TYPES OF EARTHQUAKE WAVESEarthquake shaking and damage is the result of three basic types of elastic waves. Two of the three propagate within a body of rock. The faster of these body waves is called the primary or P wave. Its motion is the same as that of a sound wave in that, as it spreads out, it alternately pushes (compresses) and pulls (dilates) the rock. These P waves are able to travel through both solid rock, such as granite mountains, and liquid material, such as volcanic magma or the water of the oceans.

The slower wave through the body of rock is called the secondary or S wave. As an S wave propagates, it shears the rock sideways at right angles to the direction of travel. If a liquid is sheared sideways or twisted, it will not spring back, hence S waves cannot propagate in the liquid parts of the earth, such as oceans and lakes.

The actual speed of P and S seismic waves depends on the density and elastic properties of the rocks and soil through which they pass. In most earthquakes, the P waves are felt first. The effect is similar to a sonic boom that bumps and rattles windows. Some seconds later, the S waves arrive with their up-and-down and side-to-side motion, shaking the ground surface vertically and horizontally. This is the wave motion that is so damaging to structures.The third general type of earthquake wave is called a surface wave, reason being is that its motion is restricted to near the ground surface. Such waves correspond to ripples of water that travel across a lake.Surface waves in earthquakes can be divided into two types. The first is called a Love wave. Its motion is essentially that of S waves that have no vertical displacement; it moves the ground from side to side in a horizontal plane but at right angles to the direction of propagation. The horizontal shaking of Love waves is particuly damaging to the foundations of structures.

The second type of surface wave is known as a Rayleigh wave. Like rolling ocean waves, Rayleigh waves wave move both vertically and horizontally in a vertical plane pointed in the direction in which the waves are travelling.

Surface waves travel more slowly than body waves (P and S); and of the two surface waves, Love waves generally travel faster than Rayleigh waves. Love waves (do not propagate through water) can effect surface water only insofar as the sides of lakes and ocean bays pushing water sideways like the sides of a vibrating tank, whereas Rayleigh waves, becasuse of their vertical component of their motion can affect the bodies of water such as lakes.P and S waves have a characteristic which effects shaking: when they move through layers of rock in the crust, they are reflected or refracted at the interfaces between rock types. Whenever either wave is refracted or reflected, some of the energy of one type is converted to waves of the other type. A common example; a P wave travels upwards and strikes the bottom of a layer of alluvium, part of its energy will pass upward through the alluvium as a P wave and part will pass upward as the converted S-wave motion. Noting also that part of the energy will also be reflected back downward as P and S waves.

Plate Tectonic - Earthquakes (5)Pre Lab

OBJECTIVES: Analyzing the type of waves produced by earthquakes. Comparing S and P waves recorded on a seismogram.VOCABULARY: earthquake "event" lithosphere primary wave secondary wave seismic waves seismogramMATERIALS: slinky ropeStudents learn about P and S waves.

Ground rupture caused by an earthquake

BACKGROUND:Earthquakes and volcanoes are evidence for plate tectonics. Earthquakes are caused when energy is released as the lithosphere (crust and upper mantle) of the Earth moves. Energy is emitted in the form of waves. There are different types of waves, some move faster, slower, sideways, or up and down. A seismograph records these waves on a seismogram. When an earthquake is recorded it is called an earthquake "event."There are two types of waves you will discuss with the students, P and S waves. P waves or primary waves, are the first waves that the seismogram records. The P wave is the "fast" wave and can be called a push-pull wave, because it moves by contracting and expanding along a horizontal path. A P-wave travels through a material as a compressional force. For example, when you speak, your voice compresses a volume of air. One of the properties of air (and just about any other material) is that it resists being compressed into a smaller volume. When your voice compresses the air, it resists by pushing against neighboring volumes of air. These volumes then resist compression, and they push back against their neighbors. This generates a wave of compression that travels through all the volumes of air between your mouth and the person hearing you.The second major type of seismic wave is called an S-wave. S-waves are shear waves. S-waves are slower than P-waves. The particle motion in shear waves is perpendicular to the direction of the wave.PROCEDURE:1. Review the divisions of inside the Earth. Earthquakes occur in the upper part of the crust and mantle. Earthquakes release energy in the form of waves.2. Demonstrate P- and S-wave motion to the class. P-waves can be demonstrated with a slinky. Pull the slinky apart and then pull in about 6 coils. Let them go. The wave will oscillate through the slinky, alternately compressing and expanding the coils.3. The S wave can be shown by using a rope attached to a wall. Hold onto the rope and move your wrist up and down. This whipping motion will generate S-waves. The motion will be up and down as the energy goes through the rope. Although you can demonstrate both types of wave with a slinky, we have found that students can distinguish the two types of waves more readily if you use different materials. If you cannot attach a rope to your classroom walls, try this demonstration with two people.4. Draw a P and S wave on the board as illustrated below. Make sure the students understand how to identify them. In addition, explain that the greater the height of the lines on the seismogram, the larger the earthquake. This holds true unless a seismograph is located very close to the epicenter of the earthquake. This causes the wave height to be exaggerated.

[Dictionary][Back to Plate Tectonic Grid][Back to Earthquakes (5)]

Each cell, under a specific grade level contains 3 lesson plans and student worksheets per week. Multimedia activities, web links, and dictionaries can also be found. Lesson plans increase in difficulty through the grades. Each grade level builds knowledge in a logical sequence. When printing directly from the Internet Explorer, the pages have to be set up before printing.Click here for instructions. Printable version and workbooks can be downloaded byclicking here. Printable version of the below Scope and Sequenceclick here.

K123456

Volcanoes(1 week)Volcanoes Produce RocksVolcanoes have Definite ShapesProducts of VolcanoesCreating Rocks from Lava3 Basic Types of VolcanoesVolcanoes produce Different RocksLocation of Volcanoes

Earthquakes(1 week)Shaking during an EarthquakeEarthquakes Release EnergyEarthquake FaultsSeismic Waves cause DamageMeasuring Earthquake IntensitiesWave Movements and SeismogramsDividing the Earth by Waves

Plate Tectonics(1 week)Continents and OceansMoving ContinentsEvidence from ContinentsPressure in the EarthDiverging, Converging, Transform BoundariesCrustal MovementDefinition of Plate Boundaries

Hazards(1 week)Earthquakes and Volcanoes cause DamageVolcanic EruptionsWhere do you go for Help?Historical Damage (Volcanoes)Damage during EarthquakesMudslides and Volcanoes"Earthquake Proof" Structures

OverviewandAcknowledgmentsTo purchase Curriculum Materials, go to the CatalogReturn to ElementaryPlate Tectonics Cycle at a Glance

In the Plate Tectonic Cycle, students learn about the Earth's dynamics as it spins on its axis, revolving around the Sun. The Earth is restless inside, as it tries to cool its interior. Material inside the Earth become viscous and flow in certain areas. Movement within the Earth's interior is reflected on the outside crust. Convection currents inside the mantle (area between the crust and the outer core) create 2 types of crustal movements. When convections currents come together, convergent plate boundaries (earthquakes) are formed on the Earth's crust. When the convection currents pull the crust apart in two different directions divergent plate boundaries (volcanoes and earthquakes) are formed. A consequence of the Earth's surface moving faster along the equator than at the poles creates tension which in part forms transform boundaries.

In the Classroom

Hands-on activities teach students how scientists investigate the Earth through earthquakes and volcanoes. They learn to challenge and think about different theories. Learning about how to cope with the disasters caused by plate tectonics is also emphasized.

LABORATORY 8

EarthquakesMAIN IDEAS Several types of faults occur in the crust.

The faults break due to accumulated stress along the fault. The sudden release of energy is called an earthquake.

The energy is released as seismic waves that travel away from the earthquake location. Two major types of waves are produced: body waves and surface waves.

The waves can be measured by an instrument named a seismometer. The timing and amplitude of the seismic waves can be used to determine the location and magnitude of the earthquake.

Earthquakes commonly occur along plate boundaries. These waves also provide information on the structure of the earth. A clear layering is recognized.INTRODUCTIONIn this lab we will study the three types of faults that can form. Next, we will look at how and earthquake forms along a fault. Then we will see how a seismometer records an earthquake and how the location and magnitude are determined. Finally we will look at the relationship between earthquakes and plate tectonics.FAULTSA fault is a fracture or zone of fractures slong which there has been displacement of the rock on either side of the fracture. Faulting is a basic mechanism by which rocks deform. Faults are generally planar and are classified according to the nature of the movements as observed perpendicular to the plane of the fault. Four common types of faults are shown below. Tensional forces cause normal faulting, whereas compressional forces cause reverse and thrust faulting. Notice how the relative movements along the faults differ and are caused from the different forces.

Click for animationSEISMOLOGYSeismology is the study of earthquakes. The principal tool to measure earthquakes is a seismometer which measures the arrival of earthquake waves. Sudden displacement along a fault (earthquake) will generate different types of waves that travel through the earth and along its surface. These waves are defined by the type of motion of a particle in the path of the wave.The study of seismic waves is an effective means of interpreting the nature of the earth's interior. The velocity of P and S waves generally increases with depth in the earth which causes the waves to bend. The waves also reflect and refract off abrupt discontinuities such as the crust/mantle boundary (Moho). Fluid layers in the earth block S waves and create "shadow zones". TYPES OF SEISMIC WAVESWaves in air, water, and rock transfer energy long distances without moving the constituent particles of these substances very far. For example, an ocean wave can travel across an ocean but each individual water molecule only moves a few meters back and forth. Similarly, a sound wave in air can go tens and hundreds of kilometers but the air molecules themselves only shift a fraction of a millimeter. Equivalent types of waves occur in solid rock as well.

Click for animation (motion not exactly to scale)P waves(or "longitudinal waves") travel through fluids, and solids. They are compression waves and rely on the compressional strength and elasticity of the materials to propagate. They are known as body waves because they travel though the body of a material in all directions and not just at the surface, as water waves do. For P waves, the motion of the meterial particles that transmit the energy move parallel to the direction of propagation. P waves travel the same way as sound waves in air. The transmission of compressional waves is due to the strong electronic between atoms that get squeezed together too tightly. P waves are the fastest seismic waves ^M and travel at roughly 6.0 km/s in the crust (more than seven times the speed of sound).

Click for animation (motion not exactly to scale)S wavesdepends on the shear strength of the material. Imagine a very long and narrow block of Jello, and then imagine shaking the end of it and then imagine shaking the end of it from side to side. A shear wave will propagate down the long length of it. You shake it from side to side but the wave travels forward and perpendicular to the direction of shaking. You can try this with a long spring or a Slinky suspended from strings also.If you give it a sudden sideways deflection and a transverse or shear wave will travel both lengths of the spring. Now try to imagine doing the same thing with water in a tank. No shear wave will propagate because gases and fluids have no shear strength. They give too easily. However, the strength of atomic bonds in solids allows them to transmit tranverse motions. S waves do not travel as fast as P waves and have a velocity of about 3.5 km/s in the crust.

Click for animation (motion not exactly to scale)Surface wavesare very similar to ocean waves as they only occur at the surface of the earth and do not penetrate into the interior deeply. There are two types of surface waves: Love waves and Rayleigh waves. Typically, it the surface waves that do the most damage during an earthquake, especially at distances far from the epicenter. Most of the damage in the 1985 Mexico City earthquake was from surface waves that had traveled over 200 kilometers from the epicenter located near the west coast of Mexico. The velocity of surface waves varies with their wavelength but always travel slower than P and S waves.

An earthquake will generate all of these types of waves and they will propagate over the surface of the earth and through the body of the earth. The waves can be distinguished by the differing velocities and particle motions. Seismometers measure the particle motion produced by these waves.

Click for animation (motion not exactly to scale)

Table 1. Main types of seismic waves.

wave typeparticle motionname

body waveslongitudinalP wave

transverseS wave

surface waveshorizontal transverseLove wave

vertical ellipticalRayleigh wave

LOCATING EARTHQUAKES WITH SEISMIC WAVESAs we have seen above, earthquakes produce all three types of seismic waves: P waves, S waves, and surface waves. Because the different waves travel at different velocities, the time it takes each wave to arrive depends on the distance to the earthquake. (just like thunder and lightning; the farther away the lightning is, the longer it takes the thunder to arrive). If we have a recording of the seismic waves made by a seismometer, we can measure the time between the P and S waves. From that time we can calculate the distance to the earthquake.

Above we see two maps showing the location of a small (magnitude 3.8) earthquake that occurred along the San Jacinto fault northeast of San Diego in 1997. The yellow triangles mark the location of some of the seismic stations that recorded the earthquake. Active faults are marked by red lines. The red dot marks the official earthquake location as calculated by the United States Geological Survey. Below we see the seismograms recorded at different stations for a this earthquake. The seismograms are the yellow sqiggles and show the vertical movement of the earth as measured by a seismometer located at the stations TRO, LVA2, FRD, and RDM. The horizontal axis is time and is marked in hours, minutes, and seconds (1997207 refers to day 207 of 1997). The vertical lines are 5 seconds apart. At TRO, the nearest station, the P wave arrived at just before 3:15. At station LVA2, which is slightly farther from the earthquake than TRO, the P wave arrived a little later, at almost exactly 3:15. The last station to record the earthquake was RDM, which recorded the P wave at 3:15:05. Notice that the gap between the P and S increases with the distance to the earthquake also.

We can measure the time separation between the S and P times to determine the location of the earthquake. (in the case, we already know where the earthquake is, but we can test the method). Below is a table showing the P and S times as measured from the seismograms above. From the P and S times we can calculate the S minus P time (in seconds). By multiplying the S minus P time by a factor of 8, we can get the approximate distance in km between the seismic station and the earthquake. For example, the S minus P time at RDM is 6.2 seconds so the distance to the earthquake is 6.2 times 8, which equals 49.6. If we draw a circle around RDM at a distance of 49.6 km on a map, we can find all possible locations of the earthquake. If we do this for all four stations, we can determine the location of the earthquake (epicenter). The map below shows circles corresponding to the distances in Table 2. All four circles intersect on the red dot. Our location has a slight error because the earthquake actually occurred at a depth of 14 km and not at the surface of the earth. Seismologists use computers to locate earthquakes but the computer programs still use the same method. Table 2. S and P wave arrival times and distance

stationP timeS timeS -P timedistance (km)

TRO3:14:59.23:15:01.92.721.8

LVA23:14:59.93:15:02.93.024.1

FRD3:15:00.73:15:04.43.029.8

RDM3:15:05.03:15:11.26.249.6

MAGNITUDE AND INTENSITYThe size, or magnitude, of an earthquake depends mainly on how large the original fault break is. For example, in the 1906 San Francisco earthquake, the fault rupture was about 200 miles long. In the biggest earthquake ever recorded (in 1960 in Chile), the broken fault was over 800 miles long. For small earthquakes, however, the size of the fault rupture might only be a few hundred feet. Because it is not always easy to measure the size of the fault directly (it might be under the ocean, or very deep), the size of the earthquake is estimated by the amplitude of the seismic waves. This can be done by measuring the P and S waves or the surface waves. One way of doing this was is called the Richter magnitude, after the person who invented it. Magnitude is expressed as number scaled to the size of the earthquake. The intensity of an earthquake measures the amount of shaking that is produced. This depends both on the distance to an earthquake and the magnitude of the earthquake. A nearby small earthquake can produce the same amount of shaking as a more distant large earthquake. The amount of movement also depends on the local geology. Soft sediment and sand tends to amplify seismic waves and create more shaking (and consequently damage). Intensity is measured on the Mercalli intensity scale, which goes from 1 to 12.EARTHQUAKES AND PLATE TECTONICS

Most earthquakes occur along plate boundaries, as the constant movement of the plates causes faults to slip. The map above shows all earthquakes above magnitude 4 recorded in the world for the year 1996. The plate boundaries are shown as thin black lines. Below is a map of Southern California with all earthquakes that occurred between 1996 and 1997. The plate boundary between the North American plate and Pacific plate lies along the San Andreas fault but we can see that considerable earthquake activity occurs along the San Jacinto and Elsinore faults as well. Earthquakes provide a good way to locate plate boundaries.

LAB EXERCISESEarthquake LocationEarthquake Magnitude

HALLWAY DISPLAYIn the hallway of the Chemistry-Geology building are three seismographs showing the seismic signals are three seismic stations in the San Diego area (Palomar mountain, Barrett Dam, and Glamis). These recorders show only the vertical component of the seismic wave that travels past it. The PC map display shows the location of earthquakes that have occurred in Southern California in the past few days.What Causes An Earthquake ?An Earthquake is a sudden tremor or movement of the earth's crust, which originatesnaturallyat or below the surface. The wordnaturalis important here, since it excludes shock waves caused by French nuclear tests, man made explosions and landslides caused by building work.There are two main causes of earthquakes.Firstly, they can be linked to explosive volcanic eruptions; they are in fact very common in areas of volcanic activity where they either proceed or accompany eruptions.Secondly, they can be triggered by Tectonic activity associated with plate margins and faults. The majority of earthquakes world wide are of this type.TerminologyAn earthquake can be likened to the effect observed when a stone is thrown into water. After the stone hits the water a series of concentric waves will move outwards from the center. The same events occur in an earthquake. There is a sudden movement within the crust or mantle, and concentric shock waves move out from that point. Geologists and Geographers call the origin of the earthquake thefocus. Since this is often deep below the surface and difficult to map, the location of the earthquake is often referred to as the point on the Earth surface directly above the focus. This point is called theepicentre.The strength, or magnitude, of the shockwaves determines the extent of the damage caused. Two main scales exist for defining the strength, the Mercalli Scale and the Richter Scale.Earthquakes are three dimensional events, the waves move outwards from the focus, but can travel in both the horizontal and vertical plains. This produces three different types of waves which have their own distinct characteristics and can only move through certain layers within the Earth. Lets take a look at these three forms of shock waves.Types of shockwavesP-WavesPrimary Waves (P-Waves) are identical in character to sound waves. They are high frequency, short-wavelength, longitudinal waves which can pass through both solids and liquids. The ground is forced to move forwards and backwards as it is compressed and decompressed. This produces relatively small displacements of the ground.P Waves can be reflected and refracted, and under certain circumstances can change into S-Waves.Particles are compressed and expanded in the wave's direction.S-WavesSecondary Waves (S-Waves) travel more slowly than P-Waves and arrive at any given pointafterthe P-Waves. Like P-Waves they are high frequency, short-wavelength waves, but instead of being longitudinal they are transverse. They move in all directions away from their source, at speeds which depend upon the density of the rocks through which they are moving. They cannot move through liquids. On the surface of the Earth, S-Waves are responsible for the sideways displacement of walls and fences, leaving them 'S' shaped.S-waves move particles at 90 to the wave's direction.L-WavesSurface Waves (L-Waves) are low frequency transverse vibrations with a long wavelength. They are created close to the epicentre and can only travel through the outer part of the crust. They are responsible for the majority of the building damage caused by earthquakes. This is because L Waves have a motion similar to that of waves in the sea. The ground is made to move in a circular motion, causing it to rise and fall as visible waves move across the ground. Together with secondary effects such as landslides, fires and tsunami these waves account for the loss of approximately 10,000 lives and over $100 million per year.L-waves move particles in a circular path.Tectonic EarthquakesTectonic earthquakes are triggered when the crust becomes subjected to strain, and eventually moves. The theory of plate tectonics explains how the crust of the Earth is made of several plates, large areas of crust which float on the Mantle. Since these plates are free to slowly move, they can either drift towards each other, away from each other or slide past each other. Many of the earthquakes which we feel are located in the areas where plates collide or try to slide past each other.The process which explains these earthquakes, known asElastic Rebound Theorycan be demonstrated with a green twig or branch. Holding both ends, the twig can be slowly bent. As it is bent, energy is built up within it. A point will be reached where the twig suddenly snaps. At this moment the energy within the twig has exceeded theElastic Limitof the twig. As it snaps the energy is released, causing the twig to vibrate and to produce sound waves.Perhaps the most famous example of plates sliding past each other is the San Andreas Fault in California. Here, two plates, the Pacific Plate and the North American Plate, are both moving in a roughly northwesterly direction, but one is moving faster than the other. The San Francisco area is subjected to hundreds of small earthquakes every year as the two plates grind against each other. Occasionally, as in 1989, a much larger movement occurs, triggering a far more violent 'quake'.Major earthquakes are sometimes preceded by a period of changed activity. This might take the form of more frequent minor shocks as the rocks begin to move,calledforeshocks, or a period of less frequent shocks as the two rock masses temporarily 'stick' and become locked together. Detailed surveys in San Francisco have shown that railway lines, fences and other longitudinal features very slowly become deformed as the pressure builds up in the rocks, then become noticeably offset when a movement occurs along the fault. Following the main shock, there may be further movements, calledaftershocks, which occur as the rock masses 'settle down' in their new positions. Such aftershocks cause problems for rescue services, bringing down buildings already weakened by the main earthquake.Volcanic EarthquakesVolcanic earthquakes are far less common than Tectonic ones. They are triggered by the explosive eruption of a volcano. Given that not all volcanoes are prone to violent eruption, and that most are 'quiet' for the majority of the time, it is not surprising to find that they are comparatively rare.When a volcano explodes, it is likely that the associated earthquake effects will be confined to an area 10 to 20 miles around its base, where as a tectonic earthquake may be felt around the globe.The volcanoes which are most likely to explode violently are those which produceacidiclava. Acidic lava cools and sets very quickly upon contact with the air. This tends to chock the volcanic vent and block the further escape of pressure. For example, in the case of Mt Pelee, the lava solidified before it could flow down the sides of the volcano. Instead it formed a spine of solid rock within the volcano vent. The only way in which such a blockage can be removed is by the build up of pressure to the point at which the blockage is literally exploded out of the way. In reality, the weakest part of the volcano will be the part which gives way, sometimes leading to a sideways explosion as in the Mt St.Helens eruption.When extraordinary levels of pressure develop, the resultant explosion can be devastating, producing an earthquake of considerable magnitude. When Krakatoa ( Indonesia, between Java and Sumatra ) exploded in 1883, the explosion was heard over 5000 km away in Australia. The shockwaves produced a series of tsunami ( large sea waves ), one of which was over 36m high; that's the same as four, two story houses stacked on top of each other. These swept over the coastal areas of Java and Sumatra killing over 36,000 people.By contrast, volcanoes producing free flowingbasiclava rarely cause earthquakes. The lava flows freely out of the vent and down the sides of the volcano, releasing pressure evenly and constantly. Since pressure doesn't build up, violent explosions do not occur.

Alfred Lothar Wegener(November 1, 1880 November 1930) was a German polar researcher,geophysicistandmeteorologist.During his lifetime he was primarily known for his achievements in meteorology and as a pioneer of polar research, but today he is most remembered as the originator of the theory ofcontinental driftby hypothesizing in 1912 that thecontinentsare slowly drifting around the Earth (Kontinentalverschiebung). His hypothesis was controversial and not widely accepted until the 1950s, when numerous discoveries such aspalaeomagnetismprovided strong support for continental drift, and thereby a substantial basis for today's model ofplate tectonics.[1][2]Wegener was involved in several expeditions toGreenlandto studypolarair circulation before the existence of thejet streamwas accepted. Expedition participants made many meteorological observations and achieved the first-ever overwintering on the inland Greenland ice sheet as well as the first-ever boring ofice coreson a moving Arctic glacier.

scientus.org Home Science Scientists Telescopes HistoryWegener and Continental Drift TheoryWe are taught that modern scientists are driven only by reason and facts. It was only early scientists like Galileo who needed to fear the reaction to their radical views. Neither of these beliefs is true. The reaction to Alfred Wegener's Continental Drift Theory demonstrates that new ideas threaten the establishment, regardless of the century.Alfred Wegener was the scientist who proposed the Continental Drift Theory in the early twentieth century. Simply put, his hypothesis proposed that the continents had once been joined, and over time had drifted apart. The jigsaw fit that the continents make with each other can be seen looking at the map of soil types below (derived fromUniversity of Idaho). South America can be dragged and rotated (rotating is tricky by touch) so you can try to see how well it joins with Africa.Wegener and his CriticsSince his ideas challenged scientists in geology, geophysics, zoogeography and paleontology, it demonstrates the reactions of different communities of scientists. These reactions eventually shut down serious discussion of the concept. The geologist Barry Willis summed it up best:further discussion of it merely incumbers the literature and befogs the mind of fellow students.The students' minds would not be befogged. The world had to wait until the 1960's for a wide discussion of the Continental Drift Theory to be restarted.Why the extreme reaction? Wegener did not even present Continental Drift as a proven theory. He knew he would need more support to convince others. His immediate goal was to have the concept openly discussed. These modest goals did not spare him. His work crossed disciplines. The authorities in the various disciplines attacked him as an amateur that did not fully grasp their own subject. More importantly however, was that even the possibility of Continental Drift was a huge threat to the authorities in each of the disciplines.Radical viewpoints threaten the authorities in a discipline. Authorities are expert in thecurrentview of their discipline. A radical view could even force experts to start over again. One of Alfred Wegener's critics, the geologist R. Thomas Chamberlain, suggested just that :"If we are to believe in Wegener's hypothesis we must forget everything which has been learned in the past 70 years and start all over again."He was right.Continental Drift Theory:Building the CaseIn spite of all the criticism, Wegener was able to keep Continental Drift part of the discussion until his death. He knew that any argument based simply on the jigsaw fit of the continents could easily be explained away. To strengthen his case he drew from the fields of geology, geography, biology and paleontology. Wegener questioned why coal deposits, commonly associated with tropical climates, would be found near the North Pole and why the plains of Africa would show evidence of glaciation. Wegener also presented examples where fossils of exactly the same prehistoric species were distributed where you would expect them to be if there had been Continental Drift (e.g. one species occurred in western Africa and South America, and another in Antartica, India and central Africa)[_1_]. The graphic below shows the striking distribution of fossils on the different continents.

Wegener used an Alexander duToit graphic to demonstrate the uncanny match of geology between eastern South America and western Africa.

Continental Drift Theory:The Fatal FlawThe picture painted of Alfred Wegener's contemporaries might not be fair. One would expect scientists to resist ideas that challenged their life's work. It doesn't explain all of the criticism. There were alternatives. To explain the unusual distribution of fossils in the Southern Hemisphere some scientists proposed there may once have been a network of land bridges between the different continents. To explain the existence of fossils of temperate species being found in arctic regions, the existence of warm water currents was proposed. Modern scientists would look at these explanations as even less credible than those proposed by Wegener, but they did help to preserve the steady state theory.New theories often have rough edges. Wegener did not have an explanation for how continental drift could have occurred. He proposed two different mechanisms for this drift. One was based on the centrifugal force caused by the rotation of the earth and another a 'tidal argument' based on the tidal attraction of the sun and the moon. These explanations could easily be proven inadequate. They opened Wegener to ridicule because they were orders of magnitude too weak. Wegener really did not believe that he had the explanation for the mechanism, but that this should not stop discussion of a hypothesis. Wegener's contemporaries disagreed. A major conference was held by the American Association of Petroleum Geologists in 1926 that was critical of the theory. Alfred Wegener died a few years later. With his death, the Continental Drift Theory was quietly swept under the rug. The existing theories of continent formation were allowed to survive, with little challenge until the 1960's.Wegener and DarwinThe main problem with Wegener's hypothesis of Continental Drift was the lack of a mechanism. He did not have an explanation for how the continents moved. Did this justify the strong reactions to his work? Charles Darwin was missing a mechanism for the inheritance of beneficial traits when he published theOrigin of Speciesin 1859. Darwin had amassed a huge amount of evidence that supported some type of adaptive process that contributed to the evolution of new species. He argued that with the natural variations that occur in populations, any trait that is beneficial would make that individual more likely to survive and pass on the trait to the next generation. If enough of theseselectionsoccured on different beneficial traits you could end up with completely new species. But he did not have a mechanism for how the traits could be preserved over the succeeding generations. The dominant theory of inheritance at the time was that the traits of the parents wereblendedin the offspring. But this would mean that any beneficial trait would be diluted out of the population within a few generations. This is because most of the blending over the next generations would be with individuals that did not have the trait.The lack of a mechanism to preserve traits didn't seem a problem. Within 5 years, Oxford University was teaching Darwin's theory as fact. The Oxford texts stated, "Though evidence might be required to show that natural selection accounts for everything ascribed to it, yet no evidence is required to show that natural selection has always been going on, is going on now, and must ever continue to go on. Recognizing this is ana prioricertainty, let us contemplate it under its two distinct aspects." At Oxford, evolution by natural selection had gone from hypothesis toa prioricertainty in the space of 5 years. Many in the scientific community simply chose to ignore the lack of mechanism. Wegener had no such luck with his own theory.[_2_].The mechanism of inheritance was explained shortly after the Origin of Species was published. It was ignored. In 1865, an obscure Augustinian monk from Moldavia presented a paper to the Natural History Society of Brunn where he discussed the results of experiments on pea plants. The results presented by this monk, Gregor Mendel, pointed to traits being inherited 'whole' (also known as particulate inheritance), and that certain traits (recessive traits) that disappear in one generation can reappear in a following generation (seeMendel and Evolution). This would have gone a long way in plugging at least one hole in the Darwin's theory. Mendel's work was largely ignored until about 1900. Shortly afterward it was incorporated into our modern view of evolution known as the 'modern synthesis'.Darwin's theory had another problem. His theory proposed a gradual evolution through successive generations. The fossil record didn't co-operate. There seemed to be a 'explosion' of different life-forms over a relatively short time span (in geologic terms) in the early Cambrian period. There also didn't seem to be any transitional forms of life preceding these species. This eventually became known as theCambrian Explosion. Darwin himself recognized this as a serious issue with his theory and he discussed it in theOrigin of Species. Darwin explained away the problem as a problem with the fossil record and not with his theory. Over the course of the twentieth century, a much better picture of the fossil record of both the Cambrian and Pre-Cambrian eras was developed. The new discoveries made the problem worse. Much worse. In the early twentieth century, the American paleontologist, Charles Walcott, discovered and excavated the Burgess Shale in British Columbia, Canada. He found 65,000 more specimens of early Cambrian life, many of which were complex multi-celled animals. At the time there still was no evidence of transitional forms in the pre-Cambrian. Only recently have they started discovering isolated examples of moderately complex multi-celled animals from the Pre-Cambrian. This still doesn't explain the step-change in the diversity of life-forms in the Cambrian.Wegener and GalileoWegener also shares much in common with Galileo. Wegener probably had at least as strong a case for Continental Drift in 1929 as Galileo had for the Copernican model in 1633. Galileo's problems over the Copernican Model are usually presented as a conflict with the Church, and not a conflict with other scientists (seeGalileo's Battle for the Heavens). Most discussions do not even mention the main problems associated with the Copernican model. It was a scientific controversy with many parallels to the Continental Drift controversy.Galileo had his own 'tidal argument' ; one that was even more embarassing than Wegener's. Galileo argued that the tides were caused by the sun. How could a great scientist who had spent his youth less than 20 kilometres from the sea be so wrong about tides! He presented an argument for Copernicism based on there only being 1 tide per day and where the tides cycle over the year and not over a month. While it took a noted geologist to show that Wegener's tidal argument was ridiculous, Galileo's tidal argument could be proven wrong by anyone living near the sea.The tidal argument wasn't the only problem with Galileo's defense of Copernicism. Wegener's critics never presented strong arguments that Continental Drift couldn't have happened. They did show that themechanismthat Wegener suggested was driving Continental Drift was inadequate. The scientists of Galileo's day did have scientifically valid reasons to doubt a moving earth. A moving earth required that a phenomenon known asstellar parallax(seeCopernicism and Stellar Parallax) would be observed . No one in Galileo's day or for two centuries after his death was able to observe this phenomenon.Another argument against Copernicism was very simple; the data didn't support it! By the time of the Galileo Affair, there was 80 years of data comparing the performance of Copernican-based tables (Prutenic) and Ptolemaic-based tables (Alphonsine). In 1551, only 8 years after Copernicus's death, the Prutenic tables were developed from the Copernican model to predict the positions of stars and planets. It didn't seem that one was much better than the other. A reasonable conclusion based on this experience is that if the Ptolemaic was wrong, then the Copernican was not right. Today we know these scientists were right to doubt the performance of the Copernican model. Modern statistical analyses shows little difference in performance between the two models[_3_]. It is the Keplerian system of planetary motion that is taught in schools, not the Copernican or the Ptolemaic. Galileo knew of Kepler's model and had never accepted it during his lifetime.Winners, Losers, Insiders, OutsidersWhy was one theory above was quickly accepted, another quickly dismissed, and the other a cause of controversy amongst scientists. Analysis from a strictly scientific basis won't help. All of the theories had strengths and weaknesses. We might have to look beyond the world of ideas to the world of people, events and things.Darwin, was the ultimate insider in English scientific circles. His grandfather, Erasmus, was an early student of evolution and his half-cousin, Francis Galton, was a noted statistician who was considered the father of eugenics. Being part of the Wedgewood-Darwin clan meant having no worries about money and established connections in the scientific world. When evolution by natural selection was under attack, Darwin could enlist the efforts of a Who's Who of mid-nineteenth century English science. The most famous of the early defenses of Darwinism was not by Darwin himself but by the famous biologist, Thomas Huxley and the social philosopher, Herbert Spencer. Darwin's ideas were adopted by supporters of laissez-faire capitalism. "Survival of the fittest" gave an ethical dimension to the no-holds barred capitalism of the late nineteenth century. Andrew Carnegie, the fabulously rich robber baron, used elements of evolution by natural selection to justify his own ruthless business practices.Alfred Wegener wasn't an insider. He had to earn all his allies. His few allies (duToit and Holmes) were no match for his many skeptics. His place of birth may have played a role. Anti-German bias was very strong in the 1910's and 1920's in English-speaking countries. This resulted in German-based names for cities, streets, foods and animal breeds being changed to names that were more 'patriotic'. Being German wasn't Wegener's only problem; the arguments he used to support his hypothesis crossed into disciplines that were not his specialty. He was trained as an astronomer and worked as a meterologist. He was considered an outsider for a reason.The early history of the Copernican model is an example of the effect of outside forces. The publication of Copernicus'de Revolutionibusdrew very little criticism from the Catholic countries. The most serious early criticisms came from the Protestant countries in Europe. The Vatican's interest in the publication had begun 10 years earlier, after a series of lectures given to Pope Clement VII on Copernicus's work. Any doubt of the church's support for Copernicus' work ended with the actual publication. The original publication included a copy of the letter from the Vatican urging him to share his work, a dedication to the pope, and a thank you to a bishop who was an important supporter of his work. The involvement of the church may have muted criticism from academics in the Catholic countries of Europe and encouraged criticism in the Protestant countries. The reverse happened after Galileo's trial in 1633. Galileo was tried for not obeying an order from 1616 to not teach the Copernican theory as true but only as a hypothesis. He was placed under house arrest in his villa in the Tuscan hills just outside Florence (seeGalileo's Battle for the Heavens).Science:A Question of FaithScience depends on facts. It also depends on reason. But fact and reason alone cannot explain how science works. The examples chosen all had some compelling support and serious shortcomings. Part of the answer may lie in the sociology of groups. Another part lies in simple faith: faith that future scientists will address a theory's shortcomings. Darwin needed an explanation for the Cambrian Explosion and a mechanism for the preservation of traits (seeMendel and Darwin) . Wegener needed a mechanism for Continental Drift. Galileo needed an explanation for the lack of stellar parallax and the poor performance of his model (seeGalileo's Battle for the Heavens) . It is not only the community that requires faith. The champions of these new theories require faith in their ideas, even when facts contradict their hypotheses. In each case above, there were facts which when combined with the current assumptions of the time clearly contradicted their hypotheses. None of these scientists let those facts get in the way. Paul Feyerabend, a modern philosopher of science, presents a similar view, where he argues that science is sometimes required to work "against the facts". His key example was how the heliocentric system made less sense than a geocentric system during Galileo's time. One irony missed by discussions of science and religion is how much both depend on faith.

Top of FormCopyright Joseph Sant (2014).

Cite this page (APA).

Bottom of Form

1. Jordan, R.G, Florida Atlantic University,The Newton Project, http://courses.science.... ,This page provides a good summary of Wegeners problems with the noted scientists of his time. It also details some of the arguments he used to support his hypothesis...back

2. Spencer, Herbert, Williams and Norgate,The Principles of Biology, ,The textbook mentioned 'natural selection' no less than 25 times. Herbert Spencer, the author, had been an important defender of Darwin when Origin of Species was first published. Principles of Biology was the biology text at University of Oxford between 1864 and 1867...back

3. Babb, Stanley E.,, Isis, Sept. 1977,Accuracy of Planetary Theories, Particularly for Mars,, , pp. 426-34In this article Stanley Babb compares the predictions of the Copernican and Ptolemaic models against the actual planetary positions using computer-based statistical analysis. The results did not show much difference between the two systems, but the earth-centred system (the Ptolemaic) did perform better for planets such as Mars...back

Reference:What is Plate Tectonics?by Becky Oskin, Senior Writer | December 04, 2014 06:04pm ET551314212Submit206Reddit

Tectonic plates of the Earth.Credit: USGSView full size image

From the deepest ocean trench to the tallest mountain, plate tectonics explains the features and movement of Earth's surface in the present and the past.Plate tectonics is the theory that Earth's outer shell is divided into several plates that glide over the mantle, the rocky inner layer above the core. The plates act like a hard and rigid shell compared toEarth's mantle. This strong outer layer is called the lithosphere.Developed from the 1950s through the 1970s, plate tectonics is the modern version ofcontinental drift, a theory first proposed by scientist Alfred Wegener in 1912. Wegener didn't have an explanation for how continents could move around the planet, but researchers do now. Plate tectonics is the unifying theory of geology, said Nicholas van der Elst, a seismologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New YorkBefore plate tectonics, people had to come up with explanations of the geologic features in their region that were unique to that particular region," Van der Elst said. "Plate tectonics unified all these descriptions and said that you should be able to describe all geologic features as though driven by the relative motion of these tectonic plates."The driving force behind plate tectonics is convection in the mantle. Hot material near the Earth's core rises, and colder mantle rock sinks. "It's kind of like a pot boiling on a stove," Van der Elst said. The convection drive plates tectonics through a combination of pushing and spreading apart atmid-ocean ridgesand pulling and sinking downward at subduction zones, researchers think. Scientists continue to study and debate the mechanisms that move the plates.Mid-ocean ridges are gaps between tectonic plates that mantle the Earth like seams on a baseball. Hot magma wells up at the ridges, forming new ocean crust and shoving the plates apart. Atsubduction zones, two tectonic plates meet and one slides beneath the other back into the mantle, the layer underneath the crust. The cold, sinking plate pulls the crust behind it downward.Many spectacular volcanoes are found along subduction zones, such as the "Ring of Fire" that surrounds the Pacific Ocean.Plate boundariesSubduction zones, or convergent margins, are one of the three types of plate boundaries. The others are divergent and transform margins.At a divergent margin, two plates are spreading apart, as at seafloor-spreading ridges or continental rift zones such as the East Africa Rift.Transform margins mark slip-sliding plates, such as California'sSan Andreas Fault, where the North America and Pacific plates grind past each other with a mostly horizontal motion.

This artist's cross-section illustrates the main types of plate boundaries.Credit: USGS/Jos F. Vigil from This Dynamic PlanetView full size imageReconstructing the pastWhile the Earth is 4.54 billion years old, because oceanic crust is constantly recycled at subduction zones, the oldest seafloor is only about 200 million years old. The oldest ocean rocks are found in the northwestern Pacific Ocean and the eastern Mediterranean Sea. Fragments of continental crust are much older, with large chunks at least 3.8 billion years found in Greenland.With clues left behind in rocks and fossils, geoscientists can reconstruct the past history of Earth's continents. Most researchers think modernplate tectonics began about 3 billion years ago, based on ancient magmas and minerals preserved in rocks from that period."We don't really know when plate tectonics as it looks today got started, but we do know that we have continental crust that was likely scraped off a down-going slab [a tectonic plate in a subduction zone] that is 3.8 billion years old," Van der Elst said. "We could guess that means plate tectonics was operating, but it might havelooked very different from today."As the continents jostle around the Earth, they occasionally come together to form giantsupercontinents, a single landmass. One of the earliest big supercontinents, called Rodinia, assembled about 1 billion years ago. Its breakup is linked to a global glaciation called Snowball Earth.A more recent supercontinent called Pangaea formed about 300 million years ago. Africa, South America, North America and Europe nestled closely together, leaving a characteristic pattern of fossils and rocks for geologists to decipher once Pangaea broke apart. The puzzle pieces left behind by Pangaea, from fossils to the matching shorelines along the Atlantic Ocean, provided the first hints that the Earth's continents move.Follow Becky Oskin@beckyoskin. Follow LiveScience@livescience,Facebook&Google+.Editor's Recommendations 50 Interesting Facts About The Earth Have There Always B1880 Wegener born, Berlin Germany1889 Mantovani:Expanding Earth/Drift Theory1908 Taylor:Crust moved by tidal forces.1912 Wegener presents Drift Theory1915 Wegener publishes Drift Theory1926 AAPG Conference attacks Drift Theory1930 Wegener dies in Greenland.1937 DuToit:On Wandering Continents1956 Paleomagnetic support for Drift Theory.1959 Discovery of sea floor spreading.1960 Tuzo Wilson:Drift Theory revived.1965 Plate Tectonics

People used to think that the Earth was static, and that it never changed. Gradually, a body of evidence was gathered that made no sense in this model. Alfred Wegener, Geologic Supersleuth, laid the groundwork for a whole new theory for the large-scale changing nature of the earth.Background on the GroundHave you ever had the experience where you see a younger relative or friend after not seeing them for a few years, and you're taken aback at how much they have grown? We have an image in our minds as to what they looked like the last time we saw them, and they are much different. If the continents of the earth move and grow, why don't we notice that? Well, for two reasons. One, we really don't see what whole continents look like in real time, and two, they move so slowly that people die before any noticeable changes can take place.As far back as 1620, people often noticed that the coastlines of North and South America looked like they fit together with Europe and Africa. These observers noticed these coastlines but had no easy explanation for how that could have occurred since everyone believed the continents were stationary. Solving this mystery would take the work of a geologic supersleuth.

Map showing the coastlines of South America and Africa

Alfred Wegener, the Geologic SupersleuthAlfred Wegener, a German meteorologist, was the first to begin to work out details to explain this interesting observation. To begin with, the current geologic theory was that the crust was all stationary, and continents were relatively unchanging - they didn't move around.Alfred Wegener knew that other people had made observations of the fit of the coastlines. He accidentally became drawn in to that topic by discovering evidence that might explain that phenomenon.Fossil EvidenceIn the fall of 1911, though, he came across a scientific paper that described the locations of identical plant and animal fossils on very different continents.These fossils includedmesosaurus, which was a freshwater reptile,lystrosaurus, a land reptile,cynognathus, a land reptile, andglossopteris, which was a tropical fern.These were on very different continents, and he wondered how could these same plants and animals be on such different land masses? How could they have migrated over such vast distances or survived in such harsh conditions? The current theories were that the continents were connected by land bridges that have since eroded away or by stepping-stone islands. (Stepping-stone islands would be a series of islands that traversed the ocean.)Wegener developed a much simpler hypothesis that stated perhaps the continents were all together at one point, thereby also explaining the fit of the coastline.

Photo of Alfred Wegener and an illustration of the supercontinent, Pangea

He then theorized a supercontinent he namedPangea, which meant 'one earth'. He realized, though, that if this idea were to be accepted, he would need much more supporting data than he had (just fossils and fits of coastlines).Other CluesOther clues came from more research. He discovered that in his Pangea model, largegeologic featuressuch asmountain rangeson separate continents often

Plate tectonicsFrom Wikipedia, the free encyclopedia

The tectonic plates of the world were mapped in the second half of the 20th century.

Remnants of theFarallon Plate, deep in Earth's mantle. It is thought that much of the plate initially went under North America (particularly the western United States and southwest Canada) at a very shallow angle, creating much of the mountainous terrain in the area (particularly the southernRocky Mountains).Key topics on

Geology

Grand Canyon

Overview[show]

Dating methods[show]

omposition and structure[show]

Historical geology[show]

Motion[show]

Hydrogeology[show]

Geophysics[show]

History of geologic science[show]

Earth Sciences PortalCategoryRelated topics

v t e

Plate tectonics(from theLate Latintectonicus, from theGreek:"pertaining to building")[1]is ascientific theorythat describes the large-scale motion ofEarth'slithosphere. This theoretical model builds on the concept ofcontinental driftwhich was developed during the first few decades of the 20th century. Thegeoscientificcommunity accepted the theory after the concepts ofseafloor spreadingwere later developed in the late 1950s and early 1960s.The lithosphere, which is the rigid outermost shell of a planet (on Earth, the crust and upper mantle), is broken up intotectonic plates. On Earth, there are seven or eight major plates (depending on how they are defined) and many minor plates. Where plates meet, their relative motion determines the type of boundary;convergent,divergent, ortransform.Earthquakes,volcanic activity,mountain-building, andoceanic trenchformation occur along these plate boundaries. The lateral relative movement of the plates typically varies from zero to 100mm annually.[2]Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind ofcrust. Along convergent boundaries,subductioncarries plates into themantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the globe remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories (that still have some supporters) propose gradual shrinking (contraction) or gradual expansion of the globe.[3]Tectonic plates are able to move because the Earth's lithosphere has greater strength than the underlyingasthenosphere. Lateral density variations in the mantle result inconvection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from the spreading ridge (due to variations in topography and density of the crust, which result indifferences in gravitational forces) anddrag, with downwardsuction, at the subduction zones. Another explanation lies in the different forces generated by the rotation of the globe and the tidal forces of theSunandMoon. The relative importance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.Contents[hide] 1Key principles 2Types of plate boundaries 3Driving forces of plate motion 3.1Driving forces related to mantle dynamics 3.2Driving forces related to gravity 3.3Driving forces related to Earth rotation 3.4Relative significance of each driving force mechanism 4Development of the theory 4.1Summary 4.2Continental drift 4.3Floating continents, paleomagnetism, and seismicity zones 4.4Mid-oceanic ridge spreading and convection 4.5Magnetic striping 4.6Definition and refining of the theory 5Implications for biogeography 6Plate reconstruction 6.1Defining plate boundaries 6.2Past plate motions 6.3Formation and break-up of continents 6.4Gallery of past configurations 7Current plates 8Other celestial bodies (planets, moons) 8.1Venus 8.2Mars 8.3Galilean satellites of Jupiter 8.4Titan, moon of Saturn 8.5Exoplanets 9See also 10References 10.1Notes 10.2Cited books 10.3Cited articles 11External links 11.1VideosKey principlesTheouter layers of the Earthare divided into thelithosphereandasthenosphere. This is based on differences inmechanical propertiesand in the method forthe transfer of heat. Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat byconduction, whereas the asthenosphere also transfers heat byconvectionand has a nearlyadiabatictemperature gradient. This division should not be confused with thechemicalsubdivision of these same layers into the mantle (comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure.The key principle of plate tectonics is that the lithosphere exists as separate and distincttectonic plates, which ride on the fluid-like (visco-elasticsolid) asthenosphere. Plate motions range up to a typical 1040mm/year (Mid-Atlantic Ridge; about as fast asfingernailsgrow), to about 160mm/year (Nazca Plate; about as fast ashairgrows).[4]The driving mechanism behind this movement is described below.Tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal material:oceanic crust(in older texts calledsimafromsiliconandmagnesium) andcontinental crust(sialfrom silicon andaluminium). Average oceanic lithosphere is typically 100km (62mi) thick;[5]its thickness is a function of its age: as time passes, it conductively cools and subjacent cooling mantle is added to its base. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed. For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6km (4mi) thick at mid-ocean ridges to greater than 100km (62mi) atsubductionzones; for shorter or longer distances, the subduction zone (and therefore also the mean) thickness becomes smaller or larger, respectively.[6]Continental lithosphere is typically ~200km thick, though this varies considerably between basins, mountain ranges, and stablecratonicinteriors of continents. The two types of crust also differ in thickness, with continental crust being considerably thicker than oceanic (35km vs. 6km).[7]The location where two plates meet is called aplate boundary. Plate boundaries are commonly associated with geological events such asearthquakesand the creation of topographic features such asmountains,volcanoes,mid-ocean ridges, andoceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate'sRing of Firebeing the most active and widely known today. These boundaries are discussed in further detail below. Some volcanoes occur in the interiors of plates, and these have been variously attributed to internal plate deformation[8]and to mantle plumes.As explained above, tectonic plates may include continental crust or oceanic crust, and most plates contain both. For example, theAfrican Plateincludes the continent and parts of the floor of theAtlanticandIndianOceans. The distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers, and continental crust is formed througharc volcanismandaccretionofterranesthrough tectonic processes, though some of these terranes may containophiolitesequences, which are pieces of oceanic crust considered to be part of the continent when they exit the standard cycle of formation and spreading centers and subduction beneath continents. Oceanic crust is also denser than continental crust owing to their different compositions. Oceanic crust is denser because it has less silicon and more heavier elements ("mafic") than continental crust ("felsic").[9]As a result of this density stratification, oceanic crust generally lies belowsea level(for example most of thePacific Plate), while continental crust buoyantly projects above sea level (see the pageisostasyfor explanation of this principle).Types of plate boundariesMain article:List of tectonic plate interactionsThree types of plate boundaries exist,[10]with a fourth, mixed type, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are:[11][12]1. Transform boundaries(Conservative)occur where two lithospheric plates slide, or perhaps more accurately, grind past each other alongtransform faults, where plates are neither created nor destroyed. The relative motion of the two plates is eithersinistral(left side toward the observer) ordextral(right side toward the observer). Transform faults occur across a spreading center. Strong earthquakes can occur along a fault. TheSan Andreas Faultin California is an example of a transform boundary exhibiting dextral motion.2. Divergent boundaries(Constructive)occur where two plates slide apart from each other. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new ocean basin. As the continent splits, the ridge forms at the spreading center, the ocean basin expands, and finally, the plate area increases causing many small volcanoes and/or shallow earthquakes. At zones of continent-to-continent rifting, divergent boundaries may cause new ocean basin to form as the continent splits, spreads, the central rift collapses, and ocean fills the basin. Active zones of Mid-ocean ridges (e.g.,Mid-Atlantic RidgeandEast Pacific Rise), and continent-to-continent rifting (such as Africa'sEast African Riftand Valley, Red Sea) are examples of divergent boundaries.3. Convergent boundaries(Destructive)(oractive margins) occur where two plates slide toward each other to form either asubductionzone (one plate moving underneath the other) or acontinental collision. At zones of ocean-to-continent subduction (e.g., Western South America, and Cascade Mountains in Western United States), the dense oceanic lithosphere plunges beneath the less dense continent. Earthquakes then trace the path of the downward-moving plate as it descends into asthenosphere, a trench forms, and as the subducted plate partially melts, magma rises to form continental volcanoes. At zones of ocean-to-ocean subduction (e.g., theAndesmountain range in South America,Aleutian islands,Mariana islands, and theJapaneseisland arc), older, cooler, denser crust slips beneath less dense crust. This causes earthquakes and a deep trench to form in an arc shape. The upper mantle of the subducted plate then heats and magma rises to form curving chains of volcanic islands. Deep marine trenches are typically associated with subduction zones, and the basins that develop along the active boundary are often called "foreland basins". The subductingslabcontains manyhydrousminerals which release their water on heating. This water then causes the mantle to melt, producing volcanism. Closure of ocean basins can occur at continent-to-continent boundaries (e.g., Himalayas and Alps): collision between masses of granitic continental lithosphere; neither mass is subducted; plate edges are compressed, folded, uplifted.4. Plate boundary zonesoccur where the effects of the interactions are unclear, and the boundaries, usually occurring along a broad belt, are not well defined and may show various types of movements in different episodes.

Three types of plate boundary.Driving forces of plate motion

Plate motion based on Global Positioning System (GPS) satellite data from NASAJPL. The vectors show direction and magnitude of motion.Plate tectonics is basically a kinematic phenomenon. Scientists agree on the observation and deduction that the plates have moved with respect to one another but continue to debate as to how and when. A major question remains as to what geodynamic mechanism motors plate movement. Here, science diverges in different theories.It is generally accepted that tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere.Dissipation of heat from the mantleis acknowledged to be the original source of the energy required to drive plate tectonics through convection or large scale upwelling and doming. The current view, though still a matter of some debate, asserts that as a consequence, a powerful source of plate motion is generated due to the excess density of the oceanic lithosphere sinking in subduction zones. When the new crust forms at mid-ocean ridges, this oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes denser with age as it conductively cools and thickens. The greaterdensityof old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate movement. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.[13]Although subduction is believed to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted. The same is true for the enormousEurasian Plate. The sources of plate motion are a matter of intensive research and discussion among scientists. One of the main points is that the kinematic pattern of the movement itself should be separated clearly from the possible geodynamic mechanism that is invoked as the driving force of the observed movement, as some patterns may be explained by more than one mechanism.[14]In short, the driving forces advocated at the moment can be divided into three categories based on the relationship to the movement: mantle dynamics related, gravity related (mostly secondary forces), and Earth rotation related.Driving forces related to mantle dynamicsMain article:Mantle convectionFor much of the last quarter century, the leading theory of the driving force behind tectonic plate motions envisaged large scale convection currents in the upper mantle which are transmitted through the asthenosphere. This theory was launched byArthur Holmesand some forerunners in the 1930s[15]and was immediately recognized as the solution for the acceptance of the theory as originally discussed in the papers ofAlfred Wegenerin the early years of the century. However, despite its acceptance, it was long debated in the scientific community because the leading ("fixist") theory still envisaged a static Earth without moving continents up until the major breakthroughs of the early sixties.Two- and three-dimensional imaging of Earth's interior (seismic tomography) shows a varying lateral density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this varying lateral density ismantle convectionfrom buoyancy forces.[16]How mantle convection directly and indirectly relates to plate motion is a matter of ongoing study and discussion in geodynamics. Somehow, thisenergymust be transferred to the lithosphere for tectonic plates to move. There are essentially two types of forces that are thought to influence plate motion:frictionandgravity. Basal drag (friction): Plate motion driven by friction between the convection currents in the asthenosphere and the more rigid overlying lithosphere. Slab suction (gravity): Plate motion driven by local convection currents that exert a downward pull on plates in subduction zones at ocean trenches.Slab suctionmay occur in a geodynamic setting where basal tractions continue to act on the plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab).Lately, the convection theory has been much debated as modern techniques based on 3D seismic tomography still fail to recognize these predicted large scale convection cells. Therefore, alternative views have been proposed:In the theory ofplume tectonicsdeveloped during the 1990s, a modified concept of mantle convection currents is used. It asserts that super plumes rise from the deeper mantle and are the drivers or substitutes of the major convection cells. These ideas, which find their roots in the early 1930s with the so-called "fixistic" ideas of the European and Russian Earth Science Schools, find resonance in the modern theories which envisagehot spots/mantle plumeswhich remain fixed and are overridden by oceanic and continental lithosphere plates over time and leave their traces in the geological record (though these phenomena are not invoked as real driving mechanisms, but rather as modulators). Modern theories that continue building on the older mantle doming concepts and see plate movements as a secondary phenomena are beyond the scope of this page and are discussed elsewhere (for example on the plume tectonics page).Another theory is that the mantle flows neither in cells nor large plumes but rather as a series of channels just below the Earth's crust, which then provide basal friction to the lithosphere. This theory, called "surge tectonics", became quite popular in geophysics and geodynamics during the 1980s and 1990s.[17]Driving forces related to gravityForces related to gravity are usually invoked as secondary phenomena within the framework of a more general driving mechanism such as the various forms of mantle dynamics described above.Gravitational sliding away from a spreading ridge: According to many authors, plate motion is driven by the higher elevation of plates at ocean ridges.[18]As oceanic lithosphere is formed at spreading ridges from hot mantle material, it gradually cools and thickens with age (and thus adds distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with increased distance from the ridge axis.This force is regarded as a secondary force and is often referred to as "ridge push". This is a misnomer as nothing is "pushing" horizontally and tensional features are dominant along ridges. It is more accurate to refer to this mechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the most prominent feature. Other mechanisms generating this gravitational secondary force include flexural bulging of the lithosphere before it dives underneath an adjacent plate which produces a clear topographical feature that can offset, or at least affect, the influence of topographical ocean ridges, andmantle plumesand hot spots, which are postulated to impinge on the underside of tectonic plates.Slab-pull: Current scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly cause motion by friction along the base of the lithosphere.Slab pullis therefore most widely thought to be the greatest force acting on the plates. In this current understanding, plate motion is mostly driven by the weight of cold, dense plates sinking into the mantle at trenches.[19]Recent models indicate thattrench suctionplays an important role as well. However, as theNorth American Plateis nowhere being subducted, yet it is in motion presents a problem. The same holds for the African,Eurasian, andAntarcticplates.Gravitational sliding away from mantle doming: According to older theories, one of the driving mechanisms of the plates is the existence of large scale asthenosphere/mantle domes which cause the gravitational sliding of lithosphere plates away from them. This gravitational sliding represents a secondary phenomenon of this basically vertically oriented mechanism. This can act on various scales, from the small scale of one island arc up to the larger scale of an entire ocean basin.[20]Driving forces related to Earth rotationAlfred Wegener, being ameteorologist, had proposedtidal forcesand pole flight force as the main driving mechanisms behindcontinental drift; however, these forces were considered far too small to cause continental motion as the concept then was of continents plowing through oceanic crust.[21]Therefore, Wegener later changed his position and asserted that convection currents are the main driving force of plate tectonics in the last edition of his book in 1929.However, in the plate tectonics context (accepted since theseafloor spreadingproposals of Heezen, Hess, Dietz, Morley, Vine, and Matthews (see below) during the early 1960s), oceanic crust is suggested to be in motionwiththe continents which caused the proposals related to Earth rotation to be reconsidered. In more recent literature, these driving forces are:1. Tidal drag due to the gravitational force theMoon(and theSun) exerts on the crust of theEarth[22]2. Shear strain of the Earth globe due to N-S compression related to its rotation and modulations;3. Pole flight force: equatorial drift due to rotation and centrifugal effects: tendency of the plates to move from the poles to the equator ("Polflucht");4. TheCoriolis effectacting on plates when they move around the globe;5. Global deformation of thegeoiddue to small displacements of rotational pole with respect to the Earth's crust;6. Other smaller deformation effects of the crust due to wobbles and spin movements of the Earth rotation on a smaller time scale.For these mechanisms to be overall valid, systematic relationships should exist all over the globe between the orientation and kinematics of deformation and the geographicallatitudinalandlongitudinalgrid of the Earth itself. Ironically, these systematic relations studies in the second half of the nineteenth century and the first half of the twentieth century underline exactly the opposite: that the plates had not moved in time, that the deformation grid was fixed with respect to the Earthequatorand axis, and that gravitational driving forces were generally acting vertically and caused only local horizontal movements (the so-called pre-plate tectonic, "fixist theories"). Later studies (discussed below on this page), therefore, invoked many of the relationships recognized during this pre-plate tectonics period to support their theories (see the anticipations and reviews in the work of van Dijk and collaborators).[23]Of the many forces discussed in this paragraph, tidal force is still highly debated and defended as a possible principle driving force of plate tectonics. The other forces are only used in global geodynamic models not using plate tectonics concepts (therefore beyond the discussions treated in this section) or proposed as minor modulations within the overall plate tectonics model.In 1973, George W. Moore[24]of theUSGSand R. C. Bostrom[25]presented evidence for a general westward drift of the Earth's lithosphere with respect to the mantle. He concluded that tidal forces (the tidal lag or "friction") caused by the Earth's rotation and the forces acting upon it by the Moon are a driving force for plate tectonics. As the Earth spins eastward beneath the moon, the moon's gravity ever so slightly pulls the Earth's surface layer back westward, just as proposed by Alfred Wegener (see above). In a more recent 2006 study,[26]scientists reviewed and advocated these earlier proposed ideas. It has also been suggested recently inLovett (2006) that this observation may also explain whyVenusandMarshave no plate tectonics, as Venus has no moon and Mars' moons are too small to have significant tidal effects on the planet. In a recent paper,[27]it was suggested that, on the other hand, it can easily be observed that many plates are moving north and eastward, and that the dominantly westward motion of the Pacific ocean basins derives simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces). In the same paper the authors admit, however, that relative to the lower mantle, there is a slight westward component in the motions of all the plates. They demonstrated though that the westward drift, seen only for the past 30Ma, is attributed to the increased dominance of the steadily growing and accelerating Pacific plate. The debate is still open.Relative significance of each driving force mechanismThe actual vector of a plate's motion is a function of all the forces acting on the plate; however, therein lies the problem regarding the degree to which each process contributes to the overall motion of each tectonic plate.The diversity of geodynamic settings and the properties of each plate result from the impact of the various processes actively driving each individual plate. One method of dealing with this problem is to consider the relative rate at which each plate is moving as well as the evidence related to the significance of each process to the overall driving force on the plate.One of the most significant correlations discovered to date is that lithospheric plates attached to downgoing (subducting) plates move much faster than plates not attached to subducting plates. The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than the plates of the Atlantic basin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates. It is thus thought that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces which determine the motion of plates, except for those plates which are not being subducted.[19]The driving forces of plate motion continue to be active subjects of on-going research withingeophysicsandtectonophysics.Development of the theoryFurther information:Timeline of the development of tectonophysicsSummary

Detailed map showing the tectonic plates with their movement vectors.In line with other previous and contemporaneous proposals, in 1912 the meteorologist Alfred Wegener amply described what he called continental drift, expanded in his 1915 bookThe Origin of Continents and Oceans[28]and the scientific debate started that would end up fifty years later in the theory of plate tectonics.[29]Starting from the idea (also expressed by his forerunners) that the present continents once formed a single land mass (which was calledPangealater on) that drifted apart, thus releasing the continents from the Earth's mantle and likening them to "icebergs" of low densitygranitefloating on a sea of denserbasalt.[30]Supporting evidence for the idea came from the dove-tailing outlines of South America's east coast and Africa's west coast, and from the matching of the rock formations along these edges. Confirmation of their previous contiguous nature also came from the fossil plantsGlossopterisandGangamopteris, and thetherapsidormammal-like reptileLystrosaurus, all widely distributed over South America, Africa, Antarctica, India and Australia. The evidence for such an erstwhile joining of these continents was patent to field geologists working in the southern hemisphere. The South AfricanAlex du Toitput together a mass of such information in his 1937 publicationOur Wandering Continents, and went further than Wegener in recognising the strong links between theGondwanafragments.But without detailed evidence and a force sufficient to drive the movement, the theory was not generally accepted: the Earth might have a solid crust and mantle and a liquid core, but there seemed to be no way that portions of the crust could move around. Distinguished scientists, such asHarold JeffreysandCharles Schuchert, were outspoken critics of continental drift.Despite much opposition, the view of continental drift gained support and a lively debate started between "drifters" or "mobilists" (proponents of the theory) and "fixists" (opponents). During the 1920s, 1930s and 1940s, the former reached important milestones proposing thatconvection currentsmight have driven the plate movements, and that spreading may have occurred below the sea within the oceanic crust. Concepts close to the elements now incorporated in plate tectonics were proposed by geophysicists and geologists (both fixists and mobilists) like Vening-Meinesz, Holmes, and Umbgrove.One of the first pieces of geophysical evidence that was used to support the movement of lithospheric plates came frompaleomagnetism. This is based on the fact that rocks of different ages show a variablemagnetic fielddirection, evidenced by studies since the midnineteenth century. The magnetic north and south poles reverse through time, and, especially important in paleotectonic studies, the relative position of the magnetic north pole varies through time. Initially, during the first half of the twentieth century, the latter phenomenon was explained by introducing what was called "polar wander" (seeapparent polar wander), i.e., it was assumed that the north pole location had been shifting through time. An alternative explanation, though, was that the continents had moved (shifted and rotated) relative to the north pole, and each continent, in fact, shows its own "polar wander path". During the late 1950s it was successfully shown on two occasions that these data could show the validity of continental drift: by Keith Runcorn in a paper in 1956,[31]and by Warren Carey in a symposium held in March 1956.[32]The second piece of evidence in support of continental drift came during the late 1950s and early 60s from data on the bathymetry of the deepocean floorsand the nature of the oceanic crust such as magnetic properties and, more generally, with the development ofmarine geology[33]which gave evidence for the association of seafloor spreading along themid-oceanic ridgesandmagnetic field reversals, published between 1959 and 1963 by Heezen, Dietz, Hess, Mason, Vine & Matthews, and Morley.[34]Simultaneous advances in earlyseismicimaging techniques in and aroundWadati-Benioff zonesalong the trenches bounding many continental margins, together with many other geophysical (e.g. gravimetric) and geological observations, showed how the oceanic crust could disappear into the mantle, providing the mechanism to balance the extension of the ocean basins with shortening along its margins.All this evidence, both from the ocean floor and from the continental margins, made it clear around 1965 that continental drift was feasible and the theory of plate tectonics, which was defined in a series of papers between 1965 and 1967, was born, with all its extraordinary explanatory and predictive power. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena and their implications in other studies such aspaleogeographyandpaleobiology.Continental driftFor more details on this topic, seeContinental drift.In the late 19th and early 20th centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as basin development and mountain ranges could be explained by vertical crustal movement, described in what is called thegeosynclinal theory. Generally, this was placed in the context of a contracting planet Earth due to heat loss in the course of a relatively short geological time.

Alfred Wegener in Greenland in the winter of 1912-13.It was observed as early as 1596 that the oppositecoastsof the Atlantic Oceanor, more precisely, the edges of thecontinental shelveshave similar shapes and seem to have once fitted together.[35]Since that time many theories were proposed to explain this apparent complementarity, but the assumption of a solid Earth made these various proposals difficult to accept.[36]The discovery ofradioactivityand its associatedheatingproperties in 1895 prompted a re-examination of the apparentage of the Earth.[37]This had previously been estimated by its cooling rate and assumption the Earth's surface radiated like ablack body.[38]Those calculations had implied that, even if it started atred heat, the Earth would have dropped to its present temperature in a few tens of millions of years. Armed with the knowledge of a new heat source, scientists realized that the Earth would be much older, and that its core was still sufficiently hot to be liquid.By 1915, after having published a first article in 1912,[39]Alfred Wegener was making serious arguments for the idea of continental drift in the first edition ofThe Origin of Continents and Oceans.[28]In that book (re-issued in four successive editions up to the final one in 1936), he noted how the east coast ofSouth Americaand the west coast ofAfricalooked as if they were once attached. Wegener was not the first to note this (Abraham Ortelius,Antonio Snider-Pellegrini,Eduard Suess,Roberto MantovaniandFrank Bursley Taylorpreceded him just to mention a few), but he was the first to marshal significantfossiland paleo-topographical and climatological evidence to support this simple observation (and was supported in this by researchers such asAlex du Toit). Furthermore, when the rockstrataof the margins of separate continents are very similar it suggests that these rocks were formed in the same way, implying that they were joined initially. For instance, parts ofScotlandandIrelandcontain rocks very similar to those found inNewfoundlandandNew Brunswick. Furthermore, theCaledonian Mountainsof Europe and parts of theAppalachian Mountainsof North America are very similar instructureandlithology.However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically, they did not see