258 Chapter 9

9.3 Actions at Plate Boundaries

Reading StrategyOutlining Before you read, make anoutline of this section. Use the greenheadings as the main topics and the blueheadings as subtopics. As you read, addsupporting details.

Key ConceptsWhat is seafloorspreading?

What is a subductionzone?

Vocabulary◆ oceanic ridge◆ rift valley◆ seafloor spreading◆ subduction zone◆ trench◆ continental

volcanic arc◆ volcanic island arc

Tremendous forces are at work where tectonic plates meet. Let’s takea closer look at what happens at the three types of plate boundaries.

Divergent BoundariesMost divergent plate boundaries are located along the crests of oceanicridges. These plate boundaries can be thought of as constructive platemargins because this is where new oceanic lithosphere is generated.Look again at the divergent boundary in Figure 7A on page 255. As theplates move away from the ridge axis, fractures are created. These frac-tures are filled with molten rock that wells up from the hot mantlebelow. Gradually, this magma cools to produce new slivers of seafloor.Spreading and upwelling of magma continuously adds oceanic litho-sphere between the diverging plates.

Oceanic Ridges and Seafloor Spreading Along well-developed divergent plate boundaries, the seafloor is elevated, formingthe oceanic ridge. The system of ocean ridges is the longest physicalfeature on Earth’s surface, stretching more than 70,000 kilometers inlength. This system winds through all major ocean basins like the seamon a baseball. The term ridge may be misleading. These features arenot narrow like a typical ridge. They are 1000 to 4000 kilometers wide.Deep faulted structures called rift valleys are found along the axes ofsome segments. As you can see in Figure 9, rift valleys and spreadingcenters can develop on land, too.

Actions at Boundaries

I. Divergent Boundaries

A.

B.

II. ?

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For: Links on plate boundaries

Visit: www.SciLinks.org

Web Code: cjn-3093

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FOCUS

Objectives9.7 Explain how seafloor spreading

and continental rifting causeformation of new lithosphere.

9.8 Describe the process of litho-sphere destruction that takesplace at subduction zones.

9.9 Differentiate amongsubduction at oceanic-continental, oceanic-oceanic,and continental-continentalconvergent boundaries.

9.10 Describe the action of plates ata transform fault boundary.

Build VocabularyWord Parts Have students break theword subduction into roots, prefixes,or suffixes. They may need to use adictionary to find the meaning of someparts. (Subduction comes from the Latinprefix sub-, meaning “below” and theLatin root word ducere, meaning “to drawor pull.” Thus, subduction means to drawor pull below.)

Reading StrategyI. Divergent Boundaries

A. Ocean Ridges and SeafloorSpreading

B. Continental RiftsII. Convergent Boundaries

A. Oceanic-ContinentalB. Oceanic-OceanicC. Continental-Continental

III.Transform Fault Boundaries

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Reading Focus

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Section 9.3

Download a worksheet on plateboundaries for students tocomplete, and find additionalteacher support from NSTA SciLinks.

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Rift valleys

Red Sea

Gulf of Aden

IndianOcean

LakeVictoria

LakeTanganyika

AFRICA AfarLowlands

Mt. KenyaMt.Kilimanjaro

ArabianPeninsula

NileR

iver

LakeNyasa

Plate Tectonics 259

Upwarping

Continental crust

Riftvalley

Linear sea

Rift

Oceanic ridge

Oceanic crust

Continentalcrust

A

B

C

D

Spreading Center

Seafloor spreading is the process by which plate tectonicsproduces new oceanic lithosphere. Typical rates of spreading aver-age around 5 centimeters per year. These rates are slow on a humantime scale. However, they are rapid enough so that all of Earth’socean basins could have been generated within the last 200 millionyears. In fact, none of the ocean floor that has been dated is olderthan 180 million years.

Figure 9 The East African riftvalleys may represent the initialstages of the breakup of acontinent along a spreadingcenter. A Rising magma forces thecrust upward, causing numerouscracks in the rigid lithosphere. BAs the crust is pulled apart, largeslabs of rock sink, causing a riftzone. C Further spreading causesa narrow sea. D Eventually, anocean basin and ridge system iscreated. Relating Cause and EffectWhat causes the continental crust to stretch and break?

INSTRUCT

Divergent BoundariesBuild Math SkillsConversion Factors Remind studentsto label the units of each factor whensolving the following problem. Doingthis ensures that all the conversionfactors are included and the answer hasthe correct units. Tell students thatseafloor spreading occurs at an averagerate of 5 cm per year. At this rate, howlong would it take for a narrow sea thatis 1 km wide to form? (about 20,000years)Logical

Use VisualsFigure 9 Point out the rising magmain each of the diagrams. Ask: Whathappens to the rising magma in thediagram? (It fills the cracks formed by thediverging plates.) Why is this processcalled seafloor spreading? Does theseafloor actually get thinner andspread out? (The seafloor does not getthinner. Rather, the seafloor spreads apartand new rock is constantly added to theridge.)Visual

Build Science SkillsUsing Models Givestudents two colors ofmodeling clay (onecolor for the magmaand the other color for the crust) andhave them model the activity thatoccurs at a divergent boundary.Kinesthetic, Visual

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Plate Tectonics 259

Customize for English Language Learners

Encourage students to work in groups tobrainstorm different types of boundaries.Their types of boundaries can come from othersciences, such as cell membranes, or fromeveryday life, such as the boundary between

a sidewalk and the strip of grass between thesidewalk and the curb. Ask students what allthese boundaries have in common and howthey are different.

Answer to . . .

Figure 9 The continental crust isstretched and broken by the upwarpingof the crust, caused by rising magma.

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Continental Rifts When spreading centers develop within acontinent, the landmass may split into two or more smaller segments.Examples of active continental rifts include the East African rift valleyand the Rhine Valley in Northwest Europe.

The most widely accepted model for continental breakup suggeststhat forces that are stretching the lithosphere must be acting on the plate.These stretching forces by themselves are not large enough to actuallytear the lithosphere apart. Rather, the rupture of the lithosphere isthought to begin in those areas where plumes of hot rock rise from themantle. This hot-spot activity weakens the lithosphere and creates domesin the crust directly above the hot rising plume. Uplifting stretches thecrust and makes it thinner, as shown in Figure 9A. Along with thestretching, faulting and volcanism form a rift valley, as in Figure 9B

The East African rift valley, shown in Figure 10, may represent thebeginning stage in the breakup of a continent. Large mountains, such asKilimanjaro and Mount Kenya, show the kind of volcanic activity thataccompanies continental rifting. If the stretching forces continue, therift valley will lengthen and deepen, until the continent splits in two. Atthis point, the rift becomes a narrow sea with an outlet to the ocean,similar to the Red Sea. The Red Sea formed when the Arabian Peninsularifted from Africa about 20 million years ago. In this way, the Red Seaprovides scientists with a view of how the Atlantic Ocean may havelooked in its infancy.

How do rifts begin to form?

Figure 10 East African RiftValley This valley may be wherethe African continent is splittingapart.Interpreting Diagrams Whatstage in the drawings on page259 does this photograph show?

260 Chapter 9

Creating aContinental RiftPurpose Students will observe howfractures grow to create a continentalrift as a result of the stretching of thelithosphere.

Materials 2 slices of individuallywrapped American cheese, dull knifeor fingernail, metric ruler

Procedure Using your fingernail or adull knife, make a small cut in the centerof a cheese slice parallel to one edge.Pull on the two cheese edges parallelto the cut. You will be pullingperpendicular to the direction of thecut. Observe how the small defect (thecut) concentrates the tearing. Observethe shape of the fracture that forms,especially the pointed tips where thetearing is taking place, and how thefracture tips move faster as the fracturegets bigger.

Now, make a cut near the center of thesecond piece of cheese. Make a secondparallel cut about 2 cm below and 2 cmto the right of the first cut. Pull on thecheese as before. Fractures will begin toform from each of the cuts. As the tipsof these fractures begin to move pasteach other, they will begin to curvetoward each other and eventually linkup into a single fracture.

Safety Do not allow students to eat thecheese.

Expected Outcome Students shouldinfer that the fractures in the cheese areanalogous to the formation of faults thatresult in the development of a rift valley.Visual, Logical

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Section 9.3 (continued)

The first rift that developed as Pangaea beganto break apart 200 million years ago resultedin the separation of North America and Africa.Large quantities of basalts were produced.These basalts can be found today asweathered rock beds along the eastern

seaboard of the United States. They are buriedbeneath rocks that form the continental shelfand have been radiometrically dated as beingbetween 200 million and 165 million yearsold. The rifting eventually formed the AtlanticOcean basin.

Facts and Figures

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Subducting oceanic lithosphere

Oceanic crust

Trench

Continentalvolcanic arc

Continental crust

Asthenosphere

Continentallithosphere

Melting

100 km

200 km

Plate Tectonics 261

Figure 11 Oceanic-ContinentalConvergent Boundary Oceaniclithosphere is subducted beneatha continental plate. Inferring Why doesn’t volcanicactivity occur closer to the trench?

Convergent BoundariesAlthough new lithosphere is constantly being added at the oceanicridges, our planet is not growing larger. Earth’s total surface area remainsthe same. How can that be? To accommodate the newly created litho-sphere, older portions of oceanic plates return to the mantle alongconvergent plate boundaries. Because lithosphere is “destroyed” at con-vergent boundaries, they are also called destructive plate margins. As twoplates slowly converge, the leading edge of one is bent downward, allow-ing it to slide beneath the other. Destructive plate margins where oceaniccrust is being pushed down into the mantle are called subduction zones.The surface feature produced by the descending plate is an ocean trench,as shown in Figure 11. A subduction zone occurs when one oceanicplate is forced down into the mantle beneath a second plate.

Convergent boundaries are controlled by the type of crust involvedand the forces acting on the plate. Convergent boundaries can formbetween two oceanic plates, between one oceanic plate and one conti-nental plate, or between two continental plates.

Oceanic-Continental When the leading edge of a continentalplate converges with an oceanic plate, the less dense continental plateremains floating. The denser oceanic slab sinks into the asthenosphere.When a descending plate reaches a depth of about 100 to 150 kilome-ters, some of the asthenosphere above the descending plate melts. Thenewly formed magma, being less dense than the rocks of the mantle,rises. Eventually, some of this magma may reach the surface and causevolcanic eruptions.

The volcanoes of the Andes, located along western South America,are the product of magma generated as the Nazca plate descendsbeneath the continent. Figure 11 shows this process. The Andes are anexample of a continental volcanic arc. Such mountains are producedin part by the volcanic activity that is caused by the subduction ofoceanic lithosphere.

ConvergentBoundariesBuild Reading LiteracyRefer to p. 420D in Chapter 15, whichprovides the guidelines for thispredicting strategy.

Predict Have students read the sectionon p. 260 about continental rifts. Ask:Predict what a rift valley might looklike if it stopped developing. (Thevalley would probably be filled withancient volcanic rocks that formed fromthe magma that rose to the surface.)Logical

Use VisualsFigure 11 Have students studythe diagram showing an oceanic-continental convergent boundary. Ask:Which plate is subducted? Whichplate floats? (The oceanic plate issubducted. The continental plate floats.)Why do the two plates in the diagramalways move the way they do?(The oceanic plate is denser than thecontinental plate, so it slides under thecontinental plate and sinks into theasthenosphere.)Visual, Logical

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Plate Tectonics 261

Answer to . . .

Figure 10 Large slabs of rock sink,causing a rift zone.

Figure 11 The plate doesn’t get deepenough for melting to occur untilfarther from the trench.

Rifts begin when thelithosphere is stretched

and a plume of hot rock from themantle weakens and then splits thelithosphere.

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Oceanic-Oceanic When two oceanic slabs converge, one descendsbeneath the other. This causes volcanic activity similar to what occursat an oceanic-continental boundary. However, the volcanoes form onthe ocean floor instead of on a continent, as shown in Figure 12. If thisactivity continues, it will eventually build a chain of volcanic structuresthat become islands. This newly formed land consisting of an arc-shaped chain of small volcanic islands is called a volcanic island arc.The Aleutian Islands off the shore of Alaska are an example of a volcanicisland arc. Next to the Aleutians is the Aleutian trench.

Continental-Continental When an oceanic plate is sub-ducted beneath continental lithosphere, a continental volcanic arcdevelops along the margin of the continent. However, if the subduct-ing plate also contains continental lithosphere, the subductioneventually brings the two continents together, as shown in Figure 13.Continental lithosphere is buoyant, which prevents it from being sub-ducted to any great depth. The result is a collision between the twocontinents, which causes the formation of complex mountains suchas the Himalayas in South Asia.

Subducting oceanic lithosphere

Oceanic crust

Trench

Continental crust

Melting

Asthenosphere

Oceanic lithosphere

Volcanic island arc

100 km

200 km

Continentallithosphere

Oceanic plate

Continentallithosphere

Asthenosphere

100 km

200 km

SutureFigure 13 Continental-Continental ConvergentBoundary Continentallithosphere cannot besubducted because it floats.The collision of twocontinental plates formsmountain ranges.

Figure 12 Oceanic-OceanicConvergent Boundary Oneoceanic plate is subductedbeneath another oceanic plate,forming a volcanic island arc. Predicting What wouldhappen to the volcanic activityif the subduction stopped?

262 Chapter 9

A commonly held misconception is thatthe volcanoes in a volcanic island arc areinterconnected and that an eruption ofone volcano in the arc will trigger erup-tions in all the volcanoes. Draw a cross-sectional diagram similar to Figure 12.Show a separate magma chamber foreach volcano in the arc.Visual

Use VisualsFigure 12 Have students study thediagram showing an oceanic-oceanicconvergent boundary. Ask: How is anoceanic-oceanic convergent boundarydifferent from an oceanic-continentalconvergent boundary? (Volcanoes formon the ocean floor in an oceanic-oceanicboundary rather than on Earth’s surface.)What is formed by sustained volcanicactivity at an oceanic-oceanic conver-gent boundary? (an island chain, calleda volcanic island arc)Visual, Verbal

Use VisualsFigure 13 Have students studythe diagram showing a continental-continental convergent boundary. Ask:Why isn’t the continental lithospheresubducted far into the asthenospherein this diagram? (The continentallithosphere is buoyant and does not sinkinto the asthenosphere to a great depth.)Why aren’t volcanoes formed in acontinental-continental convergentboundary? (Because molten magma thatforms down deep is unable to rise all theway to the tops of the mountains. Themagma cools within the cores of themountains to form large granitic plutons.)Visual, Verbal

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Section 9.3 (continued)

Only two volcanic island arcs are located in theAtlantic Ocean—the Lesser Antilles adjacent tothe Caribbean Sea and the Sandwich Islandsin the South Atlantic. There have been manyvolcanic eruptions in the Lesser Antilles. In1902 on the island of Martinique, Mount Peléerupted, killing 28,000 people and destroying

the town of St. Pierre. More recently, theSoufriere Hills Volcano on the island ofMontserrat erupted from 1995 until 1997.Although volcanic activity has since decreased,seismic activity has increased. There wereseveral earthquakes on Montserrat inearly 2004.

Facts and Figures

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Subducting oceanic lithosphere

India

Continentalshelf

deposits

Continentalcrust

Ocean basin

Continental volcanic arc

Tibet

MeltingAsthenosphere

Developingaccretionary

wedge

Plate Tectonics 263

Indiatoday

10 millionyears ago

38 millionyears ago

55 millionyears ago

71 millionyears ago

Himalayas

India(Ganges Plain)

TibetanPlateau

Suture

Asthenosphere

Figure 14 A The leading edge ofthe plate carrying India issubducted beneath the Eurasianplate. B The landmasses collideand push up the crust. C India’scollision with Asia continuestoday.

Before continents collide, the landmasses involved are separatedby an ocean basin. As the continents move toward each other, theseafloor between them is subducted beneath one of the plates. Whenthe continents collide, the collision folds and deforms the sedimentsalong the margin as if they were placed in a giant vise. A new moun-tain range forms that is composed of deformed and metamorphosedsedimentary rocks, fragments of the volcanic arc, and possibly sliversof oceanic crust.

This kind of collision occurred when the subcontinent of Indiarammed into Asia and produced the Himalayas, as shown in Figure14. During this collision, the continental crust buckled and fractured.Several other major mountain systems, including the Alps,Appalachians, and Urals, were also formed as a result of continentalcollisions.

A

B

C

Collision of India and Asia

Build Science SkillsUsing Analogies The discussion onthis page uses an analogy of a giant viseto help students visualize andunderstand what happens to thelithosphere during a continental-continental collision. Be sure studentsunderstand what a vise is. (a tool thatholds an object by squeezing two platestogether, usually by turning a large screw)If they are not familiar with the term,have them look it up in a dictionary, orhave another student describe it. Revisitthe text and discuss why the analogy isuseful. (A squeezing vise could fold anddeform material as colliding continentsfold and deform rock) Ask: What otheranalogies might be used to describecontinental-continental collisions?(Sample answers: small entry rugcrumpling as it gets caught between anopening door and a wall, two carscolliding)Intrapersonal, Logical

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Plate Tectonics 263

The Himalayas include the highest mountainson Earth. When India and Asia collided, theleading edge of the Indian plate was forcedpartially under Asia, generating an unusuallygreat thickness of continental lithosphere.

This accumulation accounts in part for thehigh elevation of the Himalayas and may alsoexplain the elevated Tibetan Plateau to thenorth.

Facts and Figures

Answer to . . .

Figure 12 If the subduction stopped,the volcanic activity would probablyalso soon stop because the source ofnew magma is the continuing subduc-tion of the oceanic plate.

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264 Chapter 9

Section 9.3 Assessment

Reviewing Concepts1. What is seafloor spreading?

2. What is a subduction zone? What types ofplate boundaries have subduction zones?

3. Describe the process that occurs whencontinents converge.

4. What actions of plate boundaries cause thedestruction of the lithosphere?

Critical Thinking5. Drawing Conclusions What evidence

supports the idea that the Earth is neithergrowing nor shrinking in size?

6. Relating Cause and Effect During thecollision between two continents, why doesn’ta subduction zone form?

7. Predicting How will the angle at which anoceanic plate is subducted affect the distancefrom the volcanic arc to the trench?

Transform fault(active)

Fracturezone

Fracturezone

Creative Writing Write a paragraph thatdescribes the rifting apart of a continent toform a new ocean. The paragraph shouldbe written from the point of view of a per-son witnessing the events.

Figure 15 A transform faultboundary offsets segments ofa divergent boundary at anoceanic ridge.

Transform FaultBoundariesThe third type of plate boundary is thetransform fault boundary. At atransform fault boundary, platesgrind past each other without destroy-ing the lithosphere. Most transformfaults join two segments of a mid-oceanridge, as shown in Figure 15. Thesefaults are present about every 100 kilo-meters along the ridge axis. Activetransform faults lie between the two

offset ridge segments. The seafloor produced at one ridge axis moves inthe opposite direction as seafloor is produced at an opposing ridge seg-ment. So between the ridge segments these slabs of oceanic crust aregrinding past each other along a transform fault.

Although most transform faults are located within the ocean basins,a few cut through the continental crust. One example is the San AndreasFault of California. Along the San Andreas, the Pacific plate is movingtoward the northwest, past the North American plate. If this movementcontinues, that part of California west of the fault zone will become anisland off the west coast of the United States and Canada. It could even-tually reach Alaska. However, a more immediate concern is theearthquake activity triggered by movements along this fault system.

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Transform FaultBoundariesBuild Science SkillsRelating Cause and Effect Remindstudents that plates in a transform faultboundary move past each other withoutproduction or destruction of lithosphere.Ask: Why does this movement causeearthquakes? (The tremendous frictioncaused by two plates grinding past eachother causes earthquakes.)Logical

ASSESSEvaluateUnderstandingTo assess students’ knowledge of sectioncontent, have them write three shortparagraphs describing the threeconvergent boundaries and whatresults when they converge.

ReteachHave students demonstrate the actionof the three convergent boundaries byusing their hands to represent theconverging plates.

Answers should be accurate and showan understanding of the process ofrifting at a divergent plate boundary.

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Section 9.3 (continued)

5. the evidence that the production anddestruction of the lithosphere is going on atabout the same rate.6. Continental lithosphere floats and can’t beforced down into the mantle at a subductionzone.7. The higher the angle of subduction, thecloser the volcanic arc will be to the trench.If the angle is shallow, the volcanic arc will belocated farther behind the trench because thedescending plate doesn’t reach a depthwhere melting occurs until farther fromthe trench.

Section 9.3 Assessment

1. Seafloor spreading is the creation of newseafloor at oceanic ridges.2. Subduction zones occur at deep-oceantrenches where slabs of oceanic lithosphereare descending into the mantle. Subductionzones are associated with convergent bound-aries, either oceanic-oceanic or oceanic-continental.3. two continental plates collide with eachother, forming a mountain range4. Lithosphere is destroyed at convergentboundaries in subduction zones.

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