global tectonics and geohazards
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Chapter Outline
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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
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0.780.90
1.061.19
1.78
2.002.082.14
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3.053.123.223.33
3.59
4.174.29
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4.814.895.01
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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
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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
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ro
r S
ea
mo
un
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ha
i n
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(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
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Den
mar
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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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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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