Plate Tectonics

The Continental Drift Hypothesis••••••••••.• i ••• Before Yo'u Read .What do you think? Read the two statements below and decidewhether you agree or disagree with them. Place an A in the Before columnifyou agree with the statement or a D ifyou disagree. After you've readthis lesson, reread the statements to see ifyou have changed your mind.

Before Statement After

1. India has always been north ofthe equator.

2. Al!the continents once formed onesupercontinent.

................ Read to Learn .Pangaea

Nearly 100 years ago, a scientist named Alfred Wegener(VAY guh nuhr) began an investigation. He wanted to knowif Earth's continents had always been in the same place, or ifthey had moved. Wegener proposed that all the continents wereonce part of a supercontinent called Pangaea (pan JEE uh). Overtime, Pangaea broke apart, and the continents slowly driftedto their present locations. Wegener proposed the hypothesisof continental dríft, The continental drift hypothesis suggestedthat continents are in constant motion on the surface ofEorth.

Wegener looked at the coastlines of continents that arenow separated by oceans. He saw similarities in their shapes.For instance, Africa and South America seemed to ñt togetherlike the pieces of a puzzle, as shown below.

¡

<,Continentalshelf

Reading Essentials

Key Concepts €;=• What evidence supports

continental drift?• Why did scientists question

the continental drifthypothesis?

Identify the Main IdeasHighlight two or threephrases in each paragraphthat summarize theinformation presented. Afteryou have finished the lesson,review the highlighted texto

mrammtMake a half-book. Use it toorganize your notes on thecontinental drift hypothesis.

E\lióef\ce" orthe COf\tiMI'\tQ\ .\) t,H \lwothe¡"

Visual Check

1. Identify With a pen orpencil, trace the area whereAfrica and South Americamatch up.

Plate Tectonics 51

REVIEW VOCABULARYfossilthe naturally preservedremaíns, imprints, or traces oforganisms that lived long ago

~Think it Ove,!

2. Specify Where didGlossopteris probably grow?

G}Visual Check3. Consider Which ofthecontinents would notsupport Glossopteris growthtoday?

52 PlateTectonics

." ,.. '

Evidence That Continents MoveWegener knew that he needed evidence to support his

hypothesis of continental drift. The most obvious evidencewas how the continents fit together like pieces of a puzzle.But othei: scientists were doubtful of his hypothesis. Wegenerneeded more evidence.

Fossil CluesThere are many animals and plants that live only on one

continent. For example, lions Iíve in Africa but not in SouthAmerica. Because oceans separate the continents, animalscannot travel from one continent to another by naturalmeans. However, fossils of similar organisms have been .discovered on several continents that are now separated byoceans.

Fossils of a plant called Glossopteris (glahs AHP tur us)have been discovered in rocks from South America, Africa,India, Antarctica, and Australia. Today these continents arefar apart and separated by oceans. The plant's seeds couldnot have traveled across the oceans.

The figure below shows how some of the continents werejoined as part of Pangaea 250 million years ago. The lighterarea on the map shows where Glossopteris fossils have beenfound. Notice that the plant once grew in parts of fivecontinents-South America, Africa, India, Antarctica,and Australia. Because these plants grew in a swampyenvironment, this region, including Antarctica, was differentfrom how it ís today. Most of Antarctica is covered in icesheets. No swampy environments are found there now.

SouthAmerica

ReadingEssentials

Climate CluesOther fossil evidence supported continental drift. Coal

beds are in Antarctica, a polar climate today. Yet coal formedfrom fossilized plants that lived long ago in warm, wetclimates. This meant that Antarctica must have been warrnerand wetter when these plants were alive. Is it possible thatAntarctica was at one time closer to the equator? DidAntarctica move to a colder climate near the South Pole?

Another climate clue used by Wegener to supportcontinental drift carne from glaciers. When Wegener piecedPangaea together, he proposed that South America, Africa,India, and Australia were located closer to the South Pole250 million years ago. He suggested that a large ice sheetcovered much ofthe continents, as shown below. When theice sheet melted as Pangaea spread apart, it left rock andsediment behind. Wegener studied the similarities of thesesediments. $

Wegener also studied glacial grooves. Glacial grooves aredeep scratches in rocks made as ice sheets move across theland. Wegener found glacial grooves on many differentcontinents. By studyíng these grooves, he was able todetermine the direction that the ice sheet moved across thejoined continents.

¡-Equator -i

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RockCluesSorne of the evidence used by Wegener to support his idea

of continental drift carne from rock formations on dífferéntcontinents. The rock formations and mountaln ranges seemedto have formed in the same way at the same time. Todaygeologists know that there were large-scale volcanic eruptionson the westem coast of Africa and on the eastem coast ofSouth America hundreds of millions of years ago. Geologistshave studied rocks from these eruptions. They found that thevolcanic rocks from both continents were identical inchemistry and age.

Reading Essentials

• Reading Check

4. State Why did Wegenersuggest that continents inthe southern hemisphere hada colder climate long ago?

~ Visual Check

5. Name the areas on Earthwhere there is evidence ofancient glaciers.

Plate Tectonics 53

GVisual Check6. Name Whichmountainrange is in Europe?

Mountain Chains More evidence carne from the rocks thatmake up two mountain chains in Europe and NorthAmerica. Locate the caledonian mountain range and theAppalachian Mountains in the figure below on the left. Thecaledonian mountain range is in northern Europe, and theAppalachian Mountains are in eastern North America.

Mountain Ranges

~ Key Concept Check7. Specify Howweresimilarrocktypes used tosupport the continental drifthypothesis?

~ Key Concept Check8. Explain Whydidscientists argue againstWegener's continental drifthypothesis?

S4 PlateTectonics

Rock Types Rocks in these two mountain chains are similarin age and structure. Both are also composed of the samerock types. If you could place North Ameríca and Europenext to each other, these mountain chains would meet. Theywould form one long, continuous mountain belt, shown inthe figure above on the right. ~

What was missing?Wegener supported his continental drift hypothesis until

his death in 1930. Wegener's ideas were not widely accepteduntil nearly 40 years later. Why were scientists skeptical ofWegener's hypothesis?

Continental drift is a slow process. Wegener could notmeasure how fast the continents moved. Wegener also couldnot explain what forces caused the continents to move. Themantle under the continents and seafloor was made of solidrock. How could continents push their way through solidrock? Wegener needed more scientific evidence to prove hishypothesis.

The evidence for drifting continents was hidden on theseafloor. During Wegener's lifetime, scientists did not havethe tools to determine what happened beneath the oceans.Wegener also could not have known what the seafloorlooked like. The evidence needed to prove continental driftwas not discovered untillong after Wegener's death. ~

ReadingEssentials

-:

Key Concepts ~• What is seafloor spreading?• What evidence is used to

support seafloor spreading?

11@N+'! ..Two-Column Notes Asyouread, organize your notes intwo columns. In the leftcolumn, write the main ideaof each paragraph. In theright column, write detailsthat support each main idea.Review your notes to helpyou remember the details ofthe lesson. .

éVisual Check1. Identify Clrcle the areaon the map that shows themid-ocean ridge.

S6 PlateTectonics

Plate Tectonics

Development of a Theory............... BeforeYou Read··············What do you think? Readthe two statements below and decidewhether you agree or disagree with them. Placean A in the Before columnif you agree with the statement or a D if you disagree. After you've readthis lesson, reread the statements to see if you have changed your mind.

Before Statement After

3. The seafloor is flat,

4. Volcanic activity occurs only on the seafloor.

................Readto Learn················Mapping the Ocean Floor

Scientists began exploring the seafloor in greater detall'during the late 1940s. They used a device called an echosounder to measure the depths of the ocean floor, An echosounder produces sound waves that travel from a ship to theseafloor. The waves echo, or bounce, off the seafloor and backto the ship. The echo sounder récords the time it takes theecho to return. When the ocean ís deeper, the time it takesfor the sound waves to bounce back is longer. Scientistscalculated ocean depths and used these data to createtopographic maps of the seafloor.

These new topographic maps showed large mountainranges that stretched for many miles along the seafloor. Themountain ranges in the middle of the oceans are called mid-oceanridges. Mid-ocean rídges, shown in the figure below, aremuch longer than any mountain range on land.

Reading Essentlals

Seafloor SpreadingBy the 1960s, scientists had discovered a new process to

help explain continental drift. This process is called seafloorspreading. Seafloor spreading is the process by which new oceaniccrust forms along a mid-ocean ridge and older oceanic crust movesaway from the ridge.

When the seafloor spreads, Earth's mantle melts andforms magma. The liquid magma is less dense than the solidmantle. The magma rises through cracks in the crust alongthe mid-ocean ridge. When magma reaches Earth's surface,it is called lava.

As the lava cools and crystallizes on the seafloor, itforms a type of rack called basalt. Oceanic crust is mostly.basalto Because the lava erupts into water, it cools rapidly.The rapidly cooling lava forms raunded structures calledpillow lava.

As the seafloor spreads apart, new crust that is formingpushes the older crust away from the mid-ocean ridge. Themid-ocean ridge, at the center of this formation, is shownbelow. The closer the crust is to a mid-ocean ridge, theyounger the oceanic crust is. Scientists concluded that as theseafloor spreads, the continents must be moving. Seafloorspreading is the mechanism that explains Wegener'shypothesis of continental drift. €t=

Make a layered book torecord your notes andillustrate seafloor spreading.

~¡,preodio9 \

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€t= Key Concept Check2. Identify What is seafloorspreading?

€6Visual Check3.lnterpret Propose apattern that exists in rocksoneither side of the mid-oceanridge.

Reading Essentials PlateTectonics 57

• Reading Check4. Describe How domountains form along amid-ocean ridge?

Topography of the SeafloorWhat determines the topography of the ocean floor? One

factor is seafloor spreading. The rugged mountains thatmake up the mid-ocean ridge system can form in twodifferent ways. Some form as large amounts of lava eruptfrom the center of the ridge. That lava cools and builds uparound the ridge. Others form as the lava cools and formsnew crust that cracks. The rocks move up or down alongthese cracks and form jagged mountains. &

Sediment also determines the topography of the oceanfloor. Close to a mid-ocean ridge, the crust is young, andthere is not much sedimento However, farther from the rídge,sediment becomes thick enough to make the seafloorsmooth. This deep, smooth part of the ocean floor, shownbelow, is called the abyssal (uh BIH sul) plain.

Visual CheckS. Compare thetopography of a mid-oceanridge to an abyssal plain.

• Reading Check6. Identify What evidencesupports seafloor spreading?

S8 PlateTectonics

Mid-ocean ridge

Moving Continents AroundThe theory of seafloor spreadíng provides a way to

explain how continents move. Continents do not movethrough the solid mantle or the seafloor. However, seafloorspreading suggests that continents move as the seafloorspreads along a mid-ocean ridge.

Development of a TheoryJust as evidence was needed to support continental drift,

evidence was needed to support seafloor spreading. Some ofthe evidence to support seafloor spreading carne from rockson the ocean floor that were not covered with sedimentoScientists studied the magnetic signatures of minerals inthese rocks. They discovered two important things. First,Earth's magnetic field changes. Second, these changes appearin rocks that make up the ocean floor. O

ReadingEssentials

Magnetic ReversalsEarth's iron-rích, liquid outer core is like a giant magnet

that creates Earth's magnetic field. The direction of thismagnetic field is not always the same. Today's magnetic fieldis described as having normal polarity. Normal polarity ts astate in which magnetized objects, such as compass needles, will orientthemselves to point north.

Sometimes a magnetic reversal occurs and tbe magnetic fieldreverses direction. The opposite of normal polarity is reversedpolarity. Reversed polarity is a state in which magnetized objectsreverse direction and orient themselves to point south. «1

Magnetic reversals have occurred hundreds of times inEarth's pasto They occur every few hundred thousand toevery few million years.

Rocks Reveal Magnetic SignatureOcean crust contains large amounts of basalto Basalt

contains íron-rích minerals that are magnetic. Each mineralacts like a small magneto The figure below shows howmagnetic minerals align themselves with Earth's magneticfield. When lava erupts along a míd-ocean ridge, it cools,crystallizes, and permanently records the direction of Earth'smagnetic fíeld at the time of the eruption. Scientists havediscovered parallel patterns in the magnetic signature ofrocks on either side of mid-ocean ridges. For example, in thefigure,below, notice the normal pattern exists closest toeither side of the míd-ocean ridge. Likewise, the reversedpolarity pattern exists at about the same distance on eitherside of the mid-ocean ridge.

Reading Essentials

• Reading Check7, Identify Does Earth'smagnetic field currently havenormal or reversed polarity?

efVisual Check8, Describe the pattern inthe magnetic stripes shownin the image to the left.

Plate Tectonics 59

·. ,"

& Reading Check9. Discuss How domagnetic minerals helpsupport the theory ofseafloor spreading?

Evidence to Support the TheoryTo support the theory of seafloor spreading, scientists

collected data about the magnetic rninerals in rocks from theseafloor. They used a magnetometer (mag nuh TAH muh tur)to measure and record the magnetic signature of these rocks.The data collected showed parallel magnetic stripes on eitherside of the mid-ocean ridge, as shown below. What do thesestripes mean?

Each pair of magnetic stripes is similar in composition,age, and magnetic character. Each stripe also records whetherEarth's magnetic field was in a period of normal or reversedpolarity when the crust formed. Notice that the strip es oneither side of the ridge are the same. This pattem supportsthe idea that ocean crust forms along míd-ocean ridges andis carried away from the center of the ridges. $

• Normalpolarity • Reversedpolarity'-_~_,~~~v..#_T'~fI"/

Mid-oceanridge//-

5 1 present 1Ageof rocks (millions of years)

4 3 2 2 3 4

Visual Check10. Determine Whatwas the polarity of Earth'srnaqnetíc field 4 millionyears ago?

Other measurements made on the seafloor confirmseafloor spreading. Scientists drilled holes in the seafloor andmeasured the temperature below the surface. Thesetemperatures show how much thermal energy leaves Earth.Scientists discovered that more thermal energy leaves Earthnear míd-ocean ridges than is released from beneath abyssalplains. In addition, studies of sediment show that sedimentclosest to a mid-ocean ridge is younger and thinner thansediment farther away from the ridge. $& Reading Check

11. Locate Where doesmore thermal energy leaveEarth-near mid-oceanridges or beneath abyssalplains?

60 PlateTectonics ReadingEssentials

5

Key Concepts ~• What is the theory of plate

tectonics?

• What are the three typesof plate boundaries?

• Why do tectonic platesmove?

Make an Outline Use themain heads in this lesson asthe main points of youroutline. Complete the outlinewith details found in thelesson. 5tudy the lesson byreviewing your outline.

~ Key Concept Check1. State What is platetectonics?

62 Plate Tectonics

Plate Tectonics

The Theory of Plate Tectonics...... ~ Before You Read .What do you think? Readthe two statements below and decidewhether you agree or disagree with them. Placean A in the Before columnif you agree with the statement or a D if you disagree. After you've readthis lesson, reread the statements to see if you have changed your mind.

Before Statement After

5. Continents drift across a molten mantle.

6. Mountain ranges can form when continentscollide .

... ..... . ...... .Read to Learn················The Plate Tectonics Theory

When you blow into a balloon, the balloon expands. Itssurface area also tncreases. As more air is added to theballoon, the balloon gets larger. Similarly, if ocean crustcontinually forms at mid-ocean ridges and is neverdestroyed, Earth's surface should be expanding. Butmeasurements of the planet show that Earth is not gettinglarger. How can this be explained?

Geologists praposed a more complete theory in the late1960s. It was called plate tectonics theory. The theory ofplate tectonics states that Earth's surface ismade of rigid slabs of

rock, or plates, that move with respect to each other, or in relation toeach other. This new theory suggested that Earths surface,the lithosphere, is divided into large pieces of rack. Thesepieces are called plates. Each plate moves slowly over Earth'shot and semiplastic mantle. ~

The word tectonic describes the forces that shape Earth'ssurface and the rack structures that form as a resulto Platetectonics explains why earthquakes occur and volcanoeserupt. When plates separate on the seafloor, earthquakesresult and a mid-ocean ridge forms. When plates cometogether, one plate can move under the other. This causesearthquakes and creates a chain of volcanoes. When platesslide past each other, earthquakes can resulto

Reading Essentials

Earth's Tectonic Plates

PadñcPlate

"-..ir.,?

SoúthAnferican

.1. ¿- Plate5, - AntarcticPlate

Tectonic PlatesEarth's surface is divided into rigid plates that move

relative to one another. Look at the map above. It showsEarth's major plates and their boundaries. Notice how someboundaries are in the middle of the oceans. Many of theseboundaries are located at mid-ocean ridges. The PacificPlate is the largest plate. The Juan de Fuca is one of thesmallest plates.

Earth's outermost layers are cold and rigid compared tothe layers within Earth's interior. The cold and rigid outerrnostrock layer is calledthe lithosphere. The crust and the solid,uppermost mantle form the Jithosphere.

The lithosphere varies in thickness. It is thin below mid-ocean ridges. It is thick below contínents. Earth's tectonicplates are large pieces of lithosphere. These plates fit togetherlike the pieces of a giant jigsaw puzzle.

Directly below the lithosphere is a very hot part of themantle, This layer of Earth is called the asthenosphere(as THEE nuh sfihr). Even though it is solid, theasthenosphere behaves like a plastic material because itis so hot.

The asthenosphere flows below Earth's plates and enablesthe plates to move. The ways in which the lithosphere andasthenosphere interact help explain plate tectonics. $

Reading Essentials

PacificPlate

,.

Plateboundary

@Visual Check

2. Locate Circlethe NazcaPlate.

SOENCE USE V. COMMON USE' . :

plasticSdence Use capable of beíng

. molded or changtng shapewithout breaking

Common Use any of numer-ous organic, synthetic, orprocessed materials rnade intoobjects

• Reading Check

3. Identify What areEarth's outermost layerscalled?

PlateTectonics 63

• Reading Check4. Describe three ways inwhich tectonic plates move.

G!m!!!DMakea layered book toorganize your notes on thethree types of plateboundaries.

~ Key Concept Check5. Name Whatare thethree types of plateboundaries?

64 PlateTectonics

Plate BoundariesImagine placing two books side by side. Imagine that

each book is a tectonic plate. The place where the edges ofthe books meet represents a plate boundary. How many wayscan you move the books along a set of boundaries? You canpul! the books away from each other. You can push thebooks together. You can slide the books past each other.Earth's tectonic plates move in much the same way as youcan move these books. ODivergent Plate Boundaries

A divergent plate boundary forms where two tectonic platesseparate. Divergent means "moving apart." Mid-ocean ridgesare located along divergent plate boundaries: When theseafloor spreads at a mid-ocean ridge, lava erupts. As the lavacools and hardens, it forms new oceanic crust. As thisprocess continúes, the plates move away from each other.

Divergent plate boundaries can also exist in the middleof a continent. At these boundaries, continents pul! apartand a rift val!ey forms. The East African Rift is one exampleof a continental rift.

Transform Plate BoundariesThe San Andreas Fault in California is a transform plate

boundary. A transform plate boundary forms where two tectonicplates slide past each other. As they move past each other, theplates might get stuck and stop moving. Stress builds upwhere the plates are stuck. When this stress is too great, therocks break and suddenly move apart. The result is a rapidrelease of energy in the form of an earthquake.

Convergent Plate BoundariesA convergent plate boundary forms where two plates collide.

The denser plate sinks below the more buoyant plate in a processcal!ed subduction. A subduction zone is the area where adenser plate descends into Earth along a convergent plateboundary. The two types of convergent plate boundaries areocean-to-continent and continent-to-continent. ~

Ocean-to-Continent Boundary When a dense oceanic plateand a less-dense continental plate collíde, the oceanic platesubducts, or sinks, under the edge of the continental plate.This creates a deep ocean trench and a line of volcanoesforms on the edge of the continent. This process can alsooccur when two oceanic plates coUide. An older and denseroceanic plate will subduct beneath a younger oceanic plate.A deep ocean trencli íorms, along with a line of volcanoes.

ReadingEssentials

Continent-to-Continent Boundary Convergent plateboundaries also form when two continental plates collide.When this happens, neither plate is subducted. The less-dense plate folds and deforms, forming mountains such asthe Himalayas in India. OEvidence for Plate Tectonics

When Wegener proposed the continental drift hypothesis,technology was not available to measure how fast thecontinents moved. Remember that continents move apart orcome together at speeds of only a few centirneters per year.This is about the length of a small paper clip.

Today, scientists can measure how fast continents move.A network of satellites orbiting Earth is used to monitor platemotion. By keeping track of the distance between satellitesand Earth, it is possible to determine how fast a tectonicplate moves. This network of satellites is called the GlobalPositioning System (GPS).

The theory of plate tectonics explains why earthquakesand volcanoes are more common in some places than inothers. Recall that when plates sepárate, collide, or slide pasteach other, stress builds. When this stress suddenly releases,earthquakes can resulto

Volcanoes can also form along a mid-ocean ridge orcontinental rifts. They also form where plates collide along asubduction zone. Mountains can form where two continentscollide. The map below shows that most earthquakes andvolcanoes occur along tectonic plate boundaries. ~

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• Pacific Ocean

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;;~..,.;,..,:....r-r-----r-------~.•~.. .Active volcano ..•.•.~ .1:Earthquake epicenterPlate boundary

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Reading Essentials

~ Reading Check6. Identify Along whattype of convergent plateboundary did the Himalayasform? (Circle the correctanswer.)

a. ocean-to-ocean

b. continent-to-continent

c. ocean-to-continent

~ Key Concept Check7. Explain Howareearthquakes and volcanoesrelated to the theory of platetectonics?

Visual Check8. Interpret Doearthquakes and volcanoesoccur anywhere away fromplate boundaries? If so,where?

. -".r:,..,'.-.,.~

Plate Tectonics 6S

..... :--

• Reading Check

9. Explain What causesconvection?

~ Key Concept Check10. Cause and EffectWhydo tectonic platesmove?

• Reading Check

11. Explain What causesridge push? (Circlethecorrect answer.)a. a plate going into the

mantleb. force on the bottom of

a platec. movement along a

mid-ocean ridge

66 PlateTectonics

Plate MotionYou learned that the main objection to Wegener's

continental drift hypothesis was that he could not explainhow or why Earth's continents move. Scientists nowunderstand that continents move because the asthenospheremoves underneath the rigid líthosphere.

Convection CurrentsThe circulation of material caused by differences in temperature and

density is called convection. For example, the upstairs floors ofhomes are often warmer because hot air ríses. Hot air is lessdense than cold aír, As the cold air. sinks, the hot air ríses. &

Convection in the mantle is related to plate tectonicactivity. The warmth for convection comes from radioactiveelements inside Earth, such as uraníum, thoríum, andpotassíum. When materials such as solid rock are heated,they expand and become less dense. Heated mantlematerial rises and comes in contact with Earth's crust.Thermal energy is transferred from hot mantle material tothe colder surface above. As the mantle cools, it becomesdenser and sinks, forming a convection current. Thesecurrents in the asthenosphere act like a conveyor beltmoving the lithosphere above it. Therefore, tectonic platesmove in response to the heating and cooling of mantlematerial. ~

Forces Causing Plate MotionHow can something as large as the Pacific Plate move?

Convection currents in the mantle produce enormous forcesthat can move Earth's massive plates, These forces are basaldrag, ridge push, and slab pullo Scientists' opinions differ onwhich force is strongest.

Basal Drag Convection currents in the mantle produce aforce on plates that causes motion called basal drag,Convection currents in the asthenosphere can drag theIíthosphere. This is similar to how a conveyor belt movesitems at a supermarket.

Ridge Push Recall that mid-ocean ridges are higher than thesurrounding seafloor. Because mid-ocean ridges are elevated,gravity pulls the surrounding rocks down and away from theridge. Rising mantle material at mid-ocean ridges creates the potentialfor plates to move away from the ridge with a force called ridge push.Ridge push moves the lithosphere in opposite directionsaway from the mid-ocean ridge .•

ReadingEssentials

Slab Pul! You learned that when tectonic plates collide, thedenser plate will sink into the mantle along a subductionzone. This sinking plate is called a slab. Because the slab isold and cold, it is denser than the surrounding mantle.Therefore, it sinks. Asa slab sinks, it pulls on the rest of the plate

with a force called slab pull. Ihis is similar to pushing atablecloth over the edge of a table. When enough of thecloth slides over the edge, it will pull the rest of the cloth offthe table.

A Theory in ProgressPlate tectonics is often said to be the unifying theory in

geology. It explains the connection between continentaldrift and the formation and destruction of crust along plateboundaries. lt also helps explain why earthquakes andvolcanoes occur and why mountains formo

The ínvestígatíon that Wegener began nearly a centuryago is still being updated. Several questions remain.

• Why is Earth the only planet in the solar system thathas plate tectonic activity? No other planet in our solarsystem is known to have active tectonic plates.

• Why do some earthquakes and volcanoes occur farfrom plate boundaries? Perhaps it is because plates arenot perfectly rigid. Different thicknesses and weaknessesexist within plates. Also, the mantle is much moreactive than scientists originally understood.

• What forces actually dorninate plate motions?Currently accepted models suggest that convection cur-rents occur in the mantle. However, there is no way tomeasure or observe them .

• How will scientists answer these questions? One topie ofinterest is creating 3-D images of seisrnic wave velocitíesin a subduction zone. This technology is called anisot-ropy. It might help scientists better understand the pro-cesses that occur within the mantle and along plateboundaries .•

Reading Essentials

Math Skills }The plates along the Mid-Atlantic Ridge spread at anaverage rate of 2.5 cm/y.How long will it take theplates to spread 1 m?Use proportions to find theanswer.a. Convert the distance to

the same unit.lm=lOOcm

b. Set up a proportion:2.5 cm 100 cm1Y=---xy

C. Cross-multiply and solvefor X as follows:

2.5 cm x xy = 100 cm X 1Y

d. Divide both sides by2.5 cm.

100 cm yxy = 2.5 cm

x=40y12. Use Proportions TheEurasianPlate travels theslowest, at about 0.7 cm/y.How long would it take theplate to travel 3 m?

• Reading Check

13. Explain Whydoes thetheory of plate tectonicscontinue to change?

PlateTectonics 67