Global Tectonics and Geohazards

Chapter Outline

Courtesy of Jim Peaco/Yellow

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ber © iStockphoto/Thinkstock.

IntroductionThe Concept of the Model

• AnAnalogy:AModelofanAircraftA Very Brief Review of the Scientific MethodModeling the Earth’s Interior

• TheImportanceofEarthquakeWaves• SummaryofEvidenceUsedtoModelthe

Earth’s InteriorThe Interior: Lithosphere, Asthenosphere, Crust, Mantle, and Core

Summary of the Earth’s InteriorRadioactive Decay and Interior HeatHeat Transfer

• Conduction• Convection

Global Tectonics and the Dynamic Earth Model• PlateTectonicsasaUnifyingTheoryofGeology• ContinentalDriftandSeafloorSpreading• PlatesandPlateBoundaries• Continent–ContinentCollisions

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■ Introduction

Inthischapter,weintroducematerialthatyoumustlearnifyouaretounder-stand,amongotherimportantconcepts,whyandwhereearthquakesandvol-canoesoccur,and,therefore,theoriginofthemanygeologicalhazardstheygenerate.First,wewilldiscusstheconceptofthegeologicalmodel.Thenwewillreviewthescientific methodandcontinuewithabrieflookatthehistory

ofrecentscientificthoughtonthenatureoftheEarth’sinterior.Finally,wepresentamodeloftheEarth’sinteriortosetthestageforourdiscussionofglobaltectonics,thegreatunifyingtheoryofgeology.

■ The Concept of the ModelIfscientistswanttoknowtheEarth’scompositionandstructureatanydepth,theeasiestwaytofindoutwouldbetodrillaholeandcollectasample( Figure 4-1 ).

IntheUnitedStatesalone,over250,000holeshavebeendrilledexploringforoilandgas!

Howdeep into the interiordoyou thinkwehavesucceeded indrilling(theradiusoftheEarthisabout6600km,or4100mi)?Science-fictionwriterstothecontrary,scientistshavesucceededindrillinglessthat1/4ofone percentofthewaytotheEarth’scenter,oronlyabout14km(8.7mi)!

ThereasonwehavenotdrilleddeeplyintotheEarthisthattemperaturesandpres-suresincreaseasdepthincreases,andtheconditionseventuallybecometooextremefordrillingmaterialtoholditsstrength.Solidsteeldrillpipestartstoactlikeangel-hairpasta.Thecostbecomesprohibitiveaswell.This illustratesavery importantpointaboutourknowledgeoftheEarth’sinterior:almost all our knowledge is indirect.

Concept Check 4-1. Recall that knowledge is not scientific if it is not observable. Discuss whether our knowledge of the Earth’s interior is scientific, given that we have drilled only 1/4 of one percent into the planet.

Becausewedon’thavedirectsamplesfromboreholesintotheEarth’scenter,wemustcontentourselveswithmakingmodelsof the interiorbasedon indirectevidence.Andwemustconstantlytestthevalidityofthesemodels,whichisthescientificstandardof“suspendedjudgment.”

An Analogy: A Model of an AircraftModels in geology are abstract representations of a general concept. As kids,manyofusputtogethermodelairplanes.Tomakethosemodels,weboughtkits

• MantlePlumesandHotSpots• VolcanoesandPlateTectonics

Case Study: An Island Arc, a Subduction Zone, and a Great Tragedy: The 2004 Sumatra-Andaman Earthquake and Its Aftermath

Chapter SummaryKey TermsReview QuestionsFootnotes

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Figure 4-1 Ships designed for deep-sea drilling. (A) The Glomar Challenger, a unique drilling vessel 122 meters long, could manage about 7.6 kilometers of drill pipe. Courtesy of Integrated Drilling Program. (B) The Joides Resolution, about 300 meters long,can handle over 9.1 kilometers of drill pipe and operate safely in heavier seas and winds than the Glomar Challenger could. Courtesy of Ocean Drilling Program, Texas A&M University. (C )The 210-meter-long Chikyu can drill in water depths up to 2.5 kilometers and carries enough drill pipe to continue 7.5 kilometers below the sea floor. © Kyodo/Landov.

consistingoftheexteriorpartsoftheplaneallreducedbythesameamounttotheoriginalairplane: inotherwords,ascale model.Butwhat ifyouhadtodesignageneral modelforanaircraft—somethingthatimmediatelywouldberecognizedbyeverybodyasanaircraft,withoutbeinganactualmodelofanyrealaircraft?Obvi-ouslyyouwouldfirsthavetodecidewhat characteristics are common to all aircraft.Perhapsyou’dconcludethatallaircrafthavewings,abody(fuselage),oneormoreengines,andatailofsomesort.So,youmightmakeageneralmodelofanaircraftwiththesefourcharacteristics.However,theymaybeofdifferentsizesandshapes.Forexample,someplaneshavestraightwingsandsomehavewingsthataresweptbackatanangle.Somehavepropellersforenginesandothershavejets.Butthesespecificvariantsdonotmeanthatyoucouldn’tmakeageneralmodelofaplanethateveryonecouldrecognizeas a plane;theywouldonlyhavetoknowsomethingaboutthevariationsinplanetypes.

This is thebasis for thegeologicaluseof themodel:we try toboildown thesystemwearetryingtomodeltothebasicandessentialcharacteristicsthatallvariantsshare.Modelsareessentialtoageologist:insedimentarygeologywemakemodelsofsedimentaryenvironments,likerivers,inordertobeabletorecognizeriversedimentfromancientsedimentaryrocks.

(C) THE CHIKYU

(A) THE GLOMAR CHALLENGER

(B) THE JOIDES RESOLUTION

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Concept Check 4-2. Find photographs of at least three different deltas. Design a model of a delta (in other words, determine common features of these deltas that would characterize all deltas). Describe your model.

■ A Very Brief Review of the Scientific Method

Let’s review the scientificmethod, becausewe use it tomake ourmodel of theEarth’sinterior.Beingscientists,geologistsusehypothesestoexplainsomenatu-ralphenomenon,andthengatherevidencetotesttheirhypotheses.Theyalsomaydesignexperiments to test them.After this, thehypothesishaseitherbeen sup-portedorrejectedand,ifsupported,geologistsmayelevateittoatheory(awidelyacceptedexplanationforaphenomenon);ifrejected,it’sbacktothedrawingboard.

■ Modeling the Earth’s InteriorA model of the Earth’s interior shows the general structure such as: interiorlayersdrawntoscale,whereinteriorboundariesare,whatthestates of matterare(whethersolid,liquidorgas,forexample),andthecompositionoftheinterior,inasmuchdetailasnecessary.Wemaynotknowtheexactcompositionofeachzone,orthelayersmayvaryincomposition,soourmodelmighttrytoshowan“average”compo-sition;orourmodelmighttrytoshowhowcompositionsvarywithdepth,orlaterally.

Tomakeourmodel,weuseallavailableevidencethatwillallowustoputcon-straintsonwhatourmodellookslike.Forexample,weknow(fromitsgravitationalinteractionswithotherbodiesintheSolarSystem)theEarth’soveralldensity(anindicatorof themassof a substance relative to its volume, inotherwords,howheavyitis)is5.52g/cm3,soourinteriormaterialmustaverage5.52indensity,eventhoughthedensityofeachzoneorlayermaybehigherorlower.

The Importance of Earthquake WavesTheenergystoredinastickwhenbentisreleasedquicklywhenthestickbreaks.Someoftheenergyisreleasedassound:asharpcrackisheard.Soundisaformofenergyandbreakingthestickgeneratessoundwaves.Inthesameway,breakingrocksalongfaults,whicharefracturesintheEarthalongwhichmovementtakesplace,alsoreleasesenergy.Severaltypesofenergywavesaregenerated(discussedelsewhereinthistext; Figure 4-2 ).

Seismicwave pathsandvelocitiesinsidetheEarthdependonthepropertyofthematerialthroughwhichthewavespass.Ifwecantrackthewavepath,wecantellalotaboutthenatureofthematerialalongitspath( Figure 4-3 ).

Summary of Evidence Used to Model the Earth’s InteriorWeuseevidencefromthefollowingsourcestomakeourmodeloftheEarth’sinterior.

• Thecompositionanddistributionofsurfacerocks.Therearethreecatego-riesof rocks: igneous,which formfromamoltenmasscalledmagma;sedimentary,whichformbyerosionofpre-existingrocksorbyorganisms,likecorals;andmetamorphic,a“hybrid”rockformedwhenapre-existingrock is subjected toheat andpressure, causing theoriginalminerals toalignthemselves,recrystallize,orboth.Besidesoutcrops,wehavehundredsof

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Figure 4-2 Types of earthquake waves, how they move, and their relative velocities. P waves are the fastest waves, and are known as compressional waves because they exert a push and pull (think of a wave moving in a Slinky toy). S waves arrive after P waves, and they displace the ground perpendicularly to the direction of propagation. Surface waves are the slowest but may be the most destructive, since they displace the ground like rolling ocean waves and/or side-side. A more complete description of these waves is found elsewhere in this book.

Figure 4-3 Changes in seismic wave velocities with depth. P waves penetrate through liquid, while S waves are reflected. Internal zones where waves abruptly change velocity are known as discontinuities. They are important internal boundaries and indicate a change of state, of composition, or both. (A) How waves would travel through an Earth with a homogeneous interior. (B) Actual wave paths, which demonstrate the increase in density with depth and the existence of the core.

Earthquake

P or Swave

S wave

P wave

P wave

P wave

P wave

Inner core

Outer core

P wave

P wave

(A) (B)

P wave

Earthquake

P Waves

DilationCompression

S Waves

Wave direction

Grounddisplacement

Surface Waves


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thousandsofsamplesoftheshallowcontinentalcrustfromboreholes,sowecanestimatetheaveragecompositionoftheEarth’scontinents.

• Meteorites,the“rawmaterial”oftheoriginalEarth.Meteorites,objectsfromspace that have survived impact onEarth, commonly are either an iron-nickelalloy(about6%),orcomposedofsilicatemineralscontainingtheele-mentssiliconandoxygen(about93%; Figure 4-4 ).

• Thousandsofdrillholesanddredgesamplesofoceanicsedimentandunder-lyingbedrock.Wealsohaveseismicdatathatshowwhereboundariesexistintheoceaniccrust.Wecanconfidentlyinferthecompositionoftheselayers.

• WeknowtheEarth’saveragedensityis5.52g/cm3,whichputsstronglimitsonthekindsofsubstancesthatcanpossiblymakeuptheinterior. Table 4-1 showsdensitiesofsomecommonsubstances,whichshouldgiveyouanideaofwhatadensityof5.52means.

Figure 4-4 A meteorite recovered from Greenland. The snow cover helped researchers spot the dark meteorites. © Mary Evans Picture Library/Alamy Images.

Table 4-1 Some Common Substances and Their Densities

Diamond 3.5

Pyrite 5.2

Gold (22 carat) 17.5

Cement 1.6

Concrete 2.4

Cast iron 7.7

Marble 2.7

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• ThephysicsoftheEarth’srotationdictatesthatthedensestmaterialmustbeattheEarth’scenteranddensitiesmustdecreaseoutward(andofcourseweknowtheaveragedensityoftheouterlayerfromdirectsamples).

• When an earthquake occurs, seismic waves are generated, which travelthroughtheEarth,makingit“ringlikeabell.”Likelightoffamirror,seis-micwavescanbounceoffsurfacesinsidetheEarthwheredensitiesabruptlychange.WethereforeknowfromthebehaviorofseismicwavesastheytravelthroughtheEarthwherethemajorboundariesarelocatedbetweensubsurfacematerials,andweknowsomethingabouttheirproperties,becauserockprop-ertiesdetermineseismicwavevelocity.Forexample,otherthingsbeingequal,thehigherthedensityofrock,thefasterseismicwavestravelthroughit.Fig-ure4-3showshowwaves, inthiscasesoundwaves,arereflectedandbent(refracted)bythevariouslayersinsidetheEarth.Internalzoneswherewavesabruptlychangevelocityareknownasdiscontinuities.Theyareimportantinternalboundariesandindicateachangeofstate,composition,orboth.

• TheEarth’spowerfulmagneticfieldputslimitsonthepossiblecompositionandstateoftheplanet’sinterior( Figure 4-5 ).

Usingsuchinformation,wecouldhypothesizethattheEarthismadeofmate-rialthatgraduallyincreasesindensityfromthatatthesurfacetoadensermassatthecenter,sothattheoverallaveragedensityis5.52.Crustalrocks,thatis,rocksneartheEarth’ssurface,havedensitiesintherangeof2.5to3.1,butnocommonnaturalrockhasadensityapproaching5.52.

Applyingdatafromseismicwaves,wefindthatthismodelwon’twork,becauseseismicwavevelocities(whichtellusthedensityofEarthmaterials)changeabruptlyatseveraldiscontinuitieswithintheEarth.Sowehavetomodifyourmodelbyhypoth-esizingseparatelayersinsidetheEarth,ratherthanmaterialthatgraduallyincreasesfromsurfacetocenter.

Concept Check 4-3. Explain why not altering our hypothesis given the existence of the seismic data would have violated the “rules” of science.

Figure 4-5 A diagram showing the lines of force of the Earth’s magnetic field. The field is generated by convection in the Earth’s core. Data from A. Cox, Science. 163 (1969): 237–245.

(B)(A)

Geomagneticnorthpole

Normal PolarityGeomagnetic

southpole

Reverse Polarity

Geomagneticnorth pole


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Concept Check 4-4. Explain the concept of a geological model. Based on the information in the preceding sections, explain the model of the Earth’s interior.

■ The Interior: Lithosphere, Asthenosphere, Crust, Mantle, and Core

OneofthemostfundamentaldiscoveriesabouttheEarthwasthattheinteriorwascomposedoflayerswithrecognizableboundaries.GeologistsatfirstdividedtheEarthintothreelayers:thecrust(outermostmaterial),anunderlyingmantle,andacoreatthecenter,basedonhypothesizeddifferencesinrockcomposition. Figure 4-6a showsthemajorzoneswithintheEarth.Materialmayvaryincomposition,andpropertiessuchas strength (whichmeasureshow it responds todeforming force),or state(whethersolid,liquid,orgas).Strengthandstateinturndependonthetempera-tureandpressuretowhichthematerialissubjected.

LaterstudyofseismicwavesandtheEarth’smagneticfield,coupledwithlaboratory research, allowed detail to be added to thismodel. The core washypothesizedtoconsistofaninnersolidportionandanouterliquidportion.Twotypesofcrustwerefound:oceanic,comprisingbasaltic(basaltisadark-colored,fine-grained,igneousrock)rocksbelowsediment,andcontinental,composedofallthreerocktypes,butonaveragehavingacompositionnearthatofgranite.Thelowermantlewasdenserthantheuppermantle.Theuppermantlewasfoundtocontainazonenearitstopwhereseismicwavevelocitieswerelowerthanaboveandbelow:Thiszonebecameknownasthelow-velocity zone,orastheno sphere.The

Figure 4-6 Earth’s internal structure. Percentages indicate the relative masses of the core, mantle, and continental crust. Where is most of the Earth’s mass concentrated? (A) Layering based on chemical composition and physical state. (B) Layering based on response to a deforming force. The lithosphere was defined to include the crust (both continental and oceanic: see arrows) and the rigid uppermost portion of the mantle. The asthenosphere lies entirely within the upper mantle.

6371 km2900 km

Hydrosphere(4 km)

Continental crust 0.4%35–50 km

Mantle 68.1%

Outer core 31.5%Inner core

Oceanic crust (3–10 km)(A) the earth’s structure (B) layered, internal structure of the earth

brittle LithosphereAsthenosphere

Mesosphere

Outer core

relatively plastic

0100–200

350

1000

2000

29003000

1300 km2200 km

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Figure 4-7 A per idotite sample, brought up by lava known to have originated in the mantle. Peridotite takes its name from the gem-quality form of the mineral olivine—peridot—which is a magnesium-rich silicate mineral. Silicon is the second most abundant element in the Earth after oxygen. It thus should be no surprise then that minerals made mainly of silicon and oxygen should be so common. © Slim Sepp/ShutterStock, Inc.

termisderivedfromtheGreekasthenos(meaningweaknessorlossofstrength),whichmeansaweak,plastic(thatis,deformable),partiallymoltenzone,todistin-guishitfromthelithosphere(Greek,lithos=rock),astrong,rigidzone,aboveit( Figure 4-6b ).Thelithospheresitsatoptheasthenosphereandincludesthecrust(bothcontinentalandoceanic)andtherigiduppermostportionofthemantle.

Theuppermantle’scompositionisbestdescribedasperidotite,amagnesium-richrockcontainingabundantolivine,basedonseverallinesofevidence( Figure 4-7 ).First,peridotiteisarock,whichonbeingpartiallymelted,yieldsabasalticmagma,andbasaltistheuniversalbedrockoftheoceanbasins(belowaveneerofsediment),soitislikelythattheoceanbasinbedrockformedfrompartiallymeltedperidotite.Furthermore,thedensityandpropertiesofperidotiteareconsistentwiththebehaviorofseismicwavestravelingthroughtheuppermantle.Finally,itispossibletoderiveperidotitefromthecompositionofstonymeteorites,theprobablerawmaterialfromwhichtheplanetwasbuiltmorethan4 billionyearsago.

■ Summary of the Earth’s InteriorOurmodelatpresentislikethis:TheEarthisalayeredbodythathasdiscretebound-ariesseparatingthelayers.Theselayers,basedoncompositionaldifferencesfromthesurfaceinwardare,crust,mantle,andcore.TheuppermostfewhundredkmoftheEartharedividedintoalithosphere(containingthecrustanduppermostmantle)andanasthenosphere,orlow-velocityzone;apartiallymoltenzoneentirelywithinthemantle.Thecrustisoftwotypes:oceanic,consistingmainlyofbasalticrocksbelowavariablelayerofsediment,andcontinental,composedofallthreerocktypes,butonaveragehavingacompositionnearthatofgranite.Theoutercoreismoltenandtheinnercoresolid.Thecompositionofthecoreismainlyiron,withsomenickelandminorelementssuchassulfur,silicon,and/orpotassium.Theuppermantleisofper-idotiticcomposition(amaterialwhich,ifpartiallymelted,yieldsabasalticmagma)andthelowermantleisprobablymadeupofdenselypackedoxideminerals,withasimilarcompositiontotheuppermantlebutmadeupofdensercrystalstructures.

■ Radioactive Decay and Interior Heat

One additional factor affects the nature and behavior of the Earth’s interior andsurface: radioactive decay ( Figure 4-8 ). Radioactivity refers to the “decay” of


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unstableatomicnuclei.(Decayheremeanstheradioactiveatomchangestoanon-radioactiveelementbyemittingparticlesandenergy.)Wediscussradioactivedecayfurtherelsewhere.

The reason we introduce radioactivity here is that it produces a great dealofheat intheEarth’s interior,andradioactiveheatpowerstheglobalprocesswecallplatetectonics,whichis introducedbelow.Thefactthatradioactivedecayisirreversiblethatis,onceanatomhasdecayeditnolongerproducesheat,helpsusunderstandhowtheplanet’scrusthasevolvedoverthepast4+billionyears.Thus,heat available to power plate tectonics, including volcanic activity, has declinedovergeologictime.

■ Heat TransferHeatproducedbyradioactivedecaymovesfromhighconcentrationstoareaswherethereislessheat,formingaheatgradient.Thesamethinghappenswhenyouputapotonahotburneronthestove.Thepotwarmsbyheatthatmigratesfromhighheat(theburner)towardlowheatregions(thepot’scontents).IngeneraltheEarth’s

Figure 4-8 Radioactive decay. The U238/Pb206 decay series. Heat produced by radioactive decay has decreased over the Earth’s history. If the Earth’s heat engine is based on radioactive decay, and the rate at which plates move is related to heat produced, how do you think plate motion rates have changed over planetary history? How do you think plate motion rates will change going into the future? Will they stay the same, increase, or decrease? On what do you base your answer?

Nuclide

Uranium 238 (U238) Radioactive Decay

uranium-234

uranium-238

thorium-234 24.1 days

thorium-230

protactinium-234 1.17 minutes

3.05 minutes

245000 years

8000 years

radium-226 1600 years

22.3 years

radon-222

polonium-218

0.000164 secondspolonium-214

26.8 minuteslead-214

lead-210

19.7 minutesbismuth-214

bismuth-210

3.823 days

5.01 days

polonium-210 138.4 days

lead-206 stable

4.47 billion years

Half-life

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interiorishotandthesurfaceiscold,soheattendstomovefromtheinteriortowardthesurface.Threewaysthatheatcanmoveare:byconduction,byconvection,andbyradiation( Figure 4-9 ).

ConductionInthesolidpartsoftheEarth’sinterior,heat transferisaccomplishedbymolecu-larmotion,likestickingacoldironbarinahotfire(Figure4-9).Thisisalsocalledconduction,anditisarelativelyslowprocess.

ConvectionIfthereisanyliquidorgaspresentinasystem,heatingcanmakeitbehavelikeapotofsouponthestove:convection cellsorplumeswillform,carryinghotsoupatthepot’sbottomtothesurface,anddisplacingcoldersoupatthetopofthepottothebottom,whereitisheated.Convectionwillcontinueaslongasthepotisbeingheatedandatleastpartofitisliquidorgas,andthisisapparentlytheprocessbywhichmuchoftheheatinthemantleistransferredtowardsthesurface.Computersimulationsofmodelsof themantle show that concentrationsofheatwill formsuchconvectioncells,asshowninFigure4-9.

Convectioncellsandplumespoweredbyradioactivedecayarethemeansbywhichnewoceanicbedrockismade,andnewoceansmaybeformedinthepro-cess.Theoceanicbedrockiscomposedofbasaltandbasalticrock.

Thegreatmassofmaterialbeingmovedbyconvection,aswellasthematerial’shighviscosity(resistancetoflow)limitsthespeedoftheconvectioncurrentstoafew

Figure 4-9 (A) Three modes of heat transfer: conduction, convection, and radiation. Radiation here does not mean radioactivity; radiation means transferring heat by electromagnetic radiation (e.g. by microwaves). (B) Formation of a convection cell in the Earth’s interior. Heat may be transferred rapidly by convection, and the material need not be entirely liquid for convection to occur.

Conduction

Convection

Radiationi.e., infrared

Outer core

Convectioncell

ConvectioncellMantle

Lithosphere

Inner core


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centimeters a year and this is not coincidentally the rate atwhich the lithosphericplates,whichwe’lldiscussbelow,aremovingatthepresenttime.Becausecontinentsareimbeddedintheseplates,theyarepassivelycarriedovertheEarth’ssurfacelikerafts.

■ Global Tectonics and the Dynamic Earth Model

Plate Tectonics as a Unifying Theory of GeologyNow,we’rereadytointroducetheconceptofplate tectonics.ThewordtectonicshasLatin/Greekoriginsandmeansof or relating to building, in this case,of thestructuresontheEarth’ssurface.PlatetectonictheorystatesthatthesurfaceoftheEarthiscoveredbysevenmajorrigidlithosphericplatesandanumberofsmallerplates( Figure 4-10 ),allofwhichareinmotion,andnotallinthesamedirectionoratthesamespeed.PlatetectonicsexplainsthemajorityoftheEarth’slarge-scalegeologicstructuresandprocesses,fromislandarcsliketheAleutianstoeventslikeearthquakesand,asonemightexpect foraconceptof suchoverarchingsignifi-cance,hasbeencalledaunifying principleofgeology.

Continental Drift and Seafloor SpreadingThe theory of plate tectonics evolved from two older ideas, continental driftandseafloor spreading.TheconceptofcontinentaldriftiscreditedtoGermanmeteorologistAlfredWegenerwho,duringtheearlytwentiethcentury,postulatedthat the locationof theEarth’s continents in thedistantpastwasdifferent fromtheircurrent location, inotherwords,thecontinentshad“drifted”overgeologi-caltime.Basedinpartonthejigsawpuzzlefitofthecurrentcontinentsthatwasevidentonmapsatthetime,forexample,theeastcoastofSouthAmericamesh-ingalmostseamlesslywiththewestcoastofAfrica,andusingevidencefromear-lier researchers,Wegener proposed the existence of a supercontinent known as

Figure 4-10 Earth’s lithospheric plates. Clusters of black dots indicate earthquake activity. Arrows show relative plate motion. Plates are generally very thin (tens of kilometers is typical) relative to their other dimensions. Therefore, they are rather like the fractured shell of an egg in scale. Data from Stowe, K.S. Ocean Science. John Wiley & Sons, Ltd., 1983.

Eurasian plate

Australianplate

Philippineplate

Pacific plate

Antarctic plate

Nazcaplate

Cocosplate

North Americanplate

South Americanplate

Caribbeanplate

African plate

Arabianplate

60° N

30° N

30° S

60° S

Antarctic Circle

Tropic of Capricorn

Arctic Circle

Tropic of Cancer

0° Equator

150° W 120° W 90° W 60° W 30° W30° E 60° E 180° 150° E120° E90° E0°

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Pangaea(fromtheGreekmeaning“allland”)surroundedbysinglelargeocean,Panthalassa (“all sea”), about200millionyearsago.Wegenermarshaledaddi-tionalsupportingevidence,includingthepresenceofthelatePaleozoicfossilplantGlossopterisonIndia,Australia,SouthAfrica,andSouthAmerica,areasdissimilarintheircurrentclimateandthusunlikelytosupporttheseplantsunlessunderasimilarclimateregimeinthepastwhentheseareaswereallconnected.Otherevi-denceincludedthecontiguousdistributionsoffossilreptilesacrossoceanbasins,aneventunlikelyunlessthebasinswereclosedandthecontinentshadbeencon-nected.Geologicalstructureslikecoalveinsandmountainsofthesameageshowedsimilardistributionsacrossoceanbasins.Therewasevidenceofancientglaciationin tropicalpartsof SouthAmerica, India,Africa, andAustralia, aphenomenonbestexplainedbytheselocationsoncehavingexistedaspartofanAntarcticsuper-continent( Figure 4-11 ).Despiteaseemingabundanceofgeologic,paleontologic,andclimatologicsubstantiationforhisidea,Wegener’sconceptofcontinentaldriftwasnotwidely accepted by the scientific community because a sound explana-tionforthemechanismbywhichthecontinentsmovedcouldnotbemade.Howcouldgraniticcontinentsplowthroughthedenserbasalticoceaniccrust,scientistswondered?

In the late 1950s–early 1960s, the advent of post-WorldWar II technology,namely SONAR (SOundNavigationAndRanging), allowed formore extensivemapping of the sea floor.Themost extraordinary finding was the existence of65,000km(40,400mi)ridgesystem—anunderwatermountainchain—ontheseaflooranddeep-oceantrenches( Figure 4-12 ).

Other evidence came from new discoveries of magnetism of fossil rocks,paleomagnetism.This new arena of study revealed that information on thedirection and intensity of the Earth’s magnetic fields at a particular time waslocked into the iron inmagmawhen it solidified intorockat theridges. Itwas

Figure 4-11 The continuous distributions of certain fossil plants and animals on present-day, widely separated continents demonstrates a pattern if the continents are rejoined. Modified from This Dynamic Earth, USGS.

AFRICAINDIA

AUSTRALIA

ANTARCTICA

SOUTHAMERICA

Fossil remains of thefreshwater reptileMesosaurus.

Fossil evidenceof the Triassicland reptileLystrosaurus.

Fossil remains ofCynognathus,a Triassic land reptileapproximately3 m long.

Fossils of the fernGlossopteris, foundin all of the southerncontinents, show thatthey were once joined.


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Figure 4-12 The mid-oceanic ridge (MOR) system, shown here in the Atlantic Ocean. Courtesy National Geophysical Data Center/NOAA.

Figure 4-13 Reversals of the Earth’s magnetic fields over the last 5 million years. Red represents periods of normal polarity. Yellow represents reversals.

thuslearnedthatthecurrentEarth’smagneticfieldhasundergonecompleterever-sals in the past, 170 during last 76million years ( Figure 4-13 ).When amag-netometer, a device developed at the Scripps Institution of Oceanography( Figure 4-14 ) tomeasuremagneticpropertiesof the seafloor,wasdeployed, itrevealedapatternofmagnetic“stripes”parallel to theridgeandsymmetricallydistributedaroundit( Figure 4-15 ).

This informationwas synthesizedbyHarryHessofPrincetonUniversity in1962.Hessproposedthetheoryofseafloorspreading.Accordingtothistheory,newseafloorisproducedattheoceanridgesfrommagma,whichsolidifiesandmovesveryslowlybilaterallyawayfromtheridge,preservinginformationonthecurrentdirectionandintensityoftheEarth’smagneticfield.Whenthefieldreversed,evi-denceofthisreversalwassimilarlylockedintothenewseafloor.Thestripedpat-ternofalternatingmagneticorientationssymmetricaltotheridgerevealedbythemagnetometerthuswasexplained(Figure4-15).

In addition to the paleomagnetic evidence, further validation of seafloorspreadingcamefromradiometricdatingoftheoceancrust,whichrevealed(1)thatoceaniccrustisgeologicallyyoung,muchyoungerthancontinentalcrust(180 mil-lionyearsfortheoldestoceaniccrustcomparedtonearly4billionyearsfortheoldestcontinentalcrust)and(2)thefartherawayfromtheridge,theoldertheoceaniccrustwas, and therewas a symmetrical distributionof ages around the ridge.Newseafloorthereforewasproducedattheridgesandsubducted(“destroyed”)atthetrenches.Moreover,thisideadidnotrelyonthecontinentsplowingthroughtheocean(recallthatthiswasamajorobjectiontoWegener’stheoryofcontinen-taldrift),butratherbothcontinentsandoceansmove.

Concept Check 4-5. Explain the concept of paleomagnetism and discuss its role in under-standing seafloor spreading.

North America

South America

Continental slope

Midoceanicridge

Islands

Africa

Europe

Continental shelf

Abyssal plain

Island arc

Trench1.0

0

Age(Ma)

0.780.90

1.061.19

1.78

2.002.082.14

2.59

3.053.123.223.33

3.59

4.174.29

4.47

4.64

4.814.895.01

5.25

2.0

3.0

4.0

5.0


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Figure 4-14 The survey ship Pioneer, which first deployed the magnetometer, steaming under the Golden Gate Bridge in the late 1950’s. The discovery of magnetic striping on the seafloor contributed to our understanding of the theory of seafloor spreading. Renowned marine geologist, H. W. Menard called the discovery of the magnetic stripes, “among most significant geophysical surveys ever made.” Courtesy of NOAA.

Figure 4-15 The pattern of magnetic stripes parallel to the ridge and symmetrically distributed around it. The oldest are shown by the pale brown color, the youngest are shown by the darkest brown color. What was the significance of these stripes? Modified from The Dynamic Earth, USGS.

Lithosphere

Normal magnetic polarity

Reversed magnetic polarity Mid-ocean ridge

Magma

(A)

(B)

(C)


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Plates and Plate BoundariesPlatesaresegmentsofthelithosphere(seeFigure4-10).Thegeological“action”—mountain-building, earthquakes, volcanoes—is mainly concentrated at plate boundaries,becausehereiswhereplatesarejostlingagainstoneanother,relievinggreatstresses.

Thethreetypesofplateboundariesare:divergent, convergent, and transform.

Divergent BoundariesIf theplatesmove away fromeachother, adivergent boundary is formed.Riftzones, like theEastAfricanRift, formwherecontinentsarerippedapart.Here,ascendingconvectioncellsreachthesurfaceandspreadoutatthebaseofthelithosphere,tuggingatit,heatingit,andtherebyweakeningit.Withenoughtime, the spreading convection cell may fracture the thinned and weakenedoverlyinglithosphere,allowingmoltenbasalticmagmatorisefromtheastheno-sphereandfillthecracksbetweenthespreadingplates(Figure4-12).

Oceansgrowandcontractasoceanfloorrockformsandisinturnconsumed,andthusmodernoceansexhibitvariousstagesofdevelopment. Figure 4-16 illustratesthe

Figure 4-16 The Wilson Cycle, which shows the evolution of ocean basin development. Data from Wilson, J.T. American Philosophical Society Proceedings 112 (1968): 309–320; Jacobs, J.A., et al. Physics and Geology. McGraw-Hill, 1974.

EMBRYONIC

STAGE MOTION

UpliftComplex system oflinear rift valleyson continent

East Africanrift valleys

Atlantic, Indian,and Arctic oceans

Red Sea

Pacific Ocean

Mediterranean Sea

Himalayas

Narrow seas withmatching coasts

Ocean basin withcontinental margins

Island arcs andtrenches aroundbasin edge

Narrow, irregularseas with youngmountains

Young to maturemountain belts

Divergence(spreading)

EXAMPLEPHYSIOGRAPHY

JUVENILE

MATURE

DECLINING

TERMINAL

SUTURING

Divergence(spreading)

Convergence(subduction)

Convergenceand uplift

Convergence(collision)and uplift


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WilsonCycle,whichshowstheevolutionofoceanbasindevelopment.Representa-tiveexamplescouldbetheRiftValleyofEastAfrica,theGulfofCalifornia,theGulfofAden,andtheNorthAtlantic.

Convergent Boundaries and Subduction ZonesWhere plates move together a convergent boundary results. At convergentboundariesoldoceaniclithosphereisrecycledbackintothemantle.Aplacewherethishappensiscalledasubduction zone( Figure 4-17 ).

Convergentboundariesmaybeofthreetypes:iftwomassesofcontinentalcrustarebroughttogetheratasubductionzone,acontinent–continent convergent bound-ary isformed.Ifoceaniclithosphereissubductedbeneathcontinentallithosphere,anocean–continent convergent boundaryisformed.Andiftwooceaniclithospheresegmentsconverge,anocean–ocean convergent boundaryisformed( Figure 4-18 ).

Modernsubductionzonesareusuallyfoundwithtrenches.Trenchesarearcu-ate(thatis,curved)topographiclowsthatformbytheincessantdownwardtuggingontheoceaniclithosphere.Earthquakesaredistributedalongsubductionzonesfromnearthesurface(shallow-focus),throughintermediatedepths(intermediate-focus),todepthswherethesubductingmaterialgetstooweaktostorestress(about700 km).Deepestearthquakesarecalleddeep-focusearthquakes.Island arcs,arcsofvolca-nicislandsformedbysubductionatocean–oceanboundaries,areoneoftheplanetsmostdistinctivefeatures.Theyalsoarethesiteofsomeoftheworld’shighestcon-centrationsofhumanpopulationsandsomeofthemostseverezonesofgeohazards.JapanandIndonesiaareexamples.

Concept Check 4-6. Name and distinguish between the types of plate boundaries.

Transform BoundariesWhenplatesslidelaterallypastoneanother,transform or shearboundariesresult.Trans-form boundariesoncontinentsarerecognizedbythepresenceofstrike-slipfaults(verticalfractureswheretheblocksslidehorizontallypasteachother)liketheSanAndreasinCalifornia( Figure 4-19 ),theAlpineFaultinNewZealand,andtheDeadSeaFaultZoneintheMiddleEastStrike-slipfaultsposeenormoushazardstogrowinghumanpopulationsandinfrastructure.Wediscussearthquakehazardselsewhereinthistext.

Continent–Continent CollisionsIftwomassesofcontinentallithospherearebroughttogetherbyplatemotionatasub-ductionzone,acontinent–continentcollisioneventuallyoccurs(Figure4-18).Colli-sionsbuildgreatmountainranges,withcontinentalcrustgreatlythickenedandusuallyintenselydeformed.Continentalcrustthickensbecauseitistoolowindensitytobesubducted,soitpilesupatthesurface.Isostatic(Greekisos=equal,stasis=standstill)forcespushthelightrockeverhigher.Tounderstandisostasy ( Figure 4-20 ),imagineblocksofslightlydifferingdensitiesfloatinginapailofwater.

The Himalayas formed when India collided with Asia, nearly doubling thethicknessofthecontinentalcrustthere.Theresultingupliftandsubsequenterosionofthelightcrustalrocksgenerateshugevolumesofsediment,whicharepresentlybeingdepositedinthefloodplainsofthegreatriversdrainingtheHimalayas.Some

Text continues on page 91.


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Figure 4-17 A model of a subduction zone. The Cascadia subduction zone and associated centers of volcanic activity. Modified from USGS.

Figure 4-18 Types of convergent plate boundaries. (A) and (C) Data from Tarbuck, E.J. and Lutgens, F.K. 2000. Earth Science 9th ed, Upper Saddle River, NJ: Prentice-Hall. Figure 7.13 (p. 194). (B) Courtesy of USGS/NPS..

Cascaderange

North America

Plate

Mt. St. Helens

Mt. HoodPortland

VancouverSubduction zone

Juan de Fuca plate

Oceanic crust

LithosphereLithosphere

Partially melted lithosphere

Continental crust

Trench

Island arc

Asthenosphere

Oceanic-oceanic convergence

(A)

Oceanic crust

Lithosphere Lithosphere

Continental crust

Volcanic arcTrench

Asthenosphere Asthenosphere

(B)

Partially melted lithosphere

Oceanic-continent convergence

Ancient oceanic crust

LithosphereLithosphere

Mountainrange

High plateau

Continental crust

Asthenosphere

Continental-continental convergence

Continental crust

(C)


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Figure 4-19 The San Andreas Fault system, a transform-fault boundary between the North American and Pacific Plates.

Figure 4-20 Two barges on the Saigon River in Vietnam, one with cargo, one without, float at different levels due isostatic equilibrium. © Pluff Mud Photography.

0

0

500 miles

800 km

CANADA

MEXICOEast Paci�c rise

Relative motion ofNorth American plate

San Andreas Fault

Explorerridge

Juan deFuca ridge

Blancofracturezone

Mendocinofracture zone

Murrayfracture zone

San Francisco

LosAngeles

Molokaifracture zone

Subduction zone

Relative motionof Paci�c plate

N


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An Island Arc, a Subduction Zone, and a Great Tragedy: The 2004 Sumatra-Andaman Earthquake and Its Aftermath

An online travel guide1 describes Sumatra thusly:

“Anchored tenuously in the deep Indian Ocean, this giant island is still as wild and unpredictable as the Victorian-era jungle-seekers dreamed. Millen-nia of chaos erupting from the earth’s toxic core or from the fierce ocean waves create and destroy in equal measure. When the earth and sea remain still, the past’s death and destruction fertilise a ver-dant future.”

Located in western Indonesia, Sumatra is the sixth largest island in the world. Despite the “millennia of chaos erupting from the Earth’s toxic core or from the fierce ocean waves,” 50 million people call Sumatra home.

From a geological perspective, Sumatra lies near a convergent plate boundary. The Indian (also known as the Indo-Australian) Plate lies to the south and east. To the north and west is the Eurasian Plate (Figure CS 1-1). The Indian Plate moves northerly at a rate of about 40–50 mm/

year (1.6–2 in/year). It subducts beneath a portion of the Eurasian plate (technically, the Burma microplate, on which also sit the Andaman and Nicobar Islands in the Bay of Bengal) creating the Sunda Trench as well as a large fault called an interplate thrust. This Sunda fault runs for approximately 5,500 km (3,300 mi).

These plates creep past each other until large portions of the plates lock or “get stuck” in some loca-tions, building up tremendous forces. When these plates rupture, the stored force is released, resulting in an earthquake (detailed elsewhere in this text).

Although earthquakes along the Sunda fault and within both the subducting Indian and overriding Eurasian Plates are not uncommon, the massive earth-quake that struck 60 km off the west coast of northern Sumatra on December 26, 2004 surprised geolo-gists. The event, known as the Sumatra-Andaman Earthquake, and the resulting tsunami, enormous water waves generated when the seismic energy

Figure Cs 1-1 Tectonic setting for the 2004 Sumatra-Andaman Earthquake.

Continued 4

Background © Hemera Technologies/AbleStock.com/Thinkstock. Title © iStockphoto/Thinkstock.

Main Shock26 December 2004

Aftershocks M ≥ 4

Plate Boundaries

INDIA

India plate

Burma microplate

Sundaplate

59 mm/yr

61 mm/yr

65 mm/yr

68 mm/yr

Australia Plate

THAILAND

CAMBODIA

MYANMAR(BURMA)

Eurasiaplate

VIETNAM

MALAYSIA

SINGAPORE

INDONESIA

SRI LANKA

Earthquakeepicenter 2004

Sunda Trench

INDIANOCEAN

BAY OFBENGAL

0 200 400 800 1,200 1,800 km


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released by the earthquake entered the ocean, killed 250,000 people in Indonesia, Thailand, India, Sri Lanka, and elsewhere around the Indian Ocean. It was one of the largest earthquakes ever recorded as well as one of the greatest natural disasters in human history.

The earthquake registered > 9.0 on the Richter scale, releasing energy said to be equivalent to that of 10,000 atomic bombs. The overriding Burma microplate was vertically displaced, elevating the seafloor by several meters and setting in motion the monstrous tsunami. Two “N-waves” traveling in opposite directions were gen-erated, a local tsunami and a distant tsunami (Figure CS 1-2). The local wave traveled toward Indonesia, Thailand, and nearby islands, while the distant tsunami moved across the Bay of Bengal in the direction of India, and Sri Lanka, and was even detected by tide gauges along the Atlantic coast of North America from Nova Scotia to Florida. The height of the wave varied from 2 to 3 m along the African coast to 10 to 15 m at Sumatra.

Concept Check 4-7. If you could use one adjective to des cribe the geological setting of Sumatra, in light of this case study, what word would you choose? Explain your choice.

At locations of the tsunami’s maximum impact, the force of the waves was so powerful that structures and people stood little chance (Figure CS 1-3). Along Sumatra’s coast-line, waves may have reached heights of 15 to 30 m over a distance of 100 km.

Further away, however, according to a report in Science magazine,2 “areas with coastal [forest] were markedly less damaged than areas without.” The trees referred to were mangroves, and they absorbed much of the energy of the waves, protecting the area landward of the mangrove forest. According to the report, mangrove area had been reduced before the tsunami (between 1980 and 2000) by 26% in the five countries most affected by the tsunami, from 5.7 to 4.2 million hectares (14.1–10.4 million acres) because of human activities that included aquaculture and devel-opment of resorts. Neil Burgess, a co-author of the study, stated,

“Just as the degradation of wetlands in Louisi-ana almost certainly increased Hurricane Katrina’s destructive powers, the degradation of mangroves in India magnified the tsunami’s destruction. Man-groves provide a valuable ecological service to the communities they protect.”

Figure Cs 1-2 (A) Diagram of the type of vertical displacement of the Burma microplate that elevated the seafloor by several meters and set in motion the disastrous tsunami. (B) The resulting N-waves of the tsunami.

Relativeplate motion

Strike-slip fault

Inter-plate thrust fault

Distanttsunami

Localtsunami

(A)

(B)


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Figure Cs 1-3 A view of the destruction caused by the 2004 Indonesian earthquake and tsunami. © Claudio Gallone/agefotostock.

Concept Check 4-8. Summarize the major points of this case study.

Value-added Question 4-1. The displacement of the fault occurred along a 1,200-km long segment. If the earthquake moved at a rate of 2.5 km/sec, how long (in minutes and seconds) did it take for the fault to propagate from its hypocenter (origin) to its extreme 1,200 km away?

Value-added Question 4-2. The initial velocity of the tsunami was 700 km/hr. Assuming that the tsunami maintained that speed (even though it slows in shallow water), how long would it take for it to reach: (A) India (approximate distance 2,000 km); (B) Sri Lanka (1,570 km); (C) Thailand (500 km); (D) Sumatra (60 km); (E) Myanmar (1,800 km); (F) Somalia (5,000 km); (G) Maldives (2,500 km)?

ofthesedimentisfindingitswayintotheIndianOcean,whereitisbuildingmassivesubmarine fans( Figure 4-21 ).

ThedeformationofthecontinentalcrustnorthoftheHimalayaextendsatleast2,000 kilometers from the collision zone. Some of the planet’smost destructiveearthquakeshaveoccurredalongfaultshere.

Mantle Plumes and Hot SpotsFigure 4-22 showstheHawaiianisland chain.IslandchainslikeHawaiiformas

aresultofmovementofoceanicplatesovermantle plumes.Plumesarebelievedto form due to abnormally high concentrations of heat that produce vertical,candle-flameshapedmassescontainingmagmaofbasalticcomposition.Astheyreachthe surface they formhot spots.Magmarises to the surfacemainlybyconvection,formshotspots,andmaypunchholesintheoverlyinglithosphere,formingvolca-nicislandsiftheoverlyinglithosphereisoceanic.


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Hotspotsmayalsoformalongdivergentboundaries.IcelandisformedbyhotspotvolcanismattheMid-Atlantic Ridge.Hotspotsunderoceaniclithosphereproducevolcanoesemittingmainlybasaltlavaandtephra(tephra,fromtheGreekforash,describesvolcanicrockfragmentsandlavaofvaryingsizesthatareexplo-sivelyblastedintotheairorliftedbyhotgases).AgoodexampleofsuchavolcanoisHawaii’sMaunaLoa,theworld’slargestvolcano.

Inanocean,alineofvolcanicislandsformedastheoceanicplatepassesoverthehotspot,ascontrastedwiththearcofislandsatasubductionzone(Figure 4-18).Theorientationof the island chain traces thedirectionof platemovement.TheHawaiianPlume ispresentlysituated immediatelysoutheastof theBig IslandofHawaii,and,inadditiontotheactivevolcanoesonHawaii,theplumeisforminganewislandcalledLoihi,presentlyentirelyunderwater(Figure4-22).

Plumesmaypersistfortensofmillionsofyears,andapparentlystaynearthesameplaceinthemantle.

Volcanoes and Plate TectonicsVolcanoeswillbediscussedinmoredetailelsewhereinthistext.Geologistsrecognizeplate boundary volcanoesandhot-spot volcanoes.Theplate tectonic settingstronglyinfluencesmagmatypeproduced,whichinturndeterminesviscosity(viscosityreferstointernalresistancetoflowandcanthoughtofasaliquid’sthickness)andultimatelyrisk,aswediscusselsewhereinthistext.

Figure 4-21 The Indus and Bengal submarine fans, formed of thousands of meters of sediment eroded at the Himalayan collision zone. The weight of the sediment depresses the oceanic crust in another example of isostatic adjustment.

INDIA

Himalayas

Tibetan Plateau

AFGANISTAN

PAKISTAN

Bengal Fan

Indus Fan

ARABIA

Arabian Sea

Bay of Bengal

Ganges riverBrah

maputra riv

er

Indus

rive

r


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Figure 4-22 (A) The Hawaiian Islands are made up of basaltic islands above seal level, and seamounts, below. Data from Dalrymple, G.B., et al., American Scientist 6 (1973): 294–308; Claque, D.A. and R.D. Jarrard, Geol. Soc. Am. Bull. 84 (1975): 1135–1154. (B) Plumes form as heat is transferred from the core/mantle boundary. The magma rises to the surface mainly by convection, forming hot spots, and may punch holes in the overlying lithosphere, forming volcanic islands if the overlying lithosphere is oceanic. Courtesy of USGS. (C) The underwater island of Loihi, the newest island in the Hawaiian chain. Courtesy of USGS.

Plate Boundary VolcanoesPlateboundaryvolcanoescanformatdivergentandconvergentboundaries.

Muchvolcanicactivityoccursatdivergentboundariesandatriftzones.Recallthatpartialmeltingofmantleperidotiteproducesbasalticmagma.Becausethereisnocontinental(granitic)crusttocontaminatethismagmaatmid-oceanridges,basaltlavaisoverwhelminglyproduced.VolcanoesofIceland( Figure 4-23 )arethemostvisibleexamplesofthesetypes,andtheyaremainlybasalt.

Volcanoesofconvergentboundariesformeitheratocean–ocean boundariesorocean–continent boundaries.

Figure 4-24 showstheIndonesianarc.Theseislandsaremadeuplargelyofvol-canicrocks.Islandarcvolcanoesderivetheirnamefromthebroadcurveoftheirvolcanicislands.Indonesia,Japan,andtheAleutiansareexamples.

Fixed “Hot Spot”

Zone of magma formation

Hawaii

(youngest)

Kauai

(oldest)

Oahu

Maui

Solid dense rock

Direction of Plate Motion

Hawaiian Ridge

PACIFIC PLATE

(B)

20 miles0

30 km0

N

Mauna Kea

Kohala

Mauna Loa

Kilauea

Loihi

Hualalai

(C)

Ages in MYBP

Em

pe

ro

r S

ea

mo

un

t

C

ha

i n

Haw a i i an R id ge

Hawaii

<2

10

2030

40

50

70 50° N

40° N

30° N

20° N

170° E 180° 170° W 160° W 150° W 140° W160° E

(A)


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Figure 4-23 (A) Location of volcanoes of Iceland. Courtesy of USGS. (B) Photo of the volcano Krafla. © iStockphoto/Thinkstock.

100 km0

100 miles0

N

ICELAND

Vatnafjoll

Katla

Kra�a

Grimsvotn(and Laki)

Vestmannaeyjar(Surtsey, Heimaey)

Hekla

Loki-Fogrufjoll

REYKJAVIK

Greenland Sea

NORTH ATLANTIC OCEAN

Den

mar

k St

rait

(A)

(B)


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Figure 4-24 (A) The Indonesian Arc, showing some of the sites of active volcanoes. (B) Diagram depicting how these islands arc formed.

400 km2000

400 Mile2000

NVIETNAM

THAILAND

MALAYSIA

Singapore

Sumatra

BRUNEI

MALAYSIA

Tangkubanparahu

Jakarta Java

Colo [Una Una]

INDONESIA

Awu

Banua Wuhu

IbuGamkonora

GamalamaMakian

Sirung

Bandi ApiSerua

Teon

Lewotolo

Iya

Tambora

Dieng Volc.Complex

Sundoro

Merapi

LamongamBatur

Slamet

Cereme

Galunggung

PapandayanGede

Salak

Kiaraberes-Gagak

Sorikmarapi

Tandikat

Sumbing

Gunung BesarSuoh

Krakatau

DempoKaba

KerinciTalang

Marapi

Tengger Caldera

Arjuno-WelirangKelut Sulawesi

Rinjani

Agung

Ijen

RaungSemeru

InielikaGunung Ranakah

Ebulobo

IlibolengLerobolengPaluweh

Sangeang Api

Nila

KelimutuEgonLewotobi

Iliwerung

PAPUANEW

GUINEA

NEWGUINEA

AUSTRALIA

Dukono

RuangKarangetang [Api Siau]

Borneo

PHILIPPINES

TongkokoMahawu

Lokon-empung

Soputan

Peuet Sague

Bur Ni Telong

CAMBODIA

INDIANOCEAN

SOUTH CHINASEA

PHILIPPINE SEA NORTH PACIFICOCEAN

CELEBESSEA

BANDASEA

TIMORSEA

ARAFURASEA

JAVASEA

STRAIT OFMALACCA

(A)

Deep ocean trench

BORNEO

SUMATRA

Oceaniccrust

Zone wheremagma originates

Plate boundaryMantle

Continental crust

(B)


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Islandarcvolcanoesareproducedbythesubductionofoceaniclithosphereasshownabove.Awidevarietyoferuptivematerial (e.g.,basalt,andesite, rhyolite,andcompositionsinbetween)canresultfromthepartialmeltingofoceaniclitho-sphereasitiscarriedintothemantle.

Recentresearchhasdiscoveredthat,counterintuitively, thepressureinsomeboreholesintotheoceaniccrustishigherattheseafloorthanintheholehundredsofmetersbelow.Thispressuredifferential canpush seawaterdown intocoolingbasalticrocks,hydratingthemandalteringtheircomposition.Thisalteredbasaltcanthenundergo“dewatering”asitissubducted( Figure 4-25 ).Thewaterdrivenoffathightemperaturesandpressurescanmeltoverlyingrocks.Eruptiveproductscanbelavaflows,pyroclasticflows(fast-movingmixturesofhotgasandrockfrag-ments),and tephraeruptions.Compared tooceanicvolcanoes, islandarcvolca-noesaremuchmoredangerous,owingtotheirhighersilicacontent.

TheCascadesoftheNorthwesternUnitedStates,isanocean–continentplateboundary.Volcanoescommonlyformatocean–continentplateboundaries,ontheedgesof continents.Ascendingmagmamaybecontaminatedbyassimilationofgraniticcontinentalcrustthroughwhichitpasses.Thiscanresultinawidevarietyoflavaandpyroclasticeruptions.

Concept Check 4-9. Write a few key terms that contrast divergent and convergent vol-cano types and processes. Which are most dangerous to humans? Why?

Figure 4-25 The dewatering of subducted basalt could induce melting in overlying rocks. Magmas of intermediate to silicic composition could result.

Oceanic lithosphere

Continentallithosphere

Meltingand risingmagma

Dewateringof

subductedbasalt

Deep ocean trench

Volcanic arc

Subduction

Zone


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© NinaM

alyna/ShutterStock, Inc.

Chapter Summary1. Becausewedon’thavedirectsamplesfromboreholesintotheEarth’scenter,

wemustcontentourselveswithmakingmodelsoftheinteriorbasedonourindirectevidence.

2. Geologistsusehypothesestoexplainsomenaturalphenomenon,andthengatherallevidencetotesttheirhypotheses.

3. Ifthehypothesisissupportedbyevidence,geologistsmayelevateittoatheory.4. AmodeloftheEarth’sinteriorshowsthegeneralinternalstructure,where

interiorboundariesare,whatthestatesofmatterare,andthecompositionoftheinterior,inasmuchdetailasnecessary.

5. WeuseevidencefromthefollowingsourcestomakeamodeloftheEarth’sinterior.SeismicwavepathsandvelocitiesinsidetheEarthdependonthepropertyofthematerialthroughwhichthewavespass.Ifwecantrackthewavepath,wecanputconstraintsonthenatureofthematerialalongitspath.Wehaveexcellentknowledgeofthecompositionanddistributionofsurfacerocks.Wealsouseevidencefrommeteorites.Wehavehundredsofdrillholesanddredge samplesofoceanicbedrock, and soweknow itsnear-surfacecompositionverywell.Weknowtheearth’saveragedensity is5.52 g/cm3.Thephysicsof theEarth’s rotationdictates that thedensestmaterialmustbeattheEarth’scenter.Theexistenceandnatureofthemagneticfieldputslimitsonthepossiblecompositionandstateoftheplanet’sinterior.

6. OneofthemostfundamentaldiscoveriesabouttheEarthwasthattheinte-riorwascomposedoflayerswithrecognizableboundaries.

7. GeologistsdividetheEarthintothreelayers:thecrust(outermostmaterial),anunderlyingmantle,andacoreatthecenter.Thecorewasfoundtoconsistofaninnersolidportionandanouterliquidportion.Theuppermantlewasfoundtocontainazonenearitstopwhereseismicwavevelocitieswerelowerthanaboveandbelow;thiszonebecameknownasthelow-velocityzone,orasthenosphere, and ispartiallymolten.Theuppermantle isofperidotiticcompositionandthelowermantleisprobablymadeupofdenselypackedoxideminerals.

8. Radioactivityreferstothedecayofunstableatomicnuclei.Radioactiveheatpowerstheglobalprocesswecallplatetectonics.Threewaysthatheatcanbetransferredare:byconduction,byconvection,andbyradiation.Convectioncellsandplumespoweredbyradioactivedecayarethemeansbywhichnewoceanicbedrockismade,andnewoceansmaybeformedintheprocess.

9. Plate tectonic theoryholds that theEarth iscoveredbysevenmajorrigidlithosphericplatesandanumberofsmallerplates.

10. PlatetectonicsexplainsthemajorityoftheEarth’slarge-scalegeologicstruc-turesandprocessesandhasbeencalledaUnifyingPrincipleofGeology.

11. Thetheoryofplatetectonicsevolvedfromtwoolderideas,continentaldriftandseafloorspreading.

12. Platesmoveatafewcentimetersayear.13. Thethreetypesofplateboundariesare:divergent,convergent,andtransform.14. Atconvergentboundariesoldoceaniclithosphereisrecycledbackintothe

mantle.Aplacewherethishappensiscalledasubductionzone.15. In2004oneoftheEarth’smostpowerfulearthquakesoccurredatthesub-

ductionzoneoftheconvergentIndianandEurasianPlates.Thequakeandresulting tsunamikilled 250,000people in Indonesia,Thailand, India, SriLanka,andelsewherearoundtheIndianOcean.

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16. Transform boundaries on continents are recognized by the presence ofstrike-slipfaultsliketheSanAndreasinCalifornia.

17. Iftwomassesofcontinentallithospherearebroughttogetherbyplatemotionatasubductionzone,acontinent–continentcollisioneventuallyoccurs.

18. IslandchainslikeHawaiiarehypothesizedtoformasaresultofmovementofoceanicplatesovermantleplumes.

19. Volcanoescanbeclassifiedasplateboundaryvolcanoesandhot-spotvolca-noes.Islandarcvolcanoesareproducedbythesubductionofoceaniclitho-sphere.Continentalvolcanoes,likeislandarcvolcanoes,aremoredangerousthanoceanicvolcanoes,andmayoccasionallyeruptwithexplosiveandevencataclysmicforce.

Key Termsasthenospherebasalticconductioncontinental crustcontinental driftconvectionconvergent

boundarycorecrustdensitydiscontinuitydivergent boundaryfaultheat transferhypothesisigneousisland arcisland chain

isostasylithospheremagmamagnetometermantlemantle plumemetamorphicMid-Atlantic ridgemodelocean trenchoceanic crustpaleomagnetismPangaeaPanthalassaperidotiteplateplate boundariesplate tectonics

plate-tectonic setting of volcanoes

pyroclasticradiationradioactivityseafloor spreadingsedimentaryseismic wavestates of mattersubduction zoneSumatra-Andaman

EarthquakeSunda Trenchtephratheorytransform boundarytrenchtsunami

Review Questions1. Ahypothesissupportedbyexperimentsandobservationcanbeelevatedtoa

a. paradigm.b. fact.c. proof.d. theory.

2. TheEarth’saveragedensityisclosesttoa. 5.5.b. 1.c. 55.d. 10.


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3. Thetwocommonestkindsofmeteoritesaremadeofa. uranium-leadandiron-nickel.b. uranium-leadandsilicateminerals.c. silicatemineralsandpotassium-sulfur.d. silicatemineralsandiron-nickel.

4. WhichistrueaboutourknowledgeoftheEarth’sinterior?a. Wehavedirectdrillsamplesthroughthemantle.b. Wehavedirectdrillsamplesoftheoutercore.c. Wehavedirectdrillsamplesoftheouterfewkmonly.d. Wehavenodirectdrillsamplesofoceanicsediments,onlythecontinents.

5. ZoneswithintheEarthwhereseismicwavesabruptlychangevelocityarecalleda. discontinuities.b. attenuatedzones.c. continuities.d. batholiths.

6. WhichisthemostacceptedmodeloftheEarth’score?a. Theinnercoreisliquidandtheoutercoresolid.b. Theinnercoreisliquidandtheoutercoregaseous.c. Theinnercoreissolidandtheoutercoreisliquid.d. Theentirecoreissolid,accordingtomagneticfielddata.

7. Anothernameforthelow-velocityzoneintheuppermantleisa. thediscontinuity.b. thelithosphere.c. theophiolitesequence.d. theasthenosphere.

8. Thecompositionoftheuppermantleismostnearlywhichofthefollowing?a. peridotiteb. basaltc. granited. iron-nickelalloy

9. Heattransferoccursinthreeways:a. conduction,induction,andradioactivity.b. conduction,convection,andradiation.c. subduction,attenuation,andshear.d. shear,transform,anddivergence.

10. Platemotion,poweredbyradioactivedecay,occursatanaveragerateofa. afewkm/year.b. afewmeters/year.c. afewcentimeters/year.d. afewnanometers/year.

11. Whichofthefollowingisconsideredaunifyingprincipleofgeology?a. evolutionb. continentaldriftc. seafloorspreadingd. basalte. theprincipleofradiometricdating

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12. Evidenceforcontinentaldriftincludesallofthefollowingexcepta. thejigsawpuzzlenatureofcontinents.b. thepresenceoftheremainsofearlyhumansonAfricaandSouthAmerica.c. thepresenceofthelatePaleozoicfossilplantGlossopterisonIndia,

Australia,SouthAfrica,andSouthAmerica.d. thecontiguousdistributionsoffossilreptilesacrossoceanbasins.

13. Inthelate1950s-early1970s,paleomagnetismshoweda. thattheEarth’spolesperiodicallyreversed.b. thatallrocksaremagnetic.c. thatseafloorisproducedatdeepseatrenchesandconsumedatmid-ocean

ridges.d. noneoftheabove.

14. Thetheoryofseafloorspreadingwasconfirmedinthe1960sbya. magneticstripingparalleltoandsymmetricalaroundtheoceanridges.b. currentweatherpatternsacrosstheplanet.c. advancesinmolecularbiology.d. alloftheabove.

15. Platesarea. segmentsoftheasthenosphere.b. segmentsofthelithosphere.c. segmentsofthebiosphere.d. formedatsubductionzones.

16. Newoceansformata_________typeofplateboundary.a. divergentb. subduction/convergentc. aseismicd. discontinuous

17. Atconvergentboundaries,olderlithosphereisa. recycledbackintothemantle.b. convertedtocore.c. convertedtoperidotite.d. alloftheabove.

18. Whenplatesslidelaterallypasteachother,a. transformorshearboundariesresult.b. noearthquakesoccur.c. earthquakesarealldeep(>500km)focus.d. earthquakesoccur,butnonehastodatecauseddamage.

19. IslandchainslikeHawaiiarehypothesizedtoformasaresultofmovementofoceanicplatesa. overmid-oceanridges.b. overoldcontinents.c. overmantleplumes.d. awayfromeachotheratadivergentboundary.

20. Islandarcvolcanoesareproducedbya. thesubductionofoceaniclithosphere.b. movementofplatesoverhotspots.c. continent–continentcollisions.d. anyoftheabove.


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21. Whichis/areaccuratecontrastsbetweencontinentalandoceanicvolcanoes?a. Continentalvolcanoesaremoreexplosiveanddangerousthanoceanic

volcanoes.b. Oceanicvolcanoestendtobebasalt,whilecontinentalvolcanoescan

varyincomposition.c. Ascendingbasalticmagmaundercontinentscanmeltcontinentalcrust

andchangecomposition.d. Alloftheabovearecorrect.e. Noneoftheaboveiscorrect.

22. DoublingthethicknessofthecontinentalcrustbyformationoftheHimala-yascausesthemtorise,andpilingsedimentderivedfromweatheringoftheHimalayasonthenorthIndianOceancausesittosink.TheseareexamplesofthePrincipleofa. Uniformity.b. PlateActivity.c. Isostasy.d. Discontinuity.

Footnotes1IntroducingSumatra. Retrieved August 19, 2012 from http://www.lonelyplanet

.com/indonesia/sumatra2Danielsen,F.,etal.2005.TheAsiantsunami:Aprotectiveroleforcoastalvegetation.

Science310:643.

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