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Chapter5 PlateTectonics PlatetectonicsisatheoryabouthowthesurfaceoftheEarthevolvesduetostronginternalforces ThesurfaceoftheEarthiscomposedofrigidplatesthataremobileandmoverelativetooneanother Platetectonicswasproposedandbegantogainacceptancefromthescientificcommunityinthe1960 s Priortothedevelopmentofthetheoryofplatetectonics mostgeologistsbelievedthattherelativelocationofthecontinentswasrelativelyunchangedthroughtime PlatetectonicsisaunifyingtheoryinGeology differentgeologicphenomenasuchasmountainbuilding earthquakes volcanoes andthedistributionoffossilsandorganismscanbeexplainedthroughplatetectonics ContinentalDriftLookingatamapoftheEarth itappearsthatthecontinentscouldfittogetherlikeajigsawpuzzle Wegenersuggestedthatasingle supercontinent calledPangaeaonceexistedinthepast AlfredWegener 1915 proposedtherevolutionaryideacalledContinentalDrift Wegenerdevelopedhisideabasedupon4differenttypesofevidence 1 FitoftheContinents2 FossilEvidence3 RockTypeandStructuralSimilarities4 PaleoclimaticEvidence EvidenceforContinentalDrift1 FitoftheContinentsItwastheamazinglygoodfitofthecontinentsthatfirstsuggestedtheideaofcontinentaldrift Inthe1960 s itwasrecognizedthatthefitofthecontinentscouldbeevenfurtherimprovedbyfittingthecontinentsattheedgeofthecontinentalslope theactualextentofthecontinentalcrust EvidenceforContinentalDrift2 FossilEvidenceWegenerfoundthatidenticalfossils suchasmesosaurus werelocateddirectlyoppositeonwidelyseparatedcontinents Thishadbeenrealizedpreviouslybuttheideaof landbridges wasthemostwidelyacceptedsolution Thisnotagenerallyreasonablesolutionforcontinentssodistantfromoneanother Wegenerfoundfossilstobeconvincingevidencethatasupercontinenthadexistedinthepast EvidenceforContinentalDrift2 FossilEvidence cont WegeneralsonotedthatGlossopteris afossilfern waswidelydispersedinregionsofAfrica SouthAmerica IndiaandAustralia IthassubsequentlyfoundinAntarcticaaswell Thecurrentclimaticzonesfortheseregionsaretoodiversetoallhavesupportedthisplant InordertoexplainthedistributionofGlossopterisduringthesametimeofEarthhistory Wegenerconcludedthatthesefossilswereconvincingevidencethatthecontinentswereoncejoinedintoa supercontinent EvidenceforContinentalDrift3 RockTypeandStructuralSimilaritiesWegenerfoundsimilarrocktypesandgeologicstructuresoncontinentsonoppositesidesoftheAtlanticOcean Themapshowsthat ifthecontinentsarejoinedtogether themountainsformacontinuousbelt Thismountainbeltis 300millionyearsoldandrepresentsthetimewhenthecontinentscollidedtoformthesupercontinentPangaea TheAppalachianMountainstrendalongtheeasternflankofNorthAmerica MountainsofsimilarageandstructurealsoappearinScotlandandtheCaledonianMountainsofScandinavia EvidenceforContinentalDrift4 PaleoclimaticEvidenceGlacialtillofthesameageisfoundinsouthernAfrica SouthAmerica IndiaandAustralia areasthatitwouldbeverydifficulttoexplaintheoccurrenceofglaciation Inaddition theareaswithextensivecoaldepositsfromthesametimeperiodoccurinregionsthatwouldhavebeenequatorial Atthesametime largecoaldepositswereformedfromtropicalswampsinN AmericaandEurope PangaeawithS AfricacenteredovertheSouthPolecouldaccountfortheconditionsnecessarytogenerateglacialiceinthesoutherncontinents Scienceisbasedonmorethanmereempiricalobservation westrivetounderstandthemechanisms Wedevelopscientifictheoriestoexplainourobservations Wegener sworkmaymorecorrectlybeanhypothesis Itwasnotuntilthe1960 sthatfurtherdataledtothedevelopmentofthetheoryofplatetectonicsthatcouldexplainthemovementofcontinents Wegener sideaofcontinentaldriftwasnotgenerallyacceptedbecausenoonecouldcomeupwithareasonablemechanismforthemovementofthecontinents Untoldtragediesofcontinentaldrift PlateTectonicsTheoceanfloorrevealsvaluablecluesaboutplatetectonics Wearefamiliarwiththewidediversityoflandformsandgeologicprocessesonthecontinents butthereisanequaldiversityintheoceanbasins Theoceanscontainvolcanoes mountainchains valleys plateaus etc Thereisaglobalmid oceanridgesystemthatwindsitswaythroughtheEarth soceansliketheseamsonabaseball Themid oceanridgesystemisanearlycontinuousvolcanicridgefoundinalloceans Wegener stheoryofContinentalDriftwasnotacceptedpartlybecausetherewasnotmechanismforhowlargeportionsoftheEarth scrustcouldmoverelativetooneanotheronthesurface TherecognitionofseafloorspreadingcausedascientificrevolutionwherescientistsbegantoreexamineWegener stheoryofContinentalDrift SeafloorSpreadingandContinentalDriftwereincorporatedintothemoderntheoryofPlateTectonics ThestoryofPlateTectonicsisanexcellentexampleofthecumulativenatureofscienceandthescientificmethod DivergentBoundariesMostdivergentboundariesarelocatedalongmid oceanridges Divergentplateboundariesareknownasconstructivemarginsbecausetheyarethesitewherenewoceaniccrust lithosphere isgenerated Aprocessknownasseafloorspreadingoccurswheremagmafromthemantlewellsupintothedivergentboundary formingnewbasalticseafloor Spreadingratesaverage 5cm year Thereisnearlycontinuousvolcanicactivitysomewherealongthemid oceanridgesystem Themid oceanridgehasanelevatedpositionontheseafloorbecauseitisformedfromrelativelyhotigneousrocks Astheseafloormovesawayfromtheridge itcoolsandcontracts thustheseafloorgenerallyisatagreaterdepthasyoumoveawayfromthemid oceanridge Althoughmostdivergentboundariesarefoundalongmid oceanridgesystems somecandeveloponcontinentsThefigureshowsthedevelopmentofacontinentalriftthateventuallyevolvestoformanewoceanbasin ThisiswhathappenedwhenNorthandSouthAmericanriftedfromEuropeandAfricatoformtheAtlanticocean TheEastAfricanRiftrepresentsamodernexampleofacontinentalrift Ifthisriftissuccessful easternAfricamaysplitofffromtherestofthecontinentandanewoceanbasinmayformbetweenthetwo Africas Thisregionissubjectedtoextensivevolcanicactivity asyouwouldexpect withlargevolcanoessuchasKilimanjaroandMt Kenya TheRedSeaandtheGulfofAdenformedwhentheArabianPeninsulariftedfromAfrica 20millionyearsago TheRedSeaandGulfofAdenwillcontinuetowidenifriftingandspreadingcontinues Earth sMajorPlatesTheEarth ssurfaceiscomposedofastrong rigidlayerknownasthelithosphere Thelithosphereisbrokenintopiecesknownastectonicplates Lithosphericplatesarethinnestintheoceans 100kmthick andmaybemorethan250kmthickonthecontinents Thereare7majorplatesandoveradozensmallerplates Thelithosphericplatesoverlieaweakerregionofthemantleknownastheasthenosphere Therocksintheasthenosphereareneartheirmeltingpointandarerelativelyweakandductile Theasthenosphereallowstheplatestomoveaboveit Platesmoveslowlybutcontinuously generallyontheorderofa 5cm year NotethattheplatesgenerallyincludeacontinentoraportionofacontinentANDaportionoftheoceanfloor Eachplatemovesasacoherentunitrelativetoothers PlateBoundariesInteractionsbetweenplatesoccuralongtheirboundaries thelocationsofmostearthquakesindicatetheplateboundarieswheretheyaremovingrelativetooneanother PlateBoundariesThereare3distincttypesofplateboundariesthataredefinedbasedupontherelativemovementbetweentheplates1 Divergentboundaries whereplatesmoveapart diverge 2 Convergentboundaries whereplatesmoveintooneanother converge 3 Transformfaultboundaries whereplatesgrindpastoneanother Thisfigureillustratesthedifferentmajortypesofplateboundaries Wejustconsideredtwotypesofdivergentboundaries mid oceanridgesandcontinentalrifts Nowwewillconsiderthedifferenttypesofconvergentboundaries ConvergentBoundaries Wediscussedthatnewcrustiscontinuallybemadeatthemid oceanridges constructivemargins SincethesizeoftheEarthremainsthesame portionsofthecrustmustbeconsumedordestroyedatapproximatelythesameratethatitisbeingmade Convergentmarginsarealsoknownasdestructivemarginssinceoceaniccrustisdestroyedorconsumed Mostconvergentmarginsareassociatedwithasubductionzonewhereoneplateisdescendingintotheasthenospherebeneathanotherplate Thesurfaceexpressionofasubductionzoneisadeep oceantrench thesetrenchesmaybethousandsofkmlong 50 100kmwide and8 12kmdeep Themapshowstheworld soceanictrenches NotethatthePacificisNearlyencircledindeep oceantrenches ConvergentBoundaries Oceanic ContinentalConvergence Oceanic continentalconvergenceoccurswhenleadingedgeofoneplateiscomposedofcontinentalrocks granitic andtheotherisoceanic basaltic Thedenseroceanicplatedivesbeneath subducts thelower densitycontinentalplate Lowerdensitygraniticrockstendtofloatintheasthenosphere Dewateringofthesubductedslabcausesmeltinginthewedgeoftheasthenosphereaboveit ThemagmathatisproducedisbuoyantandrisesthroughthemantletowardtheEarth ssurface Themagmathatisproducedintheasthenosphereisbasalticincomposition Asthemagmarises itmustpenetratethroughthethickcontinental granitic rocks Asitassimilatesthecontinentalrocks thecompositionofthemagmachangesfrommafictointermediate Themagmaresultsinvolcanicactivityalongalineparalleltothesubductionzoneknownasacontinentalvolcanicarc ExamplesofcontinentalvolcanicarcsincludetheCascadevolcanoessuchasMt RanierandMt St HelensandthevolcanoesoftheAndesmountainsalongthewestcoastofSouthAmerica Mt St Helens ConvergentBoundaries Oceanic OceanicConvergence Oceanic oceanicconvergenceoccurswhentheleadingedgeofbothplatesconsistsofoceaniccrust Theseplateboundarieshavemanyofthesamefeaturesasinoceanic continentalconvergence Inoceanic oceanicconvergence thelineofvolcanoesformsastringofislandsparalleltothesubductionzoneknownasavolcanicislandarc ExamplesofislandarcsystemsincludetheAleutianIslands Tonga Indonesia andJapan ConvergentBoundaries Continental ContinentalConvergence Continental continentalconvergencedefinesaplatemarginwheretheleadingedgeofbothplatescontainscontinentalcrust Thistypeofplateboundaryisassociatedwithmountain building Continent continentconvergenceusuallybeginsasoceanic continentalconvergence ex Andes Astheoceaniccrustissubducted acontinentalblockonthesubductingplatemayapproachthecontinent TheHimalayanmountainswereformedbythecollisionoftheIndiansubcontinentintotheAsianmainland ThesefiguresshowtheconvergenceofIndiaintoAsiaoverthelast71millionyears TransformFaultBoundaries Transformplateboundariesarewhereplatesslidepastoneanother Mosttransformboundariesareassociatedwithmid oceanridgeswheretheyformlinearbreaksintheridgesystem Theactivetransformboundaryexistsbetweenthetwooffsetridgesegments Thisanimationillustratestheoffsetportionsofamid oceanridgeandmovementalongatransformfault Althoughmosttransformboundariesoccurintheoceanbasins somecutthroughcontinentalcrust TheSanAndreasfaultsystemconnectsaspreadingcenterintheSeaofCortezwithaspreadingcenterofthecoastofnorthernCalifornia Studythisfigureandmakesurethatyouunderstandthedetails TestingandEvidenceofPlateTectonicsPlatetectonicsisascientifictheorythatallowustomakepredictions DataandobservationsabouttheEartharecomparedtopredictionsfromthetheoryofplatetectonicstotestitsvalidity Aswe veseen thedistributionofdeep oceantrenches earthquakesandvolcanoessupportstheplatetectonics Therearenumerousotherlinesofevidencethatprovidevalidityforthetheoryincluding DatafromoceandrillingHotspotsMagneticreversals Evidence OceanDrillingAnactiveprogramofsamplinganddrillingintheseafloorhasprovidedconsiderableevidenceinsupportofplatetectonics Thefigureshowstheageoftheseafloor Thepatternisasispredictedbythetheoryofplatetectonics Theseafloorisveryyoungatthemid oceanridgesandgetsprogressivelyolderasafunctionofdistancefromtheridge Evidence OceanDrillingThefigureshowsthethicknessofsedimentontheseafloorthroughouttheoceanbasins Theseaflooratthemid oceanridgesisyoungandhasessentiallynosedimentarycover Generally thesedimentarycoverincreaseswithdistancefromthemid oceanridge Evidence HotSpotsMappingoftheseafloorindicatesthattherearelinearchainsofvolcanicislandsstructuresknownasseamounts TheHawaiianIslandsarepartofachainofislandsandseamountsextendingtotheAleutiantrench TheBigIslandofHawaiiistheonlyislandinthechainwithactivevolcanism RadiometricdatingofthevolcanicrocksofthischainindicatethattheygetprogressivelyolderasafunctionofdistancefromtheIslandofHawaii Theoriginofthevolcanicislandsandseamountsofthisandotherchainsisfromananomalouslyhotportionofthemantleknownasahotspotthatremainsrelativelystationary AsthePacificplatemovesoverthehotspot volcanoesareformedfrommagmasgeneratedbythehotspot Astheplatecontinuestomove thevolcanomovesoffofthehotspotandbecomesextinctbutisreplacedbyanewonedirectlyabovethehotspot Theseobservationsareconsistentwiththetheoryofplatetectonicsandsupportit Evidence MagneticReversalsTheEarthislikeagiantbarmagnet Whenvolcanicrockscrystallize someminerals ex magnetite orientthemselvestotheEarth smagneticfield preservingarecordoftheorientationoftheEarth smagneticfield ThefigureshowsthattheEarth smagneticfieldiscurrentlyorientedsothatmagneticlinesofforceareenteringtheEarthnearthenorthpole Carefulmappingoftheremnantmagneticfieldoftheseafloorindicatesthatitcontainsstripesofalternatingmagneticpolarity Inthefigure regionsofnormalpolarityareindicatedinwhitewheremagneticnorthiscoincidentwiththegeographicnorth Theregionsinredrepresentseafloorthatcrystallizedw

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