.41 These photographs of the Grinnell Glacier in

Glacier National Park were taken over an almost70 year period. The glacier is clearly visible andwell developed in 1938. From 1981 through 2005,the amount of glacial ice has decreased and themeltwater forming the lake has increased. In 2005,icebergs are further evidence of glacial melting.

When glaciers melt more than they flowforward, they deposit all the big and small bits ofrocky material they have been carrying. In general,all these unsorted deposits of rock, formed directlyby the ice, are called glacial till. If you live in anarea where glaciers once were, you may have seenlarge boulders in the woods or even in the middleof a field. If these large rocks are a different typeof rock than the bedrock in that area, they arecalled glacial erratics (Figure below). Scientists

know only ice could carry these large bouldersgreat distances. The largest glacial erratic, calledBig Rock found in Alberta, Canada weighsthousands of tons!

Figure 10.42 A large boulder dropped by a glacier is called

a glacial erratic.Sometimes long ridges of rock are deposited at

the furthest point that the glacier reached. Theseare called terminal and end moraines. Just as theconveyor belt at the grocery store moves yourgroceries to the end of the counter, a glaciertransports rock and sediment while it flows. If youcouldn’t stop the conveyor belt at the grocery

store, you would end up with a big jumbled pile offood at the end of the counter. An end moraine is alittle bit like that. Whatever the glacier has beencarrying, all gets left behind in a pile as the icemelts away. Geologists study these materials tofigure out how far glaciers once extended and theycan also figure out how long it took them to meltaway. Long Island in New York was formed bytwo glacial end moraines. The end moraine thatformed this pile of rock and stone deposited byglaciers extends all the way out to Cape Cod,Massachusetts.

Even while a glacier is flowing slowlydownhill, it deposits a layer of sedimentunderneath the glacier, which scientists callground moraine. This layer of sediment makes athick layer of unsorted sediment under the glacierthat fills in low spots and evens out higher areas.Ground moraine is an important contribution to thefertile transported soils in many regions. Scientistsknew that all these rocks and thick soils came fromsomewhere else but for a while they did not knowthat they came from glacial ice. This is easy to

understand because the ice is not there today. Manyscientists thought the big rocks looked like they hadbeen dropped there and they thought maybeicebergs carried by a huge flood had brought themthere. Because of this early hypothesis, lots ofglacial deposits are called ‘drift’, because theywere thought to have drifted in on icebergs. Theycorrectly understood that only ice could havebrought these materials, but not that there werethick ‘rivers’ of ice moving over the Earth inplaces where no ice exists today.

Another glacial depositional landform whichforms under a glacier by water melting from the iceis an esker (Figure below). These curving ridgesof sand are deposited by streams that run within theice along the base of the glacier. A normal streamcarves its channel into the ground, forming a ‘V’shaped channel, with the wide part of the ‘V’ atground level. Because the water in this streammoves through the ice, not on the ground, only thedeposits mark where these streams flowed. Whenthe ice melts, the sediments form an upside down‘V’ on the ground.

Figure 10.43 An esker is a winding ridge of sand and gravel

deposited under a glacier by glacial meltwater.A drumlin is another type of asymmetrical hill

that glaciers form but this one is made ofsediments. A drumlin is an upside down teaspoonshaped hill which lines up with the direction the

ice moved. The sediments dropped by the glacierare thought to be formed into a long narrow hill bythe flowing glacier with the gentle sloped endpointing in the direction of ice flow. Usuallydrumlins are found in groups called drumlin fields.

Once material has been deposited by theglacier, water melting from the glacier can sort andretransport these sediments. An importantdifference between glacial deposits formeddirectly from the ice and those that form fromglacial meltwater is their degree of sorting. Ice iscapable of carrying a tremendous range of particlesizes, but solid ice does not sort any of theseparticles. So when material is dropped as icemelts, you will find very large pieces jumbled

together in an unsorted deposit along with all theother size particles it carried. A very differentsituation occurs when running water movesparticles. Liquid water cannot carry the largeparticles that ice carries. So as water movesthrough these unsorted deposits, it will select outonly the smaller bits of sand and silt that it cancarry. This produces a sorted deposit of just thesand and smaller particles transported by liquidwater. Often these deposits form layer on layer andshow the direction that rivers flowed. Thesedeposits are called stratified drift. Often a broadarea of stratified drift blankets the region justbeyond the furthest reach of the glacier, asmeltwater streams spread material out forming abroad plain called an outwash plain (Figurebelow).

Figure 10.44 Stratified drift carried by meltwater spreads

out to form an outwash plain just beyond the

furthest reaches of the glacial ice.If an isolated block of ice remains behind as

the glacier retreats, it may be surrounded andeventually covered over by these layers ofsediment. In many years time, as the ice melts, itfills the depression with water, forming smallcircular lakes called kettle lakes (Figure below).These small lakes are common in the areas whereglaciers made their farthest advances.

Figure 10.45 Small, circular lakes are common in areas of

glacial outwash. They form from blocks of ice leftbehind as the glacier retreats.

Several types of stratified deposits form in

glacial regions but are not formed directly by theice. In glaciated areas, lakes are covered by ice inthe winter. During the winter months, darker, finegrained clays sink to the bottom of the quiet watersin the lake. In the spring, with glaciers producinglots of melting water, lighter colored sands aredeposited on top of the darker layers at the bottomof the lakes. These distinctive layers, calledvarves have paired dark/light layers, with eachlayer representing one year of deposits.

Loess is a very fine grained, wind transporteddeposit which forms in areas of stratified driftglacial deposits. It is common in the middle ofNorth America as well as the eastern centralportion of Europe. This fine sediment is producedas glaciers grind the underlying rock producing afine powder called rock flour.

Lesson Summary

The movement of ice in the form of glaciershas transformed our mountainous land

surfaces with its tremendous power oferosion.U-shaped valleys, hanging valleys, cirques,horns and aretes are just a few of the featuressculpted by ice.The eroded material is later deposited aslarge glacial erratics, in moraines, stratifieddrift, outwash plains and drumlins.Varves are a very useful yearly deposit thatforms in glacial lakes.

Review Questions

1. How much of the Earth’s land surface iscovered by glaciers today? Was the Earthever covered by more ice than it is today?

2. What is the shape of a valley that has beeneroded by glaciers? How did it get thatshape?

3. What are two different features that can formas smaller side glaciers join the central mainglacier?

4. Name and describe the two processes bywhich glaciers erode the surrounding rocks.

5. Name the erosional feature that would form asseveral glaciers in a mountainous regionmove downslope in different directions froma central peak.

6. Describe the different types of morainesformed by glaciers.

7. Describe the difference between glacial tilland stratified drift. Give an example of howeach type of deposit forms.

8. Name and describe the two asymmetrical hillshaped landforms created by glaciers.

Vocabulary

abrasionScraping of the underlying bedrock, producedas ice flows against it.

arêteSteep sided, sharp edged ridge that forms as

two glaciers erode in opposite directions.

cirqueSteep sided, bowl shaped depression formedas a glacier plucks and erodes underlyingbedrock.

drumlinAn asymmetrical hill formed from sedimentsunder the flowing glacier.

end moraineUnsorted pile of glacial till that marks thefurthest reach of a glacier’s advance.

eskerLong, curving, upside down ‘V’ shaped ridgeof sediment deposited under a glacier bymeltwater.

glacial erraticLarge boulder with a different rock type ororigin from the surrounding bedrock.

glacial striationsLong, parallel scratches carved intounderlying bedrock by moving glaciers.

glacial tillAny unsorted deposit of sediment depositedby glacial ice.

ground moraineThick layer of sediment deposited under aflowing glacier.

hornSharp sided, angular peak formed as glaciersmove away from a central peak.

kettle LakeOften circular lake formed in the outwashplain by stranded ice.

moraineDeposit of unsorted, rocky material on, underor left behind by glacial ice.

pluckingRemoval of blocks of underlying bedrock bythe glacier as meltwater seeps into cracks &freezes.

roche moutonnéeAsymmetrical hill of bedrock formed byabrasion and plucking of the moving glacier.

rock flourFine sediments produced by abrasion ofglaciers scrape over bedrock.

tarnMountain lake formed by glacial meltwaterand precipitation.

varvePaired deposit of light colored, coarsersediments and darker, fine grained sediments.

Points to Consider

What features would you look for aroundwhere you live, to determine if glaciers hadever been present there?If glaciers had never formed on Earth, howwould that affect the type of soil in the middleof North America?Can the process of erosion produce landformsthat are beautiful?

Erosion and Deposition byGravity

Lesson Objectives

Describe the ways that material can movedownhill by gravity.Discuss the factors that increase thepossibility of landslides.Describe the different types of gravity drivenmovement of rock and soil.Describe ways to prevent and be aware ofpotential landslides or mudflows.

Introduction

So far in this chapter, you have learned abouterosion and deposition by moving water in riversand the ocean, erosion and deposition by glacialice and erosion and deposition by wind. With this

long list, you may think that we have covered allthe types of erosion and deposition that canpossibly occur. The force you may have forgottenis gravity! Perhaps because it is a constant force orperhaps because it is invisible, students oftenforget that gravity also acts to shape the Earth’ssurface. The examples we will consider hereinclude sudden, dramatic events like landslides, aswell as slow steady movements that happen overlong periods of time. Whatever the example, weknow that the force of gravity will always be thereand it is changing the Earth’s surface right now.

Gravity Moves Material Downhill

There are several ways that gravity can movematerial from a higher place to a lower one.Sometimes this happens along a cliff or a verysteep slope. Material that has been loosened bysome type of weathering simply falls away fromthe cliff because there is nothing to keep it inplace. If you were to keep nudging your notebook

towards the edge of your desk, eventually enoughof the notebook would be off the desk to cause it tofall. Landslides happen when large amounts ofrock suddenly fall down a cliff or mountainside.

Other times gravity simply makes things slidealong rather slowly. You may have seen this as aclassmate moves further and further down in theirseat. It’s not a very fast or dramatic movement, likeyour notebook falling to the floor, but slowly yourfriend is no longer sitting up straight in the seat.The same thing can happen to rock or even wholeparts of a hillside. This might happen over aperiod of days or even weeks. In the end, thewhole area of soil or rock has slid to a lower spot.

The last way that gravity moves material alongis when it becomes very wet. Saturated soil flowsdownhill, often removing trees, homes and bridgesthat are in the way. To help you understand howwater increases the chances of movement, thinkabout playing in sand at the beach. If you weremaking a sand castle with dry sand, you could notbuild walls very well. If you add a little bit ofwater, it helps your walls to stand. A little bit of

water helps to hold grains of sand or soil together.However, if you added lots of water, what wouldhappen? Too much water causes the sand to flowquickly away. There are a couple of ways that soilor rock can get very wet and flow. Sometimes thishappens if it has been raining for a very long timeor if it rains very hard. In the spring, snow and icebegins to melt and much of this water moves intothe ground. Springtime is a particularly dangeroustime for landslides because there are heavier andmore frequent rainfalls at this time of year and it isalso the season when snow and ice melt. Extrawater in the soil adds more weight to the slope andalso makes the grains of soil lose contact with eachother, allowing them to flow.

Contributing Factors

There are several factors that increase thechance that a landslide will occur. Some of thesewe can prevent and some we cannot. Whenever wedig into the base of a slope, this contributes to thelikelihood of a landslide. There are many reasons

why we might need to do this. We may want tobuild a house on flat ground, so we level out anarea by cutting into a hillside. Roads and railroadtracks also need to be flat and level, so excavationof a slope could be necessary in areas where wetravel frequently. This is particularly dangerouswhen the underlying rock layers also slopetowards the area that is cut away or when layers ofclay are present. If rock layers slope towards theregion that is removed, then the support for thoselayers is gone and the overlying rocks can slipaway, causing a landslide (Figure below).

Figure 10.46 The slope of underlying materials must be

considered when making road cuts.Soils rich in clay are water holding types of

soils. If you have ever worked with clay in art

class, you know that when clay is wet, it is veryslippery. If there is a lot of clay in the soil, theclays hold onto the water when it rains. Thisslippery clay layer provides an easy surface formaterials to slide over.

When construction workers need to cut intoslopes for building a home or road, it is importantto stabilize the slope to help prevent a landslide(Figure below). Some ways that you may haveseen on steep slopes along a highway includebuilding supports into the slope or plantingvegetation to keep the soil in place. Trees havedeeper roots than grasses, but each type of plantproduces benefits for particular areas. It is also agood idea to provide drainage for groundwater sothat the slope does not become saturated.

Figure 10.47 It is important to reinforce a slope that has

been cut away in order to prevent landslides.One well known cause of landslides is from

the ground shaking. Sometimes the ground shakesfrom an earthquake, a volcanic eruption or evenjust a truck going by. We can’t control earthquakesor volcanoes and some of the most devastating

landslides have been started by these other naturalhazards. Skiers and hikers need to be aware of theways they disturb the snow they travel over orthrough to avoid setting off an avalanche. Mostpeople buried by an avalanche do not survive,either because they freeze, are crushed by theweight of the snow or are unable to breathe. If youski or snow board in deep powder, you shouldcarry a small shovel in your backpack and attach along, red lightweight cord to your waist or carry aGPS radio transmitter to help rescuers locate youin the event of an avalanche.

Types of Movement Caused byGravity

Mechanical weathering loosens pieces of rockas water seeps into cracks in the rock and freezes.As these rocks fall, they form a big pile of angularrocks at the base of a cliff called a talus slope(Figure below). If you travel along a road orhighway through regions such as these, you may

see signs warning of the danger along the roadside. Sometimes as one rock falls, it hits anotherrock lower down, which hits another and so on andso forth. This is one way that a landslide or anavalanche can begin.

Figure 10.48 Pieces of rock regularly fall to the base of

cliffs and form slopes known as talus slopes.

Landslides and Avalanches

Landslides and avalanches are the mostdramatic, sudden and dangerous examples of earthmaterials moved by gravity. Usually the termlandslide is used to mean solid rock that fallssuddenly, whereas an avalanche is formed fromsnow. Most landslides happen along convergentplate boundaries, in regions of the world that aretectonically active. These regions are oftenmountainous and are places of frequentearthquakes and volcanic eruptions. When largeamounts of rock suddenly break loose from a cliffor mountainside, they move quickly and withtremendous force (Figure below). Scientistsbelieve that air gets trapped under the falling rocksand acts as a cushion that keeps the rock fromslowing down. Landslides and avalanches canmove as fast as 200 to 300 km/hour (Figurebelow).

Figure 10.49 Landslides are called rock slides by

geologists.

Figure 10.50 An avalanche of snow moves suddenly and

quickly down slope, burying everything in its path.Landslides are exceptionally destructive. They

can bury everything in their path, including entirevillages. Some landslides have created lakes whenthe rocky material dams a river or stream. Oftenhomes are destroyed as hillsides collapse. If alandslide flows into a lake or bay, they can triggera tsunami. In July of 1958, a landslide of 30.6cubic meters of rock fell from 914m up on a steepslope at the end of Lituya Bay in Alaska (Figurebelow). As that large volume of rock suddenlypushed away all the water, a 524m tsunami wasformed. The tsunami produced by the landslide

knocked down all the trees and vegetationsurrounding the bay. In the area directly oppositethe landslide, trees at elevations higher than theEmpire State Building were scoured off the valleywalls. Fortunately, this event happened in an areawhere very few people were living. Most of thepeople who witnessed this event were in boats andmost of them were able to survive because theirboats rode on top of the wave, rather than beingsmashed by it.

Figure 10.51 This photograph of Lituya Bay in Alaska shows

(in light gray) the areas damaged by the tsunamiproduced by a landslide sent 30.6 million cubic

meters of rock into the bay.Landslides occur often in dry or semi-arid

climates in areas with steep slopes or mountains.The California coastline, with its steep cliffs andyears of drought punctuated by seasons of abundantrainfall, is more prone to landslides than manyother regions. In areas where landslides are afrequent hazard, communities have put togetherwarning systems, to help people be betterprepared. Around San Francisco Bay, the NationalWeather Service and the United States GeologicalSurvey have a set of rain gauges that monitor thecondition of the soil. If soil becomes saturated, theweather service will issue a warning. Earthquakes,which can happen along western California’sabundant faults, can also trigger landslides.

Mudflows and Lahars

Mudflows and lahars are also dramatic anddangerous natural hazards produced by the force ofgravity (Figure below). Mudflows tend to follow

existing stream channels or ravines. Mudflowsoften occur on hillsides with soils rich in clay andwith little sand or gravel. Where there is little rain,there is not much vegetation to hold the soil. Thatmeans mud will flow when a large storm producesa lot of rain in a short time. The saturated soils,without plant roots to keep them in place, flowdownhill, following river channels, washing outbridges, trees and homes that are in their path.

Figure 10.52 The white areas on the otherwise green

mountainsides mark scars from numerousmudflows. Mud deposited by the flow can be seenalong the river channels.

Some mudflows are as small as a few metersin length, width and depth. Others can travel forthousands of meters, moving materials tens of

meters deep and hundreds of meters wide. Onsteep slopes, a mudflow might travel very quickly,ending abruptly when it reaches flatter ground.Thicker, more viscous mudflows move over aperiod of days or even years. The movement couldbe as slow as several millimeters/day or perhapsseveral meters/day.

A lahar is a particular type of mudflow thatflows down the steep sides of a stratovolcano(Figure below). These explosive volcanoesproduce tremendous quantities of ash and dust asthey erupt. Snow and ice from the top of thevolcano melt, producing floods of meltwater. Thisnow hot water mixes with volcanic ash to produceexceptionally hazardous flows that move as fast as60 km/hour. In Columbia, the eruption of Nevadodel Ruiz in 1985 produced a lahar that killed morethan 23,000 people as it swept over villages andflattened everything in its way. In 1991, a typhoonarrived just after Mt. Pinatubo in the Philippineserupted. The rains soaked the volcanic ash anddust that blanketed the entire region and producedlahars that killed 1,500 people and displaced

thousands more from their homes.

Figure 10.53 A lahar is a mudflow that forms from volcanic

ash and debris.

Slump and Creep

Fortunately not all types of erosion by gravitycause so many problems. Some less dramatic typesof movement, correctly called slump and creep,move earth materials slowly down a hillside.When materials slump down a hillside, they tend tomove as a large block along a curved surface. Thistype of earth movement often happens when aslope is undercut, leaving little or no support for

the overlying materials. It can also happen whentoo much weight is added to an unstable slope. It isvery unfortunate when that extra weight comesfrom building someone’s home on a slippery slope.When earth materials slump down a hillside, acrescent shaped scar marks the place they movedfrom (Figure below). A wise homeowner willlook for these crescent shaped scars alongsurrounding hillsides when considering buying anew home. If they are present, it is a goodpossibility that earth materials have slippedbefore.

Figure 10.54 Material that slumps down a hillside often

moves as a whole unit, leaving behind a crescentshaped scar.

The term creep is used to describe the verygradual movement of soil downhill, because it justbarely creeps along. Creep is such a slow way thatearth materials move that no human would likelynotice. One way to tell that earth materials areslowly moving downhill is to look at the growth oftrees. Have you ever seen a tree whose trunk bendsalmost horizontally to the ground and then growsupwards? If there are many trees growing this way,it is likely that the ground is slowly moving downhill (Figure below). Tilted telephone or powercompany poles are also signs that this type ofmotion is occurring.

Figure 10.55 Trees with curved trunks are often signs that

the hillside is slowly creeping downhill.

Prevention and Awareness

Landslides cause $1-2 billion damage in the

United States each year and cause traumatic andsudden loss of life and homes in many areas of theworld. Wherever you live, it is important to beaware of your surroundings and notice the changesin the natural world that occur. In times of heavyrainfall, look for areas of soil that are unusuallywet, cracks or bulges in soil along hillsides, tiltingof decks or patios, leaning poles or fences. Evensticking windows and doors can mean that theground is moving. As soil pushes slowly against ahouse it can put pressure on the walls that knockswindows and doors out of plumb. Areas that arevery likely to produce landslides are places wherethey have occurred before, at the top or bottom of asteep slope and anywhere where slopes have beensteepened for construction of homes or roads. Youcan help to prevent landslides around your homeby planting vegetation and trees along slopes tohelp hold soil in place. Different types of retainingwalls can help to keep a slope stable. It isimportant to install good drainage in a hill that isnear a home or road to keep the soil from gettingsaturated. Loss of life and property can be

minimized or prevented with good planning andawareness.

Lesson Summary

Gravity moves earth materials from higherelevations to lower elevations.Landslides, avalanches and mudflows arevery rapid and dangerous examples of erosionby gravity.Slump and creep happen slowly but surely,moving material downslope.Planting trees and vegetation, buildingretaining walls and providing good drainageare ways to help prevent this type of erosion.

Review Questions

1. Name three ways that gravity movesmaterials. Describe each.

2. What natural events and human actions cantrigger a landslide or avalanche?

3. What makes landslides and avalanches moveat such great speeds?

4. Compare and contrast a mudflow and a lahar.5. Name two ways that soil can move slowly

down a slope.6. What can people do to help prevent

landslides or mudflows?

Vocabulary

avalancheMass of snow that suddenly moves down amountain under the influence of gravity.

creepExceptionally slow movement of soil downhill.

laharVolcanic mudflow formed when heavy rainsor snow and ice melt and combine withvolcanic ash & dust.

landslideRapid movement downslope of rock anddebris under the influence of gravity.

mudflowSaturated soil that can flow very rapidly orslowly down a slope depending on theviscosity of the flow.

saturatedSoil that has become completely soaked withwater.

slumpDownslope slipping of a mass of soil or rock,generally along a curved surface.

talus slopeA pile of angular rock fragments formed at thebase of a cliff or mountain.

Points to Consider

Why might someone build a home on top ofland where a landslide has happened before?Could a landslide happen anywhere in theworld? What would make it likely or unlikelyin your area?What new technologies might help people toknow when a landslide will occur?

Chapter 11: Evidence AboutEarth's Past

Fossils

Lesson Objectives

Explain why it is rare for an organism to bepreserved as a fossil.Distinguish between body fossils and tracefossils.Describe five types of fossilization.Explain the importance of index fossils, andgive several examples.Describe what a living fossil is.

Introduction

Throughout human history, people have

discovered fossils and wondered about thecreatures that lived long ago. In ancient times,fossils inspired legends of monsters and otherstrange creatures. The Chinese writer Chang Qureports the discovery of “dragon bones,” whichwere probably dinosaur fossils in China 2,000years ago. The griffin, a mythical creature with alion’s body and an eagle’s head and wings, wasprobably based on skeletons of Protoceratops thatwere discovered by nomads in Central Asia(Figure below).

Figure 11.1 Griffin (left) and (right).Another fossil reminded the Greeks of the

coiled horns of a ram. The Greeks named themammonites after the ram god Ammon. Similarly,legends of the Cyclops may be based on fossilizedelephant skulls found in Crete and other

Mediterranean islands. Can you see why (Figurebelow)?

Figure 11.2 Ammonite (left) and Elephant Skull (right).Many of the real creatures whose bones

became fossilized were no less marvelous than themythical creatures they inspired (Figure below).The giant pterosaur Quetzalcoatlus had awingspan of up to 12 meters (39 feet). Thedinosaur Argentinosaurus had an estimated weightof 80,000 kg, equal to the weight of sevenelephants! Other fossils, such as the trilobite andammonite, impress us with their bizarre forms anddelicate beauty.

Figure 11.3 Kolihapeltis sp (left) and Ammonite (right).

How Fossils Form

A fossil is any remains or trace of an ancientorganism. Fossils include body fossils, left behindwhen the soft parts have decayed away, as well astrace fossils, such as burrows, tracks, or fossilizedwaste (feces) (Figure below).

Figure 11.4 Coprolite (fossilized waste or feces) from a

meat-eating dinosaur.The process of a once living organism

becoming a fossil is called fossilization.Fossilization is a very rare process: of all theorganisms that have lived on Earth, only a tinypercentage of them ever become fossils. To seewhy, imagine an antelope that dies on the Africanplain. Most of its body is quickly eaten byscavengers, and the remaining flesh is soon eatenby insects and bacteria, leaving behind onlyscattered bones. As the years go by, the bones arescattered and fragmented into small pieces,eventually turning into dust and returning their

nutrients to the soil. It would be rare for any of theantelope’s remains to actually be preserved as afossil.

On the ocean floor, a similar process occurswhen clams, oysters, and other shellfish die. Thesoft parts quickly decay, and the shells arescattered over the sea floor. If the shells are inshallow water, wave action soon grinds them intosand-sized pieces. Even if they are not in shallowwater, the shells are attacked by worms, sponges,and other animals (Figure below).

Figure 11.5 Fossil shell that has been attacked by a boring

sponge.For animals that lack hard shells or bones,

fossilization is even more rare. As a result, thefossil record contains many animals with shells,

bones, or other hard parts, and few soft-bodiedorganisms. There is virtually no fossil record ofjellyfish, worms, or slugs. Insects, which are by farthe most common land animals, are only rarelyfound as fossils. Because mammal teeth are muchmore resistant than other bones, a large portion ofthe mammal fossil record consists of teeth. Thismeans the fossil record will show many organismsthat had shells, bones or other hard parts and willalmost always miss the many soft-bodiedorganisms that lived at the same time.

Because most decay and fragmentation occursat the surface, the main factor that contributes tofossilization is quick burial. Marine animals thatdie near a river delta may be buried by sedimentcarried by the river. A storm at sea may shiftsediment on the ocean floor, covering and helpingto preserve skeletal remains.

On land, burial is rare, so consequently fossilsof land animals and plants are less common thanmarine fossils. Land organisms can be buried bymudslides or ash from a volcanic eruption, orcovered by sand in a sandstorm. Skeletons can be

covered by mud in lakes, swamps, or bogs as well.Some of the best-preserved skeletons of landanimals are found in the La Brea Tar Pits of LosAngeles, California. Although the animals trappedin the pits probably suffered a slow, miserabledeath, their bones were preserved perfectly by thesticky tar.

In spite of the difficulties of preservation,billions of fossils have been discovered,examined, and identified by thousands of scientists.The fossil record is our best clue to the history oflife on Earth, and an important indicator of pastclimates and geological conditions as well. Thefossil record also plays a key role in our lives.Fossil fuels such as coal, gas, and oil formed fromthe decayed remains of plants and animals thatlived millions of years ago.

Types of Fossils

Fossilization can occur in many ways. Mostfossils are preserved in one of five processes

(Figure below):

Preserved Remains

The most rare form of fossilization is thepreservation of original skeletal material and evensoft tissue. For example, insects have beenpreserved perfectly in amber, which is ancienttree sap. Several mammoths and even aNeanderthal hunter have been discovered frozen inglaciers. These preserved remains allow scientiststhe rare opportunity to examine the skin, hair, andorgans of ancient creatures. Scientists havecollected DNA from these remains and comparedthe DNA sequences to those of modern creatures.

Permineralization

The most common method of fossilization ispermineralization. After a bone, wood fragment,or shell is buried in sediment, it may be exposed tomineral-rich water that moves through the

sediment. This water will deposit minerals intoempty spaces, producing a fossil. Fossil dinosaurbones, petrified wood, and many marine fossilswere formed by permineralization.

Molds and Casts

In some cases, the original bone or shelldissolves away, leaving behind an empty space inthe shape of the shell or bone. This depression iscalled a mold. Later the space may be filled withother sediments to form a matching cast in theshape of the original organism. Many mollusks(clams, snails, octopi and squid) are commonlyfound as molds and casts because their shellsdissolve easily.

Replacement

In some cases, the original shell or bonedissolves away and is replaced by a differentmineral. For example, shells that were originally

calcite may be replaced by dolomite, quartz, orpyrite. If quartz fossils are surrounded by a calcitematrix, the calcite can be dissolved away by acid,leaving behind an exquisitely preserved quartzfossil.

Compression

Some fossils form when their remains arecompressed by high pressure. This can leavebehind a dark imprint of the fossil. Compression ismost common for fossils of leaves and ferns, butcan occur with other organisms, as well.

Figure 11.6 Five types of fossils: (A) Insect preserved in

amber (B) Petrified wood (permineralization) (C)Cast and mold of a clam shell (D) Pyritizedammonite (E) Compression fossil of a fern.

Exceptional Preservation

Some rock beds have produced exceptional

fossils. Fossils from these beds may showevidence of soft body parts that are not normallypreserved. Two of the most famous examples ofsoft organism preservation are the Burgess Shalein Canada and the Solnhofen Limestone inGermany. The Burgess Shale is 505 million yearsold and records the first explosion of shelledorganisms in Earth’s oceans. Many of the BurgessShale fossils are bizarre animals that seemunrelated to any other animal group. The SolnhofenLimestone is 145 million years old and containsfossils of many soft-bodied organisms that are notnormally preserved, such as jellyfish. The mostfamous Solnhofen fossil is Archaeopteryx, one ofthe earliest birds. Although it resembles a dinosaurfossil, impressions of feathers can clearly be seen(Figure below).

Figure 11.7 Fossils from Lagersttten: (left) and (right). was

an early bird. was an enormous predator (onemeter long) that lived 500 million years ago.

Index Fossils and Living Fossils

The fossil record shows clearly that over time,life on Earth has changed. Fossils in relativelyyoung rocks tend to resemble animals and plantsthat are living today. In older rocks, fossils areless similar to modern organisms.

As scientists collected fossils from differentrock layers and formations, they discovered thatthey could often recognize the rock layer by the

assemblage of fossils it contained. Some fossilsproved particularly useful in matching up rocklayers from different regions. These fossils, calledindex fossils, are widespread but only existed fora relatively brief period of time. When a particularindex fossil is found, the relative age of the bed isimmediately known.

Many fossils may qualify as index fossils.Ammonites, trilobites, and graptolites are oftenused as index fossils, as are various microfossils,or fossils of microscopic organisms. Fossils ofanimals that drifted in the upper layers of the oceanare particularly useful as index fossils, as they maybe distributed all over the world.

In contrast to index fossils, living fossils areorganisms that have existed for a tremendouslylong period of time without changing very much atall. For example, the Lingulata brachiopods haveexisted from the Cambrian period to the present, atime span of over 500 million years! Modernspecimens of Lingulata are almostindistinguishable from their fossil counterparts(Figure below).

Figure 11.8 Fossil (left) and Modern (right).

Clues from Fossils

Fossils are our best form of evidence about thehistory of life on Earth. In addition, fossils cangive us clues about past climates, the motions ofplates, and other major geological events.

The first clue that fossils can give is whetheran environment was marine (underwater) orterrestrial (on land). Along with the rock

characteristics, fossils can indicate whether thewater was shallow or deep, and whether the rateof sedimentation was slow or rapid. The amount ofwear and fragmentation of a fossil can allowscientists to estimate the amount of wave action orthe frequency of storms.

Often fossils of marine organisms are found onor near tall mountains. For example, theHimalayas, the tallest mountains in the world,contain trilobites, brachiopods, and other marinefossils. This indicates that rocks on the seabedhave been uplifted to form huge mountains. In thecase of the Himalayas, this happened when theIndian Subcontinent began to ram into Asia about40 million years ago.

Fossils can also reveal clues about pastclimate. For example, fossils of plants and coalbeds have been found in Antarctica. AlthoughAntarctica is frozen today, in the past it must havebeen much warmer. This happened both becauseEarth’s climate has changed and becauseAntarctica has not always been located at the SouthPole.

One of the most fascinating patterns revealedby the fossil record is a number of massextinctions, times when many species died off.Although the mass extinction that killed thedinosaurs is most famous, the largest massextinction in Earth history occurred at the end ofthe Permian period, about 250 million years ago.In this catastrophe, it is estimated that over 95% ofspecies on Earth went extinct! The cause of thesemass extinctions is not definitely known, but mostscientists believe that collisions with comets orasteroids were the cause of at least a few of thesedisasters.

Lesson Summary

A fossil is any remains of ancient life. Fossilscan be body fossils, which are remains of theorganism itself or trace fossils, such asburrows, tracks, or other evidence of activity.Preservation as a fossil is a relatively rareprocess. The chances of becoming a fossil are

enhanced by quick burial and the presence ofpreservable hard parts, such as bones orshells.Fossils form in five ways: preservation oforiginal remains, permineralization, moldsand casts, replacement, and compression.Rock formations with exceptional fossils arecalled very important for scientists to study.They allow us to see information aboutorganisms that we may not otherwise everknow.Index fossils are fossils that are widespreadbut only existed for a short period of time.Index fossils help scientists to find therelative age of a rock layer and match it upwith other rock layers.Living fossils are organisms that haven’tchanged much in millions of years and arestill alive today.Fossils give clues about the history of life onEarth, environments, climate, movement ofplates, and other events.

Review Questions

1. What factors make it more likely that ananimal will be preserved as a fossil?

2. What are the five main processes offossilization?

3. A scientist wants to determine the age of arock. The rock contains an index fossil and anancient relative of a living fossil. Whichfossil will be more useful for dating the rock,and why?

4. The island of Spitzbergen is in the ArcticOcean north of Norway, near the North Pole.Fossils of tropical fruits have been found incoal deposits in Spitzbergen. What does thisindicate?

Further Reading / SupplementalLinks

http://www.fossils-facts-and-finds.com/index.html

http://www.sdnhm.org/kids/fossils/index.htmlhttp://www.sdnhm.org/kids/fossils/index.htmlhttp://www.amnh.org/exhibitions/mythiccreatureshttp://www.amnh.org/exhibitions/mythiccreatures/land/griffin.phphttp://www.tonmo.com/science/fossils/mythdoc/mythdoc.phphttp://www.geo.ucalgary.ca/~macrae/Burgess_Shalehttp://www.ucmp.berkeley.edu/mesozoic/jurassic/solnhofen.html

Vocabulary

amberFossilized tree sap.

body fossilThe remains of an ancient organism.Examples include shells, bones, teeth, andleaves.

castA structure that forms when sediments fill amold and harden, forming a replica of theoriginal structure.

fossilAny remains or trace of an ancient organism.

fossil fuelA fuel that was formed from the remains ofancient organisms. Examples include coal,oil, and natural gas.

fossilizationThe process of becoming a fossil.

index fossilA fossil that identifies and shows the relativeage of the rocks in which it is found. Indexfossils come from species that werewidespread but existed for a relatively briefperiod of time.

living fossilA modern species or genus that has existed onEarth for millions of years without changingvery much.

marine

Of or belonging to the sea.

mass extinctionA period of time when an unusually highnumber of species became extinct.

microfossilA fossil that must be studied with the aid of amicroscope.

moldAn impression made in sediments by the hardparts of an organism.

permineralizationA type of fossilization in which minerals aredeposited into the pores of the original hardparts of an organism.

terrestrialOf or belonging to the land.

trace fossilEvidence of the activity of an ancient

organism. Examples include tracks, trails,burrows, tubes, boreholes, and bite marks.

Points to Consider

What are some other examples of mythicalcreatures that may be based on fossils?Why is it so rare for an animal to bepreserved as a fossil?Some organisms are more easily preservedthan others. Why is this a problem forscientists who are studying ancientecosystems?Why are examples of amazing fossilpreservation so valuable for scientists?Many fossils of marine organisms have beenfound in the middle of continents, far from anyocean. What conclusion can you draw fromthis?

Relative Ages of Rocks

Lesson Objectives

Explain Steno’s laws of superposition andoriginal horizontality.Based on a geological cross-section, identifythe oldest and youngest formations.Explain what an unconformity represents.Use fossils to correlate rock layers.

Introduction

In 1666, a young doctor named Nicholas Stenowas invited to dissect the head of an enormousgreat white shark that had been caught by localfisherman near Florence, Italy. Steno was struck bythe resemblance of the shark’s teeth to fossils,known as “tongue stones,” recovered from inlandmountains and hills (Figure below).

Figure 11.9 Fossil Shark Tooth (left) and Modern Shark

Tooth (right).While it may seem obvious today, most people

at the time did not believe that fossils were oncepart of living creatures. The reason was that thefossils of clams, snails, and other marine animalswere found in tall mountains, miles from anyocean. Two schools of thought explained thesefossils. Some religious writers believed that theshells were washed up during the Biblical flood.But this explanation could not account for the factthat fossils were not only found on mountains, buta l s o within mountains, in rocks that had beenquarried from deep below Earth’s surface. Seekingan alternate explanation, other writers proposedthat the fossils had formed within the rocks as a

result of mysterious forces. In other words, fossilshells, bones, and teeth were never a part of aliving creature!

Steno had other ideas. For Steno, the closeresemblance between fossils and modernorganisms was impossible to ignore. Instead ofinvoking supernatural forces to explain fossils,Steno concluded that fossils were once parts ofliving creatures. He then sought to explain howfossil seashells could be found in rocks far fromany ocean. As in the Tyrannosaurus rex Figurebelow, fossils resemble living organisms.

Figure 11.10 Tyrannosaurus rex fossil resembling a living

organism.

Superposition of Rock Layers

Steno first proposed that if a rock contained thefossils of marine animals, the rock was formedfrom sediments that were deposited on theseafloor. These rocks were then uplifted to becomemountains. Based on those assumptions, Stenomade a remarkable series of conjectures that arenow known as Steno’s Laws.

Original-Horizontality

Because sediments are deposited under water,they will form flat, horizontal layers (Figurebelow). If a sedimentary rock is found tilted, thelayer was tilted after it was formed.

Figure 11.11 Sedimentary layers that have been deposited

horizontally.

Lateral Continuity

Sediments were deposited in continuous sheetsthat spanned the body of water that they weredeposited in. When a valley cuts throughsedimentary layers, it can be assumed that therocks on either side of the valley were originallycontinuous.

Superposition

Sedimentary rocks are deposited one on top ofanother. Therefore, the youngest layers are found atthe top, and the oldest layers are found at thebottom of the sequence.

Cross-Cutting Relationships

A rock formation or surface that cuts acrossother rock layers is younger than the rock layers itdisturbs. For example, if an igneous intrusion goesthrough a series of metamorphic rocks, theintrusion must be younger than the metamorphicrocks that it cuts through (Figure below).

Figure 11.12 Cross cutting relationships: (A) Older banded

gneiss (B) The white granite intrusion. The granitemust be younger than the gneiss, because it cuts

across the existing gneiss.The Grand Canyon provides an excellent

illustration of Steno’s laws. Figure below showsthe many horizontal layers of sedimentary rock thatmake up the canyon. This nicely illustrates theprinciple of original horizontality. The youngestrock layers are at the top of the canyon, while theoldest are at the bottom, which is described by thelaw of superposition. Distinctive rock layers, suchas the Kaibab Limestone, can be matched acrossthe broad expanse of the canyon. We know theserock layers were once connected, which isdescribed in the rule of lateral continuity. Finally,the Colorado River cuts through all the layers ofsedimentary rock to form the canyon. Based on theprinciple of cross-cutting relationships, the rivermust be younger than all of the rock layers that itcuts through.

Figure 11.13 Grand Canyon, with the Kaibab Limestone

marked with arrows.

Determining the Relative Ages ofRocks

T h e relative age of a rock is its age incomparison with other rocks. If you know therelative ages of two rock layers, you know whichis older and which is younger, but you do not knowhow old the layers are in years. In some cases, it isvery tricky to determine the sequence of events thatleads to a certain formation. Take the example,Figure below:

Figure 11.14 Cross-section of sedimentary layers: (A-C)

Igneous intrusion (D) Cross-section (E) Fault.The principle of cross-cutting relationships

states that a fault or intrusion is younger than therocks that it cuts through. The fault labeled “E”cuts through all three sedimentary rock layers (A,B, and C) and also cuts through the intrusion (D).So the fault must be the youngest formation that isseen. The intrusion (D) cuts through the threesedimentary rock layers, so it must be younger thanthose layers.

The principle of superposition states that theoldest sedimentary rock units are at the bottom, and

the youngest are at the top. Based on this, layer Cis oldest, followed by B and A. So the fullsequence of events is as follows:

1. Layer C formed.2. Layer B formed.3. Layer A formed.4. When layers A-B-C were present,intrusion D

formed.5. Intrusion D cut through layers A – C.6. Fault E formed, shifting rocks A through C

and intrusion D.7. Weathering and erosion occurred, forming a

layer of soil on top of layer A.

Unconformities in Rock Layers

Steno discovered the rules for determining therelative age of rock beds, but he did not have agood understanding of how long it would take forthese rock formations to form. At the time, mostEuropeans believed that the Earth was around

6,000 years old, a figure that was based on theamount of time estimated for the events describedin the Bible. One of the first to question this timescale was a Scottish geologist named James Hutton(1726-1797). Often described as the founder ofmodern geology, Hutton formulated a philosophycalled uniformitarianism: The present is the keyto the past. According to uniformitarianism, thesame processes we see around us today operatedin the past as well. For example, if erosion anddeposition occur slowly now, they probably havealways occurred slowly.

Hutton discovered places where sedimentaryrock beds lie on an eroded surface. Such aformation is called an unconformity, or a gap inrock layers, where some rocks were eroded away.Hutton reconstructed the sequence of events thatled to this formation. For example, consider thefamous unconformity at Siccar Point, on the coastof Scotland (Figure below).

Figure 11.15 Huttons Unconformity on the Coast of Scotland.Based on figure 5, at least nine geological

events can be inferred:

1. A series of sedimentary beds is deposited onan ocean floor.

2. The sediments harden into sedimentary rock.3. The sedimentary rocks are uplifted and tilted,

exposing them above the ocean surface.4. The tilted beds are eroded by rain, ice, and

wind to form an irregular surface.5. A sea covers the eroded sedimentary rock

layers.

6. New sedimentary layers are deposited.7. The new layers harden into sedimentary rock.8. These layers are tilted.9. Uplift occurs, exposing the new sedimentary

rocks above the ocean surface.

Hutton realized that an enormous period oftime was needed to account for the repeatedepisodes of deposition, rock formation, uplift, anderosion that led to the formation of anunconformity, like the one at Siccar Point. Huttonrealized that the age of Earth should not bemeasured in thousands of years, but millions ofyears.

Matching Rock Layers

Superposition and cross-cutting are helpfulwhen rocks are touching one another, but areuseless when rocks are kilometers or evencontinents apart. Three kinds of clues helpgeologists match rock layers across great

distances. The first is the fact that somesedimentary rock formations span vast distances,recognizable across large regions. For example,the Pierre Shale formation can be recognizedacross the Great Plains, from New Mexico toNorth Dakota. The famous White Cliffs of Doverin southwest England can be matched to similarwhite cliffs in Denmark and Germany.

Figure 11.16 White layer of clay that marks the Cretaceous-

Tertiary Boundary.A second clue could be the presence of a key

bed, or a particularly distinctive layer of rock thatcan be recognized across a large area. Volcanicash flows are often useful as key beds because they

are widespread and easy to identify. Probably themost famous example of a key bed is a layer ofclay found at the boundary between the CretaceousPeriod and the Tertiary Period, the time that thedinosaurs went extinct (Figure above). This thinlayer of sediment, only a few centimeters thick,contains a high concentration of the elementiridium. Iridium is rare on Earth but common inasteroids. In 1980, a team of scientists led by LuisAlvarez and his son Walter proposed that a hugeasteroid struck Earth about 66 million years ago,causing forest fires, acid rain, and climate changethat wiped out the dinosaurs.

A third type of clue that helps scientistscompare different rock layers is index fossils.Recall that index fossils are the remains oforganisms that were widespread but only existedfor a relatively short period of time. If two rockunits both contain the same type of index fossil,their age is probably very similar.

As scientists collected fossils from all over theworld, they recognized that rocks of different agescontain distinctive types of fossils. This pattern led

to the creation of the geologic time scale andhelped to inspire Darwin’s theory of evolution(Figure below).

Figure 11.17

Geologic Time Scale.Each era, period, and epoch of the geologic

time scale is defined by the fossils that appeared atthat time. For example, Paleozoic rocks typicallycontain trilobites, brachiopods, and crinoid fossils.The presence of dinosaur bones indicate that arock is from the Mesozoic era, and the particulartype of dinosaur will allow the rock to beidentified as Triassic, Jurassic, or Cretaceous. TheCenozoic Era is also known as the Age ofMammals, and the Quaternary Period representsthe time when the first humans spread across Earth.

Lesson Summary

Nicholas Steno first formulated the principlesthat allow scientists to determine the relativeages of rocks in the 17th century. Steno statedthat sedimentary rocks are formed incontinuous, horizontal layers, with youngerlayers on top of older layers. A century later,James Hutton discovered the law of cross-

cutting relationships: a fault or igneousintrusion is younger than the rocks that it cutsthrough. Hutton also was the first to realizethe vast amounts of time that would be neededto create an unconformity, a place wheresedimentary rocks lie above an erodedsurface.Other methods come into play whencomparing rock layers that are separated by alarge distance. Many sedimentary rockformations are large and can be recognizedacross a region. Distinctive rock layers,called key beds, are also useful forcorrelating rock units. Fossils, especiallyindex fossils, are the most useful way tocompare different rock layers. Changes offossils over time led to the development ofthe geologic time scale.

Review Questions

1. In the 15th century, a farmer finds a rock that

looks exactly like a clamshell. What did thefarmer probably conclude about how thefossil got there?

2. Which of Steno’s Laws is illustrated by eachof the following images in Figure "Illustrationof Steno's Laws"?

3. What is the sequence of rock units in Figure"Sequence of Rock Units", from oldest toyoungest?

4. What kind of geological formation is shownin the outcrop in Figure "Outcrop", and whatsequence of events does it represent?

5. The three outcrops in Figure "Fossils" arevery far apart. Based on what you see, whichfossil is an index fossil, and why?

Figure 11.18 Illustration of Steno's Laws

Figure 11.19 Sequence of Rock Units

Figure 11.20

Figure 11.21

Further Reading / SupplementalLinks

http://pubs.usgs.gov/gip/fossils/contents.htmlhttp://www.ucmp.berkeley.edu/exhibits/index.phphttp://alan-cutler.com/excerpt.htmlhttp://www.uvm.edu/~ccoutu/evolution/qanda/?Page=time/relative.html&#38;amp;SM=time/timemenu.htmlhttp://www.ucmp.berkeley.edu/fosrec/McKinney.htmlhttp://en.wikipedia.org

Vocabulary

cross-cutting relationshipsOne of Steno’s principles that states that anintrusion or fault is younger than the rocks thatit cuts through.

geologic time scaleA division of Earth’s history into blocks oftime distinguished by geologic andevolutionary events.

key bedA distinctive, widespread rock layer thatformed at a single time.

lateral continuityOne of Steno’s principles that states that asedimentary rock layer extends sideways aswide as the basin in which it forms.

original horizontalityOne of Steno’s principles that states thatsedimentary layers were horizontal or flatlying at the time they were deposited.

relative ageThe age of an object in comparison with theage of other objects.

superpositionOne of Steno’s principles that states that in asequence of sedimentary rock layers, theoldest layer is at the bottom and the youngestlayer is at the top.

unconformityA boundary between rocks of very differentages. Unconformities are often marked by an

erosional surface.

uniformitarianismThe idea that the geologic processes thatshape the land today have acted in basicallythe same way throughout Earth’s history.

Points to Consider

In Nicholas Steno’s time, why didn’t mostpeople believe the fossils were the remains ofancient organisms?How did Steno explain the presence ofmarine fossils in high mountains?What was the significance of unconformitiesto James Hutton?How can you determine the relative age oftwo rock layers that are very far apart?

Absolute Ages of Rocks

Lesson Objectives

Define the difference between absolute ageand relative age.Describe four methods of absolute dating.Explain what radioactivity is and giveexamples of radioactive decay.Explain how the decay of radioactivematerials helps to establish the age of anobject.Estimate the age of an object, given the half-life and the amounts of radioactive anddaughter materials.Give four examples of radioactive materialsthat are used to date objects, and explain howeach is used.

Introduction

As we learned in the previous lesson, indexfossils and superposition are effective methods ofdetermining the relative age of objects. In otherwords, you can use superposition to tell you thatone rock layer is older than another. Butdetermining the absolute age of a substance (itsage in years) is a much greater challenge. Toaccomplish this, scientists use a variety ofevidence, from tree rings to the amounts ofradioactive materials in a rock.

Tree Rings

In regions outside the tropics, trees grow morequickly during the warm summer months thanduring the cooler winter. This pattern of growthresults in alternating bands of light-colored, lowdensity “early wood” and dark, high density “latewood.” Each dark band represents a winter; bycounting rings it is possible to find the age of thetree (Figure below). The width of a series ofgrowth rings can give clues to past climates and

various disruptions such as forest fires. Droughtsand other variations in the climate make the treegrow slower or faster than normal, which showsup in the widths of the tree rings. These tree ringvariations will appear in all trees growing in acertain region, so scientists can match up thegrowth rings of living and dead trees. Using logsrecovered from old buildings and ancient ruins,scientists have been able to compare tree rings tocreate a continuous record of tree rings over thepast 2,000 years. This tree ring record has provenextremely useful in creating a record of climatechange, and in finding the age of ancient structures.

Figure 11.22

Cross-section showing growth rings. The thick,light-colored part of each ring represents rapidspring and summer growth. The thin, dark part ofeach ring represents slow autumn and wintergrowth.

Ice Cores and Varves

Several other processes result in theaccumulation of distinct yearly layers that can beused for dating. For example, layers form withinglaciers because there tends to be less snowfall inthe summertime, allowing a dark layer of dust toaccumulate on top of the winter snow (Figurebelow). To study these patterns, scientists drilldeep into ice sheets, producing cores hundreds ofmeters long. Scientists analyze these ice cores todetermine how the climate has changed over time,as well as to measure concentrations ofatmospheric gases. The longest cores have helpedto form a record of polar climate stretchinghundreds of thousands of years back.

Figure 11.23 Ice core section showing annual layers.Another example of yearly layers is the

deposition of sediments in lakes, especially thelakes that are located at the end of glaciers. Rapidmelting of the glacier in the summer results in athick, sandy deposit of sediment. These thicklayers alternate with thin, clay-rich layersdeposited during the winter. The resulting layers,called varves, give scientists clues about pastclimate conditions. For example, an especiallywarm summer might result in a very thick layer ofsediment deposited from the melting glacier.Thinner varves can indicate colder summers,because the glacier doesn’t melt as much and carryas much sediment into the lake.

Age of Earth

While tree rings and other annual layers areuseful for dating relatively recent events, they are

not of much use on the vast scale of geologic time.During the 18th and 19th centuries, geologists triedto estimate the age of Earth with indirecttechniques. For example, geologists measured howfast streams deposited sediment, in order to try tocalculate how long the stream had been inexistence. Not surprisingly, these methods resultedin wildly different estimates, from a few millionyears to “quadrillions of years.” Probably the mostreliable of these estimates was produced by theBritish geologist Charles Lyell, who estimated that240 million years have passed since theappearance of the first animals with shells. Todayscientists know his estimate was too young; weknow that this occurred about 530 million yearsago.

In 1892, William Thomson (later known asLord Kelvin) calculated the age of Earth in asystematic fashion (Figure below). He assumedthat the Earth began as a ball of molten rock, whichhas steadily cooled over time. From theseassumptions, he calculated that the Earth was 100million years old. This estimate was a blow to

geologists and supporters of Charles Darwin’stheory of evolution, which required an older Earthto provide time for evolution to take place.

Figure 11.24 Lord Kelvin.Thomson’s calculations, however, were soon

shown to be flawed when radioactivity wasdiscovered in 1896. Radioactivity is the tendencyof certain atoms to decay into lighter atoms,

emitting energy in the process. Radioactivematerials in Earth’s interior provide a steadysource of heat. Calculations of Earth’s age usingradioactive decay showed that Earth is actuallymuch older than Thomson calculated.

Radioactive Decay

The discovery of radioactive materials didmore than disprove Thomson’s estimate of Earth’sage. It provided a way to find the absolute age of arock. To understand how this is done, it isnecessary to review some facts about atoms.

Atoms contain three particles: protons,neutrons, and electrons. Protons and neutrons arelocated in the nucleus, while electrons orbit aroundthe nucleus. The number of protons determineswhich element you’re examining. For example, allatoms of carbon have six protons, all atoms ofoxygen have eight protons, and all atoms of goldhave 79 protons. The number of neutrons,however, is variable. An atom of an element with a

different number of neutrons is an isotope of thatelement. For example, the isotope carbon-12contains 6 neutrons in its nucleus, while theisotope carbon-13 has 7 neutrons.

Some isotopes are radioactive, which meansthey are unstable and likely to decay. This meansthe atom will spontaneously change from anunstable form to a stable form. There are twoforms of nuclear decay that are relevant in howgeologists can date rocks (Table (below):

Types of Radioactive DecayParticle Composition Effect on Nucleus

Alpha 2 protons, 2neutrons

The nucleus contains twofewer protons and twofewer neutrons.

Beta 1 electronOne neutron decays to forma proton and an electron,which is emitted.

(Source: Kurt Rosenkrantz, License: CC-BY-SA)

If an element decays by losing an alphaparticle, it will lose 2 protons and 2 neutrons. If an

atom decays by losing a beta particle, it loses justone electron.

So what does this have to do with the age ofEarth? Radioactive decay eventually results in theformation of stable daughter products.Radioactive materials decay at known rates. Astime passes, the proportion of radioactive isotopeswill decrease and the proportion of daughterisotopes will increase. A rock with a relativelyhigh proportion of radioactive isotopes is probablyvery young, while a rock with a high proportion ofdaughter products is probably very old.

Scientists measure the rate of radioactivedecay with a unit called half-life. The half-life of aradioactive substance is the amount of time, onaverage, it takes for half of the atoms to decay. Forexample, imagine a radioactive substance with ahalf-life of one year. When a rock is formed, itcontains a certain number of radioactive atoms.After one year (one half-life), half of theradioactive atoms have decayed to form stabledaughter products, and 50% of the radioactiveatoms remain. After another year (two half-lives),

half of the remaining radioactive atoms havedecayed, and 25% of the radioactive atoms remain.After the third year (three half-lives), 12.5% of theradioactive atoms remain. After four years (fourhalf-lives), 6.25% of the radioactive atoms remain,and after 5 years (five half-lives), only 3.125% ofthe radioactive atoms remain.

If you find a rock whose radioactive materialhas a half life of one year and measure 3.125%radioactive atoms and 96.875% daughter atoms,you can assume that the substance is 5 years old.The decay of radioactive materials can be shownwith a graph (Figure below). If you find a rockwith 75% of the radioactive atoms remaining,about how old is it?

Figure 11.25 Decay of an imaginary radioactive substance

with a half-life of one year.

Radiometric Dating of Rocks

In the process of radiometric dating, severalisotopes are used to date rocks and other materials.Using several different isotopes helps scientists tocheck the accuracy of the ages that they calculate.

Carbon Dating

Earth’s atmosphere contains three isotopes ofcarbon. Carbon-12 is stable and accounts for98.9% of atmospheric carbon. Carbon-13 is alsostable and accounts for 1.1% of atmosphericcarbon. Carbon-14 is radioactive and is found intiny amounts. Carbon-14 is produced naturally inthe atmosphere when cosmic rays interact withnitrogen atoms. The amount of carbon-14 producedin the atmosphere at any particular time has beenrelatively stable through time.

Radioactive carbon-14 decays to stablenitrogen-14 by releasing a beta particle. Thenitrogen atoms are lost to the atmosphere, but theamount of carbon-14 decay can be estimated bymeasuring the proportion of radioactive carbon-14to stable carbon-12. As a substance ages, therelative amount of carbon-14 decreases.

Carbon is removed from the atmosphere byplants during the process of photosynthesis.Animals consume this carbon when they eat plantsor other animals that have eaten plants. Thereforecarbon-14 dating can be used to date plant andanimal remains. Examples include timbers from an

old building, bones, or ashes from a fire pit.Carbon dating can be effectively used to find theage of materials between 100 and 50,000 yearsold.

Potassium-Argon Dating

Potassium-40 decays to argon-40 with a half-life of 1.26 billion years. Because argon is a gas, itcan escape from molten magma or lava. Thereforeany argon that is found in a crystal probablyformed as a result of the decay of potassium-40.Measuring the ratio of potassium-40 to argon-40will yield a good estimate of the age of the sample.

Potassium is a common element found in manyminerals such as feldspar, mica, and amphibole.The technique can be used to date igneous rocksfrom 100,000 years to over a billion years old.Because it can be used to date geologically youngmaterials, the technique has been useful inestimating the age of deposits containing the bonesof human ancestors.

Uranium-Lead Dating

Two isotopes of uranium are used forradiometric dating. Uranium-238 decays to formlead-206 with a half-life of 4.47 billion years.Uranium-235 decays to form lead-207 with a half-life of 704 million years.

Uranium-lead dating is usually performed oncrystals of the mineral zircon (Figure below).When zircon forms in an igneous rock, the crystalsreadily accept atoms of uranium but reject atoms oflead. Therefore, if any lead is found in a zirconcrystal, it can be assumed that it was producedfrom the decay of uranium.

Figure 11.26 Zircon crystal.Uranium-lead dating can be used to date

igneous rocks from 1 million years to around 4.5billion years old. Some of the oldest rocks onEarth have been dated using this method, includingzircon crystals from Australia that are 4.4 billionyears old.

Limitations of Radiometric Dating

Radiometric dating can only be used onmaterials that contain measurable amounts ofradioactive materials and their daughter products.This includes organic remains (which compared torocks are relatively young, less than 100,000 yearsold) and older rocks. Ideally, several differentradiometric techniques will be used to date thesame rock. Agreement between these valuesindicates that the calculated age is accurate.

In general, radiometric dating works best forigneous rocks and is not very useful fordetermining the age of sedimentary rocks. To

estimate the age of a sedimentary rock deposit,geologists search for nearby or interlayeredigneous rocks that can be dated. For example, if asedimentary rock layer is sandwiched between twolayers of volcanic ash, its age is between the agesof the two ash layers.

Using a combination of radiometric dating,index fossils, and superposition, geologists haveconstructed a well-defined timeline of Earthhistory. For example, an overlying lava flow cangive a reliable estimate of the age of a sedimentaryrock formation in one location. Index fossilscontained in this formation can then be matched tofossils in a different location, providing a good agemeasurement for that new rock formation as well.As this process has been repeated all over theworld, our estimates of rock and fossil ages hasbecome more and more accurate.

Lesson Summary

Techniques such as superposition and index

fossils can tell you the relative age of objects,which objects are older and which areyounger. Other types of evidence are neededto establish the absolute age of objects inyears. Geologists use a variety of techniquesto establish absolute age, includingradiometric dating, tree rings, ice cores, andannual sedimentary deposits called varves.Radiometric dating is the most useful of thesetechniques—it is the only technique that canestablish the age of objects older than a fewthousand years. The concentrations of severalradioactive isotopes (carbon-14, potassium-40, uranium-235 and -238) and their daughterproducts are used to determine the age ofrocks and organic remains.

Review Questions

1. What four techniques are used to determinethe absolute age of an object or event?

2. A radioactive substance has a half-life of 5

million years. What is the age of a rock inwhich 25% of the original radioactive atomsremain?

3. A scientist is studying a piece of cloth froman ancient burial site. She determines that40% of the original carbon-14 atoms remainin the cloth. Based on the carbon decay graph(Figure below), what is the approximate ageof the cloth?

Figure 11.27 4. Which radioactive isotope or isotopes would

you use to date each of the following objects?Explain each of your choices.

1. A 4 billion year old piece of granite.2. A one million year old bed of volcanic

ash that contains the footprints ofhominids (human ancestors).

3. The fur of a woolly mammoth that wasrecently recovered frozen in a glacier.

4. A fossilized trilobite recovered from abed of sandstone that is about 500million years old.

5. The principle of uniformitarionism states thatthe present is the key to the past. In otherwords, the processes that we see happeningtoday probably worked in a similar way inthe past. Why is it important to assume that therate of radioactive decay has remainedconstant over time?

Further Reading / SupplementalLinks

http://www.pbs.org/wgbh/nova/elnino/reach/living.htmlhttp://nvl.nist.gov/pub/nistpubs/jres/109/2/j92cur.pdf

http://pubs.usgs.gov/gip/geotime/radiometric.html

Vocabulary

absolute ageThe age of an object in years.

alpha particleParticle consisting of two protons and twoneutrons that is ejected from the nucleusduring radioactive decay.

beta particleParticle consisting of a single electron that isejected from the nucleus during radioactivedecay. A beta particle is created when aneutron decays to form a proton and theemitted electron.

daughter productStable substance that is produced by thedecay of a radioactive substance. Forexample, uranium-238 decays to produce

lead-207.

half-lifeAmount of time required for half of the atomsof a radioactive substance to decay and formdaughter products.

ice coreCylinder of ice extracted from a glacier or icesheet.

isotopeAn atom of an element that has a differingnumber of neutrons.

radioactiveSubstance that is unstable and likely to emitenergetic particles and radiation.

radioactivityEmission of high-energy particles and/orradiation by certain unstable atoms.

radiometric dating

Process of using the concentrations ofradioactive substances and daughter productsto estimate the age of a material. Assubstances age, the amounts of radioactiveatoms decrease while the amounts of daughtermaterials increase.

relative ageAge of an object as compared to otherobjects.

tree ringLayer of wood in a tree that forms in oneyear. You can determine the age of a tree bycounting its rings.

varveThin layer of sediment deposited on a lakebedover the course of one year usually found atthe bottom of glacial lakes.

Points to Consider

Why are techniques like tree rings, ice cores,and varves only useful for events thatoccurred in the last few thousand years?Why was it so important for Darwin and hisfollowers to prove that the Earth was veryold?Why is it important to use more than onemethod to find the age of a rock or otherobject?

Chapter 12: Earth's History

Geologic Time ScaleIn this Earth’s history chapter, you will learn

about some of the ways that scientists study thehistory of Earth and how they use clues from rocksand fossils to piece together pictures of how theEarth has changed over billions of years. You willlearn how the Earth formed and how life graduallydeveloped on Earth. You will also gain anappreciation for how life changes on Earth andhow living things respond to changes in theirenvironments.

Lesson Objectives

Discuss how scientists know that the Earth isbillions of years old.Describe how Earth’s history can berepresented by the geologic time scale.

Introduction

How many years is a “long time?” We oftenexpress time in hours or days, and 10 or 20 yearscertainly feels like a long time. Imagine if youneeded to think about one million, 100 million, oreven several billion years. These exceptionallengths of time seem unbelievable, but they areexactly the spans of times that scientists use todescribe the Earth.

The Earth is 4 1/2 billion years old. That’s4,500,000,000 years! Have places like the GrandCanyon and the Mississippi River been around forall of those years, or were they formed morerecently? When did the giant Rocky Mountainsform and when did dinosaurs walk the Earth? Toanswer these questions, you have to think abouttimes that were millions or billions of years ago.

Historical geologists are scientists who studythe Earth’s past. They study clues left on the Earthto learn two main things: the order in which eventshappened on Earth, and how long it took for those

events to happen. For example, they have learnedthat the Mississippi River formed many millions ofyears after the Grand Canyon began forming. Theyhave also concluded that dinosaurs lived on theEarth for about 200 million years.

Scientists have put together the geologic timescale to describe the order and duration of majorevents on Earth for the last 4 1/2 billion years.Some examples of events listed on the geologictime scale include the first appearance of plant lifeon Earth, the first appearance of animals on Earth,the formation of Earth’s mountains, and theextinction of the dinosaurs.

You will learn about some of the scientificprinciples that historical geologists use to describeEarth’s past. You will also learn some of the cluesthat scientists use to learn about the past and showsyou what the geologic time scale looks like.

Evaluating Prior Knowledge

Before you work through this lesson, think

about the following questions. Be sure that you cananswer each one. They will help you betterunderstand this lesson.

What is a fossil and how does a fossil form?How does a sedimentary rock form?In what types of locations do sedimentary

rocks form?How do you determine the relative and

absolute ages of rock layers?

Geologic Time

The first principle you need to understandabout geologic time is that the laws of nature neverchange. This means that the laws describing howthings work are the same today as they werebillions of years ago. For example, water freezesat 0oC. This law has always been true and alwayswill be true. Knowing that natural laws neverchange helps you think about Earth’s past, becauseit gives you clues about how things happened verylong ago. It means that we can use present-day

processes to interpret the past. Imagine you findfossils of sea animals in a rock. The laws of naturesay that sea animals must live in the sea. That lawhas never changed, so the rock must have formednear the sea. The rock may be millions of yearsold, but the fossils in it are a clue for us todayabout how it formed.

Now imagine that you find that same rock withfossils of a sea animal in a place that is very dryand nowhere near the sea. How could that be?Remember that the laws of nature never change.Therefore, the fossil means that the rock definitelyformed by the sea. This tells you that even thoughthe area is now dry, it must have once beenunderwater. Clues like this have helped scientistslearn that Earth's surface features have changedmany times. Spots that were once covered bywarm seas may now be cool and dry. Places thatnow have tall mountains may have once been low,flat ground. These kinds of changes take place overmany millions of years, but they are still slowlygoing on today. The place where you live rightnow may look very different in the far away future.

Relative and Absolute Age Datingof Rocks

The clues in rocks help scientists put together apicture of how places on Earth have changed.Scientists noticed in the 1700s and 1800s thatsimilar layers of sedimentary rocks all over theworld contain similar fossils. They used relativedating to order the rock layers from oldest toyoungest. In the process of relative dating,scientists do not determine the exact age of a fossilbut do learn which ones are older or younger thanothers. They saw that the fossils in older rocks aredifferent from the fossils in younger rocks. Forexample, older rock layers contain only reptilefossils, but younger rock layers may also containmammal fossils.

Scientists divided Earth’s history into severalchunks of time when the fossils showed similarthings living on the Earth. They gave each chunk oftime a name to help them keep track of how Earthhas changed. For example, one chunk of time when

many dinosaurs lived is called the Jurassic. Wefind fossils of Earth’s first green plants from thechunk of time named the Ordovician. Many of thescientists who first assigned names to times inEarth’s history were from Europe. As a result,many of the names they used came from towns orother local places where they studied in Europe.

Ordering rock layers from oldest to youngestwas a first step in creating the geologic time scale.It showed the order in which life on Earth changed.It also showed us how certain areas changed overtime in regard to climate or type of environment.However, the early geologic time scale onlyshowed the order of events. It did not show theactual years that events happened. With thediscovery of radioactivity in the late 1800’s,scientists were able to measure the exact age inyears of different rocks. Measuring the amounts ofradioactive elements in rocks let scientists useabsolute dating to give ages to each chunk of timeon the geologic time scale. For example, they arenow able to state that the Jurassic began about 200million years ago and that it lasted for about 55

million years.

Geologic Time Scale

Today, the geologic time scale is divided intomajor chunks of time called eons. Eons may befurther divided into smaller chunks called eras,and each era is divided into periods. Figure belowshows you what the geologic time scale looks like.We now live in the Phanerozoic eon, the Cenozoicera, and the Quarternary period. Sometimes,periods are further divided into epochs, but theyare usually just named “early” or “late,” forexample, “late Jurassic,” or “early Cretaceous.”Note that chunks of geologic time are not dividedinto equal numbers of years. Instead, they aredivided into blocks of time when the fossil recordshows that there were similar organisms on Earth.

Figure 12.1 The Geologic Time Scale.One of the first scientists to understand

geologic time was James Hutton. In the late 1700s,he traveled around Great Britain and studiedsedimentary rocks and their fossils. He believedthat the same processes that work on Earth todayformed the rocks and fossils from the past. Heknew that these processes take a very long time, sothe rocks must have formed over millions of years.Before Hutton, most people believed the Earth wasonly several thousand years old. His work helped

us understand that the laws of nature never changeand that the Earth is very old. He is sometimescalled the “father of geology.”

The geologic time scale is often shown withillustrations of how life on Earth has changed. Itsometimes includes major events on Earth, too,such as the formation of the major mountains or theextinction of the dinosaurs. Figure below showsyou a different way of looking at the geologic timescale. It shows how Earth’s environment and lifeforms have changed.

Figure 12.2 A different way of looking at the Geologic

Time Scale.

Lesson Summary

The Earth is very old, and the study of Earth’spast requires us to think about times that weremillions or even billions of years ago.Scientists use the geologic time scale toillustrate the order in which events on Earthhave happened.The geologic time scale was developed afterscientists observed changes in the fossilsgoing from oldest to youngest sedimentaryrocks. They used relative dating to divideEarth’s past in several chunks of time whensimilar organisms were on Earth.Later, scientists used absolute dating todetermine the actual number of years ago thatevents happened. The geologic time scale isdivided into eons, eras, periods, and epochs.

Review Questions

1. How old is the Earth?

2. Why did early geologic time scales notinclude the number of years ago that eventshappened?

3. Dinosaurs went extinct about 66 million yearsago. Which period of geologic time was thelast in which dinosaurs lived?

4. Can scientists use the same principles theyuse to study Earth’s history to also study thehistory of other planets?

5. Suppose you are hiking in the mountains ofUtah and find a fossil of an animal that livedon the ocean floor. You learn that the rock thatholds the fossil is from the Mississippianperiod. What was the environment like duringthe Mississippian in Utah?

6. Why are sedimentary rocks more useful thanmetamorphic or igneous rocks in establishingthe relative ages of rock?

7. Which is likely to be more frequently found inrocks: fossils of very old sea creatures orvery old land creatures?

Vocabulary

absolute datingMethods used to determine how long agosomething happened.

extinctionWhen an organism completely dies out.

fossilsThe remains of past life, such as bones,shells, or other hard parts; may also includeevidence of past life such as footprints or leafimpressions.

geologic time scaleA timeline that illustrates Earth’s past.

relative dating methodsUsed to determine the order of geologicevents in Earth’s history.

Points to Consider

How did life on Earth change from one periodof geologic time to the next?When did life first appear on Earth?What conditions were necessary on Earth forliving things to survive?

Early Earth

Lesson Objectives

Describe how the Earth formed with otherparts of the solar system more than 4 billionyears ago.Explain how Earth’s atmosphere has changedover time.Explain the conditions that allowed the firstforms of life to develop on Earth.

Introduction

Figure 12.3 The Earth from space. The Earth looks very

different today than it did when it first formed over4 billion years ago.

Imagine that you had a movie that shows thehistory of Earth from its beginning to the presentday—as if a giant camera in space had recordedpictures of Earth over the last 4 1/2 billion years.How do you think the Earth would look in thatmovie at different times in history? How do youthink it has changed?

If you put the movie in fast-forward, you would

see lots of action and lots of change! You wouldsee that our planet has undergone remarkablechanges over billions of years (Figure above).Huge mountains have formed, been destroyed, andreplaced with new mountains. The oceans haveopened up and moved around the globe. Thecontinents have moved around, split apart fromeach other, and collided with each other, untilfinally reaching their present locations. Life onEarth has also changed tremendously. At first, theEarth was not even able to support life. There wasno oxygen in the atmosphere, and Earth’s surfacewas extremely hot. Slowly, over millions of years,the Earth changed so that plants and animals couldbegin to grow. Living things then changed the Eartheven more.

We often enjoy using our imagination to thinkabout what the Earth was like when dinosaursroamed around (Figure below). What images cometo your mind when you think about the dinosaurs?Now imagine a time on Earth before even thedinosaurs. Imagine the time before any living thingwas on Earth. What images come to mind now?

How do you think the Earth looked when it wasfirst formed? This lesson will help you understandhow the Earth formed, what it looked like duringits earliest years, and how life first developed onEarth.

Figure 12.4 The Earth and its dominant life forms have

changed throughout the Earths long history.

Evaluating Prior Knowledge

The following questions are addressed in otherchapters and will help you work through this

lesson. Research these before you move on.What are chemical elements?What conditions do plants and animals require

to live?What is the atmosphere and what is it made of?How do weathering and erosion affect the

Earth?

Formation of Earth and Our SolarSystem

We can construct the formation history of oursolar system by looking at regions where otherstars are forming now. Star formation begins whena giant cloud of gas and dust collapses under itsown gravity. As the cloud contracts, it begins tospin faster and settles into a disk-shaped structure.We see these disc-shaped objects (calledproplyds) in the Orion Nebula (Figure below),where the new stars are forming today. Most of thedusty disk material drains toward the center wherethe density gradually increases until the enormous

central pressure triggers nuclear fusion reactionsand the star is born.

Figure 12.5 Orion NebulaHowever, a relatively small fraction of the

disk material is left behind in the form of ice-coated dust grains. The icy mantles of the grainsbegin sticking together and eventually grow tometer-sized rocky boulders called planetesimals.The planetesimals collide and accrete into largerbodies that are tens of kilometers in diametercalled protoplanets. Once the protoplanets clear agap in the disk, they become bonafide planets andtheir orbits begin to stabilize (Figure below).

Figure 12.6 An artists rendition of a baby star still

surrounded by a protoplanetary disc in whichplanets are forming.

The process of planet formation is messy. Notall of the planetesimals are accreted into planets.Millions of planetesimals remain as the leftoverdebris and are now the asteroids and ice-coatedcomets in our solar system. In the first hundredmillion years after the formation of the Sun,collisions between the leftover planetesimals andthe planets were common. We see evidence forheavy bombardment by planetesimals on thesurfaces of the moon and Mercury (Figure belowand Figure below).

Figure 12.7 The surface of the moon is scarred by

collisions with debris that was meters tokilometers in diameter. Most of the planetesimalswere accreted into planets or moons, but some ofthese objects remain as meteors, asteroids, andcomets in our solar system today.

Figure 12.8 The surface of Mercury shows similar

collisional cratering. Most of the planetesimalswere accreted into planets or moons, but some ofthese objects remain as meteors, asteroids, andcomets in our solar system today.

The same types of collisions would haveoccurred on the surface of the Earth, howevererosive processes have erased all except the mostrecent of these collisions. Pictured in Figure

below is a Meteor Crater in Arizona.

Figure 12.9 Meteor crater in Arizona was formed about

40,000 years ago by the impact of a meteorite thatwas about 50 meters in diameter. Such collisionsare rare today.

About 100 million years after the formation ofthe Sun, the gravity of the planets and moons in oursolar system had swept up most of theplanetesimals. However, millions of these objectsstill remain in gravitationally stable orbits in themain asteroid belt of the solar system, in theTrojan asteroid belt, or out beyond Neptune andPluto in the Kuiper belt. Illustrated in the sketchbelow is the location of the largest reservoir ofasteroids in our solar system today (Figure

below).

Figure 12.10 This sketch shows the largest reservoir of

asteroids in our solar system today.Earth is the only object in our solar system

known to support life (Figure below). Today thereare over 1 million known species of plants andanimals on Earth.

Figure 12.11 The Earth formed at the same time as the other

planets in our solar system about 4 1/2 billionyears ago.

The materials that came together to form theEarth were made of several different chemicalelements. Each element has a different density,defined as mass per volume. Density describeshow heavy an object is compared to how muchspace the object takes up. After Earth’s earlyformation, the denser elements sank to the center.The lighter elements rose to the surface. You haveprobably seen something like this happen if youhave ever mixed oil and water in a bottle. Thewater is denser than oil. If you put both in a bottle,shake it up, and then let it sit for a while, the watersettles to the bottom and the oil rises up over thetop of the water.

Today, the Earth consists of layers thatrepresent different densities (Figure below).Earth’s center is called its core. The core is madeof very dense metal elements called iron andnickel. The outermost layer of the Earth is its crust.The crust is made mostly of light elements such assilicon, oxygen, and aluminum. More informationon the different layers of the Earth is presented inthe lesson on plate tectonics.

Figure 12.12 The Earth is made of several layers that vary in

density. The center of the Earth is the core, which

is the densest. The outermost layer is the crust,which is the least dense. The middle layers makeup the mantle.

Formation of Earth's Atmosphere

The early Earth was very different from ourEarth today. The early Earth experienced frequentimpacts from asteroids and meteorites and hadmuch more frequent volcanic eruptions. There wasno life on Earth for the first billion years becausethe atmosphere was not suitable for life. Earth’sfirst atmosphere had lots of water vapor but hadalmost no oxygen. Later, frequent volcaniceruptions put several different gases into the air(Figure below). These gases created a new type ofatmosphere for Earth. The volcanic eruptionsspewed gases such as nitrogen, carbon dioxide,hydrogen, and water vapor into the atmosphere—but no free oxygen. Without oxygen, there was stillvery little that could live on Earth.

Figure 12.13 Volcanic eruptions occurred almost constantly

on the early Earth. Eruptions put water vapor,carbon dioxide and other gases into the air thathelped create Earths early atmosphere.

Slowly, two processes changed Earth’satmosphere to one that is more oxygen-rich—likethe one we have today. First, radiation from theSun caused water vapor molecules to split apart.Remember that a molecule of water is made of theelements hydrogen and oxygen, or H2O. Radiationfrom the Sun split some of the water molecules intohydrogen and oxygen. The hydrogen escaped backto outer space. The oxygen accumulated in theatmosphere. The second process that changed

Earth’s early atmosphere was photosynthesis(Figure below). About 2.4 billion years ago, atype of organism called cyanobacteria evolved onthe early Earth and began carrying outphotosynthesis. Photosynthesis uses carbon dioxideand energy from the Sun to produce sugar andoxygen. The cyanobacteria were very simpleorganisms but performed an important role inchanging Earth’s early atmosphere. They carriedout photosynthesis to produce the materials theyneeded to grow. They gave off oxygen to theatmosphere as they did this.

Oxygen in the atmosphere is important for lifefor two main reasons. First, oxygen makes up theozone layer. The ozone layer is in the upper part ofthe atmosphere, and is made of O3 molecules — aparticular type of oxygen molecule. It blocksharmful radiation from the sun and keeps it fromreaching Earth’s surface. Without an ozone layer,intense radiation from the sun reached the earlyEarth’s surface, making life almost impossible.Secondly, oxygen in the atmosphere is necessaryfor animals, including humans, to breathe. No

animals would have been able to breathe in Earth’searly atmosphere. However, there were probablyseveral types of bacteria that lived on Earth duringthis early time. They would have been anaerobic,meaning that they did not need oxygen to live.

Figure 12.14 Bacteria capable of photosynthesis first

appeared on Earth about 2.4 billion years ago.Photosynthesis takes sunlight, carbon dioxide, and

water and produces sugar and oxygen.Photosynthesis contributed oxygen to Earths earlyatmosphere and helped change it from one rich incarbon dioxide to one rich in oxygen.

Very simple cells lived on Earth for the firstfew billion years of Earth’s history. Some of theoldest fossils of more complex organisms are fromabout 2 billion years ago. They are found inAustralia.

Besides changes in life and the atmosphere,other changes have also happened since the Earthwas first formed. Early volcanic eruptions onEarth released large amounts of water vapor intothe atmosphere. The water vapor slowlycondensed and returned to Earth’s surface inrainfall. This formed the oceans. Water began tocycle on Earth, and events like rainfall and stormsnext began to change the Earth’s surface throughweathering and erosion. The Earth's Fresh Waterchapter gives more detail on how water cycles onEarth.

The continents were in very different locationsthan they are now. Scientists do not know how

Earth’s land looked exactly after the planet’s firstformation. They do know that North America andGreenland formed one giant landmass calledLaurentia about 1.8 billion years ago. By about 1billion years ago, Antarctica may have been closeto the equator, even though it now sits at Earth’sSouth Pole. Today, Earth’s continents continue toslowly shift around the globe.

Lesson Summary

The Earth formed more than 4 billion yearsago along with the other planets in our solarsystem.The early Earth had no ozone layer and wasprobably very hot. The early Earth also hadno free oxygen.Without an oxygen atmosphere very fewthings could live on the early Earth.Anaerobic bacteria were probably the firstliving things on Earth.The early Earth had no oceans and was

frequently hit with meteorites and asteroids.There were also frequent volcanic eruptions.Volcanic eruptions released water vapor thateventually cooled to form the oceans.The atmosphere slowly became more oxygen-rich as solar radiation split water moleculesand cyanobacteria began the process ofphotosynthesis. Eventually the atmospherebecame like it is today and rich in oxygen.The first complex organisms on Earth firstdeveloped about 2 billion years ago.

Review Questions

1. Describe how the different layers of the Earthvary by density. When did the materials thatmake the Earth separate out by density?

2. Explain two reasons why having an oxygen-rich atmosphere is important for life on Earth.

3. Scientists believe that Earth’s ozone layer isshrinking because of human activities and airpollution. What affect might this have on

Earth’s life forms?4. Describe the role of cyanobacteria in

changing Earth’s early atmosphere.5. List three ways the Earth was different today

from when it was first formed.6. Suppose that the Earth had been much cooler

when it first formed. How would the earth’sinterior be different than it is today?

Vocabulary

atmosphereThe mixture of gases that surrounds the Earthand contains the air we breathe.

condensedCooled and changed from water vapor toliquid water.

densityThe measure of how much mass an object hasin a given volume.

moleculesThe smallest possible amounts of a chemicalsubstance.

radiationEnergy given off by the Sun.

speciesA group of living things that have similarcharacteristics.

waterVapor water in a gas form.

Points to Consider

How did life on Earth develop from simplebacteria to more complex organisms?When did complex organisms like fish,reptiles, and mammals appear on Earth?When did the major features of the Earth thatwe know today first form?

History of Earth’s LifeForms

Lesson Objectives

Describe how adaptations develop.Explain how the fossil record shows us thatspecies evolve over time.Describe the general development of Earth’slife forms over the last 540 million years.

Introduction

In the summer of 1909, an American scientistnamed Charles Doolittle Walcott (Figure below)was in the Rocky Mountains of British Columbia,Canada. He was a paleontologist, which is ascientist who studies past life on Earth. He wassearching for fossils. Riding on horseback, he wasmaking his way down a mountain trail when he

noticed something on the ground. He stopped topick it up. It was a fossil! He began to dig aroundthe area and found even more fossils. The fossilsthat Walcott found were of some of the mostbizarre organisms anyone had ever seen. One ofthe organisms preserved in the fossils had a softbody like a worm, five eyes, and a long nose like avacuum cleaner hose (Figure below). Most of thefossils were the remains of animals that do not livetoday. They are now extinct, which means thatnothing of their kind lives and that they are goneforever.

Figure 12.15 Charles Doolittle Walcott.

Figure 12.16 This bizarre animal with five eyes lived during

the Cambrian. Fossils of it were discovered byCharles Walcott.

The organisms in Walcott’s fossils livedduring a time of geologic history known as theCambrian. The Cambrian period began about 540million years ago. It marked the beginning of thePhanerozoic Eon. It also marked the beginning ofmany new and complex life forms appearing onEarth. In fact, the term Phanerozoic means “time ofwell-displayed life.” We still live today in thePhanerozoic Eon. However, life on Earth is verydifferent today than it was 540 million years ago.

This lesson covers some of the history of life onEarth. It will show you how living things havedeveloped and changed over the last 540 millionyears of the Phanerozoic Eon. You will learn abouthow species adapt and evolve over time.

Evaluating Prior Knowledge

Be sure that you can answer the followingquestions before you begin this lesson.

What is a fossil?How is geologic time divided?How do organisms depend on their

environment to live?

Earth's Diversity

There are over 1 million species of plants andanimals known to be currently alive on Earth(Figure below). Scientists believe there aremillions more that have not been discovered yet.Look around you and you notice that the organisms

on this planet have incredible variation. One of themost remarkable features of Earth’s organisms istheir ability to survive in their specificenvironments. For example, polar bears have thickfur coats that help them stay warm in the icy watersthat they hunt in (Figure below). Reindeer havesponge-like hoofs that help them walk on snowyground without slipping and falling. Plants that livein dry desert environments have special stems andleaves that help them conserve water.

Figure 12.17 There is an amazing diversity of organisms on

Earth.

Figure 12.18 Many animals, like this polar bear, have

special body features that help them live in acertain environment.

Other organisms have special features that helpthem hunt for food or avoid being the food ofanother organism. For example, when zebras in aherd run away from lions, the zebras’ dark stripesconfuse the lions and make it hard for them to focuson just one zebra during the chase. Hummingbirdshave long thin beaks that help them drink nectarfrom flowers. Some plants have poisonous or foul-tasting substances in them that keep animals from

eating them.

Adaptations and Evolution

The characteristics of an organism that help itsurvive in a given environment are calledadaptations. Adaptations develop when certainvariations in a population help some memberssurvive better than others (Figure below). Oftenthe variation comes from a mutation, or a randomchange in an organism’s genes. The ones thatsurvive pass favorable traits on to their offspring.

To help you understand adaptation, think abouta population of oak trees. Imagine that most of thetrees are easily killed by a certain fungus but thatevery now and then, there is one tree that has anatural ability to survive the fungus. That one treedisplays a variation that gives it a better chance ofsurviving its environment. It also has a betterchance of living to produce seeds and haveoffspring. It will reproduce and carry on thespecies, while the other trees will die off. The tree

with the natural ability to resist the fungus willpass that trait on to its offspring. The other treeswill not live and have offspring. Eventually thepopulation will change so that most of theindividual trees have the trait to survive the fungus.This is an adaptation. Adaptations are inheritedtraits that an organism gets from its parents. Overtime traits that help an organism survive becomemore common. Traits that hinder survivaleventually disappear.

Figure 12.19 An explanation of how adaptations develop.

Changes and adaptations in a speciesaccumulate over time. Eventually the descendantsare very different from their ancestors and maybecome a whole new species. Changes in aspecies over time are called evolution. We learnabout evolution from the fossil record. It shows usthat many of the life forms that live todaydeveloped from earlier, different life forms. Forexample, horse fossils show us that about 60million years ago horses were much smaller thanthey are today (Figure below). Fossils also showus that horses’ teeth and hooves have changedseveral times as horses have adapted to changes inthe environment.

Figure 12.20

The horse has evolved over the last 60 millionyears. Horses today are much larger than earlierhorses.

Studying the Fossil Record

Like the organisms that were represented inWalcott’s fossils, many of the organisms that oncelived on Earth are now extinct. Earth’s overallenvironmental conditions have changed many timessince the Cambrian, and many organisms did nothave the traits to survive the changes. Those thatdid survive the changes passed traits on to theiroffspring. They gave rise to the species that livetoday.

We study fossils to learn about how speciesresponded to change over the Earth’s long history.Fossils show us that simple organisms dominatedlife on Earth for its first 3 billion years. Then,between 1 and 2 billion years ago, the first multi-cellular organisms appeared on Earth. Life formsgradually evolved and became more complex.

During the Cambrian period, animals became morediverse and complex. We sometimes refer to thispart of the Phanerozoic Eon as the CambrianExplosion—meaning a time when the Earth“exploded” with incredible numbers of newcomplex life forms.

Phanerozoic Eon

The Phanerozoic Eon is divided into threechunks of time called eras—the Paleozoic, theMesozoic, and the Cenozoic (Table (below). Theyspan from about 540 million years ago to thepresent. We live now in the Cenozoic Era. Thetable below shows how life has changed during thelong span of the Phanerozoic Eon. Notice thatdifferent types of organisms developed at differenttimes. However, all organisms evolved from acommon ancestor. Life gradually became morediverse and new species branched out from thatcommon ancestor. Most modern organismsevolved from species that are now extinct. To get

an idea of how an organism can change from asingle common ancestor to many different types,think about all the different types of dogs. All dogsevolved from a common wolf ancestor. Todaythere are hundreds of varieties of dogs that all lookvery different.Development of Life During the Phanerozoic Eon

Era Millions ofYears Ago Major Forms of Life

Cenozoic0.2(200,000years ago)

First humans

35First grasses; grasslandsbegin to dominate theland

Mesozoic 130 First plants with flowers150 First birds on Earth200 First mammals on Earth

Age of dinosaursbegins

251

Paleozoic 300 First reptiles on Earth360 First amphibians on Earth

400

First insects onEarth

475 First plants and fungibegin growing on land

500First fish on Earth

The eras of the Phanerozoic Eon are separatedby events called mass extinctions. A massextinction occurs when large numbers of organismsbecome extinct in a short amount of time. Betweenthe Paleozoic and the Mesozoic, nearly 95% of allspecies on Earth died off. The cause or causes ofthis extinction are still being debated.

Between the Mesozoic and the Cenozoic, about50% of all animal species on Earth died off. Thismass extinction, 65 million years ago, is the one inwhich the dinosaurs became extinct. Although thereare other hypotheses, most scientists think that thismass extinction took place when a giant meteoritestruck Earth with the energy of the most powerfulnuclear weapon. The impact kicked up a massivedust cloud. When the particles rained back onto thesurface they heated the atmosphere until it becameas hot as a kitchen oven, roasting animals. Dust that

remained in the atmosphere blocked sunlight for ayear or more, causing a deep freeze and endingphotosynthesis. Sulfur from the impact mixed withwater in the atmosphere to form acid rain, whichdissolved the shells of the tiny marine plankton thatform the base of the food chain. With little foodbeing produced by land plants and plankton,animals starved. Carbon dioxide was also releasedfrom the impact and eventually caused globalwarming. Life forms could not survive thedramatic temperature swings.

Earth’s climate changed numerous times duringthe Phanerozoic Eon. Just before the beginning ofthe Phanerozoic, much of the Earth was cold andcovered with glaciers (Figure below). As thePhanerozoic began, however, the climate waschanging to a warm and tropical one (Figurebelow). The glaciers were replaced with tropicalseas. This allowed the Cambrian Explosion ofmany new life forms on Earth. During thePhanerozoic, Earth’s climate has gone through atleast 4 major cycles between times of coldglaciers and times of warm tropical seas. Some

organisms survived environmental changes in theclimate; others became extinct when the climatechanged beyond their capacity to cope with it.

Figure 12.21 Just before the Phanerozoic, many parts of

Earth were covered with glaciers. After the Earthbegan to warm and many of the glaciers melted,there was an explosion of new life on Earth. Theglaciers in this picture are from the present.However, glaciers are much less common on Earthtoday than at other times in Earths history.

Figure 12.22 The Phanerozoic Eon was often characterized

by times of warm tropical climates. The age of thedinosaurs was especially mild for most of theEarth. This allowed plants and animals to spreadover large areas of land. This picture shows plantsin a modern rainforest. Plants of the Phanerozoicmay have looked similar.

Lesson Summary

Adaptations are favorable traits thatorganisms inherit. Adaptations develop fromvariations within a population and help

organisms to survive in their givenenvironment.Changes in populations accumulate over time;this is called evolution.The fossil record shows us that present daylife forms evolved from earlier different lifeforms. It shows us that the first organisms onEarth were simple bacteria that dominated theEarth for several billion years.Beginning about 540 million years ago morecomplex organisms developed on Earth.During the Phanerozoic Eon all of the plantand animal types we know today haveevolved.Many types of organisms that once lived arenow extinct. Earth’s overall environment,especially the climate, has changed manytimes, and organisms change too over time.

Review Questions

1. Describe what is meant by adaptation.

2. The first animals on Earth had soft bodies.Gradually many animal species evolved thathad hard outer parts called exoskeletonscovering their bodies. How might anexoskeleton be a favorable adaptation?

3. Explain why unfavorable traits do not usuallyget passed to offspring.

4. List the order in which the major types ofanimals appeared on Earth.

5. How might climate have affected the abilityof plants to grow over large areas during agiven time?

6. One cause of mass extinctions is meteorite orcomet impacts. What might be someadditional causes of mass extinctions?

Vocabulary

adaptationA trait that an organism inherits that helps itsurvive in its natural environment.

evolutionThe change in an organism's traits over timesuch that a new species is often the result.

glaciersLarge sheets of flowing ice.

paleontologistA scientist who studies Earth’s past lifeforms.

tropicalA climate that is warm and humid.

variationHaving many differences.

Chapter 13: Earth's FreshWater

Water on Earth

Lesson Objectives

Describe how water is distributed on Earth.Describe what powers the water cycle andhow water moves through this cycle.

Introduction

Water is a simple compound, made of twoatoms of hydrogen and one atom of oxygen bondedtogether. More than any other substance on theEarth, water is important to life and hasremarkable properties. Without water, life couldprobably not even exist on Earth. When looking at

Earth from space, the abundance of water on Earthbecomes obvious — see Figure below. On land,water is also common: it swirls and meandersthrough streams, falls from the sky, freezes intosnow flakes, and even makes up most of you andme. In this chapter, we’ll look at the distribution ofwater on Earth, and also examine some of itsunique properties.

Figure 13.1 Earth, the Blue Marble, can be seen in this

photograph to be mostly covered with liquidwater.

Distribution of Earth’s Water

A s Figure above makes clear, water is themost abundant substance on the Earth’s surface.About 71% of the Earth’s surface is covered withwater, most of which is found in the oceans. Infact, 97% of Earth's water, nearly all of it, is in theEarth’s oceans. This means that just 3% of Earth'swater is fresh water, water with lowconcentrations of salts (Figure below). Mostfreshwater is found as ice in the vast glaciers ofGreenland and the immense ice sheets ofAntarctica. That leaves just 0.6% of Earth’s waterthat is freshwater that humans can easily use. Mostliquid freshwater is found under the Earth’ssurface as groundwater, while the rest is found inlakes, rivers, and streams, and water vapor in thesky.

Figure 13.2 Earths water is mostly in the oceans. Fresh

water is only 3% of all the Earths water, and mostof that is in the form of ice.

The Water Cycle

Water is a special substance. It is abundant onEarth and frequently appears as a gas, liquid, andsolid. It is one of the few substances on Earth thatis frequently found in all three phases of matter.Moreover, it can readily cycle through the globe:

the same molecule can travel through manydifferent regions on Earth.

Three States of Water

Part of the reason that water is unique isbecause of its melting point and boiling point.Under normal atmospheric conditions, waterfreezes at 0oC (32oF) and boils at 100oC (212oF).Because of our Earth’s position in the solarsystem, Earth’s temperature varies from far belowthe melting point of water to well above thatmelting point. Even though water does not boil atnormal temperatures, it often becomes gaseouswater vapor by evaporating. All this means thatwe frequently see water in its three phases onEarth (See Figures ice, water, and clouds).

Figure 13.3 Solid ice floating amidst liquid water. This

image shows what an iceberg might look like ifyou could see both above and below the surface.

Figure 13.4 Liquid water.

Figure 13.5 Water vapor is invisible to our eyes. However,

we can see the clouds that form when water vaporcondenses.

The Water Cycle

The water on Earth moves about the Earth inwhat is known as the water cycle (Figure below).Because it is a cycle, there truly is no beginningand no end. The very same water molecule foundin your glass of water today has probably been onthe Earth for billions of years. It may have been ina glacier or far below the ground. It may have beenhigh up in the atmosphere and deep in the belly of adinosaur. Who knows where it will end up today,

when you’re done with it!

Figure 13.6 Water on Earth is constantly in motion.Let’s study Figure above for a moment. The

Sun, many millions of kilometers away, providesthe energy which drives the water cycle. Since theocean holds most of the Earth’s water, let’s beginthere. As you can see in the illustration, water inthe ocean evaporates as water vapor into the air.The salt in the ocean does not evaporate with thewater, however, so the water vapor is fresh. Someof the invisible water vapor in the air condensesto form liquid droplets in clouds. The clouds areblown about the globe by wind. As the water

particles in the clouds collide and grow, they fallfrom the sky as precipitation. Precipitation canoccur in forms such as rain, sleet, hail, and snow.Sometimes precipitation falls right back into theocean. Other times, however, it falls onto the solidearth as freshwater.

That freshwater, now on the Earth, may befound in a solid form as snow or ice. Some of itgoes directly back into the air to form water vaporand clouds again. However, most of this solidwater sits atop mountains and slowly melts overtime to provide a steady flow of freshwater tostreams, rivers, and lakes below. Some of thatwater enters the Earth’s groundwater, seepingbelow the surface through pores in the ground. Thiswater can form aquifers that store freshwater forcenturies. Alternatively, it may come to the surfacethrough springs or find its way back to the oceans.

When water falls from the sky is rain it formstreams and rivers that flow downward to oceansand lakes. People use these natural resources astheir source of water. They also create canals,aqueducts, dams, and wells to direct water to

living areas to meet their needs (Figure below).Sometimes, our manipulation or pollution of watergreatly affects other species. Many scientists areseeking better ways of using Earth’s water in asustainable and efficient way.

Obviously, people are not the only creaturesthat rely on water. Plants and animals also dependon this vital resource. Plants play an important rolein the water cycle because they release largeamounts of water vapor into the air from theirleaves. This process of transpiration moves liquidwater from plants into the air. You can seetranspiration in action if you cover a few leaves ona plant with a plastic bag. Within a few hours,water vapor released from the leaves will havecondensed onto the surface of the bag.

Figure 13.7 Hoover Dam on the Colorado River.

Lesson Summary

Earth's surface is mostly water covered. Mostof that water is in our oceans, leaving only3% freshwater.Water exists on Earth in all three phases:solid, liquid and gas.The water cycle moves water from thehydrosphere to the atmosphere to the land andback again.The major processes of the water cycle

include evaporation and transpiration,condensation, precipitation and return to theoceans via runoff and groundwater supplies.

Review Questions

1. About what percent of the Earth’s water isfresh water?

2. About what percent of all of Earth’s water isfound in groundwater, streams, lakes, andrivers?

3. Explain the following statement: The water onother planets is present in a different formthan on Earth.

4. What powers the water cycle?5. In what state would water be found at 130oC?

What state would water be at -45oC?6. Define the words condensation and

evaporation.7. Summarize the water cycle.8. Why do you think the atmosphere is so

important to the water cycle?

9. Suppose the sun grew much stronger inintensity. How would this affect the watercycle?

Further Reading / SupplementalLinks

http://www.freshwaterlife.org/http://www.usgs.gov/

Vocabulary

aquiferA layer of rock, sand, or gravel that holdslarge amounts of groundwater. Humans oftenuse aquifers as sources of freshwater.

condenseTo turn from a gas to a liquid.

freshwaterWater with a low concentration of salts,

which can be consumed and used by humans.

groundwaterWater that is found beneath the Earth’ssurface, between soil or rock particles.

precipitationWater that falls to the Earth from the sky.Precipitation usually takes the form of rain,but can also occur as snow, sleet, or hail.

transpirationThe release of water vapor into the airthrough the leaves of plants; sometimes calledevapotranspiration.

water cycleThe cycle through which water moves aroundthe Earth, changing both its phase (betweensolid to liquid to gas) and its location (in theoceans, in clouds, in streams and lakes, and ingroundwater).

water vapor

Water in the form of a gas. Water vapor isinvisible to humans; when we see clouds, weactually are seeing liquid water in the clouds.

Points to Consider

How does precipitation affect the topographyof the Earth?What natural disasters are caused by thewater cycle?How might pollution affect creatures far fromthe source of the pollution?How might building dams disrupt the naturalwater cycle?If the temperature of the Earth increasesthrough global warming, how might the watercycle be altered?

Surface Water

Lesson Objectives

Compare streams and rivers and theirimportance.Describe what ponds and lakes are, and whythey are important.Explain why wetlands are significant in thewater cycle, and describe their biodiversity.Describe the causes of floods and theireffects.

Introduction

As we’ve learned, some of the freshwater onthe Earth is on the surface, in streams, rivers,ponds and lakes. This freshwater is tremendouslyimportant to humans, plants, and animals. Wetlandsare areas where water bodies and land meet.Wetlands contain high biodiversity and play a key

role in naturally removing pollutants from water.At times, surface waters flood, which often createshazardous conditions for people on the ground.

Streams and Rivers

A stream is a body of moving water confinedby a bottom (or bed) and earthen sides (or banks).There are many categories of streams includingcreeks, brooks, tributaries, bayous, and rivers —all of these types of streams vary in their size,depth, speed, and location. Streams are always-changing natural objects where water flowsdownhill, taking turns through hills and plains aselevation, rock type, and topography guide thestream along. They are responsible for a great dealof erosion and can create great canyons over time,as they slowly move soil, pebbles, and evenboulders downstream.

Parts of a Stream

The place at which a stream originates iscalled the source; this is often a spring but it couldbe the top of a mountain. When two streams cometogether, the point where they join is called aconfluence. The smaller of the two streams isconsidered a tributary of the larger stream. A poolin a stream is somewhat like a swimming pool —it’s a slow part in the stream where water movesmore slowly, so that the stream spreads out andbecomes deeper. Finally, the point at which astream comes into a large body of water, like anocean or a lake is called the mouth. These areasare called estuaries, and they oftentimes formunique ecosystems where water from the streamand the lake or ocean mix together (Figure below).

Figure 13.8 (Left)This estuary at Damas Island, Costa Rica,

shows how water, plants, and land all come

together in an estuary. (Right) This is a satelliteimage of the Nile Delta, showing the uniqueecosystem around the estuary, and its shape (thename delta comes from the greek letter ).

Rivers

Rivers are the largest type of stream, and movelarge amounts of water through landscapes fromhigher to lo through landscapes from higher tolower elevations. North America has severaldivides that separate the land up into separatewater basins (Figure below). In each of thesesections, rivers will eventually run to the AtlanticOcean, Pacific Ocean, the Great Lakes, ArcticOcean, or the Gulf of Mexico. Most rivers arebordered by floodplains, which are flat areas thatflood when rivers overflow their banks.

Figure 13.9 The divides of North America.Rivers generally move a lot of water. The

Amazon River, the world’s river with the greatestflow, has a flow rate of nearly 220,000 cubicmeters per second! By comparison, at NiagaraFalls, nearly 1,800 cubic meters of water fall persecond (Figure below).

Figure 13.10 The famous Horseshoe Falls at Niagara Falls

drops over 1,800 cubic meters of water persecond, down a cliff nearly 50 meters (170 feet) inheight. The falls are fed by Lake Erie and theNiagara River.

Since rivers contain so much water, humanshave used them since the beginning of civilizationas a source of water, food, transportation, defense,power, recreation, and waste disposal. The wateryou drink probably comes from a reservoir fed byrivers. The electricity in your house may also comefrom power plants that use rivers to generatepower. Obviously, the natural areas along byrivers are affected by humans use or misuse of the

rivers. Sometimes entire populations of organismscan be destroyed by pollution of a river manymiles upstream. Table below shows the 10 longestrivers in the world.

# Name ContinentRate ofFlowm3/s

ApproximateLength (km)

1 The Nile Africa 2,900 6695

2 TheAmazon

SouthAmerica 225,000 6683

3 Yangtze Asia 33,000 6380

4 MississippiRiver

NorthAmerica 13,000 5970

5 Ob River Asia 13,000 54106 Huang He Asia 2,600 48307 Congo Africa 43,000 46308 Lena Asia 17,000 44009 Amur Asia 6,000 4350

10 YeniseiRiver Asia 20,000 4106

Ponds and Lakes

Streams and rivers, by definition, are bodies ofwater that have a current; they are in constantmotion. Ponds and lakes, on the other hand, do not(Figure below). They are generally bordered byhills or low rises, so that the water is blocked fromflowing directly downhill. They represent yetanother important resource for humans and anotherarea in need of conservation.

Though the word pond refers to water that doesnot constantly flow downhill, there is disagreementabout the exact definition of a pond. It is generallyagreed, however, that a pond is a small body offreshwater. You probably wouldn’t need a boat toget across it, and you might be able to stand up init. Little or no surface water would escape fromthe pond through streams, and they are often fed byunderground springs.

Figure 13.11 Ponds are small, enclosed bodies of water.Lakes are larger bodies of freshwater formed

by some natural process like tectonic platemovement, landslides, or human actions , such asbuilding a dam. Almost all lakes are freshwater,and water usually leaves the lake through a river ora stream. All lakes lose some water toevaporation.

Some lakes are so large that they have theirown tidal systems and currents, and can affectweather patterns. The Great Lakes in the UnitedStates, for example, contain 22% of the world’sfresh surface water (Figure below). The largest ofthe Great Lakes, Lake Superior, has a tide that

rises and falls several centimeters each day. TheGreat Lakes are large enough to change the entireweather system in the Northeast region of theUnited States, in what is known as the “lakeeffect.” They are home to countless species of fishand wildlife as well.

Figure 13.12 The Great Lakes are the largest lakes in the

world. They are found along the border of theUnited States of America and Canada.

Lakes can be formed in a variety of differentways. Some lakes, like The Great Lakes filldepressions eroded as glaciers scraped soil androck out from the landscape. Lakes known as craterlakes, formed in volcanic calderas that have filledup with precipitation. Rift lakes are formed in

cracks created by tectonic faults. And subglaciallakes are found below a frozen ice cap. As a resultof geologic history and the arrangement of landmasses on the Earth, most lakes are in the NorthernHemisphere. In fact, over 60% of all the world’slakes are in Canada — most of these lakes wereformed by the glaciers that covered most ofCanada in the last Ice Age.

Limnology is the study of all bodies offreshwater and the organisms that live there. Theecosystem of a lake is divided into three distinctsections (Figure below):

1. The littoral zone, which is the sloped areaclosest to the edge of the water.

2. The open-water zone (also called the photicor limnetic zone), where sunlight is abundant.

3. The deep-water zone (also called the aphoticor profundal zone), where little or no sunlightcan reach.

Much life is found in the littoral zone, becausesunlight allows the growth of plants on the lake

bed. These plants in turn, provide food and shelterto animals like snails, insects, and fish. Otherplants and fish such as bass and trout live in theopen-water zone. The deep-water zone does notallow for plants to grow, so fewer organisms livethere. In this zone, most organisms are scavengerslike crabs and catfish, which feed on deadorganisms that fall to the bottom. Fungi andbacteria aid in the decomposition of those deadorganisms, too. Though different creatures live inthe oceans, ocean waters also have these samedivisions based on sunlight with similar types ofcreatures that live in each of the zones.

Figure 13.13 The three primary zones of a lake are the

littoral, open-water, and deep-water zones.Lakes are not always permanent features of a

landscape. Some intermittent lakes come and gowith the seasons, as water levels rise and fall.Over a longer time period, lakes can disappearwhen they are filled in with sediments, if thesprings or streams that fill them diminish, or iftheir outlets grow due to erosion. When the climateof an area alters, lakes can either expand or shrink,and lakes may disappear altogether if precipitationsignificantly diminishes.

Wetlands

The word wetland is well-named. It refers toland that holds a great deal of water for significantperiods of time, and that contains specializedplants able to grow in these wet conditions.Wetlands are created where bodies of water andbodies of land meet. They can be large flat areasor relatively small and steep areas. Wetlands tendto create unique ecosystems that rely on both theland and the water for survival. Wetlands areimportant regions of biological diversity, yet they

can also be fragile systems that are sensitive to theamounts and quality of water.

Types of Wetlands

A marsh is a type of wetland usually aroundlakes, ponds, streams, or the ocean where grassesand reeds are common but trees are not (Figurebelow). Animals present in marshes usuallyinclude frogs, turtles, muskrats, and many varietiesof birds. The water in a marsh is generally shallowand may be either freshwater or saltwater.

Figure 13.14 A marsh is a treeless wetland.

A swamp is a wetland characterized by lushtrees and vines in a low-lying area beside slow-moving rivers (Figure below). Like marshes, theyare frequently or always inundated with water.Since the water in a swamp moves slowly, oxygenin the water is often scarce, so plants and animalsmust be adapted for these low-oxygen conditions.Swamps can be freshwater, saltwater, or a mixtureof both.

Figure 13.15 A swamp is characterized by trees in still

water.An estuary is an area where saltwater from the

sea mixes with freshwater from a stream or river(Figure below). These semi-enclosed areas are

home to plants and animals that can tolerate thesharp changes in salt content that the constantmotion and mixing of waters creates. Estuariescontain brackish water, which has more salt thanfreshwater but less than sea water. Becauseestuaries contain areas of water with manydifferent levels of dissolved salt, they tend to havemany different habitats for plants and animals. As aresult, estuaries have extremely high biodiversity.

Figure 13.16 Chesapeake Bay, surrounded by Maryland and

Virginia, is the largest estuary in the United States.

Ecological Role of Wetlands

As mentioned above, wetlands are homes tomany different species of organisms. Though theymake up only 5% of the area of the United States,wetlands contain more than 30% of the plant typesfound in the United States. Many endangeredspecies live in wetlands, and therefore manywetlands are protected from human use.

Wetlands also play a key biological role byremoving pollutants from water. For example, theycan trap and use fertilizer that has rushed off afarmer’s fields, and therefore prevent that fertilizerfrom contaminating another body of water. Sincewetlands naturally purify water, preservingwetlands also helps to maintain clean supplies ofwater.

Floods

Floods are a natural part of the water cycle, butthey can be terrifying forces of destruction. Put

most simply, a flood is an overflow of water inone place. Floods can occur for a variety ofdifferent ways, and their effects can be minimizedin several different ways. Perhaps unsurprisingly,floods tend to affect low-lying areas mostseverely.

Causes of Floods

Floods usually occur when precipitationoccurs more quickly than that water can beabsorbed into the ground or carried away by riversor streams. Flooding may be sudden andunexpected, in the case of a flash flood, when veryintense rainfall occurs in an area (Figure below).This strong rainfall will fall too fast to beabsorbed into the ground, and will overflow thebanks of the streams and rivers. Alternately, floodscan occur more slowly, when a long period ofrainfall fills the ground with water and the levelsof rivers and streams gradually rises. Lesscommonly, floods can occur when a dam breaksalong a reservoir — as you might expect, this type

of flooding can be catastrophic. In California,floods commonly occur when rainfall far exceedsannual averages, such as during an El Nino year.High water levels have also caused small dams tobreak, wreaking havoc downstream.

Figure 13.17 A flash flood in England in 2004 was caused

by three and a half inches of rain that fell in just 60minutes. It devastated two villages.

Vegetation is an important factor indetermining whether a flood occurs. Plants tend toslow down the water that runs over the land, givingit time to enter the ground. Even if the ground is toowet to absorb more water, plants still slow the

water’s passage across the earth, increasing thetime between rainfall and the water’s arrival in astream. For the same reason, wetlands also play akey role in minimizing the impacts of floods; theyact as a buffer between land and high water levels.Flooding is therefore less common in areas that areheavily vegetated, and can be more severe in areasthat have been recently logged.

Effects of Floods

The most recent catastrophic flooding in UnitedStates history occurred in New Orleans in 2005, inthe aftermath of Hurricane Katrina (Figure below).This flooding occurred because of the failure of thecity’s levees, raised structures designed to holdback a river or lake. The New Orleans leveeswere poorly designed, and broke in multipleplaces after the storm, allowing water to pour intothe city (Figure below). Ultimately, over 80% ofthe city was submerged, 90% of city residentsevacuated, and over 1800 people died in the

disaster.

Figure 13.18 Hurricane Katrina in 2005 was one of the

deadliest storms in United States history.

Figure 13.19 Levees that held back flood water were broken

in many key areas around the city of New Orleans.Not all the consequences of flooding are

negative. Floods deposit sediment in theirfloodplains, and these sediments are nutrient richand good for farming. Therefore, many farmerstoday grow crops in the floodplains of majorrivers. This pattern of rain and flooding was alsoimportant to such peoples as the ancient Egyptiansalong the Nile River.

Floods are also responsible for moving largeamounts of sediments about within streams. Thesesediments provide habitats for animals, and theperiodic movement of sediment is crucial to thelives of several types of organisms. Many plants

and fish along the Colorado River, for example,depend on seasonal flooding to rearrange sandbars.

Lesson Summary

One way water returns to the oceans isthrough rivers and streams.Streams begin in higher elevations, with manytributaries joining together as it flows tolower elevations.A mature river will develop a floodplain andmay eventually form a delta where the rivermeets the ocean.Water temporarily resides in ponds and lakes,which are mostly freshwater.Scientists study lakes, wetlands and estuariesbecause they are biologically important areas.Flooding is part of the natural cycle of allrivers, which enriches floodplains withimportant nutrients.Flooding produces difficulties for humans

living on or near the floodplain and in coastalareas, particularly when levees break.

Review Questions

1. Where do streams originate?2. Compare and contrast streams and rivers.3. What is an advantage and disadvantage of

living in floodplains?4. Which of the 10 longest rivers has the greatest

rate of flow?5. Compare and contrast ponds and lakes.6. What are 3 main types of wetlands?7. Consider an animal common in swamps and

an animal common in rivers. What naturaladaptations do they each have to their habitat?

8. Deserts are places that get little rain. Why arethey in danger of flash floods at times?

Vocabulary

aphotic zone

The region in a freshwater body where nosunlight can reach. Also called the deepwaterzone or the profundal zone.

brackishWater that is a mixture of freshwater andsaltwater.

confluenceThe point where two streams join together.

divideA ridge that separates one water basin fromanother. Each water basin will be drained bystreams into a different ocean.

estuaryAn area where saltwater from the sea mixeswith freshwater from a stream or river.Estuaries often have high biodiversity.

floodplainA flat area covered by a stream or river whenit floods. Often rich in nutrients and thus good

places to farm.

intermittent lakeA lake that appears and disappearsseasonally, as water levels rise and fall.

lakeA larger body of freshwater, usually drainedby a stream. May be naturally occurring orhumanmade.

leveeA raised structure designed to hold back thewaters of a stream or river in the case of aflood.

limnologyThe study of all freshwater bodies and theorganisms that live in them.

littoral zoneThe region in a freshwater body closest toshore. Usually contains the most life in the

body of water.

marshA type of wetland around lakes, streams, orthe ocean where grasses and reeds arecommon, but there are no trees. May befreshwater, saltwater, or brackish. Water isgenerally shallow.

mouthThe point where a stream enters a larger bodyof water like a lake or an ocean.

photic zoneThe region in a freshwater body wheresunlight is abundant. Also called the open-water zone or limnetic zone.

pondA small body of freshwater, with no streamdraining it. Often fed by an undergroundspring.

pool

A deep, slow-moving part of the stream. Thestream is usually wider at the point where apool is found.

sourceThe place where a stream starts.

streamA body of moving water, contained within abank (sides) and bed (bottom).

swampA wetland in a low-lying area, where watermoves very slowly. Oxygen levels are oftenlow in swamps.

tributaryThe smaller of two streams that join togetherto make a larger stream.

wetlandA region of land that holds a great deal ofwater for significant periods of time, and that

contains specialized plants able to grow inthese wet conditions.

Points to Consider

What types of streams have you seen in yourarea?Why are bodies of water never reallypermanent?Is it possible that your home could beflooded? What would you do if it wereflooded?

Ground Water

Lesson Objectives

Define groundwater.Explain the location, use, and importance ofaquifers.Define springs and geysers.Describe how wells work, and why they areimportant.

Introduction

Although lakes and rivers are visible sourcesof water, did you know that there is water presentunderground at almost every spot on Earth? Thoughthis may be surprising, water beneath the ground iscommonplace. It bubbles to the surface at timesthrough springs and geysers. We also use wells tobring underground water to the surface, so that wecan use this important resource in places where

fresh surface water is not readily available.

Groundwater

As you have learned, most of the Earth’s wateris found in the oceans, with smaller amounts infrozen ice caps, and still smaller amounts presentin lakes and rivers. Some water is found in theatmosphere in the form of water vapor or clouds.However the most common place to find freshliquid water is under the Earth’s surface, in a formcalled groundwater (Figure below). Water fromthe surface seeps downward into the groundthrough tiny spaces or pores in the rock. At somepoint, though, it hits a layer of rock that no longerhas pores, which stops the water from travelingdownward. This rock is called impermeablebecause the water can no longer pass through it.The upper surface of the groundwater is called thewater table. The water table will fall when therehas been little rain in an area for a long time. Thewater table will also rise when it rains steadily for

a long time. It is important to know how deepbeneath the surface the water table is for anyonewho intends to dig into the surface or make a well.Because groundwater involves interaction betweenthe Earth and the water, the study of groundwater iscalled hydrogeology.

Figure 13.20 Groundwater is found beneath the solid

surface. Notice that the water table roughly mirrorsthe slope of the lands surface. A well penetratesthe water table.

Aquifers

Large collections of groundwater can be foundi n aquifers (Figure below). Aquifers are largeregions of sediment or rock that can holdsignificant amounts of groundwater. Aquifers canbe large, sustainable water resources when waterpumped out of aquifers is replenished by the watercycle. However, some aquifers are overused;people pump out more water than can be replaced.As the water is pumped out, the water table slowlyfalls, requiring people to spend more energypumping out the water from greater depths. Inaddition, some wells may go completely dry if theyare not deep enough to reach into the loweredwater table. Draining aquifers can lead to theground sinking, sometimes under houses and otherstructures. And when coastal aquifers areoverused, salt water from the ocean may enter theaquifer, contaminating the aquifer and making itless useful for drinking and irrigation.

Most land areas have some kind of aquiferbeneath them. Aquifers can occur at differentdepths and different geographic locations. Thecloser aquifers are to the surface, the more likely

they will be used by humans. However, closenessto the surface also increases the probability that theaquifers could be contaminated by surfacepollution that seeps through the porous rock alongwith the water. Aquifers are usually not openspaces like caverns or swimming pools, butinstead are porous rock and sediment. The spacesbetween the sediments or rock particles are filledin with water. Wet sand at the beach is a goodmodel for the consistency of most aquifers.

The Ogallala Aquifer is one of the world’slargest aquifers, and is a particularly importantsource of freshwater in the United States. It liesbeneath eight United States states — South Dakota,Nebraska, Wyoming, Colorado, Kansas,Oklahoma, New Mexico, and Texas. It ranges fromless than a meter deep to hundreds of meters deepand covers about 440,000 square kilometers! It iswidely used by people for municipal andagricultural needs. However, its rate ofreplenishment is only about 10% of the rate atwhich it is being used. In other words, for every100 liters of water that people withdraw from the

aquifer, only 10 liters are being naturally replacedby precipitation. This overuse of the aquifer hascreated political controversies and disputes inthose areas that depend on the aquifer.

Figure 13.21 Agricultural irrigation often depends on water

from aquifers.

Springs and Geysers

Whenever water beneath the ground meets thesurface, a spring is created (Figure below). Thisis a natural point where groundwater emerges onthe Earth’s surface. When water from a spring

flows downhill, it can create a stream. If it doesnot move downhill, it may be termed a seep, andmay create a pond or lake. Depending upon thesource of water, the spring may be either constant,or may only flow at certain times of year.

Figure 13.22 Big Spring in Missouri lets out 12,000 liters of

water per second(Left). Other springs are just tinyoutlets like this one.(Right)

Some minerals may become dissolved ingroundwater, changing the water’s flavor. Evencarbon dioxide can dissolve in the groundwater,causing the water to be naturally carbonated. Thiswater is sometimes sold as “mineral water.”

Groundwater can be heated by magma belowthe Earth's surface. The heated water can createhot springs, springs with water that is naturally hot(Figure below). Some hot springs are used as

natural hot tubs and are considered therapeutic andspiritual by some people. However, hot springscan be dangerous, too. Their temperatures can beexceedingly hot, dissolved substances can bepoisonous, and organisms like viruses and bacteriacan spread disease. Be sure a hot spring is safebefore entering one.

Figure 13.23 Green Dragon Spring is a hot spring found at

Yellowstone National Park.When heated groundwater is trapped in narrow

spaces, the pressure builds up and causes water toactually rocket upward. A geyser is the result ofsuch a pressurized spring. Most geysers do noterupt constantly, but rather in periodic spurts,

because pressure decreases during an eruption andthen increases again after an eruption. Old Faithful,probably the most famous geyser in the world, gotits name for erupting in regular cycles lasting 90minutes (Figure below). Its eruptions last for acouple of minutes and discharge 15,000 to 30,000liters of water during each eruption.

Figure 13.24

Old Faithful Geyser during an eruption.

Wells

A well is an artificial structure created bydigging or drilling in order to reach groundwaterpresent below the water table. In Figure below,you can see how a well penetrates thegroundwater. When the water table is close to thesurface, wells can be a very convenient method forextracting water. You may have made a verysimple well by digging a hole in the sand at thebeach until you see a pool of water at the bottom.When the water table is far below the surface,digging wells can be quite a challenge. Most wellsuse motorized pumps to bring water to the surface,but many wells still require people to use a bucketto draw water up.

Figure 13.25 An old-fashioned well that uses a bucket

drawn up by hand.Wells have been an important source of water

for humans through the ages. Obviously, in placesthat have little precipitation, wells are vital to life.Using groundwater at a faster rate than it can bereplenished by the water cycle, will cause thewater table in an aquifer to fall. A well using thatgroundwater might therefore go dry, as the waterthat supplies the well gets used up. It is importantto use water at a rate at which it can be naturallyreplenished. In addition, humans must be carefulnot to pollute groundwater, since pollution can

make water supplies unusable by humans.

Lesson Summary

Groundwater, water that infiltrates theground, forms our largest source of readilyavailable freshwater.The water table forms the top of the zone ofsaturation, where pore spaces in sediment orrock are completely filled with water.Aquifers are underground areas of sedimentor rock that hold groundwater.In steep areas, where groundwater intersectsthe ground surface, a spring or seep can form.If groundwater is heated by magma, it canform hot springs and geysers.In order to access groundwater supplies,humans drill wells and pump water from theground.

Review Questions

1. What is groundwater?2. What is the water table?3. What are aquifers and why are they so

important?4. Replenishing an aquifer is important because

it makes the aquifer a resource that can last along time. What do you think are ways to keepthe amount of water used and the amount ofwater replenished the same?

5. Earthquakes can often change the frequency oferuptions or the amount of water released bygeysers. Why do you think this is so?

6. Why can hot springs be dangerous?7. How does a well work?8. Groundwater is invisible to people on the

surface of the Earth. Explain one way that youmight monitor how humans are affecting theamount of groundwater in an aquifer.

Further Reading / SupplementalLinks

Inside Yellowstonehttp://www.nps.gov/archive/yell/insideyellowstone/0017oldfaithful3.htmEarth's water distribution video, University ofWaikato, New Zealandhttp://www.sciencelearn.org.nz/contexts/h2o_on_the_go/sci_media/video/earth_s_water_distribution

Vocabulary

groundwaterWater present under the ground, between thespaces in sediment or rock. Impermeable rocklies beneath the groundwater.

hot springA spring in which the water has been heatedby magma.

hydrogeologyThe study of groundwater.

impermeableSomething that water cannot penetrate.

seepA point where a small amount of groundwatermoves up onto the Earth’s surface. Seeps donot produce enough water to create a stream,but they may create a small pond or wetland.

springA point on the Earth’s surface, at which watergroundwater bubbles up.

water tableThe upper surface of the groundwater.

Points to Consider

Which fresh water source do you think wouldbe cleaner: water from a river or water froma well? Why?Why is pollution and overuse of our naturalresources always a big concern?What policies might people put in place toconserve water levels in lakes and aquifers?

Chapter 14: Earth's Oceans

Introduction to the Oceans

Lesson Objectives

Describe how the oceans formed.Explain the significance of the oceans.Describe the composition of ocean water.Define the parts of the water column andoceanic divisions.

Introduction

Have you ever heard the Earth called the “BluePlanet”? This term makes sense, because over70% of the surface of the Earth is covered withwater. The vast majority of that water (97.2%) isin the oceans. Without all that water, our worldwould be a different place. The oceans are an

important part of Earth: they help to determine themake-up of the air, they help determine the weatherand temperature, and they support great amounts oflife. The composition of ocean water is unique toits location and depth. Just as Earth’s interior isdivided into layers, the ocean separated intodifferent layers, called the water column.

How the Oceans Formed

Scientists have developed a number ofhypotheses about how the oceans formed. Thoughthese hypotheses have changed over time, one ideanow has the wide support of Earth scientists,called the volcanic outgassing theory. This meansthat water vapor given off by volcanoes eruptingover millions or billions of years, cooled andcondensed to form Earth's oceans.

Creation and Collection of Water

When the Earth was formed 4.6 billion years

ago, it would never have been called the BluePlanet. There were no oceans, there was no oxygenin the atmosphere, and no life. But there wereviolent collisions, explosions, and eruptions. Infact, the Earth in its earliest stage was molten. Thisallowed elements to separate into layers within theEarth — gravity pulled denser elements toward theEarth’s center, while less dense materialsaccumulated near the surface. This process ofseparation created the layers of the Earth as weknow them.

As temperatures cooled, the surface solidifiedand an atmosphere was created. Volcaniceruptions released water vapor from the Earth'scrust, while more water came from asteroids andcomets that collided with the Earth (Figurebelow). About 4 billion years ago, temperaturescooled enough for oceans to begin forming.

Figure 14.1 Volcanic activity was common in Earths early

stages, when the oceans had not yet begun to form.

Present Ocean Formation

As you know, the continents were not alwaysin the same shape or position as they are today.Because of tectonic plate movements, land masseshave moved about the Earth since they werecreated. About 250 million years ago, all of thecontinents were arranged in one huge mass of landcalled Pangea (Figure below). This meant thatmost of Earth’s water was collected in a hugeocean called Panthalassa. By about 180 million

years ago, Pangea had begun to break apartbecause of continental drift. This then separatedthe Panthalassa Ocean into separate but connectedoceans that are the ocean basins we see today onEarth.

Figure 14.2 Pangea was the sole landform 250 million

years ago, leaving a huge ocean calledPanthalassa, along with a few smaller seas.

Significance of the Oceans

The Earth's oceans play an important role inmaintaining the world as we know it. Indeed, theocean is largely responsible for keeping thetemperatures on Earth fairly steady. It may getpretty cold where you live in the wintertime. Someplaces on Earth get as cold as -70oC. Some placesget as hot as 55oC. This is a range of 125oC. Butcompare that to the surface temperature onMercury: it ranges from -180oC to 430oC, a rangeof 610oC. Mercury has neither an atmosphere noran ocean to buffer temperature changes so it getsboth extremely hot and very cold.

On Earth, the oceans absorb heat energy fromthe Sun. Then the ocean currents move the energyfrom areas of hot water to areas of cold water, andvice versa. Not only does ocean circulation keepthe water temperature moderate, but it also affectsthe temperature of the air. If you examine landtemperatures on the Earth, you will notice that themore extreme temperatures occur in the middle ofcontinents, whereas temperatures near the watertend to be more moderate. This is because water

retains heat longer than land. Summer temperatureswill therefore not be as hot, and wintertemperatures won’t be as cold, because the watertakes a long time to heat up or cool down. If wedidn’t have the oceans, the temperature rangewould be much greater, and humans could not livein those harsh conditions.

The ocean is home to an enormous amount oflife. This includes many kinds of microscopic life,plants and algae, invertebrates like sea stars andjellyfish, fish, reptiles, and marine mammals. Themany different creatures of the ocean form a vastand complicated food web, that actually makes upthe majority of all biomass on Earth. (Biomass isthe total weight of living organisms in a particulararea.) We depend on the ocean as a source of foodand even the oxygen created by marine plants.Scientists are still discovering new creatures andfeatures of the oceans, as well as learning moreabout marine ecosystems (Figure below).

Figure 14.3 Coral reefs are amongst the most densely

inhabited and diverse areas on the globe.Finally, the ocean provides the starting point

for the Earth’s water cycle. Most of the water thatevaporates into the atmosphere initially comesfrom the ocean. This water, in turn, falls on land inthe form of precipitation. It creates snow and ice,

streams and ponds, without which people wouldhave little fresh water. A world without oceanswould be a world without you and me.

Composition of Ocean Water

Water has oftentimes been referred to as the“universal solvent”, because many things candissolve in water (Figure below). Many thingslike salts, sugars, acids, bases, and other organicmolecules can be dissolved in water. Pollution ofocean water is a major problem in some areasbecause many toxic substances easily mix withwater.

Figure 14.4 Ocean water is composed of many substances.

The salts include sodium chloride, magnesiumchloride and calcium chloride.

Perhaps the most important substancedissolved in the ocean is salt. Everyone knows thatocean water tastes salty. That salt comes frommineral deposits that find their way to the oceanthrough the water cycle. Salts comprise about3.5% of the mass of ocean water. Depending onspecific location, the salt content or salinity canvary. Where ocean water mixes with fresh water,like at the mouth of a river, the salinity will belower. But where there is is lots of evaporation

and little circulation of water, salinity can be muchhigher. The Dead Sea, for example, has 30%salinity—nearly nine times the average salinity ofocean water. It is called the Dead Sea because sofew organisms can live in its super salty water.

The density (mass per volume) of seawater isgreater than that of fresh water because it has somany dissolved substances in it. When water ismore dense, it sinks down to the bottom. Surfacewaters are usually lower in density and less saline.Temperature affects density too. Warm water isless dense and colder waters are more dense.These differences in density create movement ofwater or deep ocean currents that transport waterfrom the surface to greater depths.

The Water Column

In 1960, one of the deepest parts of the ocean(10,910 meters) was reached by two men in aspecially designed submarine called the Trieste(Figure below). This part of the ocean has been

named the Challenger Deep. In contrast, theaverage depth of the ocean is 3,790 meters — stillan incredible depth for sea creatures to live at andfor humans to travel. What makes it so hard to liveat the bottom of the ocean? There are three majorfactors—the absence of light, low temperature, andextremely high pressure. In order to betterunderstand regions of the ocean, the scientistsdefine different regions by depth (Figure below).

Figure 14.5 The Trieste made a record dive to the

Challenger Deep in 1960. No craft exists today thatcan reach that depth.

Sunlight only penetrates water to a depth ofabout 200 meters, a region called the photic zone(photic means light). Since organisms thatphotosynthesize depend on sunlight, they can onlylive in the top 200 meters of water. Suchphotosynthetic organisms supply almost all theenergy and nutrients to the rest of the marine foodweb. Animals that live deeper than 200 metersmostly feed on whatever drops down from thephotic zone.

Beneath the photic zone is the aphotic zone,where there is not enough light for photosynthesis.The aphotic zone makes up the majority of theocean but a minority of its life forms. Descendingto the ocean floor, the water temperature decreaseswhile pressure increases tremendously. Eachregion is progressively deeper and colder, with thevery deepest areas in ocean trenches.

The ocean can also be divided by horizontaldistance from the shore. Nearest to the shore liesthe intertidal zone. In this region, you might findwaves, changes in tide, and constant motion in thewater that exposes the water to large amounts of

air. Organisms that live in this zone are adapted towithstand waves and exposure to air in low tides,by having strong attachments and hard shells. Theneritic zone includes the intertidal zone and thepart of the ocean floor that very gradually slopesdownward, the continental shelf. Lots of oceanicplants live in this zone, since some sunlight stillpenetrates to the bottom of the ocean floor in theneritic zone. Beyond the neritic zone is the oceaniczone, where the sloping sea floor takes a mucheven steeper dive and sunlight does not reach.Animals such as sharks, fish, and whales can befound in this zone. They feed on materials that sinkfrom upper levels, or consume one another. Athydrothermal vents, areas of extremely hot waterwith lots of dissolved materials allow rare andunusual producers to thrive.

Figure 14.6 The ocean environment is divided into many

regions based on factors like availability of lightand nutrients.Organisms adapt to the conditionsand resources in the regions in which they live.

Lesson Summary

Our oceans originally formed as a watervapor released by volcanic outgassing cooledand condensed.The oceans serve the very important role ofhelping to moderate Earth's temperatures.The oceans are home to a tremendous

diversity of life, and algae which are allphotosynthetic organisms.The main elements dissolved in seawater arechlorine, sodium, magnesium, sulfate andcalcium.Usual salinity for the oceans is about 3.5% or35 parts per thousand.Some regions in areas of high evaporation,like the Dead Sea, have exceptionally highsalinities.The photic zone is the surface layer of theoceans, down to about 200m, where there isenough available light for photosynthesis.Below the photic zone, the vast majority ofthe oceans lies within the aphotic zone, wherethere is not enough light for photosynthesis.On average, the ocean floor is about 3,790mbut there are ocean trenches as deep as10,910m.The ocean has many biological zonesdetermined by availability of different abioticfactors.Neritic zones are nearshore areas, including

the intertidal zone. Oceanic zones areoffshore regions of the ocean.

Review Questions

1. What was the name of the single continent thatseparated to form today’s continents?

2. From what three sources did water originateon Earth?

3. What percent of the Earth’s surface iscovered by water?

4. How do the oceans help to moderate Earth'stemperatures?

5. Over time, the Earth’s oceans have becomemore and more salty. Why?

6. What is the most common substance that isdissolved in ocean water?

7. What is density?8. Compare and contrast the photic and aphotic

zones.9. Describe the types of organisms found in the

intertidal, neritic, and oceanic zones. Give

examples of a life form you think might befound in each.

Vocabulary

aphotic zone

The zone in the water column deeper than 200m. Sunlight does not reach this region of the ocean.

biomassThe total mass of living organisms in a certainregion.

currentThe movement of water in a stream, lake, orocean.

densityMass per volume. The units for density areusually g/cm3 or g/mL.

intertidal zone

The part of the ocean closest to the shore,between low and high tide.

neritic zoneThe part of the ocean where the continentalshelf gradually slopes outward from the edgeof the continent. Some sunlight can penetratethis region of the ocean.

oceanic zoneThe open ocean, where the seafloor is deep.No sunlight reaches the floor of the oceanhere.

PangeaThe supercontinent that tectonically brokeapart about 200 million years ago, forming thecontinents and oceans that we see today onEarth.

photic zoneThe topmost region of the water column,extending from the surface down to about 200

m in depth. Sunlight easily penetrates thisregion of the water column.

salinityA measure of the amount of dissolved salt inwater.

water columnA vertical column of ocean water, which isdivided into different zones according to theirdepth.

Points to Consider

What creates the movement of water like tidesand waves?Is it possible to have a river in the middle ofthe ocean?What other factors affect the movement ofocean water? How do these factors affect tothe world’s climate and the ocean’secosystem?

Ocean Movements

Lesson Objectives

Define waves and explain their formation.Describe what causes tides.Describe how surface currents form and howthey affect the world’s climate.Describe the causes of deep currents.Relate upwelling areas to their impact on thefood chain.

Introduction

Ocean water is constantly in motion (Figurebelow). From north to south, east to west, and upand down the shore, ocean water moves all overthe place. These movements can be explained asthe result of many separate forces, including localconditions of wind, water, the position of the moonand Sun, the rotation of the Earth, and the position

of land formations.

Figure 14.7 Ocean waves transfer energy through the water

over great distances.

Waves

A wave is a disturbance that transfers energythrough matter or empty space. Sound waves movethrough the air, earthquakes send powerful wavesthrough solid earth, spacecraft radio waves travelacross millions of miles through the vacuum ofempty space, and ocean waves move through

water. All of these types of waves are able totransfer energy over great distances. The size of awave and the distance it travels depends on theamount of energy that the wave carries.

The most familiar waves occur on the ocean'ssurface. It is upon these waves that surfers playand boogie boarders ride. These waves are mostlycreated by the wind. There are three factors windthat determine the size of the wave: 1) the speed ofthe wind, 2) the distance over which the wind hasblown, and 3) the length of time that the wind hasblown. The greater each of these factors, the biggerthe wave.

Waves can be measured by their amplitude, adistance measured vertically from the crest (thetop of the wave) to the trough (the bottom of thewave). They can also be measured by theirwavelength, which is the horizontal distancebetween crests (Figure below). When wind blowsacross the water surface, energy is transferred tothe water. The transfer of that energy may createtiny ripples that disappear when the wind diesdown, or it may create larger waves that continue

until they reach the shore. Most waves reach theshore.

Scientists sometimes describe waves bymeasuring the speed of a wave. A wave’s speed isdetermined by measuring the time it takes for onewavelength to pass by. Interestingly, particles inthe ocean are not significantly moved by waves;although they are bobbled around by the waves, theparticles tend to stay where they are.

Figure 14.8 Waves are measured by their amplitude and by

their wavelength.Waves can also form when a rapid shift in

ocean water is caused by underwater earthquakes,landslides, or meteors that hit the ocean. Thesewaves, called tsunami (Figure below), can travelat speeds of 800 kilometers per hour (500 milesper hour). Tsunami have small, often unnoticeable

wave heights in the deep ocean. However as atsunami approaches the continental shelf, waveheight increases. The wave speed is also slowedby friction with the shallower ocean floor, whichcauses the wavelength to decrease, creating a muchtaller wave. Many people caught in a tsunami haveno warning of its approach. Tsunami warningsystems are important for protecting for coastalareas and low-lying countries.

Figure 14.9 An undersea earthquake caused the Boxing Day

Tsunami in 2004 which devastated Indonesia, SriLanka, India, Thailand, and Myanmar. In thisphoto, the tsunami hits the Maldives in the Indian

Ocean.Waves break when they get close to the shore.

That is due to the wave’s interaction with the seafloor. When the wave hits the shore, the energy atthe bottom of the wave is transferred to the oceanfloor, which slows down the bottom of the wave.The energy at the top of the wave, in the crest,continues at the same speed, however. Since thetop of this wave is going faster than the bottom, thecrest falls over and crashes down.

Tides

Wind is the primary force that causes oceansurface waves, but it does not cause the tides.Tides are the daily changes in the level of theocean water at any given place. The main factorsthat causes tides are the gravitational pull of theMoon and the Sun (Figure below).

Figure 14.10 High tide (left) and low tide (right) at Bay of

Fundy on the Gulf of Maine in North America. TheBay of Fundy has one of the greatest tidal rangeson Earth.

How does the Moon affect the oceans? Sincethe Moon is a relatively large object in space thatis very close to the Earth, its gravity actually pullsEarth's water towards it. Wherever the moon is, asit orbits the Earth, there is a high tide 'bulge' thatstays lined up with the Moon. The side of the Earththat is furthest from the Moon also has a high tide'bulge'. This is because the Earth is closer to themoon the water on its far side. The Moon's gravity

pulls more on the planet than the water on theopposite side. These two water bulges on oppositesides of the Earth aligned with the Moon are thehigh tides. Since ocean water is pulled higher inthe areas of the two high tides, there is less waterin between the two high tides. These areas are thelow tides (Figure below).

Figure 14.11 High tide is created by the gravitational pull of

the moon which pulls water toward it. Water onthe opposite side of the Earth is pulled least by themoon so the water bulges away from the moon.High tide occurs where the water is bulging. Lowtide occurs where it is not.

The tidal range is the difference between theocean level at high tide and the ocean at low tide

(Figure below). Some places have a greater tidalrange than others. High tides occur about twice aday, about every 12 hours and 24 minutes.

Figure 14.12 The tidal range is the difference between the

ocean level at high tide and low tide.The Moon’s gravity is mostly responsible for

our tides, but the Sun also plays a role (Figurebelow). The Sun is much larger than our Moon. Ithas a mass about 27,500,000 times greater than theMoon. A very large object like the Sun wouldproduce tremendous tides if it were as near toEarth as the Moon. However it is so far from theEarth that its effect on the tides is only about halfas strong as the Moon’s. When both the Sun andMoon are aligned, the effect of each is addedtogether, producing higher than normal tides calledspring tides. Spring tide are tides with the greatest

tidal range. Despite their name, spring tides don'tjust occur in the spring; they occur throughout theyear whenever the Moon is in a new-moon or full-moon phase, or about every 14 days.

Here is a link to see these tides in motion:http://oceanservice.noaa.gov/education/kits/tides/media/tide06a_450.gifLicense: GNU-FDL)

Figure 14.13 Spring tides occur when the Earth, the Sun, and

the Moon are aligned, increasing the gravitational

pull on the oceans. Sometimes, the Sun and Moonare on opposite sides of the Earth while at othertimes, they are on the same side.

Neap tides are tides that have the smallesttidal range, and occur when the Earth, the Moon,and the Sun form a 90o angle (Figure below). Theyoccur exactly halfway between the spring tides,when the Moon is at first or last quarter. Thishappens because the Moon's high tide occurs in thesame place as the Sun's low tide and the Moon'slow tide is added to by the Sun's high tide.

Figure 14.14 Neap tides occur when the Earth, the Sun, and

the Moon form a right angle, at first and lastquarters for the Moon. The two possible angles are

shown below.

Surface Currents

Wind that blows over the ocean water createswaves. It also creates surface currents, which arehorizontal streams of water that can flow forthousands of kilometers and can reach depths ofhundreds of meters. Surface currents are animportant factor in the ocean because they are amajor factor in determining climate around theglobe.

Causes of Surface Currents

Currents on the surface are determined by threemajor factors:the major overall global windpatterns, the rotation of the Earth, and the shape ofocean basins.

When you blow across a cup of hot chocolate,you create tiny ripples on its surface that continueto move after you’ve stopped blowing. The ripples

in the cup are tiny waves, just like the waves thatwind forms on the ocean surface. The movement ofhot chocolate throughout the cup forms a stream orcurrent, just as oceanic water moves when windblows across it.

But what makes the wind start to blow? Whensunshine heats up air, the air expands, which meansthe density of the air decreases and it becomeslighter. Like a balloon, the light warm air floatsupward, leaving a slight vacuum below, whichpulls in cooler, denser air from the sides. Thecooler air coming into the space left by the warmair is wind.

Because the Earth's equator is warmed by themost direct rays of the Sun, air at the equator ishotter than air further north or south. This hotter airrises up at the equator and as colder air moves into take its place, winds begin to blow and push theocean into waves and currents.

Wind is not the only factor that affects oceancurrents. The 'Coriolis Effect' describes howEarth’s rotation steers winds and surface currents(Figure below). The Earth is a sphere that spins on

its axis in a counterclockwise direction when seenfrom the North Pole. The further towards one of thepoles you move from the equator, the shorter thedistance around the Earth. This means that objectson the equator move faster than objects furtherfrom the equator. While wind or an ocean currentmoves, the Earth is spinning underneath it. As aresult, an object moving north or south along theEarth will appear to move in a curve, instead of ina straight line. Wind or water that travels towardthe poles from the equator is deflected to the east,while wind or water that travels toward theequator from the poles gets bent to the west. TheCoriolis Effect bends the direction of surfacecurrents.

Figure 14.15 The Coriolis Effect causes winds and currents

to form circular patterns. The direction that theyspin depend on the hemisphere that they are in.

The third major factor that determines thedirection of surface currents is the shape of oceanbasins (Figure below). When a surface currentcollides with land, it changes the direction of thecurrents. Imagine pushing the water in a bathtubtowards the end of the tub. When the water reachesthe edge, it has to change direction.

Figure 14.16 This map shows the major surface currents at

sea. Currents are created by wind, and theirdirections are determined by the Coriolis effectand the shape of ocean basins.

Effect on Global Climate

Surface currents play a large role indetermining climate. These currents bring warmwater from the equator to cooler parts of the ocean;they transfer heat energy. Let’s take the GulfStream as an example; you can find the GulfStream in the North Atlantic Ocean in Figureabove. The Gulf Stream is an ocean current thattransports warm water from the equator past theeast coast of North America and across the

Atlantic to Europe. The volume of water ittransports is more than 25 times that of all of therivers in the world combined, and the energy ittransfers is more than 100 times the world's energydemand. It is about 160 kilometers wide and abouta kilometer deep. The Gulf Stream's warm watersgive Europe a much warmer climate than otherplaces at the same latitude. If the Gulf Stream wereseverely disrupted, temperatures would plunge inEurope.

Deep Currents

Surface currents occur close to the surface ofthe ocean and mostly affect the photic zone. Deepwithin the ocean, equally important currents existthat are called deep currents. These currents arenot created by wind, but instead by differences indensity of masses of water. Density is the amountof mass in a given volume. For example, if youtake two full one liter bottles of liquid, one mightweigh more, that is it would have greater mass than

the other. Because the bottles are both of equalvolume, the liquid in the heavier bottle is denser. Ifyou put the two liquids together, the one withgreater density would sink and the one with lowerdensity would rise.

Two major factors determine the density ofocean water: salinity (the amount of salt dissolvedin the water) and temperature (Figure below). Themore salt that is dissolved in the water, the greaterits density will be. Temperature also affectsdensity: the colder the temperature, the greater thedensity. This is because temperature affectsvolume but not mass. Colder water takes up lessspace than warmer water (except when it freezes).So, cold water has greater density than warmwater.

More dense water masses will sink towardsthe ocean floor. Just like convection in air, whendenser water sinks, its space is filled by less densewater moving in. This creates convection currentsthat move enormous amounts of water in the depthsof the ocean. Why is the water temperature coolerin some places? Water cools as it moves from the

equator to the poles via surface currents. Coolerwater is more dense so it begins to sink. As aresult, the surface currents and the deep currentsare linked. Wind causes surface currents totransport water around the oceans, while densitydifferences cause deep currents to return that waterback around the globe (Figure below).

Figure 14.17 Thermohaline currents are created by

differences in density due to temperature (thermo)and salinity (haline). The dark arrows are deepcurrents and the light ones are surface currents.

Figure 14.18 Surface and deep currents together form

convection currents that circulate water from oneplace to another and back again. A water particlein the convection cycle can take 1600 years tocomplete the cycle.

Upwelling

As you have seen, water that has greaterdensity usually sinks to the bottom. However, inthe right conditions, this process can be reversed.Denser water from the deep ocean can come up tothe surface in an upwelling (Figure below).Generally, an upwelling occurs along the coastwhen wind blows water strongly away from the

shore. As the surface water is blown away fromthe shore, colder water from below comes up totake its place. This is an important process inplaces like California, South America, SouthAfrica, and the Arabian Sea because the nutrientsbrought up from the deep ocean water support thegrowth of plankton which, in turn, supports othermembers in the ecosystem. Upwelling also takesplace along the equator between the North andSouth Equatorial Currents.

Figure 14.19 An upwelling forces denser water from below

to take the place of less dense water at the surfacethat is pushed away by the wind.

Lesson Summary

Ocean waves are energy traveling through thewater.The highest portion of a wave is the crest andthe lowest is the trough.The horizontal distance between two wavecrests is the wave's length.Most waves in the ocean are wind generatedwaves. Tsunami are exceptionally longwavelength waves often caused byearthquakes.Tides are produced by the gravitational pullof the Moon and Sun on Earth's oceans.Spring tides happen at full and new moons,when the Earth, Moon, and Sun are allaligned.Neap tides are tides of lower than normaltidal range that occur at first and last quartermoons, when the Moon is at right angles to theSun.Ocean surface currents are produced by major

overall patterns of atmospheric circulation,the Coriolis Effect and the shape of eachocean basin.Each half of each ocean basin has a majorcircular pattern of surface water circulationcalled a gyre.Ocean surface circulation brings warmequatorial waters towards the poles andcooler polar water towards the equator.Deep ocean circulation is density drivencirculation produced by differences in salinityand temperature of water masses.Upwelling areas are biologically importantareas that form as ocean surface waters areblown away from a shore, causing cold,nutrient rich waters to rise to the surface.

Review Questions

1. What factors of wind determine the size of awave?

2. Define the crest and trough of a wave.

3. Why does a hurricane create big waves?4. Tsunami are sometimes incorrectly called

“tidal waves.” Explain why this is not anaccurate term for tsunami.

5. What is the principle cause of the tides?6. What is a tidal range?7. Why do you think that some places have a

greater tidal range than other places?8. Which has a greater tidal range, spring tides

or neap tides? Explain.9. What is the most significant cause of the

surface currents in the ocean?10. How do ocean surface currents affect

climate?11. What is the Coriolis Effect?12. Some scientists have hypothesized that if

enough ice in Greenland melts, the GulfStream might be shut down. Without the GulfStream to bring warm water northward,Europe would become much colder. Explainwhy melting ice in Greenland might affect theGulf Stream.

13. What process can make denser water rise to

the top?14. Why are upwelling areas important to marine

life?

Further Reading / SupplementalLinks

Learn About Ocean Currents, 5 min LifeVideopediahttp://www.5min.com/Video/Learn-about-Ocean-Currents-117529352

Vocabulary

amplitudeThe vertical height of a wave, measured fromtrough to crest.

Coriolis effectthe apparent deflection of a moving objectlike water or air caused by Earth's rotation.

crestThe highest point in a wave.

deep currentA current deep within the ocean, whichmoves because of density differences (causedby differences in water temperature andsalinity).

high tideThe maximum height reached by a tide in thecourse of a day.

low tideThe minimum height reached by a tide in thecourse of a day.

neap tideA tide that occurs when the Moon, Sun, andEarth are at 90° angles to one another. Tideshave the smallest tidal range during a neaptide.

rip current

A strong surface current of water that isreturning to the ocean from the shore.

spring tideA tide that occurs when the Moon, Sun, andEarth area all in a line. Tides will have thegreatest tidal range during a spring tide.

surface currentA horizontal movement of ocean water,caused by surface winds.

tidal rangeThe difference between the high and low tide.

tideThe daily rise and fall in the level of theocean water.

troughThe lowest point in a wave.

tsunamiA seismic sea wave generated by vertical

movement of the ocean floor underwaterearthquake, underwater volcanic eruption orlandslide or meteorite impact.

upwellingCold, nutrient-rich water that rises fromoceanic depths usually near the continents,when wind blows the overlying surface awayor along the equator.

waveA change in the shape of water caused byenergy moving through the water.

wavelengthThe horizontal distance between two troughs,or two crests in a wave.

Points to Consider

What is the bottom of the ocean like?How is the seafloor studied?How does the ocean floor contribute to the

ocean’s ecosystem?

Going Further - Applying Math

Tide Generating Force

In this chapter, you have learned some of thefundamental forces that influence tides. You canalso learn some more about tides by using anequation to calculate the tide generating force. Likethe force of gravity, the pull of the tide generatingforce is directly related to the masses of theastronomical objects involved and inverselyrelated to the square of the distance between them.Tides are caused by both the gravitational pull ofthe Moon and the gravitational pull of the Sun onthe layer of water that covers the Earth. Unlike thegravitational force, the tide generating force varieswith the distance between the Moon (or the Sun)and the Earth cubed. So the equation for the tidegenerating force is as follows: T = G (m1 . m2 / d3

) where T is the tide generating force, G is the

universal gravitational constant, m1 and m2 are themass of the Earth and the mass of the Moon (or themass of the Earth and the mass of the Sun), and d isthe distance between them.

If we plug in values for the gravitationalconstant, the mass of the Earth and the mass of theMoon, we can calculate the tide generating forcewhen the Moon is at apogee (farthest from theEarth in its orbit). Use G = 6.673 x 10 -11 m 3 / kg .

s 2; m 1 = 7.35 x 10 22 kg for the mass of the Moon,m 2 = 5.974 x 10 24 kg for the mass of the Earth;and d = 405,500 km for the distance from the Earthto the Moon at apogee.

You could use all the same values butsubstitute in d = 363,300 km for the distance fromthe Earth to the Moon when the Moon is at perigee(point when the Moon is closest to the Earth) andcompare the tide generating force each distance.

Tsunami Tag

Often students ask if they could simply outrun a

tsunami as it approaches them. How fast wouldyou have to run to do this? You can calculate howfast a tsunami travels in the ocean using theequation for the speed of a shallow water wave,which is: V = the square root of g x d, where V =wave speed (velocity), g = the acceleration ofgravity: 9.8 meters / s 2, and d = the depth of thewater. If you use d = 3,940m (the average depth ofthe Pacific Ocean), how fast does a tsunami travel?Do you think you could outrun this wave?

The Seafloor

Lesson Objectives

Describe the obstacles to studying theseafloor and methods for doing so.Describe the features of the seafloor.List the living and non-living resources thatpeople use from the seafloor.

Introduction

The ocean surface is vast and hides an entireworld underneath it. The ocean floor is sometimescalled the final frontier of the modern era. Thoughpeople have traveled on the ocean for millennia,people have explored only a tiny fraction of theocean floor. We know very little about the vastexpanse of our oceans. Today’s technology hasallowed us to learn more about the seafloor,including both its physical properties and its

effects on living organisms.

Studying the Seafloor

Ancient myth says that Atlantis was a powerfulundersea city whose warriors conquered manyparts of Europe. There is little proof that such acity existed, but human fascination with the worldunder the oceans certainly has existed forcenturies. Not much was known about the aphoticzone of the ocean until scientists developed asystem modeled after the way that bats anddolphins use echolocation to navigate in the dark(Figure below). Prompted by the need to findsubmarines during World War II, scientists learnedto bounce sound waves through the ocean to detectunderwater objects. The sound waves bounce backlike an echo off of whatever object may be in theocean. The distance of the object can be calculatedbased on the time that it takes for the sound wavesto return. Finally, scientists were able to map theocean floor.

Figure 14.20 Dolphins and whales use echolocation, a

natural sonar system, to navigate the ocean.Three main obstacles have kept us from

studying the depths of the ocean: absence of light,very cold temperatures, and high pressure. As youknow, light only penetrates the top 200 meters ofthe ocean; the depths of the ocean can be as muchas 11,000 meters deep. Most places in the oceanare completely dark, which makes it impossiblefor humans to explore without bringing a source oflight with them. Secondly, the ocean is very cold;colder than 0oC (32oF) in many places. Such coldtemperatures pose significant obstacles to humanexploration of the oceans. Finally, the pressure inthe ocean increases tremendously as you godeeper. Scuba divers can rarely go deeper than 40meters due to the pressure. The pressure on a diver

at 40 meters would be 4 kilograms/squarecentimeter (60 lbs/sq in). Even though we don'tthink about it, the air in our atmosphere has weight.It presses down on us with a force of about 1kilogram per square centimeter (14.7 lbs/ sq in). Inthe ocean, for every 10 meters of depth, thepressure increases by nearly 1 atmosphere!Imagine the pressure at 10,000 meters; that wouldbe 1,000 kilograms per square centimeter (14,700lbs/ sq in). Today’s submarines usually dive toonly about 500 meters; to go deeper than this theymust be specially designed for greater depth(Figure below).

In the 19th century, explorers mapped oceanfloors by painstakingly dropping a line over theside of a ship to measure ocean depths, one tinyspot at a time. SONAR, which stands for SoundNavigation An d Ranging, has enabled modernresearchers to map the ocean floor much morequickly and easily. Researchers send a pulse ofsound down to the ocean floor and calculate thedepth based on how long it takes the sound toreturn. Of course, some scientific research requires

actually traveling to the bottom of the ocean tocollect samples or directly observe the oceanfloor, but this is more expensive and can bedangerous.

In the late 1950s, the bathyscaphe (deep boat)Trieste was the first manned vehicle to venture tothe deepest parts of the ocean, a region of theMarianas Trench named the Challenger Deep. Itwas built to withstand 1.2 metric tons per squarecentimeter and plunged to a depth of 10,900meters. No vehicle has carried humans again tothat depth, though robotic submarines havereturned to collect sediment samples from theChallenger Deep. Alvin is a submersible used bythe United States for a great number of studies; itcan dive up to 4,500 meters beneath the oceansurface (Figure below).

Figure 14.21 Submarines are built to withstand great

pressure under the sea, up to 680 atmospheres ofpressure (10,000 pounds per square inch). Theystill rarely dive below 400 meters.

Figure 14.22 Alvin allows for a nine hour dive for up to two

people and a pilot. It was commissioned in the1960s.

In order to avoid the expense, dangers andlimitations of human missions under the sea,remotely operated vehicles or ROV’s, allowscientists to study the ocean’s depths by sendingvehicles carrying cameras and special measuring

devices. Scientists control them electronically withsophisticated operating systems (Figure below).

Figure 14.23 Remotely-operated vehicles like this one allow

scientists to study the seafloor.

Features of the Seafloor

Before scientists invented sonar, many peoplebelieved the ocean floor was a completely flatsurface. Now we know that the seafloor is far fromflat. In fact, the tallest mountains and deepestcanyons are found on the ocean floor; far taller anddeeper than any landforms found on the continents.The same tectonic forces that create geographical

features like volcanoes and mountains on landcreate similar features at the bottom of the oceans.

Look at (Figure below). If you follow theocean floor out from the beach at the top left, theseafloor gently slopes along the continental shelf.The sea floor then drops off steeply along thecontinental slope, the true edge of the continent.The smooth, flat regions that make up 40% of theocean floor are the abyssal plain. Running throughall the world's oceans is a continuous mountainrange, called the mid-ocean ridge ("submarineridge" in Figure below). The mid-ocean ridge isformed where tectonic plates are moving apartfrom each other, allowing magma to seep out in thespace where the plates pulled apart. The mid-ocean ridge system is 80,000 kilometers in totallength and mostly underwater except for a fewplaces like Iceland. Other underwater mountainsinclude undersea volcanoes (called seamounts),which may rise more than 1,000 meters above theocean floor. Those that reach the surface becomevolcanic islands, such as the Hawaiian Islands.Deep oceanic trenches are created where a

tectonic plate dives beneath (subducts) anotherplate.

Figure 14.24 The seafloor is as varied a landscape as the

continents.

Resources From the Seafloor

The seafloor provides important living andnon-living resources, which must be managedsustainably in order to maintain these resources. Itis important for us to use the resources in arenewable way and to be careful not tocontaminate the ocean because pollution affects the

very resources that we need.

Living Resources

Although most fish are caught as they swim inthe open waters of the ocean, bottom trawling is amethod of fishing that involves towing a weightednet across the seafloor to harvest fish from theocean floor. In many areas where bottom trawlingis done, ecosystems are severely disturbed by thelarge nets and other fishing gear that people use.Many species of fish are being overharvested,which means their rate of reproduction cannot keepup with the rate at which people consume them.For this reason, a few areas in the world havelaws that limit bottom trawling to waters not morethan 1,000 meters deep or waters far fromprotected and sensitive areas. Still, recent reportson fishing resources tell us that urgent action isrequired to restore species of fish that have beentaken in too great numbers.

The seafloor is home to many types of seacreatures, like clams, abalone, sea snails, and

slugs. Some of these animals are used as food bypeople. Like the rich range of life in therainforests, coral reefs in the ocean are sites ofgreat biological diversity (Figure below). Some ofthe organisms found in the ocean provide us withmedications. The ocean floor also indirectlysupports most life in the oceans, since upwellingcurrents bring important nutrients from the seafloorto plankton. These plankton in turn, provide foodfor most other creatures in the oceanic food web.

Figure 14.25 The seafloor in the coral reefs of Papua New

Guinea is home to many important species.

Non-living Resources

The most valuable non-living resources foundin the ocean are oil and natural gas. Of course,these resources are below the seafloor, and requiredrilling to reach. Oil platforms can hosts dozens ofoil wells that are drilled in places where the oceanis sometimes 2,000 meters deep (Figure below).Working on oil platforms is dangerous forworkers, who are exposed to harsh oceanconditions and gas explosions. Oil rigs also pose athreat to the ocean ecosystem, as a result of oilleaks and disruption of the natural environment.

Figure 14.26 Oil platforms like this one of the coast of the

Gulf of Mexico can be fixed or they can float. Theyare generally used on the continental shelf but newtechnology allows them to be in deeper places.

Many minerals are found on the ocean floor.They can form crystallized spheres called nodulesthat one day may be collected or mined from the

bottom of the ocean (Figure below). The nodulesmay be as small as a pea or as large as abasketball. Common mineral resources found inthese nodules include manganese, iron, copper,nickel, phosphate, and cobalt. These minerals havemany uses in the industrial world. It is estimatedthat there may be as much as 500 billion tons ofnodules on the seafloor. Currently, there is notsignificant mineral mining on the seafloor, in partbecause of expense and concerns about how thismining would disrupt the seafloor.

Figure 14.27 Manganese nodules from the seafloor are often

rich in metals like manganese, iron, nickel, copper,and cobalt.

Lesson Summary

Until the development of sonar, we knew verylittle about the ocean floor.The deep ocean is dark, very cold and hastremendous pressure from the overlyingwater.Scuba divers can explore only to about 40meters, while most submarines dive only toabout 500 meters. Scientific researchsubmersibles have explored the ocean'sdeepest trenches, but most are designed toreach only the ocean floor.Today much of our exploration of the oceanshappens using sonar and remotely operatedvehicles.Features of the ocean include the continentalshelf, slope and rise. The ocean floor iscalled the abyssal plain. Below the oceanfloor, there are a few small deeper areascalled ocean trenches. Features rising up fromthe ocean floor include seamounts, volcanic

islands and the mid-oceanic ridges and rises.The oceans provide us with both living andnon-living resources.Living oceanic resources include fish that areharvested for food as well as thephotosynthetic algae which begin the foodchain in the surface waters of the ocean.Non-living resources include oil and naturalgas found on our continental shelves andmineral resources like manganese nodulesfound on the deep ocean floor.

(Source:http://www.cdr.isa.org.jm/servlet/page?_pageid=326, License: GNU-FDL)

Review Questions

1. What are three obstacles to studying theseafloor?

2. The atmospheric pressure is about 1 kilogramper centimeter squared (14.7 pounds per

square inch or 1 atmosphere) at sea level.About what is the pressure if you are 100meters deep in the ocean?

3. What invention gave people the ability to mapthe ocean floor?

4. Which parts of the ocean floor would youexpect there to be the greatest amount ofliving organisms?

5. How much deeper did the Trieste submergethan Alvin?

6. Compare and contrast the continental shelfand the abyssal plain.

7. Why do you think mapping the seafloor isimportant to the Navy? Explain.

8. If the mid-ocean ridge is created where thetectonic plates separate, why is a mountainrange formed there?

9. Why is bottom trawling damaging to theseafloor?

10. Many people rely on the ocean to livebecause it provides them with food or work.As the world population grows, the resourcesin the ocean are used more and more. What

can we do to make sure that people use theresources in the ocean at a rate that can bereplenished?

11. What is a mineral nodule?

Vocabulary

abyssal plainThe flat bottom of the ocean floor; the deepocean floor.

bottom trawlingFishing by dragging deep nets along the oceanfloor, so that they gather up living creaturesalong the bottom of the ocean.

continental shelfThe shallow, gradually sloping seabed aroundthe edge of a continent. Usually less than 200meters in depth. The continental shelf can bethought of as the submerged edge of acontinent.

continental slopeThe sloped bottom of the ocean that extendsfrom the continental shelf down to the deepocean bottom.

mid ocean ridgeMountain range on the ocean floor wheremagma upwells and new ocean floor isformed.

seamountA mountain rising from the seafloor that doesnot reach above the surface of the water.Usually formed from volcanoes.

trenchDeepest areas of the ocean; found wheresubduction takes place.

Points to Consider

What types of organisms are found in theocean?

How does ocean life differ in differentregions of the ocean?How do ocean organism interact?

Ocean Life

Lesson Objectives

Describe the different types of oceanorganisms.Describe the interactions among differentocean organisms.

Introduction

The ocean covers an area of about 361 millionsquare kilometers, about 71% of the Earth. For thatreason, it is a home to a large portion of all life onEarth. The life in the ocean includes many differentspecies. Some species seem bizarre, others areenormous, some are delicious to eat, and others aredangerous. Living organisms can be foundthroughout the ocean, even in the most remote andharsh parts.

Types of Ocean Organisms

There is a great variety of ocean life thatranges from the smallest animals on Earth to thelargest. Some of these organisms breathe air fromthe atmosphere, while others can extract oxygenfrom the water. There are those that mostly float onthe surface and those found in the ocean’s depths.Some animals eat other organisms, while othercreatures generate food from sunlight. Theabundance of life in the oceans can seem endless.However pollution, acidification of the oceans,and overfishing can greatly reduce the diversityand abundance of ocean life. By studying andunderstanding the creatures of the ocean, humanscan better preserve these organisms. With this inmind, we’ll learn about the life forms in the oceanby dividing them into seven basic groups.

Plankton

The most abundant life forms in the ocean are

plankton; most are so small that you can’t even seethem (Figure below). These include many types ofalgae, copepods, and jellyfish. Because exploringthe oceans is much harder than studying the land,many marine organisms haven’t been extensivelystudied by humans. Scientists believe many speciesof marine organisms haven’t even been discoveredyet. The plankton are one group of organisms thathave been studied extensively. The word"plankton," which comes from the Greek forwanderer, describes how these organisms live. Allplankton float freely or drift, wandering at theocean's surface.

The first link in all marine food chains are thephytoplankton, or 'plant' plankton, which usesunlight to make sugars from carbon dioxide andwater (photosynthesis). Because they needsunlight, they can only live in the photic zone.Through photosynthesis, phytoplankton make foodfor themselves and give off oxygen, which is awaste product for them but essential for all animalson Earth. Phytoplankton produce all the food at thebottom of the ocean food chain, so they are called

primary producers. Most of the photosynthesis onEarth happens in the oceans and phytoplanktonproduce a large share of the oxygen in the air webreathe. Zooplankton, or animal plankton eatphytoplankton as their source of food. They can befound in all parts of the ocean.

Figure 14.28 Plankton are perhaps the most important part of

the food chain because they supply food for mostaquatic life.

Plants and Algae

There are only a few true plants in the oceans;

these include salt marsh grasses and mangrovetrees. But large algae, or seaweed, also usephotosynthesis to make food, just as plants do onland. These organisms have to live in the photiczone, because they require sunlight forphotosynthesis. For that reason, most plants,seaweeds and algae in the ocean are found near theocean surface or close to the shore. The largealgae kelp grows in the neritic zone (Figurebelow). Kelp tends to grow in forests, and canreach over 50 meters long. Kelp forests sustain anabundance of life, like the otter that lives in theirswaying stems. It is thought that land plantsadapted from ocean organisms some 500 millionyears ago.

Figure 14.29 Kelp is common off the shores of California.

Kelp is a crucial organism in many food webs nearthe coast.

Marine Invertebrates

The ocean includes a great variety of animals.

One major group of animals is the invertebrates.Invertebrates are animals with no spinal column.Marine invertebrates include sea slugs, seaanemones, starfish, octopi, clams, sponges, seaworms, crabs and lobsters. Most of these animalsare found close to the shore, but they can be foundthroughout the ocean. In fact, scientists wereamazed to discover invertebrates that thrived in thedeep ocean near hydrothermal(hot water) vents,including giant tube worms, crabs, and shrimps(Figure below).

Figure 14.30 Giant tube worms have been found a mile deep

in the ocean. They can grow up to 8 feet long andcan withstand the very high water temperatures

heated by hydrothermal vents.

Fish

Like us, fish are vertebrates that have a spinalcolumn and a hard skull. They are animals thathave adapted to life in the water. Most fish are"cold-blooded" animals that have fins with whichto move and steer, scales that protect them, gillswith which to extract oxygen from the water, and aswim bladder that lets them float at particulardepths within the ocean. Included among the fishare sardines, salmon, and eels, as well as thesharks and rays (which lack swim bladders)(Figure below).

Figure 14.31 The Great White Shark is a fish that preys on

other fish and marine mammals.

Reptiles

Reptiles are air-breathing, "cold-blooded"vertebrates. A few groups of reptiles have adaptedto life at sea. These include sea turtles, sea snakes,a few saltwater crocodiles, and the marine iguana,which is found only at the Galapagos Islands(Figure below). Most sea snakes bear live youngin the ocean and do not need to come on land tobreed. But turtles, crocodiles, and marine iguanasall lay their eggs on land, which makes both eggsand adults vulnerable to predation. For example,people use sea turtles or their eggs for food, fortheir shells, and for the medicinal purposes thatsome cultures believe they possess. Sea turtles areendangered species, so they are protected in manycountries around the world.

Figure 14.32 Sea turtles are found all over the oceans, but

their numbers are diminishing.

Seabirds

Everybody loves penguins, a type of birdadapted to the sea (Figure below). They do not fly;rather they are adapted to swimming and mayspend half of their time at sea looking for food.There are many other kinds of seabirds, though,like gulls, gannets, pelicans, and petrels. Seabirdsare adapted to catching fish by diving or bygrabbing them at the surface with their claws.

Figure 14.33 Many penguins live in and around Antarctica,

but some penguins live farther north on islandssuch as the Galapagos Islands (near the Equator).The Southern Rockhopper Penguin lives in theFalkland Islands off the coast of Argentina, a long

way from the ice.

Marine Mammals

Mammals are warm-blooded vertebrates thatfeed their young with milk. Most mammals havehair, ears, a jaw bone with teeth, and give birth todeveloped young. There are five types of marinemammals. The first type is termed Cetaceanswhich include whales, dolphins, and porpoises.The second type is called Sirenians which includethe manatee and the dugong. Seals, sea lions, andwalruses comprise the Pinniped group. Sea ottersare the ocean members of the fourth group, theMustelids, which also includes skunks, badgersand weasels (Figure below). The final type ofocean marine mammal is the polar bear, whichdepends heavily on the ocean for survival and isadapted to a life around the sea.

Figure 14.34 The sea otter is an marine mammal that

depends on the ocean for survival.

Interactions Among OceanOrganisms

To best understand how ocean organismsinteract, it is necessary to consider the particularenvironments in which they live. There are fourmain ocean habitats: the intertidal zone and shore,reefs, the open ocean, and the deep sea includingtrenches. Most organisms have some adaptationsspecific to their preferred habitat.

A great abundance of life can be found in theintertidal zone. Many intertidal animals can live inor out of the water; some spend one part of theirlives in the water and another out of the water.They must be adapted to frequent shifting of waterlevels and wave impacts. In response, many havehard shells and strong attachments that keep themsafe. Some animals, like marine mussels, clingsteadily to a rock for their entire lives (Figurebelow). Many young organisms get their start inestuaries, which are special ecosystems affectedby the tides, where freshwater and salt water cometogether.

Figure 14.35

Marine mussels live in the intertidal zone.How are they adapted for life in the intertidalzone?

Reefs are built up by corals and other animalsthat deposit the mineral calcium carbonate to makerock formations near the shore. They support acomplex ecosystem of ocean organisms that livewithin the coral reef. These diverse organismshave complex interactions with one another; somespecies help each other to survive. When reefs aredestroyed or polluted, certain species can beaffected more than others. Harm to one speciesmay have a domino effect on other species. Thismay cause the entire ecosystem to collapse. Coralreefs are particularly sensitive to certain threatslike temperature change and oil spills.

The open ocean refers to the large openexpanses of ocean water. This vast area is theprimary habitat for relatively few animals. Most ofthe food in the ocean is found nearer to shore, somost of these animals are just passing through.Some larger animals like whales and giantgroupers may live their entire lives in the open

water.As you know, scientists were surprised to find

life in ocean trenches, the deepest parts of theocean. How can animals survive at that depth?They have adapted to the resources availablethere, and some bacteria can even use inorganiccompounds as energy sources instead of relying onthe sun as a source of energy. This is calledchemosynthesis. Shrimp, clams, fish, and gianttube worms have been found in these extremeplaces.

Still other animals can live on floating rafts ofalgae or in frozen places, like the North and SouthPoles. No matter where you might look in the grandocean, some creature has found a way to live there.Almost all of these creatures depend on each other.Certainly all creatures depend on producers thatconvert sunlight into biomass. Our oceans arecurrently threatened by global warming, overuseand pollution. These imbalances in the ecosystemsmay someday devastate the delicate web of life onwhich humans depend.

Lesson Summaries

Our oceans are home to a tremendousdiversity of life including the very smallestbacteria to the very largest baleen whale.Some marine organisms float at the surfaceusing the sun's energy, some exist at greatocean depths transforming chemicals in thewater into food.Plankton are freely floating organisms thatinclude the photosynthetic phytoplankton aswell as the animals that eat them, thezooplankton.Virtually every phyla on Earth is representedin the ocean including invertebrate andvertebrate organisms, fish, reptiles, seabirdsand even air breathing mammals.Many creatures in the ocean live incooperation with other organisms, like coralanimals that live symbiotically withdinoflagellates in their tissues.

Review Questions

1. What are seven categories of life in theocean?

2. What does “invertebrate” mean?3. What is the group of organisms are the

primary producers in the ocean, on which allother life depends?

4. If fish require oxygen to live, why can’t theysurvive on land?

5. Some people argue that polar bears are notreally marine mammals because they don’tlive in the ocean itself. They would say polarbears are land animals like all other bears.What is your opinion? Explain.

6. What are four major habitats of oceanorganisms?

7. Describe adaptations that an organism thatlives in a reef might have. How might theseadaptations be different from an organism thatlives in the open ocean?

8. Describe the importance of maintaining the

ecosystem in the ocean.

Vocabulary

chemosynthesisUsing inorganic compounds to produce food.

invertebratesAnimals with no spinal column.

phytoplanktonPlankton that can photosynthesize andtherefore create oxygen and sugars.

planktonA diverse group of tiny animals and plantsthat freely drift in the water.

reefA large underwater structure created from thecalcium carbonate skeletons of coral.

vertebrates

Animals with a spinal column.

zooplanktonPlankton that are tiny animals; they usuallyconsume phytoplankton or other zooplanktonas food.

Points to Consider

How does the ocean interact with theatmosphere?How is energy transferred around the planetand how does this affect life on Earth?What does global warming mean for theoceans and how might this affect the entireglobe?

Chapter 15: Earth'sAtmosphere

The Atmosphere

Lesson Objectives

Describe the importance of the atmosphere toour planet and its life.Outline the role of the atmosphere in thewater cycle.List the major components of the atmosphereand know their functions.Describe how atmospheric pressure changeswith altitude.

Introduction

Earth’s atmosphere is a thin blanket of gases

and tiny particles—together called air. Without air,the Earth would just be another lifeless rockorbiting the Sun. Although we are rarely aware ofit, air surrounds us. We are most aware of airwhen it moves, creating wind. Like all gases, airtakes up space. These gases that make up our airare packed closer together near the Earth’s surfacethan at higher elevations.

All living things need some of the gases in airfor life support. In particular, all organisms rely onoxygen for respiration — even plants requireoxygen to stay alive at night or when the Sun isobscured. Plants also require carbon dioxide in theair for photosynthesis. All weather happens in theatmosphere. The atmosphere has many otherimportant roles as well. These include moderatingEarth’s temperatures and protecting living thingsfrom the Sun’s most harmful rays.

Significance of the Atmosphere

Without the atmosphere, planet Earth would be

much more like the Moon than like the planet welive on today. The Earth's atmosphere, along withthe abundant liquid water on the Earth’s surface,are keys to our planet's unique place in the solarsystem. Much of what makes Earth exceptionaldepends on the atmosphere. Let's consider some ofthe many reasons we are lucky to have anatmosphere.

Atmospheric Gases Are Indispensable for Lifeon Earth

Without the atmosphere, Earth would belifeless. Carbon dioxide (CO2) and oxygen (O2)are the most important gases for living organisms.CO2 is vital for use by plants in photosynthesis, inwhich plants use CO2 and water to convert theSun’s energy into food energy. This food energy isin the form of the sugar glucose (C6H12O6). Plantsalso produce O2. Photosynthesis is responsible fornearly all of the oxygen currently found in theatmosphere.

The chemical reaction for photosynthesis is:6CO2 + 6H2O + solar energy → C6H12O6 +

6O2By creating oxygen and food, plants have made

an environment that is favorable for animals. Inrespiration, animals use oxygen to convert sugarinto food energy they can use. Plants also gothrough respiration and consume some of thesugars they produce.

The chemical reaction for respiration is:C6H12O6 + 6O2 → 6CO2 + 6H2O + useable

energyNotice that respiration looks like

photosynthesis in reverse. In photosynthesis, CO2is converted to O2 and in respiration, O2 isconverted to CO2.

The Atmosphere is a Crucial Part of the WaterCycle

Water moves from the atmosphere onto the

land, into soil, through organisms, to the oceansand back into the atmosphere in any order. Thismovement of water is called the water cycle orhydrologic cycle (Figure below).

Water changes from a liquid to a gas byevaporation. Water vapor is the name for waterwhen it is a gas. When the Sun’s energy evaporateswater from the ocean surface or from lakes,streams, or puddles on land, it becomes watervapor. The water vapor remains in the atmosphereuntil it condenses to become tiny droplets ofliquid. The tiny droplets may come together tocreate precipitation, like rain and snow. Snowmay become part of the ice in a glacier, where itmay remain for hundreds or thousands of years.Eventually, the snow or ice will melt to formliquid water. A water droplet that falls as rain,could become part of a stream or a lake, or it couldsink into the ground and become part ofgroundwater. At the surface, the water willeventually undergo evaporation and reenter theatmosphere. If the water is taken up by a plant andthen evaporates from the plant, the process is

called evapotranspiration.

Figure 15.1 The Water Cycle.Al l weather takes place in the atmosphere,

virtually all of it in the lower atmosphere. Weatherdescribes what the atmosphere is like at a specifictime and place, and may include temperature, windand precipitation. It is the changes we experiencefrom day to day. Climate is the long-term averageof weather in a particular spot. Although theweather for a particular winter day in Tucson,Arizona may include snow, the climate of Tucsonis generally warm and dry.

The physical and chemical changes that happenon Earth's surface due to precipitation, wind and

reactions with the gases in our atmosphere arecalled weathering. Weathering alters rocks andminerals and shapes landforms at the Earth’ssurface. Without weathering, Earth’s surfacewould not change much at all. For example, theMoon has no atmosphere, water or winds, so itdoes not have weathering. The footprints thatastronauts made on the Moon decades ago willremain there until someone (human or alien)smooths them out! You would only need to spend afew minutes at the beach to know that Earth'ssurface is changing all the time.

Ozone in the Upper Atmosphere Makes Life onEarth Possible

Ozone is a molecule composed of three oxygenatoms, (O3). Ozone in the upper atmosphereabsorbs high energy ultraviolet radiation (UV)coming from the Sun. This protects living things onEarth’s surface from the Sun’s most harmful rays.Without ozone for protection, only the simplest life

forms would be able to live on Earth.

The Atmosphere Keeps Earth’s TemperatureModerate

Our atmosphere keeps Earth's temperatureswithin an acceptable range; the difference betweenthe very coldest places on Earth and the veryhottest is about 150oC (270oF). Without ouratmosphere, Earth's temperatures would be frigidat night and scorching during the day. Our dailytemperatures would resemble those seen on theMoon, where the temperature range is 310oC(560oF) because there is no atmosphere.Greenhouse gases trap heat in the atmosphere.Important greenhouse gases include carbondioxide, methane, water vapor and ozone.

Atmospheric Gases Provide the Substance forWaves to Travel Through

The atmosphere is made of gases, mostly

nitrogen and oxygen. Even though you can't seethem, gases take up space and can transmit energy.Sound waves are among the types of energy thatcan travel though the atmosphere. Without anatmosphere, we could not hear a single sound.Earth would be as silent as outer space. Of course,no insect, bird or airplane would be able to flysince there would be no atmosphere to hold it up!

Composition of Air

Air is made almost entirely of two gases. Themost common gas is nitrogen, and the second mostcommon gas is oxygen (O2). Nitrogen and oxygentogether make up 99% of the planet’s atmosphere.All other gases together make up the remaining1%. Although each of these trace gases are onlyfound in tiny quantities, many such as ozone, serveimportant roles for the planet and its life. One veryimportant minor gas is carbon dioxide, CO2, whichis essential for photosynthesis and is also a veryimportant greenhouse gas (Table (below).

Concentrations of Atmospheric Gases

Concentrations of Atmospheric Gases

Gas Symbol Concentration(%)

Nitrogen N2 78.08

Oxygen O2 20.95Argon Ar 0.93Neon Ne 0.0018Helium He 0.0005Hydrogen H 0.00006Xenon Xe 0.000009Water vapor H2O 0 to 4

Carbon dioxide CO2 0.038

Methane CH4 0.00017Krypton Kr 0.00011Nitrous oxide N2O 0.00005

Ozone O3 0.000004Particles (dust, soot) 0.000001Chlorofluorocarbons 0.00000002

(CFCs)(Source:

http://upload.wikimedia.org/wikipedia/commons/7/7a/Atmosphere_gas_proportions.svg,License: GNU-FDL)

In nature, air is never completely dry. Up to4% of the volume of air can be water vapor.Humidity is the amount of water vapor in air. Thehumidity of the air varies from place to place andseason to season. This fact is obvious if youcompare a summer day in Atlanta,Georgia wherehumidity is very high, with a winter day inPhoenix, Arizona where humidity is very low.When the air is very humid, it feels heavy orsticky. Your hair might get really curly or frizzywhen it is very humid outside. Most people feelmore comfortable when the air is dry. Thepercentage of water vapor in our atmosphere islisted with a wide range of values in the tableabove because air can be both very humid or dry.

Argon, neon, helium, xenon, and krypton arenoble gases. They are colorless, odorless,tasteless, and they do not become part of ordinarychemical reactions because they are chemically

inert. The noble gases simply exist in theatmosphere.

Some of what is in the atmosphere is not a gas.Particles of dust, soil, fecal matter, metals, salt,smoke, ash and other solids make up a smallpercentage of the atmosphere. This percentage isvariable, as anyone who has spent a windy day inthe desert knows (Figure below). Particles areimportant because they provide starting points (ornuclei) for water vapor to condense on, which thenforms raindrops. Some particles are pollutants,which are discussed in the chapter on humanactions and the atmosphere.

Figure 15.2 A dust storm in Al Asad, Iraq.

Pressure and Density

The atmosphere has different properties atdifferent elevations above sea level, or altitudes.T he density of the atmosphere (the number ofmolecules in a given volume) decreases the higheryou go. This is why explorers who climb tallmountains, like Mt. Everest, have to set up camp atdifferent elevations to let their bodies get used tothe changes. What the atmosphere is made of, orthe composition of the first 100 kilometers of theatmosphere stays the same with altitude, with oneexception: the ozone layer at about 20 - 40kilometers above the Earth. In the ozone layer,there is a greater concentration of ozone than inother portions of the atmosphere (Figure below).

Figure 15.3 The difference in air pressure between 2,000

meters and sea level caused this bottle to collapse.The bottle was originally at higher elevation,where air pressure is lower. When it was broughtdown to sea level, the higher air pressure at sealevel caused the bottle to collapse.

The molecules in gases are able to movefreely. If no force acted on a gas at all, it wouldjust escape or spread out forever. Gravity pulls gasmolecules in towards the Earth’s surface, pullingstronger closer to sea level. This means thatatmospheric gases are denser at sea level, wherethe gravitational pull is greater. Gases at sea levelare also compressed by the weight of theatmosphere above them. The weight of theatmosphere on a person’s shoulders is equal tomore than one ton. The force of the air weighingdown over a unit of area is known as itsatmospheric pressure, or air pressure. People andanimals are not crushed because molecules insideour bodies are pushing outward to compensate. Airpressure is felt from all directions, not just from

above.The atmosphere has lower atmospheric

pressure and is less dense at higher altitudes.There is less pull from gravity and there is less gasto push down from above. Without as much weightabove them, the gases expand, so the air is lighter.For each 6 km (3.7 mile) increase in altitude, theair pressure decreases by half. At 5,500 meters(18,000 feet) above sea level, the air pressure isjust less than half of what it is at sea level. Thismeans that the weight of the air on a person’sshoulders at that altitude is only one-half ton. At ahigh enough altitude, there is no gas left. Thedensity of the atmosphere at 30 km (19 miles)above sea level is only 1% that of sea level. By700 km (435 miles) from the planet’s surface, theair pressure is almost the same as that in thevacuum of deep space.

If your ears have ever 'popped,' you haveexperienced a change in air pressure. This occurswhen you go up or down in altitude quickly, suchas flying in an airplane or riding in a car as it goesup or down a mountain. Gas molecules are found

inside and outside your ears. When you changealtitude quickly, your inner ear keeps the density ofmolecules at the original altitude. The poppingoccurs when the air molecules inside your earsuddenly move through a small tube in your earequalizing the pressure. This sudden rush of air isfelt as a popping sensation.

Colder, drier places on Earth usually havehigher air pressure, while warmer, more humidplaces usually have lower air pressure. Thishappens because large areas of air move up ordown by convection. Air pressure also oftenchanges over time, as low and high pressuresystems change locations. These phenomena willbe discussed when we learn about weather.

Lesson Summary

Without its atmosphere, Earth would be avery different planet. Gases in the atmosphereallow plants to photosynthesize and animalsand plants to engage in respiration.

Water vapor, which is an atmospheric gas, isan essential part of the water cycle.All weather takes place in the atmosphere.While the amount of gases do not varyrelative to each other in the atmosphere, thereis one exception: the ozone layer. Ozone inthe upper atmosphere protects life from theSun’s high energy ultraviolet radiation.Air pressure varies with altitude, temperatureand location.

Review Questions

1. What gas is used and what gas is createdduring photosynthesis? What gas is used andwhat gas is created during respiration?

2. Describe two reasons why photosynthesis isimportant.

3. Briefly describe the movement of waterthrough the water cycle.

4. What is evapotranspiration?5. What will happen if the humidity of the

atmosphere increases?6. Is weathering more effective in a humid or a

dry climate?7. On an unusual February day in Portland,

Oregon, the temperature is 18oC (65oF) and itis dry and sunny. The winter climate inPortland is usually chilly and rainy. Howcould you explain a warm, dry day inPortland in winter?

8. What important role do greenhouse gases playin the atmosphere?

9. Why do your ears pop when you are in anairplane and the plane descends for alanding?

10. If air pressure at sea level is one ton and at5,500 m (18,000 feet) is one-half ton, what isair pressure at 11,000 m (36,000 feet)?

Vocabulary

air pressureThe force of air pressing on a given area.

altitudeDistance above sea level.

climateThe long-term average of weather.

condensesChanges state from a gas to a liquid; in thecase of water, from water vapor to liquidwater.

evaporationThe change in state of a substance from aliquid to a gas; in the case of water, fromliquid water to water vapor.

evapotranspirationWater loss by plants to the atmosphere.

greenhouse gasesGases that trap heat in the atmosphere; theseinclude water vapor, carbon dioxide, methaneand ozone.

groundwaterFresh water found beneath the ground surface.

humidityThe amount of water vapor held in the air;usually refers to relative humidity, meaningthe amount of water the air holds relative tothe total amount it could hold.

noble gasesGases that usually do not react chemically andhave no color, taste, or odor; these arehelium, neon, argon, xenon and krypton.

ozoneA molecule made of three oxygen atoms;ozone in the lower atmosphere is a pollutant,but in the upper atmosphere it filters out thesun’s most harmful ultraviolet radiation.

photosynthesisThe process in which plants produce simplesugars (food energy) from solar energy; the

process of photosynthesis changes carbondioxide to oxygen.

precipitationCondensed moisture including rain, sleet,hail, snow, frost or dew.

respirationThe process in which animals and plants useoxygen and produce carbon dioxide; inrespiration, organisms convert sugar into foodenergy they can use.

water vaporThe gas form of water.

weatherThe temporary state of the atmosphere in aregion; the weather in a location depends onthe air temperature, humidity, precipitation,wind and other features of the atmosphere.

Points to Consider

How would Earth be different if it did nothave an atmosphere?What are the most important components ofthe atmosphere?How does the atmosphere vary with altitude?

Atmospheric Layers

Lesson Objectives

List the major layers of the atmosphere andtheir temperatures.Discuss why all weather takes place in thetroposphere.Discuss how the ozone layer protects thesurface from harmful radiation.

Introduction

The atmosphere is layered, and these layerscorrespond with how the atmosphere’s temperaturechanges with altitude. By understanding the waytemperature changes with altitude, we can learn alot about how the atmosphere works. For example,the reason that weather takes place in the lowestlayer is that the Earth’s surface is the atmosphere’sprimary heat source. Heating the lowest part of the

atmosphere places warm air beneath colder air, anunstable situation that can produce violent weather.Interesting things happen higher in the atmosphere,like the beautiful aurora, which light up the skywith brilliant flashes, streaks and rolls of white orcolored light.

Air Temperature

Warm air rises: that’s a saying just abouteveryone has heard. But maybe not everyoneknows why this is true. Gas molecules are free tomove around, and the molecules can take up asmuch space as they need. When the molecules arecool, they are sluggish and do not move as much,so they do not take up as much space. When themolecules are warm, they move vigorously andtake up more space. With the same number ofmolecules in this larger volume, the air is lessdense and air pressure is lower. This warmer,lighter air is more buoyant than the cooler airabove it, so it rises. The cooler air then sinks

down, since it is more dense than the air beneath it.The rising of warmer air and sinking of cooler airis a very important concept for understanding theatmosphere.

As you learned in the previous section, thecomposition of gases is mostly the same throughoutthe first 100 km of our atmosphere. This means ifwe measure the percentages of different gasesthroughout the atmosphere, it will stay basicallythe same. However the density of the gases and theair pressure do change with altitude; they basicallydecrease with increasing altitude. The propertythat changes most strikingly with altitude is airtemperature. Unlike the change in pressure anddensity, changes in air temperature are not regular.A change in temperature with distance is called atemperature gradient.

The atmosphere is divided into layers based onhow the temperature in that layer changes withaltitude, the layer’s temperature gradient (Figurebelow). The temperature gradient of each layer isdifferent. In some layers, temperature increaseswith altitude and in others it decreases. The

temperature gradient in each layer is determined bythe heat source of the layer. The differenttemperature gradients in each of the four mainlayers create the thermal structure of theatmosphere.

There are several layers of the atmosphere.The first layer is the troposphere. It is the closestto the ground and is sometimes referred to as thelower atmosphere. The second layer is thestratosphere, and is sometimes referred to as theupper atmosphere. Most of the important processesof the atmosphere take place in one of these twolayers.

Figure 15.4 The layers of the atmosphere with altitude.

Troposphere

About three-fourths of the gases of theatmosphere are found in the troposphere becausegravity pulls most of the gases close to the Earth'ssurface. As with the rest of the atmosphere, 99% ofthe gases are nitrogen and oxygen.

The thickness of the troposphere varies around

the planet. Near the equator, the troposphere isthicker than at the poles, since the spinning of theEarth tends to shift air towards the equator. Thethickness of the troposphere also varies withseason. The troposphere is thicker in the summerand thinner in the winter all around the planet. Atthe poles in winter, the atmosphere is uniformlyvery cold and the troposphere cannot bedistinguished from other layers. The importance ofthis feature of the atmosphere will become clearwhen we learn about ozone depletion.

Earth’s surface is a major source of heat for thetroposphere. Where does the heat come from?Nearly all the heat comes from the sun, eitherdirectly or indirectly. Some incoming sunlightwarms the gases in the atmosphere directly. Butmore sunlight strikes the Earth's rock, soil, andwater, which radiate it back into the atmosphere asheat, further warming the troposphere. Thetemperature of the troposphere is highest near thesurface of the Earth and declines with altitude. Onaverage, the temperature gradient of thetroposphere is 6.5oC per 1,000 m (3.6oF per 1,000

feet) of altitude.Notice that in the troposphere, warm air is

beneath cold air. Since warm air is less dense andtries to rise, this condition is unstable. So thewarm air at the base of the troposphere rises andcool air higher in the troposphere sinks. For thisreason, air in the troposphere does a lot of mixing.This mixing causes the temperature gradient tovary with time and place. For reasons that will bediscussed in the next section, rising air cannot riseabove the top of the troposphere. The rising andsinking of air in the troposphere means that all ofthe planet’s weather takes place in the troposphere.

When there is a temperature inversion, airtemperature in the troposphere increases withaltitude and warm air sits over cold air. This iscalled an inversion because the usual situation isreversed or inverted. Inversions are very stableand they often last for several days or even weeks.Inversions commonly form over land at night or inwinter. At these times, the ground is cold becausethere is little solar energy reaching it. At night, theSun isn’t out and in winter, the Sun is at a low

angle, so little solar radiation reaches the ground.This cold ground cools the air that sits above it,making this low layer of air denser than the airabove it. An inversion also forms on the coastwhere cold seawater cools the air above it. Whenthat denser air moves inland, it slides beneath thewarmer air over the land. Since temperatureinversions are stable, they often trap pollutants andproduce unhealthy air conditions in cities (Figurebelow).

Figure 15.5 Smoke makes a temperature inversion visible.

The smoke is trapped in cold dense air that liesbeneath a cap of warmer air.

At the top of the troposphere is a thin layercalled the tropopause. Temperature in the

tropopause does not change with height. Thismeans that the cooler, denser air of the troposphereis trapped beneath the warmer, less dense air ofthe stratosphere. So the tropopause is a barrier thatkeeps air from moving from the troposphere to thestratosphere. Sometimes breaks are found in thetropopause and air from the troposphere andstratosphere can mix.

Stratosphere

The stratosphere rises above the tropopause.When a volcano erupts dust and gas that makes itsway into the stratosphere, it remains suspendedthere for many years. This is because there is solittle mixing between the stratosphere andtroposphere. Pilots like to fly in the lower portionsof the stratosphere because there is little airturbulence.

In the stratosphere, temperature increases withaltitude. The reason is that the direct heat sourcefor the stratosphere is the Sun. A layer of ozone

molecules absorbs solar radiation, which heats thestratosphere. Unlike in the troposphere, air in thestratosphere is stable because warmer, less denseair sits over cooler, denser air. As a result, there islittle mixing of air within the layer.

The stratosphere has the same composition ofgases as the rest of the atmosphere, with theexception of the ozone layer. The ozone layer isfound within the stratosphere at between 15 to 30km (9 to 19 miles) altitude. The thickness of theozone layer varies by the season and also by thelatitude. The amount of ozone present in the ozonelayer is tiny, only a few molecules per million airmolecules. Still, the concentration of ozone ismuch greater than in the rest of the atmosphere. Theozone layer is extremely important because ozonegas in the stratosphere absorbs most of the Sun’sharmful ultraviolet (UV) radiation.

How does ozone do this? High energyultraviolet light, traveling through the ozone layer,breaks apart the ozone molecule, O3 into oneoxygen molecule (O2) and one oxygen atom (O).

This process absorbs the Sun’s most harmful UVrays. Ozone is also reformed in the ozone layer:Oxygen atoms bond with O2 molecules to make O3.Under natural circumstances, the same amount ofozone is continually being created and destroyedand so the amount of ozone in the ozone layerremains the same.

The ozone layer is so effective that the highestenergy ultraviolet, the UVC, does not reach theplanet’s surface at all. Some of the second highestenergy ultraviolet, UVB, is stopped as well. Thelowest energy ultraviolet, UVA, travels through theatmosphere to the ground. In this way, the ozonelayer protects life on Earth. High energy ultravioletlight penetrates cells and damages DNA, leading tocell death (which we know as a bad sunburn).Organisms on Earth are not adapted to heavy UVexposure, which kills or damages them. Withoutthe ozone layer to reflect UVC and UVB, mostcomplex life on Earth would not survive long.

Above the stratosphere is the thin stratopause,which is the boundary between the stratospherebelow and the mesosphere above. The stratopause

is at about 50 km above the Earth’s surface.

Mesosphere

Temperatures in the mesosphere decreasewith altitude. Since there are very few gasmolecules in the mesosphere to absorb the Sun’sradiation, the heat source here is the stratospherebelow. The mesosphere is extremely cold,especially at its top, about -90oC (-130oF).

The air in the mesosphere is extremely thin:99.9% of the mass of the atmosphere is below themesosphere. As a result, air pressure is very low.Although the amount of oxygen relative to othergases is the same as at sea level, there is very littlegas and so very little oxygen. A person travelingthrough the mesosphere would experience severeburns from ultraviolet light since the ozone layerwhich provides UV protection is in thestratosphere below them. And there would bealmost no oxygen for breathing! Stranger yet, anunprotected traveler’s blood would boil at normal

body temperature because the pressure is so low.Despite the thin air, the mesosphere has enough

gas that meteors burn as they enter the atmosphere(Figure below). The gas causes friction with thedescending meteor, producing its tail. Some peoplecall them “shooting stars.” Above the mesosphereis the mesopause. Astronauts are the only peoplewho travel through the mesopause.

Figure 15.6 Meteors burn up as they hit the mesosphere.

Thermosphere and Beyond

The thermosphere rises from the mesopause.The International Space Station (ISS) orbits withinthe upper part of the thermosphere, at about 320 to380 km above the Earth (Figure below).

Figure 15.7 The International Space Station.What does an astronaut experience in the

thermosphere? Temperatures in the thermospherecan exceed 1000oC (1800oF) because oxygenmolecules in the layer absorb short wavelengthsolar energy. Yet despite these high temperatures,the atmosphere outside the ISS feels cold. This isbecause gas molecules are so few and far between

that they very rarely collide with other atoms andso little energy is transferred. The density ofmolecules is so low that one gas molecule can goabout 1 km before it collides with anothermolecule.

Within the thermosphere is the ionosphere.The ionosphere gets its name because nitrogen andoxygen molecules are ionized by solar radiation. Inthe process of ionization, the neutrally-chargedmolecules absorb high-energy, short-wavelengthenergy from the Sun. This causes the molecules tolose one or more electrons and become positively-charged ions. The freed electrons travel within theionosphere as electric currents. Because of the freeions, the ionosphere has many interestingcharacteristics.

Have you ever been out on an open road andfound a radio station on the AM dial that istransmitted from hundreds of kilometers away?The reason radio waves can travel so far at nightinvolves the ionosphere. During the day, the lowerpart of the ionosphere absorbs some of the energyfrom the radio waves and reflects some back to

Earth. But at night the waves bounce off of theionosphere, go back down to the ground, and thenbounce back up again. This does not happen duringthe day due to ionization in the ionosphere. Thisbouncing up and down allows radio waves totravel long distances.

The most spectacular feature of the ionosphereis the nighttime aurora. Spectacular light displayswith streamers, arcs, or foglike glows are visibleon many nights in the polar regions. The lights canbe white, green, blue, red or purple. The display iscalled the aurora borealis or northern lights in theNorthern Hemisphere (Figure below). It is calledt h e aurora australis or southern lights in theSouthern Hemisphere.

Figure 15.8

The Northern Lights above Bear Lake, Alaska.The aurora is caused by massive storms on the

Sun that release great quantities of protons andelectrons. These electrically charged particles flythrough space and spiral in along lines of Earth’smagnetic field. Earth’s magnetic field guides thecharged particles toward the poles, which explainswhy the auroras are seen primarily in the polarregions. When the protons and electrons enter theionosphere, they energize oxygen and nitrogen gasmolecules and cause them to light up. Each gasemits a particular color of light. Depending onwhere they are in the atmosphere, oxygen shinesgreen or red and nitrogen shines red or blue. Thefrequency and intensity of the aurora increaseswhen the Sun has more magnetic storms.

The outermost layer of the atmosphere is theexosphere. There is no real outer limit to theexosphere. If you continued traveling farther outfrom the Earth, the gas molecules would finallybecome so scarce that you would be in outerspace. There is so little gravity holding gasmolecules in the exosphere that they sometimes

escape into outer space. Beyond the atmosphere isthe solar wind. The solar wind is made of high-speed particles, mostly protons and electrons,traveling rapidly outward from the Sun.

Lesson Summary

Different temperature gradients createdifferent layers within the atmosphere. Thelowest layer is the troposphere, where mostof the atmospheric gases and all of theplanet’s weather are located.The troposphere gets its heat from the ground,and so temperature decreases with altitude.Warm air rises and cool air sinks and so thetroposphere is unstable.In the stratosphere, temperature increaseswith altitude. The stratosphere contains theozone layer, which protects the planet fromthe Sun’s harmful UV. The higher layerscontain few gas molecules and are very cold.

Review Questions

1. Why does warm air rise?2. Why doesn’t air temperature increase or

decrease uniformly with altitude, just like airpressure decreases uniformly with altitude?Give examples of the different possiblescenarios.

3. Where and when in the atmosphere is there noreal layering at all? Why is this phenomenonimportant?

4. Describe how the ground acts as the heatsource for the troposphere. What is the sourceof energy and what happens to that energy?

5. How stable is an inversion and why? Howdoes an inversion form?

6. Why doesn’t air from the troposphere and thestratosphere mix freely?

7. Where does the heat from the stratospherecome from and what is needed for that heat tobe absorbed?

8. Describe the process of ozone creation and

loss in the ozone layer. Under normalcircumstances, does one occur more than theother?

9. How and where are 'shooting stars' created?10. Why would an unprotected traveler’s blood

boil at normal body temperature in themesosphere?

Further Reading / SupplementalLinks

http://www.youtube.com/watch?v=PaSFAbATPvk&#38;feature=related

Vocabulary

auroraA spectacular light display that occurs in theionosphere near the poles; called the auroraborealis or northern lights in the NorthernHemisphere, and the aurora australis or

southern lights in the Southern Hemisphere.

exosphereThe outermost layer of the atmosphere, wheregas molecules are extremely far apart andsome occasionally escape earth’s gravity andfly off into outer space.

inversionA situation in the troposhere in which warmair lies above cold air.

ionosphereAn ionized layer contained within thethermosphere; the second to the last layer ofthe atmosphere.

mesopauseThe thin transition layer in the atmosphere, theboundary between the mesosphere and thethermosphere.

mesosphereThe layer of the atmosphere between the

stratosphere and the thermosphere;temperature decreases with altitude.

ozone layerA layer of the stratosphere where ozone gas ismore highly concentrated.

solar windHigh-speed protons and electrons that flythrough the solar system from the Sun.

stratopauseThe thin transitional layer of the atmospherebetween the stratosphere and the mesosphere.

stratosphereThe second layer of the atmosphere, wheretemperature increases with altitude due thepresence of ozone.

temperature gradientThe change in temperature with distance.

thermosphere

The second to the last layer of the atmospherewhere gases are extremely thinly distributed.

tropopauseThe thin transitional layer of the atmospherebetween the troposphere and the stratosphere.

troposphereThe lowermost layer of the atmosphere.

ultraviolet radiationnHigh energy radiation that comes from theSun; there are three types of UV radiation,UVA, UVB and UVC. The shortestwavelength, and therefore the most dangerous,is UVC.

Points to Consider

How does solar energy create theatmosphere’s layers?How does solar energy create the weather?What would be the situation for life on Earth

if there was less ozone in the ozone layer?

Energy in the Atmosphere

Lesson Objectives

Describe how energy is transmitted.Describe the Earth’s heat budget and whathappens to the Sun’s energy.Discuss the importance of convection in theatmosphere.Describe how a planet’s heat budget can bebalanced.Describe the greenhouse effect and why it isso important for life on Earth.

Introduction

Wind and precipitation, warming and coolingdepend on how much energy is in the atmosphereand where that energy is located. Much moreenergy from the Sun reaches low latitudes (nearerthe equator) than high latitudes (nearer the poles).

These energy differences cause the winds, affectclimate, and even drive ocean currents. Heat isheld in the atmosphere by greenhouse gases.

Energy, Temperature, and Heat

Every material has energy: All the moleculeswithin it vibrate. Gas molecules contain moreenergy than an equal number of liquid molecules(under the same temperature and pressureconditions) and move freely. Liquid moleculescontain more energy than solids and move morefreely than solids.

Energy travels through space or material. Youknow this because you can stand near a fire andfeel the warmth. In this situation, energy is beingtransferred as invisible waves that can travelthrough air, glass, and even the vacuum of outerspace. These waves have electrical and magneticproperties, so they are called electromagneticwaves. The transfer of energy from one object toanother through electromagnetic waves is known

a s radiation. Different types of electromagneticwaves have different wavelengths. A wavelengthis the horizontal distance from trough-to-trough orcrest-to-crest of adjacent waves (Figure below).

Figure 15.9 Waves. The high points are the crests, the low

point is the trough. The wavelength is the distancefrom crest to crest.

Humans are able to see some wavelengths oflight, the wavelengths known as 'visible light.'These wavelengths appear to us as the colors ofthe rainbow (Figure below). The longestwavelengths of visible light appear red and theshortest wavelengths appear violet. Wavelengthsthat are longer than visible red are infrared. Snakes

can see infrared energy. We can record this withspecial equipment. Wavelengths that are shorterthan violet are ultraviolet. Infrared and ultravioletwavelengths of energy are just as important as thewavelengths in visible light; we just can’t seethem.

Figure 15.10 The electromagnetic spectrum showing the

wavelengths of energy. Solar radiation that reachesthe outer atmosphere includes radio waves, alongwith visible light plus the ultraviolet range nearest

to violet and the near infrared. Other wavelengthsof electromagnetic radiation are blocked bydifferent parts of Earths atmosphere.

Some objects radiate electromagnetic waves inthe visible light spectrum. Two familiar sourcesare the Sun and a light bulb. Some objects radiateelectromagnetic waves at wavelengths that wecannot see. The glass of water sitting next to youdoes not radiate visible light, but it does radiate atiny amount of heat.

You should be aware that some objects appearto radiate visible light, but they actually do not.The moon and the planets, for example, do not emitlight of their own. They reflect the light of the Sun.Reflection is when light bounces back from asurface. Albedo is a measure of how well asurface reflects light. A surface that reflects a highpercentage of the light that strikes it has highalbedo and one that reflects a small percentage haslow albedo.

One important fact to remember is that energycannot be created or destroyed. It can only bechanged from one form to another. In

photosynthesis, for example, plants convert theSun’s energy into food energy. They do not createnew energy. When energy is transformed, oftensome becomes heat. Heat transfers betweenmaterials easily, from warmer objects to coolerones. If no more heat is added, eventually all of amaterial will reach the same temperature.

Temperature is a measure of how fast theatoms in a material are vibrating. High temperatureparticles vibrate faster than low temperatureparticles. Rapidly vibrating atoms smash together,which generates heat. As a material cools down,the atoms vibrate more slowly and collide lessfrequently. As a result, they emit less heat.

What is the difference between heat andtemperature? Temperature measures how fast amaterial’s atoms are vibrating. Heat measures thematerial’s total energy. Think of a candle flameand a bathtub full of hot water. Which has a higherheat and which has a higher temperature?Surprisingly, the flame has a higher temperature,but much less heat because the hot region is verysmall. The bathtub has lower temperature but

contains much more heat because it has many morevibrating atoms. Even though it’s at a lowertemperature, the bathtub has a greater total energy.

Heat is taken in or released when an objectchanges state, or changes from a gas to a liquid ora liquid to a solid. This heat is called latent heat.When a substance changes state, latent heat isreleased or absorbed. A substance that is changingits state of matter does not change temperature. Allof the energy that is released or absorbed goestoward changing the material’s state.

For example, imagine a pot of boiling water onthe stove: that water is at 100oC (212oF). If a cookincreases the temperature of the burner beneath thepot, more heat enters the water. But still the waterremains at its boiling temperature. The additionalenergy goes into changing the water from liquid togas. This allows the water to evaporate morerapidly. When water changes from a liquid to a gasit takes in heat. Since evaporation takes in heat,this is called evaporative cooling. Evaporativecooling is an inexpensive way to cool homes inhot, dry areas.

Substances also differ in their specific heat,the amount of energy needed to raise thetemperature of one gram of the material by 1.0oC(1.8oF). Water has a very high specific heat, whichmeans it takes a lot of energy to change thetemperature of water. Let's compare a puddle andasphalt, for example. If you are walking barefooton a sunny day, which would you rather walkacross, the shallow puddle or an asphalt parkinglot? Due to its high specific heat, the water stayscooler than the asphalt, even though it receives thesame amount of solar radiation.

Energy From the Sun

Most of the energy that reaches the Earth’ssurface comes from the Sun. The Sun emits energyin a continuous stream of wavelengths (Figurebelow). These wavelengths include visible light,infrared, ultraviolet radiation, and others. About44% of solar radiation is in the visible lightwavelengths. When viewed together, all of the

wavelengths of visible light appear white. But aprism or water droplets, for example, can breakthe white light into different wavelengths so thatyou can see separate colors (Figure below).

Figure 15.11 An image of the sun taken by the SOHO

spacecraft. The sensor is picking up only the 17.1nm wavelength, in the ultraviolet wavelengths.

Figure 15.12 A prism breaks apart white light by wavelength

so that you can see all the colors of the rainbow.Only about 7% of solar radiation is in the

ultraviolet (UV) wavelengths. Of the solar energythat reaches the outer atmosphere, UV wavelengthshave the greatest energy. There are three types ofUV energy: UVC has the shortest wavelengths andis the most energetic; UVA is the longestwavelengths and is the least energetic; and UBV isin the middle of the two. UV radiation will tan orburn human skin. The remaining solar radiation isthe longest wavelength, infrared. Most objectsradiate infrared energy, which we feel as heat(Figure below).

Figure 15.13 An image of a dog taken by an infrared sensor.

The image shows the different amounts of heatradiating from the dog.

Some of the wavelengths of solar radiationtraveling through the atmosphere may be lostbecause they are absorbed by various gases(Figure below). Ozone, for example, completelyremoves UVC, most UVB and some UVA fromincoming sunlight. O2 , CO2 and H2O also filter outother wavelengths from solar energy.

Figure 15.14 In the atmosphere, gases filter some

wavelengths from incoming solar energy. Theyellow field shows the wavelengths of energy thatreach the top of the atmosphere. The red fieldshows the wavelengths that reach sea level. Theamount of radiation is reduced overall as differentgases filter out different wavelengths. Ozone filtersout the shortest wavelength ultraviolet and oxygenfilters out most infrared, at about 750 nm.

Different parts of the Earth receive differentamounts of solar radiation. This is because theSun's rays strike the Earth's surface most directly atthe equator. As you move away from the equator,you will notice that different areas also receive

different amounts of sunlight in different seasons.But what causes the seasons?

The Earth revolves around the Sun once eachyear and spins on its axis of rotation once eachday. This axis of rotation is tilted 23.5° relative toits plane of orbit around the Sun. The axis ofrotation happens to be pointed to the star Polaris,or the North star. As the Earth orbits the Sun, thetilt of Earth's axis stays lined up with the Northstar. This means that the North Pole is tiltedtowards the Sun and the Sun's rays strike theNorthern Hemisphere more directly in summer. Atthe summer solstice, June 21 or 22 of each year,the Sun's rays are hit the Earth most directly alongthe Tropic of Cancer. This is a circle of latitudeexactly 23.5 o north of the equator. When it issummer solstice in the Northern Hemisphere, it iswinter solstice in the Southern Hemisphere. Wintersolstice for the Northern Hemisphere happens onDecember 21 or 22. The tilt of Earth's axis pointsaway from the sun in the winter and the Sun's raysstrike most directly at the Tropic of Capricorn(Figure below). The Tropic of Capricorn is a line

of latitude exactly 23.5 o south of the equator. Thelight from the Sun gets spread out over a largerarea, so that area isn't heated as much. There arealso fewer daylight hours in winter, so there isalso less time for the Sun to warm that place. Whenit is winter in the Northern Hemisphere, it issummer in the Southern Hemisphere.

Figure 15.15 Arctic winter solstice. The suns rays are

directly overhead at the Tropic of Capricorn.Sunlight is striking the south pole, but it is spreadout. No sunlight is getting to the North pole.

Halfway between the two solstices, the Sun'srays shine most directly at the equator. We callthese times an 'equinox' (Figure below). Thedaylight and nighttime hours are exactly equal on

an equinox. The autumnal equinox happens onSeptember 22 or 23 and the vernal or springequinox happens March 21 or 22 in the NorthernHemisphere. Thus the seasons are caused by thedirection Earth's axis is pointing relative to theSun.

Figure 15.16 Where sunlight reaches on spring equinox,

summer solstice, vernal equinox, and wintersolstice. The time is 9:00 pm Universal Time, atGreenwich, England.

Heat Transfer in the Atmosphere

Heat can move in three different ways. We’vealready examined radiation, in whichelectromagnetic waves transfer heat between twoobjects. Conduction is a type of heat transfer thatoccurs when heat moves from areas of more heat toareas of less heat by direct contact. Warmermolecules vibrate more rapidly than coolermolecules. They collide directly with other nearbymolecules, giving them some of their energy, whichtransfers heat. When all the molecules are movingat the same rate, the substance is the sametemperature throughout. Heat in the atmosphere istransferred by conduction. This is more effective atlower altitudes where air molecules are packedmore densely together. Conduction can transferheat upward to where the molecules are spreadfurther apart. It can also transfer heat laterally froma warmer to a cooler spot, where the moleculesare moving less vigorously.

The most important way heat is transferred in

the atmosphere is by convection currents.Convection is the transfer of heat by movement ofheated materials. The radiation of heat from theground warms the air above it. This warmer air isless dense than the air above it and so it rises. Asthe heated air rises it begins to cool, since it isfurther from the heat source. As it cools, itcontracts, becomes denser and sinks. Air moveshorizontally between warm, rising air and cooler,sinking air. This entire structure is a convectioncell.

Heat at Earth’s Surface

Not all energy coming in from the Sun makes itto the Earth’s surface. About half is filtered out bythe atmosphere. Besides being absorbed by gases,energy is reflected by clouds or is scattered.Scattering occurs when a light wave strikes aparticle and bounces off in some other direction.Of the energy that strikes the ground, about 3% isreflected back into the atmosphere. The rest warms

the soil, rock or water that it reaches. Some of theabsorbed energy radiates back into the air as heat.These infrared wavelengths can only be seen byinfrared sensors.

It might occur to you that if solar energycontinually enters the Earth’s atmosphere andground surface, then the planet must always begetting hotter. This is not true, because energy fromthe Earth escapes into space through the top of theatmosphere, just as energy from the Sun entersthrough the top of the atmosphere. If the amount thatexits is equal to the amount that comes in, thenthere is no increase or decrease in average globaltemperature. This means that the planet’s heatbudget is in balance. If more energy comes in thangoes out, the planet warms. If more energy goes outthan comes in, the planet cools.

To say that the Earth’s heat budget is balancedignores an important point. The amount ofincoming solar energy varies at different latitudes(Figure below). This is partly due to the seasons.At the equator, days are about the same length allyear and the Sun is high in the sky. More sunlight

hits the regions around the equator and airtemperatures are warmer. At the poles, the Sundoes not rise for months each year. Even when theSun is out all day and night in the summer, it is at avery low angle in the sky. This means that not muchsolar radiation reaches the ground near the poles.Because of this, during a large part of the year, thepolar areas are covered with ice and snow. Thesebrilliant white substances have a high albedo andreflect solar energy back into the atmosphere. Forall of these reasons, the region around the equatoris much warmer than the areas at the poles.

Figure 15.17 The average annual temperature of the Earth,

showing that the equatorial region is much warmerthan the polar regions. There is a roughly gradualtemperature gradient from the low to the highlatitudes.

The difference in the amount of solar energythat the planet receives at different latitudes drivesmuch of the activity that takes place at the Earth’ssurface. This includes the wind, water cycle, andocean currents. The differences in solar energyaround the globe drive the way the atmospherecirculates.

The Greenhouse Effect

The remaining factor in the Earth’s heat budgets the role of greenhouse gases. Greenhouse gaseswarm the atmosphere by trapping heat. Sunlightstrikes the ground, is converted to heat, andradiates back into the lower atmosphere. Some ofthe heat is trapped by greenhouse gases in thetroposphere, and cannot exit into space. Like ablanket on a sleeping person, greenhouse gases act

a s insulation for the planet. The warming of theatmosphere due to insulation by greenhouse gasesis called the greenhouse effect (Figure below).

Figure 15.18 The Earths heat budget, showing the amount of

energy coming into and going out of the Earthsystem and the importance of the greenhouse effect.The numbers are the amount of energy that is foundin one square meter of that location.

The greenhouse effect is very important, sincewithout it the average temperature of theatmosphere would be about -18oC (0oF). With thegreenhouse effect, the average temperature of theatmosphere is a pleasant 15oC (59oF). Without

insulation, daytime temperatures would be veryhigh and nighttime temperatures would beextremely low. This is the situation on all of theplanets and moons that have no atmosphere. If theEarth did not have insulation, temperatures wouldlikely be too cold and too variable for complexlife forms.

There are many important greenhouse gases inthe atmosphere including CO2, H2O, methane, O3,nitrous oxides (NO and NO2), andchlorofluorocarbons (CFCs). All of these gasesare a normal part of the atmosphere except CFCs,which are human-made. However, human activityhas significantly raised the levels of many of thesegases; for example, methane levels are about 2 1/2times higher as a result of human activity. Tablebelow shows how each greenhouse gas naturallyenters the atmosphere.

Different greenhouse gases have differentabilities to trap heat. For example, one methanemolecule can trap 23 times as much heat as oneCO2 molecule. One CFC-12 molecule (a type of

CFC) can trap 10,600 times as much heat as oneCO2. Still, CO2 is a very important greenhouse gasbecause it is much more abundant in theatmosphere than the others.Greenhouse Gas Where It Comes From

Carbon dioxide

Respiration, volcaniceruptions, decomposition ofplant material; burning offossil fuels

Methane

Decomposition of plantmaterial under someconditions, biochemicalreactions in stomachs

Nitrous oxides Produced by bacteriaOzone Atmospheric processes

Chlorofluorocarbons Not naturally occurring;made by humans

The greenhouse effect is very important foranother reason. If greenhouse gases in theatmosphere increase, they trap more heat and warmthe atmosphere. If greenhouse gases in the

atmosphere decrease, less heat is trapped and theatmosphere cools. The increase or decrease ofgreenhouse gases in the atmosphere affect climateand weather the world over.

Lesson Summary

All materials contain energy, which canradiate through space as electromagneticwaves. The wavelengths of energy that comefrom the Sun include visible light, whichappears white but can be broken up into manycolors.Ultraviolet waves are very high energy. Thehighest energy UV, UVC and some UVB, getsfiltered out of incoming sunlight by ozone.More solar energy reaches the low latitudesand the redistribution of heat by convectiondrives the planet's air currents.

Review Questions

1. What is the difference between temperatureand heat?

2. Give a complete description of these threecategories of energy relative to each other interms of their wavelengths and energy:infrared, visible light, and ultraviolet.

3. Why do the polar regions have high albedo?4. Give an example of the saying “energy can’t

be created or destroyed.5. Describe what happens to the temperature of

a pot of water and to the state of the water asthe dial on the stove is changed from no heatto the highest heat.

6. Describe where the Sun is relative to theEarth on summer solstice, autumnal equinox,winter solstice and spring equinox. Howmuch sunlight is the North pole getting onJune 21? How much is the South pole gettingon that same day?

7. What is the difference between conductionand convection?

8. What is a planet’s heat budget? Is Earth’s heatbudget balanced or not?

9. On a map of average annual temperature, whyare the lower latitudes so much warmer thanthe higher latitudes?

10. Why is carbon dioxide the most importantgreenhouse gas?

11. How does the amount of greenhouse gases inthe atmosphere affect the atmosphere’stemperature?

Vocabulary

albedoThe amount of light that reflects back off asurface; snow and ice have high albedo.

conductionHeat transfer between molecules in motion.

convectionHeat transfer by the movement of currents.

convection cellA heat transfer unit in which warm material

rises, cold material sinks, and material movesbetween the two to create a cell.

electromagnetic wavesRadiation travels in electromagnetic waves;waves that have both electrical and magneticproperties.

energyThe ability to work; energy is not created ordestroyed but can be transferred from oneform to another.

greenhouse effectThe trapping of heat that is radiated out fromthe planet’s surface by greenhouse gases inthe atmosphere and moderates a planet’stemperatures.

insulationA material that inhibits heat or electricityconduction so that the insulated object stays atits current temperature for longer.

latent heatThe energy taken in or released as a substancechanges state from solid to liquid or liquid togas.

radiationThe movement of energy through a material orthrough space, as carried by electromagneticwaves.

reflectionThe return of a wave from a surface, such as alight wave from a mirror.

specific heatThe amount of energy needed to raise thetemperature of 1 gram of material by 1oC(1.8oF).

Points to Consider

How does the difference in solar radiation

that reaches the lower and upper latitudesexplain the way the atmosphere circulates?How does the atmosphere protect life onEarth from harmful radiation and fromextreme temperatures?What would the consequences be if theEarth’s overall heat budget were notbalanced?

Air Movement

Lesson Objectives

List the parts of an atmospheric convectioncell and the properties of the air currentswithin it.Describe how high and low pressure cellscreate local winds and explain how severaltypes of local winds form.Discuss how global convection cells lead tothe global wind belts.

Preview Questions

1. How do high and low pressure zonesdetermine where winds blow?

2. How are land and sea breezes related tomonsoon winds?

3. What determines the directions in which theglobal wind belts blow?

Introduction

Knowing a few basic principles can give aperson a good understanding of how and why airmoves. Warm air rises, creating a low pressureregion, and cool air sinks, creating a high pressurezone. Air flowing from areas of high pressure tolow pressure creates winds. Air moving at thebases of the three major convection cells in eachhemisphere north and south of the equator createsthe global wind belts.

Air Pressure and Winds

Think back to what you learned aboutconvection cells in the previous lesson. Warm airrises, creating an upward-flowing limb of aconvection cell (Figure below). Upward flowingair lowers the air pressure of the area, forming alow pressure zone. The rising air sucks in airfrom the surrounding area, creating wind.

Figure 15.19 Papers being held up by rising air currents

above a furnace demonstrate the importantprinciple that warm air rises.

At the top of the troposphere, the air travelshorizontally from a high pressure zone to a lowpressure zone. Since it is at the top of thetroposphere, the air cools as it moves. This cold,dense air creates the downward flowing limb ofthe convection cell. Where the sinking air strikesthe ground, air pressure is relatively high. Thiscreates a high pressure zone. The sinking air isrelatively cool, since it has traveled across thetropopause.

Air that moves horizontally between high andlow pressure cells makes wind. The winds willrace from the high to low zones if the pressuredifference between them is large. If the differenceis smaller, the winds will be slower.

Convection in the atmosphere creates theplanet’s weather. It’s important to know that warmair can hold more moisture than cold air. Whenwarm air near the ground rises in a low pressure

zone, it cools. If the air is humid, it may not be ableto hold all the water it contains as vapor. Somewater vapor may condense to form clouds or evenprecipitation. Where cooler air descends at a highpressure zone, it warms. Since it can then holdmore moisture, the descending air will evaporatewater on the ground.

Air moving between large high and lowpressure systems creates the global wind belts thatprofoundly affect regional climate. Smallerpressure systems create localized winds that affectthe weather and climate of a local area.

Local Winds

Local winds are created when air moves fromsmall high pressure systems to small low pressuresystems. High and low pressure cells are createdby a variety of conditions. Some of these windshave very important effects on the weather andclimate of some regions.

Land and Sea Breezes

You learned that water has a very high specificheat: it maintains its temperature well. This meansthat water heats and cools more slowly than land.Sometimes there is a large temperature differencebetween the surface of the sea (or a large lake) andthe land next to it. This temperature differencecauses small high and low pressure regions toform, which creates local winds.

In the summer, and to a lesser degree in theday, a low pressure cell forms over the warm landand a high pressure cell forms over the coolerocean. During warm summer afternoons, windscalled sea breezes blow from the cooler oceanover the warmer land (Figure below). Sea breezesoften have a speed of about 10 to 20 km (6 to 12miles) per hour and can lower air temperaturemuch as 5 to 10oC (9 to 18oF). The effect of landand sea breezes is felt only about 50 to 100 km (30to 60 miles) inland.

The opposite occurs in the winter, the land is

colder than the nearby water due to its lowerspecific heat. The cold land cools the air above it.This causes the air to become dense and sink,which creates a high pressure cell. Meanwhile, thewarmer ocean warms the air above it and creates alow pressure cell. This occurs to a smaller degreeat night, since land cools off faster than the ocean.Winds called land breezes blow from the high tothe low pressure cell. These local winds blowfrom the cooler land to the warmer ocean. Somewarmer air from the ocean rises and then sinks onland, causing the temperature over the land tobecome warmer.

Figure 15.20 Sea and land breezes. (A) Sea breezes blow

from the cooler sea to the warmer land. This coolsthe land near shore in summer and in the daytimeand moderates coastal temperatures. (B) Landbreezes blow from the cooler land to the warmersea. This warms the land near shore in winter andat night and moderates coastal temperatures.

Land and sea breezes are very importantbecause they moderate coastal climates. In the hotsummer, sea breezes cool the coastal area. In the

cold winter, land breezes blow cold air seaward.These breezes moderate coastal temperatures.Land and sea breezes create the pleasant climatefor which Southern California is known.

Monsoon Winds

Monsoon winds are larger scale versions ofland and sea breezes; they blow from the sea ontothe land in summer and from the land onto the seain winter. Monsoon winds are incredibly strongbecause they occur in coastal areas with extremelyhigh summer temperatures. Monsoons are commonwherever very hot summer lands are next to thesea. The southwestern United States has summermonsoon rains when relatively cool moist airsucked in from the Gulf of Mexico and the Gulf ofCalifornia meets air that has been heated byscorching desert temperatures (Figure below).

Figure 15.21 The Arizona summer monsoon.The most important monsoon in the world

occurs each year over the Indian subcontinent.More than two billion residents of India andsoutheastern Asia depend on monsoon rains fortheir drinking and irrigation water. In the summer,air over the Indian subcontinent becomes extremelyhot, so it rises. Warm, humid air from the northernIndian Ocean enters the region, and it too is heatedand rises. As the rising wet air cools, it dropsheavy monsoon rains. In the winter, cool air fromover the land moves seaward. Back in the days ofsailing ships, seasonal shifts in the monsoon winds

carried goods back and forth between India andAfrica.

Mountain and Valley Breezes

Temperature differences between mountainsand valleys create mountain and valley breezes.During the day, air on mountain slopes is heatedmore than air at the same elevation over anadjacent valley. As the day progresses, warm airrises off the slopes and draws the cool air up fromthe valley. This uphill airflow is called a valleybreeze. When the Sun goes down, the mountainslopes cool more quickly than the air in the nearbyvalley. This cool air sinks, which causes amountain breeze to flow downhill.

Katabatic Winds

Katabatic winds also move up and downslopes, but they are stronger mountain and valleybreezes. Katabatic winds form over a high land

area, such as on a high plateau. The plateau isusually surrounded on almost all sides bymountains. In winter, the plateau grows cold,making the air above it extremely cold. This denseair sinks down from the plateau through gaps in themountains. Wind speeds depend on the differencein air pressure over the plateau and over thesurroundings. If a storm, which has low pressure,forms outside the plateau, there is a big differencein wind pressure and the winds will race rapidlydownslope. Katabatic winds form over manycontinental areas. Extremely cold katabatic windsblow over Antarctica and Greenland.

Foehn Wind (Chinook Winds)

Foehn winds or Chinook winds develop whenair is forced up over a mountain range. This takesplace, for example, when the westerly winds bringair from the Pacific Ocean over the Sierra NevadaMountains in California. As the relatively warm,moist air rises over the windward side of themountains, it cools and contracts. If the air is

humid, it may form clouds and drop rain or snow.When the air sinks on the leeward side of themountains, it forms a high pressure zone. Thewindward side of a mountain range is the side thatreceives the wind; the leeward side is the sidewhere air sinks.

The descending air warms and creates verystrong, dry winds. Foehn winds can raisetemperatures more than 20oC (36oF) in an hour andcause humidity to decrease. If there is snow on theleeward side of the mountain, it may disappear byquickly melting and evaporating in the dry winds.If precipitation falls as the air rises over themountains, the air will be very dry as it sinks onthe leeward size of the mountains. This dry, sinkingair causes a rainshadow effect (Figure below).Many deserts are found on the leeward side ofmountains due to rainshadow effect.

Figure 15.22 Air cools and loses moisture as it rises over a

mountain. It descends on the leeward side andwarms by compression. The resulting warm anddry winds are Foehn winds or Chinook winds. Ifthe air loses precipitation over the mountain, theleeward side of the mountain will be dry,experiencing rainshadow effect.

The name of these winds is a bit confusing.Some people refer to all of these winds as Foehnwinds, others as Chinook winds, and still others asorogenic winds. The names Foehn and Chinook aresometimes used for any of these types of winds, butare also used regionally. Foehn winds are found inthe European Alps, and Chinook winds are foundin the Rocky Mountains of North America.Although the description is apt, Chinook does not

mean “snow eater”.

Santa Ana Winds

“Deadly” is a term often used to describe theSanta Ana winds in Southern California (Figurebelow). These winds are created in the late falland winter when the Great Basin east of the SierraNevada cools. The high pressure is created whenthe Great Basin cools forces winds downhill andin a clockwise direction. The air sinks rapidly, sothat its pressure rises. At the same time, the air’stemperature rises and its humidity falls. The windsblow across the Southwestern deserts and thenrace downhill and westward toward the ocean. Airis forced through canyons cutting the San Gabrieland San Bernadino mountains. The winds areespecially fast through Santa Ana Canyon, whichgives them their name.

Figure 15.23 Santa Ana winds blow dust and smoke

westward over the Pacific from SouthernCalifornia.

The Santa Ana winds often arrive at the end ofCalifornia’s long summer drought season. The hotdry winds dry out the landscape even more. If afire starts, it can spread quickly, causing large-scale devastation. In late October 2007, Santa Anawinds fueled many fires that together burned426,000 acres of wild land and more than 1500homes (Figure below). The 2003 Santa Ana windscontributed to the loss of 721,791 acres to wildfires.

Figure 15.24 The Harris Fire burning downward on Mount

Miguel, San Diego County on October 23, 2007.The fire is being pushed along by Santa Anawinds.

Desert Winds

The image of a lonely traveler battling a duststorm in the desert is one most people have seen, atleast in cartoon form. Desert winds pick up dustbecause there is not as much vegetation to holddown the dirt and sand. Hot winds blow acrossmany deserts and most are given local names.Across the Sahara there are winds known as

leveche, sirocco and sharav.High summer temperatures on the desert create

high winds, which are often associated withmonsoon storms. A haboob forms in thedowndrafts on the front of a thunderstorm (Figurebelow). Air spins and lifts dust and sand into acloud of dirt that may include dust devils ortornadoes. Haboobs cause many sandstorms.

Figure 15.25 A haboob in the Phoenix metropolitan area,

Arizona.Dust devils, also called whirlwinds, may also

form on hot, clear desert days. The groundbecomes so hot that the air above it heats and rises.Air flows into the low pressure and begins to spin.

Dust devils are small and short-lived but they maycause damage.

Atmospheric Circulation

You have already learned that more solarenergy hits the equator than the polar areas. Theexcess heat forms a low pressure cell at theequator. Warm air rises to the top of thetroposphere where half of the warmed air movestoward the North Pole and half toward the SouthPole. The air cools as it rises and moves along thetop of the troposphere. When the cooled airreaches a high pressure zone, it sinks. Back on theground, the air then travels toward the lowpressure at the equator. The air rising at the lowpressure zone at the equator and sinking at a highpressure in the direction of the North or South Polecreates a convection cell.

If the Earth was just a ball in space and did notrotate, there would be only one low pressure zoneand it would be at the equator. There would also

be one high pressure at each pole. This wouldcreate one convection cell in the northernhemisphere and one in the southern. But becausethe planet does rotate, the situation is morecomplicated. The planet’s rotation means that theCoriolis Effect must be taken into account.

T h e Coriolis Effect causes freely movingobjects to appear to move right in the NorthernHemisphere and to the left in the SouthernHemisphere. The objects themselves are actuallymoving straight, but the Earth is rotating beneaththem, so they seem to bend or curve. An examplemight make the Coriolis Effect easier to visualize.If an airplane flies 500 miles due north, it will notarrive at the city that was due north of it when itbegan its journey. Over the time it takes for theairplane to fly 500 miles, that city moved, alongwith the Earth it sits on. The airplane willtherefore arrive at a city to the west of the originalcity (in the Northern Hemisphere), unless the pilothas compensated for the change.

A common misconception of the CoriolisEffect is that water going down a drain rotates one

way in the Northern Hemisphere and the other wayin the Southern Hemisphere. This is not truebecause in a small container like a toilet bowl,other factors are more important. These factorsinclude the shape of the bowl and the direction thewater was moving when it first entered the bowl.

But on the scale of the atmosphere and oceans,the Coriolis Effect is very important. Let’s look atatmospheric circulation in the NorthernHemisphere as a result of the Coriolis Effect(Figure below). Air rises at the equator asdescribed above. But as the air moves toward thepole at the top of the troposphere, it deflects to theright. (Remember that it just appears to deflect tothe right because the ground beneath it moves.) Atabout 30oN latitude, the air from the equator meetsrelatively cool air flowing toward the equator fromthe higher latitudes. This air is cool because it hascome from higher latitudes. Both batches of airdescend, creating a high pressure cell. Once on theground, the air returns to the equator. Thisconvection cell is called the Hadley Cell and isfound between 0o and 30oN.

Figure 15.26 The atmospheric circulation cells, showing

direction of winds at Earth's surface.There are two more convection cells in the

Northern Hemisphere. The Ferrell cell is between30oN and 50o to 60oN. This cell shares itssouthern, descending side with the Hadley cell toits south. Its northern rising limb is shared with thePolar cell located between 50oN to 60oN and theNorth Pole, where cold air descends.

There are three mirror image circulation cellsin the Southern Hemisphere. In that hemisphere, theCoriolis effect makes objects appear to deflect to

the left.

Global Wind Belts

Global winds blow in belts encircling theplanet. The global wind belts are enormous and thewinds are relatively steady (Figure below). Wewill be able to figure out how the wind in thesebelts blows using the information you just learnedabout atmospheric circulation.

In between each convection cell, where airmoves vertically, there is little wind. But whereair moves horizontally along the ground betweenthe high and low pressure zones, steady windsform. The air movement of each large circulationcell creates the major wind belts. The wind beltsare named for the directions from which the windscome. The westerly winds, for example, blowfrom west to east. Some names remain from thedays when sailing ships depended on wind fortheir power.

Figure 15.27 The major wind belts and the directions that

they blow.Let’s look at the global wind belts at the

Earth's surface in the Northern Hemisphere. In theHadley cell, air moves north to south, but isdeflected to the right by the Coriolis Effect. Thesewinds therefore blow from the northeast to thesouthwest. They are called the trade windsbecause at the time of sailing ships they were goodfor trade. Winds in the Ferrel cell blow from thesouthwest and are called the westerly winds orwesterlies. The westerlies are the reason a flight

across the United States from San Francisco toNew York City takes less time than the reversetrip. On the outbound flight, the airplane is beingpushed along by the westerlies, but on the reversetrip the airplane must fight against the air currents.In the Polar cell, the winds travel from thenortheast and are called the polar easterlies.These names hold for the winds in the wind beltsof the Southern Hemisphere as well.

The usual pattern of atmospheric circulationcells and the global wind belts determine normalglobal climate, but many other factors come intoplay locally. The high and low pressure areascreated by the six atmospheric circulation cellsgenerally determine the amount of precipitation aregion receives. In low pressure regions, where airis rising, rain is common. In high pressure cells,the sinking air causes evaporation and the region isusually dry. More specific climate affects will bedescribed in the chapter about climate.

The junction between the Ferrell and Polarcells is a low pressure zone. At this location,relatively warm, moist air that has circulated from

the equator meets relatively cold, dry air that hascome from the pole. The result is a place ofextremely variable weather, known as the polarfront. This weather is typical of much of NorthAmerica and Europe.

The polar jet stream is found high up in theatmosphere where the two cells come together. Ajet stream is a fast-flowing river of air at theboundary between the troposphere and thestratosphere. A jet stream can flow faster than 185km/hr (115 mi/hr) and be thousands of kilometerslong and a few hundred kilometers in width, butonly a few kilometers thick. Jet streams formwhere there is a large temperature differencebetween two air masses. This explains why thepolar jet stream is the world’s most powerful.

Jet streams move seasonally as the angle of theSun in the sky moves north and south. The polar jetstream moves south in the winter and north in thesummer between about 30oN and 50o to 75oN. Thelocation of the jet stream determines the weather alocation on the ground will experience. Cities tothe south of the polar jet stream will be under

warmer, moister air than cities to its north.Directly beneath the jet stream, the weather is oftenstormy and there may be thunderstorms andtornadoes.

Lesson Summary

Winds blow from high pressure zones to lowpressure zones. The pressure zones arecreated when air near the ground becomeswarmer or colder than the air nearby.Local winds may be found in a mountainvalley or near a coast.Global wind patterns are long term, steadywinds that prevail around a large portion ofthe planet.The location of the global wind belts has agreat deal of influence on the weather andclimate of an area.

Review Questions

1. Draw a picture of a convection cell in theatmosphere. Label the low and high pressurezones and where the wind is.

2. Under what circumstances will winds be verystrong?

3. Given what you know about global-scaleconvection cells, where would you travel ifyou were interested in experiencing warm,plentiful rain?

4. Describe the atmospheric circulation for twoplaces where you are likely to find deserts,and explain why these regions are relativelywarm and dry.

5. How could the Indian and southeast Asianmonsoons be reduced in magnitude? Whateffect would a reduction in these importantmonsoons have on that part of the world?

6. Why is the name “snow eater” an aptdescription of Chinook winds?

7. Why does the Coriolis Effect cause air (orwater) to appear to move clockwise in theNorthern Hemisphere? When would theCoriolis Effect cause air to appear to move

counterclockwise?8. Sailors once referred to a portion of the ocean

as the doldrums. This is a region where thereis frequently no wind, so ships would becomebecalmed for days or even weeks. Givenwhat you know about atmospheric circulation,where do you think the doldrums might be interms of latitude?

9. Imagine that the jet stream is located furthersouth than usual for the summer. What will theweather be like in regions just north of the jetstream, as compared to a normal summer?

10. Give a general description of how windsform.

Further Reading / SupplementalLinks

High and Low Pressure Systems animations,Bureau of Meteorology, AustralianGovernmenthttp://www.bom.gov.au/lam/Students_Teachers/pressure.shtml

Vocabulary

Coriolis EffectThe tendency of a freely moving object toappear to move right right in the NorthernHemisphere and left in the SouthernHemisphere.

Foehn winds (Chinook winds)Winds that form when low pressure draws airover a mountain range.

haboobDesert sandstorms that form in the downdraftsof a thunderstorm.

high pressure zoneA region where relatively cool, dense air issinking.

jet streamA fast-flowing river of air high in theatmosphere, where air masses with two very

different sets of temperature and humiditycharacteristics move past each other.

katabatic windsWinds that move down a slope.

land breezeA wind that blows from land to sea in winterwhen the ocean is warmer than the land.

low pressure zoneA region where relatively warm, less denseair is rising.

mountain breezeA wind that blows from up on a mountaindown to the valley below in the late afternoonor at night when mountain air is cooler.

polar frontThe meeting zone between cold continentalair and warmer subtropical air at around50oN and 50oS.

Santa Ana windsHot winds that blow east to west intoSouthern California in fall and winter.

sea breezeA wind that blows from sea to land in summerwhen the land is warmer than the ocean.

valley breezeAn uphill airflow.

Points to Consider

How do local winds affect the weather in anarea?How do the global wind belts affect theclimate in an area?What are the main principles that control howthe atmosphere circulates?

Chapter 16: Weather

Weather and AtmosphericWater

Lesson Objectives

Discuss the difference between weather andclimate.Describe the relationship between airtemperature and humidity, including theconcept of dew point.List the basics of the different cloud types andwhat they indicate about current and futureweather.Explain how the different types ofprecipitation form.

Introduction

If someone across country asks you what theweather is like today, you need to consider severalfactors. Air temperature, humidity, wind speed, theamount and types of clouds and precipitation areall part of a thorough weather report. In thischapter, you will learn about these many of thesefeatures in more detail.

What is Weather?

Weather is what is going on in the atmosphereat a particular place at a particular time. Weathermay be cold or hot, or wet or dry, and it changesrapidly. A warm sunny day may rapidly turn into acold and stormy one, making you wish you hadbrought your jacket. There are many factors thatinfluence the weather; a few examples include theair temperature over a region, whether there is asecond air mass nearby, and how close high andlow pressure cells are. A location’s weatherdepends on air temperature, air pressure, humidity,cloud cover, precipitation, and wind speed and

direction, which are all directly related to theamount of energy that is in the system and wherethat energy is. The ultimate source of this energy isthe sun.

Climate is the average of a region’s weatherover time. The climate for a particular place issteady, and changes only very slowly. Portland,Oregon has a mild, moist climate and Fairbanks,Alaska has a frigid, dry one. Portland or Fairbanksmay experience a warm sunny day in February, butthat doesn’t change their climate. Climate isdetermined by many factors, which are related tothe amount of energy that is found in that locationover time. Factors that determine the amount ofenergy include the angle of the sun, the likelihoodof cloud cover, the air pressure, and many others.

Humidity

Humidity is the amount of water vapor in theair in a particular spot. We usually use the term tomean relative humidity, the percentage of water

vapor a certain volume of air is holding relative tothe maximum amount it can contain. If the humiditytoday is 80%, that does not mean that 80% of themolecules in the air are water vapor. It means thatthe air contains 80% of the total amount of water itcan hold at that temperature. If the humidityincreases to more than 100%, the excess waterwill condense from the air and form precipitation.

Humidity affects weather a great deal and isimportant for weather forecasting. When humidityis high, precipitation is more likely. Thecombination of high humidity and hightemperatures can threaten people’s health. Peopleare more uncomfortable when both temperatureand humidity are high. As people and some otheranimals sweat to cool themselves off; they loseheat as the sweat evaporates. But if the air isalready saturated with water vapor, the sweat willnot evaporate and the person will not cool. Theywill simply be hot and sweaty, and uncomfortable.

The National Weather Service has developed aheat index (HI). On an HI chart (Table below),people can see what the temperature feels like,

when the air temperature and humidity are known.For example, if the temperature is 85oF, buthumidity is only 40%, the temperature feels like apleasant 84oF. But if the temperature is 85oF, butthe humidity is 90%, the air temperature feels likea very hot and sticky 101oF. This information isuseful for people who are interested in outdooractivities. High humidity cause health problems,such as sunstroke or heatstroke, to occur morequickly.

Heat Index: Temperature (oF) vs.Humidity (%)

90% 80% 70% 60% 50% 40%

80oF 85 84 82 81 80 79

85oF 101 96 92 90 86 84

90oF 121 113 105 99 94 90

95oF 133 122 113 105 98

100oF 142 129 118 109

105oF 148 133 121

110oF 135Since warm air can hold more water vapor

than cool air, raising or lowering temperature canchange air's relative humidity (Figure below). Thetemperature at which air becomes saturated withwater is called the air's dew point. This termmakes sense, because water will condense fromthe air as dew, if the air cools down overnight andreaches 100% humidity.

Figure 16.1 This diagram shows the amount of water air

can hold at different temperatures. The

temperatures are given in degrees Celsius.

Clouds

Sometimes there are lots of clouds in the skyand sometimes you can't see a cloud anywhere.Either way, clouds have a big influence onweather. Clouds affect weather in three ways: (1)preventing solar radiation from reaching theground, (2) absorbing warmth that is re-emittedfrom the ground, and (3) as the source ofprecipitation. When there are no clouds, there isless insulation. As a result, cloudless days can beextremely hot, and cloudless nights can be verycold. For this reason, cloudy days tend to have alower range of temperatures than clear days.

Clouds form when air reaches its dew point,the temperature when the air is saturated withwater vapor. This can happen in two ways. First,the air temperature can stay the same while thehumidity increases. This is common in locationsthat are warm and humid. Second, the humidity can

remain the same, but the temperature decreases.When this happens, the air will eventually coolenough so that it reaches 100% humidity, andwater droplets form. Air cools when it comes intocontact with a cold surface or when it rises. Thereare three ways that rising air can create clouds: (1)It can be warmed at or near the ground level, (2) Itcan be pushed up over a mountain or mountainrange, or (3) It can be thrust over a mass of cold,dense air.

Water vapor in the atmosphere is not visible,unless it condenses to become a cloud. Watervapor condenses around a nucleus, such as dust,smoke, or a salt crystal. This forms a tiny liquiddroplet. Billions of these water droplets togethermake a cloud. If the atmosphere is very cold, thedroplets freeze into ice. Most clouds appear whitebecause sunlight reflects off the water droplets. Ifthe clouds are thick, the droplets scatter or absorbthe light and less solar radiation can travel throughthem. This is why storm clouds are dark black orgray.

Clouds have been classified in several ways.

The most common classification used todaydivides clouds into four separate cloud groups,which are determined by their altitude (Figurebelow). High clouds, which have the prefix 'cirro-,' are found above 6,000 m (20,000 feet) inaltitude. Middle clouds, which have the prefix'alto-,' are between 2,000 to 7,000 m (6,500 to23,000 feet). Low clouds, which have the word'stratus' in their names, occur beneath 2,000 m(6,500 feet). Each of the clouds that occur in thesegroups is layered, and they grow horizontally.

Another group of clouds, which have the prefix'cumulo-,' describes clouds that grow verticallyinstead of horizontally. These impressive cloudshave their bases at low altitude and their tops athigh or middle altitude.

Figure 16.2 The four cloud types and where they are found

in the atmosphere. The symbols are shown in :Highclouds:

Cirrus(Ci)

Cirrostratus(Cs)

Cirrocumulus(Cc)

Middleclouds:

Altostratus(As)

Altocumulus(Ac)

Lowclouds:

Stratus(St)

Stratocumulus(Sc)

Nimbostratus(Ns)

Verticalclouds:

Cumulus(Cu)

Cumulonimbus(Cb)

(Source: CK-12 Foundation, License: CC-BY-SA)

High clouds form where the air is extremelycold and can hold little water vapor. The icecrystals that form create thin, wispy cirrus clouds(Figure below). Cirrus clouds may indicate anoncoming storm.

Figure 16.3 Cirrus clouds are thin wisps of ice crystals

found at high altitudes.Cirrocumulus clouds are small, white puffs

that ripple across the sky, often in rows.Cirrostratus clouds are thin, white sheets ofclouds, made up of ice crystals like cirrus clouds(Figure below). Cirrostratus clouds are sometimesso thin that they cannot be seen, unless illuminated

by the sun or moon.

Figure 16.4 Cirrostratus clouds are so thin they are

sometimes invisible unless backlit by the Sun orMoon.

Middle clouds may be made of water droplets,ice crystals or both, depending on the airtemperatures. Altocumulus clouds appear as whiteto gray, puffy stripes rolling across the sky (Figurebelow). These clouds often occur beforethunderstorms. Thick and broad altostratus cloudsare gray or blue-gray. They often cover the entiresky and usually mean a large storm, bearing a lot ofprecipitation is coming.

Figure 16.5 Altocumulus clouds are white puffs found in

the middle altitudes.Low clouds usually hold droplets of liquid

water, although they may also contain ice whentemperatures are very cold. Stratus clouds aregray sheets that cover the entire sky (Figurebelow). These clouds may produce a steadydrizzle or mist, but do not carry hard rain.Nimbostratus clouds are thick and dark. Theybring steady rain or snow. Stratocumulus cloudsare rows of large, low puffs that may be white orgray. These clouds rarely bring precipitation. but

Figure 16.6 Stratus clouds with the Alps in the distance.Clouds grow vertically when strong air

currents are rising upward. Cumulus cloudsresemble white or light gray cotton and havetowering tops (Figure below). On fair days,cumulus clouds may grow upward but produce noprecipitation. On hot summer afternoons, though,cumulus clouds may mushroom into a form thatlooks like a head of cauliflower. These clouds mayproduce light showers.

Figure 16.7 Anvil-shaped cumulus clouds floating over

Australia.If the vertical air currents are strong, a cumulus

cloud will grow upward until it develops into acumulonimbus cloud (Figure below). Tall, darkand ominous cumulonimbus clouds are associatedwith lightning and intense thunderstorms.

Figure 16.8 Cumulonimbus cloud lit up by lightning.

Fog

Fogs are clouds located at or near the ground.When humid air near the ground cools below itsdew point, fog is formed. Fogs develop differentlyfrom the way clouds form. There are several typesof fog, each of which forms in a different way.

Radiation fogs form at night when skies areclear and the relative humidity is high. As theground cools, the bottom layer of air cools also.Eventually the air temperature may be loweredbelow its dew point. If there is a light breeze, thefog will be carried upward. Radiation fog cangrow to 30 meters (100 feet) thick. One to threehours after sunrise, radiation fog burns off as theground warms. The Central Valley of Californiafrequently experiences radiation fog, which iscalled tule fog in this area. Tule fog can be so thickthat drivers cannot see the car in front of them andtheir headlights just reflect back off the sheet ofwater droplets.

San Francisco, California is famous for its

summertime advection fog (Figure below). Warm,moist air from over the Pacific Ocean blows overthe cold California current just offshore. Thiscools the eastward moving air below its dew pointand thereby creates fog. Advection fog is broughtonshore by sea breezes. If the fog is accompaniedby light wind, a thicker layer of air cools and thefog can grow to be up to 600 m (2,000 feet) thick.

Figure 16.9 Advection fog fills the gap where the Golden

Gate Bridge spans the San Francisco Bay inlet.Steam fog appears in autumn or early winter

and can make a pond or lake appear to besteaming. The “steam” forms when cool air movesover a lake that still holds some of its summer heat.Water evaporates from the lake surface and

condenses as it cools in the overlying air. Steamfog is rarely very thick.

When warm humid air travels up a hillside andcools below its dew point it creates an upslopefog (Figure below).

Figure 16.10 Upslope fog around the peaks of Mt. Lushan in

China.

Precipitation

As you know from your daily life, precipitationis an extremely important part of weather.Precipitation most commonly falls as rain or snow,but can also be sleet, hail, dew or frost. Sleet is amixture of rain and snow, and often forms whensnow partially melts as it falls. Dew forms whenmoist air comes into contact with a cold surface,like the ground or a car windshield, and coolsbelow its dew point. Frost forms under similarconditions, but when the air cools to belowfreezing (Figure below).

Figure 16.11 Hoar frost.

The other types of precipitation come fromclouds. Rain or snow droplets fall when theybecome heavy enough to escape from the rising aircurrents that hold them up in the cloud. The mostcommon way for rain or snow to droplets to grow,occurs in cold clouds, where the temperature is -10oC(14oF) or less (Figure below). Here thewater vapor freezes directly into ice crystals,which continue to grow as more water vaporfreezes onto them. When the ice crystals becomeheavy enough, they fall. Even as they fall, the icecrystals collect more moisture. If temperatures arecold, the ice will hit the ground as a snowflake. Iftemperatures near the ground are greater than 4o

C(39oF), the ice crystal may melt and become araindrop. One million cloud droplets will combineto make only one rain drop!

Figure 16.12 Snow storm in Cleveland, Ohio.Water may also precipitate from warm clouds.

Here too, water droplets get trapped in rising andfalling air currents. As the droplet travels aroundthe convection cell, it collides with other smalldroplets. At some point, the droplet is large enoughto escape the convecting air currents and it falls tothe ground as rain. If the air currents are verystrong, the droplets must be very large before theyfall.

If a raindrop falls through warm air but hits alayer of freezing air near the ground, it becomesfrozen into a small clear ice pellet known as sleet.

Sleet usually is mixed with liquid water drops thatdid not freeze as they descended from the cloud. Ifthe layer of frigid air near the ground is not thickenough for the raindrop to freeze before it reachesthe ground, the drop may freeze on the ground,forming glaze. The weight of glaze covering a treebranch can make the branch fall.

Hail forms in cumulonimbus clouds with strongupdrafts. An ice particle falling through a cloud iscaptured by an updraft and continues to grow as ittravels around the convection cell. When it finallybecomes too heavy, it drops to the ground.Although hail is usually less than 1 cm (about one-half inch), it’s not uncommon to find hail that is 5to 10 cm (2 to 4 inch) in diameter (Figure below).The largest hailstone ever measured, 14 cm (5.5inches) in diameter and weighing 766 grams (27ounces), was collected in Coffeyville, Kansas in1970.

Figure 16.13 A large hail stone, about 6 cm (2.5 inches) in

diameter.

Lesson Summary

Air temperature causes differences inpressure so that convection cells form.Air rising in a convection cell may coolenough to reach its dew point and form cloudsor precipitation if the humidity is high enough.Clouds or fog may form if warmer air meets acolder ground surface. Air temperature andhumidity also determine what sorts of cloudsand precipitation form.

These factors play a role in creating apleasant or uncomfortable day, such as whenit might be warm and dry or hot and humid.

Review Questions

1. What factors need to be included in athorough weather report?

2. If Phoenix, Arizona experiences a cool, wetday in June (when the weather is usually hotand dry), does that mean the region’s climateis changing?

3. Look back at the table that shows heat index.Which day would most people find morepleasant: An 85oF day with 90% humidity ora 90oF day with 40% humidity?

4. What happens when a batch of air reaches itsdew point? At what temperature does thisoccur?

5. What effect do clouds have on weather?6. You are standing in a location which is clear

in the morning, but in the afternoon there are

thunderstorms. There is no wind during theday, so the thunderstorms build directly aboveyou. Describe how this happens.

7. In what three ways can air rise to createclouds?

8. What are the four different cloud groups andhow are they classified?

9. How does sleet form? How does glaze form?10. What circumstances must be present for

enormous balls of hail to grow and then fall tothe ground?

Vocabulary

altocumulusGray puffy stripes of globular cloudsarranged in lines across the sky.

altostratusThicker clouds than cirrostratus; like a grayveil, may completely hide Sun or Moon.

cirrocumulus

High clouds that are small, white and puffy,arranged in groups or lines.

cirrostratusThin, whitish, veil-like clouds that produce ahalo around the Sun or Moon, but do not blurtheir outline.

cirrusHigh, wispy clouds made of ice crystals.

cloudTiny water or ice particles that are groupedtogether in the atmosphere.

cumulonimbusTall, dark clouds that produce thunderstorms.

dew pointThe temperature at which air is saturated withwater vapor, or where the air has reached100% humidity.

glaze

A layer of smooth, transparent ice that formswhen freezing rain or drizzle hit a coldsurface.

hailPellets of ice or ice and snow that form onlyin cumulonimbus clouds.

heat indexA measurement that combine the effects oftemperature and humidity; the heat index moreaccurately describes what weather willactually feel like.

humidityThe amount of water vapor in the air,sometimes used synonymously with relativehumidity.

nimbostratusThick, dark, continuous, low clouds thatbrings continuous rain or snow.

radiation fog

Fog caused by the radiation of heat on a cold,windless night.

relative humidityThe amount of water vapor in the air relativeto the maximum amount of water vapor thatthe air could contain at that temperature.

sleetPartly frozen rain or partly melted snow andice.

stratocumulusSoft, globular, low clouds in groups or linesthat rarely bring precipitation.

stratusLow clouds that are continuous and mayproduce drizzle but no hard rain.

upslope fogFog that forms from winds that blow up aslope and cool.

Points to Consider

When thinking about the weather, what factorsdo you consider important in the air thatsurrounds you?How do air temperature, humidity, andpressure differences create different sorts ofweather?Think about the types of weather described inthis lesson. Imagine types of weather that youhave not experienced, look at photos, and askfriends and relatives who’ve lived in otherplaces what their weather is like.

Changing Weather

Lesson Objectives

Describe the characteristics air masses haveand how they get those characteristics.Discuss what happens when air masses meet.List the differences between stationary, cold,warm, and occluded fronts.

Introduction

The weather in a location often depends onwhat type of air mass is over it. Another key factorrevolves around whether or not the spot is beneatha front, the meeting place of two air masses. Thecharacteristics of the air masses and theirinteractions can determine whether the weather isconstant over an area, or whether there are rapidchanges in air temperature, wind, precipitation andeven thunderstorms.

Air Masses

An air mass is a batch of air that has nearly thesame temperature and humidity (Figure below).An air mass is created above an area of land orwater known as its source region. Air massescome to have a distinct temperature and humiditywhen they remain over a region for several days orlonger. The heat and moisture leave the ground andmove into the air above it, until the overlying airtakes on the temperature and humiditycharacteristics of that particular region.

Air masses are created primarily in highpressure zones. They most commonly form in polarand tropical regions, which have very distinctivetemperature and humidity. The temperate zones areordinarily too unstable for air masses to form.Instead, air masses move across them, making themiddle latitudes the site of very interestingweather.

Air masses can be 1,600 km (1,000 miles) ormore across and several kilometers thick.

Temperature and humidity may change a bithorizontally across the air mass, but not too much.An air mass may have more changes with altitude.

Meteorologists use symbols to describe thecharacteristics of an air mass. The first symboltells whether the air mass had its origin over acontinent (c) or over an ocean (m, for maritime).As you might expect, air masses that form overoceans contain more water vapor than those thatform over land. The second symbol tells thegeneral latitude where the air mass gained itstemperature and humidity traits. The categories arearctic (A), polar (P,)tropical (T), and equatorial(E). Of course, air masses that form over polarareas are colder than those that form over tropicalregions.

Globally, the major air masses are continentalarctic or continental antarctic(cA or cAA);continental polar (cP); maritime polar (mP);continental tropical (cT); maritime tropical (mT);and maritime equatorial (mE). Maritime arctic andcontinental equatorial air masses rarely form.

Figure 16.14 The source regions of air masses found around

the world.A third symbol takes into account the

properties of an air mass relative to the ground itmoves over. If the air mass is colder than theground, it is given the designation k, for cold. If itis warmer than the ground, it is given thedesignation w. For example, a cPk is an air masswith a continental polar source region that iscolder than the region it is now moving over.

Air Mass Movement

Air masses are pushed along by high-levelwinds, although they move slower than the winds.

An air mass gets its characteristics from the groundor water it is above, and it also shares thosecharacteristics with the regions that it travels over.Therefore, the temperature and humidity of aparticular location depends partly on thecharacteristics of the air mass that sits over it.

If the air mass is very different from the groundbeneath it, storms may form. For example, when acolder air mass moves over warmer ground, thebottom layer of air is heated. That air rises,forming clouds, rain, and sometimesthunderstorms. When a warmer air mass travelsover colder ground, the bottom layer of air iscooled. This forms a temperature inversion, sincethe cold air near the ground is trapped. Inversionsmay form stratus clouds, advection fogs, or theymay trap a layer of pollution over a city.

In general, cold air masses tend to flow towardthe equator and warm air masses tend to flowtoward the poles. This brings heat to cold areasand cools down areas that are warm. It is one ofthe many processes that act towards balancing outthe planet’s temperatures.

Fronts

Two air masses meet at a front. Because thetwo air masses have different temperature andhumidity, they have different densities. Air masseswith different densities do not easily mix.Ordinarily, when fronts meet, one air mass is liftedabove the other. Rising air creates a low pressurezone. If the lifted air is moist enough, there will becondensation and precipitation. Fronts usually alsohave winds in them. If the temperature differencebetween the two air masses is high, then the windswill be strong. Fronts are the main cause of stormyweather.

The map symbols for the different types offronts are shown in (Figure below): (1) cold front,(2) warm front, (3) stationary front, (4) occludedfront, (5) surface trough, (6) squall line, (7) dryline, (8) tropical wave.

Figure 16.15 The map symbols for different types of fronts.The direction that fronts move is guided by

pressure gradients and the Coriolis Effect. In theNorthern Hemisphere, cold fronts and occludedfronts tend to move from northwest to southeast.Warm fronts move southwest to northeast. Thedirection the different types of fronts move in theSouthern Hemisphere is the mirror image of howthey move in the Northern Hemisphere. Fronts canbe slowed or stopped by a barrier such as amountain range.

The rest of this section will be devoted to fourtypes of fronts. Three of these fronts move and oneis stationary. With cold fronts and warm fronts, theair mass at the leading edge of the front gives the

front its name. In other words, a cold front is rightat the leading edge of moving cold air and a warmfront marks the leading edge of moving warm air.

Stationary Front

Most fronts move across the landscape, but atstationary fronts (3) the air masses do not move.A front may become stationary if an air mass isstopped by a barrier. For example, cold air massesmay be stopped by mountains, because the cold airmass is too dense to rise over them.

A region under a stationary front mayexperience days of rain, drizzle and fog. Thisweather may be present over a large area. Windsusually blow parallel to the front, but in oppositedirections. This results in shear stress. Shearstresses result when objects are pushed past eachother in opposite directions.

After several days, the front will break apart.T h e temperature gradient or temperaturedifference across the front may decrease, so the air

masses start to mix. Shear stresses may force thefront to break apart. Conditions may change so thatthe stationary front is overtaken by a cold front or awarm front. If the temperature gradient between theair masses increases, wind and rainy weather willresult.

Cold Fronts

When a cold air mass takes the spot of a warmair mass, there is a cold front (1) (Figure below).Since cold air is denser than the warm air, the coldair mass slides beneath the warm air mass andpushes it up. As the warm air rises, there are oftenstorms.

Figure 16.16

A cold front with cold air advancing todisplace warm air. The warm air is pushed upover the cold air.

When cold air moves underneath warm air, theground temperature drops. The humidity may alsodecrease since the colder air may also be drier.Winds at a cold front can be strong because of thetemperature difference between the two airmasses. When a cold front is on its way, there maybe a sharp change in dew point, changes in winddirection, changes in air pressure, and certaincharacteristic cloud and precipitation patterns.

Cold fronts often move rapidly across thelandscape. Fast-moving cold fronts create a line ofintense storms over a fairly short distance. Asquall line(6) is a line of severe thunderstorms thatforms along a cold front (Figure below). If thefront moves slowly, the storms may form over alarger area.

Figure 16.17 A shelf line that commonly precedes a squall.Imagine that you are standing in one spot as a

cold front approaches. Along the cold front, thedenser, cold air pushes up the warm air, causingthe air pressure to decrease. If the humidity is highenough, some types of cumulus clouds will grow.High in the atmosphere, winds blow ice crystalsfrom the tops of these clouds to create cirrostratusand cirrus clouds. At the front, there will be a lineof rain or snow showers or thunderstorms withblustery winds. Behind the front is the cold airmass. This mass is drier and so precipitation stops.The weather may be cold and clear or only partlycloudy. Winds may continue to blow into the lowpressure zone at the front.

The weather at a cold front varies with the

season. Thunderstorms or tornadoes may form inspring and summer, when the air is unstable. In thespring, the temperature gradient can be very high,causing strong winds to blow at the front. In thesummer, thunderstorms may be severe and mayalso include hailstorms. In the autumn, strong rainsfall over a large area. If the front moves slowly,enough rain may fall to cause flooding. Cold frontsin winter may bring frigid temperatures and heavysnows. The cold air mass is likely to have formedin the frigid arctic.

When the temperature gradient across a coldfront is low, a cold front has little effect on theweather. This may occur at some locations in thesummer. Along the western United States, thePacific Ocean warms and moistens cold air massesso that the temperature gradient across a cold frontis small.

Warm Fronts

A warm front (2) is found where warm air

mass slides over a cold air mass (Figure below).Since the warmer, less dense air is moving overthe colder, denser air, the atmosphere is relativelystable. Warm fronts travel much more slowly thancold fronts because the leading cold air mass isdense and sluggish.

Figure 16.18 A warm front. Warm air moves forward to take

over the position of colder air.Imagine that you are on the ground in the

wintertime under a cold winter air mass with awarm front approaching. The transition betweenthe cold air and the warm air takes place over along distance. This means that the first signs ofchanging weather appear long before the front isactually over you. In fact, weather changes mayappear hundreds of kilometers ahead of the front.

Initially, the air is cold: the cold air mass is aboveyou and the warm air mass is above it. High cirrusclouds mark the transition from one air mass to theother.

Over time, cirrus clouds become thicker andcirrostratus clouds form. As the front approaches,altocumulus and altostratus clouds appear and thesky turns gray. Since it is winter, snowflakes fall.Soon the clouds thicken and nimbostratus cloudsform. Snowfall increases. Winds grow stronger asthe low pressure approaches. As the front getscloser, the cold air mass is just above you but thewarm air mass is not too far above that. Theweather worsens. As the warm air massapproaches, temperatures rise and snow turns tosleet and freezing rain. Warm and cold air mix atthe front, leading to the formation of stratus cloudsand fog (Figure below).

Figure 16.19 Cumulus clouds build at a warm front.As the front passes over you, the temperature

and dew point rise and the rain likely ends. Windschange direction. The transition is not nearly asdramatic as when a cold front passes over, sincethere is more mixing of the two air massesoccurring in a warm front.

The Pacific Ocean also plays a role inmodifying the warm fronts that reach the west coastof the United States. These storms are so broad thatit is very difficult to spot exactly where the warmfront is!

Occluded Front

A n occluded front or occlusion (4) usuallyforms around a low pressure system (Figurebelow). The occlusion starts when a cold frontcatches up to a warm front. The air masses, inorder from front to back, are cold, warm, and thencold again. The boundary line, where the twofronts meet, curves towards the pole because of theCoriolis effect. If the air mass that arrives third iscolder than either of the first two air masses, thatair mass will slip beneath the other two airmasses. This is called a cold occlusion. If the airmass that arrives third is warm, that air mass willride over the other air mass. This is called a warmocclusion.

Figure 16.20 An occluded front with a warm front being

advanced on by a cold front. The order of airmasses from front to rear is cold, warm, and thencold.

Occluded fronts can cause drying or storms.Precipitation and shifting winds are typical. Theweather is especially fierce right at the occlusion.The Pacific coast has frequent occluded fronts. Allof these fronts are part of the mid-latitude cyclone.These weather systems will be discussed in thenext lesson.

Lesson Summary

An air mass takes on the temperature andhumidity characteristics of the location whereit originates. Air masses meet at a front.Stationary fronts become trapped in place andthe weather they bring may last for many days.At a cold front, a cold air mass takes theplace of a warm air mass and forces the warmair upwards.The opposite occurs at a warm front, exceptthat the warm air slips above the cold airmass.In an occluded front, a warm front isovertaken by a cold front, which createsvariable weather.

Review Questions

1. What type of air mass will be created if abatch of air sits over the equatorial PacificOcean for a few days? What is the symbol for

this type of air mass?2. What conditions must be present for air to sit

over a location long enough to acquire thecharacteristics of the land or water beneathit?

3. Discuss how latitude affects the creation ofair masses in the tropical, temperate andpolar zones.

4. Phoenix, Arizona is a city in the Southwesterndesert. Summers are extremely hot. Winterdays are often fairly warm but winter nightscan be quite chilly. In December, inversionsare quite common. How does an inversionform under these conditions and what are theconsequences of an inversion to thissprawling, car-dependent city?

5. Why are the directions fronts move in theSouthern Hemisphere a mirror image of thedirections they move in the NorthernHemisphere?

6. How is a stationary front different from a coldor warm front?

7. What sorts of weather will you experience as

a cold front passes over you?8. What sorts of weather will you experience as

a warm front passes over you?9. How does an occlusion form?

10. What situation creates a cold occlusion andwhat creates a warm occlusion?

Further Reading / SupplementalLinks

Cold Front animation, Goddard Space FlightCenterhttp://svs.gsfc.nasa.gov/vis/a000000/a002200/a002203/index.html

Vocabulary

air massA large mass of air with the same temperatureand humidity characteristics, although thesecharacteristics may change with altitude.

cold front

A front in which a cold air mass is replacinga warm air mass; the cold air mass pushes thewarm air mass upward.

frontThe meeting place of two air masses withdifferent characteristics.

occluded frontA front in which a cold front overtakes awarm front.

squall lineA line of thunderstorms that forms at the edgeof a cold front.

stationary frontA stalled front in which the air does notmove.

temperature gradientA change in temperature over distance.

warm front

A front in which a warm air mass is replacinga cold air mass.

Points to Consider

How do the various types of fronts lead todifferent types of weather?Why are some regions prone to certain typesof weather fronts and other regions prone toother types of weather fronts?Why does the weather sometimes change sorapidly and sometimes remain very similarfor many days?

Storms

Lesson Objectives

Describe how atmospheric circulationpatterns cause storms to form and travel.Understand the weather patterns that lead totornadoes, and identify the different types ofcyclones.Know what causes a hurricane to form, whatcauses it to disappear, and what sorts ofdamage it can do.Know the damage that heat waves anddroughts can cause.

Introduction

Weather happens every day, but only somedays have storms. Storms vary immenselydepending on whether they’re warm or cold,coming off the ocean or off a continent, occurring

in summer or winter, and many other factors. Theeffects of storms also vary depending on whetherthey strike a populated area or a natural landscape.Hurricane Katrina is a good example, since theflooding after the storm severely damaged NewOrleans, while a similar storm in an unpopulatedarea would have done little damage.

Thunderstorms

Thunderstorms are extremely common.Across the globe, there are about 14 million peryear; that’s 40,000 per day! Most come and goquickly, dropping a lot of rain on a small area, butsome are severe and highly damaging.Thunderstorms are most common when groundtemperatures are high. This tends to be in the lateafternoon or early evening in spring and summer.As temperatures increase, warm, moist air rises.These updrafts form first cumulus and thencumulonimbus clouds (Figure below).

Figure 16.21 On a warm spring or summer day, air warmed

near the ground rises and forms cumulus clouds. Ifwarm air continues to rise, cumulonimbus cloudsform.

At the top of the stratosphere, upper level

winds blow the cloud top sideways to make theanvil shape that characterizes a cloud as athunderhead (Figure below).

Figure 16.22 Winds at the top of the stratosphere blow the

top of a cumulonimbus cloud sideways to createthe classic anvil-shape of a thunderhead.

Clouds form when water vapor condenses.Remember that when water changes state from agas to a liquid, it releases latent heat. Latent heatmakes the air in the cloud warmer than the airoutside the cloud and supplies the cloud with a lotof energy. Water droplets and ice travel throughthe cloud in updrafts. When these droplets getheavy enough, they fall. This starts a downdraft,and soon there is a convection cell within the

cloud. The cloud grows into a cumulonimbus giant.Droplets traveling through the convection cellgrow. Eventually, they become large enough to fallto the ground. At this time, the thunderstorm ismature, it produces gusty winds, lightning, heavyprecipitation and hail (Figure below).

Figure 16.23 A mature thunderstorm showing updrafts and

downdrafts that reach the ground. Thisthunderstorm will no longer grow, since the baseof the cloud is being cooled too much forconvection to continue.

Once downdrafts have begun, the thunderstormcan no longer continue growing. The downdraftscool the air at the base of the cloud, so the air is nolonger warm enough to rise. As a result,convection shuts down. Without convection, watervapor does not condense, no latent heat isreleased, and the thunderhead runs out of energy. Athunderstorm usually ends only 15 to 30 minutesafter it began, but other thunderstorms may start inthe same area.

Severe thunderstorms grow larger because thedowndrafts are so intense, they flow to the ground.This sends warm air from the ground upward intothe storm. The warm air feeds the convection cellsin the cloud and gives them more energy. Rain andhail grow huge before gravity pulls them to Earth.Hail that is 1.9 cm (0.75 inch) in diameter is not

uncommon. Severe thunderstorms can last forhours and can cause a lot of damage due to highwinds, flooding, intense hail, and tornadoes.

Thunderstorms can form individually or insquall lines, which can run along a cold front forhundreds of kilometers. Individual storms withinthe line may reach an altitude of more than 15kilometers (50,000 feet). In the United States,squall lines form in spring and early summerwhere the maritime tropical (mT) air mass fromthe Gulf of Mexico meets the continental polar (cP)air mass from Canada. In the United States, severethunderstorms are most common in the Midwest.

Lightning is a huge release of electricity thatforms in cumulonimbus clouds (Figure below). Aswater droplets in the cloud freeze, positive ionsline the colder outside of the drop. Negative ionscollect in the warmer inside. If the outside of thedrop freezes, the water inside often shatters theoutside ice shell. The small, positively-charged icefragment rises in the updraft. The heavier,negatively-charged water droplet falls in thedowndraft. Soon the base of the cloud is mostly

negatively-charged and the top is mostlypositively-charged. The negative ions at the baseof the cloud drive away negative ions on theground beneath it, so the ground builds up apositive charge. Eventually the opposite chargeswill attempt to equalize, creating ground to cloudlightning. Only about 20% of lightning bolts strikethe ground. Lightning can also discharge intoanother part of the same cloud or another cloud.

Figure 16.24 Lightning over Pentagon City in Arlington,

Virginia.Lightning heats the air so that it expands

explosively. The loud clap is thunder. Lightwaves travel so rapidly that lightning is seeninstantly. Sound waves travel much more slowly,about 330 m (1,000 feet) per second. If you werewatching a lightning storm, the difference in theamount of time between seeing a lighting bolt andhearing its thunder clap in seconds times 1,000gives the approximate distance in feet of thelightning strike. For example, if 5 seconds elapsebetween the lightning and the thunder, the lightninghit about 5,000 feet or about 1 mile (1,650 m)away.

Thunderstorms kill approximately 200 peoplein the United States and injure about 550Americans per year, mostly from lightning strikes.Have you heard the common misconception thatlightning doesn't strike the same place twice? Infact, lightning strikes the New York City's EmpireState Building about 100 times per year (Figurebelow).

Figure 16.25 Lightning strikes some places many times a

year. Here, lightning is striking the Eiffel Tower inParis.

Tornadoes

Tornadoes, also called twisters, are the mostfearsome products of severe thunderstorms (Figurebelow). Tornadoes are created as air in athunderstorm rises, and the surrounding air races into fill the gap, forming a funnel. A tornado is afunnel shaped, whirling column of air extendingdownward from a cumulonimbus cloud.

Figure 16.26

The formation of this tornado outside Dimmit,Texas in 1995 was well studied.

A tornado can last anywhere from a fewseconds to several hours. The most importantmeasure of the strength of a tornado is its windspeed. The average is about 177 kph (110 mph),but some can be much higher. The average tornadois 150 to 600 m across (500 to 2,000 feet) acrossand 300 m (1,000 feet) from cloud to ground. Atornado travels over the ground at about 45 km perhour (28 miles per hour) and travels about 25 km(16 miles) before losing energy and disappearing.

Tornadoes strike a small area compared toother violent storms, but they can destroyeverything in their path. Tornadoes uproot trees,rip boards from buildings, and fling cars up intothe sky. The most violent two percent of tornadoeslast more than three hours. These monster stormshave winds up to 480 kph (300 mph). They cutpaths more than 150 km (95 miles) long and 1 km(one-half mile) wide (Figure below).

Figure 16.27 This tornado struck Seymour, Texas in 1979.Most injuries and deaths from tornadoes are

caused by flying debris. In the United States, anaverage of 90 people are killed by tornadoes eachyear, according to data from the National WeatherService. The most violent two percent of tornadoesaccount for 70% of the deaths by tornadoes(Figure below).

Figure 16.28 Tornado damage at Stoughton, Wisconsin in

2005.Tornadoes form at the front of severe

thunderstorms, so these two dangerous weatherevents commonly occur together. In the UnitedStates, tornadoes form along the front where themaritime tropical (mT) and continental polar (cP)air masses meet. In a typical year, the location oftornadoes moves along with the front, from thecentral Gulf States in February, to the southeasternAtlantic states in March and April, and on to thenorthern Plains and Great Lakes in May and June.Although there is an average of 770 tornadoes

annually, the number of tornadoes each year variesgreatly (Figure below).

Figure 16.29 The frequency of F3, F4 and F5 tornadoes in

the United States. The red region that starts inTexas and covers Oklahoma, Nebraska and SouthDakota is called Tornado Alley because it iswhere most of the violent tornadoes occur.

Meteorologists can only predict tornado dangerover a very wide region, a few hours in advance ofthe possible storm. Once a tornado is sighted onradar, its path is predicted and a warning is issuedto people in that area. The exact path is unknownbecause tornado movement is not very predictable.The intensity of tornadoes is measured on the

Fujita Scale (see Table below), which assigns avalue based on wind speed and damage.

The Fujita Scale (F Scale) of Tornado IntensityFScale (km/hr) (mph) Damage

F0 64-116 40-72 Light - tree branches falland chimneys may collapse

F1 117-180

73-112

Moderate - mobilehomes, autos pushed aside

F2 181-253

113-157

Considerable - roofstorn off houses, large treesuprooted

F3 254-332

158-206

Severe - houses tornapart, trees uprooted, carslifted

F4 333-419

207-260

Devastating - housesleveled, cars thrown

F5 420-512

261-318

Incredible - structuresfly, cars become missiles

F6 >512 >318 Maximum tornado windspeed

Cyclones

A cyclone is a system of winds rotatingcounterclockwise in the Northern Hemispherearound a low pressure center. On the east side,winds come from the south and so are warmer thanthose on the west side. The swirling air rises andcools, creating clouds and precipitation. Cyclonescan be the most intense storms on Earth. There aretwo types of cyclones: middle latitude cyclonesand tropical cyclones. Mid-latitude cyclones arethe main cause of winter storms in the middlelatitudes. Tropical cyclones are also known ashurricanes.

A n anticyclone, as you might expect, is theopposite of a cyclone. An anticyclone's windsrotate around a center of high pressure. Air fromabove sinks to the ground to fill the space leftwhen the air moved away. High pressure centersgenerally have fair weather. Anticyclone windsmove clockwise in the Northern Hemisphere,

exactly the opposite of a cyclone. Since winds onthe east side of the anticyclone come from the northand those on the west side come from the south, theeast side tends to be colder than the west side ofthe high.

Middle Latitude Cyclones

Middle latitude cyclones, sometimes calledextratropical cyclones, form at the polar frontwhen the temperature difference between two airmasses is large. These air masses blow past eachother in opposite directions. Winds are deflectedby Coriolis Effect—to the right in the NorthernHemisphere and to the left in the SouthernHemisphere. This causes the winds to strike thepolar front at an angle. Warm and cold fronts formnext to each other. Most winter storms in themiddle latitudes, including most of the UnitedStates and Europe, are caused by middle latitudecyclones (Figure below).

Figure 16.30 A hypothetical mid-latitude cyclone affecting

the United Kingdom. The arrows indicate the winddirection and its relative temperature; symbolizesthe low pressure area. Notice the warm, cold, andoccluded fronts.

The warm air at the cold front rises and createsa low pressure cell. Winds rush into the lowpressure and create a rising column of air. The airtwists, rotating counterclockwise in the NorthernHemisphere and clockwise in the SouthernHemisphere. Since the rising air is moist, rain orsnow falls.

Mid-latitude cyclones form in winter in themid-latitudes and move eastward with the westerly

winds. These two to five day storms can reach1,000 to 2500 km (625 to 1,600 miles ) in diameterand produce winds up to 125 km (75 miles) perhour. Like tropical cyclones, they can causeextensive beach erosion and flooding.

Mid-latitude cyclones are especially fierce inthe mid-Atlantic and New England states wherethey are called nor’easters, because they comefrom the northeast. About 30 nor’easters strike theregion each year. Most do little harm, but some aredeadly. The typical weather pattern of a nor’easteris familiar to anyone who has lived in this region.First, heavy snow and ice cover the ground. Then,air temperature warms and rain falls. The rain hitsthe frozen ground and freezes, cloaking everythingin ice (Figure below).

Figure 16.31 The 1993 Storm of the Century was a noreaster

that covered the entire eastern seaboard of theUnited States.

Hurricanes

Tropical cyclones have many names. They arecalled hurricanes in the North Atlantic and easternPacific oceans, typhoons in the western PacificOcean, tropical cyclones in the Indian Ocean, andwilli-willi's in the waters near Australia (Figurebelow). By any name, they are the most damagingstorms on Earth.

For a hurricane to form, sea surfacetemperature must be 28oC (82oF) or higher.Hurricanes arise in the tropical latitudes (between10o and 25oN) in summer and autumn. The warmseas create a large humid air mass. The warm airrises and forms a low pressure cell, known as atropical depression. Thunderstorms materializearound the tropical depression.

If the temperature within the cell reaches orexceeds 28oC (82oF) the air begins to rotatearound the low pressure. The rotation iscounterclockwise in the Northern Hemisphere andclockwise in the Southern Hemisphere. As the airrises, water vapor condenses, releasing energyfrom latent heat. If winds frequently shift directions

in the upper atmosphere, the storm cannot growupward. If wind shear is low, the storm builds intoa hurricane within two to three days.

Figure 16.32 A cross-sectional view of a hurricane.Hurricanes are roughly 600 km (350 miles)

across and 15 km (50,000 feet) high. Winds reachat least 118 km (74 miles) per hour. The exceptionis the relatively calm eye of the storm, which isabout 13 to 16 km (8 to 10 miles) in diameter. Theeye is calm because it is where air is risingupward.

Rainfall can be as high as 2.5 cm (1") per hour,resulting in about 20 billion metric tons of waterreleased daily in a hurricane. The release of latentheat generates enormous amounts of energy, about2,000 billion kilowatt hours per day. This amount

of energy is nearly the total annual electricalpower consumption of the United States.Hurricanes can also generate tornadoes.

Hurricanes are assigned to categories based ontheir wind speed. An estimate can be made as tothe damage that will be caused based on thecategory of storm. The categories are listed on theSaffir-Simpson hurricane scale (Table below).

Saffir - Simpson Hurricane ScaleCategory Kph Mph Damage

1 (weak) 119-153

74-95

Above normal; no realdamage to structures

2 (moderate) 154-177

96-110

Some roofing, door, andwindow damage,considerable damage tovegetation, mobilehomes, and piers

3 (strong) 178-209

111-130

Some buildingsdamaged; mobile homesdestroyedComplete roof failure

4 (verystrong)

210-251

131-156

on small residences;major erosion of beachareas; major damage tolower floors ofstructures near shore

5(devastating) >251 >156

Complete roof failureon many residences andindustrial buildings;some complete buildingfailures

Hurricanes move with the prevailing winds. Inthe Northern Hemisphere, they originate in thetrade winds and move to the west. When they reachthe latitude of the westerlies, they switch directionand travel toward the north or northeast.Hurricanes typically travel from 5 to 40 kph (3 to25 mph) and can cover 800 km (500 miles) in oneday. Their speed and direction depend on theconditions that surround them. This uncertaintymakes it hard for meteorologists to accuratelypredict where a hurricane will go and how strongit will be when it reaches land.

Damage from hurricanes tends to come fromthe high winds and rainfall, which can causeflooding. Near the coast, flooding is also causedb y storm surge (Figure below). Storm surgeoccurs as the storm's low pressure center comesonto land, causing the sea level to rise unusuallyhigh. A storm surge is often made worse by thehurricane's high winds blowing seawater acrossthe ocean onto the shoreline. Storm surge may riseas high as 7.0 to 7.5 m (20 to 25 feet) for up to 160km (100 miles) along a coastline. If a storm surgeis channeled into a narrow bay, it will greatlyincrease in height.

Figure 16.33 Storm surge effects on sea level.Waves created by a hurricane’s high winds and

high tide further increase water levels during astorm surge. Flooding can be devastating,especially along low-lying coastlines like the

Atlantic and Gulf Coasts. Hurricane Camille in1969 had a 7.3 m (24 foot) storm surge thattraveled 125 miles (200 km) inland.

Hurricanes can last from three hours to threeweeks, but 5 to 10 days is typical. Once ahurricane travels over cooler water or onto land,its latent heat source is shut down and it will soonweaken. However, an intense, fast-moving stormcan travel quite far inland before its demise. InSeptember, 1938 a hurricane made it all the way toMontreal, Canada before breaking up. When ahurricane disintegrates, it is replaced with intenserains and tornadoes.

There are about 100 hurricanes around theworld each year, plus many smaller tropicalstorms and tropical depressions. As peopledevelop coastal regions, property damage fromstorms continues to rise. However, scientists arebecoming better at predicting the paths of thesestorms and fatalities are decreasing. There is,however, one major exception to the previousstatement: Hurricane Katrina.

The 2005 Atlantic hurricane season was the

longest, costliest, and deadliest hurricane seasonso far. Although the hurricane season officiallyruns from June 1 to November 30, the 2005hurricane season was active into January 2006.Total damage from all the storms together wasestimated at more than $128 billion, with morethan 2,280 deaths. Of the 28 named storms, 15were hurricanes, including five Category 4 stormsand four Category 5 storms on the Saffir-SimpsonScale.

Hurricane Katrina was both the mostdestructive hurricane and the most costly (Figurebelow). The storm was a Category 1 hurricane asit passed across the southern tip of Florida. It waspushed westward by the trade winds, blowing overthe Gulf of Mexico where temperatures were ashigh as 32oC (89oF). The warm Gulf waters andlatent heat fueled the storm until it grew into aCategory 5. As it moved through the Gulf, themayor of the historic city of New Orleans ordereda mandatory evacuation of the city. Not everyonewas willing or able to comply.

Figure 16.34 Hurricane Katrina nears its peak strength as it

travels across the Gulf of Mexico.When Hurricane Katrina reached the Gulf

Coast, it had weakened to a Category 4 storm.Even so, it was the third strongest hurricane toever hit the United States. The eye of the stormstruck a bit east of New Orleans, buffeting the areaaround Biloxi, Mississippi with the worst directdamage. The initial reports were that New Orleanshad been spared. But as water began to rise in thelowest lying portions of the city, officials realizedthat the storm surge had caused the levee system tobreach. Eventually 80% of the city was underwater(Figure below). By the end of that horrible period,around 2,500 people were dead or missing from

the Gulf Coast, most of them from New Orleans.Over two hundred thousand of people left NewOrleans as a result of the hurricane, and many havenot returned due to loss of their homes andlivelihood.

Figure 16.35 Flooding in New Orleans after Hurricane

Katrina caused the levees to break and water to

pour through.

Blizzards and Lake Effect Snow

A blizzard is distinguished by certainconditions (Figure below):

Temperatures below –7oC (20oF); –12oC(10oF) for a severe blizzard.Winds greater than 56 kmh (35 mph); 72 kmh(45 mph) for a severe blizzard.Snow so heavy that visibility is 2/5 km (1/4mile) or less for at least three hours; near zerovisibility for a severe blizzard.

Figure 16.36 A near white out in a blizzard in Minnesota.Blizzards happen across the middle latitudes

and toward the poles. They usually develop on thenorthwest side of a mid-latitude cyclone. Blizzardsare most common in winter, when the jet streamhas traveled south and a cold, northern air masscomes into contact with a warmer, semitropical airmass. The very strong winds develop because ofthe pressure gradient between the low pressurestorm and the higher pressure west of the storm.Snow produced by the storm gets caught in thewinds and blows nearly horizontally. Blizzardscan also produce sleet or freezing rain.

The snowiest, metropolitan areas in the UnitedStates are Buffalo and Rochester, New York.These cities are prone to getting lake-effect snow.While other locations can have lake effect snow,the greatest amount is on the leeward side of theGreat Lakes. In winter, a continental polar air masstravels down from Canada. As the frigid air travelsacross one of the Great Lakes, it warms andabsorbs moisture. When the air mass reaches the

leeward side of the lake, it is very unstable and itdrops tremendous amounts of snow. Buffalo is onthe leeward side of Lake Erie and Rochester is onthe leeward side of Lake Ontario.

While lake effect snow is not a blizzard, thetwo can work together to create even greatersnows. The Great Lakes Blizzard of 1977 wascreated mostly by the passage of a cold front overthe area. The snowfall was aided by lake effectsnow coming off of Lake Ontario, which had notyet frozen that winter.

Extreme Heat and Drought

Although not technically storms, extreme heatand drought are important weather phenomena. Aheat wave is defined as extreme heat that lastslonger than normal for an area. During a heatwave, a high pressure zone sits over an area andhot air at the ground is trapped. A heat wave canoccur because the position of the jet stream makesthe area hotter than it is normally. For example, if

the jet stream is further north than usual, hotweather can also be found north of where it isusual. Winds coming from a different direction canalso make a region hotter than normal.Temperatures that would not ordinarily be too hotmay create a heat wave if the humidity rises toohigh.

More people die from extreme heat on averageeach year than in any type of storm. The ChicagoHeat Wave of 1995 killed about 600 people whodid not have access to air conditioning. The worldwas shocked in July and August 2003, whenbetween 20,000 and 35,000 died in a Europeanheat wave, mostly in France (Figure below).

Figure 16.37 Temperature anomalies (outside of the normal,

expected range) across Europe in the summer of2003. France was the hardest hit nation.

Drought strikes a region if it has less rainfallthan normal for days, weeks, or years, dependingon its location. A normally wet city enters droughtat a much greater rainfall level than a city locatedin the desert. A location may also be experiencingdrought, even if it receives rain, if the rain falls sothat it is useless to humans. For example, a heavyrain may run off a dried out landscape rather than

sinking into the soil and nourishing the plants.

Lesson Summary

Thunderstorms arise in warm weather whenupdrafts form cumulonimbus clouds that rainand hail.Lightning and thunder result when positiveand negative electrical charges in differentparts of the cloud and on the ground attempt toequalize.Tornadoes form most commonly fromthunderstorms. Although they are shorter induration and affect a smaller area than othersevere storms, they do an enormous amount ofdamage where they strike.Cyclones of all sorts are large and damaging;they include nor’easters and hurricanes.Heat waves kill more people each year thanany type of storm and mostly form in regionsbeneath an unusually high pressure zone.

Review Questions

1. Describe in detail how a thunderstorm formsand where the energy to fuel it comes from.Start with a warm day and no clouds.

2. How does a thunderstorm break apart anddisappear?

3. When and why does a severe thunderstorm getmore severe rather than losing energy anddisappearing?

4. How do lightning and thunder form?5. Discuss the pros and cons of living in an area

that is prone to tornadoes versus one that isprone to hurricanes.

6. Where are tornadoes most common in theUnited States?

7. What is a cyclone? What are the two types ofcyclone and how do they differ?

8. Describe in detail how a hurricane forms.9. What level is the most damaging hurricane on

the Saffir-Simpson scale? What sorts ofdamage do you expect from such a strong

hurricane?10. What causes damage from hurricanes?11. What could have been done in New Orleans

to lessen the damage and deaths fromHurricane Katrina?

12. Do you think New Orleans should be rebuiltin its current location?

13. Where do blizzards develop?

Vocabulary

cycloneWinds rotating around a low pressure center.

droughtA situation in which there is less precipitationthan normal for a matter of days, weeks, oryears.

lake-effect snowExtreme snowfall caused by the evaporationof relatively warm, moist air into a cold frontthat then drops its snow on the leeward side

of the lake.

lightningA huge discharge of electricity typical ofthunderstorms.

hurricaneCyclones that form in the tropics and spinaround a low-pressure center; they can be theworld’s most damaging storms.

mid-latitude cycloneA cyclone that forms in the middle latitudes atthe polar front.

Nor’easterMid-latitude cyclones that strike thenortheastern United States.

storm surgeA buildup of sea level due to wind blowingwater up against the land and water beingsucked upward by low pressure.

thunderThe loud clap produced by lightning.

thunderstormStorms caused by upwelling air andcharacterized by cumulonimbus clouds,thunder, and lightning.

tornadoViolently rotating funnel shaped clouds thatgrow downward from a cumulonimbus cloud.

tropical depressionA low pressure cell that rises in the tropics;thunderstorms materialize around the tropicaldepression

Points to Consider

Why is predicting where tornadoes will goand how strong they will be so difficult?How would the damage done by HurricaneKatrina have been different if the storm had

taken place 100 years ago?What knowledge do meteorologists need tobetter understand storms?

Weather Forecasting

Lesson Objectives

List some of the instruments thatmeteorologists use to collect weather data.Describe how these instruments are used tocollect weather data from many geographiclocations and many altitudes.Discuss the role of satellites and computers inmodern weather forecasting.Describe how meteorologists developaccurate weather forecasts.

Introduction

Weather forecasts are better than they everhave been. According to the World MeteorologicalOrganization (WMO), a 5-day weather forecasttoday is as reliable as a 2-day forecast was 20years ago! This is because forecasters now use

advanced technologies to gather weather data,along with the world’s most powerful computers.Together, the data and computers produce complexmodels that more accurately represent theconditions of the atmosphere. These models can beprogrammed to predict how the atmosphere and theweather will change. Despite these advances,weather forecasts are still often incorrect. Weatheris extremely difficult to predict, because it is avery complex and chaotic system.

Collecting Weather Data

To make a weather forecast, the conditions ofthe atmosphere must be known for that location andfor the surrounding area. Temperature, airpressure, and other characteristics of theatmosphere must be measured and the datacollected. Thermometers measure temperature.One way to do this is to use a temperature-sensitive material, like mercury, placed in a long,very narrow tube with a bulb. When the

temperature is warm, the mercury expands, causingit to rise up the tube. Cool temperatures cause themercury to contract, bringing the level of themercury lower in the tube. A scale on the outsideof the thermometer matches up with the airtemperature.

Because mercury is toxic, most meteorologicalthermometers no longer use mercury in a bulb.There are many ways to measure temperature.Some digital thermometers use a coiled stripcomposed of two kinds of metal, each of whichconducts heat differently. As the temperature risesand falls, the coil unfolds or curls up tighter. Othermodern thermometers measure infrared radiationor electrical resistance. Modern thermometersusually produce digital data that can be fed directlyinto a computer.

Meteorologists use barometers to measure airpressure (Figure below). A barometer may containwater, air or mercury. Like thermometers,barometers are now mostly digital. Air pressuremeasurements are corrected so that the numbersare given as though the barometer were at sea

level. This means that only the air pressure ismeasured instead of also measuring the effect ofaltitude on air pressure.

Figure 16.38 Barometers are Mercury columns used to

measure air pressure.A change in barometric pressure indicates that

a change in weather is coming. If air pressurerises, a high pressure cell is on the way and clearskies can be expected. If pressure falls, a lowpressure is coming and will likely bring stormclouds. Barometric pressure data over a largerarea can be used to identify pressure systems,fronts and other weather systems.

Other instruments measure differentcharacteristics of the atmosphere. Below is a listof a few of these instruments, along with what theymeasures:

5. snow gauge: the amount of solidprecipitation over a period of time4. rain gauge: the amount of liquidprecipitation over a period of time3. wind vane: wind direction2. hygrometers: humidity1. anemometers: wind speed

These instruments are placed in variouslocations so that they can check the atmosphericcharacteristics of that location. Weather stations

are located on land, the surface of the sea, and inorbit all around the world (Figure below).According to the WMO, weather information iscollected from 15 satellites, 100 stationary buoys,600 drifting buoys, 3,000 aircraft, 7,300 ships andsome 10,000 land-based stations.

Figure 16.39 A land-based weather station. Since some of

the instruments must be protected fromprecipitation and direct heat, they are held behinda screen.

Instruments are also sent into the atmosphere inweather balloons filled with helium or hydrogen.As the balloon ascends into the upper atmosphere,the gas in the balloon expands until the balloonbursts. The specific altitude at which the balloonbursts depends on its diameter and thickness, but isordinarily about 40 km (25 miles) in altitude. Thelength of the flight is ordinarily about 90 minutes.Weather balloons are intended to be used onlyonce, and the equipment they carry is usually notrecovered.

Weather balloons contain radiosondes thatmeasure atmospheric characteristics, such astemperature, pressure and humidity (Figurebelow). Radiosondes in flight can be tracked toobtain wind speed and direction. Radiosondes usea radio to communicate the data they collect to acomputer.

Figure 16.40 A weather balloon with a radiosonde beneath

it. The radiosonde is the bottom piece and theparachute that will bring it to the ground, is aboveit.

Radiosondes are launched from around 800

sites around the globe twice daily (at 0000 and1200 UTC; UTC is Coordinated Universal Time; itis the same as Greenwich Mean Time — the timein the city of Greenwich, England) at the same timeto provide a profile through the atmosphere.Special launches are done when needed for specialprojects. Radiosondes can be dropped from aballoon or airplane to make measurements as theyfall. This is done to monitor storms, for example,since they are dangerous places for airplanes tofly.

Weather information can also come fromremote sensing, particularly radar and satellites(Figure below). Radar stands for Radio Detectionand Ranging. In radar, a transmitter sends out radiowaves. The radio waves bounce off the nearestobject and then return to a receiver. Weather radarcan sense many characteristics of precipitation: itslocation, motion, intensity, and the likelihood offuture precipitation. Most weather radar isDoppler radar, which can also track how fast theprecipitation falls. Radar can outline the structureof a storm and in doing so estimate the possibility

that it will produce severe weather.

Figure 16.41 Radar view of a line of thunderstorms.Weather satellites have been increasingly

important sources of weather data since the firstone was launched in 1952. Weather satellites arethe best way to monitor large scale systems, likestorms. Satellites can also monitor the spread ofash from a volcanic eruption, smoke from fires,and pollution. They are able to record long-termchanges, such as the amount of ice cover over theArctic Ocean in September each year.

Weather satellites may observe all energy fromall wavelengths in the electromagnetic spectrum.Most important are the visible light and infrared

(heat) frequencies. Visible light images recordimages the way we would see them, includingstorms, clouds, fires, and smog. Infrared imagesmeasure heat. These images can record clouds,water and land temperatures, and features of theocean, such as ocean currents. Weather patternslike the El Niño are monitored in infrared imagesof the equatorial Pacific Ocean.

Two types of weather satellites aregeostationary and polar orbiting (Figure below).Geostationary satellites orbit the Earth at the samerate that the Earth rotates; therefore, they remainfixed in a single location above the equator at analtitude of about 36,000 km (22,000 miles). Thisallows them to constantly monitor the hemispherewhere they are located. A geostationary satellitepositioned to monitor the United States will have aconstant view of the mainland, plus the Pacific andAtlantic Oceans, as it looks for hurricanes andother potential hazards.

Figure 16.42 One of the geostationary satellites that monitors

conditions over the United States.Polar orbiting satellites orbit much lower in

the atmosphere, at about 850 km (530 miles) inaltitude. They are not stationary but continuouslyorbit making loops around the poles, passing overthe same point at around the same time twice eachday. Since these satellites are lower, they get amore detailed view of the planet.

Forecasting Methods

There are many ways to create a forecast, somesimple and some complex. Some use only current,

local observations, while others deal withenormous amounts of data from many locations atdifferent times. Some forecasting methods arediscussed below.

Perhaps the easiest way to forecast weather iswith the 'persistence' method. In this method, weassume that the weather tomorrow will be like theweather today. The persistence method works wellif a region is under a stationary air mass or if theweather is consistent from day to day. Forexample, Southern California is nearly alwayswarm and sunny on summer days, and so that is afairly safe prediction to make. The persistencemethod can also be used for long-term forecasts inlocations where a warm, dry month is likely tolead to another warm dry month, as in a SouthernCalifornia summer.

The 'climatology 'method assumes that theweather will be the same on a given date as it wason that date in past years. This is often not veryaccurate. It may be snowing in Yosemite one NewYear’s Day and sunny and relatively warm on thenext. Using the 'trend' method, forecasters look at

the weather upwind of their location. If a cold frontis moving in their direction at a regular speed, theycalculate when the cold front will arrive. Ofcourse, the front could slow down, speed up, orshift directions, so that it arrives late, early, in astrengthened or weakened state, or never arrives atall. Forecasters use the 'analog' method when theyidentify a pattern. Just like an analogy comparestwo similar things, if last week a certain pattern ofatmospheric circulation led to a certain type ofweather, the forecaster assumes that the samepattern this week will lead to the same weather.There are lots of possible variations in patternsand changes often occur, so this method is also notentirely accurate.

Numerical Weather Prediction

The most accurate weather forecasts are madeby advanced computers, with analysis andinterpretation added by experiencedmeteorologists. These computers have up-to-date

mathematical models that can use much more dataand make many more calculations than would everbe possible by scientists working with just mapsand calculators. Meteorologists can use theseresults to give much more accurate weatherforecasts and climate predictions.

In Numerical Weather Prediction (NWP),atmospheric data from many sources are pluggedinto supercomputers running complex mathematicalmodels. The models then calculate what willhappen over time at various altitudes for a grid ofevenly spaced locations. The grid points areusually between 10 and 200 kilometers apart.Using the results calculated by the model, theprogram projects that weather further into thefuture. It then uses these results to project theweather still further into the future and so on, as faras the meteorologists want to go. The final forecastis called a prognostic chart or prog.

Certain types of progs are better at particulartypes of forecasts and experienced meteorologistsknow which to use to predict different types ofweather. In addition to the prog, scientists use the

other forecasting methods mentioned above. Withso much data available, meteorologists use acomputerized system for processing, storage,display and telecommunications. Once a forecast ismade, it is broadcast by satellites to more than1,000 sites around the world.

NWP produces the most accurate weatherforecasts, but as anyone knows, even the bestforecasts are not always right. Some of the reasonsfor this are listed below:

Not enough data was initially entered into theprogram. This is most likely to happen for aregion near an ocean or a remote area.The computer program makes certainassumptions about how the atmosphereoperates, which may not always be correct.The programs only deal with weather locally,which means errors are likely at the edges ofthe area studied. A global model would bemore accurate, but producing one wouldrequire an incredible number of calculations.The weather system may be too small to show

up on the grid. If a system is small, like athunderstorm, it will not be modeled. Ifdistances between grid points are reduced,many more calculations and therefore morepowerful computers are needed.There is always the possibility that conditionschange unpredictably. Weather is a chaoticsystem and small, unpredictable things alwayshappen. The farther into the future a modeltries to forecast, the more unpredictablethings arise to change the forecast.

Weather Maps

Weather maps simply and graphically depictmeteorological conditions in the atmosphere.Weather maps may display only one feature of theatmosphere or multiple features. They can depictinformation from computer models or from humanobservations. Weather maps are found innewspapers, on television, and on the Internet.

On a weather map, each weather station will

have important meteorological conditions plotted.These conditions may include temperature, currentweather, dew point, the amount of cloud cover, sealevel air pressure, and the wind speed anddirection. On a weather map, meteorologists usemany different symbols. These symbols give thema quick and easy way to put information onto themap. Figure below shows some of these symbolsand explains what they mean (Figure below).

Figure 16.43 Explanation of some symbols that may appear

on a weather map.

Figure 16.44 Different types of weather that can be shown

using a weather symbol.Once conditions have been plotted, points of

equal value can be connected. This is like thecontour line on a topographic map, in which allpoints at a certain elevation are joined. Weathermaps can have many types of connecting lines. Forexample:

Lines of equal temperature are calledisotherms. Isotherms show temperaturegradients and can indicate the location of afront. The 0°C (32°F) isotherm will show

where rain is likely to give way to snow.Isobars are lines of equal average airpressure at sea level (Figure below). Closedisobars represent the locations of high andlow pressure cells. High pressure cells areshown as H’s and low pressure cells as L’s.A thick, brown dashed line is often placedinside a long low pressure trough.

Figure 16.45 Lines of equal pressure drawn on a weather

map are isobars. Isobars can be used to helpvisualize high and low pressure cells.

Isotachs are lines of constant wind speed.Where the minimum values occur high in theatmosphere, tropical cyclones may develop.The highest wind speeds can be used to locatethe jet stream.

Surface weather analysis maps are weathermaps that only show conditions on the ground(Figure below). These maps show sea level meanpressure, temperature and amount of cloud cover.This information will reveal features such as highand low pressure cells.

Figure 16.46 Surface analysis map of the contiguous United

States and southern Canada.Weather maps can also depict conditions at

higher altitudes. Aviation maps show conditions inthe upper atmosphere, particularly those that are ofinterest to pilots. These include current weather,cloud cover, and regions where ice is likely toform.

Lesson Summary

Weather forecasts are more accurate thanever before. Older instruments and datacollection methods such as radiosondes andweather balloons are still used.These techniques have now been joined bysatellites and computers to create much moredetailed and accurate forecasts.Still, forecasts are often wrong, particularlythose that predict the weather for severaldays.

Meteorologists are working hard to improveweather forecasts one to two weeks inadvance of potentially hazardous weather.

Review Questions

1. What types of instruments would you expectto find at a weather station and what do theseinstruments measure?

2. How does a thermometer work?3. How could a barometer at a single weather

station be used to predict an approachingstorm?

4. Why are weather balloons important forweather prediction? What information do theygive that isn’t obtainable in other ways?

5. How does radar work, and what is its valuein weather prediction?

6. What type of weather satellite is best to usefor monitoring hurricanes that may causeproblems in the United States and why?

7. Imagine that your teacher asks you to predict

what the weather will be like tomorrow. Youcan go outside, but can’t use a TV orcomputer. What method will you use?

8. Imagine that you need to predict tomorrow’sweather and you are allowed to use atelephone, but no other electronics. Who willyou call and what method will you use?

9. Okay, now you need to predict tomorrow’sweather and you have access to electronics,but not to weather forecasts. That is, you canlook at information such as weather maps andradar images but you cannot look at theinterpretations made by a meteorologist. Nowwhat method are you using?

10. No rain is in the forecast, but it’s pouringoutside. How could the NWP weatherforecast have missed this weather event?

11. What does it mean to say that weather is achaotic system? How does this affect theability to predict the weather?

Further Reading / Supplemental

Links

National Doppler Radar Siteshttp://radar.weather.gov/Google Earth Visualizations, Barnabuhttp://www.barnabu.co.uk/global-cloud-animations-update/

Vocabulary

barometerAn instrument for measuring atmosphericpressure.

isobarsLines connecting locations that have equal airpressure.

isotachsLines connecting locations that have equalwind speed.

isothermsLines connecting locations that have equaltemperatures.

prognostic chart (prog)A chart showing the state of the atmosphere ata given time in the future.

radarRadio detection and ranging device that emitsradio waves and receives them after theybounce on the nearest surface. This creates animage of storms and other nearby objects.

radiosondeA group of instruments that measure thecharacteristics of the atmosphere—temperature, pressure, humidity, etc. — asthey move through the air.

weather mapA map showing weather conditions over awide area at a given time, it collects data

from many locations.

weather satelliteA human made object that orbits the Earth andsenses electromagnetic waves, mostly in thevisible light and infrared spectra.

Points to Consider

With so much advanced technology available,what is the role of meteorologists in creatingaccurate weather forecasts?With so much advanced technology available,why are weather forecasts so often wrong?What advances do you think will be necessaryfor meteorologists to create accurate weatherforecasts one- to two-weeks in advance of amajor weather event?

Chapter 17: Climate

Climate and Its Causes

Lesson Objectives

Describe the effect of latitude on the solarradiation a location receives and how thisinfluences climate.Diagram the Hadley, Ferrell and Polaratmospheric circulation cells and show howthey influence the climate of variouslocations.Discuss the other important location factorsthat influence a location’s climate: position inthe global wind belts, proximity to a largewater body, position relative to a mountainrange and others.

Introduction

While almost anything can happen with theweather, climate is more predictable. The weatheron a particular winter day in San Diego may becolder than on the same day in Lake Tahoe, but, onaverage, Tahoe’s winter climate is significantlycolder than San Diego’s. Climate then is the long-term average of weather. Good climate is why wechoose to vacation in Hawaii in February, eventhough the weather is not guaranteed to be good!

What is Climate?

Weather is what is happening in theatmosphere at a particular location at the moment.Climate is the average of weather in that location

over a long period of time, usually for at least 30years. A location’s climate can be described by itsair temperature, humidity, wind speed anddirection, and the type, quantity, and frequency ofprecipitation. The climate of a location depends onit position relative to many things. Most importantis latitude, but other factors include global andlocal winds, closeness to an ocean or other largebodies of water, nearness to mountains, andaltitude. Climate can change, but only over longperiods of time.

The term climate also refers to Earth’s entireclimate system. The climate system is influencedby the movement of heat around the globe. Heat iscarried by currents within the atmosphere andoceans. The type and amount of vegetation alsoaffects climate. Plants absorb heat and retainwater, which may increase or decrease rainfall.The composition of the atmosphere also controlsclimate. If the concentration of greenhouse gasesincrease or decrease, the heat-trapping abilities ofthe atmosphere rise or fall.

Latitude

The amount of solar energy a particularlocation receives is the most important factor indetermining that location’s temperature. Theamount of sunlight that strikes the ground isdifferent at each latitude. The lower the latitude,the more sunlight an area will receive. At theequator, days are equally long year-round and thesun is just about directly overhead at midday. Atthe poles, during the winter, nights are long and thesun never rises very high in the sky. Sunlight filtersthrough a thick wedge of atmosphere, making thesunlight much less intense. Ice and snow at highlatitudes also reflect a good portion of the sun'slight, giving these regions much greater albedo.

This animation shows the average surfacetemperature across the planet as it changes throughthe year

Monthly Mean Temperatures(http://upload.wikimedia.org/wikipedia/commons/b/b3/MonthlyMeanT.gif)

From all this information you can understand

why the tropics are warmer than the polar areas.The temperate regions are in between, both inlatitude and average air temperature. The air inEarth's atmosphere moves as the Sun warms someareas more than others. The main reason we havedifferent climates at various latitudes is alsodetermined by the amount of sunlight that hits eachplace (Figure below).

Figure 17.1 This map of annual average temperatures

shows how dramatically temperature decreasesfrom the low latitudes to the high latitudes.

Prevailing Winds

There are winds that usually blow in oneparticular direction, called the global wind belts.These winds are called the trade winds, thewesterlies and the polar easterlies. The directionthese winds blow is different at various latitudes.In the Earth's Atmosphere chapter, you learned thatair rises at low pressure areas, which form at 0o

and again at 50o to 60o north and south of theequator. Air sinks at high pressure areas, whichform at around 30o N and S and at the poles. Theselow and high pressure zones represent the upwardand downward flowing regions of the Hadley,Ferrell and Polar atmospheric circulation cells(Figure below). Low pressure areas form whereair is moving upwards or rising. High pressureareas form where cooler, drier air sinks. Areas ofhigh pressure often have climates that are coolerand drier. Low pressure zones often have climatesthat are warm and rainy.

Figure 17.2 The atmospheric circulation cells and their

relationships to air movement on the ground.The low pressure area near the equator is

located at the boundary between the two HadleyCells. In both these cells, air rises up at the equatorand then travels away from the equator. This bandof rising air is called the IntertropicalConvergence Zone (ITCZ) (Figure below). Asthe air rises, it cools and condenses to createclouds and rain. Climate along the ITCZ istherefore warm and wet. In an area where the air ismostly rising, there is not much wind. Early

mariners called this region the doldrums becausetheir ships were often unable to sail without steadywinds.

Figure 17.3 The ITCZ can be easily seen where

thunderstorms are lined up north of the equator.The ITCZ migrates slightly with the season.

Land areas heat more quickly than the oceans.Since there are more land areas in the NorthernHemisphere, the ITCZ is influenced by the heatingeffect of the land. In Northern Hemisphere summer,it is approximately 5 o north of the equator while inthe winter, it shifts back and is approximately atthe equator. As the ITCZ shifts, the major windbelts also shift slightly north in summer and southin winter, which causes the wet and dry seasons inthis area (Figure below).

Figure 17.4 Seasonal differences in the location of the

ITCZ are shown on this map.At the high pressure zone where the Hadley

cell and Ferrell cells meet, at about 30oN and30oS, the air is fairly warm since much of it camefrom the equator. It is also very dry for tworeasons: (1) The air lost much of its moisture at theITCZ, and (2) Sinking air is more likely to causeevaporation than precipitation. Mariners had avery grim reason for naming this region the horselatitudes. Often the lack of wind would cause theirships to be delayed for so long that they would runout of water and food for their livestock. Sailorswould toss horses and other animals over the sideof the ship after they died. Sailors sometimesdidn’t make it either. On land, these high pressure

regions mark the locations of many of the world’sgreat deserts, including the Sahara in Africa andthe Sonora in North America.

The other low pressure zone is between theFerrell and Polar Cells at around 50-60o. This isthe usual location of the polar jet stream, wherecold air from the poles meets warmer air from thetropics and storms are common. As the Earth orbitsthe Sun, the angle of the Sun shifts between 23.5oNand 23.5oS; in turn, this shift causes the polar jetstream to move. Like the ITCZ, the position of thepolar jet stream moves seasonally, creatingseasonal weather changes in the mid-latitudes.

The direction of the prevailing winds greatlyinfluences the climate of a region. These windsform the bases of the Hadley, Ferrell and PolarCells. Winds often bring the weather from thelocations they come from. For example, inCalifornia, the predominant winds are thewesterlies. These winds blow in from the PacificOcean. The stability of the ocean’s temperaturesmoderates California temperatures, so that

summers are cooler and winters are warmer. In themiddle of the continent, the winds bring morevariable conditions. Local winds also influencelocal climate. For example, land breezes and seabreezes moderate coastal temperatures.

Continental Position

When a particular location is near an ocean orlarge lake, the body of water plays an extremelyimportant role in affecting the region’s climate.When a location has a maritime climate itsclimate is strongly influenced by the nearby sea:summers are not too hot and winters are not toocold. Temperatures also do not vary much betweenday and night. For a location to have a truemaritime climate, the winds must most frequentlycome off the sea. A continental climate is moreextreme, with greater temperature differencesbetween day and night and between summer andwinter.

The ocean’s influence in moderating climate

can be seen in the following temperaturecomparisons. Each of these cities is located at37°N latitude, within the westerly winds. SanFrancisco is on the Pacific coast; Wichita, Kansasis in the middle of the North American continent;and Virginia Beach, Virginia is on the Atlanticcoast. San Francisco is cooler in July and warmerin January than either of the other two cities. Thisis typical of a maritime climate; not too hot, not toocold. Wichita has the greatest range oftemperatures; the hottest temperatures in July andcoldest in January, which is typical for acontinental climate. Although Virginia Beach islocated on the Atlantic Ocean, it has a mostlycontinental climate since the westerly winds comeoff the continent (Table below).

Average Temperature

Location City July: high(oC/oF)

January:low (oC/oF)

West CoastSanFrancisco,CA

CentralUnitedStates

Wichita,KS

East Coast VirginiaBeach, VA

(Source: wikipedia.org)

Ocean Currents

The temperature of the water offshore willinfluence the temperature of a coastal location,particularly if the winds come off the sea. TheCalifornia Current runs from north to south alongthe length of the western coast of North America.The cool waters of the California Current bringcooler temperatures to the California coastalregion. Coastal upwelling also brings cold, deepwater up to the ocean surface off of California,which contributes to the cool, coastaltemperatures. Further north, in southern Alaska, theupwelling actually raises the temperature of thesurrounding land because the ocean water is much

warmer than the land.The California Current is part of the global

system of surface ocean currents that spread theSun’s heat around the world. These currents bringcool water from high latitudes to the low latitudesand warm water from low latitudes to the highlatitudes. Just as the California current bringscooler temperatures into the temperate regions, theGulf Stream raises air temperatures along thesoutheastern United States as it brings warmequatorial water north along the Western Atlanticocean (Figure below).

Figure 17.5 The Gulf Stream can be identified in this image

of the Atlantic Ocean off of the eastern UnitedStates and Canada. The warm current appears inorange and yellow.

The Gulf Stream even has a large effect on theclimate of Northern Europe. After it travels pastCanada, the current moves eastward across theAtlantic and then splits. One portion flowsnorthward between Great Britain and NorthernEurope, the other moves south along Europe and

northern Africa. These warm waters raisetemperatures in the North Sea, which raises airtemperatures over land between 3 to 6oC (5 to11oF). London, U.K. for example, is at the samelatitude as Quebec, Canada. However, London’saverage January temperature is 3.8oC (38oF),while Quebec’s is only -12oC (10oF). Because airtraveling over the warm water in the Gulf Streampicks up a lot of water, London gets a lot of rain. Incontrast, Quebec is much drier and receives itsprecipitation as snow.

Altitude and Mountain Ranges

All else being equal, air temperature decreasesat higher altitudes. A town at 3000 meters in themountains will be much cooler, on average, than atown at the base of the same mountains. Gravitypulls air molecules closer together at sea levelthan at higher altitudes. The closer molecules arepacked together, the more likely they are tocollide. Collisions between molecules give off

heat, which warms the air. At higher altitudes, theair is less dense and air molecules are morespread out and less likely to collide.

Mountain ranges have two effects on theclimate of the surrounding region. One is therainshadow effect. As moist air rises over amountain, it cools and drops precipitation. The airthen descends down the leeward side of the range.This process creates a high pressure region on theback side of the mountain where evaporationexceeds precipitation. The result is that thewindward side of the mountain range is wet but theleeward side is dry. The other effect occurs whenthe mountain range separates the coastal regionfrom the rest of the continent. In this case, theocean can only influence the coastal area beforethe mountain range. The coastal area near theocean will have a maritime climate but just overthe mountain, the inland area will have acontinental climate.

California has two mountain ranges that exhibitboth effects on climate: the Coast Range rightalong the coast and the Sierra Nevada, further east.

The predominant winds here are the westerlies,winds that blow from the west over the ocean ontothe continent. Both ranges trap cool air from thePacific so that it has a difficult time movingeastward. As this moist ocean air rises over theCoast Range, it drops a lot of rain on thewindward side. A rainshadow is created as the airthen descends into the Central Valley, some ofwhich receives so little rainfall it is classified as adesert. As the air continues eastward, it rises overthe Sierra Nevada Mountains and drops more rainand snow on the west side of these mountains. Onthe leeward side, the air then descends intoNevada. The Sierra Nevada rain shadow createsthe Great Basin desert, covering Nevada, westernUtah and a small part of southeastern Oregon(Figure below).

Figure 17.6 The Bonneville Salt Flats are part of the very

dry Great Basin. They receive so little rainfallbecause they are on the leeward side of the SierraNevada, part if its rainshadow.

Lesson Summary

Many factors influence the climate of aregion, all of them somehow related to theregion’s position.Latitude determines how much solar energy aparticular place receives during a day or ayear.Latitude is directly related to location within

one of the global wind belts, thereforelatitude determines if a location is beneath ahigh or low pressure cell, where winds arelow.If a region is near a large water body, itsclimate will be influenced by that water body.Mountain ranges can separate land areas fromthe oceans and can create rainshadow effects,which also influence climate.

Review Questions

1. Describe the weather of the location whereyou are right now. How is the weather todaytypical or atypical of your usual climate?

2. In what two ways could a desert be found at30°N?

3. Could a desert form at 45oN latitude?4. Why is there so little wind in the locations

where the atmospheric circulation cells meet?5. If it is windy at 30oN where there is normally

little wind, does that mean the model of the

atmospheric circulation cells is wrong?6. What is the Intertropical Convergence Zone

(ITCZ)? What winds do you expect to findhere?

7. How does the polar jet stream move fromsummer to winter? How would this affect theclimate of the locations where it moves?

8. How would the climate of a city at 45oN nearthe Pacific Ocean differ from one at the samelatitude near the Atlantic Coast?

9. Why does the ocean water off California coolthe western portion of the state, while thewater off the southeastern United Stateswarms that region?

10. Think about what you know about surfaceocean currents. How would you expect theclimate of western South America to beinfluenced by the Pacific Ocean? Could thissame effect happen in the NorthernHemisphere?

11. The Andes Mountains line western SouthAmerica. How do you think they influence theclimate of that region and the lands to the east

of them?

Vocabulary

climateWeather averaged over a long period of time,usually about 30 years.

continental climateA location in which the climate is dominatedby a vast expanse of land. Continentalclimates are more variable.

Intertropical Convergence Zone (ITCZ)A low pressure zone where the Hadley Cellsin the northern and southern hemispheresmeet. The trade winds meet at the ITCZ.

maritime climateA location in which the climate is dominatedby a nearby ocean. Maritime climates are lessvariable than continental climates.

rainshadowDry conditions that are created on theleeward side of a mountain range.

upwellingUpward flow of deep water to the surfacebecause the deep water is less dense than thesurface water.

Points to Consider

Describe how two cities at the same latitudecan have very different climates. Forexample, Tucson, Arizona has a hot, drydesert climate and New Orleans, Louisianahas a warm, muggy climate even though bothcities are at approximately the same latitude.How does climate influence the plants andanimals that live in a particular place?Would you expect climate at similar latitudesto be the same or different on the oppositeside of the equator, e.g. how would the

climate of a city at 45oN be similar ordifferent to one at 45oS latitude?

World Climates

Lesson Objectives

Describe the relationship between the climatezones and the factors that influence climate.Discuss the relationship between climatezones and biomes.Discuss the different biomes based on ageneral description.

Introduction

A climate zone results from the climateconditions of an area: its temperature, humidity,amount and type of precipitation, and the season. Aclimate zone is reflected in a region’s naturalvegetation. Perceptive travelers can figure outwhich climate zone they are in by looking at thevegetation, even if the weather is unusual for theclimate on that day!

Climate Zones and Biomes

The major factors that influence climate alsodetermine the different climate zones. The sametype of climate zone will be found at similarlatitudes and in similar positions on nearly allcontinents, both in the Northern and SouthernHemispheres. The one exception to this pattern isthe climate zones called the continental climates,which are not found at higher latitudes in theSouthern Hemisphere. This is because the SouthernHemisphere land masses are not wide enough toproduce a continental climate.

Climate zones are classified by the Köppenclassification system (Figure below). This systemis based on the temperature, the amount ofprecipitation and the times of year whenprecipitation occurs. Since climate determines thetype of vegetation that grows in an area, vegetationis used as an indicator of climate type. A climatetype and its plants and animals make up a biome.The organisms of a particular biome share certain

characteristics around the world, because theirenvironment has similar advantages andchallenges. The organisms have adapted to thatenvironment in similar ways over time. Forexample, different species of cactus live ondifferent continents, but they have adapted to theharsh desert in similar ways.

The Köppen classification system recognizesfive major climate groups, each with a distinctcapital letter A through E. Each lettered group isdivided into subcategories. Some of thesesubcategories are forest (f), monsoon (m), andwet/dry (w) types, based on the amount ofprecipitation and season when that precipitationoccurs.

Figure 17.7 This world map of the Kppen classification

system gives a good general idea of where theclimate zones and major biomes are located.Where the groups are not represented by stripes(like in the western United States) the situation iscomplicated by geographic features such asmountains. All of the c

Tropical Moist Climates (Group A)

Tropical Moist (Group A) climates are foundin a band about 15° to 25° N and S of the equator(Figure below). Intense sunshine means that thetropics are warm year-round: each month has anaverage temperature of at least 18°C (64°F).Rainfall is abundant — at least 150 cm (59 inches)per year. The subcategories of this zone are basedon when the rain falls.

Figure 17.8 Tropical Moist Climates (Group A) are shown

in green. The main vegetation for this climate is thetropical rainforest, which are found in the Amazonin South America, the Congo in Africa and thelands and islands of southeast Asia.

Tropical Wet (Af)

The wet tropics lie in a band around theequator, covering about 10% of the Earth's land.The wet tropics are consistently warm, with almostno annual temperature variation. Great amounts ofrain fall year-round, between 175 and 250 cm (65and 100 inches) per year. These conditions supportthe tropical rainforest biome (Figure below). Theforest is dominated by densely packed, broadleaf

evergreen trees. Many habitats are found inrainforests, as a result of the high number of planttypes and the different environments within thelayers of the forest. Rainforests have the highestnumber of species or biodiversity of anyecosystem.

Figure 17.9 The Amazon river and rainforest in Brazil.

Tropical Monsoon (Am)

The tropical monsoon climate resembles thetropical wet biome (Af) but has very lowprecipitation for one to two months each year.

During these months, less than 6 cm (2.4 inches) ofrain falls. Rainforests can grow here because thedry period is short, and the trees can be supportedby moisture trapped in the soil. This climate isfound where the monsoon winds blow, primarily insouthern Asia, western Africa, and northeasternSouth America.

Tropical Wet and Dry (Aw)

The tropical wet and dry climate lies northand south of the tropical wet climate, betweenabout 5° and 10° latitude to around 15° to 20°latitude. The average annual temperature is similarto the wet tropics, but the temperature range isgreater. This climate zone receives less rain thanthe wet tropics, about 100 to 150 cm (40 to 60inches). For more than two months each year,rainfall is less than 6 cm (2.4 inches). Wet and dryseasons are related to the location of the ITCZ. Inthe summer, when the ITCZ drifts northward, thezone is wet. In the winter, when the ITCZ movesback toward the equator, the region is dry. This

climate exists where strong monsoon winds blowfrom land to sea, such as in India.

Rainforests cannot survive the months of lowrainfall, so the typical vegetation is savanna(Figure below). This biome consists mostly ofgrasses, with widely scattered deciduous trees andrare areas of denser forests. Central Africa isfamous for its savanna and the unique animals thatlive there.

Figure 17.10 A male lion stalks the African savanna.

Dry Climates (Group B)

The Dry Climates (Group B) generally haveless precipitation than evaporation and are further

from the equator than Tropical (Group A) climates.They have cooler winters and longer, harsher dryseasons. Rainfall is irregular; several years ofdrought are often followed by a single year ofabundant rainfall. Summer temperatures are high,and much of the rain evaporates before it reachesthe ground. Dry climate zones cover about 26% ofthe world’s land area. Low latitude deserts formas a result of the Ferrell cell high pressure zone.Higher latitude deserts occur within continents orin rain shadows where the air has little humidity.

Arid Desert (Bw)

Low-latitude, arid deserts are found between15° to 30° N and S latitudes. This is where warmdry air sinks at high pressure zones. True desertsmake up around 12% of the world’s lands. Desertsare found in southwestern North America, Africa,Australia and central Asia. Humidity is low, andas a result there are few clouds in the sky. TheSahara is the world’s largest desert (Figure

below).

Figure 17.11 The Sahara desert takes up much of northern

Africa. The green band below the sandy Sahara isthe Congo.

In the Sonora Desert of the southwesternUnited States and northern Mexico, most weatherstations record sunshine 85% of the time, both insummer and winter (Figure below). Clear skiesallow the ground to heat rapidly during the day andcool rapidly at night. The summer sun can bemerciless and daytime temperatures are extremelyhot. Temperatures often plunge when the Sun goesdown and the daily temperature range may be 15°to 25°C (27° to 45°F). Annual rainfall is mostlyless than 25 cm (10 inches). The Sonora desertgets much of its rain in the winter, when storms

come in from the Pacific. Summer monsoon rainsdrop a great deal of rain in some areas. Theexistence of two wet seasons a year allows aunique group of plants and animals to survive inthe southwestern deserts.

Figure 17.12

The Sonora desert is home to many well-adapted plants including cacti, scrubby bushes, andperennials.

Vegetation is limited, since water is scarce.Desert plants are adapted to surviving long periodsof drought. Cacti and shrubby plants have wide ordeep roots to reach water after a rain. Many desertplants are able to store water. Some plants liedormant as seeds, and bloom after rain falls.

Semi-arid or Steppe (Bs)

Deserts in continental interiors or in rainshadows are found at higher latitudes. Semi-ariddeserts often receive more rain than the ariddeserts, between 20 to 40 cm (8 to 16 inches)annually. Because land areas change temperatureeasily, these areas have lower annual averagetemperatures plus greater annual temperatureranges. In the winter, these climate zones get verycold under cold high pressure cells. In the summer,they heat up, allowing air to rise and rain to fall.

In the United States, the Great Plains, portions

of the southern California coast and the GreatBasin are semi-arid deserts. The biome is calledsteppe, which has short bunch grass, and scatteredlow bushes, or sagebrush (Figure below). Asteppe has few or no trees because there is notenough rain for trees to grow.

Figure 17.13 This photo of the Great Basin in Utah

illustrates the steppe biome.

Moist Subtropical Mid-latitude(Group C)

The Moist Subtropical Mid-latitude (Group

C) climates are found along the coastal areas in theUnited States. Seasons are distinct, althoughwinters are mild. The average temperature of thecoldest month ranges from just below freezing toalmost balmy, between -3°C and 18°C (27o to64°F). Summers are often mild with averagetemperatures above 10°C (50°F). There is plentifulannual rainfall.

Dry Summer Subtropical or MediterraneanClimates (Cs)

The Dry Summer Subtropical climate is foundon the western sides of continents between 30° and45° latitude. Annual rainfall is 30 to 90 cm (14 to35 inches), most of which comes in the winter.This climate is also called the Mediterraneanclimate because it is found around theMediterranean Sea (Figure below). The climate isalso typical of coastal California, which sitsbeneath a summertime high pressure for about fivemonths each year. Land and sea breezes make

winters moderate and summers cool. The mildwinters and foggy summers of San Franciscorepresent a coastal Mediterranean climate. Furtherinland in Sacramento, summer temperatures aremuch higher, representing an interiorMediterranean climate. Vegetation must survivelong summer droughts. This vegetation type iscalled chaparral. Scrubby, woody plants and treesare common (Figure below).

Figure 17.14 A map showing locations which have a

Mediterranean climate. The areas around theMediterranean Sea are the best representatives ofthis climate type.

Figure 17.15 The scrubby plants that are typical of a

Mediterranean climate. This photo is from southernFrance.

Humid Subtropical (Cfa)

The Humid Subtropical climate zone is foundmostly on the eastern sides of continents (Figurebelow). Rain falls throughout the year with annualaverages between 80 and 165 cm (31 and 65inches). Summer days are humid and hot, from thelower 30's up to 40°C (mid-80's up to 104°F).Afternoon and evening thunderstorms are common.These conditions are due to warm tropical air

passing over the hot continent. Winters are mild,but middle-latitude storms called cyclones maybring snow and rain. The southeastern UnitedStates, with its hot humid summers and mild, butfrosty winters, is typical of this climate zone.

Figure 17.16 The humid subtropical climate zone is shown

in green.Forests grow thickly in much of this region,

due to the mild temperatures and high humidity.Pine forests are common in the lower latitudes,while oak are more common at higher latitudes(Figure below).

Figure 17.17 Pine forests are common in the humid

subtropical climate zone.

Marine West Coast Climate (Cfb)

This climate lines western North Americabetween 40° and 65° latitude, an area known as thePacific Northwest (Figure below). Ocean windsbring mild winters and cool summers. Thetemperature range between seasons and betweenday and night is fairly small. Rain falls year-round,although summers are drier as the jet stream movesnorthward. Low clouds, fog, and drizzle are

typical. Snowfall is infrequent and short-lived. Themountain ranges that line the western U.S. keep thisclimate from extending far inland.

Figure 17.18 The west coast marine climate zone.Dense forests of Douglas fir thrive in the

heavy rain and mild temperatures (Figure below).In Western Europe the climate covers a largerregion since no high mountains are near the coastto block wind blowing off the Atlantic.

Figure 17.19 A Douglas fir forest in the Pacific Northwest.

Continental Climates (Group D)

Continental (Group D) climates are found inmost of the North American interior from about

40°N to 70°N. In this climate, summers are cool-to-warm and winters are very cold and stormy.The average temperature of the warmest month ishigher than 10°C (50°F) and the coldest month isbelow -3°C (-27°F). Snowfall is common and thecold temperatures mean that snow stays on theground for long periods of time. Trees grow incontinental climates, even though winters areextremely cold, because the average annualtemperature is fairly mild. Continental climates arenot found in the Southern Hemisphere due to theabsence of a continent large enough to generate thiseffect.

Humid Continental (Dfa, Dfb)

Humid continental climates are found aroundthe polar front at about 60oN latitude withincontinental North America and Europe (Figurebelow). In the winter, middle-latitude cyclonesbring chilly temperatures and snow. In the summer,westerly winds bring continental weather and

warm temperatures. The average July temperatureis often above 20°C (70°F). The region is typifiedby deciduous trees, which protect themselves inwinter by losing their leaves.

Figure 17.20 The humid continental climate zone covers

much of the northeastern United States,southeastern Canada and middle Europe.

The two variations of this climate are based onsummer temperatures. In the humid continentalclimate with long, hot summers (Dfa), the longsummers and high humidity foster plant growth(Figure below). Summer days may be over 38°C(100°F), nights are warm and the temperaturerange is large, perhaps as great as 31°C (56°F)! Inthe humid continental climate with long, coolsummers (Dfb), summertime temperatures andhumidity are lower. Winter temperatures are

below -18°C (0°F) for long periods. For example,Winnipeg, Canada has a 38°C (68°F) annualtemperature range.

Figure 17.21 Parts of South Korea lies in the humid

continental climate zone.

Subpolar (Dfc)

T he subpolar climate is found between thehumid continental and the polar tundra climates(Figure below). This climate is dominated bylong, very cold winters with very littleprecipitation. Continental polar air masses form

when air stagnates in this zone. Snowfall is light,but temperatures are so cold that snowfall remainson the ground for months. Most of theapproximately 50 cm (20 inches) of annualprecipitation falls during summer cyclonic storms.The angle of the Sun's rays is low but the Sun isvisible in the sky for most or all of the day, sotemperatures may get warm, but are rarely hot.These continental regions have very high annualtemperature ranges. The climate of Fairbanks,Alaska is a typical subarctic climate.

Figure 17.22 The location of the subpolar climate.The boreal coniferous forests of this climate

are called taiga (Figure below). These vastforests stretch across North America from westernAlaska to Newfoundland, and across Eurasia from

Norway to the Pacific coast.

Figure 17.23 The vast taiga is known for its small, hardy,

and widely-spaced trees. This photo is of theAlaska Range in Alaska.

Polar Climates (Group E)

In the polar regions, winters are entirely darkand bitterly cold. In the summer, days are long, butthe sun is low on the horizon. Summers are coolwith the average temperature of the warmest monthat less than 10°C (50°F). Winters are extremelycold, so the annual temperature range is large.Polar climates receive less than 25 cm (10 inches)

precipitation, mostly during the summer. Thisclimate is found across the continents that borderthe Arctic Ocean, Greenland and Antarctica.

Polar Tundra (ET)

The polar tundra climate is continental, withsevere winters (Figure below). Temperatures areso cold that a layer of permanently frozen ground,called permafrost forms below the surface. Thisfrozen layer can extend hundreds of meters deep.The average temperature of the warmest months isabove freezing, so summer temperatures defrost theuppermost portion of the permafrost. In winter, thepermafrost prevents water from drainingdownward. In summer, the ground is swampy.Although the precipitation is low enough in manyplaces to qualify as a desert, evaporation rates arealso low, so the landscape receives more usablewater than a desert.

Figure 17.24 Tundra loses its green in the fall, as mosses

and leaves turn brown.The only plants that can survive the harsh

winters and soggy summers are small ground-hugging plants like mosses, lichens, small shrubsand scattered small trees that make up the tundra(Figure below). Due to the lack of ice-free landnear the South Pole, there is very little tundra in theSouthern Hemisphere. The area surrounding theArctic Ocean is the only part of the globe withmuch tundra.

Figure 17.25 The tundra biome is shown in this map.

Ice Cap

Ice caps are found mostly on Greenland andAntarctica, about 9% of the Earth's land area(Figure below). Ice caps may be thousands ofmeters thick. Ice cap areas have extremely lowaverage annual temperatures, e.g. -29°C (-20°F) atEismitte, Greenland. Precipitation is low, since theair is too cold to hold much moisture. Snowoccasionally falls in the summer.

Figure 17.26 A composite satellite image of Antarctica.

Almost all the continent is covered with an icecap.

Microclimates

When climate conditions in a small area aredifferent from those of the surroundings, theclimate of the small area is called a microclimate.The microclimate of a valley may be cool relative

to its surroundings since cold air sinks. The groundsurface may be hotter or colder than the air a fewfeet above it, since rock and soil gain and lose heatreadily. Different sides of a mountain will havedifferent microclimates. In the NorthernHemisphere, a south-facing slope receives moresolar energy than a north-facing slope, and so eachside supports different amounts and types ofvegetation.

Altitude mimics latitude in climate zones.Climates and biomes typical of higher latitudesmay be found in other areas of the world at highaltitudes. Mt. Kilimanjaro in Africa lies near theequator in the tropics, but the top of the mountain isin the tundra biome and there is also a smallglacier at the top.

Lesson Summary

A climate zone depends on a region’slatitude, continental position, and relationshipto prevailing winds, large water bodies and

mountains, among other factors.The temperature, rainfall, length of dryseason, and other features of the climate zonedetermine which plants can grow.When living organisms develop in similarclimates, they must adapt to that sameenvironment. So the organisms in similarclimate zones resemble each other, no matterhow geographically distant they are. Becausethe organisms are so similar, a climate zoneand its organisms make up a biome.

Review Questions

1. Why are most climate zones found in similarlocations on continents within the NorthernHemisphere?

2. What are some reasons that climate zonesdiffer between continents, even thoughlocations are similar?

3. Why do organisms in the same biome oftenlook the same even though they are not the

same species? Think about desert plants, forexample. Why are the plants that live in lowlatitude deserts on different continents sosimilar?

4. Why is the length of the dry season importantin distinguishing different types of climatezones? Give an example.

5. Since the equator receives the most solarradiation over the course of a year, why arethe hottest temperatures found in the low-latitude deserts? Why are low-latitude desertsoften chilly at night, even in the summer?

6. What are the differences between arid andsemi-arid deserts?

7. What conditions bring about the hot andhumid summer days found in the AmericanSouth?

8. What is the most important factor indetermining the presence of a forest?

9. Look at Figure 7 and see which major climatetypes are found in California (ignore the 3rd

letter on each symbol). Look at a geographicmap at the same time if you need to. Which

climate type is the most common and where isit found? Which other two types of climatesare found and where? Why does Californiahave so many major climate types?

10. Polar regions receive little precipitation. Whyare they not considered deserts?

11. What is permafrost? Does it stay the sameyear-round?

12. Why are microclimates important to livingthings?

Vocabulary

biodiversityThe number of species of plants, animals andother organisms within a particular habitat.

biomeThe living organisms that are found within aclimate zone that make that zone distinct, suchas rainforests, arid deserts, tundra, and icecaps.

chaparralScrubby woody plants and widely scatteredtrees typical of the Mediterranean climate.

desertAreas receiving very little precipitation, lessthan 25 cm (10") per year; found in aridclimates; plants are infrequent.

Douglas firA coniferous, evergreen tree found inenormous forests in western North America.

ice capPermanent ice that is found mostly aroundGreenland and Antarctica.

permafrostPermanently frozen ground that is found in thepolar regions, where temperatures do not riseabove freezing most months.

rainforestThe tropical wet biome where temperatures

are warm and rain falls nearly every day.

savannaThe tropical wet and dry biome, typified bygrasses and widely scattered deciduous trees.

steppeThe biome found in semi-arid deserts,typified by bunch grasses, scattered lowbushes and sagebrush.

taigaVast, boreal forests of small, more widelyspaced trees that are typical of the subpolarclimate.

tundraThe treeless area of the arctic with very cold,harsh winters. The only polar climate thatcontains living organisms.

Points to Consider

Why aren’t biomes always determined bylatitude? What geographic features or otherfactors affect the climate?Climate zones and biomes depend on manyclimate features. If climate changes, which ofthese features changes too?If global warming is increasing averageglobal temperatures, how would you expectbiomes to be affected?

Climate Change

Lesson Objectives

Describe some ways that climate change hasbeen an important part of Earth history.Discuss what factors can cause climate tochange and which of these can be exacerbatedby human activities.Discuss the consequences of risinggreenhouse gas levels in the atmosphere, theimpacts that are already being measured, andthe impacts that are likely to occur in thefuture.

Introduction

For the past two centuries, climate has beenrelatively stable. People placed their farms andcities in locations that were in a favorable climatewithout thinking that the climate could change. But

climate has changed throughout Earth history, and astable climate is not the norm. In recent years,Earth’s climate has begun to change again. Most ofthis change is warming due to human activities thatrelease greenhouse gases into the atmosphere. Theeffects of warming are already being seen and willbecome more extreme as temperature rise.

Climate Change in Earth History

Climate has changed throughout Earth history.At times, the Earth's climate was hotter and morehumid than it is today, but climate has also beencolder than it is today, when glaciers coveredmuch more of the planet. The most recent ice ageswere in the Pleistocene Epoch, between 1.8million and 10,000 years ago (Figure below).Glaciers advanced and retreated in cycles, knownas glacial and interglacial periods. With so muchof the world’s water bound into the ice, sea levelwas about 125 meters (395 feet) lower than it istoday. We are currently in a warm, interglacial

period that has lasted about 10,000 years.

Figure 17.27 The maximum extent of glaciers in the Northern

Hemisphere during the Pleistocene epoch.

It is likely that the average global temperatureduring glacial periods was only 5.5°C (10°F) lessthan Earth’s current average temperature.Temperatures during the interglacial periods wereabout 1.1°C (2.0°F) higher than today (Figurebelow). Notice that fairly small temperaturechanges can have major effects on global climate.Over the last 900,000 years, Earth's averagetemperature has varied less than 5 o C. Somescientists think that glaciers will advance again,but not for thousands of years.

Since the end of the Pleistocene, the globalaverage temperature has risen about 4°C (7°F).Glaciers are retreating and sea level is rising. Theclimate has been relatively mild and stable whencompared with much of Earth’s history. Climatestability has been beneficial for human civilization.Stability has allowed the expansion of agricultureand the development of towns and cities. Whileclimate is getting steadily warmer, there have beena few more extreme warm and cool times in thelast 10,000 years. The Medieval Warm Periodfrom 900 to 1300 A.D. allowed Vikings to

colonize Greenland and Great Britain to growwine grapes. When the climate cooled in the TheLittle Ice Age, from the 14th to 19th centuries, theVikings were forced out of Greenland and humanshad to plant crops further south.

Figure 17.28 The graph is a compilation of 10

reconstructions (the colored lines) of meantemperature changes over the past 2,000 years, andone graph of instrumentally recorded data of meantemperature changes (black). This illustrates thehigh temperatures of the Medieval Warm Period,the lows of the Little Ice Age, and the very high(and climbing) temperature of this decade.

Short-Term Climate Oscillations

Short-term changes in climate are common asconditions oscillate (or change) from one state toanother (Figure below). The largest and mostimportant of these is the Southern Oscillationbetween El Niño and La Niña conditions. Thisoscillation drives changes in climate that are feltaround the world about every two to seven years.

In a normal year, the trade winds blow acrossthe Pacific Ocean near the equator from east towest (in the direction of Southeast Asia), piling upwarm water in the western Pacific Ocean andactually raising sea levels there by half a meter.Meanwhile, along the western coast of SouthAmerica, the Peru Current carries cold waternorthward, and cold, nutrient-rich waters upwellfrom the deep sea. When the Peru Current nears theequator, it flows westward across the PacificOcean with the trade winds.

Figure 17.29 Normal conditions in the Southern Oscillation

have a low pressure region in the western PacificOcean and warm water (shown in red) building upthere as well. Notice that continents are shown inbrown in the image. North and South America areon the right in this image.

When water temperature reaches around 28°C(82°F), the trade winds weaken or reversedirection and blow east (towards South America).An El Niño cycle has begun (Figure below).Warm water is dragged back across the PacificOcean, heating the central Pacific Ocean and thesurface waters off the west coast of SouthAmerica. With warm, low density water at the

surface, no upwelling occurs along the coast ofSouth America. Without upwelling, nutrients arescarce and plankton populations decline. Sinceplankton form the base of the food web, fish cannotfind food, and fish numbers decrease as well. Allthe animals that eat fish, including birds andhumans are affected by the decline in fish.

Figure 17.30 In El Nio conditions, the trade winds weaken

or reverse directions. Warm water moveseastward across the Pacific Ocean and piles upagainst South America.

El Niño events change global climate whencirculation patterns in the atmosphere and oceanschange. Some regions receive more than average

rainfall, including the west coast of North andSouth America; the southern United States; andWestern Europe. Drought occurs in other parts ofSouth America, the western Pacific, southern andnorthern Africa, and southern Europe.

An El Nino cycle lasts one to two years andends when the warm mass of central Pacific waterhas moved eastward once more. Normalcirculation patterns resume, but sometimes they arequicker and more energetic. This pattern, withunusually cold water in the eastern Pacific Ocean,is called La Niña (Figure below). El Niño eventstake place every three to seven years but vary intheir strength.

Figure 17.31

During a La Nia, ocean temperatures along thecoast of South America are colder than normal(instead of warmer, as in El Nio) and cold waterreaches farther into the western Pacific thannormal. As in a normal year, trade winds movingfrom east to west and warm water piles up in thewestern Pacific Ocean.

Other important oscillations are smaller andhave a local, rather than global, effect. The NorthAtlantic Oscillation mostly alters climate inEurope. The Mediterranean also goes throughcycles, varying between being dry at some times,and warm and wet at others.

Causes of Climate Change

Many natural processes cause climate tochange. There can be changes in the amount ofenergy the Sun produces. As Earth orbits the Sun,there are also changes over thousands of years inthe tilt of Earth's axis and orbit. Over millions ofyears, the positions of our continents change,

driven by plate tectonic motions. Randomcatastrophic events, like a large asteroid impactcan cause sudden, dramatic changes in climate.Human activities have greatly increased the amountof greenhouse gases in the atmosphere, whichimpacts global climate by warming the Earth.

Solar Variation

The amount of energy the Sun radiates isvariable. Sunspots are magnetic storms on theSun’s surface that increase and decrease over an11-year cycle (Figure below). When the number ofsunspots is high, solar radiation is also relativelyhigh. But the entire variation in solar radiation istiny relative to the total amount of solar radiationthat there is and there is no known 11-year cycle inclimate variability. The Little Ice Agecorresponded to a time when there were nosunspots on the Sun.

Figure 17.32 Sunspots on the face of the Sun.

Plate Tectonics

Plate tectonic movements can alter climate.Over millions of years as seas open and close,ocean currents may distribute heat differently. Forexample, when all the continents are joined intoone supercontinent (like Pangea), nearly alllocations experience a continental climate. Whenthe continents separate, heat is more evenlydistributed. Plate tectonic movements may helpstart an ice age. When continents are located nearthe poles, ice can accumulate, which may increasealbedo and lower global temperature. Low enoughtemperatures may start a global ice age.

Plate tectonics also triggers volcanic eruptions,which release dust and CO2 into the atmosphere.Ordinary eruptions, even large ones, have only ashort-term effect on weather. Ash from the 1991eruption of Mount Pinatubo in the Philippines

cooled global temperature by around 0.5°C (0.9°F)for a year. Massive eruptions of a fluid type oflava can flood the surface, releasing much moregas and dust, changing climate for many years. Thistype of eruption is exceedingly rare; none hasoccurred since humans have lived on Earth.

Asteroid Impacts

If a large asteroid hits the Earth, it may triggera mass extinction. This likely happened at the endof the Cretaceous period, around 65 million yearsago. An asteroid 10 kilometers (6 miles) indiameter struck the Yucatan Peninsula insoutheastern Mexico. About 85% of all speciespresent on Earth at that time became extinct,including the dinosaurs. Dust that was kicked highinto the atmosphere came together as rocks and fellto the ground. The organisms that survived had toendure extreme cold, as dust blocked the Sun formany years. Photosynthesis slowed down and theplanet cooled to levels that were intolerable for

many species.

Milankovitch Cycles

The most extreme climate of recent Earthhistory was the Pleistocene. Scientists attribute aseries of ice ages to variation in the Earth'sposition relative to the Sun, known asMilankovitch cycles.

The Earth goes through regular variations in itsposition relative to the Sun:

1. The shape of the Earth's orbit changes slightlyas it goes around the Sun. Our orbit variesfrom more circular to more elliptical in acycle lasting between 90,000 and 100,000years. When the orbit is more elliptical, thereis a greater difference in solar radiationbetween winter and summer.

2. The planet wobbles on its axis of rotation. Atone extreme of this 27,000 year cycle, theNorthern Hemisphere points toward the Sun,when the Earth is closest to the Sun. Summers

are much warmer and winters are muchcolder than now. At the opposite extreme, theNorthern Hemisphere points toward the Sunwhen it is farthest from the Sun. This resultsin chilly summers and warmer winters.

3. The planet’s tilt on its axis varies between22.1° and 24.5°. Seasons are caused by thetilt of Earth's axis of rotation, which is at a23.5° angle now. When the tilt angle issmaller, summers and winters differ less intemperature. This cycle lasts 41,000 years.

When these three variations are charted out, aclimate pattern of about 100,000 years emerges.Ice ages correspond closely with Milankovitchcycles. Since glaciers can only form over land, iceages only occur when landmasses cover the polarregions. Therefore, Milankovitch cycles are alsoconnected to plate tectonics.

Rising Atmospheric Greenhouse Gases

Greenhouse gases in the atmosphere trap the

heat that radiates off the planet’s surfaces.Therefore, a decrease in greenhouse gas levelslowers the average air temperature. An increase ingreenhouse gases raises air temperature.Greenhouse gas levels have varied throughoutEarth history. For example, CO2 been present inEarth's atmosphere at concentrations less than 200parts per million (ppm) and more than 5,000 ppm.But for 650,000 years or more, CO2 has neverrisen above 300 ppm, during either glacial orinterglacial periods. CO2 levels are higher duringinterglacial than glacial periods (Figure below).

Figure 17.33 CO is high during interglacial periods, when

temperatures are high. CO is low during glacialperiods, when temperatures are low. BP meansyears before present. Glacial periods are shown inblue and interglacial periods are shown in yellow.Current carbon dioxide levels are at 387 ppm, thehighest level for the last 650,000 years.

Greenhouse gases are added to the atmosphereby natural processes, like volcanic eruptions, andthe decay or burning of organic matter. Greenhousegases are also removed from the atmosphere whenCO2 is absorbed by plant tissue. When plants dieand are turned into fossil fuels - coal, oil, naturalgas - deep in the Earth, the CO2 they hold is storedwith them. Storing CO2 in the ground removes thegreenhouse gas from the atmosphere, loweringEarth’s average temperature.

Human activities are now releasing much ofthis stored CO2 into the atmosphere. Althoughpeople have been burning wood and coal to meettheir energy needs for hundreds of thousands ofyears, fossil fuel usage has increased dramaticallyin the past 200 years, since the Industrial

Revolution. Fossil fuel use has skyrocketed in thepast few decades as population has grown, andthere are more and more cars, homes, andindustries to power. Burning rainforests, to clearland for agriculture prevents the growing treesfrom removing CO2 from the atmosphere andreleases all the CO2 stored in rainforest trees. Withmore people to feed, the destruction of rain forestshas increased.

CO2 is the most important greenhouse gas thathuman activities affect. And, after water vapor,CO2 is the most abundant. But other greenhousegases are increasing as well. Methane is releasedfrom raising livestock, rice production, and theincomplete burning of rainforest plants.Chlorofluorocarbons (CFC’s) are human-madechemicals that were invented and used widely inthe 20th century. Tropospheric ozone, mostly fromvehicle exhaust, has more than doubled since 1976.All of these gases act as greenhouse gases as wellas CO2.

Global Warming

With more greenhouse gases trapping heat,average annual global temperatures are rising. Thisis known as global warming. There is now nearly40% more CO2 in the atmosphere than there was200 years ago, before the Industrial Revolution.About 65% of that increase has occurred since thefirst CO2 measurements were made on Mauna LoaVolcano, Hawaii in 1958 (Figure below).

Figure 17.34 The Keeling Curve shows the upward trend in

atmospheric CO on Mauna Loa volcano sincemeasurements began in 1958. The blue line shows

yearly averaged CO. The red line shows seasonalvariations in CO.

The United States has been the largest emitterof greenhouse gases, with about 20% of totalemissions in 2004 (Figure below). China has beenthe second highest emitter (18.4%), followed bythe European Union (11.4%). As a result ofChina’s rapid economic growth, in early 2008 itsCO2 emissions probably surpassed those of theUnited States. However, it’s also important to keepin mind that the US has only about 1/5 thepopulation of China. Therefore, the average UScitizen produces far more greenhouse gases thanthe average Chinese person. If nothing is done todecrease the rate of CO2 emissions, by 2030, CO2emissions are projected to be 63% greater thanthey were in 2002.

Figure 17.35 Global CO emissions are rising rapidly. The

industrial revolution began about 1850 andindustrialization has been accelerating.

How much CO2 levels will rise in the nextdecades is unknown. It depends on how muchCO2emissions in developing nations increase. Italso depends on how much technological advancesor lifestyle changes increase or decrease emissionsin developed nations. If nothing is done to controlgreenhouse gas emissions and they continue toincrease at current rates, the surface temperature ofthe Earth can be expected to increase between

0.5°C and 2.0°C (0.9°F and 3.6°F) by 2050 andbetween 2° and 4.5°C (3.5° and 8°F) by 2100,with CO2 levels over 800 parts per million (ppm)(Figure below). On the other hand, if severe limitson CO2 emissions begin soon, temperatures couldrise less than 1.1°C (2°F) by 2100. Whatever thetemperature increase, it will not be uniform aroundthe globe. A rise of 2.8°C (5°F) would result in0.6° to 1.2°C (1° to 2°F) at the equator, but up to6.7°C (12°F) at the poles. So far, global warminghas affected the North Pole more than the SouthPole.

Figure 17.36 Various climate prediction models show how

temperature is likely to rise by 2100.Changes in the Earth and organisms as a result

of global warming are already being observed.While temperatures have risen since the end of thePleistocene, 10,000 years ago, this rate of increasehad been more rapid in the past century, and haseven risen even faster since 1990. The eightwarmest years on record have all occurred since1998, and the 14 warmest years have occurredsince 1990 (through 2007) (Figure below).

Figure 17.37 Recent temperature increases show how much

temperature has risen since the IndustrialRevolution began.

As a result of these high temperatures, glaciers

are melting and ice caps are breaking apart at theedges (Figure below). Permafrost is melting,causing swamps in locations that were once frozensolid. Melting ice caps add water to the oceans, sosea level is rising (Figure below). Alsocontributing to sea level rise is that water slightlyexpands as it warms — this expansion of wateraccounts for about one-quarter to one-half of theobserved sea level change. Since warmer air canhold more moisture, storms are becoming moreintense. Weather is therefore likely to be moresevere with more heat waves and droughts. Morerainfall sometimes results in increased flooding.

Figure 17.38 Sea level has been rising in recent decades.

Twenty-three different geologically stable tidegauge sites with long-term records are representedhere.

Figure 17.39 The Boulder Glacier has melted back

tremendously since 1985. Other mountain glaciersaround the world are also melting.

Plants and animals are feeling the effects of thechanging climate. Winters are shorter so someanimals are changing their seasonal behaviors:migrating earlier in the spring, for example. Coralreefs are dying worldwide due in part to theincrease in surface ocean temperatures. Forests arealso dying in some places because warm weather

has allowed insect pests to expand their rangesinto areas that were once too cold. As surface seaswarm, phytoplankton productivity has decreased.Some regions that were already marginal foragriculture are no longer farmable, because theyhave become too warm or dry.

As greenhouse gases increase, changes will bemore extreme. Oceans will become slightly moreacidic, making it more difficult for creatures withcarbonate shells to grow, including coral reefs. Astudying monitoring ocean acidity in the PacificNorthwest found ocean acidity increasing ten timesfaster than expected and 10 to 20 percent ofshellfish (mussels) replaced by acid tolerant algae.

Plant and animal species seeking coolertemperatures will need to move poleward 100 to150 km (60 to 90 miles) or upward 150 m (500feet) for each 1.0°C (8°F) rise in globaltemperature. There will be a tremendous loss ofbiodiversity because forest species can’t migratethat rapidly. Even if they could, humandevelopment would block their spread and stopthem from colonizing many new areas. Parks and

wildlife refuges might be left protecting nothing.And biologists have already documented theextinction of high altitude species that havenowhere higher to go.

Decreased snowpacks, shrinking glaciers, andthe earlier arrival of spring will all lessen theamount of water available in some regions of theworld, including the western United States andmuch of Asia. Ice will continue to melt and sealevel is predicted to rise 18 to 97 cm (7 to 38inches) by 2100 (Figure below). An increase thislarge will gradually flood coastal regions whereabout one-third of the world's population lives,forcing billions of people to move inland.

Figure 17.40 Sea ice thickness around the North Pole has

been decreasing in recent decades and willcontinue to decrease in the coming decades.

There will be more severe heat waves andheat-related illnesses and deaths. Drought couldmake many marginal regions uninhabitable. Somemodelers predict that the Midwestern United Stateswill become too dry to support agriculture and theareas that currently produce our best agriculturewould move into Canada. In all, about 10 to 50%of current cropland worldwide may becomeunusable if CO2 doubles.

Although scientists do not all agree, hurricanesare likely to become more severe and possiblymore frequent. Hurricanes cause a tremendous lossof life in developing nations and a loss of propertyin developed ones. Tropical and subtropicalinsects will expand their ranges. This will result inthe spread of tropical diseases such as malaria,encephalitis, yellow fever, and dengue fever.

You may notice that the numerical predictionsabove contain wide ranges. Sea level, for example

is expected to rise somewhere between 18 and 97cm — quite a wide range. This uncertainty ispartly because scientists cannot predict exactlyhow the Earth will respond to increased levels ofgreenhouses gases. How quickly greenhouse gasescontinue to build up in the atmosphere depends inpart on the choices we make.

Lesson Summary

Climate has changed throughout Earth history.In general, when greenhouse gas levels arehigh, temperature is high.Greenhouse gases are now increasing due tohuman activities, especially fossil fuel use.We are already seeing the effects of theserising greenhouse gases in highertemperatures and changes to physical andbiological systems.Society must choose to reduce greenhouse gasemissions or face more serious consequences.

Review Questions

1. Why is the climate currently warming?2. Why does sea level rise and fall during

interglacial and glacial periods?3. How can the human history of Greenland be

related to climate cycles?4. Why did human civilization not develop

significantly until the Pleistocene ended?5. If climate has been much warmer in Earth

history, why do we need to worry aboutglobal warming now?

6. When the weather along coastal California isespecially rainy with many winter storms,what is likely to be happening in theequatorial Pacific in terms of the SouthernOscillation?

7. The Peruvian anchovy fishery collapsed in1972. Using what you know about climate andfood webs, can you devise an explanation forthis event?

8. What two events must occur for there to be an

ice age?9. What human activities are responsible for

increasing greenhouse gases in theatmosphere?

10. Why are CO2 emissions projected to increaseby so much during the next few decades?

11. What role do the developed nations play arole in increasing CO2 emissions in the nextfew decades?

12. Why do storms increase in frequency andintensity as global temperatures increase?

13. Earth is undergoing some important changes,some of which are known about andmonitored by satellites. Describe the sort ofglobal change that satellites can monitor.

14. What will happen if sea level rises by 60 cm(2 feet0 by the end of this century? Whichlocations will be hardest hit?

15. What can be done to reduce greenhouse gasemissions?

Vocabulary

asteroidA chunk of rock or ice that moves through thesolar system.

El NiñoPart of the Southern Oscillation in which thetrade winds weaken or reverse directions,and warm water accumulates on the oceansurface off of South America.

global warmingThe global increase in average globaltemperature that has been taking place due tohuman activities.

La NiñaPart of the Southern Oscillation in which thetrade winds are stronger than normal andsurface water off of South America is cold

mass extinctionAn extinction in which 25% or more of theplanet’s species die out in a fairly short

period of time.

Milankovitch cyclesCycles in Earth’s position relative to the sunthat affect global climate, resulting in a cycleof around 100,000 years.

Southern OscillationA reversal of normal atmospheric low andhigh pressure conditions in the Pacific Ocean.

sunspotSunspots are magnetic solar storms on the sunthat cause solar radiation to decrease slightly.Sunspots come and go over an 11-year cycle.

Points to Consider

Nearly all climate scientists agree that humanactivities are causing the acceleratedwarming of the planet that we see today. Whydo you suppose that the media is still talkingabout the controversy in this idea when

scientists are almost entirely in agreement?If greenhouse gas emissions must be loweredto avoid some of the more seriousconsequences of global warming, why havehumans not done something to lower theseemissions instead of letting them increase?In what ways can progress be made inreducing greenhouse gas emissions? Thinkabout this on a variety of scales: forindividuals, local communities, nations, andthe global community.

Chapter 18: Ecosystems andHuman Populations

Ecosystems

Lesson Objectives

Discuss the importance of chemical andphysical factors to living organisms.Describe the role of different species in anecosystem.Describe the function of an ecosystem, andhow different species fill different roles indifferent ecosystems.Describe energy transfer from the lowest tothe highest trophic level in a chain, includingenergy loss at every trophic level.Discuss how materials are cycled betweentrophic levels and how they can enter or leavea food web at any time.

Introduction

An ecosystem is made up of the livingcreatures and the nonliving things that thosecreatures need within an area. Energy movesthrough an ecosystem in one direction. Nutrientscycle through different parts of the ecosystem andcan enter or leave the ecosystem at many points.

Biological Communities

A population consists of all individuals of asingle species that occur together at a given placeand time. A species is a single type of organismsthat can interbreed and produce fertile offspring.All of the populations living together in the samearea make up a community. An ecosystem is allof the living things in a community and the physicaland chemical factors that they interact with. Theliving organisms within an ecosystem are its bioticfactors. Living things include bacteria, algae, fungi,plants (Figure below) and animals, including

invertebrates (Figure below), animals withoutbackbones and vertebrates (Figure below),animals with backbones.

Figure 18.1 The horsetail is a primitive plant.

Figure 18.2 Insects are among the many different types of

invertebrates.

Figure 18.3 A giraffe is an example of a vertebrate.Physical and chemical features are abiotic

factors. Abiotic factors include resources livingorganisms need like light, oxygen, water, carbondioxide, good soil, and nitrogen, phosphorous andother nutrients. Abiotic factors also includeenvironmental features that are not materials or

living things, like living space and the righttemperature range.

Organisms must make a living, just like alawyer or a ballet dancer. This means that eachindividual organism must acquire enough foodenergy to live and hopefully reproduce. A species'way of making a living is called its niche. Anexample of a niche is making a living as a topcarnivore, an animal that eats other animals, but isnot eaten by any other animals. This niche can befilled by a lion in a savanna, a wolf in the tundra,or a tuna in the oceans. Every species fills a niche,and niches are almost always filled in anecosystem.

An organism’s habitat is where it lives. Theimportant characteristics of a habitat includeclimate, the availability of food, water and otherresources, and other factors, such as weather. Ahabitat may be a hole in a cactus or the undersideof a fern in a rainforest. It may be a large area ofsavanna.

Roles in Ecosystems

There are many different types of ecosystems(Figure below). A few examples of someecosystems are a rainforest, chaparral, tundra, anddesert. These words are the same words used forbiomes. This is because climate conditionsdetermine which ecosystems are found in whichlocation. A particular biome encompasses all ofthe ecosystems that have similar climate andorganisms.

Figure 18.4 Coral reefs are complex and beautiful

ecosystems.Different organisms live in each different type

of ecosystems. Lizards thrive in deserts, but noreptiles can survive at all in polar ecosystems.Large animals generally do better in cold climatesthan in hot climates. Despite this, every ecosystemhas the same general roles that living creatures fill.It’s just the organisms that fill those niches that aredifferent. For example, every ecosystem must havesome organisms that produce food in the form ofchemical energy. These organisms are primarilyalgae in the oceans, plants on land, and bacteria atdeep sea hot springs.

The organisms that produce food are extremelyimportant in every ecosystem. The mostfundamental distinction between types oforganisms is whether they are able to produce theirown energy or not. Organisms that produce theirown food are called producers. In contrast,organisms that use the food energy that was createdby producers are named consumers.

There are two types of producers. Nearly allproducers take energy from the Sun and make itinto chemical energy (food) by the process ofphotosynthesis. Photosynthesizing organisms use

carbon dioxide (CO2) and water (H2O) to producesugar (C6H12O6) and oxygen (O2). This food canbe used immediately or stored for future use.

A tiny group of producers create usablechemical energy from chemicals, without using anysunlight. At the bottom of the ocean, at deep-seahot springs known as hydrothermal vents, a fewtypes of bacteria break down chemicals to producefood energy. This process is calledchemosynthesis (Figure below).

Figure 18.5 Tube worms have a symbiotic relationship

with chemosynthetic bacteria. The bacteriaprovide the worms with food and the worm tubesprovide the bacteria with shelter.

There are many types of consumers.

Herbivores eat producers directly (Figurebelow). These animals break down the plantstructures to get the materials and energy they need.Many other consumers eat animals. These areknown as carnivores. Carnivores can eatherbivores or they can eat other carnivores.Omnivores eat plants and animals, as well asfungi, bacteria and organisms from the otherkingdoms.

Figure 18.6 Deer are herbivores.There are many types of feeding relationships

between organisms. A predator is an animal thatkills and eats another animal (Figure below). The

animal it kills is its prey.

Figure 18.7 This South China Tiger is a predator.Scavengers are animals that eat organisms that

are already dead. Vultures and hyenas are just twotypes of scavengers. Decomposers break apartdead organisms or the waste material of livingorganisms, returning the nutrients to the ecosystem.Many decomposers are bacteria, but there areothers as well, including fungi (Figure below).Decomposers are recyclers; they make nutrientsfrom dead organisms available for livingorganisms.

Figure 18.8 Fungi are decomposing this tree.

Flow of Energy in Ecosystems

Energy cannot be created or destroyed. Energycan only be changed from one form to another. This

is such a fundamental law in nature that it has itsown name: The Law of Conservation of Energy.Plants do not create chemical energy from nothing.Instead, they create chemical energy from abioticfactors that include sunlight. So they transformsolar energy into chemical energy. Organisms thatuse chemosynthesis start with chemical energy tocreate usable chemical energy. After the producerscreate the food energy, it is then passed on toconsumers, scavengers, and decomposers.

Energy flows through an ecosystem in only onedirection. Energy enters the ecosystem with theproducers. In nearly all ecosystems, sunlight is theoriginal energy source. This energy is passed fromorganisms at one trophic level or energy level, toorganisms in the next trophic level. Producers arealways the first trophic level, herbivores thesecond, the carnivores that eat herbivores the third,and so on.

An average of 90% of the energy that reaches atrophic level is used to power the organisms at thattrophic level. They need it for locomotion, heatingthemselves, and reproduction. So animals at the

second trophic level have only about 10% as muchenergy available to them as do organisms at thefirst trophic level. They use about 90% of whatthey receive, and so those at the third level haveonly 10% as much available to them as those at thesecond level. This 10% rule continues up thetrophic levels, so much less energy is available atthe next higher trophic level in an ecosystem.

The set of organisms that pass energy from onetrophic level to the next is described as the foodchain (Figure below). In this simple depiction, allorganisms eat at only one trophic level. Animals atthe 3rd trophic level only eat from the 2nd trophiclevel and those at the 2nd eat only from the 1st. Butmany omnivores feed at more than one trophiclevel, with plants and animals in their diets.

Figure 18.9 A simple food chain from a Swedish lake. Not

pictured: algae eaten by the shrimp; Shrimp areeaten by a small fish, a bleak, which is eaten by aperch, which is eaten by a northern pike, which iseaten by an osprey.

Since only 10% of the energy is passed up thefood chain, each level can support fewerorganisms. A top predator, like a jaguar, must havea very large range in which to hunt so that it can getenough energy to live. Top carnivores are quiterare relative to herbivores for this reason. Theresult of this is that the number of organisms ateach trophic level looks like a pyramid. There aremany more organisms at the base of the pyramid, atthe lower trophic levels than at the top of thepyramid, the higher trophic levels.

Food chains usually have only four or fivetrophic levels because there is not enough energyto support organisms in a sixth trophic level. Foodchains of ocean animals are longer than those ofland-based animals because ocean conditions aremore stable. Organisms at higher trophic levels

also tend to be larger than those at lower levels.The reason for this is simple: a whale must be ableto eat a plankton, but the plankton does not have tobe able to eat the whale. Sometimes multiplesmaller predators will act together to take down alarger prey, so the organisms at the higher level aresmaller than those at the lower level. This is trueof a pack of wolves, which acts together as one tohunt a moose.

Since some organisms feed at more than onetrophic level, the food chain does not adequatelydescribe the passage of energy in an ecosystem.The more accurate representation is a food web(Figure below). A food web recognizes that manyorganisms eat at multiple trophic levels. A foodweb includes the relationships between producers,consumers and decomposers.

Figure 18.10 An arctic food web. Besides the living

organisms, some abiotic components (nitrogen,mineral salts) and nonliving parts (dung) areincluded.

All organisms depend on two global foodwebs that are interconnected. The base of one isphytoplankton, microscopic ocean producers.These tiny organisms are eaten by zooplankton.The zooplankton are tiny animals which in turn areeaten by small fish and then larger fish. Land plantsform the base of the second food web. They areeaten by herbivores, that are eaten by carnivoresand so on. Birds or bears that live on land may eat

fish, which connects the two food webs. Humansare an important part of both of these food webs;we are at the top of a food web since nothing eatsus. That means that we are top predators.

Flow of Matter in Ecosystems

The flow of matter in an ecosystem is not likeenergy flow. Matter can enter an ecosystem at anylevel and can leave at any level. It cycles freelybetween trophic levels and between the ecosystemand the physical environment. Nutrients are ionsthat are crucial to the growth of living organisms.Nutrients, like nitrogen and phosphorous, areimportant for plant cell growth. Animals use silicaand calcium to build shells and skeletons. Cellsneed nitrates and phosphates to create proteins andother biochemicals. From nutrients, organismsmake tissues and complex molecules likecarbohydrates, lipids, proteins and nucleic acids.

Nutrients may enter an ecosystem from thebreakdown of rocks and minerals. They enter the

soil and are taken up by plants. Nutrients can bebrought in from other regions, perhaps carried to alake by a stream. When one organism eats anotherorganism, it receives all of its nutrients. Nutrientscan also cycle out of an ecosystem. Decayingleaves may be transported out of an ecosystem by astream. Nutrients can blow out of an ecosystem onthe wind.

Decomposers play a key role in makingnutrients available to organisms. After scavengerseat dead organisms, they almost always leavesome parts of the dead animal or plant behind.Decomposers complete the process of breakingdown dead organisms. They convert deadorganisms into nutrients and carbon dioxide, whichthey respire into the air. These left over nutrientsare then available for other organisms to use.Without decomposers, life on Earth would not beable to continue. Dead tissue would remain as it isand eventually nutrients would run out.Decomposers break apart tissue and return thenutrients to the ground. Without decomposers, lifeon earth would have died out long ago.

Relationships Between Species

Species have different types of relationshipswith each other. Competition occurs betweenspecies that are trying to use the same resources.When there is too much competition, one speciesmay move or adapt so that it uses slightly differentresources. It may live at the tops of trees and eatleaves that are somewhat higher on bushes, forexample. If the competition does not end, onespecies will die out. Each niche can only beinhabited by one species.

Some relationships between species arebeneficial to at least one of the two interactingspecies. These relationships are known assymbiosis and there are three types. In mutualism,the relationship benefits both species (Figurebelow). Most plant-pollinator relationships aremutually beneficial. The pollinator, such as ahummingbird, gets food. The plant get its pollencaught in the bird’s feathers, so that pollen isspread to far away flowers helping them

reproduce.

Figure 18.11 This hummingbird and flower each benefit

from the mutualism of their relationship.I n commensalism, the relationship is

beneficial to one species, but does not harm orhelp the other (Figure below). A bird may build anest in a hole in a tree. This neither harms norbenefits the tree, but it provides the bird and itsyoung with protection.

Figure 18.12 The relationship between these barnacles and

the humpback whale is an example ofcommensalism. The barnacles receive protectionand get to move to new locations and the whale isnot harmed.

In parasitism, the parasite species benefits andthe host is harmed (Figure below). Parasites donot usually kill their hosts because a dead host isno longer useful to the parasite. A visible exampleof parasite and host is mistletoe on an oak tree.The mistletoe gains water and nutrients through aroot that it sends into the tree’s branch. The tree isthen supporting the mistletoe, but the tree is notkilled, even though its growth and reproduction are

slightly harmed by the parasite. Humans can hostparasites, like the flatworms that causeschistosomiasis.

Figure 18.13 These tiny mites are parasitic on a harvestman.

Lesson Summary

Each species fills a niche within anecosystem. Each ecosystem has the same

niches, although the same species doesn’talways fill them.Each ecosystem has producers, consumers,and decomposers. Decomposers break downdead tissue to make nutrients available forliving organisms.Energy is lost at each trophic level, so toppredators are scarce. Feeding relationshipsare much more complicated than a food chain,since some organisms eat from multipletrophic levels.As a result, food webs are needed to show allthe predator/prey interactions in anecosystem.

Review Questions

1. What is the difference between a population,a community and an ecosystem?

2. What is the difference between a niche and ahabitat?

3. Why are the roles in different ecosystems the

same but the species that fill them oftendifferent?

4. Why are there no producers in the deep seaecosystem? Without producers, where doesthe energy come from? What is the ultimatesource of the energy?

5. Is a predator an herbivore, carnivore oromnivore? How about a prey?

6. Biologists have been known to say thatbacteria are the most important living thingson the planet. Why would this be true?

7. Why are you so much more likely to see arabbit than a lion when you’re out on a hike?

8. How much energy is available to organismson the 5th trophic level compared with thoseon the 1st? How does this determine how longa food chain can be?

9. Why is a food web a better representation ofthe feeding relationships of organisms than afood chain?

10. Why is energy only transferred in one way inan ecosystem, but nutrients cycle around?

11. Why does a predator kill its prey but a

parasite rarely kills its host?

Vocabulary

abioticNonliving features of an ecosystem includespace, nutrients, air, and water.

bioticLiving features of an ecosystem includeviruses, plants, animals, and bacteria.

carnivoreAnimals that only eat other animals for food.

chemosynthesisThe creation of food energy by breaking downchemicals.

commensalismA relationship between two species in whichone species benefits and the other species isnot harmed.

communityAll of the living creatures of an ecosystem; allof the populations of all of the species thatlive together.

competitionA rivalry between two species, or individualsof the same species, for the same resources.

consumerAn organism that does not create its ownchemical energy, but uses other organisms forfood.

decomposerAn organism that breaks down the tissues of adead organism into its various components,including nutrients that can be used by otherorganisms.

ecosystemAll of the living things in a region and thephysical and chemical factors that they need

to live.

food chainAn energy pathway that includes allorganisms that are linked as they pass alongfood energy, beginning with a producer andmoving on to consumers.

food webInterwoven food chains that show eachorganism eating from different trophic levels,which more closely reflects reality.

habitatWhere an organism lives; habitats havedistinctive features like climate or resourceavailability.

herbivoreAn animal that only eats producers.

invertebrateAnimals without backbones.

mutualismA symbiotic relationship between twospecies in which both species benefit.

nicheAn organism’s “job” within its community.

omnivoreAn organism that consumes both plants(producers) and other consumers (animals)for food.

parasitismA symbiotic relationship between twospecies in which there is a parasite and ahost. The parasite gains nutrition from thehost. The host in a parasitic relationship isharmed but usually not killed.

photosynthesisThe process in which plants use carbondioxide and water to produce sugar andoxygen: 6CO2 + 12H2O + solar energy

C6H12O6 + 6O2 + 6H2O.

populationAll the individuals of a species that occurtogether in a given place and time.

predatorAn animal that kills and eats other animals.

preyAn animal that could be killed and eaten by apredator.

producerAn organism that creates chemical energy tobe used as food. Most producers usephotosynthesis but a very small number usechemosynthesis.

scavengerAnimals that eat animals that are alreadydead.

speciesA classification of organisms that includesthose that can or do interbreed and producefertile offspring; members of a species sharethe same gene pool.

symbiosisRelationships between two species in whichat least one species benefits.

trophic levelEnergy levels within a food chain or foodweb.

vertebrateAnimals with backbones.

Points to Consider

What happens if two species attempt to fillthe same niche?There is at least one exception to the rule thateach ecosystem has producers, consumers and

decomposers. Excluding hydrothermal vent,what does the deep sea ecosystem lack?Where do humans fit into a food web?Most humans are omnivores, but a lot of whatwe eat is at a high trophic level. Sinceecosystems typically can support only a fewtop predators relative to the number or lowerorganisms, why are there so many people?

The Carbon Cycle and theNitrogen Cycle

Lesson Objectives

Describe the short term cycling of carbonthrough the processes of photosynthesis andrespiration.Identify carbon sinks and carbon sources.Describe short term and long term storage ofcarbon.Describe how human actions interfere withthe natural carbon cycle.Describe the nitrogen cycle.

Introduction

Carbon is a very important element. It is not themost abundant element in the universe or even onthe Earth, but it is the second most common

element in the human body. You could not livewithout carbon. If something you eat has protein orcarbohydrates or fats, then it contains carbon.When your body breaks down that food to produceenergy, you breathe out carbon dioxide. Carbon isalso a very important element on Earth. Carbon isprovided by the environment, moves throughorganisms and then returns to the environmentagain. When all this happens in balance, theecosystem remains in balance too. In this section,let’s follow the path of a carbon atom over manyyears and see what happens.

Nitrogen is also a very important element.Nitrogen must be converted to a useful form so thatplants can grow. Without "fixed" nitrogen, plantsand therefore animals could not exist as we knowthem.

Short Term Cycling of Carbon

The short term cycling of carbon begins withcarbon dioxide and the process of photosynthesis.

Our atmosphere is mostly made of nitrogen andoxygen, but there is a small amount of carbondioxide in the air too. Plants and algae use thiscarbon dioxide, along with water and energy fromsunlight to produce their own food. This is a littlemiracle that is happening everywhere around youeach and every day. Plants and algae have theability to take the inorganic carbon in carbondioxide and make it into organic carbon, which isfood. That is something that we cannot do at all!Imagine the difference between what wouldhappen if you tried to eat a piece of coral or a shelland what happens when we eat sugar. We can’t getenergy from the bits of rock at all, but you knowhow quickly sugar can be used for energy in ourbodies.

Through photosynthesis, carbon dioxide pluswater and energy from sunlight is transformed intofood with oxygen given off as a waste product.Chemists write shorthand equations for differenttypes of chemical reactions. The equation forphotosynthesis looks like this (Figure below):

Figure 18.14 The amazing transformation that has happened

here is changing energy from sunlight into chemicalenergy that plants and animals can use as food(Figure below).

Figure 18.15 This diagram of the carbon cycle shows some

of the places a carbon atom might be found. Theblack numbers indicate how much carbon is storedin various reservoirs, in billions of tons ("GtC"stands for gigatons of carbon; figures are circa2004). The purple numbers indicate how muchcarbon moves between reservoirs each year. Thesediments, as defined in this diagram, do not

include the ~70 million GtC of carbonate rock andkerogen.

Carbon Can Also Cycle in the LongTerm

As described above, an individual carbon atomcould cycle very quickly if the plant takes incarbon dioxide to make food and then is eaten byan animal, which in turn breathes out carbondioxide. Carbon might also be stored as chemicalenergy in the cells of the plant or the animal. If thishappens, the carbon will stay stored as part of theorganic material that makes up the plant or animaluntil it dies. Some of the time, when a plant oranimal dies, it decomposes and the carbon isreleased back into the environment. Other times,the organic material of the organism is buried andtransformed over millions of years into coal, oil,or natural gas. When this happens, it can takemillions of years before the carbon becomesavailable again.

Another way that carbon is stored for longperiods of time happens when carbon is used byocean organisms. Many ocean creatures usecalcium carbonate (CaCO3) to make their shells orto make the reef material where coral animals live.When algae die, their organic material becomespart of the ocean sediments, which may stay at thebottom of the ocean for many, many years. Overmillions of years, those same ocean sediments canbe forced down into the mantle when oceanic crustis consumed in deep ocean trenches. As the oceansediments melt and form magma, carbon dioxide iseventually released when volcanoes erupt.

Carbon Sinks and Carbon Sources

We can think of different areas of theecosystem that use and give back carbon as carbonsources and carbon sinks. Carbon sources areplaces where carbon enters into the environmentand is available to be used by organisms. Onesource of available carbon in the environment

happens when an animal breathes out carbondioxide. So carbon dioxide added to ouratmosphere through the process of respiration is acarbon source. Carbon sinks are places wherecarbon is stored because more carbon dioxide isabsorbed than is emitted. Healthy living forestsand our oceans act as carbon sinks.

In the natural situation, the amount of carbondioxide in the atmosphere is very low. This meansthat we can quickly change the amount of carbondioxide in our atmosphere. Scientists can use datafrom air bubbles trapped in the ice of glaciers todetermine what the natural level of carbon dioxidewas before the Industrial Revolution, when humansbegan to use lots of fossil fuels. Measurements ofthe different gases in the air bubbles tell us that thenatural level of carbon dioxide was about 280parts per million. Today the amount of carbondioxide in our atmosphere is 388 parts per millionand that amount continues to rise every year.Scientists have been making measurements in themiddle of the Pacific Ocean, far from any largeland areas for fifty years. The graph (Figure

below) of this data shows that the amount ofcarbon dioxide has been steadily increasing everyyear.

Figure 18.16 The Keeling curve of atmospheric CO

concentrations measured at Mauna LoaObservatory.

Human Actions Impact the CarbonCycle

Humans have changed the natural balance ofthe carbon cycle because we use coal, oil, andnatural gas to supply our energy demands.

Remember that in the natural cycle, the carbon thatmakes up coal, oil, and natural gas would bestored for millions of years. When we burn coal,oil, or natural gas, we release the stored carbon inthe process of combustion. That means thatcombustion of fossil fuels is also a carbon source.

The equation for combustion of propane, whichis a simple hydrocarbon looks like this (Figurebelow):

Figure 18.17 The equation shows that when propane burns,

it uses oxygen and the result is carbon dioxide andwater. So each time we burn a fossil fuel, weincrease the amount of carbon dioxide in theatmosphere. Another way that carbon dioxide isbeing added to our atmosphere is through thecutting down of trees, called deforestation(Figure below). Trees are very large plants, whichnaturally use carbon dioxide while they are alive.When we cut down trees, we lose their ability toabsorb carbon dioxide and we also add the carbonthat was stored in the tree into the environment.

Healthy living forests act as a carbon sink, butwhen we cut them down, they are a carbon source.

Figure 18.18 This forest in Mexico has been cut down and

burned to clear forested land for agriculture.Coal, oil, and natural gas as well as calcium

carbonate rocks and ocean sediments are long termcarbon sinks for the natural cycling of carbon.When humans extract and use these resources,combustion makes them into carbon sources.

Why Do We Need to Know Aboutthe Carbon Cycle?

You may wonder why scientists study thecarbon cycle or why we would be concerned aboutsuch small amounts of carbon dioxide in ouratmosphere. Carbon dioxide is a greenhouse gas(Figure below). Different gases in our atmosphereabsorb infrared energy, the longer wavelengths ofthe Sun’s reflected rays. These gases hold ontoheat energy that would otherwise radiate out intospace. As the heat is held in our atmosphere, itwarms the Earth. This is just like what happens ina greenhouse. The glass that makes up thegreenhouse holds in heat that would otherwiseradiate out.

Figure 18.19 This diagram explains the role of greenhouse

gases in our atmosphere.When our atmosphere holds onto more heat

than it would in the natural situation, it producesglobal warming. As our Earth continues to warm,there are many potential consequences. Onepossibility is that the current weather patterns willchange. With rain falling in different areas, wewon’t be able to grow crops in the same regionswhich will impact our ability to grow food.Another possibility is that our polar ice caps willmelt. We can already see this happening today.Glaciers all over the world are retreating as theymelt away. Another possible consequence is thatsome species of plants and animals could becomeextinct. Polar bears have recently been added tothe endangered species list as threatened becausethey need sea ice in order to hunt (Figure below).

Figure 18.20 Polar bears depend on sea ice for hunting.As continental glacial ice melts, this will cause

sea levels to rise, which will cause flooding oflow lying coastal areas. That would be a bigproblem because many of our biggest cities arealong coastlines.

The Nitrogen Cycle

Nitrogen (N2) is also vital for life on Earth asan essential component of organic materials.Nitrogen is found in all amino acids, proteins, andnucleic acids such as DNA and RNA. Chlorophyllmolecules in plants, which are used to create food

by photosynthesis forming the basis of the foodweb, contain nitrogen.

Although nitrogen is the most abundant gas inthe atmosphere, it is not in a form that plants canuse. To be useful, nitrogen must be “fixed,” orconverted into a more useful form. Although somenitrogen is fixed by lightning or blue-green algae,much is modified by bacteria in the soil. Thesebacteria combine the nitrogen with oxygen orhydrogen to create nitrates or ammonia.

Nitrogen fixing bacteria either live free or in asymbiotic relationship with leguminous plants(peas, beans, peanuts). The symbiotic bacteria usecarbohydrates from the plant to produce ammoniathat is useful to the plant. Plants use this fixednitrogen to build amino acids, nucleic acids (DNA,RNA) and chlorophyll. When these legumes die,the fixed nitrogen they contain fertilizes the soil.

Animals eat plant tissue and create animaltissue. After a plant or animal dies or an animalexcretes waste, bacteria and some fungi in the soilfix the organic nitrogen and return it to the soil asammonia. Nitrifying bacteria oxidize the ammonia

to nitrites, other bacteria oxide the nitrites tonitrates, which can be used by the next generationof plants. In this way, nitrogen does not need toreturn to a gas. Under conditions when there is notoxygen, some bacteria can reduce nitrates tomolecular nitrogen.

Usable nitrogen is sometimes the factor thatlimits how many organisms can grow in anecosystem. Modern agricultural practices increaseplant productivity adding nitrogen fertilizers to thesoil. This can have unintended consequences asexcess fertilizers run off the land, end up in water,and then cause nitrification of ponds, lakes andnearshore oceanic areas. Also, nitrogen fromfertilizers may return to the atmosphere as nitrousoxide or ammonia, both of which have deleteriouseffects. Nitrous oxide contributes to the breakdownof the ozone layer and ammonia contributes to

smog and acid rain.

Lesson Summary

The carbon cycle begins with the process ofphotosynthesis, which transforms inorganiccarbon into organic carbon.Our forested areas and our oceans are carbonsinks. When carbon is trapped in oceansediments, or fossil fuels, it is stored formillions of years.Humans have changed the natural carboncycle by burning fossil fuels, which releasescarbon dioxide to the atmosphere. Burning offossil fuels and deforestation are carbonsources.One potential consequence of increasedcarbon dioxide in the atmosphere is globalwarming.The nitrogen cycle begins with nitrogen gas inthe atmosphere then goes through nitrogen-fixing micro organisms to plants, animals,

decomposers and into the soil.

Review Questions

1. Describe the process of photosynthesis.2. How can carbon cycle very quickly back into

the environment?3. Name two ways that carbon is stored for a

very long time in the natural cycle.4. Describe what makes a carbon sink and what

makes a carbon source and give an exampleof each.

5. Name two ways that humans interfere with thenatural carbon cycle.

6. Do we need carbon dioxide in ouratmosphere?

7. Is global warming something that couldimpact you in your lifetime?

Vocabulary

algae

Photosynthetic organisms in the ocean;includes one celled organisms and seaweeds.

carbohydratesAn organic compound that supplies energy tothe body; includes sugars, starches andcellulose.

carbon sinkAn area of an ecosystem that absorbs morecarbon dioxide than it produces.

carbon sourceAn area of an ecosystem that emits morecarbon dioxide than it absorbs.

deforestationCutting down and/or burning trees in aforested area.

greenhouse gasGases like carbon dioxide that absorb andhold heat from the sun’s infrared radiation.

global warmingWarming of the Earth brought about by addingadditional greenhouse gases to theatmosphere.

hydrocarbonAn organic compound that contains onlyhydrogen and carbon.

photosynthesisThe process using carbon dioxide, water, andenergy from sunlight by which plants andalgae produce their own food.

Human Populations

Lesson Objectives

Describe how changes in a limiting factor canalter the carrying capacity of a habitat.Discuss how humans have increased thecarrying capacity of Earth for our species andhow we may have exceeded it.Discuss how human activities like agricultureand urbanization have impacted the planet.Describe what sustainable development is.

Introduction

Improvements in agriculture, sanitation, andmedical care have enabled the human population togrow enormously in the last few 100 years. As thepopulation grows, consumption, waste, and theoveruse of resources also grow. People arebeginning to discuss and carry out sustainable

development that decreases the impact humanshave on the planet.

Populations

The population size of a species depends onthe biotic and abiotic factors present in thatecosystem. Biotic factors include the amount offood that is available to that species and thenumber of organisms that use that species as food.For life to thrive, a specific amount of abioticfactors are necessary. For example, too little watermay cause land plants or animals to becomedehydrated. Too much water, however, may causedrowning.

A population grows when the number of birthsis greater than the number of deaths. It shrinks, ifdeaths exceed births. For a population to grow,there must be ample resources and no majorproblems. A population can shrink either becauseof biotic or abiotic limits. An increase inpredators, the emergence of a new disease, or the

loss of habitat are just three possible problems thatwill decrease a population. A population may alsoshrink if it grows too large for the resourcesrequired to support it.

When the number of births equals the numberof deaths, the population is at its carryingcapacity for that habitat. In a population at itscarrying capacity, there are as many organisms ofthat species as the habitat can support. Thecarrying capacity depends on biotic and abioticfactors. If these factors improve, the carryingcapacity increases. If the factors become lessplentiful, the carrying capacity drops. If resourcesare being used faster than they are beingreplenished, then the species has exceeded itscarrying capacity. If this occurs, the populationwill then decrease in size.

Every stable population has one or morefactors that limit its growth. A limiting factordetermines the carrying capacity for a species. Alimiting factor can be any biotic or abiotic factor: anutrient, space, and water availability areexamples. The size of a population is tied to its

limiting factor. If the limiting factor decreases, thepopulation decreases. If the limiting factorincreases, the population increases. If a limitingfactor increases a lot, another factor will mostlikely become the new limiting factor.

This may be a bit confusing so let’s look at anexample of limiting factors. Say you want to makeas many chocolate chip cookies as you can with theingredients you have on hand. It turns out that youhave plenty of flour and other ingredients, but onlytwo eggs. You can make only one batch of cookies,because eggs are the limiting factor. But then yourneighbor comes over with a dozen eggs. Now youhave enough eggs for seven batches of cookies, andenough other ingredients but only two pounds ofbutter. You can make four batches of cookies, withbutter as the limiting factor. If you get more butter,something else will be limiting.

Species ordinarily produce more offspring thantheir habitat can support. If conditions improve,more young survive and the population grows. Ifconditions worsen, or if too many young are born,there is competition between individuals. As in

any competition, there are some winners and somelosers. Those individuals that survive to fill theavailable spots in the niche are those that are themost fit for their habitat.

Human Population Growth

Human population growth over the past 10,000years has been tremendous (Figure below). Thehuman population was about 5 million in 8000B.C., 300 million in A.D. 1, 1 billion in 1802, 3billion in 1961, and 6.7 billion in 2008. As thehuman population continues to grow, differentfactors may emerge limiting human population indifferent parts of the world. Space may be alimiting factor or having enough clean air, cleanwater or food to feed everyone are concerns weare already facing.

Figure 18.21 Human population from 10,000 BC through

2000 AD showing the exponential increase inhuman population that has occurred in the last fewcenturies.

Not only has the population increased, but therate of growth has also increased (Figure below).It took all of human history for the population toreach the first 1 billion people, in around 1802.The second billion was added 125 years later, in1927. It took 33 years for there to be 3 billionpeople in 1960, and only 15 years for there to be 4billion people in 1975. Another billion was addedby 1987, just twelve years later, and it took onlyanother twelve years for the population to reach 6billion people in 1999. Estimates are that thepopulation will reach 7 billion in 2012, 13 years

after reaching 6 billion.

Figure 18.22 The amount of time between the addition of

each one billion people to the planets populationincluding speculation about the future.

Although population continues to grow rapidly,the rate of growth has declined. Still, it is likelythat there will be between 9 and 10 billion peoplesharing this planet by the middle of the century.The total added will be about 2.5 billion people,which is more than were even in existence asrecently as 1950.

With so many more people on the planet thanever before, we must ask whether humans now areexceeding Earth’s carrying capacity for ourspecies. Many anthropologists say that the carryingcapacity of humans on the planet without

agriculture is about 10 million. This populationwas reached about 10,000 years ago. At the time,people lived together in small bands of hunters andgatherers. Commonly women gathered nuts andvegetables and men hunted animals and fished.People within a band shared their resources.Although they had trading networks with outsidegroups, trading was limited by what couldreasonably be carried. For the most part, peoplerelied on the resources that they could find wherethey lived.

As you can see, human populations have blownpast this hypothetical carrying capacity. By usingour brains, our erect posture, and our hands, wehave been able to do things that no other specieshas ever done. About 10,000 years ago, wedeveloped the ability to grow our own food.Farming allowed us to grow the plants we wantedto eat and to have food available year-round. Wedomesticated animals to have meat when wewanted. With agriculture, people could settledown, so that they no longer needed to carry alltheir possessions. They could develop better

farming practices and store food for when it wasdifficult to grow. Agriculture allowed people tosettle in towns and cities. Early farmers couldgrow only enough food for their families, withperhaps a bit extra to sell, barter, or trade. Moreadvanced farming practices allowed a singlefarmer to grow food for many more people. Beingfreed from having to gather or grow food allowedpeople to do other types of work.

The next major stage in the growth of thehuman population was the Industrial Revolution,which started in the late 1700’s. Increasedefficiency in farming freed up large numbers ofpeople available to work in factories. This majorhistorical event marks when products were firstmass produced and when fossil fuels were firstwidely used for power.

Every major advance in agriculture allowedglobal population to increase. Irrigation, the abilityto clear large swaths of land for farmingefficiently, and the development of farm machinespowered by fossil fuels allowed people to growmore food and transport it to where it was needed.

Currently about 70% of the world’s fresh water isused for agriculture.

The biggest advance in agriculture in recentdecades is called the Green Revolution. It is thisadvance that has allowed the population to growso rapidly. The first focus of the Green Revolutionwas to improve crops. A tremendous increase inthe use of artificial fertilizers, nutrients that helpplants to grow and chemical pesticides, chemicalsthat kill pests followed. About 23 times morefertilizer and 50 times more pesticides are usedaround the world than just 50 years ago. Mostagricultural work is now done by machines:plowing, tilling, fertilizing, picking, andtransporting (Figure below). About 17% of theenergy used each year in the US is for agriculture.

Figure 18.23 Rows of a single crop and heavy machinery are

normal sights for modern day farms.The Green Revolution has increased the

productivity of farms immensely. A century ago, asingle farmer produced enough food for 2.5people, but now a farmer can feed more than 130people. Due to this increased productivity, theGreen Revolution is credited for feeding 1 billionpeople that would not otherwise have been able tolive.

The flip side of this is that for the population tocontinue to grow, more advances in agriculturewill be needed. We’ve increased the carrying

capacity for humans by our genius: growing crops,trading for needed materials, and designing waysto exploit resources that are difficult to get at, likegroundwater. The question is, even though we haveincreased the carrying capacity of the planet, havewe now exceeded it? Are humans on Earthexperiencing overpopulation?

There are many different opinions about humanpopulation growth. In the eighteenth century,Thomas Malthus predicted that human populationwould continue to grow until we had exhausted ourresources. At that point, humans would becomevictims of famine, disease or war. Some scientiststhink that the carrying capacity of the planet isaround 1 billion people, not the almost 7 billionpeople we have today. How did we get to wherewe are today? Many of our limiting factors havechanged as we have used our intelligence andtechnology to expand our resources. Can wecontinue to do this into the future? Do we nowhave more people and more impacts on ourenvironment than the Earth can handle?

Humans and the Environment

Along with the increases in food that havecome from the Green Revolution have comeenormous impacts on the planet. More food hasallowed the human population to explode. Naturallandscapes have been altered to create farmlandand cities. Already, half of the ice free lands havebeen converted to human uses. Estimates are thatby 2030, that number will be more than 70%.Forests and other landscapes have been cleared forfarming or urban areas. Rivers have been dammedand the water is transported by canals forirrigation and domestic uses. Ecologicallysensitive areas have been altered: wetlands arenow drained and coastlines are developed.

Modern agricultural practices produce a lot ofpollution (Figure below). Some pesticides aretoxic. Fertilizers drain off farmland and introducenutrients into lakes and coastal areas, causing fishto die. Farm machines and vehicles used totransport crops produce air pollutants. Pollutants

enter the air, water, or are spilled onto the land.Moreover, many types of pollution easily movesbetween air, water, and land. As a result, nolocation or organism—not even polar bears in theremote Arctic—is free from pollution.

Figure 18.24

Pesticides are hazardous in large quantities andsome are toxic in small quantities.

The increased numbers of people have otherimpacts on the planet. Humans do not just needfood. They also need clean water, secure shelter,and a safe place for their wastes. These needs aremet to different degrees in different nations andamong different socioeconomic classes of people.For example, about 1.2 billion of the world’speople do not have enough clean water fordrinking and washing each day (Figure below).

Figure 18.25 The percentage of people in the world that live

in abject poverty is decreasing somewhat globally,but increasing in some regions, like Sub-SaharanAfrica.

A large percentage of people expect muchmore than to have their basic needs met. For aboutone-quarter of people, there is an abundance offood, plenty of water, and a secure home.Comfortable temperatures are made possible byheating and cooling systems, rapid transportation isavailable by motor vehicles or a well developedpublic transportation system, instantcommunication takes place by phones and email,and many other luxuries are available that were noteven dreamed of only a few decades ago. All ofthese need resources to produce and fossil fuels topower. Their production, use and disposal allproduce wastes. Many people refer to theabundance of luxury items in these people’s livesa s over-consumption. People in developednations use 32 times more resources than people inthe developing countries of the world.

There are many problems worldwide thatresult from overpopulation and over-consumption.

One such problem is the advance of farms andcities into wild lands, which diminishes the habitatof many organisms. In addition, water also must betransported for irrigation and domestic uses. Thismeans building dams on rivers or drilling wells topump groundwater. Large numbers of people livingtogether need effective sanitation systems. Manydeveloping countries do not have the resources toprovide all of their citizens with clean water. It isnot uncommon for some of these children to die ofdiseases related to poor sanitation. Improvingsanitation in many different areas—sewers,landfills, and safe food handling—are important toprevent disease from spreading.

Wildlife is threatened by fishing, hunting andtrading as population increases. Besides losingtheir habitat as land is transformed, organisms arethreatened by hunting and fishing as humanpopulation grows. Hunting is highly regulated indeveloped nations, but many developing nationsare losing many native animals due to hunting.Wild fish are being caught at too high a rate andmany ocean fish stocks are in peril.

Humans also cause problems with ecosystemswhen they introduce species that do not belong in aha b i t a t . Invasive species are sometimesintroduced purposefully, but often they arrive byaccident like rats on a ship. Invasive species oftenhave major impacts in their new environments. Asad example is the Australian Brown Tree Snakethat has wiped out 9 of the 13 native species on theisland of Guam (Figure below).

Figure 18.26 An Australian Brown Tree Snake perched on a

post in Guam.Pollution is a by-product of agriculture,

urbanization, and the production and consumption

of goods. Global warming is the result of fossilfuel burning.

Let’s return to the question of whether humanshave exceeded Earth’s carrying capacity for ourspecies. Carrying capacity is exceeded ifresources are being used faster than they are beingreplenished. It is also exceeded if the environmentis being damaged.

The answer to our original question thereforeappears to be yes. Many resources are being usedfar in excess of the rate at which they are beingreplaced. The best farmland is already in use andmore marginal lands are being developed. Mostrivers in the developed nations and many indeveloping nations are already dammed.Groundwater is being used far more rapidly than itis being replaced. The same is true for fossil fuelsand many mineral resources. Forests are beingchopped down; and wild fish are beingoverharvested. Human have caused the rate ofextinction of wild species to increase to about atleast 100 times the normal extinction rate.

In addition, the stability of the environment

decreases as landscapes are transformed,organisms die out and the planet becomes polluted.Although many more people are alive in the worldthan ever before, many of these people do not havesecure lives. Many people in the world live inpoverty, with barely enough to eat. They often donot have safe water for drinking and bathing.Diseases kill many of the world’s children beforethey reach five years of age.

Sustainable Development

A topic generating a great deal of discussionthese days is sustainable development. This isdevelopment that attempts to help people out ofpoverty, while protecting the environment, withoutusing natural resources faster than they can bereplaced. It is development that allows people touse resources no faster than the rate at which theyare regenerated.

One of the most important steps to achieving amore sustainable future is to reduce humanpopulation growth. This has been happening in

recent years. Studies have shown that the birth ratedecreases as women become educated. Educatedwomen tend to have fewer and healthier children.

Science can be an important part of sustainabledevelopment. When scientists understand howEarth’s natural systems work, they can recognizehow people are impacting them. Scientists canwork to develop technologies that can be used tosolve problems wisely. An example of a practicethat can aid sustainable development is fishfarming, as long as it is done in environmentallysound ways. Engineers can develop cleaner energysources to reduce pollution and greenhouse gasemissions.

Citizens can change their behavior to reducethe impact they have on the planet by demandingproducts that are produced sustainably. Whenforests are logged, new trees should be planted.Mining should be done so that the landscape is notdestroyed. People can consume less and think moreabout the impacts of what they do consume.

Lesson Summary

Populations of organisms are kept to ahabitat’s carrying capacity by factors thatlimit their growth.By developing agriculture and othertechnologies, the human population has grownwell past any natural population limits.Many people on Earth live in poverty, withoutenough food or clean water or shelter.Overpopulation and over-consumption arecausing resources to be overused and muchpollution to be generated.Society must choose development that is moresustainable, in order to secure a long termfuture for our species and the other speciesthat we share the planet with.

Review Questions

1. If phosphorous is limiting to a species in anecosystem and the amount of phosphorous is

increased, what will happen to the populationof that species? What will happen to thecarrying capacity?

2. Name some factors that could cause apopulation to increase. Try to include as manytypes of factors as possible.

3. In terms of numbers of births and deaths,explain in detail why you think humanpopulation is growing so tremendously?

4. If all people on Earth were allowed only toreplace themselves (that is, each person couldonly have one child or each couple twochildren), what would happen to the planet’spopulation in the next decade? Would itdecrease, increase, or remain exactly thesame as it is now?

5. What role has agriculture played in humanpopulation and why?

6. Discuss the good and bad points about theGreen Revolution.

7. In the United States, 17% of energy is usedfor agriculture? How is this possible, if plantsphotosynthesize with sunlight?

8. What is more threatening to the future of theplanet: overpopulation or overconsumption?How does an increase in the standard ofliving for people living in poverty affect theplanet?

9. What evidence is there that humans areexceeding Earth’s carrying capacity for ourspecies?

10. What is sustainable development?

Vocabulary

carrying capacityThe number of individuals of a given speciesa particular environment can support.

Green RevolutionChanges in the way food is produced sinceWorld War II that have resulted in enormousincreases in production.

invasive speciesA species of organism that spreads in an area

where it is not native, and negatively impactsthe native vegetation. People often introduceinvasive species either purposefully or byaccident.

limiting factorThe one factor that limits the population of aregion. The limiting factor can be a nutrient,water, space, or any other biotic or abioticfactor that species need.

over-consumptionResource use that is unsustainable in the longterm; obtaining many more products thanpeople need.

overpopulationWhen the population of an area exceeds itscarrying capacity or when long-term harm isdone to resource availability or theenvironment.

pesticide

A chemical that kills a certain pest that wouldotherwise eat or harm plants that humans wantto grow.

sustainable developmentEconomic development that helps people outof poverty, use resources at a rate at whichthey can be replaced, and protects theenvironment.

Points to Consider

How much impact on the planet does an infantborn in the United States have during itslifetime, compared with one born in Senegal?How does consuming less impact globalwarming?Can ordinary people really make a differencein changing society toward more sustainableliving?

Chapter 19: Human Actionsand the Land

Loss of Soils

Lesson Objectives

Explain how human actions accelerate soilerosion.Describe ways that we can prevent soilerosion.

Introduction

Have you ever seen muddy rain or snow fallingfrom the sky? Can you imagine what it might belike if the water that came down as rain and snowwas muddy and brown? In May 1934, a huge windstorm picked up and blew away massive amounts

of topsoil from the Central United States (Figurebelow). The wind carried the soil eastward toChicago. Some of the soil then fell down to theground like a snowstorm made of mud. The rest ofit continued blowing eastward, and reached all theway to New York and Washington, D.C. Thatwinter, states like New York and Vermont actuallyhad red snow because of all the dusty soil in theair.

Figure 19.1 Soil loss from the dust storms of 1934 and

1935 came mostly from the states shown here ingreen in the Central United States. The soil blewall the way to the east coast of the United States.

A little less than one year later, in April 1935,

another such storm happened (Figure below). Itwas called a Black Blizzard. It made the day turndark as night; people could not see right in front ofthem because of all the soil blown up by the windstorm. The storm caused tremendous damage andled to many people leaving the central UnitedStates to find other places to live. Many peoplebecame sick from breathing the soil in the air.

Figure 19.2 This wind storm blew huge amounts of soil

into the air in Texas on April 14, 1935.These storms are sometimes called the Dust

Bowl storms. They continued on and off until about1940. They are extreme examples of soil erosion,

which is the process of moving soil from one placeto another. Soil erosion is a serious problembecause it takes away a valuable resource that weneed to grow food. Several factors contributed tothe Dust Bowl storms. First, farmers in the CentralUnited States had plowed grasslands there to growfood crops. They left the crop fields bare in thewinter months. This left the soil exposed to wind.Secondly, a long drought in the 1930’s left theexposed soil especially dry. When the springwinds began blowing, the dry exposed soil waseasily picked up and blown away.

We learned many lessons from the Dust Bowlstorms. Today, we encourage farming practicesthat keep the soil covered even during the winter,so that it is not exposed and vulnerable to erosion.We have also learned of ways to prevent erosionin cities and towns as well as on farmlands. In thislesson, you will learn about some human activitiesthat lead to erosion. You will then learn some ofthe specific ways we can prevent soil erosion.

Causes of Soil Erosion

Soil erosion occurs when water, wind, ice orgravity moves soil from one place to another.Running water is the leading cause of erosion,since it can easily take soil with it as the waterflows downhill or moves across the land. Wind isthe next leading cause of erosion. Just as in theDust Bowl storms of the 1930’s, wind can blowsoil many hundreds of kilometers away. Soil isespecially vulnerable to erosion if it is bare orexposed. Plants therefore serve a tremendous rolein preventing soil erosion. If the soil is coveredwith plants, erosion is slowed down. But whensoil is bare, the rate of erosion speeds uptremendously. What are some human activities thatleave the soil exposed and speed up erosion? Wespeed up erosion through the following actions:

AgricultureGrazing animalsLogging and miningConstruction

Recreational activities, like driving vehiclesoff-road or hiking

Agriculture, is probably the most significanthuman action that accelerates, or speeds up,erosion (Figure below). We first plow the land toplant fields of crops. This takes away the naturalvegetative cover of an area and replaces it withrows of crop plants mixed with bare areas. It alsocreates an area where there may not be anythinggrowing in the winter, because in most areas, foodcrops only grow in the spring and summer. Thebare areas of a field are very susceptible toerosion. Without anything growing on them, thesoil is easily picked up and carried away. Thefields also experience more erosion in the winter ifno plants are growing on them and they are just leftas bare soil. In addition, farmers sometimes makedeep grooves in the land with their tractor tires.These grooves act like small channels that giverunning water a path. This speeds up erosion fromwater.

Figure 19.3 The bare areas of farmland are especially

vulnerable to erosion.Some parts of the world use an agricultural

practice called slash and burn. This involvescutting and burning forests to create fields andpastures. It is one of the worldwide leading

causes of excessive soil erosion. It is mostcommonly practiced in developing countries intropical areas of the world, as people create moreland for agriculture.

Grazing animals are animals that live on largeareas of grassland (Figure below). They wanderover the area and eat grasses and shrubs. They canremove large amounts of the plant cover for anarea. If too many animals graze the same land area,once the tips of grasses and shrubs have beeneaten, they will use their hooves to pull plants outby their roots.

Figure 19.4 Grazing animals can cause erosion if they are

allowed to overgraze and remove too much or all

of the vegetation in a pasture.When an area is logged, large areas of trees

are cut down and removed for human use (Figurebelow). When the trees are taken away, the land isleft exposed to erosion. Even more importantly,logging results in the loss of leaf litter, or deadleaves, bark, and branches on the forest floor. Leaflitter decreases because no trees are left to dropleaves or other plant parts to the ground. The leaflitter plays an important role in protecting forestsoils from erosion.

Figure 19.5 Logging exposes large areas of land to erosion.Mining is another activity that speeds up

erosion (Figure below). When we mine we aredigging in the Earth for mineral resources, likecopper or silver. The huge holes dug by miningoperations leave large amounts of ground exposed.In addition, most of the rock removed when miningis not actually the precious mineral, but tailings, orunwanted rock that is left next to the mine after thevaluable minerals are removed. These tailings areusually piled up next to a mine, and are easilyeroded downhill.

Figure 19.6 This large coal mining pit in Germany, and

other mines like it, are major causes of erosion.Constructing human buildings and roads also

causes much soil erosion. This development

involves changing forest and grassland into cities,buildings, roads, neighborhoods, and other human-made features. Any time we remove naturalvegetation, we make the soil more susceptible toerosion. In addition, features like roads,sidewalks, and parking lots do not let water runthrough them into the ground because they are hardand impermeable (Figure below). Since the watercannot enter the ground, it then runs over theground faster than usual. This can speed up watererosion.

Figure 19.7 Urban areas and parking lots result in less

water entering the ground. Therefore, more water

runs over the land and quickly forms channels thatcan speed up erosion.

Humans also cause erosion throughrecreational activities, like hiking and riding off-road vehicles. An even greater amount of erosionoccurs when people drive off-road vehicles overan area. The area eventually develops bare spotswhere no plants can grow. Erosion becomes aserious problem in these areas.

Human-caused Erosion

Some erosion is a natural process and hasalways happened on Earth. However, humanactivities like those discussed above, haveaccelerated soil erosion, which may occur about10 times faster than its natural rate. As the humanpopulation grows, we increase our impact on soilerosion. In order to support Earth's humanpopulation, we need to create more and morefarmland, we develop more areas and build morecities, and we use much more of the land for

recreation. Human population growth can lead todegradation of the natural environment.

Human impact on erosion differs throughout theworld. In developed countries like the UnitedStates, we have learned good agricultural practicesthat greatly slow down agriculture’s impact onerosion. However, we still experience mucherosion from the development of urban areas andconstruction of new cities. In developing countries,many people are very poor and just want to be ableto grow food and make a simple living. They carryout slash and burn agriculture because it quicklygives them land to grow food crops on. Poverty isa big contributing factor to environmentalproblems like soil erosion in developing countries.

Preventing Soil Erosion

Soil is a renewable, natural resource necessaryfor growing food. However, it renews itselfslowly: it can take hundreds or thousands of yearsto replenish lost soil. When we lose valuable soil,

we also lose an important natural resource. Manyof the farmers affected by the Dust Bowl storms ofthe 1930’s lost their homes because they could nolonger grow crops and earn money to live, oncetheir topsoil had all blown away. Whileagriculture can cause erosion, it is also necessaryfor human life. We have learned many goodagricultural practices that reduce erosion, insteadof speeding it up.

Table below shows some seps that we cantake to prevent erosion. Which of these things canyou do in your own personal life? Can you think ofany other steps we can take to slow down erosion?Notice that many of the things listed here involveways that we use the land. Land use alwaysrequires humans to make choices.Source ofErosion Strategies for Prevention

Leave leaf litter on the groundin the winterGrow cover crops, special

Agriculture

crops grown in the winter tocover the soilPlant tall trees around fields tobuffer the effects of windDrive tractors as little aspossibleUse drip irrigation that putssmall amounts of water in theground frequentlyAvoid watering crops withsprinklers that make big waterdrops on the groundKeep fields as flat as possibleto avoid soil eroding downhill

GrazingAnimals

Move animals throughout theyear, so they don’t consumeall the vegetation in one spotKeep animals away fromstream banks, where hills areespecially prone to erosion

Logging andMining

Reduce the amount of land thatwe log and mineReduce the number of roadsthat are built to access loggingareasAvoid logging and mining onsteep landsCut only small areas at onetime and quickly replantlogged areas with newseedlings

Development

Reduce the amount of land thatwe turn into cities, urbanareas, parking lots, etc.Keep as much “green space”in cities as possible, such asstrips of trees where plantscan growInvest in and use new

technologies for parking lotsthat make them permeable towater in order to reduce runoffof water

RecreationalActivities

Avoid using off-road vehicleson hilly landsStay on designated trails

BuildingConstruction

Avoid building on steep hillsGrade surrounding land todistribute water rather thancollecting it in one placeWhere water collects, drain tocreeks and riversLandscape with plants thatminimize erosion

Lesson Summary

Soil erosion is a natural process, but humanactivities have greatly accelerated soilerosion.We accelerate erosion through agriculture,grazing, logging and mining, development,and recreation.Soil is an important natural resourcenecessary for plant growth and should be keptsafe from erosion as much as possible.There are many ways that we can slow downor prevent erosion, but practicing theseinvolves making decisions about how we useland resources. It also requires striking abalance between economic needs and theneeds of the environment.

Review Questions

1. Many farmers harvest their crops in the falland then let the leftover plant material stay onthe ground over winter. How does this helpprevent erosion?

2. List five ways human activity has acceleratedsoil erosion.

3. How do urban areas contribute to soilerosion?

4. What is the connection between poverty andsoil erosion in developing countries?

5. What is one way you can prevent soil erosionwhen you are hiking?

6. You often see stone barriers or cage-likematerials set up along coastal shores andriver banks. How do you think these serve toprevent erosion? Why are areas like thisprone to erosion?

7. How can your own activities affect theenvironment, especially soil erosion?

8. What can we do to help solve environmentalproblems in developing countries? Whatresponsibility do you have to help solve thisproblem?

Further Reading / SupplementalLinks

People who lived during the Dust Bowl talkabout their experiences, the Ganzel Grouphttp://www.livinghistoryfarm.org/farminginthe30s/water_02.html

Video of the Dust Bowlhttp://www.weru.ksu.edu/vids/dust002.mpg

Vocabulary

cover cropA special crop grown by a farmer in thewintertime to reduce soil erosion. Covercrops often also add nitrogen to the soil.

developmentThe construction of new buildings, roads, andother human-made features in a previouslynatural place.

impermeableNot allowing water to flow through it.

leaf litter

Dead leaves, branches, bark, and other plantparts that accumulate on the floor of a forest.

pastureLand that is used for grazing animals.

topsoilThe very important top few inches of soil,where much of the nutrients are foundnecessary for plant growth; Part of the Ahorizon

Points to Consider

Is soil a renewable resource or anonrenewable resource? Explain the ways itcould be either.Could humans live without soil?What could you do to help to conserve soil?

Pollution of the Land

Lesson Objectives

Define hazardous waste and describe itssources.Describe some of the impacts of hazardouswaste on human health and on theenvironment.Detail some ways that we can controlhazardous wastes.

Introduction

Sometimes human activities lower the qualityor degrade the land by putting hazardous substancein the soil and water. A well-known example ofthis is the story of Love Canal in New York. Thestory began in the 1950’s, when a local chemicalcompany put dangerous chemicals in steel drumcontainers. They buried the containers in Love

Canal, an abandoned waterway near Niagara Falls,New York ( Figure below). They then covered thecontainers with soil and sold the land to the localschool system.

Figure 19.8 Steel barrels like these were used to contain

the hazardous chemicals at Love Canal. Afterseveral years, they began to leak the chemicals intothe soil and groundwater, which caused manypeople to become sick.

The school system built a school on the land.The city of Niagara Falls also built more than 800homes near Love Canal. Several years later,people who lived there began to notice bad

chemical smells in their homes. Childrendeveloped burns after playing in the soil, and theywere often sick. A woman living in the area,named Lois Gibbs, organized a group of citizenscalled the 'Love Canal Homeowners Association'to try to find out why their children kept gettingsick (Figure below). They discovered that theirhomes and school were sitting on top of the sitewhere the dangerous chemicals had been buried.They believed that the old steel drums used tocontain the dangerous chemicals were leaking andmaking them and their children sick. Theydemanded that the government take action to cleanup the area and remove the chemicals.

Figure 19.9 A resident of Love Canal protests the

hazardous waste contamination in herneighborhood.

By 1979, the United States government fullyrealized that the old drums were indeed leakingdangerous chemicals into the soil and water where

the people lived and went to school. Thegovernment gave money to many of the people tomove somewhere safer and began cleaning up thesite. The work of Lois Gibbs was important inbringing the problem of hazardous chemicalpollution to peoples’ attention. After the LoveCanal problem, the U. S. government created a lawcalled the Superfund Act. This law requirescompanies to be responsible for hazardouschemicals that they put into the environment. It alsorequires them to pay the money needed to clean uppolluted sites, which can often be hundreds ofmillions of dollars. As a result, companies todayare more careful about how they deal withhazardous substances.

This lesson describes some of the sources ofhazardous wastes throughout the world. It thendiscusses the effects these wastes have on humanhealth and the environment. Finally, this lessoncovers ways that we can control hazardous wastes.

What is Hazardous Waste?

Hazardous waste is any waste material that isdangerous to human health or that degrades theenvironment. Hazardous waste materials includesubstances that are:

1. Toxic: something that causes serious harm,death or is poisonous.

2. Chemically active: something that causesdangerous or unwanted chemical reactions,like dangerous explosions.

3. Corrosive: something that destroys otherthings by chemical reactions.

4. Flammable: something that easily catches fireand may send dangerous smoke into the air.

Hazardous waste may be solid or liquid. Itcomes from many sources, and you may besurprised to learn that you probably have somesources of hazardous waste right in your ownhome. Several cleaning and gardening chemicalsare hazardous if not used properly. These includechemicals like drain cleaners and pesticides thatare toxic to humans and many other creatures.

When we use, store, and dispose of them, we haveto be careful. We have to protect our bodies fromexposure to them and make sure they do not enterthe environment (Figure below). If they are thrownaway or disposed of improperly, they becomehazardous to the environment. Others sources ofhazardous waste are shown in Table below.

Figure 19.10 This farm worker wears special clothes for

protection from the hazardous pesticide in thecontainer.Type ofHazardousWaste

Example Why it isHazardous

Toxic to humans

Chemicalsfrom theautomobileindustry

Gasoline, usedmotor oil, batteryacid, brake fluid

and otherorganisms; oftenchemicallyactive; oftenflammable

BatteriesCar batteries,householdbatteries

Contain toxicchemicals; areoften corrosive

Medicalwastes

Surgical gloves,wastescontaminated withbody fluids suchas blood, x-rayequipment

Toxic to humansand otherorganisms; maybe chemicallyactive

Paints

Paints, paintthinners, paintstrippers, woodstains

Toxic;flammable

Drycleaningchemicals

Many variouschemicals

Toxic; manycause cancer inhumansToxic to humans;

Agriculturalchemicals

Pesticides,herbicides,fertilizers

can harm otherorganism; pollutesoils and water

Impacts of Hazardous Waste

Many hazardous waste materials have seriousimpacts on human health. They often cause cancerand can also cause birth defects. They can makepeople sick for very long times. Breathing the airor drinking the water that is contaminated withhazardous waste is a major health threat.

Two chemicals that are especially toxic in theenvironment are lead and mercury. Lead harmspeople by damaging their brain and nervoussystem. Lead is especially harmful in childrenunder the age of six; about 200 children die everyyear from lead poisoning. Lead was once acommon ingredient in gasoline and paint (Figurebelow). In the 1970’s and 1980’s, the UnitedStates government passed laws completely banninglead in gasoline and paint. This has prevented the

lead poisoning of millions of children in the UnitedStates. However, several other countries still usegasoline with lead in it. Also, homes built beforethe 1970’s may contain paint that has lead in it.These still pose a threat to human health.

Figure 19.11 In the United States, automotive gasoline must

now be unleaded, or free from lead.Mercury is a pollutant affecting the whole

world (Figure below). Mercury enters theenvironment from volcanic eruptions, burning coaland from waste products like old batteries andelectronic switches. It is also found in olddiscarded electronic appliances like television

sets. Like lead, mercury also damages the brainand impairs nervous system function. Mercuryoften accumulates in fish, so people and otheranimals that eat the fish then are in danger ofgetting the mercury in their own bodies.

Figure 19.12 This graph shows historic increases of mercury

in the atmosphere. Events in blue are volcaniceruptions. Events in brown, purple and pink arehuman-caused. Notice the effect ofindustrialization on mercury levels in theatmosphere (the red region of the graph).

Preventing Hazardous WastePollution

The United States is currently the world’slargest producer of hazardous wastes. However, asChina becomes more industrialized, it may takeover the number one spot. Countries with moreindustry produce more hazardous waste than thosewith little industry. Hazardous wastes can enter theair when we burn things like batteries containingmercury or old tires. Hazardous waste can enterthe water when chemicals are dumped on theground, or are buried and then leak. Substancesburied in the ground often leak from their

containers after a number of years. The chemicalsthen move through the soil until they reachgroundwater. Hazardous chemicals are especiallydangerous once they reach our groundwaterresources. Sites like the one at Love Canal arenow referred to as Superfund sites. They arefound throughout the country. Many of them havebeen identified and cleaned up. We now have strictlaws to prevent new sites like the Love Canal sitefrom ever forming in the first place.

In the United States, we have several laws thathelp control hazardous waste. The ResourceConservation and Recovery Act requires anycompany that produces hazardous materials to keepcareful track of what happens to it. The governmenthas passed special rules for how these materialscan be disposed of. Companies must ensure thathazardous waste is not allowed to enter theenvironment in dangerous amounts. They have toprotect their workers from the hazards of thematerials. They must keep a record of how theydispose of hazardous wastes, and show thegovernment that they did so in a safe way.

Individual people can also do much to controlhazardous wastes. We can choose to use materialsthat are not hazardous in the first place. We canmake sure that we dispose of materials properly.We can control the amount of pesticides that weuse. We can make sure to not pour toxic chemicalsover the land, or down the drain or toilet, or eveninto the trashcan. We can also use hazardousmaterials less often. We can find safer alternativesfor many of the chemicals we use. For example,we can use vinegar and water to clean windowsinstead of the usual glass-cleaning chemicals.

Lesson Summary

Hazardous wastes are dangerous to humanhealth and the environment. They come frommany sources, such as household chemicals,gasoline, paints, old batteries, discardedappliances, and industrial chemicals.Once in the air or buried on land, they cancause human health problems or even death

and degrade the environment for otherorganisms.Developed countries like the United Statesproduce most of the world’s hazardous waste.We have passed laws that require carefuldisposal of hazardous materials and that maketheir producers financially responsible forthem if they pollute the environment.

Review Questions

1. How does the United States Superfund Acthelp control hazardous wastes?

2. What is the difference between corrosive andflammable?

3. Organic farming is a method of growing foodcrops with natural alternatives to chemicalpesticides. How does organic farming helpcontrol hazardous wastes?

4. What is one disadvantage of storinghazardous wastes in barrels buried deep inthe ground?

5. Scientists who work with hazardous wastesoften wear special clothing like gloves andmasks. Why do you think they wear theseitems?

6. Which do you think is easiest and hardest tokeep track of: hazardous waste that is presentas a gas, liquid, or solid? Why?

Further Information /Supplemental Links

Love Canal Pathfinder, Nathan Tallmanhttp://www.nathantallman.org/pathfinders/lovecanal.html

Superfund Sites Where You Livehttp://www.epa.gov/superfund/sites/index.htm

Vocabulary

degradeTo lower the quality of something.

pesticidesChemicals used to kill or harm unwantedpests such as insects that damage food crops.

superfund actA law passed by the US Congress in 1980that held companies responsible for anyhazardous chemicals that they might create.

superfund siteA site where hazardous waste has beenspilled. Under the Superfund act, the companythat created the hazardous waste isresponsible for cleaning up the waste.

Points to Consider

What are the best ways to either prevent orsafely dispose of hazardous materials?If humans are the ones who mostly createhazardous materials, whose responsibility isit to clean them up?Is it important for each generation to leave the

world a safe place? If one generation doesn'tdo this, who pays the price?

Chapter 20: Human Actionsand Earth's Resources

Use and Conservation ofResources

Lesson Objectives

Discuss some natural resources used to makecommon objects.Describe some ways to conserve naturalresources.

Figure 20.1 The Monongahela National Forest in West

Virginia supplies us with many natural resources,including, timber, wildlife, coal, gas, recreation,and fishing.

Introduction

In the Monongahela National Forest of WestVirginia (Figure above), scientists have a mysteryto solve: the mystery of the missing plantnutrients, which are substances in the soil thatplants need to grow. For several years, the treesthere have not grown as well as they should. Soil

scientists believe that the soil is missing many ofthe important nutrients that the trees and otherplants there need to grow. They have conductedmany years of research to determine why thenutrients are disappearing and why the trees arenot growing like they should.

Mary Lusk was one of the soil scientists whoworked to solve the mystery of the missingnutrients in the forest. She gathered samples of thesoil and tested it for important nutrients. She sawthat the soil has very low levels of plant nutrients,such as magnesium and calcium. If these nutrientsare not in the soil, the trees cannot grow well. Shewondered why the soil had such low levels ofthese nutrients. After a little more research, shedeveloped the hypothesis that air pollution fromnearby factories has been putting chemicals in theenvironment that are removing the nutrients fromthe soil. In a sense, the pollution is “snatching” thenutrients and carrying them out of the soil.

Scientists in the Monongahela National Forestare still researching the missing plant nutrients.They are trying to learn what they can do to help

keep the nutrients in the soil, so the trees will growbetter. The forest is an important natural resource.A natural resource is something from nature thatwe depend on. We depend on the MonongahelaNational Forest for many reasons, including:

Recreation, such as hiking, camping, andpicnics.The forest is vital habitat for many animals,including 9 endangered species and 50different species of rare plants.The forest contains 207 kilometers (129miles) of streams for fishing, particularlytrout fishing.Hunters use the forest for hunting deer,squirrels, turkeys, rabbits, mink, and foxes.The forest contains materials that we use,such as coal, gas, limestone, and gravel.The forest has abundant hardwood trees usedfor timber, which is sold for over 7 milliondollars a year.

Like the Monongahela National Forest, we use

many parts of the Earth for many reasons (Figurebelow). We depend on materials from the Earth forfood, water, building materials, timber, recreation,and energy. However, human activities candegrade these natural resources, just like airpollution from factories is speeding up the loss ofsoil nutrients in West Virginia ( Figure below). Weneed to conserve our natural resources so theywill always be around. When we practiceconservation, we make sure resources will beavailable in the future, both for ourselves and forother organisms.

Figure 20.2 We use Earths resources for many purposes,

including recreation and natural beauty.

Figure 20.3 Severe pollution can lead to drastic

environmental damage and loss of naturalresources. This forest in Europe was damaged byair pollution.

Renewable versus Non-Renewable

Resources

Natural resources may be classified asrenewable or non-renewable. Renewableresources are those that can be regenerated, whichmeans new materials can be made or grown againat the same rate as they are being used. Forexample, trees are a renewable resource becausenew trees can be grown to replace trees that arecut down for use. Other examples of renewableresources include soil, wildlife, and water.However, some resources, like soil, have veryslow rates of renewal, so we still need to conservethem. It is also important to realize that while theseresources are in most cases renewable, we canstill pollute them, damage them or over-use them tothe point that they are not fit for use anymore. Fishare considered a renewable resource because wecan take some fish but leave others to reproduceand create new fish for later use. Imagine,however, what can happen if we over-fish, or taketoo many fish at one time. If we over-harvest our

trees or wildlife resources, we may not leaveenough to let the resource renew itself.

Non-renewable resources are resources thatrenew themselves at such slow rates that,practically, they cannot be regenerated. Once weuse them up, they are gone for good - or at least fora very, very long time. Coal, oil, natural gas andminerals are non-renewable resources. It takesmillions of years for these materials to form, so ifwe use them to the point of depletion, newresources will not be made for millions moreyears. We can run out of these resources.

Common Materials We Use Fromthe Earth

What do a CD, a car, a book, a soda can, abowl of cereal, and the electricity in your home allhave in common? They are all made using naturalresources. For example, a CD and a soda can aremade of metals that we mine from the Earth. Abowl of cereal comes from wheat, corn, or rice

that we grow in the soil. The milk on the cerealcomes from cows that graze on fields of grass. Wedepend on natural resources for just abouteverything that we eat and use to keep us alive, aswell as the things that we use for recreation andluxury. In the United States, every person usesabout 20,000 kilograms (40,000 pounds) ofminerals every year for a wide range of productssuch as cell phones, TV’s, jewelry, and cars.Table below shows some common objects, thematerials they are made from and whether they arerenewable or non-renewable.

CommonObject

Natural ResourcesUsed

Are TheseResourcesRenewableor Non-renewable?

Cars

15 different metals,such as iron, lead,and chromium tomake the body

Non-renewable

Precious metals

Jewelrylike gold, silver, andplatinum; Gems likediamonds, rubies,emeralds, turquoise

Non-renewable

ElectronicAppliances(TV’s,computers,DVD players,cell phones,etc.)

Many differentmetals, like copper,mercury, gold

Non-renewable

Clothing

Soil to growfibers such as cottonSunlight for theplants to growAnimals for fur andleather

Renewable

FoodSoil to grow

plants Wildlife andagricultural animals

Renewable

Water fromstreams or springs Non-

Bottled Water Petroleum productsto make plasticbottles

renewableandRenewable

Gasoline Petroleum drilledfrom wells

Non-renewable

HouseholdElectricity

Coal, natural gas,solar power, windpower, hydroelectricpower

Non-renewableandRenewable

Paper Trees SunlightSoil Renewable

Houses

Trees for timberRocks and mineralsfor constructionmaterials, forexample, granite,gravel, sand

Non-renewableandRenewable

Human Population and ResourceUse

As the human population grows, so does theuse of our natural resources. A growing populationcreates a demand for more food, more clothing,more houses and cars, etc. Population growth putsa strain on natural resources. For example, nearly500 people move into the Tampa, Florida areaevery week (Figure below). Tampa's population isgrowing quickly. The Tampa area may have over 3million people by 2010. One of Tampa’s rivers,the Hillsborough River, is pumped for drinkingwater to support all the people. Too much water isbeing taken from the river. The river is becomingsalty, as water from the near-by Gulf of Mexicostarts to take the place of the freshwater beingpumped out. This hurts wildlife and may eventuallymake the river water unsuitable for human use.Many other examples like this are taking placeworldwide.

Figure 20.4 Downtown Tampa, Florida is growing at an

enormous rate. The growing human population putsa strain on natural resources, like rivers and otherbodies of water.

Resource Availability

You can see from the table above that many ofthe resources we depend on are non-renewable.We will not be able to keep taking them from theEarth forever. Also, non-renewable resources varyin their availability. Some are very abundant andothers are rare. Precious gems, like diamonds and

rubies, are valuable in part because they are sorare. They are found only in small areas of theworld. Other materials, like gravel or sand areeasily located and used. Whether a resource is rareor abundant, what really determines its value ishow easy it is to get to it and take it from the Earth.If a resource is buried too deep in the Earth or issomehow too difficult to get, then we don’t use itas much. For example, the oceans are filled withan abundant supply of water, but it is too salty fordrinking and it is difficult to get the salt out, so wedo not use it for most of our water needs.

Resource availability also varies greatlyamong different countries of the world. Forexample, 11 countries (Algeria, Indonesia, Iran,Iraq, Kuwait, Libya, Nigeria, Qatar, Saudi Arabia,the United Arab Emirates, and Venezuela) havenearly 80% of all the world’s oil (Figure below).However, none of these is the world’s biggest userof oil. In fact, the biggest users of oil, the UnitedStates, China, and Japan, are all located outsidethis oil-rich region. This difference in availabilityand use of resources can be a source of economic

and political trouble throughout the world. Nationsthat have abundant resources often export them toother countries, while countries that lack aresource must import it from somewhere else.

Figure 20.5 The nations in green are the 11 biggest

producers of worldwide oil. They have almost80% of the worlds current oil supply, even thoughthe United States, China, and Japan are the worldsbiggest users of oil.

In developed countries like the United Statesand most of Europe, we often use many morenatural resources than we need just to live. Wehave many luxury and recreational materials madefrom resources. We also tend to throw things awayquickly because we can afford to replace them.Discarding materials not only leads to more

resource use, but it also leads to more waste thathas to be disposed of in some way. Pollution fromdiscarded materials degrades the land, air, andwater (Figure below). As our cities andneighborhoods grow, we use more and moreresources and produce more and more waste.Natural resource use is generally lower indeveloping countries because people cannot affordto use as much. Still, developing countries need toactively protect their resources by adoptingsustainable practices as they develop.

Figure 20.6 Pollution from discarded materials degrades

the environment and reduces the availability of

natural resources.

Conserving Natural Resources

We need to conserve natural resources so thatwe can continue to use them in the future, and sothat they will be safe for use. While renewableresources will not run out, they can becomedegraded or polluted. For example, water is arenewable resource, but we can pollute it to thepoint that it is not safe for use. Reducing use andrecycling materials is a great way to conserveresources (Figure below). Many people are alsoresearching ways to find renewable alternatives tonon-renewable resources. Here is a checklist ofsome things we can do to conserve resources:

Figure 20.7 Recycling can help conserve natural resources.

Purchase less stuff (use items as long as youcan, ask yourself if you really need somethingnew.)Reduce excess packaging (for example, drinkwater from the tap instead of buying it inplastic bottles).Recycle materials like metal cans, old cellphones, and plastic bottles.Purchase products made from recycledmaterials.Keep air and water clean by not polluting inthe environment.

Prevent soil erosion.Plant new trees to replace ones that we cutdown.Drive cars less, take public transportation,bicycle, or walk.Conserve energy at home (for example, byturning out lights when they are not needed).

Lesson Summary

We use natural resources for many things.Natural resources give us food, water,recreation, energy, building materials, andluxury items.Many resources vary in their availabilitythroughout the world. Some are rare, difficultto get or in short supply.We need to conserve our natural resources,protecting them from pollution and overuse.We can use materials less or recycle toconserve resources.We can also make efforts to reduce pollution

and soil erosion in order to conserveresources.

Review Questions

1. List five general things we get from naturalresources.

2. We depend on forests as habitat for wildlife.How does this make a forest an importantresource for people?

3. How could human life be affected if a largeamount of soil erosion affected our soilresources?

4. How does discarding products lead to moreresource use?

5. How does choosing to walk or ride a bicycleinstead of riding in a car help conserveresources?

Further Reading / SupplementalLinks

Maps of Renewable Resources in the UnitedStates http://www.nrel.gov/gis/maps.htmlWhat to Recycle [* Maps of RenewableResources in the United Stateshttp://www.nrel.gov/gis/maps.html

Vocabulary

conserveTo keep things safe and ensure that they willalways be around.

exportTo send out to another country.

importTo receive from another country.

macronutrientsNutrients that are needed by an organism in alarge amount.

non-renewable

A resource that cannot be regenerated; once itis used up, it cannot be replaced within ahuman lifetime.

timberTrees that are cut for wood to be used forbuilding or some other purpose.

nutrientsSubstances that a living thing needs to grow.

renewableA resource that can be regenerated, new onescan be made or grown to replace ones that getused.

Points to Consider

Could a renewable resource ever becomenonrenewable?How many resources do you use every day?Which is more sustainable: using renewableresources or nonrenewable resources? Why?

Energy Conservation

Lesson Objectives

Discuss why it takes energy to get energy andwhy some forms of energy are more usefulthan others.Describe some ways to conserve energy or touse energy more efficiently.

Introduction

Imagine that someone offers you a $100 billthat you can use for whatever you want. Thatwould be a pretty good deal, wouldn’t it? Nowimagine that the person attaches a condition to theiroffer: in order to get the $100 bill you have to paythem $75. You would still come out ahead, but thistime you would only be getting $25. Does it makesense to spend money to get money? That dependson how much you get back for what you spend.

Getting and using natural energy sources is alot like spending money to get money. We use a lotof energy just to get energy (Figure below). Wehave to find an energy source, extract it from theEarth, transport it to the places where it will beused, and often process or convert it into adifferent form of energy. All of these steps ofgetting energy require energy use themselves. Forexample, we use petroleum to make gasoline forour cars. To get the petroleum, we often have tobuild huge drilling facilities and drill downhundreds of meters into the Earth. It takes energy todo this. We then use trucks or ships to transport theoil all over the world, which also takes energy.We then have to heat the petroleum to its boilingpoint to make different products from it, likegasoline and automotive oil and this takes evenmore energy.

Figure 20.8 It takes energy to get energy. This is an oil

platform used for drilling oil from deepunderground. It took lots of energy to build theplatform and to run it.

In this lesson, you will learn that differentsources of energy all require adding some otherenergy before they can be made useful. You will

be able to compare various sources of energy interms of their usefulness. You will also learn someways that we can conserve energy or use it moreefficiently.

Obtaining Energy

It takes energy to get energy. Net energy is theamount of useable energy available from aresource after subtracting the energy used to extractit from the Earth and make it useable by humans.We just discussed someone giving you $100 butrequiring you to pay them back $75. In this case,your net pay would be $25, or $100 minus $75.Net energy is calculated the same way. Forexample, for every 5 barrels of oil that we takefrom the Earth, we have to use 1 barrel for theextraction and refining process. This leaves us anet supply of only 4 barrels (5 barrels minus 1barrel).

Remember that oil is a non-renewableresource. Imagine what would happen if the energy

needed to extract and refine oil increased. Whatmight happen if it took 4 barrels of oil being usedto get 5 barrels of new oil? Then our net supplywould only be 1 barrel. Our supply of oil wouldbegin to dwindle away even faster than the currentrate.

We sometimes use the expression net energyratio to demonstrate the difference between theamount of energy available in a resource and theamount of energy used to get it. If we get 10 unitsof energy from a certain amount of oil, but use 8units of energy to extract, transport, and refine theoil, then the net energy ratio is 10/8 or 1.25. A netenergy ratio larger than 1 means that we are stillgetting some usable energy. A net energy ratiosmaller than one means there is an overall energyloss. Table below shows several energy sourcescommonly used for heating our homes and schools.It shows their net energy ratios. Higher ratios meanthat the source provides more useable energy thanthose with lower ratios.Energy Source Net Energy Ratio

Solar Energy 5.8Natural Gas 4.9Petroleum 4.5Coal-fired Electricity 0.4

Notice from the table that renewable solarenergy gives you much more net energy than othersources and that coal-fired electricity actuallyconsumes more energy than it produces. Why doyou think this is so? Burning coal for electricityrequires a large input of energy to get energy. Wehave to find the coal, mine the coal, transport thecoal, and build power plants to burn the coal(Figure below). All of these take energy andreduce the net energy available for us to use. Solarenergy, however, requires very little energy to getin the first place. We don’t have to mine it ortransport it in trucks. Sunshine is abundant globallyand can be used in the same place where it iscollected.

Figure 20.9 Transporting coal requires a large input of

energy. It takes energy to run the train thattransports the coal.

Energy Efficiency

The discussion above on net energy shows youthat it takes energy to get energy and that somesources of energy require more input than others toget usable energy. After we get the energy, we thenuse it for some purpose. Energy efficiency is aterm that describes how much usable energy wehave available to do work from every unit of

energy that we use. Higher energy efficiency isdesirable because it means we are wasting lessenergy and getting more use out of the energysources that we take from the Earth. Higher energyefficiency also lets us extend our non-renewablesources and make them last longer.

Nearly 85% of the energy used in the UnitedStates comes from non-renewable fossil fuels.Since these exist in limited supplies, we need to beespecially concerned about using them efficiently.Sometimes our choices affect energy efficiency.For example, transportation needs require hugeamounts of energy. Forms of transportation such ascars and airplanes are less efficient thantransportation by boats and trains. Fluorescent lightbulbs are more efficient than regular, incandescentlight bulbs. Hydroelectric power plants are moreefficient than nuclear fission reactors.

Energy Conservation

Energy conservation involves reducing or

eliminating the unnecessary use of energy. Thisimproves energy efficiency. Energy conservationsaves us money and it also ensures that our energysupplies will last longer. There are two main waysto conserve energy: use less energy and use energymore efficiently. The pie chart (Figure below)shows how energy is used in the United States.

Figure 20.10 This pie chart shows how energy is used in the

United States.

Almost one-half of the energy used in theUnited States is for transportation and home use.This means that individual people can do much toconserve energy on a national basis. Table belowshows some ways that we can decrease energy useand use energy more efficiently in transportation,residences, industries, and office settings.

Where Energyis Used

How We Can UseLess Energy

How We Can UseEnergy MoreEfficiently

Transportation

Ride a bikeor walkinstead oftaking a carReduce thenumber oftrips youmake

Increase fuelefficiency incarsBuy anddrive smallercarsBuild carsfrom lighterand strongermaterialsDrive at

Use publictransportation

speeds at orbelow 90kilometersper hour (55miles perhour)

Residential

Turn offlights whennot in a roomOnly runapplianceswhennecessaryUnplugapplianceswhen not inuseWear asweaterinstead ofturning up

Replace oldapplianceswith newermoreefficientmodelsInsulate yourhomeMake surewindows anddoors arewell sealedUse LED

heatRead a bookor playoutsideinstead ofwatch TVRely onsunlightinstead ofartificial light

bulbs ifavailable, orcompactfluorescentlight bulbs(and disposeof properly!)

Industrial

Recyclematerials likesoda cansand steelReduce useof plastic,paper, andmetalmaterials

Practiceconservationin factoriesReusematerialsDesignequipment tobe moreefficient

Commercial(businesses,shoppingareas, etc.)

Turn offappliancesandequipmentwhen not inuse

UsefluorescentlightingSetthermostatstoautomaticallyturn off heator airconditioningwhenbuildings areclosed

Using less energy or using energy moreefficiently will help conserve our energyresources. Since many of the energy resources wedepend upon are non-renewable, we need to makesure that we waste them as little as possible.

Lesson Summary

It takes energy to get energy. We use the term'net energy' to refer to the amount of energyleft for use after we expend energy to get,transport and refine other forms of energy.Once the energy is available, we use it forsome purpose, but sometimes do soinefficiently.We can conserve energy resources byreducing energy use.We can also use energy more efficiently bygetting more work out of the energy that weuse.Examples of this include driving smaller carsand using fluorescent light bulbs.

Review Questions

1. Define net energy?2. Why does solar power have a higher net

energy ratio than coal-fired electricity?3. Coal-fired electricity has a net energy ratio of

0.40. Explain why this means that getting

electricity from burning coal is an undesirableoption for energy use.

4. What are two ways you can use less energy inyour home?

5. Why is it especially important to not wasteenergy from fossil fuels?

6. Trains are much more efficient than trucks intransporting items around. Why do you thinkthis might be so?

Vocabulary

energyThe ability to do work which we can get froma fuel.

extractionThe process of taking oil out of the Earth.

fluorescentA type of lighting that uses less energy thanregular light bulbs.

net energy ratioThe ratio between the useful energy present ina type of fuel, and the energy used to extractand process the fuel.

refiningThe process of removing impurities from oiland to make it usable.

Points to Consider

If it takes energy to get energy, then what arethe best choices for types of energy?Put each of these actions in order from mostimportant to least: choosing a sustainableform of energy, increasing energy efficiency,conserving energy use. Explain the order youchose.Could everyone in the world use as muchenergy as a person in the United States doeseach day? Why or why not?

Chapter 21: Human Actionsand Earth's Waters

Humans and the WaterSupply

Learning Objectives

Learn how humans use water.Discuss how much water is taken up by eachwater use.Explain the difference between consumptiveand non-consumptive water uses.Discuss three of the most serious issueshumans face today, including shortages offresh water, lack of safe drinking water andwater pollution.

Discuss why humans are facing water

shortages.Discuss how water shortages can lead todisputes and even battles between states andcountries bordering on the same water source.Explain why one fifth of the human populationdoes not have access to safe drinking water.Describe the relationship between diseaseand exposure to unsafe drinking water.What is the origin of California's fresh watersupply?

Human Uses of Water

All forms of life need water to survive. Ashumans, we need water to drink or we need to getit from the foods we eat. We also use water foragriculture, industry, household uses, andrecreation. Water is continually cycled andrecycled through the environment.

Some ways that we use water consume a lot ofwater that then is lost to the ecosystem and someways we use water put less demand on our water

supplies. Understanding how water cycles and isreplaced is important, especially when we look forways to use less water. Currently, agricultural usesthe most water. Considering different methods ofirrigation and times of day to water crops canimprove this situation. Farming, growing crops andraising livestock uses more than two thirds of thewater used by humans globally.

When water is used but not recycled, the wateruse is called consumptive. That water is lost to theecosystem. When excess water is captured orrecycled, it is called non-consumptive. As wemove to a more sustainable future, we want to besure as much of our water use is non-consumptiveas possible.

What is the most important thing for all life onEarth? Not gold or diamonds. It is water! From thesmallest bacteria to the largest trees, all forms oflife on Earth depend on water for survival. Ashumans, we could not survive for more than a fewdays without drinking water or getting water fromthe foods we eat.

In addition to our basic survival need for water

to drink, people also use freshwater foragriculture, industry and household needs. Acrossthe world, different communities also use water formany kinds of recreational and environmentalactivities.

Which human activity uses the most water? Notshowers, baths, washing dishes or other householduses. On average, agriculture uses more than twothirds of the water that humans use across theworld. Industry and household uses average 15%each. Recreational use and environmental usesaverage 1% each. (See Figure below)

Figure 21.1 Proportion of water used for home, industrial,

and agricultural purposes across the world.

Some ways that people use water do not use upthe water. When you swim in a lake, you do not useup the water. The water is still in the lake whenyou climb out. In some cases, water can berecycled for reuse. For example, the water you useto brush your teeth or take a bath can be collectedthrough your household pipes and the sewersystem, purified and then redistributed for reuse.These are examples of non-consumptive wateruse. By recycling water, we ultimately reduce ouroverall water consumption.

Unlike the previous examples, water sprinklersare called consumptive, because much of thewater is lost to the air as evaporation. None of thelost water can be captured and reused.

Agricultural Water Uses

Have you ever watched huge sprinklerswatering large fields of crops (Figure below)? Ifyou have, try to imagine how much water it takes towater a field compared to taking a shower or bath.

You may be surprised to learn that agriculture usesmore than two thirds (69%) of the water humansuse, globally.http://authors.ck12.org/wiki/index.php/File:Ear-2101-02.jpg

Figure 21.2 Agricultural Water Use: Overhead sprinklers

need to use large quantities of water on cropsbecause much of the water is lost to evaporationand runoff.

Two of the most popular irrigation methods areoverhead sprinklers and trench irrigation. Trenchirrigation systems are just that: trench canals thatcarry water from a water source to the fields.Farmers often chose these methods because they

relatively inexpensive. Unfortunately, they are alsowasteful of water. Roughly fifteen to thirty-sixpercent of the water never reaches the crops,because it evaporates into the air or is lost asrunoff. When rain or irrigation water is notabsorbed by the soil, often it washes valuable soilaway.

Giving up irrigation is not a choice for mostfarmers. A farmer living in a dry region, such as adesert, needs irrigation, just to grow crops. Afarmer living in a wetter place would useirrigation to produce more crops or to grow moreprofitable crops. In some cases, farmers canchoose to grow crops that match the amount of rainthat falls in that region naturally.

Figure 21.3

Drip Irrigation uses a series of pipes and tubesto deliver water to the base of each plant. Becauselittle water is lost to evaporation and runoff, thismethod uses less water than sprinklers andtrenches.

Instead of giving up irrigation, farmers can useless water by choosing more efficient irrigationmethods, such as drip irrigation (Figure above).This irrigation system uses pipes and tubes todeliver small amounts of water directly to the soilat the roots of each plant or tree. It wastes lesswater than sprinklers and trenches, because almostall of the water goes directly to the soil and plantroots.

You might wonder why any farmer would notswitch to efficient irrigation methods, since theywould save so much water. There are two reasons.First, drip irrigation and other efficient irrigationmethods cost more than trenches and sprinklers.Second, in some countries, such as the UnitedStates, the government pays for much of the cost ofthe water that is used for agriculture. Because,farmers do not have to pay the full price of the

water they use, they do not have any financialreason or incentive to use less water.

Aquaculture

Aquaculture is the name for the type of farmingyou might do if you were raising fish, shellfish,algae or aquatic plants (Figure below). This is afarming practice where plants and animals that livein water are raised. As the supplies of fish fromlakes, rivers, and the oceans dwindle, people aregetting more fish from aquaculture. Raising fishinstead of hunting for them is a different way ofincreasing our food resources. The next time youpass the fish display in the grocery store, look forlabels for “farm raised” fish. These fish wouldhave been raised in an aquaculture setting.

Figure 21.4 Aquaculture: Workers at a fish farm harvest

fish they will sell to stores.Some of the most productive aquaculture

farming takes place in wetland areas alongcoastlines. Rivers and streams carry nutrient-rich

water into these wetlands, so fish and other animallife thrives. A good supply of nutrients is importantwhen raising a large community of plants oranimals. We need to be careful about the wastesthat are added to our coastal waters when weincrease plant and animal populations in theseareas. Aquaculture can be considered a non-consumptive use of water, as long as we keep ourcoastal waters in good condition.

Industrial Water Use

Figure 21.5 Industrial water use: A power plant in Poland

sits on the edge of a lake with easy access to water

for cooling and other purposes.However, industrial water use accounts for an

estimated fifteen percent of worldwide water use.Industries include power plants that use water tocool their equipment, and oil refineries that usewater for chemical processes (Figure above).Industry also uses water in many manufacturingprocesses. Looking at water use in a completelydifferent way, hydroelectric power plants are builtalong rivers and streams to generate energy. Thisis a very efficient way to use water that is alsonon-consumptive.

Household Use

Figure 21.6 Domestic water use.Starting from when you wake up in the

morning, count the ways you use water at home(Figure above). You will need to count the wateryou drink, water used in cooking, bathing, flushingtoilets, and even gardening. You will be surprisedto notice how many times a day you use water.Have you ever had to go without water? TheUnited States is a developed country. In developedcountries, people use a lot of water each day.People living in lesser developed countries use farless water than people in the United States.Globally, household or personal water use isestimated to account for fifteen percent of world-

wide water use.Some household water uses are considered

non-consumptive, because water is recaptured insewer systems, treated and returned to surfacewater supplies for reuse. Watering lawns withsprinklers is an exception. Just like sprinklerirrigation on farms, yard sprinklers areconsumptive and use large amounts of water.

We all have many ways to lower the amount ofwater we use at home. Hardware stores sell water-efficient home products, such as drip irrigation towater lawns and gardens, low flow shower headsand low flow toilets. What other ways can you useless water at home?

Recreational Use

Figure 21.7 Recreational Water: Many recreational

activities, such as swimming and fishing, are non-

consumptive water activities; which wont depletethe water supply.

Which sports use water? Swimming, fishing,and boating are easy examples to think about(Figure above). Do you think playing golf requireswater? Actually it does, because we irrigate thegolf course in order to keep it nice and green! Theamount of water that most recreational activitiesuse is low: less than one percent of all the waterwe use.

Most recreational water uses are non-consumptive. That would include swimming,fishing, and boating. We can swim, fish, and boatwithout reducing the water supply. The same is nottrue for playing golf, which is the biggestrecreational water consumer. Golf courses requirelarge amounts of water. Water used for golfcourses is generally consumptive, since most of itis lost to evaporation, soil, and runoff.

Environmental Use

Figure 21.8 Environmental Water Use: Wetlands and other

environments depend on clean water to survive.Water shortages are a leading cause of globalbiodiversity loss.

Environmental uses include activities to createhabitat for wildlife, such as building lakes and fishladders to help fish spawn (Figure above). Mostenvironmental uses are non-consumptive; theyaccount for even less water use than recreation.

California Water Resources

California has a rich water supply from manysources. The winter snow pack in the Sierra

Nevada and other mountain ranges feeds rivers thatcrisscross the state. Many of these streams feedinto the Sacramento River in the northern part ofthe Central Valley, and the San Joaquin River inthe southern portion (Figure below). Virtually allof these rivers are dammed, some more than once,to supply power and water to the cities andfarmland of the state.

Figure 21.9 California rivers.

Groundwater is also an important source ofwater in California. In a normal year about 40% ofthe state’s water supply comes from groundwater.In a drought year, the number can rise to 60% ormore. The largest groundwater reservoirs arefound in the Central Valley where thousands ofyears of snow melt has fed the aquifers. In manylocations, much more groundwater is used eachyear than is available to recharge the aquifer.Subsidence of the land is common in these regions.

Despite these vast water sources, the stateslarge population and enormous agriculturallandscape put a strain on the water supply. Waterrights in California are complex and controversial.Although about 75% of the water resources are inthe northern one-third of the state, the largestusage, about 80%, is in the southern two-thirds.Besides projects that exist to distribute waterwithin the state, a large source of water is theColorado River, which California must share withfive other states and Mexico. The distribution ofwater resources in the Western United States willbe a topic of much discussion in the coming

decades.

Lesson Summary

Human water use can be lumped into fivecategories. The uses are arranged in order ofgreatest to the least amounts of total water useon Earth:

Agriculture (sixty-nine percent)Industry uses (fifteen percent of global wateruse)Home and Personal use (fifteen percent)Recreation uses (less than one percent)Environmental use (less than one percent)

Despite California’s abundant water supplyfrom surface streams and groundwater, thestate has a number of water rights issues thatwill be important long into the future.

Review Questions

1. Describe the three water uses that consumethe most fresh water.

2. Explain why humans are limited to using lessthan one percent of all the water on Earth forour needs.

3. List two reasons why human water use hasincreased tremendously during the pastcentury.

4. Describe four consequences of watershortages.

5. What does the phrase 'water is more valuablethan gold' mean?

6. Describe why some water uses are calledconsumptive.

7. Describe drip irrigation and why it wastesless water than irrigating with sprinklers.

8. Describe why droughts are more serious inarid regions of the world than in wetterregions.

9. What is the origin of California’s fresh watersources?

Vocabulary

consumptiveWater use where water is 'lost' toevaporation.

drip irrigationPipes & tubes that deliver small amounts ofwater directly to the soil at the roots.

incentiveA financial benefit for taking a particularaction.

non-consumptiveWater use that does not 'use up' the watersupply.

Points to Consider

How could fresh water be more valuable thangold or a diamond?

Which human activity uses more water thanall other activities combined?Why don’t all farmers use drip irrigation andother water efficient irrigation methods?

Problems with WaterDistribution

Learning Objectives

Explain why water shortages are increasinglyfrequent throughout the world.Discuss why 1.1 billion people (one fifth ofthe people on Earth) do not have access tosafe drinking water.Explain why humans can use less than onepercent of all water on Earth.Discuss the ways in which human waterdemands are unsustainable.

Introduction

Humans are facing a worldwide water crisisaccording to the United Nations. The crisisincludes worldwide shortages of fresh water that

humans can access, scarcity of safe drinking watersupplies and water pollution.

World Water Supply andDistribution

Water is everywhere. More than 70% of theEarth’s surface is covered by water. The Earth hasa limited supply of water that we can use. Thereare supplies of freshwater in lakes, rivers, streams,swamps, reservoirs, and even underground waterrich regions of soil and rock, called aquifers.Almost anywhere you stand, there is watersomewhere beneath you. Sometimes that water isjust several meters below you, sometimes it isdeeper within the Earth.

Still, this supply of freshwater is less than 1%of all of the water on Earth. Why is so little wateravailable for human use? Two reasons:

For most of our needs, humans cannot usesaltwater, which makes up 97-98% of all

water on Earth.Humans cannot use most of the freshwater onEarth, because is frozen in glaciers andicebergs, mainly in Greenland and Antarctica(Figure below).

Figure 21.10 Most fresh water on Earth is in the form of

frozen icebergs and glaciers.A common misconception is that water

shortages can be solved by desalination, removingsalt from seawater. This is because thedesalination process requires so much energy andis so costly, that it is not an economical way toincrease freshwater resources.

Water Distribution

Look closely at the climates of differentregions around the Earth. Some places have waterrich climates, while many others do not. Roughly40% of the land on Earth is arid or semiarid,which means it receives little or almost no rainfall.

Water Distribution: Water is unevenlydistributed across the world. The blue areas arethe most water rich regions of the world. Thesalmon pink areas are desert areas. (Source:http://earthtrends.wri.org/maps_spatial/maps_detail_static.php?map_select=264&#38;theme=2. CC-BY-SA)

Projected Water Distribution in 2025: The blueareas are the most water rich regions of the world.The salmon pink areas are desert areas. (Source:http://earthtrends.wri.org/maps_spatial/maps_detail_static.php?map_select=265&#38;theme=2. CC-BY-SA)

Global warming affects patterns of rainfall andwater distribution. As the Earth warms, regionsthat currently receive an adequate supply of rainmay shift. Regions of Earth that normally are lowpressure areas may become areas where highpressure dominates. That would completely changethe types of plants and animals that can livesuccessfully in that region.

Figure 21.11 Water Supply Compared to Population.In 1995, about 40% of the world’s population

faced water scarcity (Figure above). Scientistsbelieve that by the year 2025, nearly half of theworld’s people won’t have enough water to meettheir daily needs. Nearly one quarter of the peoplein the world will have less than 500 m3 of waterper person to use in an entire year. A cubic meterof water equals 1,000 liters. That means in certainareas of the world, many people will have lesswater available in a year than some people in the

United States use in one day.

Water Shortages

Figure 21.12 Water Scarcity Projections: If world-wide

water use and population growth continues to growat the current rate many people in the world willface serious water shortages.

Water Shortages Projections for 2025As we continue to use our precious freshwater

supplies, scientists expect that we will encounterseveral different types of problems. We currentlyirrigate our crops using supplies of groundwater inaquifers underground. When we have used up these

groundwater supplies, we will not be able to growas many different types of crops or we will havelower yields of the crops we grow. Using ourfreshwater often adds many different types ofdissolved materials to the freshwater supply. Thisuse may lead to pollution of our water resourcesand cause harm not only to humans but to many lifeforms, reducing our biodiversity. Mostimportantly, as our water supplies become scarce,there will be conflicts between individuals whohave enough clean water and those who do not(Figure above). As with any limited resource, thisconflict could produce warfare.

Two of the most serious problems facinghumans today are shortages of fresh water and thelack of safe drinking water.

Humans use six times as much water today aswe did a hundred years ago. As the number ofpeople on Earth continues to rise, our demand forwater grows. Also, people living in developedcountries use more water per person thanindividuals in lesser developed countries. This isbecause most of our activities today, such as

farming, industry, building, and lawn care, are allwater-intense practices, practices that requirelarge amounts of water.

Droughts occur when for months or years, aregion experiences unusually low rainfall (Figurebelow). Periods of drought naturally make watershortages worse. Human activities, such asdeforestation, can contribute to how often droughtsoccur. Trees and other land plants add water backinto the atmosphere through transpiration. Whentrees are cut down, we break this part of the watercycle. Some dry periods are normal and canhappen anywhere in the world. Droughts are alonger term event and can have seriousconsequences for a region. Because it is difficult topredict when droughts will happen, it is difficultfor countries to predict how serious watershortages will be each year.

Figure 21.13 Extended periods with lower than normal

rainfall cause droughts.Water shortages hurt human health, agriculture

and the environment. What happens when watersupplies run out? In undeveloped regions in the

world, people are often forced to move to a placewhere there is water. This can result in seriousconflicts, even wars, between groups of peoplecompeting for water.

Water disputes happen in developed countriesas well. Water-thirsty regions may buildaqueducts, large canals or pathways to importwater from other locations. For example, severalcities in arid regions of the United States importwater from the Colorado River. So much water istaken from the river that it can end as just a tricklewhen it reaches Mexico. Years ago, Mexico coulddepend on the river supplying water for irrigationand other uses. Today that water resource is gonefrom importing water upstream.

Some of the biggest legal battles in the UnitedStates have been over water rights, includingaccess to the Colorado River. Water disputes mayhave lead to some of the earliest wars known.

Problems with Water Quality

Figure 21.14 Access to improved drinking water and

progress towards achieving the internationallyagreed goals on water and sanitation.

Scarcity of Safe Drinking Water

The next time you get water from your faucet,imagine life in a country that cannot afford thetechnology to treat and purify water. What would itbe like if your only water came from a pollutedriver where sewage was dumped? Your onlychoice would be to drink polluted water. One fifthof all people in the world, more than 1.1 billionpeople, do not have access to safe water for

drinking, personal cleanliness and domestic use.Unsafe drinking water can carry many disease-causing agents, such as infectious bacteria, toxicchemicals, radiological hazards, and parasites.

One of the leading causes of death worldwideis waterborne disease, disease caused by unsafedrinking water. It is also the leading cause of deathfor children under the age of five. Many childrendie when they only have unsafe drinking water andlack of clean water for personal hygiene. Abouteighty-eight percent of all diseases are caused bydrinking unsafe water. At any given time, half ofthe world's hospital beds are occupied by patientssuffering from a waterborne disease. The wateryou get from a faucet is safe because it has gonethrough a series of treatment and purificationprocesses to remove contaminants.

Economic Considerations

A glass of water may be free in a restaurant,but this does not reflect its value as a resource.Water is often regarded as more valuable than

gold, because human survival depends on havingsteady access to it.

Water scarcity can have dire consequences forthe people, the economy and the environment.Without adequate water:

Crops and livestock dwindle and people gohungry.Industrial, construction and economicdevelopment is halted.The risk of regional conflicts over scarcewater resources rises.Ultimately some people die from lack ofwater.

Finding safe drinking water poses furtherchallenges. What does it take for a country toprovide its people with access to safe drinkingwater? It takes sophisticated technology to purifywater, which removes harmful substances andpathogens, disease-causing organisms. Mostdeveloping countries lack the finances and thetechnology needed to supply their people with

purified drinking water.

Figure 21.15 The states and Canadian Provinces surrounding

the Great Lakes have created a pact to controlwater in the lakes, preventing other states fromover-draining the lakes.

Water resources are so valuable, that warshave been fought over water rights throughouthistory. In many cases, water disputes add totensions between countries where differingnational interests and withdrawal rights have beenin conflict (Figure above).

Some of today’s greatest tensions arehappening in places where water is scarce. Water

disputes are happening along 260 different riversystems that cross national boundaries includingwater disputes between:

Iraq, Iran, and SyriaHungary and CzechoslovakiaNorth and South KoreaIran and SyriaIsrael and JordanEgypt and Ethiopia

International water laws, such as the HelsinkiRules, help interpret water rights among countries.

Lesson Summary

Water is a renewable resource, but it is notunlimited. Humans are limited to less than onepercent of the water on Earth. Also, water isnot evenly distributed across the globe.

Water is so valuable that countries havefought each other over water rights throughout

history. Water shortages and water pollutionhave become so serious across the world, thatsome organizations call our water status a“water crisis.” The crisis is blamed onoverpopulation, overuse of water, pollution,and global warming.

Undeveloped countries are rarely able toafford water treatment and purificationfacilities, unless other countries andinternational organizations help.

Review Questions

1. If most of the Earth is covered with water,how can there be water shortages?

2. Why are waterborne diseases more commonin less developed countries than developedcountries?

3. Why does the United Nations describe thecurrent water status today as a crisis?

4. How do droughts affect water supplies?

5. Why do water disputes happen?6. Give two reasons why water shortages are

happening around the world today?

Vocabulary

aquiferRegions of soil or rock that are saturated withwater.

aridRegions without enough water for things togrow.

droughtA long period of lower than normal rainfallfor a particular region.

pathogenDisease causing organisms.

Points to Consider

What can we do to help the one fifth of thepeople on Earth who do not have access tosafe drinking water?How can we reduce water shortages due tooveruse, overpopulation, and drought?Water is so valuable that wars have beenfought over it throughout history. Couldconserving freshwater now help avoid futurewars?

Water Pollution

Learning Objectives

Discuss the risks that water pollution poses tohuman and environmental health.Explain where fresh and saltwater pollutioncome from.Discuss how pathogen born diseases arecaused by water pollution.Describe why conserving water andprotecting water quality is important to humanhealth and the environment.Describe how water pollution reduces theamount of safe drinking water available.Discuss who is responsible for preventingand cleaning up water pollution.

Freshwater and ocean pollution are seriousglobal problems that affect the availability of safedrinking water, human health and the environment.Waterborne diseases from water pollution kill

millions of people in undeveloped countries everyyear.

Sources of Water Pollution

Water pollution can make our current watershortages even worse than they already are.Imagine that all of your drinking water came from ariver polluted by industrial waste and sewage. Inundeveloped countries throughout the world, rawsewage is dumped into the same water thatundeveloped people drink and bathe in. Withoutthe technology to collect, treat and distributewater, people do not have access to safe drinkingwater. Throughout the world, more than 14,000people die every day from waterborne diseases,like cholera which is spread through pollutedwater.

Even in developed countries that can afford thetechnology to treat water, water pollution affectshuman and environmental health.

Water pollution includes any contaminant that

gets into lakes, streams and oceans. The mostwidespread source of water contamination inundeveloped countries is raw sewage dumped intolakes, rivers and streams. In developed countries,the three main sources of water pollution are:

Agriculture, including fertilizers, animalwaste and other waste, pesticides, etc.Industry, including toxic and nontoxicchemicalsMunicipal uses, including yard and humanwaste

Types of Water Pollution

Municipal Pollution

Figure 21.16 Municipal and agricultural pollution.Wastewater usually contains many different

contaminants. This makes it difficult for theEnvironmental Protection Agency (EPA) toidentify the main source when toxic chemicals arefound in wastewater. The pollution coming fromhomes, stores and other businesses is calledmunicipal pollution (Figure above). Contaminantscome from:

Sewage disposal (some sewage isinadequately treated or untreated)

Storm drainsSeptic tanks: sewage from homesBoats that dump sewageYard runoff (See agriculture discussion offertilizer waste)

Industrial Pollution

Figure 21.17 Industrial Waste Water: Polluted water coming

from a factory in Mexico. The different colors offoam indicate various chemicals in the water andindustrial pollution.

Many kinds of pollutants from factories and

hospitals end up in our air and waterways (Figureabove). Some of the most hazardous industrialpollutants include:

Radioactive substances from nuclear powerplants, as well as medical and scientific uses.Other chemicals in industrial waste, such asheavy metals, organic toxins, oils, and solids.Chemical waste from burning high sulfurfossil fuels that cause acid rain.Inadequately treated or untreated sewage andsolid wastes from inappropriate wastedisposal.Oil and other petroleum products fromsupertanker spills and offshore drillingaccidents.Heated water from industrial processes suchas power stations.

Agricultural Pollution

Agriculture includes crops, livestock andpoultry farming. Most agricultural contaminants are

carried by runoff that carries fertilizers, pesticides,and animal waste into nearby waterways (Figurebelow). Soil and silt erosion also contribute tosurface water contamination.

Figure 21.18 Many types of agriculture add pollutants to

groundwater.Animal wastes expose humans and the

environment to some of the most harmful diseasecausing organisms or pathogens. These includebacteria, viruses, protozoa, and parasites.Pathogens are especially harmful to humans,because they can cause many illnesses includingtyphoid and dysentery as well as minor respiratoryand skin diseases.

You may be surprised to learn that even thefertilizers we use on our lawns and farm fields are

extremely harmful to the environment. Fertilizersfrom lawns and farm fields wash into nearbyrivers, lakes and the oceans. Fertilizers containnitrates that promote tremendous plant growth inthe water. Consequences of this accelerated plantgrowth include:

Lakes, rivers and bays become clogged with acarpet of aquatic plants that block light fromentering the water.Without light reaching plants in the waterbelow, these organisms die.As the plants die, their decomposition uses upall the oxygen in the water. Without enoughdissolved oxygen in the water, large numbersof plants, fish and bottom-dwelling animalsdie.

Every year you can see dead zones, hundredsof kilometers of ocean without fish or plant life(Figure below). These dead zones occur in theGulf of Mexico and other river delta areas due towater polluted with fertilizers. In 1999, a dead

zone in the Gulf of Mexico reached over 7,700square miles.

Figure 21.19 Agricultural Waste Water: A Dead Zone is a

large area of water where fertilizer runoff pollutesfarms and yards, ultimately killing off aquatic life.The size of the dead zone in the Gulf of Mexicovaries at different times of the year.

Ocean Water Pollution

Most (80 %) of ocean pollution comes asrunoff from agriculture, industry, and domesticuses (Figure below). These same kinds of runoffalso pollute freshwater. The remaining 20% ofwater pollution comes from oil spills and peopledumping sewage directly into the water.

Figure 21.20 In some areas of the world, ocean pollution is

all too obvious.Coastal pollution can make coastal water

unsafe for humans and wildlife. After rainfall,there can be enough runoff pollution that beaches

are closed to prevent the spread of disease frompollutants.

A large proportion of the fish stocks we rely onfor food live in the coastal wetlands. Coastalrunoff from farm waste often carries water-borneorganisms that cause lesions that kill fish. Humanswho come in contact with polluted waters andaffected fish can also experience harmfulsymptoms. More than one-third of the shellfish-growing waters of the United States are adverselyaffected by coastal pollution.

Thermal Water Pollution

Thermal pollution is anything that causeswater temperatures to rise or fall (Figure below).For example, power plants and other industriesoften use water to cool equipment. Once the waterabsorbs heat from a power plant or industry, theheated water is returned to the natural environmentat a higher temperature. Cold water pollution canbe observed when very cold water is released

from reservoirs.

Figure 21.21 The Macquarie perch is now extinct in most of

its upland river habitats partially due to thermalpollution by dams.

Why would changing water temperature harmthe environment? Fish and other aquatic organismsare often vulnerable to even small temperaturechanges. Heated water kills fish and otherorganisms by decreasing oxygen supply in thewater. Frigid water has a severe effect on fish(particularly eggs and larvae), macro invertebratesand river productivity.

Lesson Summary

Industrial, agricultural, and municipal sourcesproduce harmful water pollutants such astoxic chemicals, radiological agents, andanimal wastes. Thousands of people die fromwaterborne diseases every year.

Review Questions

1. What do the initials 'EPA' stand for?2. What is runoff and why is it a problem?3. Who is responsible for reducing water

pollution?4. Explain what a dead zone is and where you

might find one?5. What is the leading cause of death for

children around the world?

Vocabulary

dead zoneA region hundreds of kilometers wide withoutfish or plant life due to lack of oxygen in the

water.

thermal pollutionWater pollution created by added heat towater.

Points to Consider

Water pollution not only harms human healthand the environment. Consider how thisreduces the amount of water available tohumans.Fifty percent of all infectious diseases arecaused by water pollution. What can be doneto reduce the number of pathogens that reachour freshwater supplies?Ocean pollution harms some of the mostproductive sources of marine life. How canwe change our behaviors to protect marinelife?

Protecting the Water Supply

Learning Objectives

Describe several ways water can beconserved.Discuss how water is treated to eliminateharmful particles.State what governments and internationalorganizations can do to reduce waterpollution.

Water Treatment

The goal of water treatment is to make watersuitable for such uses as drinking water, medicine,agriculture and industrial processes.

People living in developed countries sufferfrom few waterborne diseases and illness, becausethey have extensive water treatment systems tocollect, treat, and redeliver clean water to their

people. Many undeveloped nations have few or nowater treatment facilities.

Figure 21.22 Wastewater Treatment: Most wastewater

treatment facilities separate contaminants fromwater by passing wastewater through a series ofsettlement containers. At each step, solids andparticles are separated from water. Chemical andbiological agents are also used to remove anyremaining impurities.

Water treatment is any process used to removeunwanted contaminants from water (Figureabove). Water treatment processes are designed toreduce harmful substances such as suspendedsolids, oxygen-demanding materials, dissolved

inorganic compounds, and harmful bacteria.Ideally, water treatment produces both liquids andsolid materials that are not harmful the naturalenvironment.

Water can contain hundreds of contaminants.Not all treatment processes are able to remove allof these particles and not all treated water is pureenough to qualify as safe drinking water. Sewagetreatment is any process that removescontaminants from sewage or wastewater. Waterpurification is any process used to producedrinking water for humans by removingcontaminants from untreated water. Purificationprocesses remove bacteria, algae, viruses, andfungi, unpleasant elements such as iron andsulphur, and man-made chemical pollutants.

The choice of treatment method used dependson the kind of wastewater being treated. Mostwastewater is treated using a series of steps,increasingly purifying the water at each step.Treatment usually starts with separating solidsfrom liquids. Water may then be filtered or treatedwith chlorine. With each subsequent step, the

water has fewer contaminants and the effluent isincreasingly pure.

Reducing Water Pollution

How can people reduce water pollution? Andwho is responsible for doing it?

People have two ways to reduce any kind ofpollution: We can prevent people from pollutingwater. And, we can use science to cleancontaminants from water that is already polluted.

Governments can:

Pass laws to control pollution emissions fromdifferent sources, such as factories andagriculture.Pass laws that require polluters to clean upwater they pollute.Provide money to build and run watertreatment facilities (and fund research toimprove water quality technology).Educate the public, teach them how to prevent

and clean up water pollution.Enforce laws.

The United Nations and other internationalgroups have established organizations to improveglobal water quality standards. Some internationalorganizations provide developing nations with thetechnology and education to collect, treat, anddistribute water. Another priority is educating thepeople in these countries about how they can helpimprove the quality of the water they use.

In the United States, legislators passed theClean Water Act which gives the EnvironmentalProtection Agency the authority to sets standardsfor water quality for industry, agriculture anddomestic uses (Figure below).

Figure 21.23 Scientists control water pollution by sampling

the water and studying the pollutants are in thewater.

One of the toughest problems is enforcement,catching anyone who is not following waterregulations. Scientists are working to createmethods to accurately track the source of waterpollutants. Monitoring (tracking) methods allowthe government to identify, catch and punishviolators.

Who is responsible for reducing waterpollution? Everyone who pollutes water isresponsible for helping to clean it up. This

includes individuals, communities, industries, andfarmers. Just a few of the things you can do toprotect water quality include:

Find approved recycling or disposal facilitiesfor motor oil and household chemicals sothese substances do not end up in the water.Use lawn, garden, and farm chemicalssparingly and wisely.Repair automobile or boat engine leaksimmediately.Keep litter, pet waste, leaves, and grassclippings out of gutters and storm drains.

Controlling Ocean Pollution

Controlling seawater pollution and fresh waterpollution are similar, but not exactly the same. Wecan try to prevent polluters from further spoilingthe ocean and we can require polluters to clean upany pollution they cause. Government andinternational agencies can pass laws, provide

funding, and enforce laws to prevent and clean upocean pollution (Figure below).

Figure 21.24 Many forms of marine pollution can be

controlled by regulation such as the pumping ofballast water from ships.

Several national and international agenciesmonitor and control ocean pollution. The agencies

include the National Oceanic and AtmosphericAdministration (NOAA), the EnvironmentalProtection Agency, the Department of Agricultureas well as other federal and state agencies.

When runoff pollution does cause problems,NOAA scientists help track down the exact causesand find solutions. This organization is also one ofmany organizations trying to educate the public onways to prevent ocean pollution.

Conserving Water

Figure 21.25 Low flow showerheads reduce the amount

water used during showers.As human population growth continues, water

conservation will become increasingly importantglobally (Figure above). Yet, the methods toconserve water are likely to differ betweendeveloping nations and developed countries.

For example, some people in undevelopedcountries use so little water, that they may not gainmuch water by reducing their personal use.Meanwhile, large quantities of water can beconserved in the United States by finding ways tostop overconsumption of water.

At Earth Summit 2002 many governmentsapproved a Plan of Action to address the scarcityof water and safe drinking water in developingcountries. One goal of this plan is to cut in half, thenumber of people without access to safe drinkingwater by 2015.

Developed countries have many options toreduce water consumption. A farmer can cut waterconsumption drastically by using more efficientirrigation methods. People also have manyopportunities to reduce our personal and household

water demand with such measures as low flowshower heads, toilets that use less water, and dripirrigation to water lawns.

During prolonged droughts and other watershortages, some communities ration water use andprohibit such water intensive uses as wateringlawns during the day and hosing down sidewalks.Often legislation is needed to provide incentivesfor individuals to reduce their water consumption.

Lesson Summary

Many technologies are available to conservewater as well as to prevent and treat waterpollution. Yet, most undeveloped countriescannot afford the technology they need tocollect, treat and distribute water to theirpeople.

Developing countries may be able to affordwater treatment systems, but people still needincentives to use conservation steps.

Review Questions

1. What is the purpose of water treatment andpurification?

2. How can governments and internationalorganizations help to reduce water pollution?

3. Name three things that a person could do toreduce pollution? Use lawn, garden orfarming chemicals sparingly or use short term,specific chemicals, rather than long termbroad spectrum chemicals. Repair engineleaks immediately. Keep litter, pet waste,leaves and grass clippings out of stormdrains. Use an approved recycling center todispose of motor oil and household chemicalsand batteries.

4. Name three ways that you could reduce yourpersonal water use.

Further Reading / SupplementalLinks

The American Association for theAdvancement of Science, AAAS Atlas ofPopulation and Environment. University ofCalifornia Press, 2000.www.globalchange.umich.edu/globalchange2/current/lectures/freshwater_supply/freshwater.htmlhttp://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/12/09/IN2HTP07B.DTLhttp://www.wri.org/#http://www.who.int/en/http://www.worldbank.org/http://www.epa.gov/r5water/cwa.htmhttp://www.epa.gov/lawsreg/laws/cwa.htmlhttp://en.wikibooks.org/wiki/Main_Page

Vocabulary

sewage treatment;Any process that removes contaminants fromsewage or wastewater.

water purificationAny process used to produce safe drinking

water by removing contaminants.

Points to Consider

Who is responsible for controlling waterpollution?What can governments and internationalorganizations do to control pollution?It is usually cheaper to dump polluted waterwithout spending money to treat and purify thewater. What incentives would convinceindustry to control water pollution?

Chapter 22: Human Actionsand the Atmosphere

Air Pollution

Lesson Objectives

Describe the different types of air pollutants.Discuss what conditions lead some cities tobecome more polluted than others.Describe the sources of air pollutants.

Introduction

Earth’s atmosphere supports life by providingthe necessary gases for photosynthesis andrespiration. The ozone layer protects life on Earthfrom the Sun’s ultraviolet radiation. People alsouse the atmosphere as a dump for waste gases and

particles. Pollutants include materials that arenaturally-occurring but present in larger quantitiesthan normal. In addition, pollutants consist ofhuman-made compounds that have never beforebeen found in the atmosphere. Pollutants dirty theair, change natural processes in the atmosphere,and harm living things. Excess greenhouse gasesraise global temperatures.

Air Quality

Air pollution problems began centuries agowhen fossil fuels began to be burned for heat andpower. The problem grew into a crisis in thedeveloped nations in the mid-20th century. Coalsmoke and auto exhaust combined to create toxicsmog that in some places caused lung damage andsometimes death. In Donora, Pennsylvania inOctober 1948, 20 people died and 4,000 becameill when coal smoke was trapped by an inversion.Even worse, in London in December 1952, the“Big Smoke” killed 4,000 people over five days,

and it is likely that thousands more died of healthcomplications from the event in the next severalmonths (Figure below).

Figure 22.1 A film crew recreates London smog in the

Victorian Era.A different type of air pollution became a

problem in Southern California after World War II.

Although there was no coal smoke, cars andabundant sunshine produced photochemical smog.This smog is the result of a chemical reactionbetween some of the molecules in auto exhaust oroil refinery emissions, and sunshine.Photochemical smog consists of more than 100compounds, most importantly ozone.

In the United States, these events led to thepassage of the Clean Air Act in 1970. The act nowregulates 189 pollutants. The six most importantpollutants are ozone, particulate matter, sulfurdioxide, nitrogen dioxide, carbon monoxide, andthe heavy metal lead. Other important regulatedpollutants include benzene, perchloroethylene,methylene chloride, dioxin, asbestos, toluene, andmetals such as cadmium, mercury, chromium, andlead compounds. Some of these will be discussedin the following section.

Besides human-caused emissions, air quality isaffected by environmental factors. A mountainrange can trap pollutants on its leeward side.Winds can move pollutants into or out of a region.Pollutants can become trapped in an air mass as a

temperature inversion traps cool air beneath warmair. If the inversion lasts long enough, pollution canreach dangerous levels. Pollutants remain over aregion until they are transported out of the area bywind, diluted by air blown in from another region,transformed into other compounds, or carried tothe ground when mixed with rain or snow.

As a result of the Clean Air Act, air in theUnited States is much cleaner. Visibility is betterand people are no longer incapacitated byindustrial smog. Still, in the United States,industry, power plants and vehicles put 160million tons of pollutants into the air each year.Some of this smog is invisible and somecontributes to the orange or blue haze that affectsmany cities (Figure below).

Table below lists the smoggiest cities in 2007:six of the 10 are in California. The state has theright conditions for collecting pollutants includingmountain ranges that trap smoggy air, arid andsometimes windless conditions, and lots and lotsof cars.

Figure 22.2 Smog over Los Angeles as viewed from the

Hollywood Hills.Smoggiest Cities, 2007

Rank City, State1 Los Angeles, California2 Bakersfield, California3 Visalia-Porterville, California4 Fresno, California5 Houston, Texas6 Merced, California7 Dallas-Fort Worth, Texas8 Sacramento, California9 New York, New York10 Philadelphia, Pennsylvania

(Source: American Lung Association)

Types of Air Pollution

Most air pollutants enter the atmospheredirectly; these are primary pollutants. Secondarypollutants become pollutants only after undergoinga chemical reaction. Primary pollutants includetoxic gases, particulates, compounds that reactwith water vapor to form acids, heavy metals,ozone, and greenhouse gases. Ozone is one of themajor secondary pollutants. It is created by achemical reaction that takes place in exhaust and inthe presence of sunlight.

Primary Pollutants

Some primary pollutants are natural, such asdust and volcanic ash, but most are caused byhuman activities. Primary pollutants are directemissions from vehicles and smokestacks. Some ofthe most harmful pollutants that go directly into theatmosphere from human activities include:

Carbon oxides include carbon monoxide(CO) and carbon dioxide (CO2). Both arecolorless, odorless gases. CO is toxic to bothplants and animals. CO and CO2 are bothgreenhouse gases.Nitrogen oxides are produced when nitrogenand oxygen from the atmosphere cometogether at high temperatures. This occurs inhot exhaust gas from vehicles, power plantsor factories. Nitrogen oxide (NO) andnitrogen dioxide (NO2) are greenhouse gases.Nitrogen oxides contribute to acid rain.Sulfur oxides include sulfur dioxide (SO2)and sulfur trioxide (SO3). These form whensulfur from burning coal reaches the air.Sulfur oxides are components of acid rain.Particulates are solid particles, such as ash,dust and fecal matter. They are commonlyformed from combustion of fossil fuels, andcan produce smog. In addition, particulatematter can contribute to asthma, heart disease,and some types of cancers.

Lead was once widely used in automobilefuels, paint, and pipes. This heavy metalcauses can cause brain damage or bloodpoisoning.Volatile organic compounds (VOCs) aremostly hydrocarbons, compounds made ofhydrogen and carbon. Important VOCsinclude methane (a naturally occurringgreenhouse gas that is increasing due tohuman activities), chlorofluorocarbons(human-made compounds that are beingphased out because of their effect on theozone layer), and dioxin (a byproduct ofchemical production that serves no usefulpurpose, but is harmful to humans and otherorganisms).

Photochemical Smog

Any city can have photochemical smog, but itis most common in arid locations. A rise in thenumber of vehicles in cities worldwide has

increased photochemical smog. This smog formswhen car exhaust is exposed to sunlight. Nitrogenoxides are created in car combustion chambers. Ifthere is sunshine, the NO2 splits and releases anoxygen atom (O). The oxygen ion then combineswith an oxygen molecule (O2) to form ozone (O3).This reaction can also go in reverse: Nitric oxide(NO) removes an oxygen atom from ozone to makeit O2. The direction the reaction proceeds dependson how much NO2 and NO there is. If NO2 is threetimes more abundant than NO, ozone will beproduced. If nitrous oxide levels are high, ozonewill not be created.

Ozone is an acrid-smelling, whitish gas.Warm, dry cities surrounded by mountains, such asLos Angeles, Phoenix, and Denver, are especiallyprone to photochemical smog (Figure below).Photochemical smog peaks at midday on the hottestdays of summer. Other compounds in addition toozone are found in photochemical smog. Ozone isalso a greenhouse gas.

Figure 22.3 Counties with such high ozone levels that they

do not attain federal air quality standards.

Causes of Air Pollution

Most air pollutants come from burning fossilfuels or plant material. Some are the result ofevaporation from human-made materials. Nearlyhalf (49%) of air pollution comes fromtransportation, 28% from factories and powerplants, and the remaining pollution from a varietyof other sources.

Fossil Fuels

Fossil fuels are burned in most motor vehiclesand power plants (Figure below). They fuelmanufacturing and other industries. Pure coal andpetroleum can theoretically burn cleanly, emittingonly carbon dioxide and water, which are bothgreenhouse gases. But most of the time, these fossilfuels do not completely burn, so these incompletechemical reactions produce pollutants. In addition,few fossil fuels are pure and so other pollutantsare usually released. These pollutants includecarbon monoxide, nitrogen dioxide, sulfur dioxideand hydrocarbons.

Figure 22.4

A power plant and its emissions beforeemission control equipment was added.

In large car-dependent cities such as LosAngeles and Mexico City, 80% to 85% of airpollution is from motor vehicles. Auto emissionsare the most common source of ozone. Carbonmonoxide is toxic in enclosed spaces like tunnels.Nitrous oxides come from the exhaust from avehicle or a factory. Lead was once put in gasolineto improve engine knock, but is now banned in theUnited States. Still, enormous quantities of leadare released into the air every year from othersources.

A few pollutants come primarily from powerplants or industrial plants. They pour out ofsmokestacks that burn coal or oil. Sulfur dioxide(SO2) is a major component of industrial airpollution. It is released whenever coal andpetroleum are burned. SO2 mixes with H2O in theair to produce sulfuric acid (H2SO4). The heavymetal mercury is released when coal and sometypes of wastes are burned. Mercury is emitted as

a gas, but as it cools, it becomes a droplet.Mercury droplets eventually fall to the ground. Ifthey fall into sediments, bacteria convert them tothe most dangerous form of mercury: methylmercury. Highly toxic, methyl mercury is one of themetal’s organic forms.

Biomass Burning

Fossil fuels are ancient plants and animals thathave been converted into usable hydrocarbons.Burning plant and animal material directly alsoproduces pollutants. Biomass is the total amount ofliving material found in an environment. Thebiomass of a rainforest is the amount of livingmaterial found in that rainforest.

The primary way biomass is burned is byslash-and-burn agriculture (Figure below). Therainforest is slashed down and then the waste isburned to clear the land for farming. Biomass fromother biomes, like savannah, is also burned toclear farmland. The pollutants are much the same

as from burning fossil fuels: CO2, carbonmonoxide, methane, particulates, nitrous oxide,hydrocarbons, and organic and elemental carbon.Burning forests increase greenhouse gases in theatmosphere by releasing the CO2 stored in thebiomass and also by removing the forest so that itcannot store CO2 in the future. As with all forms ofair pollution, the smoke from biomass burningoften spreads far and pollutants can plagueneighboring states or countries.

Figure 22.5 A forest that has been slash-and-burned to

make new farmland.Particulates result when anything is burned.

About 40% of the particulates that enter theatmosphere above the United States are fromindustry and about 17% are from vehicles.Particulates also occur naturally from volcaniceruptions or windblown dust. Like other pollutants,they travel all around the world on atmosphericcurrents.

Evaporation

Volatile organic compounds (VOCs) enter theatmosphere by evaporation. VOCs evaporate fromhuman-made substances, such as paint thinners, drycleaning solvents, petroleum, wood preservatives,and other liquids. Naturally occurring VOCsevaporate off of pine and citrus trees. Theatmosphere contains tens of thousands of differentVOCs, nearly 100 of which are monitored. Themost common is methane, a greenhouse gas.Methane occurs naturally, but human agriculture isincreasing the amount of methane in theatmosphere.

Lesson Summary

Industrial pollution causes health problemsand even death, though the Clean Air Act hasdecreased these health problems in the UnitedStates by forcing industry to clean theiremissions.The increase in motor vehicles in arid citieshas increased ozone and other secondarypollutants in these regions.Burning fossil fuels is the greatest source ofair pollution.Biomass burning is also a large source,especially in places where slash-and-burnagriculture is practiced.

Review Questions

1. What is the difference between the type ofsmog experienced by cities in the easternUnited States and that found in SouthernCalifornia?

2. London has suffered from terrible airpollution for at least seven centuries. Why isthe city so prone to its famous “London fog?”What did London do to get rid of its airpollution?

3. Imagine two cities of the same size with thesame amount of industrialization and the samenumber of motor vehicles. City A isincredibly smoggy most of the time and City Busually has very little air pollution. Whatfactors are important for creating these twodifferent situations?

4. What might be a reason why the city of SanFrancisco and its metropolitan area not on thelist of smoggiest cities for 2007?

5. Why are naturally-occurring substances, likeparticulates or carbon dioxide, sometimesconsidered pollutants?

6. How does ozone form from vehicle exhaust?7. What are the necessary ingredients for ozone

creation, excluding those that are readilyavailable in the atmosphere? Why could therebe a city with a lot of cars but relatively little

ozone pollution?8. Some people say that we need to phase out

fossil fuel use and replace it with cleanenergy. Why is fossil fuel use becomingundesirable?

9. Mercury is not particularly toxic as a metalbut it is very dangerous in its organic form.How does mercury convert from the metal tothe organic form?

10. In what two ways does deforestationcontribute to air pollution?

Vocabulary

leadA heavy metal found in a large number ofproducts. Exposure to too much lead causeslead poisoning, which harms people’s brainand blood.

mercuryA heavy metal that enters the atmosphere

primarily from coal-burning power plants.Mercury that has been converted to an organicform (methylmercury) is highly toxic.

ozoneThree oxygen atoms bonded together in an O3molecule. Ozone in the lower atmosphere is apollutant but in the upper atmosphere protectslife from ultraviolet radiation.

particulatesParticles like ash, dust and fecal matter in theair. Particulates may be caused by naturalprocesses, such as volcanic eruptions or duststorms, or they may be caused by humanactivities, like burning fossil fuels orbiomass.

photochemical smogThis type of air pollution results from achemical reaction between pollutants in thepresence of sunshine.

slash-and-burn agricultureIn the tropics, rainforest plants are slasheddown and then burned to clear the land foragriculture.

volatile organic compounds (VOCs)Pollutants that evaporate into the atmospherefrom solvents and other humanmadecompounds. Some VOCs occur naturally.

Points to Consider

Despite the Clean Air Act, the air over manyregions in the United States is still not clean.Why?How do pollutants damage human health?In what ways does air pollution harm theenvironment?

Effects of Air Pollution

Lesson Objectives

Describe the damage that is being done bysmog.Discuss how acid rain is formed and thedamage it does.Discus how chlorofluorocarbons destroy theozone layer.

Introduction

People in developing countries often do nothave laws to protect the air that they breathe. TheWorld Health Organization estimates that 22million people die each year from complicationsdue to air pollution. Even in the United States,more than 120 million Americans live in areaswhere the air is considered unhealthy. This lessonlooks at the human health and environmental

problems caused by different types of air pollution.

Smog

All air pollutants cause some damage to livingcreatures and the environment. Different types ofpollutants cause different types of harm.Particulates reduce visibility. For example, in thewestern United States, people can now ordinarilysee only about 100 to 150 kilometers (60 to 90miles), which is one-half to two-thirds the natural(pre-pollution) range on a clear day. In the East,visibility is worse. People can only see about 40to 60 kilometers (25-35 miles), which is one-fifththe distance they could see without any airpollution.

Particulates reduce the amount of sunshine thatreaches the ground. Since plants also receive lesssunlight, there may be less photosynthesis.Particulates also form the nucleus for raindrops,snowflakes or other forms of precipitation. Anincrease in particles in the air seems to increase

the number of raindrops, but often decreases theirsize. By reducing sunshine, particulates can alsoalter air temperature. In the three days after theterrorists attacks on September 11, 2001, jetairplanes did not fly over the United States.Without the gases from jet contrails blockingsunlight, air temperature increased 1°C (1.8°F)across the U.S (Figure below). Imagine how muchall of the sources of particulates combine to reducetemperatures.

Figure 22.6 Jet contrails block sunlight.Ozone damages some plants. Since ozone

effects accumulate, plants that live a long timeshow the most damage. Some species of trees

appear to be the most susceptible. If a forestcontains ozone-sensitive trees, they may die outand be replaced by species that are not as easilyharmed. This can change an entire ecosystem, sinceanimals and plants may not be able to survivewithout the habitats created by the native trees.

Some crop plants show ozone damage. Whenexposed to ozone, spinach leaves become spotted.Soybeans and other crops have reducedproductivity. In developing nations, where gettingevery last bit of food energy out of the agriculturalsystem is critical, any loss is keenly felt. Many ofthese nations, like China and India, also haveheavy air pollution. Some pollutants have apositive effect on plant growth. Increased CO2seems to lessen ozone damage to some plants andit may promote growth. Unfortunately, CO2 andother greenhouse gases cause other problems thatharm the ecosystem and reduce growth of someplants.

Other air pollutants damage the environment(Figure below). NO2 is a toxic, orange-brown

colored gas that gives air a distinctive orangecolor and an unpleasant odor. Nitrogen and sulfur-oxides in the atmosphere create acids that fall asacid rain. Human health suffers in locations withhigh levels of air pollution. Lead is the mostcommon toxic material for humans and isresponsible for lead poisoning. Carbon monoxideis a toxic gas and can kill people in poorlyventilated spaces, like tunnels. Nitrogen andsulfur-oxides cause lung disease and increase ratesof asthma, emphysema, and viral infections likeflu. Ozone also damages the human respiratorysystem, causing lung disease. High ozone levelsare also associated with increased heart diseaseand cancer. Particulates enter the lungs and causeheart or lung disease. When particulate levels arehigh, asthma attacks are more common. By someestimates, 30,000 deaths a year in the United Statesare caused by fine particle pollution.

Although not all cases of asthma can be linkedto air pollution, many can. During the 1996Olympic Games, Atlanta, Georgia closed off theirdowntown to private vehicles. As a result, ozone

levels decreased by 28%. At the same time, therewere 40% fewer hospital visits for asthma.

Figure 22.7 Smog in New York City.Lung cancer among non-smokers is also

increasing. One study showed that the risk of beingafflicted with lung cancer increases directly with aperson’s exposure to air pollution. The studyconcluded that no level of air pollution should beconsidered safe. Exposure to smog also increasedthe risk of dying from any cause, including heartdisease.

Children are more vulnerable to problems frombreathing dirty air than adults because their lungsare still growing and developing. Children take in50% more air for their body weight than adults.

Children spend more time outside in unfiltered airand are more likely to breathe hard from playing orexercising. One study found that in the UnitedStates, children develop asthma at more than twicethe rate of two decades ago and at four times therate in Canada. Adults also suffer from airpollution-related illnesses that include lungdisease, heart disease, lung cancer, and weakenedimmune systems. The asthma rate worldwide isrising 20% to 50% every decade.

Especially dangerous are pollutants that remainin an organism throughout its life, calledbioaccumulation. In this process, an organismaccumulates the entire amount of a toxic compoundthat it consumes over its lifetime. Not allsubstances bioaccumulate. A person who takes adaily dose of aspirin only has that day’s worth ofaspirin in her system, because aspirin does not staywithin her system. When a compoundbioaccumulates, the person has all of thatcompound she’s ever eaten in her system.Compounds that bioaccumulate are usually storedin the organism’s fat.

Mercury is a good example of a substance thatbioaccumulates. Bacteria and plankton store all ofthe mercury from all of the seawater they ingest. Asmall fish that eats bacteria and planktonaccumulates all of the mercury from all of the tinycreatures it eats over its lifetime. A big fishaccumulates all of the mercury from all of thesmall fish it eats over its lifetime. The organismsthat accumulate the most mercury are the largepredators that eat high on the food chain. Tunapose a health hazard to anything that eats thembecause their bodies are so high in mercury. Thisis why the government recommends limits on theamount of tuna that people eat. These limits areespecially important for children and pregnantwomen, since mercury particularly affects youngpeople. If the mercury just stayed in fat, it wouldnot be harmful, but that fat is used when a womanis pregnant or nursing a baby, or when she burnsthe fat while losing weight. Methyl mercurypoisoning can cause nervous system or braindamage, especially in infants and children.Children may experience brain damage or

developmental delays. Like mercury, other metalsand VOCS can bioaccumulate, causing harm toanimals and people high on the food chain.

Acid Rain

Acid rain is caused by sulfur and nitrogenoxides. These pollutants are emitted into theatmosphere from power plants or metal refineries.The oxides come out of smokestacks that have beenbuilt tall so that pollutants don’t sit over cities. Thehigh smokestacks allow the emissions to rise highinto the atmosphere and travel up to 1000 km (600miles) downwind. As they move, these pollutantscombine with water vapor to form sulfuric andnitric acids. The acid droplets form acid fog, rain,snow, or they may be deposited dry. Most typicalis acid rain (Figure below).

Figure 22.8 How acid rain is formed. Anthropogenic

pollutants are those that are human-made.Deposition of a pollutant occurs when it is placedon a surface. Rain can bring wet deposition or apollutant can be blown onto the ground for drydeposition.

Acid rain water is more acidic than normalrain water. To be called acid rain, it must have apH of less than 5.0. Acidity is measured on the pHscale, which goes from 1 to 14. A value of 7 isneutral. Lower numbers are more acidic and highernumbers are less acidic (also called morealkaline). The strongest acids are at the low end ofthe scale and the strongest bases are at the high

end. Natural rain is somewhat acidic with a pH of5.6. The acid comes from carbonic acid that formswhen CO2 combines with water in the atmosphere.A small change in pH represents a large change inacidity: rain with a pH of 4.6 is 10 times moreacidic than normal rain (with a pH of 5.6). Rainwith a pH of 3.6 is 100 times more acidic.

Regions that have a lot of coal-burning powerplants have the most acidic rain. The acidity ofaverage rainwater in the northeastern United Stateshas fallen to between 4.0 and 4.6. Acid fog haseven lower pH with an average of around 3.4. Onefog in Southern California in 1986 had a pH of 1.7,equal to toilet bowl cleaner. In arid climates, likein Southern California, acids deposit on the grounddry. Acid precipitation ends up on the land surfaceand in water bodies. Some forest soils in thenortheast are 5 to 10 times more acidic than theywere two or three decades ago. Acid dropletsmove down through acidic soils to lower the pH ofstreams and lakes even more. Acids strip soil ofmetals and nutrients, which collect in streams andlakes. As a result, stripped soils may no longer

provide the nutrients that native plants need.Acid rain takes a toll on ecosystems (Figure

below). Plants that are exposed to acids becomeweak and are more likely to be damaged by badweather, insect pests, or disease. Snails die in acidsoils, so songbirds do not have as much food toeat. Young birds and mammals do not build bonesas well and may not be as strong. Eggshells mayalso be weak and break more easily.

Figure 22.9 Acid rain has killed trees in this forest in the

Czech Republic.The nitrates found in acid rain cause some

plants to grow better. These nitrate-lovers can

drive out other plants, which may cause theecosystem to change. Nitrates also fertilize theoceans, which makes more algae grow. The algaeuse up all the oxygen in the water, which can bringabout disastrous ecological changes, including thedeaths of many fish. As lakes become acidic,organisms die off. If the pH drops below 4.5, allthe fish die. Organic material cannot decay, andmosses take over the lake. Wildlife that depend onthe lake for drinking water suffer populationdeclines. Crops are damaged by acid rain. This ismost noticeable in poor nations where people can’tafford to fix the problems with fertilizers or othertechnology. Buildings and monuments are damagedby acid precipitation (Figure below). Theseinclude the U.S. Capital and many buildings inEurope, such as Westminster Abbey.

Figure 22.10 Acid rain damages cultural monuments like

buildings and statues.Carbonate rocks can neutralize acids and so

some regions do not suffer the effects of acid rainnearly as much. The Midwestern United States isprotected by the limestone rocks throughout thearea, which are made up of calcium carbonate.One reason that the northeastern United States is sovulnerable to acid rain damage is that the rocks arenot carbonates.

Because pollutants can travel so far, much ofthe acid rain that falls hurts states or nations otherthan ones where the pollutants were released. Allthe rain that falls in Sweden is acidic and fish in

lakes all over the country are dying. The pollutantscome from the United Kingdom and WesternEurope, which are now working to decrease theiremissions. Canada also suffers from acid rain thatoriginates in the United States, a problem that isalso improving. Southeast Asia is experiencingmore acid rain between nations as the regionindustrializes.

Ozone Depletion

At this point you might be asking yourself, “Isozone bad or is ozone good?” There is no simpleanswer to that question: It depends on where theozone is located. In the troposphere, ozone is apollutant. Higher up, in the stratosphere, ozonescreens out high energy ultraviolet radiation andthus makes Earth habitable. This protective ozoneis found in the ozone layer.

The ozone layer is being attacked by human-made chemicals that break ozone molecules apartin the stratosphere. The most common of these

chemicals are chlorofluorocarbons (CFCs), butincludes others such as halons, methyl bromide,carbon tetrachloride, and methyl chloroform. CFCswere once widely used because they are cheap,nontoxic, nonflammable, and non-reactive. Theywere used as spray-can propellants, refrigerants,and in many other products.

Once they are released into the air, CFCs floatup to the stratosphere. Air currents move themtoward the poles. In the winter, they freeze ontonitric acid molecules in polar stratospheric clouds(PSC). PSCs form only where the stratosphere iscoldest, and are most common above Antarctica inthe wintertime. In the spring, the sun’s warmthstarts the air moving, and ultraviolet light breaksthe CFCs apart. The chlorine atom floats away andattaches to one of the oxygen atoms on an ozonemolecule. The chlorine pulls the oxygen atomaway, leaving behind an O2 molecule, whichprovides no UV protection. The chlorine thenreleases the oxygen atom and moves on to destroyanother ozone molecule. One CFC molecule candestroy as many as 100,000 ozone molecules.

Ozone destruction creates the ozone holewhere the layer is dangerously thin (Figurebelow). As air circulates over Antarctica in thespring, the ozone hole expands northward over thesouthern continents, including Australia, NewZealand, southern South America, and southernAfrica. UV levels may rise as much as 20%beneath the ozone hole. The hole was firstmeasured in 1981 when it was 2 million square km(900,000 square miles)). The 2006 hole was thelargest ever observed at 28 million square km(11.4 million square miles). It had the lowestozone levels ever recorded and also lasted thelongest. The difference in the size of the ozone holeeach year depends on many factors, includingwhether conditions are right for the formation ofpolar stratospheric clouds.

Figure 22.11 The September 2006 ozone hole, the largest

ever observed. Blue and purple colors showparticularly low levels of ozone.

Ozone loss also occurs over the north polarregion, but it is not enough for scientists to call it ahole. The region of low ozone levels is smallbecause the atmosphere is not as cold and PSCs donot form as readily. Still, springtime ozone levelsare relatively low. This low moves south oversome of the world’s most populated areas inEurope, North America, and Asia. At 40°N, the

latitude of New York City, UV-B has increasedabout 4% per decade since 1978. At 55°N, theapproximate latitude of Moscow and Copenhagen,the increase has been 6.8% per decade since 1978.

Ozone losses in population centers increasesunburns, cataracts (clouding of the lens of theeye), and skin cancers. A loss of ozone of only 1%is estimated to increase skin cancer cases by 5 to6%. People may also suffer from decreases in theirimmune system’s ability to fight off infectiousdiseases. Ozone loss may reduce crop yields, sincemany plants are sensitive to ultraviolet light.Excess UV appears to be decreasing theproductivity of plankton in the oceans. A decreaseof 6 to 12% has been measured around Antarctica,which may be at least partly related to the ozonehole. The effects of excess UV on other organismsis not known. When the problem with ozonedepletion was recognized, world leaders tookaction. CFCs were banned in spray cans in somenations in 1978. The greatest production of CFCswas in 1986, but has declined since then. This willbe discussed more in the next lesson.

Lesson Summary

Air pollutants damage human health and theenvironment. Particulates reduce visibility,alter the weather, and cause lung problemslike asthma attacks.Ozone damages plants and can also causelung disease. Acid rain damages forests,crops, buildings and statues.The ozone hole, caused by ozone-destroyingchemicals, allows more UV radiation to strikethe Earth.This can cause plankton populations todecline and skin cancers in humans toincrease, along with other effects.

Review Questions

1. Why is visibility so reduced in the UnitedStates?

2. Why do health recommendations suggest thatpeople limit the amount of tuna they eat?

3. Why might ozone pollution or acid rainchange an entire ecosystem?

4. Why does air pollution cause problems indeveloping nations more than in developedones?

5. Why are children more vulnerable to theeffects of air pollutants than adults?

6. Describe bioaccumulation.7. How does pollution indirectly kill or harm

plants?8. What do you think the effect is of jet airplanes

on global warming?9. Why is air pollution a local, regional and

global problem?10. How do CFCs deplete the ozone layer?

Vocabulary

acid rainRain that has a pH of less than 5.0.

alkaline

Also called basic. Substances that have a pHof greater than 7.0.

bioaccumulationThe accumulation of toxic substances withinorganisms so that the concentrations increaseup the food web.

ozone holeA region around Antarctica in which ozonelevels are reduced in springtime, due to theaction of ozone-destroying chemicals.

pH scaleA scale that measures the acidity of asolution. A pH of 7 is neutral. Smallernumbers are more acidic and larger numbersare more alkaline.

polar stratospheric clouds (PSC)Clouds that form in the stratosphere when it isespecially cold; PSCs are necessary for thebreakup of CFCs.

Points to Consider

Since mercury bioaccumulates and coal-firedpower plants continue to emit mercury intothe atmosphere, what will be the consequencefor people who like to eat tuna and other largepredatory fish?What are the possible causes of rising asthmarates in children?A ban has been imposed on CFCs and someother ozone-depleting substances. How willthe ozone hole change in response to this ban?

Reducing Air Pollution

Lesson Objectives

Describe the major ways that energy use canbe reduced.Discuss new technologies that are beingdeveloped to reduce air pollutants, includinggreenhouse gases.Describe the difference between placing capson emissions and reducing emissions.

Introduction

The Clean Air Act of 1970 and the amendmentssince then have done a great job in requiringpeople to clean up the air over the United States.Emissions of the six major pollutants regulated bythe Clean Air Act, carbon monoxide, lead, nitrousoxides, ozone, sulfur dioxide, and particulates,have decreased by more than 50%. Cars, power

plants, and factories individually release lesspollution than they did in the mid-20th century. Butthere are many more cars, power plants andfactories. Many pollutants are still being releasedand some substances have been found to bepollutants that were not known to be pollutants inthe past. There is still much work to be done tocontinue to clean up the air.

Ways to Reduce Air Pollution

Air pollution can be reduced in a number ofways. Using less fossil fuel is one way to lessenpollution. People use less fuel by engaging inconservation, which means not using a resource orusing less of it. For example, riding a bike orwalking instead of driving doesn’t use any fossilfuel. Taking a bus uses less than driving or ridingby yourself in a car, as does carpooling. If youneed to drive, buying a car that has greater fuelefficiency is important. You can conserveelectricity (and thus fossil fuels) at home by turning

off light bulbs and appliances when they are not inuse, using energy efficient light bulbs andappliances, and even buying less stuff. All theseactions reduce the amount of energy that powerplants need to produce.

There are many reasons for people in NorthAmerica and Europe to try to reduce their use offossil fuels. As you have already seen, airpollution has tremendous health and environmentalcosts. There are other reasons as well. Much of theoil we use comes from the Middle East, which is apolitically unstable region of the world. Also,fossil fuels are running out, although some will runout sooner than others. The most easily accessiblefossil fuels are mostly already gone and harder touse or recover fuels are now being used. There areother types of fossil fuels that can eventuallyreplace coal and petroleum, such as tar sands andoil shale. But these have even more environmentalproblems than traditional fossil fuels have: miningthem from the ground causes severe environmentaldamage and burning them releases pollutants,including greenhouse gases.

Alternative energy sources are important. Theycurrently are not a large part of the energy supply,but they will increase rapidly over the comingyears and decades. Several sources of alternativeenergy, including solar and wind are not currentlybeing used much because the technologies are notwell enough developed. Converting sunlight intousable solar power, for example, is still veryexpensive relative to using fossil fuels. For solarto be used more widely, technology will need toadvance so that the price falls. Also, solar poweris not practiced in all parts of the United Statesbecause some areas get low amounts of sunlight.These locations will need to develop differentpower sources. While the desert Southwest willneed to develop solar, the Great Plains can usewind energy as its energy source. Perhaps somelocations will rely on nuclear power plants,although current nuclear power plants have majorproblems like safety and waste disposal.

Some pollutants can be filtered out of theexhaust stream before they are released into theatmosphere. Other pollutants can be broken down

into non-toxic compounds before they are released.Some of these technologies will be described inthe following sections.

Reducing Air Pollution fromVehicles

Reducing air pollution from vehicles can bedone in a number of ways. Pollutants can bebroken down before they are released into theatmosphere. The vehicles can be more fuelefficient. New technologies can be developed sothat they do not rely on fossil fuels at all.

Motor vehicles emit less pollution than theyonce did due to catalytic converters (Figurebelow). Catalytic converters are placed on moderncars in the United States. These devices reduceemissions of nitrous oxides, carbon monoxide andVOCs. A catalyst speeds up chemical reactionswithout being used up in the reaction itself. Fornitrous oxides, the catalyst breaks the nitrogen andoxygen atoms apart. The nitrogen then combines

with another nitrogen ion to form nitrogen gas (N2)and the oxygen forms O2. VOCs and CO aresimilarly broken apart into the greenhouse gasesH2O and CO2. Catalytic converters only workwhen they are hot, so a lot of exhaust escapes asthe car is warming up.

Figure 22.12 A large catalytic converter on an SUV.There are several simple ways to make a

vehicle more fuel efficient. Lighter vehicles needless energy to move. Streamlined vehiclesexperience less resistance from the wind. So,

small, lightweight, streamlined cars get muchbetter gas mileage than chunky, heavy SUVs.Hybrid vehicles are among the most efficientvehicles that are now widely available. Hybridshave a small internal combustion engine that workslike an ordinary car. They also have an electricmotor and a rechargeable battery. During braking,a normal car loses the energy it has because it is inmotion. In a hybrid, that energy is instead funneledinto charging the battery. When the car acceleratesagain, it uses the power stored in the battery. Theinternal combustion engine only takes over whenpower in the battery has run out. Hybrids getexcellent gas mileage in cities where the vehiclefrequently stops and starts. Hybrid vehicles alsohave catalytic converters: the battery preheats theconverter so that it begins to work much soonerafter the car is turned on. Hybrids can reduce autoemissions by 90% or more. Unfortunately, in manyhybrid vehicles the hybrid technology is used toimprove acceleration more than gas mileage.

A new technology that is in development is aplug-in hybrid. The vehicle is plugged into an

electricity source when it is not in use, perhaps ina garage. The car uses the power stored in thatbattery when it is next used. Plug-in hybrids areless polluting than regular hybrids, since they canrun for a longer time on electricity. Automakersexpect that plug-in hybrids will become availablearound 2010.

Fuel cells are another technology that is indevelopment (Figure below). Hydrogen fuel cellsharness the energy released when hydrogen andoxygen come together to create water. Fuel cellsare extremely efficient and they produce nopollutants. But developing fuel cell technology hasits problems. The oxygen the fuel cell uses comesfrom the atmosphere, but there is no easy source ofhydrogen. Natural gas is a source, but converting itinto usable hydrogen decreases the efficiency ofthe fuel cells and increases pollution, includinggreenhouse gases. Natural gas also has otherimportant uses. A few fuel cell cars are now beingproduced as models. Right now these cars areextremely expensive and fueling stations are rare.Some automakers say that for fuel cell vehicles to

become widespread the cost of production mustdecrease to 1% of its current price.

Figure 22.13 A hydrogen fuel cell car looks like a gasoline-

powered car.

Reducing Industrial Air Pollution

Pollutants are removed from the exhauststreams of power plants and industrial plantsbefore they enter the atmosphere. Particulates canbe filtered out, while sulfur and nitric oxides arebroken down by catalysts. Removing these oxidesreduces the pollutants that cause acid rain.

Particles are relatively easy to remove fromemissions. Baghouses work like a giant vacuumcleaner bag, filtering dust as it streams past.

Baghouses collect about 98% of dry particulates.Cyclones are air streams that rotate quickly througha container shaped like a cylinder or a cone. Largeparticles are forced toward the edges of the airstream. When they hit the outside wall of thecontainer, they fall to the bottom and are swept up.Smaller particles can be picked up as the radius ofthe cyclone is reduced. Particles can also becollected and removed by static electricity. Theseelectrostatic precipitators are useful for removingmaterials from very hot gases.

Scrubbers remove particles and waste gasesfrom exhaust (Figure below). Wet scrubbers use aliquid solution to scrub pollutants. Dry scrubbersuse alkali or other materials to neutralize acid gaspollutants. Other techniques are used to eliminateother toxic gases. Nitrogen oxides, for example,can be broken down at very high temperatures.

Figure 22.14 Diagram of one type of scrubber.Gasification is a developing technology. This

method removes some of the toxins present in coalbefore they are released into the atmosphere. Ingasification, coal is heated to extremely hightemperatures. The gas that is produced is filteredand the energy goes on to drive a generator. About80% less pollution is released over regular coal

plants, and greenhouse gases are also lower. Cleancoal plants do not need scrubbers or otherpollution control devices. Although the technologyis ready, clean coal plants are more expensive toconstruct and operate and so they are seldom built.Also, heating the coal to high enough temperaturesuses a great deal of energy, so the technology notvery energy efficient. In addition, large amounts ofthe greenhouse gas CO2 are still released evenwith clean coal technology.

Reducing Ozone Destruction

One success story in reducing pollutants thatharm the atmosphere concerns ozone-destroyingchemicals. In 1973, scientists calculated that CFCscould reach the stratosphere and break apart. Thiswould release chlorine atoms, which would thendestroy ozone. Based only on their calculations,the United States and most Scandinavian countriesbanned CFCs in spray cans in 1978.

More confirmation that CFCs break down

ozone was needed before more was done to reduceproduction of ozone-destroying chemicals. In1985, members of the British Antarctic Surveyreported that a 50% reduction in the ozone layerhad been found over Antarctica in the previousthree springs. Two years later, the 'MontrealProtocol on Substances that Deplete the OzoneLayer' was ratified by nations all over the world.

The Montreal Protocol controls the productionand consumption of 96 chemicals that damage theozone layer. Hazardous substances are phased outfirst by developed nations and one decade later bydeveloping nations. More hazardous substancesare phased out more quickly. CFCs have beenmostly phased out since 1995, although some willbe used in developing nations until 2010. TheProtocol also requires that wealthier nationsdonate money to develop technologies that willreplace these chemicals.

If CFCs were not being phased out, by 2050they would have been probably been 10 timesmore abundant than they were in 1980. The resultwould have been about 20 million more cases of

skin cancer in the United States and 130 millioncases globally. Even though governments haveacted to reduce CFC’s, they take many years toreach the stratosphere and they can survive there along time before they break down. So the ozonehole will probably continue to grow for some timebefore it begins to shrink. The ozone layer willreach the same levels it had before 1980 in around2068 and 1950 levels in one or two centuries.

Reducing Greenhouse Gases

Reducing greenhouse gas emissions is relatedto air pollution control. Unlike many other airpollutants, climate change is a global problem.Climate scientists agree that all nations must cometogether to reduce greenhouse gas emissions. Sofar, this has not occurred.

The first attempt to cap greenhouse gasemissions was the Kyoto Protocol. The KyotoProtocol limits greenhouse gas emissions fordeveloped nations to below 1990 levels. Kyoto

has not achieved the success of the MontrealProtocol for several reasons. The largest emitter ofgreenhouse gases, the United States, did not signand was not bound by the agreement. Developingnations, most notably China, signed the treaty butare not obligated to make changes in theirgreenhouse gas emissions. Of the nations thatagreed to reduce their emissions, few are on trackto achieve their target. More importantly, severalyears have passed since this process was begunand climate scientists agree that the Protocol doesnot reduce emissions nearly enough. Some say thatreductions 40 times those required by Kyoto areneeded to avoid dangerous climate change. Plansare now being made to replace the Kyoto Protocolwith a more effective treaty in 2012.

The Kyoto Protocol set up a cap-and-tradesystem. Each participating nation was given a capon greenhouse gas emissions that it should not goover. If a nation is likely to go over its cap, it canbuy credits from a nation that will emit lessgreenhouses gases than allowed by the cap. Cap-and-trade provides a monetary incentive for

nations to develop technologies that will reduceemissions and to conserve energy. Some states andcities within the United States have begun theirown cap-and-trade systems, since they believe thatthe federal government is not doing enough toaddress the problem of climate change.

However it is done, climate scientists andmany others agree that greenhouse gas emissionsmust be lowered. The easiest and quickest way isto increase energy efficiency. A carbon tax can beplaced on CO2 emissions to encourageconservation. The tax would be placed ongasoline, carbon dioxide emitted by factories, andhome energy bills to encourage conservation. Forexample, when people make a purchase of a newcar, they will be more likely to purchase an energyefficient model. The money from the carbon tax canthen be used for research into alternative energysources. All plans for a carbon tax allow a taxcredit for people who cannot afford to pay morefor energy, so that they do not suffer unfairly.

More energy efficient vehicles and appliancescan be developed. Some, like hybrid cars are

currently available. Agricultural practices thatlessen the amount of methane produced can beused.

Beyond increasing efficiency, newtechnologies can be developed. Alternative energysources, like solar and wind can be developed andexpanded. Biofuels can replace gasoline invehicles, but they must be developed sensibly(Figure below). So far much of the biofuel isproduced from crops like corn. But when foodcrops are used for fuel, the price of food goes up.Also modern agriculture is extremely reliant onfossil fuels for pesticides, fertilizers and the workof farming. This means that not much energy isgained from using a biofuel over using the fossilfuels directly. More promising crops for biofuelsare now being researched. Surprisingly, algae isbeing investigated as a source of fuel! The algaecan be grown in areas that are not useful foragriculture, and it also contains much more useableoil than crops like corn.

Figure 22.15 A bus that runs on soybean oil shows the

potential of biofuels.Greenhouse gases can also be removed from

the atmosphere after they are emitted. Carbonsequestration occurs when carbon dioxide isremoved from the atmosphere. Carbon issequestered naturally in forests, but unfortunately,more forest land is currently being lost than gained.Another idea is to artificially sequester carbon.For example, carbon can be captured from theemissions from gasification plants. That carbon isthen stored underground in salt layers or coalseams, which keeps it out of the atmosphere. Whilesome small sequestration projects are underway,

no large-scale sequestration has yet beenattempted. While it is a promising new technology,carbon sequestration is also untested and may notprove to be significant in fighting global warming.

Just as individuals can diminish other types ofair pollution, people can fight global warming byconserving energy. Also, people can becomeinvolved in local, regional and national efforts tomake sound choices on energy policy.

Lesson Summary

Air pollutants can be reduced in many ways.The best method is to not use the energy thatproduces the pollutants by conservation orincreasing energy efficiency.Alternative energy sources are another goodway to reduce pollution. Most of thesealternate energy technologies are still beingrefined (solar, wind) and some have otherproblems associated with them (nuclear,biofuels).

Pollutants can be removed from an exhauststream by being filtered out or broken down.Some pollutants are best not released at alllike CFCs.

Review Questions

1. Since the Clean Air Act was passed in 1970,why is the air still not clean?

2. What are some ways that you can conserveenergy?

3. How does reducing air pollutants, asdescribed in the Clean Air Act of 1970, affectgreenhouse gas emissions?

4. What has to be done before alternative energysources can replace fossil fuels?

5. What are catalytic converters?6. Why are hybrid vehicles more energy

efficient than regular vehicles powered byinternal combustion engines?

7. Why aren’t fuel cell vehicles widelyavailable yet?

8. How does a cyclone reduce particulatepollution?

9. How can coal power be made so that it hasnearly zero carbon contribution to theatmosphere?

10. Why is it that the ozone hole will not behealed for several decades?

11. Many people think that biofuels are thesolution to a lot of the problem of climatechange, but others disagree. Whatrequirements would biofuels have to meet ifthey were to be really effective at replacinggasoline in motor vehicles?

Vocabulary

biofuelA fuel made from living materials, usuallycrop plants.

carbon sequestrationRemoval of carbon dioxide from the

atmosphere, so that it does not act as agreenhouse gas in the atmosphere.

catalystA substance that increases (or decreases) therate of a chemical reaction but is not used upin the reaction.

catalytic converterFound on modern motor vehicles, thesedevices use a catalyst to break apartpollutants.

fuel cellAn energy cell in which chemical energy isconverted into electrical energy.

gasificationA technology that cleans coal before it isburned, which increases efficiency andreduces emissions.

hybrid vehicleA very efficient vehicle that is powered by an

internal combustion engine, an electric motorand a rechargeable battery.

Points to Consider

Why is it important to reduce air pollution?What can you do in your own life to reduceyour impact on the atmosphere?Why is a worldwide effort needed to reducethe threat of global climate change?

Chapter 23: Observing andExploring Space

Telescopes

Lesson Objectives

Explain how astronomers use the wholeelectromagnetic spectrum to study theuniverse beyond Earth.Identify different types of telescopes.Describe historical and modern observationsmade with telescopes.

Introduction

Many scientists can interact directly with whatthey are studying. Biologists can collect cells,seeds, or sea urchins and put them in a controlled

laboratory environment. Physicists can subjectmetals to stress or smash atoms into each other.Geologists can chip away at rocks to see what isinside. But astronomers, scientists who study theuniverse beyond Earth, rarely have a chance fordirect contact with their subject. Instead,astronomers have to observe their subjects at adistance, usually a very large distance!

Electromagnetic Radiation

Earth is separated from the rest of the universeby very large expanses of space. Occasionally,matter from the outside reaches Earth, such aswhen a meteorite makes it through the atmosphere.But for the most part, astronomers have one mainsource for their data—light. Light can travel acrossempty space, and as it does so, it carries bothenergy and information. Light is one type ofelectromagnetic (EM) radiation, or energytransmitted through space as a wave.

The Speed of Light

Light travels faster than anything else in theuniverse. In the almost completely empty vacuumof space, light travels at a speed of approximately300,000,000 meters per second (670,000,000miles per hour). To give you an idea of how fastthat is, a beam of light could travel from New Yorkto Los Angeles and back again nearly 40 times injust one second. Even though light travelsextremely fast, objects in space are so far awaythat it takes a significant amount of time for lightfrom those objects to reach us. For example, lightfrom the Sun takes about 8 minutes to reach Earth.

Light-Years

Because astronomical distances are so large, ithelps to have a unit of measurement that is good forexpressing those large distances. A light-year is aunit of distance that is defined as the distance thatlight travels in one year. One light-year is

approximately equal to 9,500,000,000,000 (9.5trillion) kilometers, or 5,900,000,000,000 (5.9trillion) miles. That’s a long way! By astronomicalstandards, it’s actually a pretty short distance.

Proxima Centauri, the closest star to us afterthe Sun, is 4.22 light-years away. That means thelight from Proxima Centauri takes 4.22 years toreach us. The galaxy we live in, the Milky WayGalaxy, is about 100,000 light-years across. So,how long does it take light to travel from one sideof the galaxy to the other? 100,000 years! Even100,000 light years is a short distance on the scaleof the whole universe. The most distant galaxieswe have detected so far are more than 13 billionlight-years away. That’s over a hundred-billion-trillion (100,000,000,000,000,000,000,000)kilometers!

Looking Back in Time

When we look at astronomical objects such asstars and galaxies, we are not just seeing overgreat distances—we are also seeing back in time.

Because light takes time to travel, the image wesee of a distant galaxy is an image of how thegalaxy used to look. For example, the AndromedaGalaxy, shown in Figure below, is about 2.5million light years from Earth. If you look at theAndromeda Galaxy in a telescope, you will see thegalaxy as it was 2.5 million years ago. If you wantto see the galaxy as it is now, you will have to waitand look again 2.5 million years into the future!

Figure 23.1 This recent picture of the Andromeda Galaxy

actually shows the galaxy as it was about 2.5million years ago.

Electromagnetic Waves

Earlier, we said that light is one type ofelectromagnetic (EM) radiation. That means lightis energy that travels in the form of anelectromagnetic wave. Figure below shows adiagram of an electromagnetic wave. An EM wavehas two components: an electric field and amagnetic field. Each of these componentsoscillates between positive and negative values,which is what makes the “wavy” shape in thediagram.

Figure 23.2 An electromagnetic wave consists of

oscillating electric and magnetic fields. Thedistance between two adjacent oscillations iscalled wavelength.

Notice the horizontal arrow at the top left ofthe diagram. This measurement corresponds to the

wavelength, or the distance between two adjacentpoints on the wave. A related value is frequency,which measures the number of wavelengths thatpass a given point every second. Wavelength andfrequency are reciprocal, which means that as oneincreases, the other decreases.

The Electromagnetic Spectrum

Visible light—the light that human eyes can see—comes in a variety of colors. The color ofvisible light is determined by its wavelength.Visible light ranges from wavelengths of 400 nm to700 nm, corresponding to the colors violet throughred. But what about EM radiation withwavelengths shorter than 400 nm or longer than700 nm? Such radiation exists all around you—youjust can’t see it! Visible light is part of a largerelectromagnetic spectrum, as Figure belowillustrates.

Figure 23.3 Visible light is part of a larger electromagnetic

spectrum. The EM spectrum ranges from gammarays with very short wavelengths, to radio waveswith very long wavelengths.

What does the electromagnetic spectrum haveto do with astronomy? Every star, including ourSun, emits light at a wide range of wavelengths, allacross the visible spectrum, and even outside thevisible spectrum. Astronomers can learn a lot fromstudying the details of the spectrum of light from astar.

Some very hot stars emit light primarily atultraviolet wavelengths, while some very coolstars emit mostly in the infrared. There areextremely hot objects that emit X-rays and evengamma rays. Light from some of the faintest, most

distant objects is in the form of radio waves. Infact, a lot of the objects most interesting toastronomers today can’t even be seen with thenaked eye. Astronomers use telescopes to detectthe faint light from distant objects and to seeobjects at wavelengths all across theelectromagnetic spectrum.

Types of Telescopes

Optical Telescopes

Humans have been making and using lenses formagnification for thousands and thousands ofyears. However, the first true telescopes weremade in Europe in the late 16th century. Thesetelescopes used a combination of two lenses tomake distant objects appear both nearer and larger.The term telescope was coined by the Italianscientist and mathematician Galileo Galilei (1564–1642). Galileo built his first telescope in 1608 andsubsequently made many improvements to

telescope design.Telescopes that rely on the refraction, or

bending, of light by lenses are called refractingtelescopes, or simply refractors. The earliesttelescopes, including Galileo’s, were allrefractors. Many of the small telescopes used byamateur astronomers today are refractors with adesign similar to Galileo’s. Refractors areparticularly good for viewing details within oursolar system, such as the surface of Earth’s moonor the rings around Saturn. Figure below showsthe biggest refracting telescope in the world.

Figure 23.4 The largest refracting telescope in the world is

at the University of Chicagos Yerkes Observatoryin Wisconsin. This telescope was built in 1897. Itslargest lens has a diameter of 102 cm.

Figure 23.5 The telescope still looks much the same today.Around 1670, another famous scientist and

mathematician—Sir Isaac Newton (1643–1727)—built a different kind of telescope. Figure aboveshows a telescope similar in design to Newton’s.Newton’s telescope used curved mirrors instead oflenses to focus light. Telescopes that use mirrors

are called reflecting telescopes, or reflectors(Figure below). The mirrors in a reflectingtelescope are much lighter than the heavy glasslenses in a refractor. This is significant, becausethick glass lenses in a telescope mean that thewhole telescope must be much stronger to supportthe heavy glass. In addition, it’s much easier toprecisely make mirrors than to precisely makeglass lenses. For that reason, reflectors can bemade larger than refractors. Larger telescopes cancollect more light, which means they can studydimmer or more distant objects. The largest opticaltelescopes in the world today are reflectors, likethe one in Figure below.

Figure 23.6 Reflecting telescopes used by amateur

astronomers today are similar to the one designedby Isaac Newton in the 17 century.

Figure 23.7 The South African Large Telescope (SALT) is

one of the largest reflecting telescopes on Earth.SALTs primary mirror consists of 91 smallerhexagonal mirrors, each with sides 1 m long.

Many consumer telescopes today use acombination of mirrors and lenses to focus light.These telescopes are called catadioptrictelescopes. By using both kinds of elements,catadioptric telescopes can be made with largediameters but shorter lengths so they are lessawkward to move around. Figure below shows atypical catadioptric telescope.

Figure 23.8 Many amateur astronomers today use

catadioptric telescopes. These telescopes havelarge mirrors to collect a lot of light, but shorttubes for portability.

Radio Telescopes

Notice it says above that the largest opticaltelescopes in the world are reflectors. Opticaltelescopes are designed to collect visible light.There are even larger telescopes that collect lightat longer wavelengths—radio waves. Thesetelescopes are called—can you guess?—radiotelescopes. Radio telescopes look a lot likesatellite dishes. In fact, both are designed to do thesame thing—to collect and focus radio waves ormicrowaves from space.

The largest single telescope in the world is atthe Arecibo Observatory in Puerto Rico (seeFigure below). This telescope is located in anaturally-occurring sinkhole that formed whenwater flowing underground dissolved thelimestone rock. If this telescope were notsupported by the ground, it would collapse underits own weight. The downside of this design is thatthe telescope cannot be aimed to different parts ofthe sky—it can only observe the part of the sky thathappens to be overhead at a given time.

Figure 23.9 The radio telescope at the Arecibo

Observatory in Puerto Rico has a diameter of 305m.

A group of radio telescopes, such as thoseshown in Figure below, can be linked togetherwith a computer so that they are all observing thesame object. The computer can combine the datafrom each telescope, making the group functionlike one single telescope.

Figure 23.10 The Very Large Array in New Mexico has 27

radio dishes, each 25 meters in diameter. When allthe dishes are spread out and pointed at the sameobject, they act like a single telescope with adiameter of 22.3 mi.

Space Telescopes

Telescopes on Earth all have one significantlimitation: the electromagnetic radiation theygather must pass through Earth’s atmosphere. Theatmosphere blocks some radiation in the infraredpart of the spectrum and almost all radiation in theultraviolet and higher frequency ranges.Furthermore, motion in the atmosphere distortslight. You see evidence of this distortion when you

see stars twinkling in the night sky. To minimizethese problems, many observatories are built onhigh mountains, where there is less atmosphereabove the telescope. Space telescopes avoid suchproblems completely because they are outsideEarth’s atmosphere altogether—in space.

The Hubble Space Telescope (HST), shown inFigure below, is perhaps the best known spacetelescope. The Hubble was put into orbit by theSpace Shuttle Atlantis in 1990. Once it was inorbit, scientists discovered that there was a flaw inthe shape of the mirror. A servicing mission to theHubble by the Space Shuttle Endeavor in 1994corrected the problem. Since that time, the Hubblehas provided huge amounts of data that havehelped to answer many of the biggest questions inastronomy.

Figure 23.11 The Hubble Space Telescope orbits Earth at an

altitude of 589 km (366 mi). It collects data invisible, infrared, and ultraviolet wavelengths.

In addition to the Hubble, the NationalAeronautics and Space Administration (NASA)has placed three other major space telescopes inorbit: the Compton Gamma-Ray Observatory(CGRO), the Chandra X-Ray Observatory (CXO) ,and the Spitzer Space Telescope (SST). Together,these four telescopes comprise what NASA callsthe 'Great Observatories'. Figure below showshow each of these telescopes specializes in adifferent part of the electromagnetic spectrum. Of

these, all but the Compton are still in orbit andactive. NASA is planning for another telescope,the James Webb Space Telescope, to serve as areplacement for the aging Hubble. The JamesWebb is scheduled to launch no earlier than 2013.

Figure 23.12 NASAs four space based Great Observatories

were designed to view the universe in differentranges of the electromagnetic spectrum. A. HubbleSpace Telescope: visible light, B. ComptonGamma Ray Observatory: gamma ray, C. Spitzer

Space Telescope: infrared, D. Chandra X-rayObservatory: X-ray.

Observations with Telescopes

Ancient Astronomers

Humans have been studying the night sky forthousands of years. Observing the patterns andmotions in the sky helped ancient peoples keeptrack of time. This was important to them becauseit helped them know when to plant crops. Theyalso timed many of their religious ceremonies tocoincide with events in the heavens.

The ancient Greeks made careful observationsof the locations of stars in the sky. They noticedthat some of what they thought were 'stars' movedagainst the background of other stars. They calledthese bright spots in the sky planets, which inGreek means “wanderers.” Today we know thatthe planets are not stars, but members of our solarsystem that orbit the Sun. The Greeks also

identified constellations, patterns of stars in thesky. They associated the constellations with storiesand myths from their culture. Constellations stillhelp astronomers today; they are used to identifydifferent regions of the night sky.

Galileo’s Observations

Ancient astronomers knew a lot about thepatterns of stars and the movement of objects in thesky, but they did not know much about what theseobjects actually were. All of that changed in theyear 1610, when Galileo turned a telescopetoward the heavens. Using a telescope, Galileomade the following discoveries (among others):

There are more stars in the night sky than thenaked eye can see.The band of stars called the Milky Way,consists of many stars.The Moon has craters (See Figure below).Venus has phases like the Moon.Jupiter has moons orbiting around it.

There are dark spots that move across thesurface of the Sun.

Figure 23.13 Galileo was the first person known to look at

the Moon through a telescope. Galileo made thedrawing on the left in 1610. The image on the rightis a modern photograph of the Moon.

Galileo’s observations challenged people tothink in new ways about the universe and Earth’splace in it. About 100 years before Galileo,Nicolaus Copernicus had proposed a controversialnew model of the universe. According toCopernicus’s model, Earth and the other planetsrevolve around the Sun. In Galileo's time, most

people believed that the Sun and planets revolvedaround Earth. Galileo’s observations provideddirect evidence to support Copernicus’ model.

Observations with Modern Telescopes

Today, equipped with no more than a good pairof binoculars, you can see all of the things Galileosaw, and more. You can even see sunspots, but youneed special filters on the lenses to protect youreyes. Never look directly at the Sun without usingthe proper filters! With a basic telescope like thoseused by many amateur astronomers, you can alsosee polar caps on Mars, the rings of Saturn, andbands in the atmosphere of Jupiter.

We now know that all of these objects arewithin our solar system. You can also see manytimes more stars with a telescope than without atelescope. However, stars seen in a telescope stilllook like single points of light. Because they are sofar away, stars continue to appear as points of lightin even the most powerful professional telescopes.Figure below shows one rare exception.

Figure 23.14 This is an ultraviolet image of the red

supergiant star Betelgeuse taken with the HubbleSpace Telescope in 1996. This was the first directimage taken of the disk of a star other than the Sun.

Today, very few professional astronomers lookdirectly through the eyepiece of a telescope.Instead, they attach sophisticated instruments totelescopes. These instruments capture and processthe light from a telescope, and astronomers thenlook at the images or data shown on theseinstruments. Most of the time, the instruments then

pass the data on to a computer where the data canbe stored for later use. It can take an astronomerweeks or months to analyze all the data collectedfrom just a single night!

A spectrometer is a tool that astronomerscommonly use to study the light from a telescope.A spectrometer uses a prism or other device tobreak light down into its component colors. Thisproduces a spectrum like the one shown in Figurebelow. The dark lines in the spectrum of light froma star are caused by gases in the outer atmosphereof the star absorbing light. This spectrum can beobserved directly, captured on film, or storeddigitally on a computer.

Figure 23.15 This is a simplified example of what light from

a star looks like after it passes through aspectrometer.

From a single spectrum of a star, an astronomercan tell:

How hot the star is (by the relative brightness

of different colors).What elements the star contains (by thepattern of dark lines).Whether and how fast the star is movingtoward or away from Earth (by how far thedark lines are shifted from their normalpositions).

Using telescopes, astronomers can also learnhow stars evolve, what kind of matter is foundthroughout the universe, and how it is distributed,and even how the universe might have formed.

Lesson Summary

Astronomers study light from distant objects.Light travels at 300,000,000 meters persecond—faster than anything else in theuniverse.A light-year is a unit of distance equal to thedistance light travels in one year, 9.5 trillionkilometers.

When we see distant objects, we see them asthey were in the past, because their light hasbeen traveling to us for many years.Light is energy that travels as a wave.Visible light is part of the electromagneticspectrum.Telescopes make distant objects appear bothnearer and larger. You can see many morestars through a telescope than with theunaided eye.Optical telescopes are designed to collectvisible light. The three main types of opticaltelescopes are reflecting telescopes,refracting telescopes, and catadioptrictelescopes.Radio telescopes collect and focus radiowaves from distant objects.Space telescopes are telescopes orbitingEarth. They can collect wavelengths of lightthat are normally blocked by the atmosphere.Galileo was the first person known to use atelescope to study the sky. His discoverieshelped change the way humans think about the

universe.Modern telescopes collect data that can bestored on a computer.A spectrometer produces a spectrum fromstarlight. Astronomers can learn a lot about astar by studying its spectrum.

Review Questions

1. Proxima Centauri is 4.22 light-years fromEarth. Light travels 9.5 trillion kilometers inone year. How far away is Proxima Centauriin kilometers?

2. Identify four regions of the electromagneticspectrum that astronomers use whenobserving objects in space.

3. List the 3 main types of optical telescopes,and describe their differences.

4. Explain the advantages of putting a telescopeinto orbit around Earth.

5. Describe two observations that Galileo wasthe first to make with his telescope.

6. List 3 things that an astronomer can learnabout a star by studying its spectrum.

Further Reading / SupplementalLinks

http://science.nasa.gov/headlines/y2002/08feb_gravlens.htmhttp://www.nasa.gov/audience/forstudents/postsecondary/features/F_NASA_Great_Observatories_PS.htmlhttp://www.stargazing.net/David/constel/howmanystars.htmlhttp://www.astronomics.com/main/category.asp/catalog_name/Astronomics/category_name/V1X41SU50GJB8NX88JQB360067/Page/1http://galileo.rice.edu/sci/instruments/telescope.htmlhttp://www.nrao.edu/whatisra/index.shtmlhttp://www.astronomy.pomona.edu/archeo/http://www.colorado.edu/physics/PhysicsInitiative/Physics2000/quantumzone/http://en.wikipedia.org

Vocabulary

catadioptric telescopeTelescopes that use a combination of mirrorsand lenses to focus light.

constellationsPatterns of stars as observed from Earth.

electromagnetic radiationEnergy transmitted through space as a wave.

electromagnetic spectrumThe full range of electromagnetic radiation.

frequencyThe number of wavelengths that pass a givenpoint every second.

gamma raysA penetrating form of electromagneticradiation.

infrared lightElectromagnetic waves with frequenciesbetween radio waves and red light; about 1mm to 750 nanometers.

light-yearThe distance light can travel in one year; 9.5

trillion kilometers.

microwavesThe shortest wavelength radio waves.

planetsAround celestial object orbiting a star that hascleared its neighboring region ofplanetesimals.

radio telescopeA radio antenna that collects radio waves.

radio wavesThe longest wavelengths of theelectromagnetic spectrum; from 1 mm to morethan thousands of kilometers.

reflecting telescopeTelescopes that use mirrors to collect andfocus light.

refracting telescopeTelescopes that use convex lenses to collect

and focus light.

space telescopeTelescopes in orbit above Earth'satmosphere.

spectrometerA tool that uses a prism to break light into itscomponent colors.

ultravioletElectromagnetic radiation having wavelengthsshorter than the violet.

wavelengthHorizontal distance measured from wavecrest to wave crest, or wave trough to wavetrough.

visible lightThe portion of light in the electromagneticspectrum that is visible to humans.

X rays

A band of electromagnetic radiation betweengamma and ultraviolet.

Points to Consider

Radio waves are used for communicatingwith spacecraft. A round-trip communicationfrom Earth to Mars takes anywhere from 6 to42 minutes. What challenges might thispresent for sending unmanned spacecraft andprobes to Mars?The Hubble Space Telescope is a veryimportant source of data for astronomers. Thefascinating and beautiful images from theHubble also help to maintain public supportfor science. However, the Hubble is growingold. Missions to service and maintain thetelescope are extremely expensive and put thelives of astronauts at risk.Do you think there should be anotherservicing mission to the Hubble?

Early Space Exploration

Lesson Objectives

Explain how a rocket works.Describe different types of satellites.Outline major events in early spaceexploration, including the Space Race.

Introduction

Humans have long dreamed of traveling intospace. Greek mythology tells of Daedelus andIcarus, a father and son who took flight using wingsmade of feathers and wax. Daedelus warned hisson not to fly too close to the sun, but Icarus,thrilled with the feel of flying, drifted higher andhigher. When he got too close to the Sun, the waxmelted, and Icarus fell into the sea. This myth isoften interpreted to be about foolishness orexcessive pride, but we can also relate to the

excitement Icarus would have felt. Much later,science fiction writers, such as Jules Verne (1828–1905) and H.G. Wells (1866–1946), wrote abouttechnologies that might make the dream of travelingbeyond Earth into space possible.

Rockets

Humans did not reach space until the secondhalf of the 20th century. However, the maintechnology that makes space exploration possible,the rocket has been around for a long time. Arocket is a device propelled by particles flying outof it at high speed. We do not know exactly whobuilt the first rocket, or when, but there are recordsof the Chinese using rockets in war against theMongols as early as the 13th century. The Mongols,in turn, spread rocket technology in their attacks onEastern Europe. Early rockets were also used tolaunch fireworks and for other ceremonialpurposes.

How Rockets Work

Rockets were used for centuries before anyonecould explain exactly how they work. The theory toexplain this did not arrive until 1687, when IsaacNewton (1643–1727) described three basic lawsof motion, now referred to as Newton’s Laws ofMotion:

1. An object in motion will remain in motionunless acted upon by a net force.

2. Force equals mass multiplied by acceleration.3. To every action, there is an equal and

opposite reaction.

Newton’s third law of motion is particularlyuseful in explaining how a rocket works. To betterunderstand this law, consider the ice skater inFigure below. When the skater pushes the wall, theskater’s force—the “action”—is matched by anequal force by the wall on the skater in theopposite direction—the reaction.

Figure 23.16 When the skate boarder pushes against the

wall, the wall exerts an equal force on the skateboarder in the opposite direction.

Once the skater is moving, however, she hasnothing to push against. Imagine now that the skateris holding a fire extinguisher. When she pulls thetrigger on the extinguisher, a fluid or powder flies

out of the extinguisher, and she moves backward.In this case, the action force is the pressure pushingthe material out of the extinguisher. The reactionforce of the material against the extinguisherpushes the skater backward.

For a long time, many believed that a rocketwouldn’t work in space because there would benothing for the rocket to push against. However, arocket in space moves like the skater holding thefire extinguisher. Fuel is ignited in a chamber,which causes an explosion of gases. The explosioncreates pressure that forces the gases out of therocket. As these gases rush out the end, the rocketmoves in the opposite direction, as predicted byNewton’s Third Law of Motion. The reaction forceof the gases on the rocket pushes the rocketforward, as shown in Figure below. The forcepushing the rocket is also called thrust.

Figure 23.17 Explosions in a chamber create pressure that

pushes gases out of a rocket. This in turn producesthrust that pushes the rocket forward. The rocketshown here is a Saturn V rocket, used for theApollo 11 missionthe first to carry humans to theMoon.

A Rocket Revolution

For centuries, rockets were powered bygunpowder or other solid fuels. These rocketscould travel only fairly short distances. At the endof the 19th century and the beginning of the 20th

century, several breakthroughs in rocketry wouldlead to rockets that were powerful enough to carryrockets—and humans—beyond Earth. During thisperiod, three people independently came up withsimilar ideas for improving rocket design.

The first person to establish many of the mainideas of modern rocketry was a Russianschoolteacher, named Konstantin Tsiolkovsky(1857–1935). Most of his work was done evenbefore the first airplane flight, which took place in1903. Tsiolkovsky realized that in order forrockets to have enough power to escape Earth’sgravity, they would need liquid fuel instead ofsolid fuel. He also realized that it was important tofind the right balance between the amount of fuel arocket uses and how heavy the rocket is. He cameup with the idea of using multiple stages whenlaunching rockets, so that empty fuel containers

would drop away to reduce mass. Tsiolkovsky hadmany great ideas and designed many rockets, buthe never built one.

The second great rocket pioneer was anAmerican, named Robert Goddard (1882–1945).He independently came up with some of the sameideas as Tsiolkovsky, such as using liquid fuel andusing multiple stages. He also designed a systemfor cooling the gases escaping from a rocket,which made the rocket much more efficient.Goddard was more practical than Tsiolkovsky andbuilt rockets to test his ideas. Figure below showsGoddard with the first rocket to use liquid fuel.This rocket was launched on March 16, 1926 inMassachusetts. Over a lifetime of research,Goddard came up with many innovations that arestill used in rockets today.

Figure 23.18 (Left) Robert Goddard launched the first

liquid-fueled rocket on March 16, 1926; (Right)This schematic shows details of Goddard's rocket.

The third great pioneer of rocket science was aRomanian-born German, named Hermann Oberth(1894–1989). In the early 1920’s, Oberth came upwith many of the same ideas as Tsiolkovsky andGoddard. His early work was not taken seriouslyby most scientists. Nonetheless, Oberth built aliquid-fueled rocket, which he launched in 1929.Later, he joined a team of scientists that designedthe rocket shown in Figure below for the Germanmilitary. This rocket, first called the A-4 and laterthe V-2, played a major role in World War II. The

Germans used the V-2 as a missile to bombnumerous targets in Belgium, England, and France.In 1942, the V-2 was launched to an altitude of 176km (109 miles), making it the first human-madeobject to travel into space. An altitude of 100 km(62 miles) is generally considered to be thedividing line between Earth’s atmosphere andspace.

Figure 23.19 V-2 Rocket: Explosions in a chamber create

pressure that pushes gases out of a rocket. This inturn produces thrust that pushes the rocket forward.

The leader of the team that built the V-2 rocketwas a German scientist, named Wernher vonBraun. von Braun later fled Germany and came tothe United States, where he helped the UnitedStates develop missile weapons and then joinedthe National Aeronautics and Space

Administration (NASA) to design rockets forspace travel. At NASA, von Braun designed theSaturn V rocket (Figure above), which waseventually used to send the first humans to theMoon.

Satellites

One of the first uses of rockets in space was tolaunch satellites. A satellite is an object that orbitsa larger object. To orbit something just means totravel in a circular or elliptical path around it. Thispath is also called an orbit. When you think of asatellite, you probably picture some kind ofmetallic spacecraft orbiting Earth, but the Moon isalso a satellite. Human-made objects put into orbitare called artificial satellites. Natural objects inorbit, such as moons, are called natural satellites.

Newton’s Law of Universal Gravitation

Isaac Newton, whose third law of motion

explains how rockets work, also came up with thetheory that explains why satellites stay in orbit.Newton’s law of universal gravitation describeshow every object in the universe is attracted toevery other object. The same gravity that makes anapple fall to the ground, and keeps you fromfloating away into the sky, also holds the Moon inorbit around Earth, and Earth in orbit around theSun.

Newton used the following example to explainhow gravity makes orbits possible. Consider acannonball launched from a high mountain, asshown in Figure below. If the cannonball islaunched at a slow speed, it will fall back to Earth,as in paths A and B in the figure. However, if it islaunched at a fast enough speed, the Earth belowwill curve away at the same rate that thecannonball falls, and the cannonball will go into acircular orbit, as in path C. If the cannonball islaunched even faster, it could go into an ellipticalorbit (D) or leave Earth’s gravity altogether (E).

Figure 23.20 Isaac Newton explained how a cannonball

fired from a high point with enough speed couldorbit Earth.

Note that Newton’s idea would not actuallywork in real life; a cannonball launched from Mt.Everest, the highest mountain on Earth, would burnup in the atmosphere if launched at the speedrequired to put the cannonball into orbit. However,a rocket can launch straight up, then steer into anorbit. A rocket can also carry a satellite above theatmosphere and then release the satellite into orbit.

Types of Satellites

Since the launch of the first satellite over 50years ago, thousands of artificial satellites havebeen put into orbit around Earth. We have even putsatellites into orbit around the Moon, the Sun,Venus, Mars, Jupiter, and Saturn. Imaging satellitesare designed for taking pictures of Earth’s surface.The images can be used by the military, when takenby spy satellites or for scientific purposes, such asmeteorology, if taken by weather satellites.Astronomers use imaging satellites to study andmake maps of the Moon and other planets.Communications satellites, such as the one inFigure below, are designed to receive and sendsignals for telephone, television, or other types ofcommunications. Navigational satellites are usedfor navigation systems, such as the GlobalPositioning System (GPS). The largest artificialsatellite is the International Space Station,designed for humans to live in space whileconducting scientific research.

Figure 23.21 This is a Milstar communications satellite used

by the U.S. military. The long, flat solar panelsprovide power for the satellite. Most of the otherinstruments you can see are antennas for sending orreceiving signals.

Types of Orbits

The speed of a satellite depends on how high itis above Earth or whatever object it is orbiting.Satellites that are relatively close to Earth are saidto be in low Earth orbit (LEO). Satellites in LEOare also often in polar orbit, which means they

orbit over the North and South Poles,perpendicular to Earth’s spin. Because Earthrotates underneath the orbiting satellite, a satellitein polar orbit is over a different part of Earth’ssurface each time it circles. Imaging satellites andweather satellites are often put in low-Earth, polarorbits.

A satellite placed at just the right distanceabove Earth--35,786 km (22,240 miles)— orbits atthe same rate that Earth spins. As a result, thesatellite is always in the same position overEarth’s surface. This type of orbit is called ageostationary orbit (GEO). Manycommunications satellites are put in geostationaryorbits.

The Space Race

From the end of World War II in 1945 to thebreakup of the Soviet Union (USSR) in 1991, theSoviet Union and the United States were inmilitary, social, and political conflict. This period

is known as the Cold War. While there were veryfew actual military confrontations, the twocountries were in an arms race—continuallydeveloping new and more powerful weapons aseach country tried to have more powerful weaponsthan the other. While this competition had manysocial and political consequences, it did also helpto drive technology. The development of missilesfor war significantly sped up the development ofrocket technologies.

Sputnik

On October 4, 1957, the Soviet Union launchedSputnik 1, the first artificial satellite ever put intoorbit. Sputnik 1, shown in Figure below, was 58cm in diameter and weighed 84 kg (184 lb).Antennas trailing behind the satellite sent out radiosignals, which were detected by scientists andamateur radio operators around the world. Sputnik1 orbited Earth in low Earth orbit on an ellipticalpath every 96 minutes. It stayed in orbit for about 3months, until it slowed down enough to descend

into Earth’s atmosphere, where it burned up as aresult of friction with Earth’s atmosphere.

Figure 23.22 The Soviet Union launched Sputnik 1, the first

artificial satellite,.on October 4, 1957.The launch of Sputnik 1 started the Space

Race between the Soviet Union and the UnitedStates. Many people in the U.S. were shocked thatthe Soviets had the technology to put the satellite in

orbit, and they worried that the Soviets might alsobe winning the arms race. On November 3, 1957,the Soviets launched Sputnik 2, which carried thefirst animal to go into orbit—a dog named Laika.

The Race Is On

In response to the Sputnik program, the U.S.launched their own satellite, Explorer I, on January31, 1958. Shortly after that—March 17 1958—theU.S. launched another satellite, Vanguard 1. Laterthat year, the U.S. Congress and PresidentEisenhower established the National Aeronauticsand Space Administration (NASA).

The Soviets still managed to stay ahead of theUnited States for many notable “firsts.” On April12, 1961, Soviet cosmonaut Yuri Gagarin becameboth the first human in space and the first human inorbit. Less than one month later—May 5, 1961—the U.S. sent their first astronaut into space: AlanShepherd. The first American to orbit Earth wasJohn Glenn, in February 1962. The first woman inspace was a Soviet: Valentina Tereshkova, in June

1963. The timeline in Table below shows manyother Space Race firsts.

Space Race Timeline

Date Accomplished Country Name ofMission

October 4,1957

First artificialsatellite, firstsignals fromspace

USSR Sputnik 1

November3, 1957

First animal inorbit (the dogLaika)

USSR Sputnik 2

January31, 1958

USA’s firstartificial satellite USA Explorer I

January 4,1959

First human-made object toorbit the Sun

USSR Luna 1

September13, 1959

First impact intoanother planet ormoon (the Moon)

USSR Luna 2

First manned

April 12,1961

spaceflight andfirst mannedorbital flight(Yuri Gagarin)

USSR Vostok 1

May 5,1961

USA’s firstspaceflight withhumans (AlanShepherd)

USA

Mercury-Redstone 3(Freedom7)

February20, 1962

USA’s firstorbital flightwith humans(John Glenn)

USA

Mercury-Atlas 6(Friendship7)

December14, 1962

First planetaryflyby (Venus) USA Mariner 2

June 16,1963

First woman inspace, firstwoman in orbit(ValentinaTereshkova)

USSR Vostok 6

March 18,

First extra-vehicular activity(“spacewalk”) USSR Voskhod 2

1965 (AlekseiLeonov)

February3, 1966

First soft landingon another planetor moon (theMoon), firstphotos fromanother world

USSR Luna 9

March 1,1966

First impact intoanother planet(Venus)

USSR Venera 3

April 3,1966

First artificialsatellite aroundanother world(the Moon)

USSR Luna 10

June 2,1966

USA’s first softlanding on theMoon, USA’sfirst photos fromthe Moon

USA Surveyor 1

First humans to

December21, 1968

orbit anotherworld (theMoon) (JamesLovell, FrankBorman, BillAnders)

USA Apollo 8

July 21,1969

First humans onthe Moon (NeilArmstrong, BuzzAldrin)

USA Apollo 11

(Source:http://en.wikipedia.org/wiki/Timeline_of_space_explorationand David Bethel, License: GNU-FDL and CC-BY-SA)

The Space Race between the United States andthe Soviet Union reached a peak in 1969 when theU.S. put the first humans on the Moon. However,the competition between the two countries’ spaceprograms continued for many more years.

Reaching the Moon

On May 25, 1961, shortly after the firstAmerican went into space, President John F.Kennedy presented the following challenge to theU.S. Congress:

"I believe that this nation should commit itselfto achieving the goal, before this decade isout, of landing a man on the Moon andreturning him back safely to the Earth. Nosingle space project in this period will bemore impressive to mankind, or moreimportant for the long-range exploration ofspace; and none will be so difficult orexpensive to accomplish."

Eight years later, NASA’s Apollo 11 missionachieved Kennedy’s ambitious goal. On July 20,1969, astronauts Neil Armstrong and Buzz Aldrinwere the first humans to set foot on the moon, asshown in Figure below.

Figure 23.23 Neil Armstrong took this photo of Buzz Aldrin

on the Moon during the Apollo 11 mission.Armstrong and the Lunar Module can be seen in thereflection in Aldrins helmet.

Following the Apollo 11 mission, four otherAmerican missions successfully put astronauts onthe Moon. The last manned mission to the moonwas Apollo 17, which landed on December 11,1972. To date, no other country has put a person onthe Moon.

In July 1975, the Soviet Union and the United

States carried out a joint mission called theApollo-Soyuz Test Project. During the mission, anAmerican Apollo spacecraft docked with a SovietSoyuz spacecraft, as shown in Figure below. Manyconsidered this to be the symbolic end of the SpaceRace.

Figure 23.24 The docking of an Apollo spacecraft with a

Soyuz spacecraft in 1975 was a symbolic end tothe Space Race.

Exploring Other Planets

Both the United States and the Soviet Unionalso sent probes to other planets during the SpaceRace. A space probe is a spacecraft that is sentwithout a crew to collect data by flying near orlanding on an object in space, such as a planet,

moon, asteroid, or comet. In the Venera missions,the USSR sent several probes to Venus, includingsome that landed on the surface. The U.S. sentprobes to Mercury, Venus, and Mars in theMariner missions, and landed two probes on Marsin the Viking missions.

In the Pioneer and Voyager missions, the U.S.also sent probes to the outer solar system,including flybys of Jupiter, Saturn, Uranus, andNeptune. The Pioneer and Voyager probes are stilltraveling, and are now beyond the edges of oursolar system. We have lost contact with the twoPioneer probes, but expect to have contact with thetwo Voyager probes until at least 2020.

Lesson Summary

Rockets have been used for warfare andceremonies for many centuries.Newton’s third law explains how a rocketworks. The action force of the engine on thegases is accompanied by a reaction force of

the gases on the rocket.Konstantin Tsiolkovsky, Robert Goddard, andHermann Oberthall came up with similarideas for improving rocket design. Theseincluded using liquid fuel and using multiplestages.A satellite is an object that orbits a largerobject. Moons are natural satellites. Artificialsatellites are made by humans.Newton’s law of universal gravitationexplains how the force of gravity works, bothon Earth and across space. Gravity holdsatellites in orbit.Artificial satellites are used for imaging Earthand other planets, for navigation, and forcommunication.The launch of the Sputnik 1 satellite started aSpace Race between the United States and theSoviet Union.The United States’ Apollo 11 mission put thefirst humans on the Moon.The U.S. and Soviet Union also sent severalprobes to other planets during the Space

Race.

Review Questions

1. Use Newton’s third law to explain how arocket moves.

2. List the three great pioneers of rocket science.3. What is the difference between a rocket and a

satellite? How are they related?4. What is the name of Earth’s natural satellite?5. Explain why a satellite in polar orbit will be

able to take pictures of all parts of the Earthover time.

6. Describe three different types of orbits.7. What event launched the Space Race?8. What goal did John F. Kennedy set for the

United States in the Space Race?9. What are the advantages of a multi-stage

rocket instead of a single-stage rocket?

Further Reading / SupplementalLinks

http://exploration.grc.nasa.gov/education/rocket/TRCRocket/history_of_rockets.htmlhttp://www.solarviews.com/eng/rocket.htmhttp://www.thespaceplace.com/history/rocket2.htmlHermann_Oberth; Wernher_von_Braun; V-2_rocket; Satellites; Natural_satellite;Newton_cannonball; Sputnik_1;Sputnik_program; Space_Race; Cold_War;John_F._Kennedy; Apollo_program;List_of_planetary_probes.http://www.thespacesite.com/space_contents.htmlhttp://www.thespaceplace.com/history/space.htmlhttp://www.aero.org/education/primers/space/history.htmlhttp://science.howstuffworks.com/satellite.htmhttp://ctd.grc.nasa.gov/rleonard/index.htmlhttp://www.nasm.si.edu/exhibitions/gal114/gal114.htmhttp://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_25th.htmlhttp://en.wikipedia.org/

Vocabulary

geostationary orbitA satellite place at just the right distance

above Earth to orbit at the same rate thatEarth spins.

low Earth orbitSatellites that orbit relatively close to Earth.

orbitTo travel in a circular or elliptical patharound another object.

polar orbitA path for a satellite that goes over the Northand South Poles, perpendicular to Earth'sspin.

rocketA device propelled by particles flying out ofit at high speed.

satelliteAn object, either natural or human made, thatorbits a larger object.

space probe

A spacecraft that is sent without a crew tocollect data by flying near or landing on anobject in space.

Space RaceA competition between the United States andthe Soviet Union to have the best spacetechnology.

thrustThe forward force produced by gasesescaping from a rocket engine.

Points to Consider

The Space Race and the USA’s desire to getto the Moon brought about many advances inscience and technology.Can you think of any challenges we face todaythat are, could be, or should be a focus ofscience and technology?If you were in charge of NASA, what newgoals would you set for space exploration?

Recent Space Exploration

Lesson Objectives

Outline the history of space stations and spaceshuttles.Describe recent developments in spaceexploration.

Space Shuttles and Space Stations

While the United States continued missions tothe Moon in the early 1970s, the Soviets hadanother goal: to build a space station. A spacestation is a large spacecraft on which humans canlive for an extended period of time.

Early Space Stations

The Soviet Union put the first space station,Salyut 1, into orbit on April 19, 1971. At first, the

station had no crew. Three cosmonauts boarded thestation on June 7, 1971, and stayed for 22 days.Unfortunately, the cosmonauts died during theirreturn to Earth, when the return capsule lostpressure while still in the airless vacuum of space.Salyut 1 left orbit on October 11, 1971, and burnedup as a result of friction with the Earth’satmosphere.

Between 1971 and 1982, the Soviets put a totalof seven Salyut space stations into orbit. Figurebelow shows the last of these, Salyut 7. Thesewere all temporary stations that were launched andlater inhabited by a human crew. Three of theSalyut stations were used for secret militarypurposes. The others were used to study theproblems of living in space and for a variety ofexperiments in astronomy, biology, and Earthscience. Salyut 6 and Salyut 7 each had twodocking ports, so one crew could dock aspacecraft to one end, and later a replacementcrew could dock to the other end.

Figure 23.25 The Soviet Salyut 7 space station was in orbit

from 1982 to 1991.The U.S. only launched one space station

during this time—Skylab, shown in Figure below.Skylab’s design was based on a segment of theSaturn V rockets that were used in the Apollo

missions to the Moon. Skylab was launched intolow Earth orbit in May 1973. It was damaged as itpassed out of Earth’s atmosphere, but repairs weremade when the first crew arrived.

Figure 23.26 This image of Skylab was taken as the last

crew left the station in January of 1974.Three crews visited Skylab, all within its first

year in orbit. Skylab was used to study the effectsof staying in space for long periods. It was alsoused for studying the Sun. Skylab reentered Earth’satmosphere in 1979, sooner than expected. It wasso large that Skylab did not completely burn as it

reentered the Earth’s atmosphere. As a result,pieces of it fell across a large area, including someof western Australia. News headlines read, “TheSkylab is Falling!”

Modular Space Stations

The first space station designed for very long-term use was the Mir space station (Figurebelow). Mir was a modular space station, whichmeans it was launched in several separate piecesand put together in space. The core of Mir waslaunched by the Soviet Union in 1986. Mir was puttogether in several phases between 1986 and 1996.Mir holds the current record for the longestcontinued presence in space. There were peopleliving on Mir continuously for almost 10 years,falling short of the 10-year mark by just eight days.Mir was taken out of orbit in 2001; it fell into thePacific Ocean, as the Russians had planned.

Figure 23.27 The Soviet/Russian space station Mir was

designed to have several different parts attached toa core.

Mir was the first major space project in whichthe United States and Russia (after the fall of theUSSR) worked together. In 1993, U.S. VicePresident, Al Gore and Russian prime minister,Viktor Chernomyrdin announced plans for a new

space station, which would later be called theInternational Space Station, or ISS. They alsoagreed that the U.S. would be involved in the Mirproject in the years ahead. Space shuttles wouldtake part in the transport of supplies and people toand from Mir. In addition, American astronautswould live on Mir for many months. Thiscooperation with Russia allowed the United Statesto learn from Russia’s experience with longduration space flights. Figure below shows Mirwith an American space shuttle attached.

Figure 23.28 American space shuttles visited Mir several

times during the 1990s. This picture was taken by

Russian cosmonauts from their Soyuz spacecraft,as they flew around Mir and the Space Station tocheck on the space station.

The International Space Station

Early space exploration was driven bycompetition between the United States and theSoviet Union. However, since the end of the ColdWar, space technology and space exploration havebenefited from a spirit of cooperation. TheInternational Space Station, shown in Figurebelow, is a joint project between the spaceagencies of the United States (NASA), Russia(RKA), Japan (JAXA), Canada (CSA) and severalEuropean countries (ESA). The Brazilian SpaceAgency also contributes.

The International Space Station is a very largestation with many different sections. It is still beingassembled. The first piece was launched in 1997.The first crew arrived in 2000, and the station hashad people on board ever since. American spaceshuttles carry most of the supplies and equipment

to the station, while Russian Soyuz spacecraftcarry people. The primary purpose of the station isscientific research, especially in biology,medicine, and physics.

Figure 23.29 This photograph of the International Space

Station was taken by the space shuttle Atlantis inJune 2007. Construction of the station is scheduledto be finished in 2010.

Space Shuttles

The spacecraft that NASA used for the Apollo

missions were very successful, but they were veryexpensive, could not carry much cargo, and couldbe used only once. After the Apollo missions to theMoon, NASA wanted a new kind of space vehicle.They wanted this vehicle to be reusable and ableto carry large pieces of equipment, such assatellites, space telescopes, or sections of a spacestation. The resulting spacecraft was a spaceshuttle, shown in Figure below. Although thisvehicle is sometimes referred to as “the spaceshuttle,” the U.S. has actually had five workingspace shuttles—Columbia, Challenger, Discovery,Atlantis, and Endeavor. The Soviet Union built asimilar shuttle called Buran, but it never flew amission with humans aboard.

Figure 23.30 Since 1981, the space shuttle has been the

United States primary vehicle for carrying peopleand large equipment into space. This photo showsthe space shuttle Atlantis on the launch pad in2006.

A space shuttle has three main parts, althoughyou are probably most familiar with the orbiter,the part that has wings like an airplane. When aspace shuttle launches, the orbiter is attached to ahuge fuel tank that contains liquid fuel. On thesides of the fuel tank are two large boosterrockets.

Figure below shows the stages of a normalspace shuttle mission. The launch takes place atCape Canaveral, in Florida. The booster rocketsprovide extra power to get the orbiter out ofEarth’s atmosphere. When they are done, theyparachute down into the ocean so they can berecovered and used again. When the fuel tank isempty, it also falls away, but it burns up in theatmosphere. Once in space, the orbiter can be usedto release equipment such as a satellite or suppliesto the International Space Station, to repair existingequipment such as the Hubble Space Telescope, orto do experiments directly on board the orbiter.

Figure 23.31 In a typical space shuttle mission, the orbiter

takes off like a rocket and lands like an airplane.When the orbiter is done with its mission, it re-

enters Earth’s atmosphere. As it passes through theatmosphere, the outside of the orbiter heats up toover 1,500oC. The rockets do not fire during re-entry, so the shuttle is more like a glider than aregular airplane. Pilots have to steer the shuttle tothe runway very precisely. Space shuttles usuallyland at Kennedy Space Center in Cape Canaveral,Florida, or at Edwards Air Force Base inCalifornia. However, if weather is bad at boththese landing sites, a shuttle can land at one ofmany backup sites around the world. It can later behauled back to Florida on the back of a jet

airplane.

Space Shuttle Disasters

The space shuttle program has been verysuccessful. Space shuttles have made possiblemany scientific discoveries and other greatachievements in space. However, the program hasalso had some tragic disasters.

From the first flight in 1981 to the end of 1985,space shuttles flew over 20 successful missions,including many satellite launches and missions forscientific research. On January 28, 1986, the spaceshuttle Challenger launched carrying seven crewmembers, including Christa McAuliffe, who was tobe the first teacher in space. Just 73 seconds afterlaunch, the Challenger started to break apart, andmost of it disintegrated in mid-air, as shown inFigure below. All seven crew members on boarddied. Later study showed that the problem was dueto an O-ring, a small part in one of the rocketboosters. Because of this disaster, space shuttlemissions were put on hold while NASA studied

the problem and improved the safety of theshuttles.

Figure 23.32 Plume of smoke from the Challenger disaster.

The space shuttle Challenger broke apart 73seconds after its launch on January 28, 1986.

Shuttle missions started again in 1988, andthere were over 87 consecutive missions without amajor accident. However, during the takeoff ofspace shuttle Columbia on January 16, 2003, asmall piece of insulating foam broke off the fueltank. The foam smashed into one wing of theorbiter and damaged a tile on the front edge of theshuttle’s wing. These tiles are heat shield tiles that

protect the shuttle from extremely hightemperatures. When Columbia returned to Earth onFebruary 3, 2003, it could not withstand the hightemperature, and broke apart. Pieces of the shuttlewere found throughout the southern United States,especially in Texas. As in the Challenger disaster,all seven crew members died.

After the Columbia disaster, shuttle missionswere stopped for over two years while NASAworked on the problem. One year after thedisaster, President Bush announced that the spaceshuttle program was to end by the year 2010, and anew Crew Exploration Vehicle would take itsplace. The Crew Exploration Vehicle, now knownas Orion is currently expected to be ready by 2014.All the remaining shuttle missions will be to theInternational Space Station, except for one repairmission to the Hubble Space Telescope.

Recent Space Missions

Since the 1986 Challenger disaster NASA has

focused on missions without a crew, except for theInternational Space Station missions. These recentmissions are less expensive and less dangerousthan missions with a crew, yet still provide a greatdeal of valuable information.

Earth Science Satellites

In recent years, NASA and space agenciesfrom other countries have launched dozens ofsatellites that collect data on the current state ofEarth’s systems. For example, NASA’s Landsatsatellites take detailed images of Earth’s continentsand coastal areas, such as those in Figure below.Other satellites study the oceans, the atmosphere,the polar ice sheets, and other Earth systems. Thisdata helps us to monitor climate change andunderstand how Earth’s systems affect one another.

Figure 23.33 The two images above are from NASAs

Landsat 7 satellite. The left shows New Orleansand Lake Pontchartrain on April 26, 2000. Theright shows the same area on August 30, 2005,shortly after Hurricane Katrina flooded the city.Dark blue areas in the city are underwater.

Space Telescopes

Some of the greatest astronomical discoveries—and greatest pictures, like the one in Figurebelow, have come from the Hubble SpaceTelescope. The Hubble was the first telescope inspace. It was put into orbit by the space shuttleDiscovery in 1990. Since then, four shuttlemissions have gone to the Hubble to make repairs

and upgrades. A final repair mission to the Hubbleis scheduled for 2008.

Figure 23.34 This image taken by the Hubble Space

Telescope shows the Cats Eye Nebula. Hubble hasproduced thousands of beautiful pictures that arealso very valuable for scientific research.

NASA has also put several other telescopes inspace, including the Spitzer Space Telescope, theChandra X-Ray Observatory, and the ComptonGamma Ray Observatory. The biggest and mostadvanced space telescope yet, the James Webb

Telescope, is scheduled to be launched into orbitaround 2013. The James Webb will replace theHubble Space Telescope and will have an evengreater ability to view distant objects. Othercountries, including Russia, Japan, and severalEuropean countries have also put space telescopesin orbit.

Solar System Exploration

We have continued to explore the solar systemin recent years. In 1997, the Mars Pathfinder roverlanded on Mars. A rover is like a spacecraft onwheels (Figure below). It can move around on thesurface of a moon or planet and collect data fromdifferent locations. Two more rovers—Spirit andOpportunity—landed on Mars in 2004, and as of2008 are still sending data back to Earth.Amazingly, both rovers were only designed toexplore Mars for 90 days — they have nowworked for more than 15 times their intendedlifespan. Several spacecraft are currently in orbit

around Mars, studying its surface and thinatmosphere.

Figure 23.35 This artists painting of one of the two Mars

rovers shows the six wheels, as well as a set ofinstruments being extended forward by a roboticarm.

The Cassini mission has been studying Saturn,including its rings and moons, since 2004. TheHuygens probe, built by the European SpaceAgency, is studying Saturn’s moon Titan. Titan hassome of the conditions that are needed to supportlife.

Some missions are studying the smaller objects

in our solar system. The Deep Impact probe wassent to collide with a comet, collecting data all theway. When it hit the comet, the impact made acloud of dust. Space telescopes and telescopes onEarth all collected data after the impact. TheStardust mission collected tiny dust particles fromanother comet. Missions are currently underway tostudy some of the larger asteroids and Pluto.Studies of smaller objects in the solar system mayhelp us to understand how the solar system formed.

Future Missions

In 2004, President Bush proposed a “newvision for space exploration.” He set the goal ofputting humans on the Moon again by 2020. Unlikethe Apollo missions, however, Bush proposed thatreaching the Moon would only be the beginning.He also proposed building a permanent station onthe Moon, which could serve as a base formissions taking humans to Mars and beyond. Heannounced that the space shuttle program would beretired after the International Space Station was

complete (around 2010). A new kind of spacevehicle, now called Orion, will be developed totake humans to space.

President Bush also explained we would meetthese goals cooperatively, more like theInternational Space Station than the missionsduring the Space Race. He said, “We'll invite othernations to share the challenges and opportunities ofthis new era of discovery. The vision I outlinetoday is a journey, not a race, and I call on othernations to join us on this journey, in a spirit ofcooperation and friendship.” Meanwhile, China,Russia, and Japan have all said they are planningto send humans to the Moon and establish Moonbases of their own.

Lesson Summary

The Soviet Union put seven Salyut spacestations into orbit between 1971 and 1982.The United States’ first space station wasSkylab. Skylab was in orbit from 1973 to

1979.The Soviet (later Russian) space station Mirwas the first modular space station. BothRussian and American crews lived on Mir.The International Space Station is a hugeproject that involves many countries. It is stillbeing assembled.Space shuttles are reusable vehicles forAmerican astronauts to get into space. Aspace shuttle takes off like a rocket and landslike a glider plane.The space shuttle program has had two majordisasters—the Challenger disaster in 1986and the Columbia disaster in 2003. In eachcase, the spacecraft was destroyed and acrew of 7 people died.Recent space missions have mostly usedsmall spacecraft, such as satellites and spaceprobes, without crews.The United States plans to send humans to theMoon again by 2020, build a base on theMoon, then send humans to Mars.

Review Questions

1. Which space station was built and launchedby the United States alone?

2. How many years was the Mir space station inorbit?

3. Which space station was the first to involveseveral countries working together?

4. Describe two ways in which space shuttleswere an improvement over the spacecraftused for the Apollo missions?

5. Name the five fully functional space shuttlesthat the United States built. Which of thesewere destroyed?

6. Describe the space shuttle Columbia disaster,including its cause.

7. Describe two recent or ongoing spacemissions.

8. Is the Space Shuttle more like a rocket or aplane? Explain your answer.

Further Reading / Supplemental

Links

http://science.hq.nasa.gov/missions/earth.htmlhttp://landsat.gsfc.nasa.gov/images/archive/e0004.htmlhttp://www.whitehouse.gov/news/releases/2004/01/20040114-3.htmlhttp://www.nytimes.com/2007/09/25/science/space/25china.html?_r=1&#38;amp;ref=world&#38;amp;oref=sloginhttp://en.wikipedia.org

http://starchild.gsfc.nasa.gov/docs/StarChild/space_level2/skylab.htmlhttp://spaceflight.nasa.gov/station/;http://www.nasa.gov/mission_pages/station/main/index.htmlhttp://imagine.gsfc.nasa.gov/docs/features/news/pinksox.htmlhttp://spaceflight.nasa.gov/living/index.htmlhttp://science.howstuffworks.com/space-shuttle.htm;http://www.nasa.gov/mission_pages/shuttle/main/index.html;http://www.space.com/space-shuttle/http://www.nasa.gov/missions/current/index.htmlhttp://www.nasa.gov/mission_pages/exploration/main/index.html;http://en.wikipedia.org/wiki/Orion_%28spacecraft%29

Vocabulary

orbiterThe main part of the space shuttle that haswings like an airplane.

space shuttleA reusable spacecraft capable of carryinglarge pieces of equipment or pieces of aspace station.

space stationA large spacecraft in space on which humanscan live for an extended period of time.

Points to Consider

To date, a total of 22 people have died onspace missions. In the two space shuttledisasters alone, 14 people died. However,space exploration and research have led tomany great discoveries and new technologies.

Do you think sending people into space isworth the risk? Why or why not?In the past several years, private companieshave been developing vehicles and launchsystems that can take people into space.What applications can you think of for suchvehicles? What advantages and disadvantagesare there to private companies building andlaunching spacecraft?

Chapter 24: Earth, Moon,and Sun

Planet Earth

Lesson Objectives

Recognize that Earth is a sphere, and describethe evidence for this conclusion.Describe what gravity is, and how it affectsEarth in the solar system.Explain what causes Earth’s magnetism, andthe effects that magnetism has on the Earth.

Introduction

The Earth and Moon revolve around each otheras they orbit the Sun. As planet Earth rotates andrevolves, we experience cycles of day and night as

well as seasons. Earth has a very large moon foran inner planet. We are the only inner planet thatdoes, so how did the Moon form and what is thesurface of the Moon like? We revolve around anaverage, ordinary star, the Sun. It does not looklike any of the other stars we see in the sky. Whatcan we discover about our amazing Sun?

Earth’s Shape, Size, and Mass

Every day you walk across some part ofEarth’s surface, whether that surface is your yardor the sidewalks by your school. For most of us, awalk outside means walking on fairly flat ground.We don’t usually stop and realize that the Earth isa sphere, an object similar in shape to a ball. Howdo we know that Earth is a sphere? How could youconvince someone that even though the surfaces wewalk on look flat, the Earth as a whole is round?

One of the most convincing pieces of evidencefor a spherical Earth are the pictures we have of itfrom Space. Astronauts aboard the Apollo 17

shuttle took one of the most famous photographs inhistory, called “The Blue Marble” (Figure below).This outstanding image shows Earth as it looksfrom about 29,000 kilometers (18,000 miles) awayin Space. The picture shows us that Earth isspherical and looks like a giant blue and whiteball. Hundreds of years before humans ever madeit into space, we knew the Earth was round. Whatways have you been able to see this for yourself?

Figure 24.1 Photograph entitled The Blue Marble taken by

Apollo 17 Crew.The Sun and the other planets of our solar

system are also spheres. The Sun is found in thecenter of the solar system, and the planets travelaround the Sun in regular paths called orbits. Earthis the third planet from the Sun, and its mass isapproximately 6.0 x 1024 kilograms. In contrast, thevolume of planet Jupiter is about 1,000 timesgreater than Earth’s volume, and the Sun’s volumeis about 1,000 times greater than Jupiter’s (Figurebelow).

Figure 24.2 Planets and dwarf planets of the solar system.While the outer planets in the solar system are

giant balls of swirling gas with very low densities,Earth is an inner planet. The inner planets are

relatively small, denser, rockier planets than theouter planets. Three-fourths of Earth’s rockysurface is covered with water. As far as we know,Earth is also the only planet that carries liquidwater, another important requirement for life. Theentire planet is also surrounded by a thin layer ofair called the atmosphere. Earth’s atmosphere isunique in the solar system in that it contains just theright amount of oxygen to support animal life.Therefore, Earth is the only planet in the solarsystem on which life is found. The ‘Our SolarSystem’ chapter will discuss the features of otherplanets in more detail.

When describing Earth, it’s useful to name anddefine the many components of our planet. SinceEarth is a sphere, the layers that make it up arealso referred to as spheres (Figure below).

They are:

Atmosphere—the thin layer of air thatsurrounds the Earth.Hydrosphere--the part of Earth’s surface thatconsists of water.

Biosphere—the part of the Earth that supportslife. The biosphere includes all the areaswhere life is found.Lithosphere--the solid part of Earth. Thelithosphere consists of mountains, valleys,continents and all of the land beneath theoceans. Only one-fourth of Earth’s surface island, but solid rock makes up more than 99%of Earth’s total mass.

Earth’s layers all come into contact with eachother and interact. Therefore, Earth’s surface isconstantly undergoing change.

Figure 24.3

Earth has a hydrosphere, lithosphere,atmosphere, and biosphere.

Earth’s GravityWe know that the Earth orbits the Sun in a

regular path (Figure below). The Earth’s Moonalso orbits the Earth in a regular path. Gravity isthe force of attraction between all objects. Gravitykeeps the Earth and Moon in their orbits. IsaacNewton was one of the first scientists to explorethe idea of gravity. He understood that the Mooncan only circle the Earth because some force ispulling the Moon toward Earth’s center.Otherwise, the Moon would continue moving in astraight line off into space. Newton also came tounderstand that the same force that keeps the Moonin its orbit is the same force that causes objects onEarth to fall to the ground.

Figure 24.4 The planets orbit the Sun in regular paths.Newton defined the Universal Law of

Gravitation, which states that a force of attraction,called gravity, exists between all objects in theuniverse (Figure below). The strength of thegravitational force depends on how much mass theobjects have and how far apart they are from eachother. The greater the objects’ mass, the greater theforce of attraction; in addition, the greater thedistance between the objects, the smaller the forceof attraction.

Figure 24.5 The force of gravity exists between all objects

in the universe; the strength of the force depends onthe mass of the objects and the distance betweenthem.

Earth’s MagnetismEarth has a magnetic field (Figure below). It

may be helpful to imagine that the Earth has agigantic bar magnet inside of it. A bar magnet has anorth and south pole and a magnetic field thatextends around it. Earth's magnetic field also has anorth and south pole and a magnetic field thatsurrounds it. Scientists believe Earth’s magneticfield arises from the movements of molten metalsdeep inside Earth's outer liquid iron core. Iron andnickel flow within the Earth’s core, and theirmovement generates Earth's magnetic field. Earth’s

magnetic field extends several thousand kilometersinto space.

Figure 24.6 Earths Magnetic Field

Figure 24.7

Earths magnetic field protects the Earth fromradiation from the sun; Earth, on the right, is tiny bycomparison to the Sun.

Earth’s magnetic field serves an importantrole. It shields the planet from harmful types ofradiation from the Sun (Figure above). If you havea large bar magnet, you can tie a string to it, hang itfrom the string, and then watch as it aligns itself ina north-south direction, in response to Earth’smagnetic field. This concept allows a compass towork, so that people can navigate by findingmagnetic north (Figure below).

Figure 24.8 A compass; one end of the needle is pointing to

the north.

Lesson Summary

The Earth and other planets in our solarsystem are rotating spheres, that also revolvearound the Sun in fixed paths called orbits.The inner four planets are small, dense rockyplanets like Earth. The next four planets arelarge, gaseous planets like Jupiter.The balance between gravity and our motionaround the Sun, keep the planets in orbit atfixed distances from the Sun.Earth has a magnetic field, created by motionwithin Earth's outer, liquid iron core thatshields us from harmful radiation.

Review Questions

1. When you watch a tall ship sail over thehorizon of the Earth, you see the bottom partof it disappear faster than the top part. With

what you have learned about the shape of theEarth, describe why this happens.

2. What are two reasons that Earth is able tosupport life?

3. The planet Jupiter is gaseous and lacks asolid surface. How does this compare toEarth?

4. Give one example of how Earth’s lithosphereand hydrosphere can interact, and canexchange material.

5. How do mass and distance influence the forceof gravity?

6. Why are we able to use magnets to determinenorth-south directions on Earth?

Vocabulary

atmosphereThe thin layer of air that surrounds the Earth.

biosphereAll parts of the Earth that supports life.

gravityAn attractive force that exists between allobjects in the universe; gravity is responsiblefor planets orbiting the Sun, and for moonsorbiting those planets.

hydrosphereThe layer of Earth consisting of water.

lithosphereThe solid layer of Earth, made of rock.

magnetic fieldA region of space surrounding an object, inwhich an attractive magnetic force can bedetected.

orbitThe path of an object, such as a planet ormoon, around a larger object such as the Sun.

sphereAn object similar in shape to a ball.

Points to Consider

What would other planets need to have if theywere able to support life?Would life on Earth be impacted if Earth lostits magnetic field?Could a large gas planet like Jupiter or Saturnsupport life?

Earth’s Motions

Lesson Objectives

Describe Earth’s rotation on its axis.Describe Earth’s revolution around the Sun.

Introduction

Imagine a line passing through the center ofEarth that goes through both the North Pole and theSouth Pole. This imaginary line is called an axis.Earth spins around its axis, just as a top spinsaround its spindle. This spinning movement iscalled Earth’s rotation. At the same time that theEarth spins on its axis, it also orbits, or revolvesaround the Sun. This movement is calledrevolution.

Earth’s Rotation

In 1851, a French scientist named LéonFoucault took an iron sphere and swung it from awire. He pulled the sphere to one side and thenreleased it, letting it swing back and forth in astraight line. A ball swinging back and forth on astring is called a pendulum. A pendulum set inmotion, will not change its motion, so it will notchange the direction of the swinging. However,Foucault observed that his pendulum did seem tochange direction. He knew that the pendulum itselfcould not change its motion, so he concluded thatthe Earth, underneath the pendulum was moving.Figure below shows how this might look.

Figure 24.9 Imagine a pendulum at the North Pole. The

pendulum always swings in the same direction butbecause of Earths rotation; its direction willappear to change to observers on Earth.

It takes 23 hours, 59 minutes and 4 seconds forthe Earth to make one complete rotation on its axis,if we watch Earth spin from out in space. Because

Earth is moving around the Sun at the same timethat it is rotating, Earth has to turn just a little bitmore to reach the same place relative to the Sun,so we experience each day on Earth as 24 hours.At the equator, the Earth rotates at a speed of about1,700 kilometers per hour. Thankfully, we do notnotice this movement, because it would certainlymake us dizzy.

Earth’s RevolutionEarth’s revolution around the Sun takes much

longer than its rotation on its axis. One completerevolution takes 365.24 days, or one year. TheEarth revolves around the Sun because gravitykeeps it in a roughly circular orbit around the Sun.The Earth’s orbital path is not a perfect circle, butrather an ellipse, which means that it is like aslight oval in shape (Figure below). This createsareas where the Earth is sometimes farther awayfrom the Sun than at other times. We are closer tothe Sun at perihelion (147 million kilometers) onabout January 3rd and a little further from the Sun(152 million kilometers) at aphelion on July 4th.Students sometimes think our elliptical orbit

causes Earth's seasons, but this is not the case. If itwere, then the Northern Hemisphere wouldexperience summer in January!

Figure 24.10 Earth and the other planets in the solar system

make regular orbits around the Sun; the orbital pathis an ellipse and is controlled by gravity.

During one revolution around the Sun, the Earthtravels at an average distance of about 150 millionkilometers. Mercury and Venus take shorter timesto orbit the Sun than the Earth, while all the otherplanets take progressively longer times dependingon their distance from the Sun. Mercury only takesabout 88 Earth days to make one trip around theSun. While Saturn, for example, takes more than 29Earth years to make one revolution around the Sun.

Earth revolves around the Sun at an average

speed of about 27 kilometers (17 miles) persecond. Our planet moves slower when it is fartheraway from the Sun and faster when it is closer tothe Sun. The reason the Earth (or any planet) hasseasons is that Earth is tilted 23 1/2 o on its axis.This means that during the Northern hemispheresummer the North pole points toward the Sun, andin the Northern hemisphere winter the North Poleis tilted away from the Sun (Figure below). Theseason we experience depends on where the Earthis in its revolutionary orbit around the Sun.

Figure 24.11 The Earth tilts on its axis.

Lesson Summary

Earth rotates or spins on its axis once eachday and revolves around the Sun once everyyear.The tilt of Earth's axis produces seasons.

Review Questions

1. Describe the difference between Earth’srotation and its revolution.

2. What is the force that keeps the Earth andother planets in their orbital paths?

3. The planet Jupiter is about 778,570,000kilometers from the Sun; Earth is about150,000,000 kilometers from the Sun. DoesJupiter take more or less time to make onerevolution around the sun? Explain youranswer.

4. In its elliptical orbit around the Sun, the Earthis closest to the Sun in January. Even thoughEarth is closest to the Sun in January, people

in the Northern hemisphere experience winterweather. Using your understanding of how theEarth is tilted on its axis, why do you thinkpeople in the Northern Hemisphere havewinter in January?

5. Where on Earth would Foucault’s pendulumappear to not be moving? Why?

Vocabulary

axisAn imaginary line that runs from the NorthPole to South Pole, and includes the centerEarth.

ellipseA shape that looks like a slightly squashedcircle.

hemisphereOne half of a sphere.

revolution

The Earth’s movement around the Sun in anorbital path.

rotationThe motion of the Earth spinning on its axis.

Points to Consider

What type of experiment could you create toprove that the Earth is rotating on its axis?If you lived at the equator, would youexperience any effects due to Earth's tiltedaxis?If Earth suddenly increased in mass, whatmight happen to its orbit around the Sun?

Earth’s Moon

Lesson Objectives

Explain how scientists believe the Moonformed.Describe the features of the Moon.

Introduction

On July 20, 1969 hundreds of millions ofpeople all over the world excitedly sat in front oftheir televisions and witnessed something that hadnever happened before in the history of the world.On that day, two American astronauts named NeilArmstrong and Edwin “Buzz” Aldrin landed theEagle on the surface of the Moon (Figure below).Neil Armstrong was the commander of the mission,and he was the first human to ever step foot on theMoon. No other place in space, besides Earth, hasbeen touched by humans. Even today, the Moon

remains the only other body in space that humanshave visited.

Between 1969 and 1972, six pilotedspaceships were sent to land on the Moon. Theyare often referred to as lunar expeditions, the wordlunar meaning “related to the Moon.” On somemissions, the astronauts brought back soil and rocksamples from the Moon. Once back at Earth, thesamples were studied to help scientists learn aboutthe surface features of the Moon. No astronautshave visited the Moon since 1972, but in 2004 theUnited States President George W. Bush called fora return to Moon exploration by the year 2020.Maybe you can be one of the astronauts to return tothe Moon!

This lesson focuses on how the Moon wasformed and gives a description of the features andcharacteristics of the Moon—many of which wereinvestigated and discovered during the major yearsof lunar exploration in the 1960s and 1970s.

Figure 24.12 Astronaut Buzz Aldrin walks on the Moon on

July 20, 1969.

How the Moon Formed

Astronomers have carried out computersimulations showing that the collision of a Mars-sized object with the Earth could have resulted inthe formation of the Moon. Additional data showsthat the surface of the Moon dates to about 4.5

billion years ago, suggesting that the collisionoccurred during the heavy bombardment period,about 70 million years after the Earth formed. TheMoon also has a relatively small core and appearsto be largely comprised of the same basalt materialfound in the Earth mantle. Such a collision wouldhave been incredibly powerful, producing oceansof liquid magma over much of the surface of theEarth.

The explosive impact that likely led to theformation of the Moon would have produced ahuge amount of energy, leaving the surface of theMoon in an initially molten state. This means thatits surface would have been hot and fluid, likemagma inside the Earth today. The magmaeventually cooled and hardened so that the Moonnow has a solid surface.

Lunar Characteristics

The Moon is Earth’s only natural satellite. Asatellite is a body that moves around a larger body

in space. The Moon orbits Earth in the same waythat the Earth orbits the Sun, and the Moon remainsclose to Earth because of the strength of Earth’sgravity. The Moon is 3,476 kilometers in diameter,about one-fourth the size of Earth. Because theMoon is not as dense as the Earth, gravity on theMoon is only one-sixth as strong as it is on Earth.You could jump six times as high on the Moon asyou can on Earth.

If you watch the Earth and the Moon fromspace, the Moon makes one complete orbit aroundthe Earth every 27.3 days. The Moon also rotateson its axis once every 27.3 days. Thus, the sameside of the Moon always faces Earth. This meansfrom Earth we always see the same side of theMoon. The side of the Moon that faces Earth iscalled the near side (Figure below). The side ofthe Moon that faces away from Earth is called thefar side (Figure below). The Moon makes no lightof its own, but instead only reflects light from theSun.

Figure 24.13 The near side of the Moon, the one that we see,

has a thinner crust with many more maria.

Figure 24.14 The far side of the Moon has a thicker crust

and far fewer maria.

The Lunar Surface

The Moon has no atmosphere. The averagesurface temperature during the day isapproximately 225°F and can reach temperaturesas high as 253°F. At night the average temperaturedrops to -243°F and has been measured as low as -

397°F. These extremely cold temperatures occur incraters in the permanently shaded south polar basinand are amongst the coldest temperatures recordedin our entire solar system.

There are no lakes, rivers, or even smallpuddles anywhere to be found on the Moon'ssurface. (However, it should be noted that in 2009,NASA scientists believe they discovered that inthe top few millimeters of the Moon's surface,there is a large number of water molecules mixedin with dirt and rocks — you can stay up-to-datewith their latest findings at http://www.nasa.gov).Yet, despite the possible presence of water, with alack of atmosphere and extreme temperatures, itcomes as no surprise to scientists that there hasbeen zero evidence of life naturally occurring onthe Moon.

Although there are no "naturally occurring"signs of life on the Moon, there are signs that lifehas encountered the Moon — that is, there arefootprints of astronauts on the Moon's surface. It'slikely that these footprints will remain unchangedfor thousands of years, because there is no wind,

rain, or living thing to disturb them. Only a fallingmeteorite or other matter from space coulddestroy them. A meteorite is a piece of rock thatreaches the Moon from space. Meteorites also hitthe Earth sometimes.

Earth has mountains, valleys, plains and hills.This combination of all of the surface features ofan area of land is called a landscape. Thelandscape of the Moon is very different from thatof Earth. The lunar landscape is covered bycraters caused by the impacts of asteroids andmeteorites that crashed into the Moon from space(Figure below). The craters are bowl-shapedbasins on the Moon’s surface. Because the Moonhas no water, wind, or weather the craters remainunchanged. If Earth did not have plate tectonics,which continually alters the planet’s surface, or anatmosphere, which makes erosion possible, ourplanet’s surface would be at least as covered withmeteorite craters as the Moon’s. The surfaces ofmany other moons orbiting other planets have beenshaped by asteroid impacts.

Figure 24.15 A crater on the surface of the Moon.When you look at the Moon from Earth you

notice dark areas and light areas. The dark areasare called maria. They are solid, flat areas ofbasaltic lava. From about 3.0 to 3.5 billion yearsago the Moon was continually bombarded bymeteorites. Some of these meteorites were so largethat they broke through the Moon’s newly formed

surface, then magma flowed out and filling thecraters. Scientists estimate volcanic activity on theMoon ceased about 1.2 billion years ago.

The lighter parts are the Moon is called terraeor highlands (Figure below). They are higher thanthe maria and include several high mountainranges. They are believed to be the rims of ancientimpact craters.

Figure 24.16 A close-up of the Moon, showing maria (the

dark areas) and terrae (the light areas); mariacovers around 16% of the Moons surface, mostlyon the side of the Moon we see.

Interior of the Moon

Like the Earth, the Moon has a distinct crust,

mantle, and core. The crust is composed of igneousrock rich in the elements oxygen, silicon,magnesium, and aluminum. The Moon’s crust isabout 60 kilometers thick on the near side of theMoon and about 100 kilometers thick on the farside. The mantle is composed of the mineralsolivine and orthopyroxene. Analysis of Moonrocks indicates that there may also be high levelsof iron and titanium in the lunar mantle. The Moonhas a small core, perhaps 600 to 800 kilometers indiameter. The composition of the Moon’s core isnot known, but it is probably made mostly of ironwith some sulfur and nickel. This information isgathered both from rock samples gathered byastronauts and from unpiloted spacecraft sent to theMoon.

Lesson Summary

Many scientists believe the Moon formedwhen a Mars sized planet collided with Earth.The Moon makes one rotation on its axis in

the same number of days it takes for it to orbitthe Earth.The Moon has dark areas, called mariasurrounded by lighter colored highland areas,called terrae.Because the Moon is geologically inactiveand doesn't have an atmosphere, it has manythousands of craters on its surface.The Moon is made of many materials similarto Earth and has a crust, mantle and core, justlike the Earth.

Review Questions

1. What is one piece of evidence that supportsthe idea that the Moon was formed bymaterials that were once part of the Earth?

2. Why is there no weather on the Moon?3. Rusting is a process that happens when

oxygen reacts chemically with iron, in thepresence of water. Can rusting occur on theMoon? Explain your answer.

4. What is the difference between maria andterrea?

5. How much do landscape features on the Moonchange over time compared to landscapefeatures on Earth? Explain your answer.

6. Why is the force of gravity on your bodyweaker on the Moon than on the Earth?

Vocabulary

cratersBowl-shaped depressions on the surface ofthe Moon caused by impact from meteorites.

giant impact hypothesisThe idea that the Moon was formed when aplanet sized object from space collided withthe Earth about 4.5 billion years ago and senttrillions of tons of material into Earth’s orbit;the material eventually came together andformed the Moon.

landscape

The surface features of an area.

lunarRelated to the Moon.

mariaThe dark parts of the Moon’s surface, madeup of ancient basaltic eruptions.

meteoritesPieces of rock that hit the Moon, Earth, oranother planet from space.

satelliteA body that orbits a larger body in space.

terraeThe light parts of the Moon’s surface,composed of high crater rims.

Points to Consider

What things would be different on Earth if

Earth did not have a moon?If the Moon rotated on its axis once every 14days, would we see anything different than wedo now?How do we know that the Moon has beengeologically inactive for billions of years?

The Sun

Lesson Objectives

Describe the layers of the Sun.Describe the surface features of the Sun.

Introduction

Consider the Earth, the Moon, and all the otherplanets in our solar system. Think about the massthat all those objects must have when they are alladded together. Added all together, however, theyaccount for only 0.2% of the total mass of the solarsystem. The Sun makes up the remaining 99.8% ofall the mass in the solar system (Figure below)!The Sun is the center of the solar system and thelargest object in the solar system. Our Sun is a starthat provides light and heat and supports almost alllife on Earth.

In this lesson you will learn about the features

of the Sun. We will discuss the composition of theSun, its atmosphere, and some of its surfacefeatures.

Figure 24.17 The Sun.

Layers of the Sun

The Sun is a sphere, but unlike the Earth andthe Moon, is not solid. Most atoms in the Sun existas plasma, or a fourth state of matter made up ofsuperheated gas with an electrical charge. Our Sunconsists almost entirely of the elements hydrogenand helium. Because the Sun is not solid, it does

not have a defined outer boundary. It does,however, have a definite internal structure. Thereare several identifiable layers of the Sun:

The core is the innermost or central layer ofthe Sun. The core is plasma, but movessimilarly to a gas. Its temperature is around27 million degrees Celsius. In the core,nuclear reactions combine hydrogen atoms toform helium, releasing vast amounts of energyin the process. The energy released thenbegins to move outward, towards the outerlayers of the Sun.

The convection zone surrounds the radiativezone. In the convection zone, hot materialfrom near the Sun’s center rises, cools at thesurface, and then plunges back downward toreceive more heat from the radiative zone.This movement helps to create solar flaresand sunspots, which we’ll learn more about ina bit. These first three layers make up whatwe would actually call ‘the Sun’. The next

three layers make up the Sun’s atmosphere. Ofcourse, there are no solid layers to any part ofthe Sun, so these boundaries are fuzzy andindistinct.The radiative zone is just outside the core,which has a temperature of about 7 milliondegrees Celsius.The energy released in thecore travels extremely slowly through theradiative zone. Particles of light calledphotons can only travel a few millimetersbefore they hit another particle in the Sun, areabsorbed and then released again. It can takea photon as long as 50 million years to travelall the way through the radiative zone.

The Sun’s ‘Atmosphere’

The corona is the outermost layer of the Sunand is the outermost part of its atmosphere. Itis the Sun’s halo or ‘crown.’ It has atemperature of 2 to 5 million degrees Celsiusand is much hotter than the visible surface of

the Sun, or photosphere. The corona extendsmillions of kilometers into space. If you everhave the chance to see a total solar eclipse,you will be able to see the Sun’s corona,shining out into space.The chromosphere is the zone about 2,000kilometers thick that lies directly above thephotosphere. The chromosphere is a thinregion of the Sun’s atmosphere that glows redas it is heated by energy from thephotosphere. Temperatures in thechromosphere range from about 4000oC toabout 10,000oC. Jets of gas fire up through thechromosphere at speeds up to 72,000kilometers per hour, reaching heights as highas 10,000 kilometers.The photosphere is the visible surface of theSun (Figure below). This is the region of theSun that emits sunlight. It’s also one of thecoolest layers of the Sun — only about 6700°C. Looking at a photograph of the Sun’ssurface, you can see that it has several

different colors; oranges, yellow and reds,giving it a grainy appearance. We cannot seethis when we glance quickly at the Sun. Oureyes can’t focus that quickly and the Sun istoo bright for us to look at for more than abrief moment. Looking at the Sun for anylength of time can cause blindness, so don’ttry it! Sunlight is emitted from the Sun’sphotosphere. A fraction of the light thattravels from the Sun reaches Earth. It travelsas light in a range of wavelengths, includingvisible light, ultraviolet and infraredradiation. Visible light is all the light we cansee with our eyes. We can’t see ultravioletand infrared radiation, but their effects canstill be detected. For example, a sunburn iscaused by ultraviolet radiation when youspend too much time in the Sun.

Figure 24.18 The layers of the Sun.In the Sun’s core, nuclear fusion reactions

generate energy by converting hydrogen to helium.Fusion is a process where the nuclei of atoms jointogether to form a heavier chemical element.Fusion reactions in the Sun’s core produce energy,which we experience as heat and light. The rest ofthe Sun is heated by movement of heat energyoutward from the core. Light energy from the Sunis emitted from the photosphere. It travels throughspace, and some of it reaches the Earth. The Sun is

the source of almost all the energy on Earth andsunlight powers photosynthesis, as well aswarming and illuminating our Earth.

Surface Features of the Sun

The most noticeable surface feature of the Sunis the presence of sunspots, which are cooler,darker areas on the Sun’s surface (Figure below).Sunspots are only visible with special light-filtering lenses. They exhibit intense magneticactivity. These areas are cooler and darkerbecause loops of the Sun’s magnetic field breakthrough the surface and disrupt the smooth transferof heat from lower layers. Sunspots usually occurin pairs. When a loop of the Sun’s magnetic fieldbreaks through the surface, it usually creates asunspot both where it comes out and one where itgoes back in again. Sunspots usually occur in 11year cycles, beginning when the number ofsunspots is at a minimum, increasing to a maximumnumber of sunspots and then gradually decreasing

to a minimum number of sunspots again.

Figure 24.19 If a loop of the sun’s magnetic field snaps and

breaks, it creates solar flares, which are violentexplosions that release huge amounts of energy(Figure below). They release streams of highlyenergetic particles that make up the solar wind.The solar wind can be dangerous to spacecraft andastronauts. It sends out large amounts of radiation,which can harm the human body. Solar flares haveknocked out entire power grids and can disturbradio, satellite and cell phone communications.

Figure 24.20 A Solar Flare.Another highly visible feature on the Sun are

solar prominences (see video, below). If plasmaflows along a loop of the Sun’s magnetic fieldfrom sunspot to sunspot, it forms a glowing archthat reaches thousands of kilometers into the Sun’satmosphere. Prominences can last for a day toseveral months. Prominences are also visibleduring a total solar eclipse.

This compilation of video shows some of thefirst imagery and data sent back from NASA'sSolar Dynamics Observatory (SDO). Most of theimagery comes from SDO's AIA instrument, anddifferent colors are used to represent differenttemperatures, a common technique for observingsolar features. SDO sees the entire disk of the Sunin extremely high spacial and temporal resolutionand this allows scientists to zoom in on notableevents like flares, waves, and sunspots. Enjoy the

imagery and post in the comments the parts youfound most captivating. To learn more about theSDO mission, visit: http://sdo.gsfc.nasa.gov Tofind these videos for download, check out:http://www.nasa.gov/mission_pages/sdo/news/briefing-materials-20100421.htmlhttp://svs.gsfc.nasa.gov/Gallery/SDOFirstLight.htmlLaunched on Feb. 11, 2010, SDO is the mostadvanced spacecraft ever designed to study thesun. During its five-year mission, it will examinethe sun's magnetic field and also provide a betterunderstanding of the role the sun plays in Earth'satmospheric chemistry and climate. Since launch,engineers have been conducting testing andverification of the spacecrafts components. Nowfully operational, SDO will provide images withclarity 10 times better than high-definitiontelevision and will return more comprehensivescience data faster than any other solar observingspacecraft. Like our videos? Subscribe to NASA'sGoddard Shorts HD podcast:http://svs.gsfc.nasa.gov/vis/iTunes/f0004_index.htmlOr get tweeted by NASA:

http://twitter.com/NASAGoddard Or find us onfacebook:http://www.facebook.com/NASA.GSFC(Watch onYoutube)

A beautiful and mysterious effect of the Sun’selectrically charged particles are auroras, whichform around the polar regions high in Earth’satmosphere. Gases in Earth’s atmosphere areexcited by the electrically charged particles of thesolar wind and glow producing curtains of light,which bend and change as you watch.

Lesson Summary

The mass of the Sun is tremendous. It makesup 99.8% of the mass of our solar system.The Sun is mostly made of hydrogen withsmaller amounts of helium in the form ofplasma.The main part of the Sun has three layers: thecore, the radiative zone and the convectionzone.

The Sun's atmosphere also has three layers:the photosphere, the chromosphere and thecorona.Nuclear fusion of hydrogen in the core of theSun produces tremendous amounts of energythat radiate out from the Sun.Some features of the Sun's surface includesunspots, solar flares, and prominences.

Review Questions

1. In what way does the Sun support all life onEarth?

2. Which two elements make up the Sun almostin entirety?

3. Which process is the source of heat in the Sunand where does it take place?

4. Some scientists would like to plan a trip totake humans to Mars. One of the thingsstanding in the way of our ability to do this issolar wind. Why will we have to beconcerned with solar wind?

5. Describe how movements in the convectionzone contribute to solar flares.

6. Do you think fusion reactions in the Sun’score will continue forever and go on with noend? Explain your answer.

Vocabulary

chromosphereThin layer of the Sun’s atmosphere that liesdirectly above the Photosphere; glows red.

convection zoneLayer of the Sun that surrounds the RadiativeZone; energy moves as flowing cells of gas.

coreInnermost or central layer of the Sun.

coronaOutermost layer of the Sun; a plasma thatextends millions of kilometers into space.

nuclear fusionThe merging together of the nuclei of atoms toform new, heavier chemical elements; hugeamounts of nuclear energy are released in theprocess.

photosphereLayer of the Sun that we see; the visiblesurface of the Sun.

photosynthesisThe process that green plants use to convertsunlight to energy.

plasmaA high energy, high temperature form ofmatter. Electrons are removed from atoms,leaving each atom with an electrical charge.

radiationElectromagnetic energy; photons.

radiative zoneLayer of the Sun immediately surrounding the

core; energy moves atom to atom aselectromagnetic waves.

solar flareA violent explosion on the Sun’s surface.

solar windA stream of radiation emitted by a solar flare.The solar wind extends millions of kilometersout into space and can even reach the Earth.

sunspotsCooler, darker areas on the Sun’s surface thathave lower temperatures than surroundingareas; sunspots usually occur in pairs.

Points to Consider

If something were to suddenly cause nuclearfusion to stop in the Sun, how would weknow?Are there any types of dangerous energy fromthe Sun? What might be affected by them?

If the Sun is all made of gases like hydrogenand helium, how can it have layers?

Going Further - Applying Math

Have you ever wondered how to measuresomething that you cannot reach or touch? Theanswer is that you can use simple geometry. Wecan measure the diameter of the Sun, even thoughwe cannot go to the Sun and even though the Sun ismuch too large for a human being to measure. Wecan do this using the rules of similar triangles. Thesides of similar triangles are proportional to eachother. By setting up one very small triangle that isproportional to another, very large triangle, we canfind an unknown distance or measurement as longas we know three out of four of the parts of theequation. If you make a pinhole in an index cardand project an image of the Sun onto a clipboardheld 1 meter from the index card, the diameter ofour projected image of the Sun will beproportional to the true diameter of the Sun. Here's

the equation: s / d = S / D where s = diameter ofthe projected image of the Sun, S = true diameterof the Sun. We also need to know the true distancebetween the Earth and the Sun, D = 1.496 x 10 8km and the distance (d = 1 meter) between theclipboard and the index card. Before you cancorrectly solve this equation, you will need tochange all of your measurements to the same unit -in this case, change all your measurements to km.Try this out and see how accurately you canmeasure the true diameter of the Sun.

The Sun and the Earth-Moon System

Lesson Objectives

Describe how Earth’s movements affectseasons and cause day and night.Explain solar and lunar eclipses.Describe the phases of the Moon and explainwhy they occur.Explain how movements of the Earth andMoon affect Earth’s tides.

Introduction

The solar system is made up of the Sun, theplanets that orbit the Sun, their satellites, dwarfplanets and many, many small objects, likeasteroids and comets. All of these objects moveand we can see these movements. We notice the

Sun rises in the eastern sky in the morning and setsin the western sky in the evening. We observedifferent stars in the sky at different times of theyear. When ancient people made theseobservations, they imagined that the sky wasactually moving while the Earth stood still. In1543, Nicolaus Copernicus (Figure below)proposed a radically different idea: the Earth andthe other planets make regular revolutions aroundthe Sun. He also suggested that the Earth rotatesonce a day on its axis. Copernicus’ idea slowlygained acceptance and today we base our view ofmotions in the solar system on his work. We alsonow know that everything in the universe ismoving.

Figure 24.21 Nicholas Copernicus.In this lesson you will learn about how the

movements of the Earth, Moon, and Sun affectdifferent phenomena on Earth, including day andnight, the seasons, tides, and phases of the Moon.

Positions and Movements

Earlier we discussed Earth’s rotation andrevolution. The Earth rotates once on its axis aboutevery 24 hours. If you were to look at Earth fromthe North Pole, it would be spinningcounterclockwise. As the Earth rotates, observerson Earth see the Sun moving across the sky fromeast to west with the beginning of each new day.We often say that the Sun is “rising” or “setting,”but actually it is the Earth’s rotation that gives usthe perception of the Sun rising up or setting overthe horizon. When we look at the Moon or the starsat night, they also seem to rise in the east and set inthe west. Earth’s rotation is also responsible forthis. As Earth turns, the Moon and stars changeposition in our sky.

Earth’s Day and Night

Another effect of Earth’s rotation is that wehave a cycle of daylight and darknessapproximately every 24 hours. This is called aday. As Earth rotates, the side of Earth facing the

Sun experiences daylight, and the opposite side(facing away from the Sun) experiences darknessor nighttime. Since the Earth completes onerotation in about 24 hours, this is the time it takesto complete one day-night cycle. As the Earthrotates, different places on Earth experience sunsetand sunrise at a different time. As you movetowards the poles, summer and winter days havedifferent amounts of daylight hours in a day. Forexample, in the Northern hemisphere, we beginsummer on June 21. At this point, the Earth’s NorthPole is pointed directly toward the Sun. Therefore,areas north of the equator experience longer daysand shorter nights because the northern half of theEarth is pointed toward the Sun. Since the southernhalf of the Earth is pointed away from the Sun atthat point, they have the opposite effect—longernights and shorter days.

For people in the Northern hemisphere, winterbegins on December 21. At this point, it is Earth’sSouth Pole that is tilted toward the Sun, and sothere are shorter days and longer nights for thosewho are north of the equator.

Earth’s Seasons

It is a common misconception that summer iswarm and winter and cold because the Sun iscloser to Earth in the summer and farther awayfrom it during the winter. Remember that seasonsare caused by the 23 1/2 o tilt of Earth’s axis ofrotation and Earth’s yearly revolution around theSun (Figure below). This results in one part of theEarth being more directly exposed to rays from theSun than the other part. The part tilted away fromthe Sun experiences a cool season, while the parttilted toward the Sun experiences a warm season.Seasons change as the Earth continues itsrevolution, causing the hemisphere tilted awayfrom or towards the Sun to change accordingly.When it is winter in the Northern hemisphere, it issummer in the Southern hemisphere, and viceversa.

Figure 24.22 The Earths tilt on its axis leads to one

hemisphere facing the Sun more than the otherhemisphere and gives rise to seasons.

Solar Eclipses

A solar eclipse occurs when the new moonpasses directly between the Earth and the Sun(Figure below). This casts a shadow on the Earthand blocks our view of the Sun. A total solar

eclipse occurs when the Moon’s shadowcompletely blocks the Sun (Figure below). Whenonly a portion of the Sun is out of view, it is calleda partial solar eclipse. Solar eclipses are rareevents that usually only last a few minutes. That isbecause the Moon’s shadow only covers a verysmall area on Earth and Earth is turning veryrapidly. As the Sun is covered by the moon’sshadow, it will actually get cooler outside. Birdsmay begin to sing, and stars will become visible inthe sky. During a solar eclipse, the corona andsolar prominences can be seen.

Figure 24.23 A Solar Eclipse.

Figure 24.24 Photo of a Total Solar Eclipse.

A Lunar Eclipse

A lunar eclipse occurs when the full moonmoves through the shadow of the Earth (Figurebelow). This can only happen when the Earth isbetween the Moon and the Sun and all three arelined up in the same plane, called the ecliptic. Theecliptic is the plane of Earth’s orbit around the

Sun. The Earth’s shadow has two distinct parts: theumbra and the penumbra. The umbra is the inner,cone shaped part of the shadow, in which all of thelight has been blocked. The outer part of Earth’sshadow is the penumbra where only part of thelight is blocked. In the penumbra, the light isdimmed but not totally absent. A total lunar eclipseoccurs when the Moon travels completely inEarth’s umbra. During a partial lunar eclipse, onlya portion of the Moon enters Earth’s umbra. Apenumbral eclipse happens when the Moon passesthrough Earth’s penumbra. The Earth’s shadow isquite large, so a lunar eclipse lasts for hours andcan be seen by anyone with a view of the Moon atthe time of the eclipse.

Figure 24.25 The Formation of a Lunar Eclipse.Partial lunar eclipses occur at least twice a

year, but total lunar eclipses are less common. Thenext total lunar eclipse will occur December 21,2010. The moon glows with a dull red coloringduring a total lunar eclipse.

The Phases of the Moon

The Moon does not produce any light of itsown — it only reflects light from the Sun. As theMoon moves around the Earth, we see different

parts of the near side of the Moon illuminated bythe Sun. This causes the changes in the shape of theMoon that we notice on a regular basis, called thephases of the Moon. As the Moon revolves aroundEarth, the illuminated portion of the near side ofthe Moon will change from fully lit to completelydark and back again.

A full moon is the lunar phase seen when thewhole of the Moon’s lit side is facing Earth. Thisphase happens when Earth is between the Moonand the Sun. About one week later, the Moonenters the quarter-moon phase. At this point, theMoon appears as a half-circle, since only half ofthe Moon’s lit surface is visible from Earth. Whenthe Moon moves between Earth and the Sun, theside facing Earth is completely dark. This is calledthe new moon phase, and we do not usually see theMoon at this point. Sometimes you can just barelymake out the outline of the new moon in the sky.This is because some sunlight reflects off the Earthand hits the moon. Before and after the quarter-moon phases are the gibbous and crescent phases.During the gibbous moon phase, the moon is more

than half lit but not full. During the crescent moonphase, the moon is less than half lit and is seen asonly a sliver or crescent shape. It takes about 29.5days for the Moon to revolve around Earth and gothrough all the phases (Figure below).

Figure 24.26 The Phases of the Moon. Note that the Sun

would be above the top of this picture, and thus,the Suns rays would be directed downward.

The Tides

Tides are the regular rising and falling ofEarth’s surface water in response to gravitationalattraction from the Moon and Sun. The Moon’sgravity causes the oceans to bulge out in thedirection of the Moon. In other words, the Moon’sgravity is pulling upwards on Earth’s water,producing a high tide. On the other side of the

Earth, there is another high tide area, producedwhere the Moon’s pull is weakest. As the Earthrotates on its axis, the areas directly in line withthe Moon will experience high tides. Each placeon Earth experiences changes in the height of thewater throughout the day as it changes from hightide to low tide. There are two high tides and twolow tides each tidal day. Figure below and Figurebelow will help you better understand how tideswork.

Figure 24.27 A Spring Tide.

Figure 24.28 A Neap Tide.The first picture shows what is called a spring

tide. Confusingly, this tide has nothing to do withthe season ‘Spring’, but means that the tide watersseem to spring forth. During a spring tide, the Sunand Moon are in line. This happens at both the newmoon and the full moon. The Sun’s gravity pulls onEarth’s water, while the Moon’s gravity pulls onthe water in the same places. The high tideproduced by Sun adds to the high tide produced bythe Moon. So spring tides have higher than normal

high tides. This water is shown on the picture asthe gray bulges on opposite sides of the Earth.Notice that perpendicular to the gray areas, thewater is at a relatively low level. The placeswhere the water is being pulled out experiencehigh tides, while the areas perpendicular to themexperience low tides. Since the Earth is rotating onits axis, the high-low tide cycle moves around theglobe in a 24-hour period.

The second picture shows a neap tide. A neaptide occurs when the Earth and Sun are in line butthe Moon is perpendicular to the Earth. Thishappens when the moon is at first or last quartermoon phase. In this case, the pull of gravity fromthe Sun partially cancels out the pull of gravityfrom the Moon, and the tides are less pronounced.Neap tides produce less extreme tides than thenormal tides. This is because the high tideproduced by the Sun adds to the low tide area ofthe Moon and vice versa. So high tide is not ashigh and low tide is not as low as it usually mightbe.

Lesson Summary

As the Earth rotates on its axis and revolvesaround the Sun, several different effects areproduced.When the new moon comes between the Earthand the Sun along the ecliptic, a solar eclipseis produced.When the Earth comes between the full moonand the Sun along the ecliptic, a lunar eclipseoccurs.Observing the Moon from Earth, we see asequence of phases as the side facing us goesfrom completely darkened to completelyilluminated and back again once every 29.5days.Also as the Moon orbits Earth, it producestides aligned with the gravitational pull of theMoon.The Sun also produces a smaller solar tide.When the solar and lunar tide align, at newand full moons, we experience higher than

normal tidal ranges, called spring tides.At first and last quarter moons, the solar tideand lunar tide interfere with each other,producing lower than normal tidal rangescalled neap tides.

Review Questions

1. The globe is divided into time zones, so thatany given hour of the day in one time zoneoccurs at a different time in other time zones.For example, New York City is in one timezone and Los Angeles is in another time zone.When it is 8 am in New York City, it is only 5am in Los Angeles. Explain how Earth’smotions cause this difference in times.

2. Explain how Earth’s tilt on its axis accountsfor seasons on Earth.

3. Explain how the positions of the Earth, Moon,and Sun vary during a solar eclipse and alunar eclipse.

4. Draw a picture that shows how the Earth,

Moon, and Sun are lined up during the newmoon phase.

5. Why are neap tides less extreme than springtides?

Further Reading / SupplementalLinks

Demonstration of Why Earth has Seasonshttp://www.youtube.com/watch?v=DuiQvPLWziQ&#38;feature=relatedSolar and Lunar Eclipseshhttp://www.youtube.com/watch?v=tIE1MTGz4eI&feature=related

Vocabulary

crescentPhase of the moon when it is less than halffull but still slightly lit.

gibbous

Phase of the moon when it is more than half litbut not completely full.

lunar eclipseAn eclipse that occurs when the Moon movesthrough the shadow of the Earth and isblocked from view.

neap tideType of tide event when the Sun and Earth arein line and the Moon is perpendicular to theEarth.

penumbraOuter part of shadow that remains partially litduring an eclipse.

solar eclipseOccurs when moon passes directly betweenthe Earth and Sun; the Moon’s shadow blocksthe Sun from view.

spring tideAn extreme tide event that happens when the

Earth, Moon, and the Sun are aligned;happens at full and new moon phases.

tideThe regular rising and falling of Earth’ssurface waters twice a tidal day as a result ofthe Moon’s and Sun’s gravitational attraction.

umbraInner cone shaped part of a shadow when alllight is blocked during an eclipse.

Points to Consider

Why don't eclipses occur every single monthat the full and new moons?The planet Mars has a tilt that is very similarto Earth's. What does this produce on Mars?Venus comes between the Earth and the Sun.Why don't we see an eclipse when thishappens?

Chapter 25: The SolarSystem

Introduction to the SolarSystem

Lesson Objectives

Describe historical views of the solar system.Name the planets, and describe their motionaround the sun.Explain how the solar system formed.

Changing Views of the SolarSystem

People have not always known about all theobjects in our solar system. The ancient Greeks

were aware of five of the planets. They did notknow what these objects were; they just noticedthat they moved differently than the stars did. Theyseemed to wander around in the sky, changing theirposition against the background of stars. In fact, theword “planet” comes from a Greek word meaning“wanderer.” They named these objects after godsfrom their mythology. The names we use now forthe planets are the Roman equivalents of theseGreek names: Mercury, Venus, Mars, Jupiter, andSaturn.

The Geocentric Universe

The ancient Greeks believed that Earth was atthe center of the universe, as shown in Figurebelow. This view is called the geocentric modelof the universe. Geocentric means “Earth-centered.” The geocentric model also describedthe sky, or heavens, as having a set of sphereslayered on top of one another. Each object in thesky was attached to one of these spheres, and

moved around Earth as that sphere rotated. FromEarth outward, these spheres contained the Moon,Mercury, Venus, the Sun, Mars, Jupiter, Saturn,and an outer sphere which contained all the stars.The planets appear to move much faster than thestars and so the Greeks placed them closer toEarth.

Today, powerful telescopes can actually seethe surfaces of planets in our solar system. Eventhough the closest stars have diameters that arehundreds of times larger than the Earth, the distantstars appear as tiny dots that cannot be resolved.

Figure 25.1 Model of a geocentric universe. This diagram

of the universe from the Middle Ages shows Earthat the center, with the Moon, the Sun, and theplanets orbiting Earth.

The geocentric model may seem strange to usnow, but at the time, it worked quite well. Itexplained why all the stars appear to rotate aroundEarth once per day. It also explained why theplanets move differently from the stars, and fromeach other. One problem with the geocentric model

was resolved around 150 A.D. by the astronomerPtolemy. At times, some planets seemed to movebackwards (in retrograde) instead of in their usualforward motion around the Earth. Ptolemyresolved this problem by using a system of circlesto describe the motion of planets (Figure below).In Ptolemy’s system, a planet moved in a smallcircle, called an epicycle. This circle in turnmoved around Earth in a larger circle, called adeferent. Ptolemy’s version of the geocentricmodel worked so well that it remained theaccepted model of the universe for more than athousand years.

Figure 25.2 Diagram of an epicycle and deferent.

According to Ptolemy, a planet moves on a smallcircle that in turn moves on a larger circle aroundEarth.

The Heliocentric Universe

Ptolemy’s geocentric model worked prettywell, but it was complicated and occasionallymade errors in predicting the movement of planets.

At the beginning of the 16th century A.D., NicolausCopernicus proposed a different model in whichEarth and all the other planets orbited the Sun.Because this model put the Sun at the center, it iscalled the heliocentric model of the universe.Heliocentric means “sun-centered.” Figure belowshows the heliocentric model compared to thegeocentric model. Copernicus’ model explainedthe motion of the planets about as well asPtolemy’s model, but it did not requirecomplicated additions like epicycles anddeferents.

Figure 25.3 Unlike the geocentric model (top image), the

heliocentric model (lower image), had the Sun atthe center, and did not require epicycles.

Although Copernicus’ model worked moresimply than Ptolemy’s, it still did not perfectlydescribe the motion of the planets. The problemwas that, like Ptolemy, Copernicus still thoughtplanets moved in perfect circles. Not long afterCopernicus, Johannes Kepler refined theheliocentric model. He proposed that planets movearound the Sun in ellipses (ovals), not circles. Thismodel matched observations perfectly.

Because people were so used to thinking ofEarth at the center of the universe, the heliocentricmodel was not widely accepted at first. However,when Galileo Galilei first turned a telescope to theheavens in 1610, he made several strikingdiscoveries. He found that the planet Jupiter hasmoons orbiting around it. This was the firstevidence that objects could orbit somethingbesides Earth. He also discovered that Venus hasphases like our moon does. The phases of Venus

provided direct evidence that Venus orbits the Sun.Galileo’s discoveries caused many more people toaccept the heliocentric model of the universe. Theshift from an Earth-centered view to a Sun-centered view of the universe is referred to as theCopernican Revolution.

The Modern Solar System

Today, we know that our solar system is justone tiny part of the universe as a whole. NeitherEarth nor the Sun are at the center of the universe—in fact, the universe has no true center.However, the heliocentric model does accuratelydescribe our solar system. In our modern view ofthe solar system, the Sun is at the center, andplanets move in elliptical orbits around the Sun.The planets do not emit their own light, but insteadreflect light from the Sun.

Extrasolar Planets or Exoplanets

Since the early 1990s, astronomers havediscovered other solar systems, with planetsorbiting stars other than our own Sun (called“extrasolar planets” or simply “exoplanets”).Although a handful of exoplanets have now beendirectly imaged, the vast majority have beendiscovered by indirect methods. One techniqueinvolves detecting the very slight motion of a starperiodically moving toward and away from usalong our line-of-sight (also known as a star’s“radial velocity”). This periodic motion can beattributed to the gravitational pull of a planet (or,sometimes, another star) orbiting the star. Anothertechnique involves measuring a star’s brightnessover time. A temporary, periodic decrease in lightemitted from a star can occur when a planetcrosses in front of (or “transits”) the star it isorbiting, momentarily blocking out some of thestarlight. As of February 2010, over 420exoplanets have been confirmed with more beingdiscovered at an ever-increasing rate.

Planets and Their Motions

Since the time of Copernicus, Kepler, andGalileo, we have learned a lot more about oursolar system. We have discovered two moreplanets (Uranus and Neptune), four dwarf planets(Ceres, Makemake, Pluto and Eris), over 150moons, and many, many asteroids and other smallobjects.

Figure 4 shows the Sun and the major objectsthat orbit the Sun. There are eight planets. From theSun outward, they are: Mercury, Venus, Earth,Mars, Jupiter, Saturn, Uranus, and Neptune. TheSun is just an average star compared to other stars,but it is by far the largest object in the solarsystem. The Sun is more than 500 times the mass ofeverything else in the solar system combined!Table below gives more exact data on the sizes ofthe sun and planets relative to Earth.

Sizes of Solar System Objects Relative toEarth

Object Mass (Relative Diameter of Planet

to Earth) (Relative to Earth)

Sun 333,000 Earthmasses 109.2 Earth diameters

Mercury 0.06 Earth'smass 0.39 Earth's diameter

Venus 0.82 Earth'smass 0.95 Earth's diameter

Earth 1.00 Earthmass 1.00 Earth diameter

Mars 0.11 Earth'smass 0.53 Earth's diameter

Jupiter 317.8 Earthmasses 11.21 Earth diameters

Saturn 95.2 Earthmasses 9.41 Earth diameters

Uranus 14.6 Earthmasses 3.98 Earth diameters

Neptune 17.2 Earthmasses 3.81 Earth diameters

(Sources: http://en.wikipedia.org/wiki/Planets,

http://en.wikipedia.org/wiki/Sun, License: GNU-FDL)

What Is (and Isn’t) a Planet?

So what exactly is a planet? Simply put, aplanet is a massive, round body orbiting a star.For our solar system, this star is the Sun. A moonis an object that orbits a planet.

“Isn’t Pluto a planet?” you may wonder. Whenit was discovered in 1930, Pluto was considered aninth planet. When we first saw Pluto, ourtelescopes actually saw Pluto and its moon,Charon as one much larger object. With bettertelescopes, we realized that Pluto had a moon andPluto was much smaller than we thought! With thediscovery of many objects like Pluto, and one ofthem, Eris, even larger than Pluto, in 2006,astronomers refined the definition of a planet.According to the new definition, a planet must:

orbit a starbe big enough that its own gravity causes it to

be shaped like a spherebe small enough that it isn’t a star itselfhave cleared the area of its orbit of smallerobjects

Objects that meet the first three criteria but notthe fourth are called dwarf planets. Mostastronomers now consider Pluto to be a dwarfplanet, along with the objects Ceres and Eris. Evenbefore astronomers decided to change thedefinition of a planet, there were many aspects ofPluto that did not fit with the other planets in oursolar system.

Figure 25.4 Relative sizes of the Sun, planets & dwarf

planets. The largest objects in the solar system arethe Sun, the eight planets, and the three known

dwarf planets. In this figure, the relative sizes arecorrect but the relative distances are not correct.

The Size and Shape of Orbits

Figure above shows the Sun and planets in thecorrect relative sizes. However, the relativedistances are not correct. Figure below shows therelative sizes of the orbits. The image in the upperleft shows the orbits of the inner planets. Theupper left image also shows the asteroid belt, acollection of many small objects between theorbits of Mars and Jupiter. The image in the upperright shows the orbits of the outer planets. Thisupper right image also shows the Kuiper belt,another group of objects beyond the orbit ofNeptune. In general, the farther away from the Sun,the greater the distance from one planet’s orbit tothe next.

Figure 25.5 This figure shows the relative sizes of the

orbits of planets in the solar system. The innersolar system is on the upper left. The upper rightshows the outer planets of our solar system.

In Figure 5, you can see that the orbits of theplanets are nearly circular. In fact, the orbits arenot quite circular, but are slightly elliptical. Theorbit of Pluto is a much longer ellipse. Someastronomers think Pluto was dragged into itscurrent orbit by Neptune.

Something else Kepler discovered was a

relationship between the time it takes a planet tomake one complete orbit around the Sun (this isalso called an "orbital period") and the distancefrom the Sun to the planet. So, if the orbital periodof a planet is known, then it is possible todetermine how far away from the Sun the planetorbits. This is how we can measure the distancesto other planets within our own solar system.

Distances in the solar system are oftenmeasured in astronomical units (AU). Oneastronomical unit is defined as the distance fromEarth to the Sun. 1 AU equals about 150 millionkm, or 93 million miles. Table below shows thedistances to the planets (the average radius oforbits) in AU. The table also shows how long ittakes each planet to spin on its axis (the length of aday) and how long it takes each planet to completean orbit (the length of a year); in particular, noticehow slowly Venus rotates relative to Earth.Distances to the Planets and Properties of Orbits

Relative to Earth’s OrbitAverageDistance Length of Length of

Planet from Sun(AU)

Day (InEarth Days)

Year (InEarth Years)

Mercury 0.39 AU 56.84 days 0.24 yearsVenus 0.72 243.02 0.62Earth 1.00 1.00 1.00Mars 1.52 1.03 1.88Jupiter 5.20 0.41 11.86Saturn 9.54 0.43 29.46Uranus 19.22 0.72 84.01Neptune 30.06 0.67 164.8

(Source: http://en.wikipedia.org/wiki/Planets,License: GNU-FDL)

The Role of Gravity

Planets are held in their orbits by the force ofgravity. Imagine swinging a ball on a string in acircular motion. If you were to let go of the string,the ball would go flying out in a straight line. Butthe force of the string pulling on the ball keeps the

ball moving in a circle. The motion of a planet isvery similar, except the force pulling the planet isthe attractive force of gravity between the planetand the Sun.

Every object is attracted to every other objectby gravity. The force of gravity between twoobjects depends on how much mass the objectshave and on how far apart they are. When you aresitting next to a friend, there is a gravitational forcebetween you and your friend, but it is far too weakfor you to detect. In order for the force of gravity tobe strong enough to detect, at least one of theobjects has to have a lot of mass. You can feel theforce of gravity between you and Earth becauseEarth has a lot of mass. This force of gravity iswhat keeps you from floating off the ground. Thedistances from the Sun to the planets are verylarge. But the force of gravity between the Sun andeach planet is very large because the Sun and theplanets are very large objects. The force of gravityalso holds moons in orbit around planets.

The moon orbits the Earth, and the Earth-moonsystem orbits the Sun. But Earth and its moon are

not the only things that orbit the Sun. There are alsoother planets and smaller objects, such asasteroids, meteoroids, and comets that also orbitthe Sun. The solar system consists of the Sun andall the objects that revolve around the sun as aresult of gravity.

Formation of the Solar System

There are two key features of the solar systemwe haven’t mentioned yet. First, all the planets liein nearly the same plane, or flat disk like region.Second, all the planets orbit in the same directionaround the Sun. These two features are clues tohow the solar system formed.

A Giant Nebula

The most widely accepted explanation of howthe solar system formed is called the nebularhypothesis. According to this hypothesis, the solarsystem formed about 4.6 billion years ago from the

collapse of a giant cloud of gas and dust, called anebula. The nebula was made mostly of hydrogenand helium, but there were heavier elements aswell.

The nebula was drawn together by gravity. Asthe nebula collapsed, it started to spin. As itcollapsed further, the spinning got faster, much asan ice skater spins faster when he pulls his arms tohis sides during a spin move. This effect, called“conservation of angular momentum,” along withcomplex effects of gravity, pressure, and radiation,caused the nebula to form into a disk shape, asshown in Figure below. This is why all the planetsare found in the same plane.

Figure 25.6 The nebular hypothesis describes how the

solar system formed from a cloud of gas and dust

into a disk with the Sun at the center. This paintingwas made by an artist; its not an actual photographof a protoplanetary disk.

Formation of the Sun and Planets

As gravity pulled matter into the center of thedisk, the density and pressure increased at thecenter. When the pressure in the center was highenough that nuclear fusion reactions started in thecenter, a star was born—the Sun.

Meanwhile, the outer parts of the disk werecooling off. Small pieces of dust in the disk startedclumping together. These clumps collided andcombined with other clumps. Larger clumps,cal led planetesimals, attracted smaller clumpswith their gravity. Eventually, the planetesimalsformed the planets and moons that we find in oursolar system today.

The outer planets—Jupiter, Saturn, Uranus andNeptune—condensed farther from the Sun fromlighter materials such as hydrogen, helium, water,ammonia, and methane. Out by Jupiter and beyond,

where it’s very cold, these materials can formsolid particles. But in closer to the Sun, these samematerials are gases. As a result, the inner planets—Mercury, Venus, Earth, and Mars—formed fromdense rock, which is solid even when close to theSun.

Lesson Summary

The solar system consists of the Sun and allthe objects that are bound to the Sun bygravity.There are eight planets in the solar system:Mercury, Venus, Earth, Mars, Jupiter, Saturn,and Neptune. Ceres, Makemake, Pluto andEris are considered dwarf planets.The ancient Greeks believed in a geocentricmodel of the universe, with Earth at the centerand everything else orbiting Earth.Copernicus, Kepler, and Galileo promoted aheliocentric model of the universe, with thesun at the center and Earth and the other

planets orbiting the Sun.Planets are held by the force of gravity inelliptical orbits around the Sun.The nebular hypothesis describes how thesolar system formed from a giant cloud of gasand dust about 4.6 billion years ago.The nebular hypothesis explains why theplanets all lie in one plane and orbit in thesame direction around the Sun.

Review Questions

1. What does geocentric mean?2. Describe the geocentric model and

heliocentric model of the universe.3. How was Kepler’s version of the heliocentric

model different from Copernicus’?4. Name the eight planets in order from the Sun

outward.5. What object used to be considered a planet,

but is now considered a dwarf planet?6. What keeps planets and moons in their orbits?

7. How old is the solar system?8. Use the nebular hypothesis to explain why the

planets all orbit the Sun in the same direction.

Further Reading / SupplementalLinks

http://www.youtube.com/watch?v=FHSWVLwbbNw&#38;NR=1http://sse.jpl.nasa.gov/planets/index.cfmhttp://www.iau.org/iau0602.423.0.htmlhttp://starchild.gsfc.nasa.gov/docs/StarChild/solar_system_level2/solar_system.htmlhttp://sse.jpl.nasa.gov/planets/index.cfmhttp://www.solarviews.com/eng/homepage.htmhttp://www.nineplanets.org/http://www.teachingideas.co.uk/science/orderingplanets.htmhttp://www.classzone.com/books/earth_science/terc/content/visualizations/es2701/es2701page01.cfm?chapter_no=27http://www.windows.ucar.edu/tour/link=/our_solar_system/formation.htmlhttp://www.solarviews.com/cap/misc/ssanim.htmhttp://en.wikipedia.org/

Vocabulary

geocentric modelModel used by the ancient Greeks that putsthe Earth at the center of the universe.

heliocentric modelModel proposed by Copernicus that put theSun at the center of the universe.

moonA celestial object that orbits a larger celestialobject.

nebulaAn interstellar cloud of gas and dust.

nebular hypothesisThe hypothesis that our solar systemdeveloped from a spinning cloud of gas anddust, or a nebula.

planet

Around, celestial object that orbits a star andhas cleared its orbit of smaller objects.

solar systemThe Sun and all the objects that revolvearound the Sun as a result of gravity.

Points to Consider

Would you expect all the planets in the solarsystem to be made of similar materials? Whyor why not?The planets are often divided into twogroups: the inner planets and the outerplanets. Which planets do you think are ineach of these two groups? What do membersof each group have in common?

Inner Planets

Lesson Objectives

Describe key features of each of the innerplanets.Compare each of the inner planets to Earthand to one another.

The Inner Planets

The four planets closest to the sun - Mercury,Venus, Earth and Mars are the inner planets, alsocalled the terrestrial planets because they aresimilar to Earth. Figure below shows the relativesizes of these four planets. All of the inner planetsare small, relative to the outer planets. All of theinner planets are solid, dense, rocky planets. Theinner planets either do not have moons or have justone (Earth) or two (Mars). None of the innerplanets has rings. Compared to the outer planets,

the inner planets have shorter orbits around theSun, but all the inner planets spin more slowly.Venus spins the slowest of all the planets. At onetime, all the inner planets have been geologicallyactive. They are all made of cooled igneous rockwith inner iron cores.

Mercury

Mercury, shown in Figure below, is the planetclosest to the Sun. Mercury is the smallest planet,and it has no moon. As Figure below shows, thesurface of Mercury is covered with craters, likeEarth’s moon. The presence of impact craters thatare so old means that Mercury hasn’t changedmuch geologically for billions of years and, withonly a trace of an atmosphere, has no weather towear down the ancient craters.

Figure 25.7 This composite shows the relative sizes of the

four inner planets. From left to right, they areMercury, Venus, Earth, and Mars.

Because Mercury is so close to the Sun, it isdifficult to observe from Earth, even with atelescope. However, the Mariner 10 spacecraft,shown in Figure below, visited Mercury in 1974–1975. In January 2008, the Messenger missionreturned to Mercury and took much more detailedpictures. One of these images can be seen inFigure below.

Figure 25.8 Mariner 10 made three flybys of Mercury in

1974 and 1975.

Figure 25.9 Mercury is covered with craters, like Earths

moon.

Short Year, Long Days

Mercury is named for the Roman messengergod, who could run extremely fast. Likewise,Mercury moves very fast in its orbit around theSun. A year on Mercury—the length of time ittakes to orbit the Sun—is just 88 Earth days.

Mercury has a very short year, but very longdays. A day is defined as the time it takes a planetto turn on its axis. Mercury rotates slowly on itsaxis, turning exactly three times for every twotimes it orbits the Sun. Therefore, each day onMercury is 58 Earth days long. In other words, onMercury, a year is only a Mercury day and a halflong!

Extreme Temperatures

Mercury is very close to the Sun, so it can getvery hot. However, Mercury has virtually noatmosphere and it rotates very slowly. Becausethere is no atmosphere and no water to insulate thesurface, temperatures on the surface of Mercuryvary widely. In direct sunlight, the surface can beas hot as 427 oC (801 oF). On the dark side, or inthe shadows inside craters, the surface can be ascold as –183 oC (–297 oF)! Although most ofMercury is extremely dry, scientists believe theremay be a small amount of water in the form of iceat the poles of Mercury, in areas which neverreceive direct sunlight.

A Liquid Metal Core

Figure below shows a diagram of Mercury’sinterior. Mercury is one of the densest planets.Scientists believe the interior contains a relativelylarge, liquid core made mostly of melted iron.Mercury's core takes up about 42% of the planet'svolume. Mercury's highly cratered surface is

evidence that Mercury is not geologically active.

Figure 25.10 Mercury contains a thin crust, a mantle, and a

large, liquid core that is rich in iron.

Venus

The second planet out from the Sun, Venus, isour nearest neighbor. Not only is it closer to Earththan any other planet, but it also is the most similar

to Earth in size. Named after the Roman goddess oflove, it is the only planet named after a female.Venus is sometimes called Earth’s “sister planet.”But just how similar is Venus to Earth?

A Harsh Environment

Viewed through a telescope, Venus lookssmooth and featureless. That’s because Venus iscovered by a thick layer of clouds, as shown inpictures of Venus taken at ultraviolet wavelengths,such as Figure below. Because of the thick, cloudyatmosphere, we cannot take ordinary photos of thesurface of Venus, even from spacecraft orbiting theplanet. However, we can make maps of the surfaceusing radar. Figure below shows a topographicalmap of Venus produced by the Magellan probeusing radar.

Unlike clouds on Earth, Venus's clouds are notmade of water vapor. They are made of carbondioxide and sulfur dioxide—and they also containlarge amounts of corrosive sulfuric acid!

Figure 25.11 This ultraviolet image from the Pioneer Venus

Orbiter shows thick layers of clouds in theatmosphere of Venus.

The atmosphere of Venus is so thick that theatmospheric pressure on the surface of Venus is 90times greater than the atmospheric pressure onEarth’s surface. The thick atmosphere also causesa strong greenhouse effect, which traps heat fromthe Sun. As a result, Venus is the hottest planet,even hotter than Mercury. Temperatures at the

surface reach 465oC (860 oF). That’s hot enough tomelt lead!

Volcanoes

Venus has more volcanoes than any otherplanet. Planetary scientists have estimated thatVenus has up to 100,000 or even a millionvolcanoes. Although these volcanoes contributedcarbon dioxide in the past, most of the volcanoesare now dead. Venus doesn’t seem to have tectonicplates like the Earth's. It’s surface is covered withdead volcanoes and ancient craters.

Figure 25.12 This topographic map of Venus was made from

radar data collected by the Magellan probebetween 1990 and 1994.

Orbiting spacecraft have used radar to revealmountains, valleys, and canyons on Venus. Most ofthe surface, however, has large areas of volcanoessurrounded by plains of lava. Figure below is animage made by a computer using radar data. Itshows a volcano called Maat Mons, with lavabeds in the foreground. The reddish-orange coloris close to what scientists think the color of

sunlight would look like on the surface of Venus.

Figure 25.13 This image of Maat Mons was generated from

radar data. The surface of Venus has manymountains, volcanoes, and plains of lava.

Motion and Appearance

Venus is the only planet that rotates clockwiseas viewed from its North pole, in a directionopposite to the direction it orbits the Sun. It turnsslowly in the reverse direction, making one turnevery 243 days. This is longer than a year onVenus—it takes Venus only 224 days to orbit the

Sun.Because the orbit of Venus is inside Earth’s

orbit, Venus always appears close to the Sun.When Venus rises early in the morning, just beforethe Sun rises, it is sometimes called “the morningstar.” When it sets in the evening, just after the Sunsets, it may be called “the evening star.” Venus’clouds reflect sunlight very well. As a result,Venus is very bright. When it is visible, Venus isthe brightest object in the sky besides the sun andthe Moon.

Like Mercury, Venus has no moon.

Earth

The third planet out from the Sun is shown inFigure below. Does it look familiar? It’s Earth!Because it is our home planet, we know a lot moreabout Earth than we do about any other planet. Butwhat are key features of Earth when viewed as amember of our solar system?

Figure 25.14 This famous image of Earth was taken during

the Apollo 17 mission to the moon.

Oceans and Atmosphere

As you can see in Figure above, Earth has vastoceans of liquid water, large masses of land, icecovering the poles, and a dynamic atmosphere withclouds of water vapor. Earth’s average surfacetemperature is 14oC (57oF). As you know, water is

a liquid at this temperature. The oceans and theatmosphere help keep Earth’s surface temperaturesfairly steady.

Earth is the only planet known to have life. Theconditions on Earth, especially the presence ofliquid water, are ideal for life as we know it. Theatmosphere filters out radiation that would beharmful to life, such as ultraviolet radiation and Xrays. The presence of life has changed Earth’satmosphere, so it has much more oxygen than theatmospheres of other planets.

Plate Tectonics

The top layer of Earth’s interior—the crust—contains numerous plates, known as tectonicplates. These plates move on the convecting mantlebelow, so they slowly move around on the surface.Movement of the plates causes other geologicalactivity, such as earthquakes, volcanoes, and theformation of mountains. Earth is the only planetknown to have plate tectonics.

Earth’s Motions and Moon

Earth rotates on its axis once per day. In fact,the time of this rotation is how people havedefined a day. Earth orbits the Sun once every365.24 days, which is also how we have defined ayear. Earth has one large moon, which orbits Earthonce every 29.5 days, a period known as a month.

Earth’s moon is the only large moon around aterrestrial planet in the solar system. The Moon iscovered with craters, and also has large plains oflava. There is evidence that the Moon formedwhen a very large object—perhaps as large as theplanet Mars—struck Earth in the distant past.

Mars

Mars, shown in Figure below, is the fourthplanet from the Sun, and the first planet beyondEarth’s orbit. The Martian atmosphere is thinrelative to Earth’s and with much loweratmospheric pressure. Unlike Earth’s neighbor on

the side nearer the sun, Mars has only a weakgreenhouse effect, which raises its temperatureonly slightly above what it would be if the planetdid not have an atmosphere.

Although Mars is not the closest planet toEarth, it is the easiest to observe. Therefore, Marshas been studied more thoroughly than any otherplanet besides Earth. Humans have sent manyspace probes to Mars. Currently, there are threescientific satellites in orbit around Mars, and twofunctioning rovers on the surface. No humans haveever set foot on Mars. However, both NASA andthe European Space Agency have set goals ofsending people to Mars sometime between 2030and 2040.

Figure 25.15 This image of Mars, taken by the Hubble Space

Telescope in August, 2003, shows the planetsreddish color and a prominent polar ice cap.

A Red Planet

Viewed from Earth, Mars is reddish in color.The ancient Greeks and Romans named the planetafter the god of war. They may have associated theplanet with war because its red color reminded

them of blood. Mars appears red because thesurface of the planet really is a reddish-orange rustcolor, due to large amounts of iron in the soil.Mars has only a very thin atmosphere, made upmostly of carbon dioxide.

Surface Features

Mars is home to the largest mountain in thesolar system—Olympus Mons, shown in Figurebelow. Olympus Mons is a shield volcano, similarto the volcanoes that make up the Hawaiian islandson Earth. Olympus Mons is about 27 km (16.7miles/88,580 ft) above the normal Martian surfacelevel. That makes it more than three times tallerthan Mount Everest. At its base, Olympus Mons isabout the size of the entire state of Arizona!

Figure 25.16 The Martian volcano Olympus Mons is the

largest mountain in the solar system.Mars also has the largest canyon in the solar

system, Valles Marineris (Figure below). Thiscanyon is 4,000 km (2,500 miles) long, as long asEurope is wide, and one-fifth the circumference ofMars. The canyon is 7 km (4.3 miles) deep. Bycomparison, the Grand Canyon on Earth is only446 km (277 miles) long and about 2 km (1.2miles) deep.

Figure 25.17 The Martian canyon Valles Marineris is the

largest canyon in the solar system.Although Mars has mountains, canyons, and

other features similar to Earth, it doesn’t have asmuch geological activity as Earth. There is noevidence of plate tectonics on Mars. There arealso more craters on Mars than on Earth, thoughfewer than on the Moon.

Is There Water on Mars?

Water cannot stay in liquid form on Marsbecause the pressure of the atmosphere is too low.However, there is a lot of water in the form of ice.Figure above shows a prominent ice cap at thesouth pole of Mars. Scientists also believe there isalso a lot of ice water present just under theMartian surface. This ice can melt when volcanoes

erupt, and water can flow across the surfacetemporarily.

Scientists have reason to think that water onceflowed over the surface of Mars because they cansee surface features that look like water-erodedcanyons, and the Mars rover collected roundclumps of crystals that, on Earth, usually form inwater. The presence of water on Mars, even thoughit is now frozen as ice, suggests that it might havebeen possible for life to exist on Mars in the past.

Two Martian Moons

Mars has two very small moons, Phobos andDeimos. As you can see in Figure below, thesemoons are not spherical in shape, but instead justlook like irregular rocks. Phobos and Deimos werediscovered in 1877. They are named aftercharacters in Greek mythology—the two sons ofAres, who followed their father into war. Ares isequivalent to the Roman god Mars.

Figure 25.18 Mars has two small moons, Phobos (left) and

Deimos (right).

Lesson Summary

The four inner planets, or terrestrial planets,have solid, rocky surfaces.Mercury is the smallest planet and the closestto the Sun. It has an extremely thinatmosphere, with surface temperaturesranging from very hot to very cold. Like theMoon, it is covered with craters.Venus is the second planet from the Sun andthe closest planet to Earth, in distance and in

size. It has a very thick, corrosiveatmosphere, and the surface temperature isextremely high.Radar maps of Venus show that it hasmountainous areas, as well as volcanoessurrounded by plains of lava.Venus rotates slowly in a direction oppositeto the direction of its orbit.Earth is the third planet from the Sun. It is theonly planet with large amounts of liquidwater, and the only planet known to supportlife. Earth has a large moon, the only largemoon around a terrestrial planet.Mars is the fourth planet from the Sun. It hastwo small moons. Mars is reddish in colorbecause of oxidized iron (rust) in its soil.Mars has the largest mountain and the largestcanyon in the solar system.There is a lot of water ice in the polar icecaps and under the surface of Mars.

Review Questions

1. Name the inner planets in order from the Sunoutward. Then name them from smallest tolargest.

2. Why do the temperatures on Mercury varywidely?

3. Venus is farther from the Sun than Mercury.Why does Venus have higher temperaturesthan Mercury?

4. How are maps of Venus made?5. Name two major ways in which Earth is

unlike any other planets.6. Why does Mars appear to be red?7. Suppose you are planning a mission to Mars.

Identify two places where you might be ableto get water on the planet.

Further Reading / SupplementalLinks

http://www.nasa.gov/worldbook/venus_worldbook.htmlhttp://solarsystem.nasa.gov/planetselector.cfm?Object=Mercury

http://solarsystem.jpl.nasa.gov/planets/profile.cfm?Object=Mercury&#38;amp;Display=Kidshttp://mars.jpl.nasa.gov/extreme/http://www.google.com/mars/http://www.youtube.com/watch?v=U8-DTJpygykhttp://www.youtube.com/watch?v=U8-DTJpygykhttp://www.youtube.com/watch?v=HqFVxWfVtoo&#38;feature=relatedhttp://www.youtube.com/watch?v=M-KfYEQUg2s

Vocabulary

day

The time it takes a planet to rotate once on itsaxis.

inner planets

The four planets inside the asteroid belt of our

solar system; Mercury, Venus, Earth and Mars.

terrestrial planets

The solid, dense, rocky planets that are likeEarth.

year

The time it takes for a planet to orbit the Sun.

Points to Consider

We are planning to send humans to Marssometime in the next few decades. What doyou think it would be like to be on Mars?Why do you think we are going to Marsinstead of Mercury or Venus?Why do you think the four inner planets arealso called terrestrial planets? What might aplanet be like if it weren’t a terrestrialplanet?

Outer Planets

Lesson Objectives

Describe key features of the outer planets andtheir moons.Compare the outer planets to each other andto Earth.

Introduction

The four planets farthest from the sun—Jupiter,Saturn, Uranus, and Neptune—are called the outerplanets of our solar system. Figure below showsthe relative sizes of the outer planets and the Sun.Because they are much larger than Earth and theother inner planets, and because they are madeprimarily of gases and liquids rather than solidmatter, the outer planets are also called gas giants.

Figure 25.19 This image shows the four outer planets and the

Sun, with sizes to scale. From left to right, theouter planets are Jupiter, Saturn, Uranus, andNeptune.

The gas giants are made up primarily ofhydrogen and helium, the same elements that makeup most of the Sun. Astronomers believe thathydrogen and helium gases were found in largeamounts throughout the solar system when it firstformed. However, the inner planets didn’t haveenough mass to hold on to these very light gases.As a result, the hydrogen and helium initially onthese inner planets floated away into space. Onlythe Sun and the massive outer planets had enoughgravity to keep hydrogen and helium from driftingaway.

All of the outer planets have numerous moons.All of the outer planets also have planetary rings,which are rings of dust and other small particlesencircling a planet in a thin plane. Only the rings ofSaturn can be easily seen from Earth.

Jupiter

Figure 25.20 This image of Jupiter was taken by Voyager 2

in 1979. The colors were later enhanced to bring

out more details.Jupiter, shown in Figure above, is the largest

planet in our solar system, and the largest object inthe solar system besides the Sun. Jupiter is namedfor the king of the gods in Roman mythology.Jupiter is truly a giant! It is much less dense thanEarth—it has 318 times the mass of Earth, but over1,300 times Earth’s volume. Because Jupiter is solarge, it reflects a lot of sunlight. When it isvisible, it is the brightest object in the night skybesides the Moon and Venus. This brightness is allthe more impressive, since Jupiter is quite far fromthe Earth — 5.20 AUs away. It takes Jupiter about12 Earth years to orbit once around the Sun.

A Ball of Gas and Liquid

If a spaceship were to try to land on the surfaceof Jupiter, the astronauts would find that there is nosolid surface at all! Jupiter is made mostly ofhydrogen, with some helium, and small amounts ofother elements. The outer layers of the planet aregas. Deeper within the planet, pressure compresses

the gases into a liquid. Some evidence suggeststhat Jupiter may have a small rocky core at itscenter.

A Stormy Atmosphere

The upper layer of Jupiter’s atmospherecontains clouds of ammonia (NH3) in bands ofdifferent colors. These bands rotate around theplanet, but also swirl around in turbulent storms.The Great Red Spot, shown in Figure below, isan enormous, oval-shaped storm found south ofJupiter’s equator. It is more than three times aswide as the entire Earth! Clouds in the storm rotatein a counterclockwise direction, making onecomplete turn every six days or so. The Great RedSpot has been on Jupiter for at least 300 years. It ispossible, but not certain, that this storm is apermanent feature on Jupiter.

Figure 25.21 This image of Jupiters Great Red Spot (upper

right of image) was taken by the Voyager 1spacecraft. The white storm just below the GreatRed Spot is about the same diameter as Earth.

Jupiter’s Moons and Rings

Jupiter has a very large number of moons. Asof 2008, we have discovered over 60 naturalsatellites of Jupiter. Of these, four are big enoughand bright enough to be seen from Earth, using nomore than a pair of binoculars. These four moons

—named Io, Europa, Ganymede, and Callisto—were first discovered by Galileo in 1610, so theyare sometimes referred to as the Galilean moons.

Figure below shows the four Galilean moonsand their sizes relative to the Great Red Spot. TheGalilean moons are larger than the dwarf planetsPluto, Ceres, and Eris. In fact, Ganymede, which isthe biggest moon in the solar system, is even largerthan the planet Mercury!

Figure 25.22 This composite image shows the the four

Galilean moons and Jupiter. From top to bottom,the moons are Io, Europa, Ganymede and Callisto.Jupiters Great Red Spot is in the background. Sizesare to scale.

Scientists are particularly interested in Europa,

the smallest of the Galilean moons, because it maybe a likely place to find extraterrestrial life. Thesurface of Europa is a smooth layer of ice.Evidence suggests that there is an ocean of liquidwater under the ice. Europa also has a continualsource of energy—it is heated as it is stretched andsquashed by tidal forces from Jupiter. Because ithas liquid water and a continual heat source,astrobiologists surmise that life might have formedon Europa much as it did on Earth. Numerousmissions have been planned to explore Europa,including plans to drill through the ice and send aprobe into the ocean. However, no such missionhas yet been attempted.

In 1979, two spacecrafts—Voyager 1 andVoyager 2—visited Jupiter and its moons. Photosfrom the Voyager missions showed that Jupiter hasa ring system. This ring system is very faint, so it isvery difficult to observe from Earth.

Saturn

Saturn, shown in Figure below, is famous forits beautiful rings. Saturn’s mass is about 95 timesthe mass of Earth, and its volume is 755 timesEarth’s volume, making it the second largest planetin the solar system. Despite its large size, Saturn isthe least dense planet in our solar system. It is lessdense than water, which means if there could be abathtub big enough, Saturn would float. In Romanmythology, Saturn was the father of Jupiter. So, itis an appropriate name for the next planet beyondJupiter. Saturn orbits the Sun once about every 30Earth years.

Figure 25.23 This image of Saturn and its rings is a

composite of pictures taken by the Cassini orbiterin 2004.

Saturn’s composition is similar to Jupiter. It is

made mostly of hydrogen and helium, which aregases in the outer layers and liquids at deeperlayers. It may also have a small solid core. Theupper atmosphere has clouds in bands of differentcolors. These rotate rapidly around the planet, butthere seems to be less turbulence and fewer stormson Saturn than on Jupiter. One interestingphenomena that has been observed in the storms onSaturn is the presence of thunder and lightning (seevideo, below).

Cassini scientists waited years for the rightconditions to produce the first movie that showslightning on another planet -- Saturn.(Watch onYoutube)

A Weird Hexagon

There is a strange feature at Saturn’s north pole—the clouds form a hexagonal pattern, as shown inthe infrared image in Figure below. This hexagonwas viewed by Voyager 1 in the 1980s, and againby the Cassini Orbiter in 2006, so it seems to be along-lasting feature. Though astronomers havehypothesized and speculated about what causesthese hexagonal cloud, no one has yet come upwith a convincing explanation.

Figure 25.24 This infrared image taken of Saturns north pole

shows that the clouds are in a hexagon (six-sided)shape.

Saturn’s Rings

The rings of Saturn were first observed byGalileo in 1610. However, he could not see themclearly enough to realize they were rings; hethought they might be two large moons, one oneither side of Saturn. In 1659, the Dutchastronomer Christiaan Huygens was the first to

realize that the rings were in fact rings. The ringscircle Saturn’s equator. They appear tilted becauseSaturn itself is tilted about 27 degrees to the side.The rings do not touch the planet.

The Voyager 1 spacecraft visited Saturn in1980, followed by Voyager 2 in 1981. TheVoyager probes sent back detailed pictures ofSaturn, its rings, and some of its moons. From theVoyager data, we learned that Saturn’s rings aremade of particles of water and ice, with a little bitof dust as well. There are several gaps in the rings.Some of the gaps have been cleared out by moonsthat are within the rings. Scientists believe themoons’ gravity caused ring dust and gas to falltowards the moon, leaving a gap in the rings. Othergaps in the rings are caused by the competinggravitational forces of Saturn and of moons outsidethe rings.

Saturn’s Moons

As of 2008, over 60 moons have beenidentified around Saturn. Most of them are very

small. Some are even found within the rings. In asense, all the particles in the rings are like littlemoons, too, because they orbit around Saturn. Onlyseven of Saturn’s moons are large enough forgravity to have made them spherical, and all butone are smaller than Earth’s moon.

Saturn’s largest moon, Titan, is about one and ahalf times the size of Earth’s Moon and is alsolarger than the planet Mercury. Figure belowcompares the size of Titan to Earth. Scientists arevery interested in Titan because it has anatmosphere that is similar to what Earth’satmosphere might have been like before lifedeveloped on Earth. Titan may have a layer ofliquid water under a layer of ice on the surface.Scientists now believe there are also lakes on thesurface of Titan, but these lakes contain liquidmethane (CH4) and ethane (C2H6) instead of water!Methane and ethane are compounds found innatural gas, a mixture of gases found naturally onEarth and often used as fuel.

Figure 25.25 This composite image compares Saturns

largest moon, Titan (right) to Earth (left). Titan hasan atmosphere similar to what Earths might havebeen like before life formed on Earth.

Uranus

Figure 25.26 This image of Uranus was taken by Voyager 2

in 1986.Uranus, shown in Figure above, is named for

the Greek god of the sky. In Greek mythology,Uranus was the father of Cronos, the Greekequivalent of the Roman god Saturn. By the way,astronomers pronounce the name “YOOR-uh-nuhs.”

Uranus was not known to ancient observers. Itwas first discovered by the astronomer WilliamHerschel in 1781. Uranus can be seen from Earth

with the unaided eye, but it was overlooked forcenturies because it is very faint. Uranus is faintbecause it is very far away, not because it is small.It is about 2.8 billion kilometers (1.8 billion miles)from the Sun. Light from the Sun takes about 2hours and 40 minutes to reach Uranus. Uranusorbits the Sun once about every 84 Earth years.

An Icy Blue-Green Ball

Like Jupiter and Saturn, Uranus is composedmainly of hydrogen and helium. It has a thick layerof gas on the outside, then liquid further on theinside. However, Uranus has a higher percentageof icy materials, such as water, ammonia (NH3),and methane (CH4), than Jupiter and Saturn do.When sunlight reflects off Uranus, clouds ofmethane filter out red light, giving the planet ablue-green color. There are bands of clouds in theatmosphere of Uranus, but they are hard to see innormal light, so the planet looks like a plain blueball.

Uranus is the lightest of the outer planets, witha mass about 14 times the mass of Earth. Eventhough it has much more mass than Earth, it is muchless dense than Earth. At the “surface” of Uranus,the gravity is actually weaker than on Earth’ssurface. If you were at the top of the clouds onUranus, you would weigh about 10% less thanwhat you weigh on Earth.

The Sideways Planet

Most of the planets in the solar system rotateon their axes in the same direction that they movearound the Sun. Uranus, though, is tilted on its sideso its axis is almost parallel to its orbit. In otherwords, it rotates like a top that was turned so that itwas spinning parallel to the floor. Scientists thinkthat Uranus was probably knocked over by acollision with another planet-sized object billionsof years ago.

Rings and Moons of Uranus

Uranus has a faint system of rings, as shown inFigure below. The rings circle the planet’sequator, but because Uranus is tilted on its side,the rings are almost perpendicular to the planet’sorbit.

Figure 25.27 This image from the Hubble Space Telescope

shows the faint rings of Uranus. The planet is tiltedon its side, so the rings are nearly vertical.

Uranus has 27 moons that we know of. All buta few of them are named for characters from theplays of William Shakespeare. The five biggest

moons of Uranus—Miranda, Ariel, Umbriel,Titania and Oberon—are shown in Figure below.

Figure 25.28 These Voyager 2 photos have been resized to

show the relative sizes of the five main moons ofUranus.

Neptune

Neptune, shown in Figure below, is the eighthplanet from the Sun. It is the only major planet thatcan’t be seen from Earth without a telescope.Scientists predicted the existence of Neptunebefore it was actually discovered. They noticedthat Uranus did not always appear exactly where itshould appear. They knew there must be anotherplanet beyond Uranus whose gravity was affectingUranus’ orbit. This planet was discovered in 1846,in the position that had been predicted, and it was

named Neptune for the Roman god of the sea due toits blush color.

Figure 25.29 This image of Neptune was taken by Voyager 2

in 1989. The Great Dark Spot seen on the leftcenter in the picture has since disappeared, but asimilar dark spot has appeared on another part ofthe planet.

Neptune has slightly more mass than Uranus,but it is slightly smaller in size. In many respects, itis similar to Uranus. Uranus and Neptune are oftenconsidered “sister planets.” Neptune, which is

nearly 4.5 billion kilometers (2.8 billion miles)from the Sun, is much farther from the Sun thaneven distant Uranus. It moves very slowly in itsorbit, taking 165 Earth years to complete one orbitaround the Sun.

Extremes of Cold and Wind

Neptune is blue in color, with a few darker andlighter spots. The blue color is caused byatmospheric gases, including methane (CH4). WhenVoyager 2 visited Neptune in 1986, there was alarge dark-blue spot south of the equator. This spotwas called the Great Dark Spot. However, whenthe Hubble Space Telescope took pictures ofNeptune in 1994, the Great Dark Spot haddisappeared. Instead, another dark spot hadappeared north of the equator. Astronomersbelieve both of these spots represent gaps in themethane clouds on Neptune.

The changing appearance of Neptune is due toits turbulent atmosphere. The winds on Neptune

are stronger than on any other planet in the solarsystem, reaching speeds of 1,100 km/h (700 mi/h),close to the speed of sound. This extreme weathersurprised astronomers, since the planet receiveslittle energy from the Sun to power weathersystems. Neptune is also one of the coldest placesin the solar system. Temperatures at the top of theclouds are about –218oC (–360oF).

Neptune’s Rings and Moons

Like the other outer planets, Neptune has ringsof ice and dust. These rings are much thinner andfainter than those of Saturn. Some evidencesuggests that the rings of Neptune may be unstable,and may change or disappear in a relatively shorttime.

Neptune has 13 known moons. Triton, shown inFigure below, is the only one of them that hasenough mass to be spherical in shape. Triton orbitsin the direction opposite to the orbit of Neptune.Scientists think Triton did not form around

Neptune, but instead was captured by Neptune’sgravity as it passed by.

Figure 25.30 This image Triton, Neptunes largest moon, was

taken by Voyager 2 in 1989.

Pluto

Pluto was once considered one of the outerplanets, but when the definition of a planet waschanged in 2006, Pluto became one of the leaders

of the dwarf planets. It is one of the largest andbrightest objects that make up this group. Look forPluto in the next section in the discussion of dwarfplanets.

Lesson Summary

The four outer planets—Jupiter, Saturn,Uranus, and Neptune—are all gas giants madeprimarily of hydrogen and helium. They havethick gaseous outer layers and liquidinteriors.All of the outer planets have numerous moons,as well as planetary rings made of dust andother particles.Jupiter is by far the largest planet in the solarsystem. It has bands of different coloredclouds, and a long-lasting storm called theGreat Red Spot.Jupiter has over 60 moons. The four biggestwere discovered by Galileo, and are calledthe Galilean moons.

One of the Galilean moons, Europa, may havean ocean of liquid water under a layer of ice.The conditions in this ocean might be right forlife to have developed.Saturn is smaller than Jupiter, but similar incomposition and structure.Saturn has a large system of beautiful rings.Saturn’s largest moon, Titan, has anatmosphere similar to Earth’s atmospherebefore life formed.Uranus and Neptune were discovered inmodern times. They are similar to each otherin size and composition. They are bothsmaller than Jupiter and Saturn, and also havemore icy materials.Uranus is tilted on its side, probably due to acollision with a large object in the past.Neptune is very cold and has very strongwinds. It had a large dark spot thatdisappeared, then another dark spot appearedon another part of the planet. These dark spotsare storms in Neptune’s atmosphere.Pluto is no longer considered one of the outer

planets. It is now considered a dwarf planet.

Review Questions

1. Name the outer planets a) in order from theSun outward, b) from largest to smallest bymass, and c) from largest to smallest by size.

2. Why are the outer planets called gas giants?3. How do the Great Red Spot and Great Dark

Spot differ?4. Name the Galilean moons, and explain why

they are called that.5. Why might Europa be a likely place to find

extraterrestrial life?6. What causes gaps in Saturn’s rings?7. Why are scientists interested in the

atmosphere of Saturn’s moon Titan?8. What liquid is found on the surface of Titan?9. Why is Uranus blue-green in color?

10. What is the name of Neptune’s largest moon?

Further Reading / Supplemental

Links

http://www.nasa.gov/worldbook/jupiter_worldbook.htmlhttp://solarsystem.nasa.gov/planetselector.cfm?Object=Jupiterhttp://www.youtube.com/watch?v=5iVw72sX3Bghttp://www.youtube.com/watch?v=iLXeUVCNoX8http://www.youtube.com/watch?v=29wfzotaBIghttp://www.youtube.com/watch?v=FqX2YdnwtRc

Vocabulary

Galilean moonsThe four largest moons of Jupiter discoveredby Galileo.

gas giantsThe four large outer planets composed of the

gases hydrogen and helium.

Great Red SpotAn enormous, oval shaped storm on Jupiter.

outer planetsThe four large planets beyond the asteroidbelt in our solar system.

planetary ringsRings of dust and rock encircling a planet in athin plane.

Points to Consider

The inner planets are small and rocky, whilethe outer planets are large and gaseous. Whymight the planets have formed into two groupslike they are?We have discussed the Sun, the planets, andthe moons of the planets. What other objectscan you think of that can be found in our solarsystem?

Other Objects in the SolarSystem

Lesson Objectives

Locate and describe the asteroid belt.Explain where comets come from and whatcauses their tails.Differentiate between meteors, meteoroids,and meteorites.

Introduction

When our solar system formed, most of thematter ended up in the Sun, the star at the center ofthe system. Material spinning in a disk around theSun clumped together into larger and larger pieces,forming the eight planets and their moons. Butsome of the smaller pieces of matter in the solarsystem never joined one of these larger bodies. In

this lesson, we will talk about some of these otherobjects in the solar system.

Asteroids

Asteroids are very small, rocky bodies thatorbit the Sun. “Asteroid” means “star-like,” and ina telescope, asteroids look like points of light, justlike stars. Asteroids are also sometimes calledplanetoids or minor planets, because in someways they are similar to miniature planets. Unlikeplanets, though, asteroids are irregularly shapedbecause they do not have enough gravity to becomeround like planets. They do not have atmospheres,and they are not geologically active. The onlygeological changes to an asteroid are due tocollisions, which may break up the asteroid orcreate craters on the asteroid’s surface. Figurebelow shows a typical asteroid.

Figure 25.31 Asteroid 951 Gaspra was the first asteroid

photographed at close range. This picture wastaken in 1991 by the Galileo probe on its way toJupiter. 951 Gaspra is a medium-sized asteroid,measuring about 19 by 12 by 11 kilometers (12 by7.5 by 7 miles).

The Asteroid Belt

Hundreds of thousands of asteroids have beendiscovered in our solar system. They are stillbeing discovered at a rate of about 5,000 new

asteroids per month! The majority of the asteroidsare found in between the orbits of Mars andJupiter, in a region called the asteroid belt, asshown in Figure below. Although there are manythousands of asteroids in the asteroid belt, theirtotal mass adds up to only about 4% of Earth’smoon.

Figure 25.32 The asteroid belt is a ring of many asteroids

between the orbits of Mars and Jupiter. The whitedots in the figure are asteroids in the main asteroidbelt. Other groups of asteroids closer to Jupiter are

called the Hildas (orange), the Trojans (green),and the Greeks (also green).

Scientists believe that the bodies in theasteroid belt formed there during the formation ofthe solar system. However, they have never beenable to form into a single planet because thegravity of Jupiter, which is very massive,continually disrupts the asteroids.

Near-Earth Asteroids

Near-Earth asteroids are asteroids whoseorbits cross Earth’s orbit. Any object whose orbitcrosses Earth can collide with Earth. There areover 4,500 known near-Earth asteroids; between500 and 1,000 of these are over 1 kilometer indiameter. Small asteroids do in fact collide withEarth on a regular basis—asteroids 5–10 m indiameter hit Earth on average about once per year.Evidence suggests that large asteroids hitting Earthin the past have caused many organisms to die andmany species to go extinct. Astronomers arealways on the lookout for new asteroids, and

follow the known near-Earth asteroids closely, sothey can predict a possible collision as early aspossible.

Asteroid Missions

Scientists are interested in asteroids in partbecause knowing more about what they are madeof can tell us about our solar system and how itmight have formed. They may also eventually bemined for rare minerals or for constructionprojects in space. Some asteroids have beenphotographed as spacecraft have flown by on theirway to the outer planets. A few missions have beensent out to study asteroids directly. In 1997, theNEAR Shoemaker probe went into orbit around anasteroid called 433 Eros, and finally landed on itssurface in 2001. The Japanese Hayabusa probe iscurrently studying an asteroid and may returnsamples of its surface to Earth. In 2007, NASAlaunched the Dawn mission, which is scheduled tovisit some of the largest asteroids in 2011-2015.

Meteors

If you have spent much time looking at the skyon a dark night, you have probably seen a 'shootingstar', like in Figure below. A shooting star is astreak of light across the sky. The proper scientificname for a shooting star is a meteor. Meteors arenot stars at all. Rather, they are small pieces ofmatter burning up as they enter Earth’s atmospherefrom space.

Figure 25.33 This photo captures a meteor - also called a

'shooting star,' streaking across the sky to the right

of the Milky Way.

Meteoroids

Before these small pieces of matter enterEarth’s atmosphere, they are called meteoroids.Meteoroids are like asteroids, only smaller.Meteoroids range from the size of boulders downto the size of tiny sand grains. Objects larger thanmeteoroids are considered asteroids, and objectssmaller than meteoroids are consideredinterplanetary dust. Meteoroids are sometimesfound clustered together in long trails. These areremnants left behind by comets. When Earth passesthrough one of these clusters, there is a meteorshower, an increase in the number of brightmeteors in a particular region of the sky for aperiod of time.

Meteorites

Suppose a small rocky object—a meteoroid—

enters Earth’s atmosphere. Friction in theatmosphere heats the object quickly so it starts tovaporize, leaving a trail of glowing gases. At thispoint, it has become a meteor. Most meteoroidsvaporize completely before they ever reach Earth’ssurface, but larger meteoroids may have a smallcore of material that travels all the way through theatmosphere and hits the Earth’s surface. The solidremains of a meteoroid found on Earth’s surface iscalled a meteorite.

Meteorites provide clues about our solarsystem. Many meteorites come from meteoroidsthat formed when the solar system formed (Figurebelow). Some are from the insides of asteroids thathave split apart. A few meteorites are made ofmaterials more like the rocks on Mars. Scientistsbelieve the material in these meteorites wasactually knocked off the surface of Mars by anasteroid impact, and then entered Earth’satmosphere as a meteor.

Figure 25.34 A lunar-meteorite

Comets

Comets are small, icy objects that orbit theSun in very elliptical orbits. Their orbits carrythem from the outer solar system to the inner solarsystem, close to the Sun. When a comet gets closeto the Sun, the outer layers of ice melt andevaporate. The gas and dust released in this wayforms an atmosphere—called a coma—around the

comet. Radiation and particles streaming from theSun also push some of this gas and dust into a longtail, which always points away from the Sun nomatter which way the comet is moving. Figurebelow shows Comet Hale-Bopp, which shonebrightly for several months in 1997.

Figure 25.35 Comet Hale-Bopp, also called the Great Comet

of 1997, shone brightly for several months in 1997.The comet has two visible tails: a bright, curveddust tail and a fainter, straight tail of ions (chargedatoms) pointing directly away from the Sun.

Gases in the coma and tail of a comet glow,and also reflect light from the Sun. Comets arevery hard to see except when they have their comasand tails. For this reason, comets appear for only ashort time when they are near the Sun, then seem todisappear again as they move back to the outersolar system. The time between one appearance ofa comet and the next is called the comet’s period.For example, the first comet whose period wasknown, Halley’s comet, has a period of 75 years. Itlast traveled through the inner solar system in1986, and will appear again in 2061.

Where Comets Come From

Comets that have periods of about 200 years orless are considered short period comets. Mostshort-period comets come from a region beyondthe orbit of Neptune. This area, which contains not

only comets but also asteroids and at least twodwarf planets, is called the Kuiper belt. (Kuiper ispronounced “KI-per,” rhyming with “viper.”)

Some comets have much longer periods, aslong as thousands or even millions of years. Mostlong-period comets come from a very distantregion of the solar system called the Oort cloud,which is about 50,000–100,000 AU from the Sun(50,000–100,000 times the distance from the Sunto Earth). Comets carry materials from the outersolar system to the inner solar system. Comets mayhave brought water and other substances to Earthduring collisions early in Earth’s history.

Dwarf Planets

The dwarf planets of our solar system areexciting proof of how much we are learning aboutour solar system. With the discovery of many newobjects in our solar system, in 2006, astronomersrefined the definition of a planet. According to thenew definition, a planet must:

1. orbit a star2. be big enough that its own gravity causes it to

be shaped like a sphere3. be small enough that it isn’t a star itself4. have cleared the area of its orbit of smaller

objects

At the same time, astronomers defined a newtype of object: dwarf planets. A dwarf planet is anobject that meets numbers 1–3 above, but notnumber 4. There are four dwarf planets in oursolar system: Ceres, Pluto, Makemake and Eris.

Figure below shows Ceres, a rocky, sphericalbody that is by far the largest object in the asteroidbelt. Before 2006, Ceres was considered thelargest of the asteroids. Ceres has enough mass thatits gravity causes it to be shaped like a sphere.Still, Ceres only has about 1.3% of the mass of theEarth’s Moon. Ceres orbits the Sun, is round and isnot a star but the area of its orbit is full of othersmaller bodies, so Ceres fails the fourth criterionfor being a planet, and is now considered a dwarfplanet.

Figure 25.36 This composite image compares the size of the

dwarf planet Ceres to Earth and the Moon.

Pluto

From the time it was discovered in 1930 until2006, Pluto was considered the ninth planet of thesolar system. However, it was always thought ofas an oddball planet. Unlike the other outer planetsin the solar system, which are all gas giants, it issmall, icy and rocky. Its diameter is about 2400kilometers. It is only about 1/5 the mass of Earth’sMoon. Its orbit is tilted relative to the other planetsand is shaped like a long, narrow ellipse,

sometimes even passing inside the orbit ofNeptune.

Starting in 1992, many objects have beendiscovered in the same area as Pluto’s orbit, anarea now known as the Kuiper belt. The Kuiperbelt begins outside the orbit of Neptune andcontinues out at least 500 AU. We have discoveredmore than 200 million Kuiper belt objects. Plutoorbits within this region. When the definition of aplanet was changed in 2006, Pluto failed the test ofclearing out its orbit of other bodies, so it is nowconsidered a dwarf planet.

Pluto has 3 moons of its own. The largest,Charon, is big enough that the Pluto-Charon systemis sometimes considered to be a double dwarfplanet (Figure below). Two smaller moons, Nixand Hydra, were discovered in 2005.

Makemake

Makemake is the third largest and secondbrightest dwarf planet we have discovered so far

(Figure below). It is about three quarters the sizeof Pluto. Its diameter is estimated to be between1300 and 1900 kilometers. Makemake is namedafter the deity that created humanity in themythology of the people of Easter Island. It orbitsthe Sun in 310 years at a distance between 38.5 to53 AU. It is believed to be made of methane,ethane and nitrogen ices.

Figure 25.37 Largest Known Trans-Neptunian Objects.

Eris

Eris is the largest known dwarf planet in the

solar system — about 27% more massive thanPluto (Figure above). It was not discovered until2003 because it is extremely far away from theSun. Although Pluto, Makemake and Eris are in theKuiper belt, Eris is about 3 times farther from theSun than Pluto is, and almost 100 times fartherfrom the Sun than Earth is. When it was firstdiscovered, it was considered for a short time tobe the “tenth planet” in the solar system. Thediscovery of Eris helped prompt the new definitionof planets and dwarf planets in 2006. Eris also hasa small moon, Dysnomia that orbits it once aboutevery 16 days.

Astronomers already know there may be otherdwarf planets in the outer reaches of the solarsystem. Look for Haumea, Quaoar, Varuna andOrcus to be possibly added to the list of dwarfplanets in the future. We still have a lot to discoverand explore!

Lesson Summary

Asteroids are irregularly-shaped, rockybodies that orbit the Sun. Most of them arefound in the asteroid belt, between the orbitsof Mars and Jupiter.Meteoroids are smaller than asteroids,ranging from the size of boulders to the size ofsand grains. When meteoroids enter Earth’satmosphere, they vaporize, creating a trail ofglowing gas called a meteor. If any of themeteoroid reaches Earth, the remaining objectis called a meteorite.Comets are small, icy objects that orbit theSun in very elliptical orbits. When they areclose to the Sun, they form comas and tails,which glow and make the comet more visible.Short-period comets come from the Kuiperbelt, beyond Neptune. Long-period cometscome from the very distant Oort cloud.Dwarf planets are spherical bodies that orbitthe Sun, but that have not cleared their orbit ofsmaller bodies. Ceres is a dwarf planet in theasteroid belt. Pluto, Makemake and Eris aredwarf planets in the Kuiper belt.

Review Questions

1. Arrange the following from smallest tolargest: asteroid, star, meteoroid, planet,dwarf planet.

2. Where are most asteroids found?3. What is the difference between a meteor, a

meteoroid, and a meteorite?4. What kind of objects would scientists study to

learn about the composition of the Oortcloud?

5. Why is Pluto no longer considered a planet?6. Name the four known dwarf planets in our

solar system.

Further Reading / SupplementalLinks

http://www.nasa.gov/worldbook/asteroid_worldbook.htmlhttp://www.iau.org/iau0602.423.0.htmlhttp://en.wikipedia.org/

Vocabulary

asteroid

Rocky objects larger than a few hundredmeters that orbit the Sun in the region known as theasteroid belt.

asteroid belt

Region between the orbits of Mars and Jupiterwhere many asteroids are found.

comet

Asmall, icy, dusty object in orbit around theSun.

dwarf planet

Around celestial object orbiting the Sun thathas not cleared its orbit of other objects.

Kuiper belt

Aregion of space around the Sun beyond theorbit of Neptune that contains millions of frozenobjects.

meteor

Material from outer space that burns up as itenters Earth's atmosphere.

meteorite

The solid portion of a meteor that hits Earth'ssurface.

meteoroid

Asmall rock in interplanetary space that has notyet entered Earth's atmosphere.

meteor shower

An area of frequent meteors appearing to

originate in a particular part of the sky.

Points to Consider

In 2006, astronomers changed the definitionof a planet and created a new category ofdwarf planets. Do you think planets, dwarfplanets, moons, asteroids, and meteoroids areclearly separate groups?What defines each of these groups, and whatdo objects in these different groups have incommon? Could an object change from beingin one group to another? How?We have learned about many different kindsof objects that are found within our solarsystem. What objects or systems of objectscan you think of that are found outside oursolar system?

Chapter 26: Stars, Galaxies,and the Universe

Stars

Lesson Objectives

Define constellation.Describe the flow of energy in a star.Classify stars based on their properties.Outline the life cycle of a star.Use light-years as a unit of distance.

Introduction

When you look at the sky on a clear night, youcan see dozens, perhaps even hundreds, of tinypoints of light. Almost every one of these points oflight is a star, a giant ball of glowing gas at a very,

very high temperature. Some of these stars aresmaller than our Sun, and some are larger. Exceptfor our own Sun, all stars are so far away that theyonly look like single points, even through atelescope.

Constellations

For centuries, people have seen the same starsyou can see in the night sky. People of manydifferent cultures have identified constellations,which are apparent patterns of stars in the sky.Figure below shows one of the most easilyrecognized constellations. The ancient Greeksthought this group of stars looked like a hunterfrom one of their myths, so they named it Orionafter him. The line of three stars at the center of thepicture is "Orion's Belt".

Figure 26.1 The constellation Orion is a familiar pattern of

stars in the sky.The patterns in constellations and in groups or

clusters of stars, called asterisms, stay the samenight after night. However, in a single night, thestars move across the sky, keeping the same

patterns. This apparent nightly motion of the starsis actually due to the rotation of Earth on its axis. Itisn't the stars that are moving; it is actually Earthspinning that makes the stars seem to move. Thepatterns shift slightly with the seasons, too, asEarth revolves around the Sun. As a result, you cansee different constellations in the winter than in thesummer. For example, Orion is a prominentconstellation in the winter sky, but not in thesummer sky.

Apparent Versus Real Distances

Although the stars in a constellation appearclose together as we see them in our night sky, theyare usually at very different distances from us, andtherefore they are not at all close together out inspace. For example, in the constellation Orion, thestars visible to the naked eye are at distancesranging from just 26 light-years (which isrelatively close to Earth) to several thousand light-years away. A light-year is the distance that lightcan travel in one year; it is a large unit of distance

used to measure the distance between objects inspace.

Energy of Stars

Only a small portion of the light from the Sunreaches Earth; yet that light is enough to keep theentire planet warm and to provide energy for allthe living things on Earth. The Sun is a fairlyaverage star. The reason the Sun appears so muchbigger and brighter than any of the other stars isthat it is very close to us. Some other stars producemuch more energy than the Sun. How do starsgenerate so much energy?

Nuclear Fusion

Stars are made mostly of hydrogen and helium.These are both very lightweight gases. However,there is so much hydrogen and helium in a star thatthe weight of these gases is enormous. In the centerof a star, the pressure is great enough to heat the

gases and cause nuclear fusion reactions. In anuclear fusion reaction, the nuclei, or centers oftwo atoms join together and create a new atomfrom two original atoms. In the core of a star, themost common reaction turns two hydrogen atomsinto a helium atom. Nuclear fusion reactionsrequire a lot of energy to get started, but once theyare started, they produce even more energy.

The energy from nuclear reactions in the corepushes outward, balancing the inward pull ofgravity on all the gas in the star. This energyslowly moves outward through the layers of thestar until it finally reaches the outer surface of thestar. The outer layer of the star glows brightly,sending the energy out into space aselectromagnetic radiation, including visible light,heat, ultraviolet light, and radio waves.

Scientists have built machines calledaccelerators that can propel subatomic particlesuntil they have attained almost the same amount ofenergy as found in the core of a star. When theseparticles collide with each other head-on, newparticles are created. This process simulates the

nuclear fusion that takes place in the cores of stars.It also simulates the conditions that allowed for thefirst Helium atom to be produced from thecollision of two hydrogen atoms when theUniverse was only a few minutes old. Two well-known accelerators are SLAC in California, USAand CERN in Switzerland.

How Stars Are Classified

Stars come in many different colors. If youlook at the stars in Orion (as shown in Figureabove), you will notice that there is a bright, redstar in the upper left and a bright, and a blue star inthe lower right. The red star is named Betelgeuse(pronounced BET-ul-juice), and the blue star isnamed Rigel.

Color and Temperature

If you watch a piece of metal, such as a coil ofan electric stove as it heats up, you can see how

color is related to temperature. When you first turnon the heat, the coil looks black, but you can feelthe heat with your hand held several inches fromthe coil. As the coil gets hotter, it starts to glow adull red. As it gets hotter still, it becomes abrighter red, then orange. If it gets extremely hot, itmight look yellow-white, or even blue-white. Likea coil on a stove, a star’s color is determined bythe temperature of the star’s surface. Relativelycool stars are red, warmer stars are orange oryellow, and extremely hot stars are blue or blue-white.

Classifying Stars by Color

The most common way of classifying stars isby color. Table below shows how thisclassification system works. The class of a star isgiven by a letter. Each letter corresponds to acolor, and also to a range of temperatures. Notethat these letters don’t match the color names; theyare left over from an older system that is no longerused.

Classification of Stars By Color and Temperature

Class Color Temperaturerange Sample Star

O Blue 30,000 K ormore Zeta Ophiuchi

B Blue-white 10,000–30,000 K Rigel

A White 7,500–10,000 K Altair

F Yellowish-white

6,000–7,500K Procyon A

G Yellow 5,500–6,000K Sun

K Orange 3,500–5,000K Epsilon Indi

M Red 2,000–3,500K

Betelgeuse,Proxima Centauri

(Sources:http://en.wikipedia.org/wiki/Stellar_classification;http://en.wikipedia.org/wiki/Star, License: GNU-

FDL)For most stars, surface temperature is also

related to size. Bigger stars produce more energy,so their surfaces are hotter. Figure below shows atypical star of each class, with the colors about thesame as you would see in the sky.

Figure 26.2 Typical stars by class, color and size. For most

stars, size is related to class and to color. Thisimage shows a typical star of each class. Thecolors are approximately the same as you wouldsee in the sky.

Lifetime of Stars

As a way of describing the stages in a star'sdevelopment, we could say that stars are born,grow, change over time, and eventually die. Most

stars change in size, color, and class at least onceduring this journey.

Formation of Stars

Stars are born in clouds of gas and dust callednebulas, like the one shown in Figure below. InFigure above, the fuzzy area beneath the centralthree stars across the constellation Orion, oftencalled Orion’s sword, contains another nebulacalled the Orion nebula.

Star formation starts when gravity starts to pullgas and dust in the nebula together. As the gas anddust falls together, it forms into one or morespheres. As one of these spheres collapses further,the pressure inside increases. As the pressureincreases, the temperature of the gas alsoincreases. Eventually, the pressure and temperaturebecome great enough to cause nuclear fusion tostart in the center. At this point, the ball of gas hasbecome a star.

Figure 26.3 The Eagle Nebula and the Pillars of Creation.

The pillars of gas and dust shown here are in theEagle Nebula.

The Main Sequence

For most of a star’s life, the nuclear fusion inthe core combines hydrogen atoms to form heliumatoms. A star in this stage is said to be a mainsequence star, or to be on the main sequence. This

term comes from the Hertzsprung-Russell diagram,that plots a star's surface temperature against itstrue brightness or magnitude. For stars on the mainsequence, the hotter they are, the brighter they are.The length of time a star is on the main sequencedepends on how long a star is able to balance theinward force of gravity with the outward forceprovided by the nuclear fusion going on in its core.More massive stars have higher pressure in thecore, so they have to burn more of their hydrogen“fuel” to prevent gravitational collapse. Becauseof this, more massive stars have highertemperatures, and also run out of hydrogen soonerthan smaller stars do.

Our Sun, which is a medium-sized star, hasbeen a main sequence star for about 5 billionyears. It will continue to shine without changing forabout 5 billion more years. Very large stars may beon the main sequence for “only” 10 million yearsor so. Very small stars may be main sequence starsfor tens to hundreds of billions of years.

Red Giants and White Dwarfs

As a star begins to use up its hydrogen, it thenbegins to fuse helium atoms together into heavieratoms like carbon. Eventually, stars contain fewerlight elements to fuse. The star can no longer holdup against gravity and it starts to collapse inward.Meanwhile, the outer layers spread out and cool.The star becomes larger, but cooler on the surfaceand red in color. Stars in this stage are called redgiants.

Eventually, a red giant burns up all of thehelium in its core. What happens next depends onhow massive the star is. A typical star like the Sun,stops fusion completely at this point. Gravitationalcollapse shrinks the star's core to a white, glowingobject about the size of Earth. A star at this point iscalled a white dwarf. Eventually, a white dwarfcools down and its light fades out.

Supergiants and Supernovas

A star that has much more mass than the Sunwill end its life in a more dramatic way. When

very massive stars leave the main sequence, theybecome red supergiants . The red star Betelgeusein Orion is a red supergiant.

Unlike red giants, when all the helium in a redsupergiant is gone, fusion does not stop. The starcontinues fusing atoms into heavier atoms, untileventually its nuclear fusion reactions produce ironatoms. Producing elements heavier than ironthrough fusion takes more energy than it produces.Therefore, stars will ordinarily not form anyelements heavier than iron. When a star exhauststhe elements that it is fusing together, the coresuccumbs to gravity and collapses violently,creating a violent explosion called a supernova. Asupernova explosion contains so much energy thatsome of this energy can actually fuse heavy atomstogether, producing heavier elements such as gold,silver, and uranium. A supernova can shine asbrightly as an entire galaxy for a short time, asshown in Figure below.

Figure 26.4 Supernova 1994D: When very large stars stop

nuclear fusion, they explode as supernovas. Abright supernova, like the one in the bottom left ofthe figure, can shine as brightly as an entire galaxyfor a short time.

Neutron Stars and Black Holes

After a large star explodes in a supernova, theleftover material in the core is extremely dense. If

the core is less than about four times the mass ofthe Sun, the star will be a neutron star, as showni n Figure below. A neutron star is made almostentirely of neutrons. Even though it is moremassive than the sun, it is only a few kilometers indiameter.

Figure 26.5 After a supernova, the remaining core may end

up as a neutron star.If the core remaining after a supernova is more

than about 5 times the mass of the Sun, the corewill collapse so far that it becomes a black hole.Black holes are so dense that not even light canescape their gravity. For that reason, black holes

cannot be observed directly. But we can identify ablack hole by the effect that it has on objectsaround it, and by radiation that leaks out around itsedges.

Measuring Star Distances

The Sun is much closer to Earth than any otherstar. Light from the Sun takes about 8 minutes toreach Earth. Light from the next nearest star,Proxima Centauri, takes more than 4 years to reachEarth. Traveling to Proxima Centauri in spacecraftsimilar to those we have today would take tens ofthousands of years.

Light-years

Because astronomical distances are so large, ithelps to use units of distance that are large as well.A light-year is defined the distance that lighttravels in one year. One light-year is9,500,000,000,000 (9.5 trillion) kilometers, or

5,900,000,000,000 (5.9 trillion) miles. ProximaCentauri is 4.22 light-years away, which meansthat its light takes 4.22 years to reach us.

One light-year is approximately equal to60,000 AU and 4.22 light-years is almost 267,000AU. Recalling that Neptune, the farthest planetfrom the Sun, orbits roughly 30 AU from the Sun,we can realize that the distance from the Earth tostars other than our own Sun is much greater thanthe distance from the Earth to other planets withinour own solar system.

Parallax

So how do astronomers measure the distance tostars? Distances to stars that are relatively close tous can be measured using parallax. Parallax is anapparent shift in position that takes place when theposition of the observer changes.

To see an example or parallax, try holding yourfinger about 1 foot (30 cm) in front of your eyes.Now, while focusing on your finger, close one eyeand then the other. Alternate back and forth

between eyes, and pay attention to how your fingerappears to move. The shift in position of yourfinger is an example of parallax. Now try movingyour finger closer to your eyes, and repeat theexperiment. Do you notice any difference? Thecloser your finger is to your eyes, the greater theposition changes due to parallax.

As Figure below shows, astronomers use thissame principle to measure the distance to stars.However, instead of a finger, they focus on a star.And instead of switching back and forth betweeneyes, they use the biggest possible difference inobserving position. To do that, they first look at thestar from one position, and they note where the starappears to be relative to more distant stars. Then,they wait 6 months; during this time, Earth movesfrom one side of its orbit around the Sun to theother side. When they look at the star again,parallax will cause the star to appear in a differentposition relative to more distant stars. From thesize of this shift, they can calculate the distance tothe star.

Parallax

Figure 26.6

Parallax is used to measure the distance tostars that are relatively nearby.

Other Methods

For stars that are more than a few hundred lightyears away, parallax is too small to measure, evenwith the most precise instruments available. Forthese more distant stars, astronomers use moreindirect methods of determining distance. Most ofthese other methods involve determining howbright the star they are looking at really is. Forexample, if the star has properties similar to theSun, then it should be about as bright as the Sun.Then, they can compare the observed brightness tothe expected brightness. This is like asking, “Howfar away would the Sun have to be to appear thisdim?”

Lesson Summary

Constellations and asterisms are apparent

patterns of stars in the sky.Stars in the same constellation are often notclose to each other in space.A star generates energy by nuclear fusionreactions in its core.The color of a star is determined by itssurface temperature.Stars are classified by color and temperature.The most common system uses the letters O(blue), B (bluish white), A (white), F(yellowish white), G (yellow), K (orange),and M (red), from hottest to coolest.Stars form from clouds of gas and dust callednebulas. Stars collapse until nuclear fusionstarts in the core.Stars spend most of their lives on the mainsequence, fusing hydrogen into helium.Typical, Sun-like stars expand into red giants,then fade out as white dwarfs.Very large stars expand into red supergiants,explode in supernovas, then end up as neutronstars or black holes.Astronomical distances can be measured in

light-years. A light year is the distance thatlight travels in one year. 1 light-year = 9.5trillion kilometers (5.9 trillion miles).Parallax is an apparent shift in an object’sposition when the position of the observerchanges. Astronomers use parallax to measurethe distance to relatively nearby stars.

Review Questions

1. What distinguishes a nebula and a star?2. What kind of reactions provide a star with

energy?3. Which has a higher surface temperature: a

blue star or a red star?4. List the seven main classes of stars, from

hottest to coolest.5. What is the primary reaction that occurs in the

core of a star, when the star is on the mainsequence?

6. What kind of star will the Sun be after itleaves the main sequence?

7. Suppose a large star explodes in a supernova,leaving a core that is 10 times the mass of theSun. What would happen to the core of thestar?

8. What is the definition of a light-year?9. Why don’t astronomers use parallax to

measure the distance to stars that are very faraway?

Further Reading / SupplementalLinks

http://www.ianridpath.com/startales/contents.htmhttp://hsci.cas.ou.edu/exhibits/exhibit.php?exbgrp=3&#38;amp;exbid=20&#38;amp;exbpg=0http://www.nasa.gov/worldbook/star_worldbook.htmlhttp://imagine.gsfc.nasa.gov/docs/science/know_l1/stars.htmlhttp://www.spacetelescope.org/science/formation_of_stars.htmlhttp://hurricanes.nasa.gov/universe/science/stars.htmlhttp://starchild.gsfc.nasa.gov/docs/StarChild/questions/parallax.htmlhttp://imagine.gsfc.nasa.gov/YBA/HTCas-size/parallax1-more.html

http://www-spof.gsfc.nasa.gov/stargaze/Sparalax.http://www.youtube.com/watch?v=VMnLVkV_ovc

Vocabulary

asterismA group or cluster of stars that appear closetogether in the sky.

black holeThe super dense core left after a supergiantexplodes as a supernova.

constellationAn apparent pattern of stars in the night sky.

light-yearThe distance that light travels in one year; 9.5trillion kilometers.

main sequence star

A star that is fusing hydrogen atoms to helium;a star in the main portion of its "life."

nebulaAn interstellar cloud of gas and dust.

neutron starThe remnant of a massive star after itexplodes as a supernova.

nuclear fusion reactionWhen nuclei of two atoms fuse together,giving off tremendous amounts of energy.

parallaxA method used by astronomers to calculatethe distance to nearby stars, using theapparent shift relative to distant stars.

red giantStage in a star's development when the innerhelium core contracts while the outer layersof hydrogen expand.

supernovaA tremendous explosion that occurs when thecore of a star is mostly iron.

starA glowing sphere of gases that produces lightthrough nuclear fusion reactions.

Points to Consider

Although stars may appear to be closetogether in constellations, they are usually notclose together out in space. Can you think ofany groups of astronomical objects that arerelatively close together in space?Most nebulas contain more mass than a singlestar. If a large nebula collapsed into severaldifferent stars, what would the result be like?

Galaxies

Lesson Objectives

Distinguish between star systems and starclusters.Identify different types of galaxies.Describe our own galaxy, the Milky WayGalaxy.

Introduction

Compared to your neighborhood, your country,or even planet Earth, the solar system is anextremely big place. But there are even biggersystems in the universe; groups of two, twohundred, or two billion stars! Small groups of starsare called star systems, and somewhat largergroups are called star clusters. There are evenlarger groups of stars, called galaxies. Our solarsystem is in the Milky Way Galaxy, which is just

one galaxy in the universe. There are severaldifferent types of galaxies and there are possiblybillions of galaxies in the universe.

Star Systems and Star Clusters

Constellations are patterns of stars that we seein the same part of the night sky, but these starsmay not be close together at all out in space.However, some stars are actually grouped closelytogether in space. These small groups of stars arecalled star systems and larger groups of hundredsor thousands of stars are called star clusters.

Star Systems

Our solar system has only one star, the Sun. Butmany stars—in fact, more than half of the brightstars in our galaxy—are in systems of two or morestars. A system of two stars orbiting each other iscalled a binary star. A system with more than twostars is called a multiple star system. In a multiple

star system, each of the stars orbits around theothers.

Often, the stars in a multiple star system are soclose together that you can only tell there aremultiple stars using binoculars or a telescope.Figure below shows Sirius A, the brightest star inthe sky. Sirius A is a very large star. If you look tothe lower left of Sirius A in the figure, you can seea much smaller star. This is Sirius B, a whitedwarf companion to Sirius A.

Figure 26.7

The bright star Sirius is actually a binarysystem with one large star (Sirius A) and one smallstar (Sirius B). This image from the Hubble SpaceTelescope shows Sirius B to the lower left ofSirius A. As you might guess, Sirius A is much,much brighter than Sirius B. Sirius B once wasbrighter than its companion, but it became a redgiant and collapsed into its current dim state about100-125 million years ago.

Star Clusters

Star clusters are divided into two main types,open clusters and globular clusters. Open clustersare groups of up to a few thousand stars that areloosely held together by gravity. The Pleiades,shown in Figure below, is a well-known opencluster. The Pleiades are also called the SevenSisters, because you can see seven stars in thecluster without a telescope, but with good vision.Using a telescope reveals that the Pleiades hasclose to a thousand stars.

Figure 26.8 The Pleiades is an open cluster containing

several hundred stars surrounded by gas. Note thatthe stars are mostly blue.

Open clusters tend to be blue in color and oftencontain glowing gas and dust. That is because thestars in an open cluster are young stars that formedfrom the same nebula. Eventually, the stars may bepulled apart by gravitational attraction to otherobjects.

Figure below shows an example of a globularcluster. Globular clusters are groups of tens tohundreds of thousands of stars held tightly togetherby gravity. Unlike open clusters, globular clustershave a definite, spherical shape. Globular clusters

contain mostly old, reddish stars. As you get closerto the center of a globular cluster, the stars arecloser together. Globular clusters don’t have muchdust in them — the dust has already formed intostars.

Figure 26.9 M80 is a large globular cluster containing

hundreds of thousands of stars. Note that the clusteris spherical and contains mostly red stars.

Types of Galaxies

The biggest groups of stars are called galaxies.Galaxies can contain anywhere from a few millionstars to many billions of stars. Every star you cansee in the sky is part of the Milky Way Galaxy, thegalaxy we live in. Other galaxies are extremely faraway, much farther away than even the most distantstars you can see. The closest major galaxy—theAndromeda Galaxy, shown in Figure below—looks like only a dim, fuzzy spot to the naked eye.But that fuzzy spot contains one trillion stars; thatis a thousand billion, or 1,000,000,000,000 stars!

Figure 26.10 The Andromeda Galaxy is the closest major

galaxy to our own. Andromeda is a large spiralgalaxy that contains about a trillion stars.

Spiral Galaxies

Galaxies are divided into three types accordingto shape: spiral galaxies, elliptical galaxies, andirregular galaxies. Spiral galaxies rotate or spin,so they have a rotating disk of stars and dust, abulge in the middle, and several arms spiraling outfrom the center. Spiral galaxies have lots of gasand dust and lots of young stars. Figure belowshows a spiral galaxy from the side, so you can seethe disk and central bulge.

Figure 26.11 The Sombrero Galaxy is a spiral galaxy that

we see from the side.Figure below shows a spiral galaxy face-on,

so you can see the spiral arms. The spiral arms of

a galaxy contain lots of dust. New stars form fromthis dust. Because they contain lots of young stars,spiral arms tend to be blue.

Figure 26.12 The Pinwheel Galaxy is a spiral galaxy we see

face-on. Note the blue spiral arms.

Elliptical Galaxies

Figure below shows a typical ellipticalgalaxy. As you might have guessed, ellipticalgalaxies are elliptical, or egg-shaped. The smallestelliptical galaxies are as small as some globularclusters. Giant elliptical galaxies, on the other

hand, can contain over a trillion stars. Ellipticalgalaxies are reddish to yellowish in color becausethey contain mostly old stars.

Figure 26.13 The large, reddish-yellow object in the middle

of this figure is a typical elliptical galaxy. Can youfind other galaxies in the figure? What kind?

Typically, elliptical galaxies contain very littlegas and dust because the gas and dust has alreadyformed into stars. However, some ellipticalgalaxies, like the one shown in Figure below,

contain lots of dust. Astronomers believe that thesedusty elliptical galaxies form when two galaxies ofsimilar size collide.

Figure 26.14 This elliptical galaxy probably formed when

two galaxies of similar size collided with eachother.

Irregular Galaxies and Dwarf Galaxies

Look at the galaxy in Figure below. Do you

think this is a spiral galaxy or an elliptical galaxy?It is neither one! Galaxies that are not clearlyelliptical galaxies or spiral galaxies are calledirregular galaxies. Most irregular galaxies wereonce spiral or elliptical galaxies that were thendeformed either by gravitational attraction to alarger galaxy or by a collision with another galaxy.

Figure 26.15 This galaxy, called NGC 1427A, is an

irregular galaxy. It has neither a spiral nor anelliptical shape.

Dwarf galaxies are small galaxies containing

“only” a few million to a few billion stars. Mostdwarf galaxies are irregular in shape. However,there are also dwarf elliptical galaxies and dwarfspiral galaxies. Dwarf galaxies are the mostcommon type in the universe. However, becausethey are relatively small and dim, we don’t see asmany dwarf galaxies as we do their full-sizedcousins.

Look back at Figure 4. In the figure, you cansee two dwarf elliptical galaxies that arecompanions to the Andromeda Galaxy. One is abright sphere to the left of center, and the other is along ellipse below and to the right of center.Dwarf galaxies are often found near largergalaxies. They sometimes collide with and mergeinto their larger neighbors.

The Milky Way Galaxy

If you look up in the sky on a very clear night,you may see a milky band of light stretching acrossthe sky, as in Figure below. This band is called the

Milky Way, and it consists of millions of starsalong with a lot of gas and dust. This band is thedisk of a galaxy, the Milky Way Galaxy, which isour galaxy. The Milky Way Galaxy looks differentto us than other galaxies because we are actuallyliving inside of it!

Figure 26.16 The band of light called the Milky Way can be

seen on a clear, dark night. Looking at this band,you are looking along the main disk of our galaxy.

Shape and Size

Because we live inside the Milky Way Galaxy,it is hard to know exactly what it looks like. Butastronomers believe the Milky Way Galaxy is atypical spiral galaxy that contains about 100billion to 400 billion stars. Figure below showswhat our Galaxy would probably look like if seenfrom the outside.

Figure 26.17 This artists rendering shows what we currently

think the Milky Way Galaxy would look like seenfrom above. The Sun and solar system (and you!)are a little more than halfway out from the center.

Like other spiral galaxies, our galaxy has adisk, a central bulge, and spiral arms. The disk isabout 100,000 light-years across and 3,000 light-years thick. Most of the Galaxy’s gas, dust, youngstars, and open clusters are in the disk.

The central bulge is about 12,000 to 16,000light-years wide and 6,000 to 10,000 light-yearsthick. The central bulge contains mostly older starsand globular clusters. Some recent evidencesuggests the bulge might not be spherical, but isinstead shaped like a bar. The bar might be as longas 27,000 light-years long. The disk and bulge aresurrounded by a faint, spherical halo, which alsocontains old stars and globular clusters. Someastronomers believe there is a gigantic black holeat the center of the Galaxy.

The Milky Way Galaxy is a big place. If oursolar system were the size of your fist, theGalaxy’s disk would still be wider than the entireUnited States!

Where We Are

Our solar system, including the Sun, Earth, andall the other planets, is within one of the spiralarms in the disk of the Milky Way Galaxy. Most ofthe stars we see in the sky are relatively nearbystars, that are also in this spiral arm. We are about26,000 light years from the center of the Galaxy. Inother words, we live a little more than halfway outfrom the center of the Galaxy to the edge, as shownin Figure 11.

Just as Earth orbits the Sun, the Sun and solarsystem orbit the center of the Galaxy. One orbit ofthe solar system takes about 225 to 250 millionyears. The solar system has orbited 20 to 25 timessince it formed 4.6 billion years ago.

Lesson Summary

Most stars are in systems of two or morestars.Open clusters are groups of young stars

loosely held together by gravity.Globular clusters are spherical groups of oldstars held tightly together by gravity.Galaxies are collections of millions to manybillions of stars.Spiral galaxies have a rotating disk of starsand dust, a bulge in the middle, and severalarms spiraling out from the center. The diskand arms contain many young, blue stars.Typical elliptical galaxies are egg-shaped,reddish, and contain mostly old stars.Galaxies that are not elliptical or spiralgalaxies are called irregular galaxies. Oftenthese galaxies were deformed by othergalaxies.The band of light called the Milky Way is thedisk of our galaxy, the Milky Way Galaxy,which is a typical spiral galaxy.Our solar system is in a spiral arm of theMilky Way Galaxy, a little more than halfwayfrom the center to the edge of the disk. Mostof the stars we see are in our spiral arm.

Review Questions

1. What is a binary star?2. Compare globular clusters with open clusters.3. Name the three main types of galaxies.4. List three main features of a spiral galaxy.5. Suppose you see a round galaxy that is

reddish in color and contains very little dust.What kind of galaxy is it?

6. What galaxy do we live in, and what kind ofgalaxy is it?

7. Describe the location of our solar system inour galaxy.

Further Reading / SupplementalLinks

http://www.cfa.harvard.edu/press/2006/pr200611.htmlhttp://hypertextbook.com/facts/2000/MarissaWager.shtmlhttp://seds.lpl.arizona.edu/messier/more/mw.htmlhttp://www.space.com/scienceastronomy/050816_milky_way.htmlhttp://seds.org/messier/cluster.html;

http://hubblesite.org/newscenter/archive/releases/star-cluster/http://stardate.org/resources/btss/galaxies/;http://casswww.ucsd.edu/public/tutorial/Galaxies.html;http://www.smv.org/hastings/student1.htmhttp://en.wikipedia.org

Vocabulary

binary starOne of two stars that orbit each other.

elliptical galaxyAn oval or egg shaped galaxy with older starsand little gas and dust.

galaxyA very large group of stars held together bygravity; few million to a few billion stars.

globular clusterGroups of tens to hundreds of thousands ofstars held together by gravity.

irregular galaxyA category of galaxy that is neither a spiralnor an elliptical galaxy.

Milky WayThe name of our galaxy; also the whitish bandof stars visible in the night sky.

open clusterGroups of up to a few thousand stars looselyheld together by gravity.

spiral armRegions of gas and dust plus young stars thatwind outward from the central area of a spiralgalaxy.

spiral galaxyA rotating type of galaxy with a central bulgeand spiral arms with young stars, gas anddust.

star systemSmall groups of stars.

star clusterLarger groups of hundreds of thousands ofstars.

Points to Consider

Objects in the universe tend to be groupedtogether. What forces or factors do you thinkcause objects to form and stay in groups?Some people used to call galaxies “islanduniverses.” Are they really universes? Why orwhy not?Can you think of anything, either an object ora group of objects, that is bigger than agalaxy?

The Universe

Lesson Objectives

Explain the evidence for an expandinguniverse.Describe the formation of the universeaccording to the Big Bang Theory.Define dark matter and dark energy.

So far we have talked about bigger and biggersystems, from stars to star systems to star clustersand galaxies. The universe contains all thesesystems, including all the matter and energy thatexists now, that existed in the past, and that willexist in the future. The universe also includes all ofspace and time.

Our understanding of the universe has changeda lot over time. The ancient Greeks thought theuniverse contained only Earth at the center, theSun, the Moon, five planets, and a sphere to whichall the stars were attached. Most people had this

basic idea of the universe for centuries, untilGalileo first used a telescope to look at the stars.Then people realized that Earth is not the center ofthe universe, and there are many more stars thanthought before. Even as recently as the early1900s, some scientists still thought the universewas no larger than the Milky Way Galaxy.

In the early 20th century, an astronomer namedEdwin Hubble (Figure below) discovered that the“Andromeda Nebula” is actually over 2 millionlight years away—many times farther than thefarthest distances we had measured before. Herealized that many of the objects astronomerscalled nebulas were not clouds of gas, butcollections of millions or billions of stars—whatwe now call galaxies. Our view of the universechanged again—we now knew that the universewas much larger than our own galaxy. Today, weknow that the universe contains about a hundredbillion galaxies—about the same number ofgalaxies as there are stars in the Milky WayGalaxy.

Figure 26.18 Edwin Hubble used the 100-inch reflecting

telescope at the Mount Wilson Observatory inCalifornia to show that some distant specks of lightseen through telescopes are actually other galaxies.He also measured these distances to hundreds ofgalaxies, and discovered that the universe isexpanding.

Expansion of the Universe

After discovering that there are galaxiesoutside our own, Edwin Hubble went on tomeasure the distance to hundreds of other galaxies.His data would eventually show us how theuniverse is changing, and even give us clues as tohow the universe formed.

Redshift

If you look at a star through a prism, you willsee a spectrum, or a range of colors through therainbow. Interestingly, the spectrum will havespecific dark bands where elements in the starabsorbed light of certain energies. By examiningthe arrangement of these dark absorption lines,astronomer can actually determine which elementsare in a distant star. In fact, the element helium wasfirst discovered in our Sun — not on Earth — byanalyzing the absorption lines in the spectrum ofthe Sun.

When astronomers started to study the spectrumof light from distant galaxies, they noticedsomething strange. The dark lines in the spectrum

were in the patterns they expected, but they wereshifted toward the red end of the spectrum, asshown in Figure below. This shift of absorptionbands toward the red end of the spectrum is knownas redshift.

Figure 26.19 Redshift is a shift in absorption bands toward

the red end of the spectrum. Redshift occurs when

the light source is moving away from you or whenthe space between you and the source is stretched.

Redshift occurs when the source of light ismoving away from the observer. So whenastronomers see redshift in the light from a galaxy,they know that the galaxy is moving away fromEarth. The strange part is that almost every galaxyin the universe has a redshift, which means thatalmost every galaxy is moving away from us.

An analogy to redshift is the noise a sirenmakes as it passes by you. You may have noticedthat an ambulance lowers the pitch of its siren afterit passes you. The sound waves shift towards alower pitch when the ambulance speeds away fromyou. Though redshift involves light instead ofsound, a similar principle operates in bothsituations.

The Expanding Universe

Edwin Hubble combined his measurements ofthe distances to galaxies with other astronomers’measurements of redshift. He noticed a

relationship, which is now called Hubble’s Law:The farther away a galaxy is, the faster it is movingaway from us. In other words, the universe isexpanding!

Figure below shows a simplified diagram ofthe expansion of the universe. Another way topicture this is to imagine a balloon covered withtiny dots. Each dot represents a galaxy. When youinflate the balloon, the dots slowly move awayfrom each other because the rubber stretches in thespace between them. If it were a giant balloon andyou were standing on one of the dots, you wouldsee the other dots moving away from you. Not onlythat, but dots farther away from you on the balloonwould move away faster than dots nearby.

Expansion of the universe diagram

Figure 26.20 This is a simplified diagram of the expansion

of the universe over time. Note that the distancebetween galaxies gets bigger as you go forward intime, but the size of each galaxy stays about thesame.

An inflating balloon is not exactly like theexpanding universe. For one thing, the surface of aballoon has only two dimensions, while space hasthree dimensions. But it is true that space itself isstretching out between galaxies like the rubberstretches when a balloon is inflated. Thisstretching of space, which causes the distance

between galaxies to increase, is what astronomersmean by the expansion of the universe.

One other difference between the universe andour balloon model involves the actual size of thegalaxies. On the inflating balloon, the dots youmade will become larger in size as you inflate it.In our universe, however, the galaxies stay thesame size; it is just the space between the galaxiesthat increases as the universe expands.

Formation of the Universe

The discovery that the universe is expandingalso told astronomers something about how theuniverse might have formed. Before this discovery,there were many ideas about the universe, most ofthem thinking of the universe as constant. Oncescientists learned that the universe is expanding,the next logical thought is that at one time it had tohave been smaller.

The Big Bang Theory

The Big Bang theory is the most widelyaccepted scientific explanation of how the universeformed. To understand this theory, start bypicturing the universe expanding steadily. Then,reverse the direction of time, like pressing the“rewind” button on a video player. Now theuniverse is contracting, getting smaller andsmaller. If you go far enough back in time, you willreach a point when the universe was squeezed intoa very small volume.

According to the Big Bang theory, the universebegan about 13.7 billion years ago, wheneverything in the universe was squeezed into avery small volume, as described above. There wasan enormous explosion—a big bang—whichcaused the universe to start expanding rapidly. Allthe matter and energy in the universe—and evenspace itself—came out of this explosion.

After the Big Bang

In the first few moments after the Big Bang, theuniverse was extremely hot and dense. As the

universe expanded, it became less dense and itcooled. After only a few seconds, the universe hadcooled enough that protons, neutrons, and electronscould form. After a few minutes, hydrogen couldform and the energy in the universe was greatenough to allow for nuclear fusion, creating heliumatoms in the same way we learned that a star canmake helium out of hydrogen atoms, even thoughthere were no stars at this point in the universe'shistory. The first neutral atoms with neutrons,protons, and electrons, did not form until about380,000 years after the big bang.

The matter in the early universe was notsmoothly distributed across space. Some parts ofthe universe were more dense than others. Theseclumps of matter were held close together bygravity. Eventually, these clumps became the gasclouds, stars, galaxies, and other structures that wesee in the universe today.

Dark Matter and Dark Energy

The Big Bang theory is still the best scientificmodel we have for explaining the formation of theuniverse. However, recent discoveries inastronomy have shaken up our understanding of theuniverse. Astronomers and other scientists are nowwrestling with some big unanswered questionsabout what the universe is made of and why it isexpanding like it is.

Dark Matter

Most of the things we see out in space areobjects that emit light, such as stars or glowinggases. When we see other galaxies, we are seeingthe glowing stars or gases in that galaxy. However,scientists think that matter that emits light onlymakes up a small part of the matter in the universe.The rest of the matter is called dark matter.

Because dark matter doesn’t emit light, wecan’t observe it directly. However, we know it isthere because its gravity affects the motion ofobjects around it. For example, when astronomersmeasure how spiral galaxies rotate, they find that

the outside edges of a galaxy rotate at the samespeed as parts closer to the center. This can onlybe explained if there is a lot of extra matter in agalaxy that we cannot see.

So what is dark matter? Actually, we don’treally know. One possibility is that it could just beordinary matter—protons, neutrons, and electrons,like what makes up the Earth and all the matteraround us. The universe could contain lots ofobjects that don’t have enough mass to glow ontheir own, such as large planets and brown dwarfs,objects larger than Jupiter but smaller than thesmallest stars. Or, there could be large numbers ofundetected black holes.

Another possibility is that the universe containsa lot of matter that is unlike anything we have everencountered. For example, scientists haveproposed that there might be particles that havemass but don’t interact much with other matter.Scientists call these theoretical particles WIMPs,which stands for Weakly Interactive MassiveParticles. WIMPs would have a gravitationaleffect on other matter because of their mass. But

because they don’t interact much with ordinarymatter, they would be very difficult or impossibleto detect directly.

Most scientists who study dark matter believethat the universe’s dark matter is a combination ofordinary matter and some kind of exotic matter thatwe haven’t discovered yet. Most scientists alsothink that ordinary matter is much less than half ofthe total matter in the universe. Researching darkmatter is clearly an active area of scientificresearch, and astronomers’ knowledge about darkmatter changing rapidly.

Dark Energy

Astronomers who study the expansion of theuniverse are interested in finding out just how fastthe universe is expanding. For years, the bigquestion was whether the universe was expandingfast enough to overcome the attractive pull ofgravity. If yes, then the universe would expandforever, although the expansion would slow downover time. If no, then the universe would someday

start to contract, and eventually would getsqueezed together in a big crunch, the opposite ofthe Big Bang.

Recently, however, these astronomers havemade a strange discovery: the rate at which theuniverse is expanding is actually increasing. Inother words, the universe is expanding faster nowthan ever before, and in the future it will expandeven faster! This answers the old question: theuniverse will keep expanding forever. But it alsoproposes a perplexing new question: what iscausing the expansion of the universe toaccelerate?

One possible hypothesis involves a new, as-yet-undiscovered form of energy called darkenergy. We know even less about dark energy thanwe know about dark matter. However, somescientists believe that dark energy makes up morethan half the total content of the universe. Otherscientists have other hypotheses about why theuniverse is continuing to expand; the causes of theuniverse’s expansion is another unansweredquestion that scientists are researching.

Lesson Summary

The universe contains all the matter andenergy that exists now, that existed in the past,and that will exist in the future. The universealso includes all of space and time.Redshift is a shift of element lines toward thered end of the spectrum. Redshift occurs whenthe source of light is moving away from theobserver.Light from almost every galaxy is redshifted.The farther away a galaxy is, the more itslight is redshifted, and the faster it is movingaway from us.The redshift of galaxies tells us that theuniverse is expanding.The current expansion of the universesuggests that in the past the universe wassqueezed into a very small volume.The Big Bang theory proposes that theuniverse formed in an enormous explosionabout 13.7 billion years ago.

Recent evidence shows that there is a lot ofmatter in the universe that we cannot detectdirectly. This matter is called dark matter.The rate of the expansion of the universe isincreasing. The cause of this increase isunknown; one possible explanation involves anew form of energy called dark energy.

Review Questions

1. What is redshift, and what causes it to occur?2. What is Hubble’s law?3. What is the most widely accepted scientific

theory of the formation of the universe called?4. How old is the universe, according to the Big

Bang theory?5. Describe two different possibilities for the

nature of dark matter.6. What makes scientists believe that dark matter

exists?7. What observation caused astronomers to

propose the existence of dark energy?

Further Reading / SupplementalLinks

http://cdms.berkeley.edu/Education/DMpages/index.shtmlhttp://stardate.org/resources/btss/cosmology/http://hurricanes.nasa.gov/universe/science/bang.htmlhttp://imagine.gsfc.nasa.gov/docs/science/know_l1/dark_matterhtmlhttp://www.youtube.com/watch?v=gCgTJ6ID6ZAhttp://en.wikipedia.org

Vocabulary

Big Bang TheoryThe hypothesis that all matter and energywere at one time compresses into a verysmall volume; then there was an explosionthat sent everything moving outward, causingthe universe to expand.

dark energy

An as yet undiscovered form of energy thatwe cannot see.

dark matterMatter in the universe that doesn't emit light.

redshiftShift of wavelengths of light towards the redend of the spectrum; happens as a light sourcemoves away from us.

universeEverything that exists; all matter and energy;also includes all of space and time.

Points to Consider

The expansion of the universe is sometimesmodeled using a balloon with dots marked onit, as described earlier in the lesson. In whatways is this a good model, and it what waysdoes it not correctly represent the expandinguniverse? Can you think of a different way to

model the expansion of the universe?The Big Bang theory is currently the mostwidely accepted scientific theory for how theuniverse formed. What is another explanationof how the universe could have formed? Isyour explanation one that a scientist wouldaccept?

TABLE OFCONTENTS

CK-12 LicenseChapter 1: What is Earth Science?Chapter 2: Studying Earth’s SurfaceChapter 3: Earth's MineralsChapter 4: RocksChapter 5: Earth's EnergyChapter 6: Plate TectonicsChapter 7: EarthquakesChapter 8: VolcanoesChapter 9: Weathering and Formation of SoilChapter 10: Erosion and DepositionChapter 11: Evidence About Earth's PastChapter 12: Earth's HistoryChapter 13: Earth's Fresh WaterChapter 14: Earth's OceansChapter 15: Earth's AtmosphereChapter 16: WeatherChapter 17: Climate

Chapter 18: Ecosystems and HumanPopulationsChapter 19: Human Actions and the LandChapter 20: Human Actions and Earth'sResourcesChapter 21: Human Actions and Earth'sWatersChapter 22: Human Actions and theAtmosphereChapter 23: Observing and Exploring SpaceChapter 24: Earth, Moon, and SunChapter 25: The Solar SystemChapter 26: Stars, Galaxies, and the Universe

CK-12 Earth Science