HSES 1eTE C11.qxd 5/19/04 6:35 AM Page 317 11.3 Mountain...

Mountain Building 317

11.3 Mountain Formation

Reading StrategyOutlining As you read, make an outline ofthe important ideas in this section. Use thegreen topic headings as the main topics andthe blue headings as subtopics.

Key ConceptsWhat mountains areassociated withconvergent plateboundaries?

What mountains areassociated with divergentplate boundaries?

How is isostaticadjustment involved inmountain formation?

Vocabulary◆ accretionary wedge◆ accretion◆ terrane◆ isostasy◆ isostatic adjustment

Mountain building still occurs in several places worldwide. Forexample, the Himalayas began to form 45 million years ago and are stillrising. Older mountain ranges, such as the Appalachians in the easternUnited States, are deeply eroded, but they have many features found inyounger mountains.

Many hypotheses have been proposed to explain mountain for-mation. One early proposal suggested that mountains are wrinkles inEarth’s crust, produced as the planet cooled from its early semi-moltenstate. People believed that as Earth cooled, it contracted and shrank. Inthis way, the crust was deformed the way an apple peel wrinkles as itdries out. However, this early hypothesis and many others were notable to withstand careful analysis and had to be discarded.

Mountain Building atConvergent BoundariesWith the development of the theory of platetectonics, a widely accepted model for oroge-nesis became available. Most mountainbuilding occurs at convergent plate bound-aries. Colliding plates provide thecompressional forces that fold, fault, and metamorphose the thicklayers of sediments deposited at the edges of landmasses. The partialmelting of mantle rock also provides a source of magma that intrudesand further deforms these deposits.

Figure 11 Young MountainsThe Grand Tetons of Wyoming arean example of relatively youngmountains.

I. Mountain FormationA. Mountain Building at Convergent Boundaries

1. Ocean-Ocean Convergence

2. a.

3. b.B. Mountain Building at Divergent Boundaries

?

?

FOCUS

Section Objectives11.8 Identify the type of mountains

associated with convergentplate boundaries.

11.9 Distinguish betweenmountains formed by ocean-ocean convergence andmountains formed by ocean-continental convergence.

11.10 Identify the type of mountainsassociated with divergent plateboundaries.

11.11 Explain how isostaticadjustment is involved inmountain formation.

Build VocabularyParaphrase After students have readthe definition of accretionary wedge onp. 319, but before they have learned thedefinition of accretion on p. 321, havethem write a short paragraph describingin their own words the process ofaccretion. After they have read thedefinition of accretion, ask if theywould change their definitions.

Reading StrategyI. Mountain Formation

A. Mountain Building at ConvergentBoundaries1. Ocean-Ocean Convergence2. Ocean-Continental

Convergence3. Continent-Continent

ConvergenceB. Mountain Building at Divergent

BoundariesC. Non-Boundary MountainsD. Continental Accretion

1. Terranes2. Mountains from Accretion

E. Principle of Isostasy1. Isostatic Adjustment for

Mountains

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Reading Focus

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Section 11.3

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318 Chapter 11

Subduction zone

Asthenosphere

Melting

Volcanicisland arc

100 km

Subducting oceanic lithosphere

Subduction zone

Melting

Volcanic island arc

Sedimentation

Asthenosphere

100 km


A B

Figure 12 A A volcanic island arcdevelops due to the convergenceof two oceanic plates. B Continuedsubduction along this type ofconvergent boundary results in the development of volcanicmountains.

Ocean-Ocean Convergence

Ocean-Ocean Convergence Ocean-ocean convergence occurswhere two oceanic plates converge and one is subducted beneath theother, as shown in Figure 12. The converging plates result in partialmelting of the mantle above the subducting plate and can lead to thegrowth of a volcanic island arc on the ocean floor. Because they areassociated with subducting oceanic lithosphere, island arcs are typicallyfound on the margins of a shrinking ocean basin, such as the Pacific.These features tend to be relatively long-lived. Here, somewhat sporadicvolcanic activity, the depth of magma, as well as the accumulation ofsediment that is scraped off the subducting plates, increases the volumeof the crust. An example of an active island arc is the Aleutian arc, whichforms the Aleutian Islands in Alaska. Some volcanic island arcs, such asJapan, appear to have been built up by two or three different periods of subduction. As shown by Japan, the continued development of a volcanic island arc can result in the formation of mountains made up of belts of igneous and metamorphic rocks. Ocean-oceanconvergence mainly produces volcanic mountains.

Ocean-Continental Convergence Mountain building alongcontinental margins involves the convergence of an oceanic plate and aplate whose leading edge contains continental crust. A good example isthe west coast of South America. In this area, the Nazca plate is beingsubducted beneath the South American plate along the Peru-Chiletrench. As shown by the Andes Mountains, ocean-continental conver-gence results in the formation of a continental volcanic arc inland ofthe continental margin.

318 Chapter 11

INSTRUCT

Mountain Buildingat ConvergentBoundariesUse VisualsFigure 12 Have students study thediagrams of ocean-ocean convergence.Ask: What is the result of theconvergence of two oceanic plates?(formation of a volcanic island arc) Whydoes the volume of the crust increasein this type of convergence? (Volcanicmagma is added to the crust. Sedimentthat is scraped off the subducting plateaccumulates to increase crustal volume.)Visual, Logical

Partial MeltingPurpose Students observe the processthat generates magma during boundaryconvergence.

Materials can of frozen grape juiceconcentrate (inexpensive brands workbest), can opener, plastic tub, rubbergloves, apron, water, plastic pitcher,paper towels for clean-up

Procedure Allow the grape juice tothaw slightly, but do not let it liquefy.Use a can opener or pull tab to removeone end of the can. Squeeze the juiceconcentrate out of the can through yourhands, letting it fall into the plastic tub.Tell students to look for evidence ofpartial melting in the concentrate. Besure to wear an apron and rubbergloves. Use the water, pitcher, andpaper towels for cleaning up.

Expected Outcomes The sugary juicehas a melting point of about �40°C,whereas the ice crystals in the juice meltat 0°C. If the juice is thawed to atemperature of about �5°C, part of themixture is liquid, but the ice crystals arestill solid. Explain to students that whena plate is subducted, it experiencespartial melting. Sometimes the mantlewedge above the subducting plate alsocan melt when magma coming up froma subducted slab doesn’t get all the wayto the surface immediately. This meltingand migration generates magma andcauses the formation of volcanic arcs.Kinesthetic, Visual

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Section 11.3 (continued)

Customize for English Language Learners

Have students use a dictionary to look up theorigin of the term isostasy. (Iso- means “equal”and stasis means “standing.”) Ask students tobrainstorm words that begin with iso-. Askthem how the words are similar or different in

meaning. (Students may know the word isobar[equal air pressure] and isotherm [equaltemperature] from meteorology, isotonic [equalconcentration] from chemistry, and isometricand isosceles [equal sides] from geometry.)



The convergence of the con-tinental block and the subductionof the oceanic plate leads to defor-mation and metamorphism ofthe continental margin. Partialmelting of mantle rock above thesubducting slab generates magmathat migrates upward. This melt-ing and fluid migration occursonce the oceanic plate moves downto about 100 kilometers. During thedevelopment of this continental vol-canic arc, sediment derived from theland and scraped from the subductingplate is stuck against the landward side of the trench. This accumulationof different sedimentary and metamorphic rocks with some scraps ofocean crust is called an accretionary wedge. A long period of subduc-tion can build an accretionary wedge of rock that is large enough to standabove sea level, as shown in Figure 13.

Ocean-continental convergence produces mountain ranges com-posed of two roughly parallel belts. The continental volcanic arcdevelops on the continental block. The arc consists of volcanoes andlarge intrusive bodies mixed with high-temperature metamorphicrocks. The seaward belt is the accretionary wedge. It consists of folded,faulted sedimentary and metamorphic rocks. The types of moun-tains formed by ocean-continental convergence are volcanicmountains and folded mountains.

Rates of Mountain Building

2. Calculating Assuming a rate of uplift of1 centimeter per year, how much highercould the Himalayas be in one million years?

3. Applying Concepts If the convergence oftectonic plates is causing the Himalayas to risein elevation, what common surface processesare working to decrease their elevations?

4. Inferring Do you think it is reasonable forthe Himalayas to continue to rise in elevationindefinitely? Explain your answer.

The mighty Himalayas between India and Tibet arethe tallest mountains on Earth, rising to more than8 kilometers. These mountains are still rising atabout 1 centimeter per year. Mount Everest is thetallest peak with an elevation of 8848 metersabove sea level. The Himalayas formed as a resultof India colliding with the Eurasian plate.

1. Calculating If you assume that theHimalayas will continue to be uplifted at thecurrent rate of 1 centimeter per year, howlong will it take the mountains to rise another500 meters?

Figure 13 Ocean-ContinentalConvergence Plate convergencegenerates a subduction zone, andpartial melting produces acontinental volcanic arc.Continued convergence andigneous activity further deformsthe crust and forms a roughlyparallel folded mountain belt.Observing What type ofmountains result from the partial melting?


Trench

Accretionarywedge

Asthenosphere

100 km

Continentalvolcanic arc

Melting

Rates of MountainBuilding1. 1 cm/year � 0.01 m/year;500 m � 0.01 m/year � 50,000 years2. 1,000,000 years � 0.01 m/year �10,000 m or 10 km3. the processes of weathering anderosion4. Accept all reasonable answers. Apossible answer could be as follows:No, the mountains probably can’t keepgrowing taller forever. Erosion wouldoccur at a faster pace than mountainbuilding. In addition, Earth’s mantleprobably can support only a particularamount of lithospheric thickness. Thearea of the mountains may begin tospread out or the convergence will stop.Logical

For Extra HelpReview conversion factors anddimensional analysis with students. Havestudents practice making conversionsfrom centimeters to meters and frommeters to centimeters by usingdimensional analysis. Remind studentsto label units for each factor and to makesure all unwanted units cancel out toyield a final answer in the desired units.

Ask students if Mount Everest will alwaysbe the highest mountain on Earth. Ifthey say yes, they may think thatmountain-building processes are nolonger taking place and that mountainswill remain the same size as they arenow. Because the Indian and Eurasianplates continue to push against eachother—at a rate of about 2 cm a year—the Himalayas are continuing to grow.According to data from recent GPSexperiments, the Himalayas are growingat a rate of 5 mm a year. Ask: Whatprocess could prevent the Himalayasfrom growing? (erosion) What wouldhappen if the Indian and Eurasianplates stop moving? (The Himalayas willstop growing and will decrease in heightdue to erosion.)Logical

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For years, scientists thought that the elevationof Mount Everest was 8848 m above sea level.A number of attempts over several years weremade to set up Global Position System (GPS)equipment so the mountain could be measuredusing satellite-based technology. A team ofseven climbers successfully measured the

mountain from the summit on May 5, 1999.The data were collected from various GPSsatellite receivers, one of which had to beplaced in bedrock, at the top of Mount Everest.Using GPS technology, Mount Everest finallywas re-measured and was found to be 8850 mabove sea level.

Facts and Figures

Answer to . . .

Figure 13 volcanic mountains


320 Chapter 11

Continent-ContinentConvergence Continental

crust floats too much to be sub-ducted. At a convergent

boundary between twoplates carrying continen-tal crust, a collisionbetween the continentalfragments will result andform folded mountains.

An example of such a collision began about 45 million years ago whenIndia collided with the

Eurasian plate, as shown in Figure 14. Before this event, India was oncepart of Antarctica, and it had split from that continent over the courseof millions of years. It slowly moved a few thousand kilometers duenorth. The result of the collision was the formation of the spectacularHimalaya Mountains and the Tibetan Plateau. Most of the oceanic crustthat separated these landmasses before the collision was subducted, butsome was caught up in the collision zone, along with the sediment alongthe shoreline. Today these sedimentary rocks and slivers of oceanic crustare elevated high above sea level.

A similar but much older collision is believed to have taken placewhen the European continent collided with the Asian continent toproduce the Ural Mountains in Russia. Before the theory of plate tec-tonics, geologists had difficulty explaining mountain ranges such asthe Urals, which are located far within continents.

Why can’t continental crust be subducted?

Figure 14 Continental-Continental Convergence Theongoing collision of India andAsia started about 45 millionyears ago and produced themajestic Himalayas. A Convergingplates generated a subductionzone, producing a continentalvolcanic arc. B Eventually the twolandmasses collided, whichdeformed and elevated themountain range.


India Continentalshelf

deposits

Continentalcrust

Ocean basin

Continental volcanic arc

Tibet

MeltingAsthenosphere

Developingaccretionary

wedge

Himalayas

India(Ganges Plain)

TibetanPlateau

Suture

Asthenosphere

A

B

320 Chapter 11

Use VisualsFigure 14 Have students examinethe diagram of continent-continentconvergence. Ask: Why are theresedimentary rocks and oceanic crusthigh above sea level in the Himalayas?(They come from sediment and bits ofcrust that were scraped off an oceanic slabas it subducted under India. When Indiaand Asia collided, this material wasuplifted.) What caused the formationof a continental volcanic arc in theHimalayas? (partial melting of theoverlying mantle rocks triggered by thesubducting oceanic slab)Visual, Logical

Build Science SkillsInferring Have students infer how amountain range can occur far inland.(Two continental plates must have collidedsometime in the past.)Logical

Use CommunityResourcesInvite a civil engineer to your class, andhave students interview him or her tofind out what a civil engineer needs toknow about local landforms whenplanning new roads. Have studentsprepare questions for the guest speakerin advance of his or her visit.Verbal

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Numerous earthquakes recorded off thesouthern coast of India indicate that a newsubduction zone may be forming. If formed, itwould provide a subduction site for the floorof the Indian Ocean, which is continually

being produced at a spreading center locatedto the southwest. Should this occur, India’snorthward journey, relative to Asia, wouldcome to an end, and the growth of theHimalayas would cease.

Facts and Figures



Mountain Building at DivergentBoundariesMost mountains are formed at convergent boundaries, but some areformed at divergent boundaries, usually on the ocean floor. Thesemountains form a chain that curves along the ocean floor at the oceanridges. This mountain chain is over 65,000 kilometers long and rises to2000 to 3000 meters above the ocean floor. The mountains thatform along ocean ridges at divergent plate boundaries are fault-blocktype mountains. The mountain chain that makes up the Mid-AtlanticRidge is an example.

Non-Boundary MountainsEven though most mountains are formed at plate boundaries, some arefound far from any boundaries. Some upwarped mountains, fault-blockmountains, and volcanic mountains are not formed at plate boundaries.Volcanic mountains such as the Hawaiian Islands are formed at a hotspot, far from any plate boundary. Many fault-block mountains occur inareas that are undergoing regional extension or stretching. These areasmay possibly become a plate boundary if the plate rifts apart.

Continental AccretionPlate tectonics theory originally suggested two major mechanisms fororogenesis at convergent boundaries: continental collisions and sub-duction of oceanic lithosphere to form volcanic arcs. Further studieshave lead to another mechanism in which smaller crustal fragmentscollide and merge with continental margins. When the fragments col-lide with a continental plate they become stuck to or embedded intothe continent in a process called accretion. Many of the mountainousregions rimming the Pacific have been produced through the processof collision and accretion.

Where is the longest mountain range?

Continentallandmass

TrenchInactive volcanic

island arc

Asthenosphere


Collision ofvolcanic

island arcand continent

Asthenosphere

Continentallandmass

Asthenosphere


Inactive volcanicisland arc Trench

Mountain Building by Continental Accretion

Figure 15 This sequenceillustrates the collision of aninactive volcanic island arc withthe margin of a continental plate.The island arc becomes embeddedor accreted onto the continentalplate.

A B C

For: Links on mountain building

Visit: www.SciLinks.org

Web Code: cjn-3113

Mountain Building atDivergent BoundariesBuild Science SkillsProblem Solving Have studentsdescribe a process whereby mountainscan form at divergent boundaries.Students should draw a series of diagramsshowing the formation of an ocean ridgeand describe what is happening in eachdiagram.Visual

Non-BoundaryMountainsBuild Reading LiteracyRefer to p. 124D in Chapter 5, whichprovides the guidelines for summarizing.

Summarize Have students reread thedescription in Chapter 9 of how hotspots cause the formation of volcanicmountain arcs. Students should write ashort summary of the process. Thenthey should write a paragraph abouthow the formation of hot spots relatesto this section.Verbal

Continental AccretionUse VisualsFigure 15 Have students examine thediagram of continental accretion. Ask:What is the crustal fragment in thesediagrams? (the volcanic island arc) Willthe mountains that are formed by thisprocess be as large as mountainsformed by converging continentalplates? (No, there is much less crustalmaterial in the fragment.)Visual, Logical

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Download a worksheet onmountain building for students tocomplete, and find additionalteacher support from NSTASciLinks.

Answer to . . .

It is too buoyant.

the Mid-Atlantic Ridgeon the Atlantic Oceanfloor


322 Chapter 11

Terranes Geologists refer to accreted crustalblocks as terranes. A terrane is any crustal frag-ment that has a geologic history distinct from thatof the adjoining terranes. Terranes come in manyshapes and sizes. Some are no larger than volcanicislands, while others are immense, such as the onemaking up the entire Indian subcontinent. Beforetheir accretion to a continental block, some of thefragments may have been microcontinents similarto the present-day island of Madagascar, locatedin the Indian Ocean east of Africa. Many otherswere island arcs like Japan and the Philippines.

As oceanic plates move, they carry the embed-ded volcanic island arcs and microcontinentsalong with them. Eventually a collision betweenthe crustal fragment and the continent occurs.Relatively small crustal pieces are peeled from theoceanic plate at a subduction zone, and thin sheetsof the crustal fragment are thrust onto the conti-nental block. This newly added material increasesthe width and thickness of the continental crust.The material may later be displaced farther inlandby the addition of other fragments.

Mountains from Accretion The accre-tion of larger crustal fragments, such as a matureisland arc, may result in a mountain range. Thesemountain ranges are much smaller than the onesthat result from a continent–continent collision.Because of its buoyancy, or ability to float, anisland arc will not subduct beneath the continen-tal plate. Instead, it will plow into the continentand deform both blocks.

The idea that mountain building occurs inconnection with the accretion of crustal fragmentsto a continental mass came mainly from studies

in western North America. See Figure 16. Areas in the mountains ofAlaska and western Canada were found to contain rocks, fossils, andstructures that were different from those in surrounding areas. Theseareas have been accreted to the western margin of North America.

What is a terrane?

CanadaUnited States

Craton

United StatesMexico0 600 km

Accreted Terranes

Island arc

Submarinedeposits

Ancientocean floor

DisplacedcontinentalfragmentsW

rangellia Terrane

SonomaTerrane

Figure 16 Accretion inWestern North America Theseterranes are thought to havebeen added to western NorthAmerica during the past 200million years. Interpreting Maps What do theareas in blue represent?

322 Chapter 11


Students often confuse a terrane with aterrain. The term terrane is used todesignate a distinct and recognizableseries of rock formations that has beentransported by plate tectonic processes.Since geologists who mapped theserocks were unsure where they camefrom, these rocks were sometimes called“exotic,” “suspect,” “accreted,” or“foreign” terranes. The term terraindescribes the shape of the surfacetopography or “lay of the land.” Havestudents use the two terms in sentencesso that their meanings are clear.Verbal

Use VisualsFigure 16 Have students examine themap showing terranes added to westernNorth America. Ask: What mechanismof plate tectonics might have causedthe addition of so many terranes tothe West Coast? (The North Americanplate could have moved westward,overriding the Pacific Basin and picking upcrustal fragments as it moved.) What wasthe origin of the Baja Peninsula? (Itwas originally an island arc.) What doesthis map imply about the number oftimes crustal material has been addedto the West Coast? (Based on the manysources of crustal material, it is likely thatmaterial has been added many times.)Visual, Logical

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Imagine 45-million-year-old tree stumps of50-m-tall trees existing next to glaciers. Thesetrees resemble the remains of a redwood forestthat once grew in northern California. Fossilsof palm trees and tropical rain forest plantshave all been found in Alaska within 800 km ofthe North Pole. Not only is the present climatetoo cold to support life of these organisms,but they would not be able to live in a region

that has limited sunlight five months of theyear. Either the climate of Earth was differentwhen the palms grew 45 million years ago orelse the rocks in which the fossils occur movedin from someplace else. Most paleontologiststhink the rocks were once part of terranes thatwere added to the Pacific coast of NorthAmerica.

Facts and Figures



Principle of IsostasyIn addition to the horizontal movements of lithos-pheric plates, gradual up-and-down motions of thecrust are seen at many locations around the globe.Although much of this vertical movement occurs alongplate margins and is linked to mountain building, someof it is not. The up-and-down motions also occur in theinteriors of continents far from plate boundaries.

Earth’s crust floats on top of the denser and moreflexible rocks of the mantle. The concept of a floatingcrust in gravitational balance is called isostasy (iso � equal and stasis �

standing). One way to understand the concept of isostasy is to thinkabout a series of wooden blocks of different heights floating in water,as shown in Figure 17. Note that the thicker wooden blocks float higherthan the thinner blocks. In a similar way, many mountain belts standhigh above the surface because they have buoyant (less dense) crustal“roots” that extend deep into the denser mantle. The denser mantlesupports the mountains from below.

What would happen if another small block of wood were placed ontop of one of the blocks in Figure 17? The combined block would sinkuntil a new isostatic balance was reached. However, the top of the com-bined block would actually be higher than before, and the bottomwould be lower. This process of establishing a new level of gravitationalequilibrium is called isostatic adjustment.

Figure 17 IsostaticAdjustment This drawingillustrates how wooden blocks ofdifferent thicknesses float inwater. In a similar manner, thicksections of crustal material floathigher than thinner crustal slabs.Inferring Would a denserwooden block float at a higher orlower level?

Isostatic Adjustment in Mountains

Mountain range

Oceaniccrust

Continentalcrust

Mountain remnant

Continental crustContinentalcrust

Erosion

Uplift

DepositionDeposition

Sinking

A

B C

Figure 18 This sequence illustrates how thecombined effect of erosion and isostaticadjustment results in a thinning of the crust inmountainous regions.

A When mountains are young, thecontinental crust is thickest.

B As erosion lowers the mountains,the crust rises in response to thereduced load.

C Erosion and uplift continue untilthe mountains reach “normal”crustal thickness.

Principle of Isostasy

Modeling IsostasyPurpose Students model the principleof isostasy by measuring how highblocks of varying density float in water.

Materials large plastic container, water,balance, metric ruler, several blocks ofvarying sizes and kinds of wood

Procedure Wear apron and goggles.Fill a large plastic container approximatelyhalf full with water. Determine the massand volume of each of the woodenblocks. Calculate the density of each.Place each block in the water. Use a rulerto measure how much of each block issupported above the water. Stack two ormore blocks on top of the other blocksin the water. Measure how much of theblocks are supported above the water.

Expected Outcomes The thickerblocks floated higher than the thinnerblocks. The blocks that had lowerdensities also floated higher thanthe denser blocks.Visual, Kinesthetic

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Archimedes, born in Sicily in 287 B.C., iscredited with discovering the principle ofbuoyancy. He was ordered by King Hiero todetermine whether a crown was pure gold orwhether the goldsmith who had made it hadreplaced some of the gold with cheaper silver.Archimedes realized while taking a bath thathe could solve the puzzle using water displace-ment. After determining the volume of waterdisplaced by the crown, a piece of pure gold

could easily be made to match the volumeof the displaced water, and thus the volumeof the crown. In theory, if the volume of thecrown and the volume of the gold block arethe same, they should also have the samemass. The only reason they would not havethe same mass is if one of them was not puregold. Archimedes tested the crown and foundthat it, indeed, weighed less than the block ofgold. The goldsmith confessed.

Facts and Figures

Answer to . . .

Figure 16 ancient ocean floor deposits

Figure 17 at a lower level

A terrane is an area thathas a different geologic

history from surrounding areas.

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324 Chapter 11

Section 11.3 Assessment

Reviewing Concepts1. What types of mountains are associated

with convergent plate boundaries?

2. What mountains are associated withdivergent plate boundaries?

3. How is isostatic adjustment involved inmountain building?

4. How is accretion involved in mountainformation?

Critical Thinking5. Comparing and Contrasting Compare

mountain building along an ocean-continentconvergent boundary and a continent-continent convergent boundary.

6. Drawing Conclusions How does the theoryof plate tectonics help explain the existence ofmarine fossils in sedimentary rocks on top ofthe Himalayas?

7. Applying Concepts How would theaccretion of a large microcontinent affect theisostatic adjustment of the region around amountain range?

Isostatic Adjustment for Mountains Applying the con-cept of isostasy, we should expect that when weight is added to thecrust, the crust responds by subsiding. Also when weight is removed,the crust will rebound. Evidence of crustal subsidence followed bycrustal rebound is provided by the continental ice sheets that coveredparts of North America during the Pleistocene epoch. The addedweight of a 3-kilometer-thick mass of ice depressed Earth’s crust byhundreds of meters. In the 8000 years since the last ice sheet melted,uplift of as much as 330 meters has occurred in Canada’s Hudson Bayregion, where the ice was thickest.

Crustal buoyancy can account for considerable vertical movement.Most mountain building causes the crust to shorten and thicken.

Because of isostasy, deformed and thickened crust will undergoregional uplift both during mountain building and for a long periodafterward. As the crust rises, the processes of erosion increase, and thedeformed rock layers are carved into a mountainous landscape.

As erosion lowers the summits of mountains, the crust will rise inresponse to the reduced load, as shown in Figure 18 on page 323. Theprocesses of uplifting and erosion will continue until the mountainblock reaches “normal” crustal thickness. When this occurs, the moun-tains will be eroded to near sea level, and the once deeply buriedinterior of the mountain will be exposed at the surface.

How are ice sheets related to isostatic adjustment?

Creative Writing Describe a trip througha mountain range like the Andes that hasformed at an ocean-continent convergentboundary.

324 Chapter 11

Build Science SkillsPredicting When you place a blockof wood in a pail of water, the blockdisplaces some of the water, and thewater level rises. Ask: If you couldmeasure the mass of the water thatthe block displaces, what would youfind? (The mass of the water equals themass of the block.) If 1 million kg ofice were added to a land mass, howmuch mantle would be displaced?(1 million kg)Logical

ASSESSEvaluateUnderstandingAsk students to describe two events thatwould cause crust to subside. (Acceptany event that increases the mass of thecrust, such as formation of large icesheets, accumulation of large amountsof sediments, or formation of volcanicmountains.)

ReteachHave students design an experimentthat models the formation of an upliftedmountain. Students may use a bicyclepump, balloon, fabric, and any othermaterials they choose when designingtheir model.

Students’ descriptions will vary. Be surethey correctly describe the volcanic arcand another mountain range inland ofthe continental margin.

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4. At a subduction zone, if an oceanic plate iscarrying any island arcs or small continentalfragments, they will be stuck to or embed-ded into the margin of the continental plate.This process of accretion deforms the conti-nental plate and can form mountains.5. Ocean-continent convergent boundary:subduction of oceanic crust beneath conti-nental crust, development of a continentalvolcanic arc, and formation of folded moun-tains as accretionary wedge is deformed andfolded. Continent-continent convergentboundary: no subduction because the conti-

Section 11.3 Assessment

1. mainly volcanic mountains2. Fault-block mountains at ocean ridges areassociated with divergent boundaries.3. As the crust is thickened due to mountainbuilding, the lithosphere will sink deeper intothe mantle. But the lithosphere is less densethan the mantle, so the lithosphere will standhigher. To balance the added thickness, thecrust will rise as a new level of gravitationalequilibrium is reached.

nental lithosphere is too buoyant, continentalplates collide and form folded mountainswith a lot of deformation, shortening, andthrust faulting, and little volcanism.6. The ocean basin between India and theEurasia was subducted before the collisionof India with the Eurasian plate. The sedi-mentary rocks on the ocean floor and alongthe shoreline were scraped off and upliftedand folded up into the mountains.7. The addition or accretion of a large frag-ment onto a mountain range would load thecrust and cause the crust to subside.

Answer to . . .

The weight of added icecauses crust to subside.

When ice melts, and weight is removed,crust rebounds.



The San Andreas Fault System

The San Andreas fault system, asshown in the map, trends in a north-westerly direction for nearly 1300kilometers through much of westernCalifornia. In many places, a lineartrough marks the trace of the SanAndreas fault. From the air, linearscars, offset stream channels, andelongated ponds mark the locationof the fault. On the ground, how-ever, evidence of the fault is harderto find. Some of the most distinctivelandforms include long, straightcliffs, narrow ridges, and sag pondsformed by the settling of blockswithin the fault zone.

Transform BoundaryMountainsThe San Andreas fault is a transformfault boundary separating twocrustal plates that move very slowly.The Pacific plate, located to thewest, moves northwestward in rela-tion to the North American plate.

Some large blocks of crust withinthe fault zone are pushed up form-ing hills or mountains of varioussizes. Other blocks are forced downand form depressions called sag ponds. The faulttrace is not straight. It has many bends along itslength. In one of these major bends, the force of thetwo sides of the fault moving past one another hascaused the uplift of the San Gabriel Mountains northof Los Angeles.

Fault SystemDifferent segments of the San Andreas behave differ-ently. Some portions creep slowly with littlenoticeable seismic activity. Other segments regularly

slip, producing small earthquakes. Still other seg-ments seem to store elastic energy for hundreds ofyears and rupture in great earthquakes.

Because of the great length and complexity of theSan Andreas fault, it is more appropriately referred toas a “fault system.” This major fault system consistsprimarily of the San Andreas fault and several majorbranches, including the Hayward and Calaveras faultsof central California and the San Jacinto and Elsinorefaults. By matching rock units across the fault, geolo-gists have determined that the total displacementfrom earthquakes and creep along the San Andreas isgreater than 560 kilometers.

The San Andreas, the largest fault system in NorthAmerica, first attracted attention after the 1906 SanFrancisco earthquake. But this dramatic event is just

one of many thousands of earthquakes that haveresulted from movements along the San Andreasover the last 29 million years.

California

SanFrancisco

HaywardFault

CalaverasFault

Andreas

Fault

Garlock Fault

Los Angeles

San Jacinto Fault

Elsinore Fault

Coyote Creek FaultImperial Fault

San

San Gabriel Mountains

The San AndreasFault SystemBackgroundThe 1906 San Francisco earthquakeresulted from a 5-m displacement alongthe San Andreas Fault. The earthquakehad a magnitude of 8.25, one of thetwo largest along this fault. The other8.25-magnitude earthquake occurred in1857 in Fort Tejon, near Los Angeles. Asmuch as 80 percent of the damage inthe San Francisco earthquake wascaused by fire.

Teaching Tips• Use an overhead projector to project

the map of California. Use blocks ofwood or rigid plastic foam to demon-strate the movement of the fault.Emphasize that the Pacific plate, alongwith the California coast, is movingnorthwest in relation to the NorthAmerican plate.

• Have students think about what willeventually happen to southwesternCalifornia if present movementscontinue. Examining a globe andthe map on this page will help themmake predictions. Refer to AddressMisconceptions on p. 313 aboutCalifornia’s coast sliding into thePacific Ocean. (Los Angeles will movenorthward and become close to SanFrancisco. Eventually, the Baja Peninsulamay become an island off the coast ofOregon or Washington.)

• Tell students that each segment ofthe San Andreas Fault exhibitssomewhat different behavior. Someportions exhibit a slow creep with littlenoticeable seismic activity. Othersegments regularly slip, producingsmall earthquakes. Still other segmentsseem to store elastic energy for hun-dreds of years and then rupture ingreat earthquakes. Have studentsresearch how various segments of thefault system behave. Ask them tomake predictions for future seismicactivity around Los Angeles and SanFrancisco.

Verbal, Visual

L2



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