Physical Geography - Version 1 Unit 13 Earths Dynamic Surface Plate Tectonics

Figure Viewing the Road Cut , a 90 Slice Through Sediments , The San Andreas Meeting between the North American Pacific Plates . Image by Jeremy is used under a license . UNIT 13 EARTHS DYNAMIC SURFACE PLATE TECTONICS Goals Objectives of this unit Compare and describe each of these Earth layers lithosphere , oceanic crust , and continental crust . Describe how convection takes place in the mantle and compare the two parts ofthe core and describe why they are different from each other . Explain the concepts of the following hypothesis continental drift hypothesis , seafloor spreading hypothesis , and the theory of plate tectonics . Describe the three types of tectonic plates , and how the processes lead to changes in Earth surface features . GEOGRAPHY

SEEING EARTH INTERIOR Before you can learn about plate tectonics , you need to know something about the layers that are found inside Earth . These layers are divided by composition into core , mantle , and crust or by mechanical properties into the lithosphere and asthenosphere . Scientists use information from earthquakes and computer modeling to learn about Earth interior . Humans have never drilled past Earth crust , and yet we know a lot about the composition of the earth interior . Rocks yield some clues , but they only reveal information about the outer crust . In rare instances , a mineral , such as a diamond , comes to the surface from deeper down in the crust or the mantle . To learn about Earth interior , scientists use energy , recorded by seismographs , to see the different layers of the Earth , just like doctors can use an MRI , scan , or to see inside our bodies . Seismic Waves One ingenious way scientists learn about Earth interior is by looking at how energy travels from the point of an earthquake , called seismic waves . Seismic waves travel outward in all directions from where the ground breaks at an earthquake . Seismograph stations measure the energy released by these earthquakes , but there are two that scientists are most interested regarding understanding the interior of the Earth . Surface wave Surface wave Seismograph station Figure Example , Surface Waves . Image by OER team is used under a license . GEOGRAPHY

Primary waves ( also called ) are fastest , traveling at about to kilometers ( about miles ) per second , so they arrive first at the seismometer . move in a compression or expansion type motion , squeezing and earth materials as they travel . bend slightly when they travel from one layer into another . Seismic waves move faster through denser or more rigid material . As encounter the liquid outer core , which is less rigid than the mantle , they slow down . This makes the arrive later and further away than would be expected . The result is a shadow zone . No are picked up at seismographs 1040 to 1400 from the earthquake focus . Secondary waves ( also called ) are about half as fast as , traveling at about ( miles ) per second , and arrive second at seismographs . move in an up and down motion perpendicular to the direction of wave travel . This produces a change in shape for the earth materials they move through . Only solids resist a change in shape , so are only able to propagate through solids . can not travel through liquid . I I I I I I I Horizontal ( I I I Horizontal ( lIl Velocity ( Figure Velocity of Seismic Waves in the Earth , Versus Depth . Image under open copyright . By tracking seismic waves , scientists have learned what makes up the planet interior . slow down at the mantle core boundary , so we know the outer core is less rigid than the mantle . disappear at the mantle core boundary , so the outer core is liquid . Other clues to Earth interior include the fact that we know that Earth overall density is higher than the GEOGRAPHY

density of crustal rocks , so the core must be made of something dense , like metal . Also , since Earth has a magnetic field , there must be metal within the planet . Iron and nickel are both magnetic . Finally , meteorites are the remains of the material that formed the early solar system and are thought to be similar to material in Earth interior . THE COMPOSITION STRUCTURE OF EARTH Core , mantle , and crust are divisions based on composition . The crust makes up less than of Earth by mass , consisting of oceanic crust and continental crust is often more rock . The mantle is hot and represents about 68 of Earth mass . Finally , the core is mostly iron metal . The core makes up about 31 of the Earth . The lithosphere and asthenosphere are divisions based on mechanical properties . The lithosphere is composed of both the crust and the portion of the upper mantle that behaves as a brittle , rigid solid . The asthenosphere is partially molten upper mantle material that behaves plastically and can flow . Pin It ! Layers This animation Earth shows the la ers and mechanical . crust mick ( and most solid mantle ) Mantle To scale Figure A Slice Exposing the Structure Composition . Image is in the public domain . GEOGRAPHY

Crust Lithosphere Earth outer surface is its crust a cold , thin , brittle outer shell made of rock . The crust is very thin , relative to the radius of the planet . There are two very different types of crust , each with its own distinctive physical and chemical properties . Oceanic crust is composed of magma that erupts on the seafloor to create basalt lava flows or cools deeper down to create the intrusive igneous rock gabbro . Sediments , primarily and the shells of tiny sea creatures , coat the seafloor . Sediment is thickest near the shore where it comes off the continents in rivers and on wind currents . Continental crust is made up of many different types of igneous , metamorphic , and sedimentary rocks . The average composition is granite , which is much less dense than the mafic igneous rocks of the oceanic crust . Because it is thick and has relatively low density , continental crust rises higher on the mantle than oceanic crust , which sinks into the mantle to form basins . When filled with water , these basins form the planet oceans . The lithosphere is the outermost mechanical layer , which behaves as a brittle , rigid solid . The lithosphere is about 100 kilometers thick . The definition ofthe lithosphere is based on how earth materials behave , so it includes the crust and the uppermost mantle , which are both brittle . Since it is rigid and brittle , when stresses act on the lithosphere , it breaks . This is what we experience as an earthquake . Mantle The two most important things about the mantle are It is made of solid rock . It is hot . Scientists know that the mantle is made of rock based on evidence from seismic waves , heat flow , and meteorites . The properties fit the rock peridotite , which is made of the and silicate minerals . Peridotite is rarely found at Earth surface . Scientists know that the mantle is extremely hot because of the heat flowing outward from it and because of its physical properties . Heat flows in two different ways within the Earth conduction and convection . Conduction is defined as the heat transfer that occurs through rapid collisions of atoms , which can only happen if the material is solid . Heat flows from warmer to cooler places until all are the same temperature . The mantle is hot mostly because of heat conducted from the core . Convection is the process of a material that can move , and flow may develop convection currents . Convection in the mantle is the same as convection in a pot of water on a stove . Convection currents within Earth mantle form as material near the core heats up . As the core heats the bottom layer of mantle material , particles move more rapidly , decreasing its density and causing it to rise . The rising material begins the convection current . When the warm material reaches the surface , it spreads horizontally . The material cools because it is no longer near the core . It eventually becomes cool and dense enough to GEOGRAPHY

sink back down into the mantle . At the bottom of the mantle , the material travels horizontally and is heated by the core . It reaches the location where warm mantle material rises , and the mantle convection cell is complete . 1000 2000 3000 4000 5000 6000 7000 8000 ( Figure This is a Snapshot of One in a Model of Mantel Convection . Image is used under a Alike license . Convection in the mantle is the same as convection in a pot of water on a stove . Convection currents within Earth mantle form as material near the core heats up . As the core heats the bottom layer of mantle material , particles move more rapidly , decreasing its density and causing it to rise . The rising material begins the convection current . When the warm material reaches the surface , it spreads horizontally . The material cools because it is no longer near the core . It eventually becomes cool and dense enough to sink back down into the mantle . At the bottom of the mantle , the material travels horizontally and is heated by the core . It reaches the location where warm mantle material rises , and the mantle convection cell is complete . Core At the planet center lies a dense metallic core . Scientists know that the core is metal for a few reasons . The density of Earth surface layers is much less than the overall density of the planet , as calculated from the planet rotation . If the surface layers are less dense than average , then the interior must be denser than average . Calculations indicate that the core is about 85 iron metal with making up much of the remaining 15 . Also , metallic meteorites are thought to be representative of the core . If Earth core were not metal , the planet would not have a magnetic field . Metals such as iron are magnetic , but rock , which makes up the mantle and crust , is not . Scientists know that the outer core is liquid , and the inner core is solid because stop at the inner core . The strong magnetic field is caused by convection in the liquid outer core . Convection currents in the outer core are due to heat from the even hotter inner core . The heat that keeps the outer core from solidifying is produced by the breakdown of radioactive elements in the inner core . GEOGRAPHY

THEORY OF CONTINENTAL DRIFT The continental drift hypothesis was developed in the early part of the century , mostly by Alfred . said that continents move around on Earth surface and that they were once joined together as a single supercontinent . While was alive , scientists did not believe that the continents could move . Find a map of the continents and cut each one out . Better yet , use a map where the edges of the continents show the continental shelf . That the true size and shape of a continent and many can be pieced together like a puzzle . The easiest link is between the eastern Americas and western Africa and Europe , but the rest can fit together too . Figure a Few Years Before His Death in 1930 . Image is in the public domain . Alfred proposed that the continents were once united into a single supercontinent named , meaning all earth in ancient Greek . He suggested that broke up long ago and that the continents then moved to their current positions . He called his hypothesis continental drift . pursued his theory with determination by combing the libraries , consulting with colleagues , and making observations , looking for evidence to support it . He relied heavily on matching geological patterns across oceans , such as sedimentary strata in South America matching those in Africa , North American matching those in Europe , and the mountains of Atlantic Canada matching those of northern Britain , both in morphology and rock GEOGRAPHY

type . also referred to the evidence for the Carboniferous and Permian ( Ma ) Glaciation in South America , Africa , India , Antarctica , and Australia . He argued that this could only have happened if these continents were once all connected as a single supercontinent . He also cited evidence ( based on his observations and interpretations ) that showed that the continents were moving with respect to each other and determined a separation rate between and of 11 per year , although he admitted that the measurements were not accurate . In fact , they were even close , the separation rate is about per year . evidence of the land reptile AU remains of Fossils of the fern . a Fol remains of the . in Triad land reptile freshwater reptile ' the approximately ' 910 ' that . they were once joined . Figure 13 . The Distribution of Several Permian Terrestrial Fossils that are Present in Various Parts of Continents that are now Separated by Oceans . Image by Steven Earl used under a international license . first published his ideas in 1912 in a short book called Die der ( The Origin of Continents ) and then in 1915 in Die der und ( The Origin of Continents and Oceans ) He revised this book several times up to 1929 . It was translated into French , English , Spanish , and Russian in 1924 . Alfred died in in 1930 while carrying out studies related to glaciation and climate . At the time of his death , his ideas were tentatively accepted by only a small minority of geologists and soundly rejected by most . However , within a few decades , that was all to change . GEOGRAPHY

Evidence for Continental Drift Besides the way the continents fit together , and his supporters collected a great deal of evidence for the continental drift hypothesis . For one , identical rocks of the same type and age are found on both sides of the Atlantic Ocean . said the rocks had formed side and that the land had since moved apart . Mountain ranges with the same rock types , structures , and ages are now on opposite sides of the Atlantic Ocean . The of the eastern United States and Canada , for example , are just like mountain ranges in eastern , Ireland , Great Britain , and Norway . concluded that they formed as a single mountain range that was separated as the continents drifted . Ancient fossils ofthe same species of extinct plants and animals are found in rocks of the same age but are on continents that are now widely separated . proposed that the organisms had lived side by side , but that the lands had moved apart after they were dead and fossilized . He suggested that the organisms would not have been able to travel across the oceans . For example , the fossils of the seed fern were too heavy to be carried so far by the wind . The reptile could only swim in freshwater . and were land reptiles and were unable to swim . and rock deposits left by ancient glaciers are found today on different continents very close to the equator . This would indicate that the glaciers either formed in the middle of the ocean covered most of the Earth . Today glaciers only form on land and nearer the poles . thought that the glaciers were centered over the southern landmass close to the South Pole and the continents moved to their present positions later on . Coral reefs and swamps are found in tropical and subtropical environments , but ancient coal seams and coral reefs are found in locations where it is much too cold today . suggested that these creatures were alive in warm climate zones and that the fossils and coal later had drifted to new locations on the continents . Although evidence was sound , most geologists at the time rejected his hypothesis of continental drift . Scientists argued that there was no way to explain how solid continents could plow through solid oceanic crust . idea was nearly forgotten until technological advances presented even more evidence that the continents moved and gave scientists the tools to develop a mechanism for drifting continents . Magnetic Polarity on The Same Continent with Rocks of Different Ages Puzzling new evidence came in the from studies on the Earth magnetic history . Scientists used , devices capable of measuring the magnetic field intensity , to look at the magnetic properties of rocks in many locations . Geologists noted important things about the magnetic polarity of different aged rocks on the same continent . Magnetite crystals in fresh volcanic rocks point to the current magnetic north pole no matter what continent or GEOGRAPHY

where on the continent the rocks are located . Older rocks that are the same age and are located on the same continent point to the same location , but that location is not the current north magnetic pole . Older rocks that are of different ages do not point to the same locations or the current magnetic north pole . In other words , although the magnetite crystals were pointing to the magnetic north pole , the location of the pole seemed to wander . Scientists were amazed to find that the north magnetic pole changed location through time . There are three possible explanations for this The continents remained and the north magnetic pole moved . The north magnetic pole stood still , and the continents moved . Both the continents and the North Pole moved . Normal magnetic polarity CI Reversed magnetic Figure The Ocean Floor Shows Patterns of Magnetic Variation That is not Random , This is an Example of the Magnetic Stripping . Image is in the public domain . Magnetic Polarity on Different Continents with Rocks of The Same Age Geologists noted that for rocks ofthe same age but on different continents , the little magnets pointed to different magnetic north poles . For example , magnetite in Europe pointed to a different north magnetic pole than the magnetite in North America . Around 250 million years ago , the north poles were also different for the two continents . The scientists looked again at the three possible explanations . Only one can be correct . Ifthe continents had remained fixed while the north magnetic pole moved , there must have been two separate north poles . Since there is only one north pole today , the only GEOGRAPHY

reasonable explanation is that the north magnetic pole has remained fixed but that the continents have moved . To test this , geologists fitted the continents together as had done and behold , it worked . There has only been one magnetic north pole and the continents have drifted . They named the phenomenon of the magnetic pole that seemed to move but did not apparent polar wander . This evidence for continental drift gave geologists renewed interest in understanding how continents could move about on the planet surface . SEAFLOOR SPREADING The discovery of magnetic striping naturally prompted more questions How does the magnetic striping pattern form ?

And why are the stripes symmetrical around the crests ofthe ridges ?

These questions could not be answered without also knowing the significance ofthese ridges . In 1961 , scientists began to theorize that ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest . New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create a new oceanic crust . This process , later called seafloor spreading , operating over many millions of years has built the systems of ocean ridges . Figure Simplified Map Showing Areas Spreading . Image is in the public domain . GEOGRAPHY

Seafloor bathymetry World War II gave scientists the tools to find the mechanism for continental drift that had eluded . Maps and other data gathered during the war allowed scientists to develop the seafloor spreading hypothesis . This hypothesis traces oceanic crust from its origin at a ocean ridge to its destruction at a trench and is the mechanism for continental drift . During World War II , battleships and submarines carried echo to locate enemy submarines . Echo produce sound waves that travel outward in all directions , bounce off the nearest object , and then return to the ship . By knowing the speed of sound in seawater , scientists calculate the distance to the object based on the time it takes for the wave to make a . During the war , most of the sound waves ricocheted off the ocean bottom . After the war , scientists pieced together the ocean depths to produce bathymetric maps , which reveal the features of the ocean floor as if the water were taken away . Even scientists were amazed that the seafloor was not completely flat . What was discovered was a large chain of mountains along the deep seafloor , are called ridges . Scientists also discovered deep sea trenches along the edges of continents or in the sea near chains of active volcanoes . Finally , large , flat areas called abyssal plains we found . When they first observed these bathymetric maps , scientists wondered what had formed these features . Pin It ! Pictures of the Seafloor Interact with this echo website to learn more . Figure Bathymetry Map Flower Garden Bank , Offshore of Texas . Image is in the public domain . Seafloor Magnetism Sometimes , for reasons unknown , the magnetic poles switch positions . North becomes south and south becomes north . During normal polarity , the north and south poles are aligned as they GEOGRAPHY

are now . With reversed polarity , the north and south poles are in the opposite position . During , attached to ships to search for submarines located an astonishing feature the normal and reversed magnetic polarity of seafloor creates a pattern . Stripes of normal polarity and reversed polarity alternate across the ocean bottom . These stripes also form a mirror image of itself on either side of the ridges . But the stripes end abruptly at the edges of continents , sometimes at a trench . Pin It ! Sediment Thickness Interact with this ma of sediment thickness to learn more . The oldest seafloor is near the edges of continents or trenches and is less than 180 million years old . Since the oldest ocean crust is so much younger than the oldest continental crust , scientists realized that seafloor was being destroyed in a relatively short time . Seafloor Spreading Hypothesis Scientists brought these observations together in the early to create the seafloor spreading hypothesis . In this hypothesis , a hot buoyant mantle rises up a ridge , causing the ridge to rise upward . The hot magma at the ridge erupts as lava that forms new seafloor . When the lava cools , the magnetite crystals take on the current magnetic polarity and as more lava erupts , it pushes the seafloor horizontally away from the ridge axis . The magnetic stripes continue across the seafloor . As oceanic crust forms and spreads , moving away from the ridge crest , it pushes the continent away from the ridge axis . If the oceanic crust reaches a trench , it sinks into the trench and is lost into the mantle . Scientists now know that the oldest crust is coldest and lies deepest in the ocean because it is less buoyant than the hot new crust . Seafloor spreading is the mechanism for drifting continents . Convection currents within the mantle take the continents on a ride of oceanic crust that over millions of years takes them around the planet surface . EARTH TECTONIC PLATES When the concept of seafloor spreading came along , scientists recognized that it was the mechanism to explain how continents could move around Earth surface . Scientific data and observation now allow us to merge the ideas of continental drift and seafloor spreading into the theory of plate tectonics . Seafloor and continents move around on Earth surface , but GEOGRAPHY

what is moving ?

What portion of the Earth makes up the plates in plate tectonics ?

This question was also answered because of technology developed during the Cold War . The plates are made up of the lithosphere . During the and early , scientists set up seismograph networks to see if enemy nations were testing atomic bombs . These seismographs also recorded all of the earthquakes around the planet . The seismic records could be used to locate an earthquake epicenter , the point on Earth surface directly above the place where the earthquake occurs . Earthquake outline these tectonic plates . ridges , trenches , and large faults mark the edges of these plates along with where earthquakes occur . The lithosphere is divided into a dozen major and several minor plates . The plates edges can be drawn by connecting the dots that mark earthquakes . A single plate can be made of all oceanic lithosphere or all continental lithosphere , but nearly all plates are made of a combination of both . Movement of the plates over Earth surface is termed plate tectonics . Plates move at a rate of a few centimeters a year , about the same rate fingernails grow . omit Juan de Fum Figure Map of Earth Plates Their Corresponding Tectonic Boundaries . Image is in the public domain . How Plates Move If seafloor spreading drives the plates , what drives seafloor spreading ?

Picture two convection cells in the mantle . Hot mantle from the two adjacent cells rises at the ridge axis , GEOGRAPHY creating new ocean crust . The top limb of the convection cell moves horizontally away from the ridge crest , as does the new seafloor . Outer Core Figure Example Mantle Convection Cells . Image by is used Alike license . The outer limbs ofthe convection cells plunge into the deeper mantle , dragging oceanic crust as well . This takes place at the trenches . The material sinks to the core and moves horizontally . The material heats up and reaches the zone where it rises again . TECTONIC BOUNDARIES Plate boundaries are the edges where two plates meet . Most geologic activities , including volcanoes , earthquakes , and mountain building , take place at plate boundaries . How can two plates move relative to each other ?

Divergent plate boundaries the two plates move away from each other . Convergent plate boundaries the two plates move towards each other . Transform plate boundaries the two plates slip past each other . The type of plate boundary and the type of crust found on each side of the boundary determines what sort of geologic activity will be found there . DIVERGENT Plates move apart at ridges where new seafloor forms . Between the two plates is a rift valley . Lava flows at the surface cool rapidly to become basalt , but deeper in the crust , the magma cools more slowly to form gabbro . So the entire ridge system is made up of igneous rock that is either extrusive or intrusive . Earthquakes are common at ridges since the movement of magma and oceanic crust results in crustal shaking . The vast majority of ocean ridges are located deep below the sea . GEOGRAPHY

Pin It ! The Interact with this animation of continental to learn more . As divergence occurs , shallow earthquakes can occur along with volcanoes along the rift areas . When the process begins , a valley will develop such as the Great Rift Valley in Africa . Over time that valley can fill up with water creating linear lakes . If divergence continues , a sea can form like the Red Sea and finally an ocean like the Atlantic Ocean . Check out the eastern half of Africa and notice the lakes that look linear . Eastern Africa is tearing apart from these linear lakes , to the Great Rift Valley , and up to the Red Sea . The ultimate divergent boundary is the Atlantic Ocean , which began when broke apart . Figure The Mid Atlantic Ridge . Image from Google Earth , Data , Navy , used under Google Earth guidelines . CONVERGENT When two plates converge , the result depends on the type of lithosphere the plates are made of . No matter what , smashing two enormous slabs of lithosphere together results in the creation of magma and earthquakes . convergence occurs when oceanic crust converges with continental crust , forcing the denser oceanic plate to plunge beneath the GEOGRAPHY

continental plate . This process called subduction occurs along oceanic trenches called subduction zones where lots of intense earthquakes and volcanic eruptions can occur . The denser , plate begins to heat up under extreme pressure near the mantle and melts to create causes melting in the volcanoes . These coastal volcanic mountains are found in a line above the plate . The volcanoes are known as a continental arc . The movement of crust and magma causes earthquakes . Pin It ! Earthquake View this presented by the . This is a collection of along subduction zones ! The volcanoes of northeastern California , Peak , Mount , and Medicine Lake volcano are all along with the rest ofthe Cascade Mountains of the Pacific Northwest are the result of subduction of the Juan de plate beneath the North American plate . The Juan de plate is created by seafloor offshore at the Juan de Ridge . If the magma at a continental arc is , it may be too viscous ( thick ) to rise through the crust . The magma will cool slowly to form granite or granodiorite . These large bodies of intrusive igneous rocks are called , which may someday be uplifted to form a mountain range . An plate boundary occurs when two oceanic plates converge , causing the older , denser plate will subduct into the mantle . An ocean trench marks the location where the plate is pushed down into the mantle . The line of volcanoes that grows on the upper oceanic plate is an island arc . The Ring of Fire is a ring around the Pacific Ocean of subduction zones , which most are convergence . Pin It ! Oceanic Plate Boundary View this Along these subduction zones , volcanic islands ( also called volcanic arcs ) form . Examples of these regions include Japan , Indonesia , and the Aleutian Islands . GEOGRAPHY

Figure The Himalayan Mountains . Image from Google Earth , Data , Navy , used under Google Earth guidelines . When two continental plates converge , instead of subduction , the two similar tectonic plates will buckle up to create large mountain ranges like a massive car . This is called convergence , and geologically creates intense folding and faulting rather than volcanic activity . Pin It ! Convergent Boundary to learn more about ent boundaries . Watch this Examples of mountain ranges created by this process are the Himalayan mountains as India is colliding with Asia , the Alps in Europe , and the Appalachian Mountains in the United States as the North American plate collided with the African plate when was forming . The Kashmir India earthquake of 2005 that killed over people occurred because of this process . And most recently , the 2008 earthquake in China which killed nearly people before the Summer Olympics was because of this tectonic force . The Appalachian Mountains are the remnants of a large mountain range that was created when North America rammed into about 250 million years ago . TRANSFORM Transform plate boundaries occur when two tectonic plates slide ( or grind ) past parallel to each other . The most famous transform boundary is the San Andreas Fault where the Pacific plate GEOGRAPHY

that Los Angeles and Hawaii are on is grinding past the North American plate that San Francisco and the rest of the United States are on at the rate of inches a year . Recently , geologists have stated that San Francisco should expect another disastrous earthquake in the next 30 years . Another important transform boundary is the North Anatolian Fault in Turkey . This powerful Figure The San Andreas Fault ( Google Earth used under Google Earth reproduction guidelines . TE BOUNDARIES A small amount of geologic activity , known as an activity , does not take place at plate boundaries but within a plate instead . Mantle plumes are pipes of hot rock that rise through the mantle . The release of pressure causes melting near the surface to form a hotspot . Eruptions at the hotspot create a volcano . Hotspot volcanoes are found in a line . Can you figure out why ?

Hint The youngest volcano sits above the hotspot and volcanoes become older with distance from the hotspot . Geologists use some hotspot chains to tell the direction and the speed a plate is moving . Hotspot rarely penetrate through thick continental crust . One exception is the Yellowstone hotspot . Pin It ! Hot Spots View this Animation of Hot Sot formations . GEOGRAPHY

UNIT 13 SUMMARY The evidence for continental drift in the early century included the matching of continental shapes on either side of the Atlantic and the geological and fossil between continents that are now thousands of kilometers apart . The established theories of global geology were and , but neither of these theories was able to explain some ofthe evidence that supported the idea of continental drift . Earth lithosphere is made up of over 20 plates that are moving in different directions at rates of between and 10 . The three types of plate boundaries are divergent ( plates moving apart and new crust forming ) convergent ( plates moving together , and one being ) and transform ( plates moving side by side ) Divergent boundaries form where existing plates are apart , and it is hypothesized that this is caused by a series of mantle plumes . Subduction zones are assumed to form where the accumulation of sediment at a passive margin leads to the separation of oceanic and continental lithosphere . form and break up through these processes . It is widely believed that and are the main mechanisms for plate motion , as opposed to traction by mantle convection . Mantle convection is a key factor for producing the conditions necessary for and GEOGRAPHY