5.1 Weathering

Reading StrategyBuilding Vocabulary Copy the table. Asyou read the section, define each vocabularyterm.

Key ConceptsWhat is mechanicalweathering?

What is chemicalweathering?

What factors affect therate of weathering?

Vocabulary◆ mechanical

weathering◆ frost wedging◆ talus◆ exfoliation◆ chemical

weathering

E arth’s surface is constantly changing. Internal forcesgradually raise some parts of the surface through moun-tain building and volcanic activity. At the same time,external processes continually break rock apart and movethe debris to lower elevations, as shown in Figure 1. Thebreaking down and changing of rocks at or near Earth’ssurface is called weathering. Weathering is a basic part ofthe rock cycle and a key process in the Earth system. Thereare two types of weathering—mechanical and chemical.Though these processes are different, they are at work atthe same time.

Mechanical WeatheringMechanical weathering occurs when physical forces

break rock into smaller and smaller pieces withoutchanging the rock’s mineral composition. Each piece hasthe same characteristics as the original rock. Breaking arock into smaller pieces increases the total surface area ofthe rock. Look at Figure 2. When rock is broken apart,more surface area is exposed to chemical weathering.

In nature, three physical processes are especiallyimportant causes of mechanical weathering: frost wedg-ing, unloading, and biological activity.

Vocabulary Term Definition

Mechanical weathering a.

Frost wedging b.

Talus c.

Exfoliation d.

Chemical weathering e. ?

?

?

?

?

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Figure 1 Weathering Ice, rain, and wind are slowly breaking down the rock in thismountain. The rock fragments accumulate insloped deposits at the base of the mountain.

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FOCUS

Section Objectives5.1 Define mechanical weathering.5.2 Explain chemical weathering.5.3 Identify the factors that affect

the rate of weathering.

Build VocabularyVocabulary Rating Chart Have eachstudent construct a chart with fourcolumns labeled Term, Can Define orUse It, Heard or Seen It, and Don’tKnow. Students should then write thevocabulary terms mechanical weathering,frost wedging, talus, exfoliation, andchemical weathering in column 1 andrate their knowledge of each term byputting a check in one of the othercolumns. Before reading the text, askquestions that help students anticipatethe meaning of the terms based on priorknowledge. For example, ask: What is awedge used for? (to force objects apart)After students have read the section,have them revise their charts.

Reading Strategya. process in which physical forces breakrock into pieces without changing therock’s mineral compositionb. process in which water enters cracksin a rock, enlarges the cracks by freezingand expanding, and breaks the rockc. large piles of rock fragments thatusually form at the base of cliffsd. process in which slabs of outer rockseparate and break loosee. the transformation of rock into oneor more new compounds

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Frost Wedging When liquid water freezes, it expands by about 9percent, exerting a tremendous outward force. This force is greatenough to burst water pipes during the winter. In nature, water worksits way into every crack in rock. When water freezes and expands, itenlarges the cracks. After many freeze-thaw cycles, the rock breaks intopieces. This process, which is shown in Figure 3, is called frost wedg-ing. Frost wedging is most common in mountainous regions in themiddle latitudes. Here daily freezing and thawing often occur. Sectionsof rock that are wedged loose may tumble into large piles called talus,which typically form at the base of steep, rocky cliffs.

Explain how water can cause mechanical weathering.

Figure 2 By breaking a rock into smaller pieces, mechanicalweathering increases the rock’ssurface area that can be exposedto chemical weathering. Calculating Calculate the totalsurface area if each of the 64 cubesshown in the right diagram werebroken into 8 equal-sized cubes.

Figure 3 Frost Wedging Rainwaterentered cracks in this boulder. Eachtime the water froze, it expanded.Eventually, the boulder split.

16 square units �6 sides �1 cube �96 square units

4 square units �6 sides �8 cubes �192 square units

1 square unit �6 sides �64 cubes �384 square units

16 square units

4 squareunits

4

4

21

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Mechanical Weathering and Surface AreaINSTRUCT

MechanicalWeatheringBuild Reading LiteracyRefer to p. 334D in Chapter 12, whichprovides the guidelines for outlining.

Outline Have students create anoutline of Section 5.1 (pp. 126–132).Outlines should follow the headstructure used in the text. Majorheadings are shown in green, andsubheadings are shown in blue. Ask:Based on your outlines, what do youexpect to learn about in this section?(mechanical weathering, chemicalweathering, and rates of weathering)Verbal

Use VisualsFigure 2 Ask: Which of the threediagrams has the most exposedsurface area? (the third one) If thediagrams were actual rocks, whichone would experience the leastchemical weathering? (the first one)Visual

Build Science SkillsDesigningExperimentsHave students work insmall groups to designexperiments showing how waterexpands when frozen. Studentsshould write down each step of theirprocedures, listing materials used andsafety measures. A sample experimentmight involve pouring water into acontainer, placing the container in afreezer overnight, then observing theeffect of the frozen water on thecontainer. If time permits, allow studentsto perform their experiments. Be sure toapprove all procedures beforehand.Logical, Group

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Customize for English Language Learners

Tailor your presentation by speaking directlyand simplifying the terms and sentencestructures used to explain key concepts. Forexample, split a cause-and-effect sentenceinto two sentences labeled Cause and Effect.A sample cause sentence might be Waterexpands when it freezes. A sample effect

sentence might be The frozen water enlargescracks in rocks. Other techniques include usingvisual aids and body language when appropriateto emphasize important concepts. For example,you can clasp your hands together, then movethem apart when discussing how waterexpands when it freezes.

Answer to . . .

Figure 2 64 cubes � 8 � 512 cubes;1 square unit/side � 6 sides/cube �512 cubes � 768 square units

Water works its wayinto cracks in rock.

When water freezes, it expands,enlarging the cracks. After manyfreeze-thaw cycles, the rock breaks.

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Figure 5 The roots of this tree arecausing mechanical weathering bywidening the cracks in the rock.

Unloading Large masses of igneous rock may be exposed throughuplift and erosion of overlying rocks. When that happens, the pressureexerted on the igneous rock is reduced. This is known as unloading.As illustrated in Figure 4A, unloading causes the outer layers of therock to expand more than the rock below. Slabs of outer rock separatelike the layers of an onion and break loose in a process calledexfoliation. Exfoliation is especially common in rock masses madeof granite. It often produces large, dome-shaped rock formations.Figure 4B shows one of these formations. Other important exfolia-

tion domes are Stone Mountain, Georgia, andLiberty Cap also in Yosemite National Park.

A striking example of the weathering effects ofunloading is shown in deep underground mining.Newly cut mine tunnels suddenly reduce the pres-sure on the surrounding rock. As a result, large rockslabs sometimes explode off the walls of the tunnels.

Biological Activity The activities of organ-isms, including plants, burrowing animals, andhumans, can also cause mechanical weathering. AsFigure 5 shows, plant roots grow into cracks in rock,wedging the rock apart as they grow. Burrowing ani-mals move rocks to the surface, where weathering ismore rapid. Decaying organisms produce com-pounds called acids that cause chemical weathering.

Figure 4 Unloading andExfoliation A Uplift and erosionexpose a buried mass of igneousrock. Reduced pressure on therock, called unloading, causes theouter rock layers to expand. Theyseparate from the rest of the rockmass. This process is calledexfoliation. B The granite layersof Half Dome in YosemiteNational Park, California, areundergoing exfoliation.

B

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Uplift ofigneous rock

Expansionand

exfoliation

ErosionA

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Use CommunityResourcesArrange for students to take a walkingtour of your community to look for signsof mechanical weathering. If possible,bring along a camera or camcorder todocument the field trip. Have studentsattempt to classify each example ofmechanical weathering as frost wedging,unloading, or biological activity.Visual

Modeling ExfoliationPurpose Students will observe a modelof the process of exfoliation.

Materials large onion, small kitchenknife

Procedure Stand before the class andhold up a large, unpeeled onion. Cut offthe top of the onion and show studentsits inner layers. Peel the onion, layer bylayer. Wash your hands afterwards.

Expected Outcomes Have studentsrelate the activity to Figure 4, whichshows an example of exfoliation. Havethem point out the onionlike layers ofthe Half Dome in Yosemite National Park.Visual

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Humans accelerate mechanical weathering through deforestation andblasting in search of minerals or in the creation of new roads.

Chemical WeatheringChemical weathering is the transformation of rock into one or

more new compounds. The new compounds remain mostly unchangedas long as the environment in which they formed does not change. Youcan contrast chemical weathering and mechanical weathering with asheet of paper. Tearing the paper into small pieces is like mechanicalweathering of rock. Burning the paper, which changes it into carbondioxide and water, is like chemical weathering.

Water Water is the most important agent of chemical weathering.Water promotes chemical weathering by absorbing gases from theatmosphere and the ground. These dissolved substances then chemi-cally react with various minerals. Oxygen dissolved in water reactseasily with certain minerals, forming oxides. For example, iron-richminerals get a yellow to reddish-brown coating of iron oxide whenthey react with oxygen. Iron oxide is the rust that forms when iron-containing objects are exposed to water. Figure 6A shows this rust onbarrels.

Water absorbs carbon dioxide when rain falls through the atmos-phere. Water that seeps through the ground also picks up carbondioxide from decaying organic matter. The carbon dioxide dissolved inwater forms carbonic acid. This is the weak acid in carbonated softdrinks. Carbonic acid reacts with many common minerals.

How are water, oxygen, and carbon dioxideinvolved in chemical weathering?

B C

Figure 6 A Oxygen reacted withthe iron in these barrels, formingiron oxide, or rust. B This granitegravestone, placed in 1868,shows little evidence of chemicalweathering. C The inscriptiondate (1872) on this marblegravestone is nearly illegibledue to chemical weathering.

A

Chemical WeatheringBuild Science SkillsObserving Obtain anew, unrusted nail anda thoroughly rustednail. Allow students tocarefully examine both. Break the rustednail into two pieces to emphasize howweak it is. Ask: How do the two nailsdiffer? (They differ in strength,appearance, and color.) Have studentsrelate their observations to chemicalweathering. Ask: What could you inferabout the minerals in a rock that hadthe same color as the rusted nail? (Theminerals likely contain iron that reactedwith oxygen and water.)Visual, Kinesthetic

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Weathering creates many important mineraldeposits by concentrating minor amountsof metals that are scattered throughoutunweathered rock into economically valuableconcentrations. Such a transformation cantake place in two ways. In one situation,chemical weathering coupled with downward-percolating water removes undesired materials

from decomposing rock, leaving the desiredelements enriched in the upper zones of thesoil. The second way is basically the reverse ofthe first. The desirable elements that are foundin low concentrations near the surface areremoved and carried to lower zones, wherethey are redeposited and become moreconcentrated.

Facts and Figures

Answer to . . .

Water dissolves someof the minerals in rock.

Oxygen dissolved in water reacts withcertain minerals, forming oxides.Carbon dioxide dissolved in waterforms carbonic acid, which reactswith many common minerals.

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Water in the atmosphere alsoabsorbs sulfur oxides and nitrogenoxides. These oxides are produced bythe burning of coal and petroleum.Through a series of chemical reactions,these pollutants are converted intoacids that are the major cause of acidprecipitation. Acid precipitation accel-erates the chemical weathering ofstone monuments and structures, suchas the one shown in Figure 7.

Chemical Weathering ofGranite To illustrate how chemicalweathering can change the propertiesof rock, let’s consider granite. Recallthat granite consists mainly of theminerals feldspar and quartz. Whengranite is exposed to water containing

carbonic acid, the feldspar is converted mostly to clay minerals. Quartz,in contrast, is much more resistant to carbonic acid and remainsunchanged. As the feldspar slowly changes to clay, the quartz grainsare released from the granite. Rivers transport some of this weathereddebris to the sea. The tiny clay particles may be carried far from shore.The quartz grains are deposited near the shore where they become themain component of beaches and sand dunes.

Chemical Weathering of Silicate Minerals Recall thatsilicate minerals make up most of Earth’s crust and are composed largelyof just eight elements. When silicate minerals undergo chemical weath-ering, the sodium, calcium, potassium, and magnesium they containdissolve and are carried away by groundwater. Iron reacts with oxygen,producing iron oxide. The three remaining elements are aluminum, sili-con, and oxygen. These elements usually combine with water andproduce clay minerals. See Table 1 for a list of products of weathering.

Spheroidal Weathering Chemical weathering canchange the physical shape of rock as well as its chemical com-position. For example, when water enters along the joints ina rock, it weathers the corners and edges most rapidly. Theseparts of the rock have a greater surface area than the faceshave. As a result, the corners and edges become morerounded. The rock takes on a spherical shape, as shown inFigure 8A. This process is called spheroidal weathering.

Figure 7 One Effect of AcidPrecipitation Acid precipitationcontributed to the chemicalweathering of this stone buildingfacade in Leipzig, Germany.

MaterialsMineral Residual Products in Solution

Table 1 Products of Weathering

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Quartz Quartz grains Silica

Feldspars Clay minerals SilicaK+, Na+, Ca2+

Amphibole Clay minerals Silica(hornblende) Limonite Ca2+, Mg2+

Hematite

Olivine Limonite SilicaHematite Mg2+

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During a student’s education, a gooddeal of emphasis is placed on thescientific method and proper invest-igation techniques. For this reason,many students mistakenly think thatscientific knowledge advances mainlythrough experimentation. In fact, inseveral fields, including geology,observation is the key to understandingnatural processes. This section offers anexcellent opportunity to point out therole of observation in science. Challengestudents to brainstorm other fieldswhere observation is crucial tounderstanding natural processes.(astronomy, biology, ecology)Verbal

Integrate ChemistryWeathering of Granite Theweathering of the potassium feldsparcomponent of granite takes place asfollows:

2KAlSi3O8 � 2(H+ + HCO3�) � H2O →

Al2Si2O5(OH)4 � 2K+ � 2HCO3� �

4SiO2.

In this reaction, the hydrogen ions (H+)attack and replace potassium ions (K+)in the feldspar structure, therebydisrupting the crystalline network. Onceremoved, the potassium is available as anutrient for plants or becomes the solublesalt potassium bicarbonate (KHCO3),which may be incorporated into otherminerals or carried to the ocean indissolved form by streams. Havestudents summarize the chemicalreaction in their own words. (Potassiumfeldspar reacts with carbonic acid andwater to produce mainly clay minerals.)Logical

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Figure 8 SpheroidalWeathering A The edges ofthese granite rocks in California’sJoshua Tree National Monumentwere rounded through spheroidalweathering. B Spheroidalweathering has caused the outerlayers of this rock to loosen andseparate.

BA

As Figure 8B shows, spheroidal weathering sometimes causes theouter layers of a rock to separate from the rock’s main body. This canhappen when the minerals in the rock turn to clay, which swells by addingwater. The swelling exerts a force that causes the layers to break loose andfall off. This allows chemical weathering to penetrate deeper into theboulder.Although the effects of this type of spheroidal weathering resem-ble exfoliation, the two processes are different. Spheroidal weathering isa form of chemical weathering. Exfoliation is caused by unloading. Thelayers that separate from the rock are not chemically changed.

Rate of WeatheringMechanical weathering affects the rate of chemical weathering. Bybreaking rock into smaller pieces, mechanical weathering accelerateschemical weathering by increasing the surface area of exposed rock.

Two other factors that affect the rate of weathering are rock char-acteristics and climate.

Rock Characteristics Physical characteristics of rock, such ascracks, are important in weathering because they influence the abil-ity of water to penetrate rock. However, a rock’s mineral compositionalso dramatically affects its rate of weathering. You can see this byvisiting a cemetery and comparing old gravestones made from dif-ferent rock types. Gravestones made of granite, like the one in Figure6B on page 129, are relatively resistant to chemical weathering. Youcan easily read the inscriptions on a granite gravestone that is over100 years old. In contrast, marble gravestones undergo much morerapid chemical weathering, as shown in Figure 6C on page 129.Marble is composed of calcite (calcium carbonate), which easily dis-solves even in weak acids.

Use VisualsFigure 8 Have students compareFigure 8 to Figure 4 on p. 128.Have them discuss the similaritiesand differences between spheroidalweathering and exfoliation. (Both aretypes of weathering that give rock alayered appearance. Spheroidalweathering is a form of chemicalweathering. Thus, the composition of therock changes. Exfoliation is a type ofmechanical weathering; the compositionof the rock does not change.)Visual, Verbal

Rate of WeatheringBuild Science SkillsInferring Hold up an atlas in frontof the class. Select an area and brieflydescribe its climate in terms of hot andwet, cold and dry, hot and dry, and soon. Have students infer rates ofweathering for each location. (Ingeneral, areas that are hot and wethave higher rates of weathering thando areas that are cooler and drier.)Logical, Visual

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Section 5.1 Assessment

Reviewing Concepts1. What happens to a rock’s mineral

composition during mechanical weathering?

2. What is unloading? How does it contribute toweathering?

3. How does chemical weathering affect thecompounds in rock?

4. Name two rock characteristics and twoclimatic factors that affect the rate ofweathering.

Critical Thinking5. Using Analogies Think about the following

processes: dissolving a piece of rock salt in apan of water and grinding a peach pit in agarbage disposal. Which process is more likemechanical weathering, and which is morelike chemical weathering?

6. Applying Concepts The level of carbondioxide in the atmosphere is increasing.How might this affect the rate of chemicalweathering of Earth’s surface rocks? Explainyour reasoning.

7. Suppose frost wedging splits a sphericalrock 2 m in diameter into two equal-sizedhemispheres. Calculate the total surfacearea of the original rock and of the twohemispheres. (The area of a circle � �r2,and the surface area of a sphere � 4�r2,where r is the radius.)

Silicates are the most abundant mineral group. Silicates weather inthe same sequence as their order of crystallization. Olivine crystallizesfirst and weathers most rapidly. Quartz, which crystallizes last, is themost resistant to weathering.

Climate Climatic factors, especially temperature and moisture, havea strong effect on the rate of weathering. For example, these factorscontrol the frequency of freeze-thaw cycles, which affect the amount offrost wedging. Temperature and moisture also affect the rate of chem-ical weathering. They influence the kind of vegetation and how muchis present. Regions with lush vegetation generally have a thick layer ofsoil rich in decaying organic matter that releases acids into the water.

The climate most favorable for chemical weathering has high tem-peratures and abundant moisture. So, chemical weathering is very slowin arid regions. It is also slow in polar regions because the low tem-peratures there keep moisture locked up as ice.

Differential Weathering Different parts of a rock mass oftenweather at different rates. This process, called differential weathering,has several causes. Differences in mineral composition are one cause.More resistant rock protrudes as pinnacles, or high peaks, such as thoseshown in Figure 9. Another cause is the variations in the number andspacing of cracks in different parts of a rock mass.

Figure 9 These boldly sculptedpinnacles in Bryce Canyon NationalPark show differential weathering.Drawing Conclusions In whichparts of these formations isweathering happening mostrapidly?

132 Chapter 5

Build Science SkillsCommunicating Results Showstudents a copy or transparency ofBowen’s reaction series. Have them notethe order of crystallization of minerals.Students can then use the series tosequence the resistance of silicates toweathering, starting with the leastresistant mineral. (olivine, pyroxene,amphibole, biotite mica, potassiumfeldspar, muscovite mica, and quartz)Verbal

ASSESSEvaluateUnderstandingHave students describe the relationshipbetween burning coal and chemicalweathering. (Oxides produced by burningcoal react with water in the atmosphere tocreate acid precipitation. Acid precipitation,in turn, accelerates rates of chemicalweathering.)

ReteachGive students examples of differenttypes of weathering, such as unloadingand spheroidal weathering, and havethem classify the examples asmechanical or chemical.

Solutions7. diameter � 2 m; radius � 1 m; areaof sphere � 4�(1 m)2 � 4� m2 �12.57 m2; area of two hemispheres �area of sphere � (2 � area of circle) �4� m2 � [2 � �(1 m)2] � 4� m2 +2� m2 = 6� m2 = 18.85 m2

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Section 5.1 (continued)

Answer to . . .

Figure 9 The parts where theformations are thinnest weathermost rapidly.

5. mechanical weathering—grinding a peachpit in a garbage disposal; chemical weather-ing—dissolving a piece of rock in a pan ofvinegar6. An increased level of carbon dioxideshould result in higher levels of carbonic acid,which reacts with many common minerals.Therefore, the rate of chemical weatheringshould increase.

Section 5.1 Assessment

1. It does not change.2. Unloading is the reduction in pressure thathappens when a mass of rock is exposed. Itcauses the outer layers of the rock to expand,which makes slabs of outer rock separate andbreak loose.3. It transforms them into one or more newcompounds.4. Rock characteristics include cracks andmineral composition; climatic factors includetemperature and moisture.

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