(岩石圈).ppt

Chapter2GlobalTectonicsOurDynamicPlanet,Introduction,Eachrockybody,whetherplanetormoon,startedwithahotinterior.Eachhasbeenkeptwarmovertimebyenergyreleasedbythedecayofradioactiveisotopes.Despiteradioactiveheating,rockybodieshavecooledconsiderablysincetheiration,sothattheirouterlayershavestiffenedintolithospheres岩石圈.,Introduction2,TheinteriorofEarthremainshotandgeologicallyactive.ThemantlesofEarthlosesinternalheatbyconvection對流,theslowflowofsolidrock.Hotrockrisesupwardtonearthesurface.Earth’sstifflithosphereisbrokenintoacollectionofnear-rigidplates.,Introduction3,Mostlarge-scalegeologicevents,likeearthquakesorvolcaniceruptions,originatewithinEarth’sinterior.ManyotherprocessesintheEarthsystem,suchasthehydrologicandbiogeochemicalcycles,areprofoundlyaffectedbyplatetectonics板塊運動.,PlateTectonics板塊學說FromHypothesistoTheory,PlatetectonicsisascientifictheorythatexplainstwocenturiesofoftenpuzzlingobservationsandhypothesesaboutourplanetEarth.Thecontinentsaredriftingveryslowlyacrossthefaceofourplanet.Continentaldrift大陸漂移isaconceptwithalonghistory.,PlateTectonicsFromHypothesistoTheory2,AcenturyagogeologistspuzzledoverthefitoftheshorelinesofAfricaandSouthAmerica.Theynotedthatfossilsofextinctland-boundplantsandanimals,glacialdeposits冰河沉積,andancientlavaflows熔岩流couldbematchedtogetheralongcoastlinesthattodayarethousandsofkilometersapart.CoalwasfoundinAntarctica.Coalsintropicalclimates,implyingthatAntarcticahasmovedinthepast.,PlateTectonicsFromHypothesistoTheory3,AlfredWegenerproposedthemostcomprehensiveearlyhypothesisfor“ContinentalDrift大陸漂移假說”in1912.HistheorywaswidelyrejectedbecauseOceanfloorwastoostrongtobeplowedaside.Wegenerhadnotproposedaplausibleforcethatcouldinducethecontinentstodrift.,PlateTectonicsFromHypothesistoTheory4,Aftermanyyearsofscientificobservations,thetheoryofPlateTectonicswasbornin1960.PlatetectonicsistheprocessbywhichEarth’shotinteriorlosesheat.Nowadays,wecanmeasuretheslowdriftofplatesworldwideusingsatellitenavigationsystems.Thebasicpremisesofplatetheoryaresecurebecausetheycanbetestedagainstawidevarietyofobservations.,ContinentalDriftversusPlateTectonics,WhatEarth’sSurfaceFeaturesTellUs,Therocksbeneathourfeetaresolid,buttheyarenotrigid.Topographythereliefandofthelandabovesealevel.Bathymetrytopographyontheoceanfloor.Earthbulgesarounditsequatorandisslightlyflattenedatthepoles.,IsostasyWhySomeRocksFloatHigherThanOthers,Thecontinentsaverageabout4.5kmelevationabovetheoceanfloor.Theystandnotablyhigherthantheoceanbasinsbecausethethickcontinentalcrust大陸地殼isrelativelylightaveragedensity2.7g/cm3.Thethinoceaniccrust海洋地殼isrelativelyheavyaveragedensity3.0g/cm3.Thelithosphere岩石圈floatsontheasthenosphere軟流圈.,Isostasy地殼均衡說-similartoPrincipleofArchimedesappliedtotheearth-firstnotedwhenFrenchBouguerinthe18thcenturysurveyedtheshapeoftheearth,Isostasy2,Theprincipleofisostasygovernstheriseorsubsidence.Allpartsofthelithosphereareinafloatingequilibrium.Low-densitywoodblocksfloathighandhavedeep“roots,”whereashigh-densityblocksfloatlowandhaveshallow“roots.”,Fig.2.2,Earth’sSurfaceLandVersusWater,Theoceancovers71percentofEarth’ssurface.Sealevelfluctuatesovertime.WhenclimateiscolderandwaterisstoredasiceSealevelfalls.Theshorelinemovesseaward.WhenclimategetswarmerTheicemelts.Sealevelrises.Theshorelineadvancesinland.,Fig.2.3,Fig.2.1,Earth’sSurfaceLandVersusWater2,Underseamid-oceanridgesacontinuousfeaturemorethan60,000kmlong.Mid-oceanridgesmarkwheretwooceanicplatesspreadapart.Newlithospheresinthegap.,Fig.2.4Continentalshelvesandslopeslightbluetake25ofthemassofthecontinentalcrust.,Fig.2.5TopographyacrossthenorthernAtlanticOcean.TheAtlanticcoastlineisatypicalexampleforpassivecontinentalmargin.,Earth’sSurfaceLandVersusWater3,Thecontinentalshelf大陸棚steepensslightlyat100-200metersbelowsealevel.Thecontinentalslope大陸斜坡isthefloodedcontinentalmargin.Thecontinentalrise大陸隆起descendsmoregentlyfromthebaseofthecontinentalslope,Earth’sSurfaceLandVersusWater4,Oceantrenches海溝occurswhereoceaniclithosphereandcontinentaloroceaniclithosphereconvergeattheboundarybetweentwoplatese.g.,Ryukyutrench,Marianatrench.Becauseoceaniclithosphereisthedenserofthetwo,itdescendsundertheactivecontinentalmarginandsinksintothedeepermantle.Thelarge,flatabyssalfloors深海海床oftheopenoceanlie3to6kmbelowsealevel.,WhatEarth’sInternalPhenomenaTellUs,Heatconduction熱傳導andconvection對流.Conductionisdominantwhenthetemperaturegradientinrocksislarge.earth’ssurfaceandcore-mantleboundaryRocksarepoorconductorsofheat,sotheinternalheatistransferredbymovingtherockitself.Thecirculationofhotrockismaintainedbymantleconvection地涵對流或地幔對流.,Fig.2.6mantleconvectionthatshapestheearth’ssurface.Heatsourcecomesfromcoolingoftheearthitselfsince4.55Byranddecayofradioactiveelements.,mid-oceanridge,oceanictrench,HeatConduction熱傳導,Conductionistheprocessbywhichheatmovesthroughsolidrockviamolecularcollisions.It’sadiffusive擴散processwhereinmoleculestransmittheirkineticenergytoothermoleculesbycollidingwiththem.Heatisconductedthroughamediuminwhichthereisaspatialvariationinthetemperatureorasteeptemperaturegradient.Thelossoftheearth’sinternalheatthroughoceaniccrustandlithosphereislargelycontrolledbyconduction.,MantleConvection1,,,Earth’sheatcanmoveinasecondprocesscalledconvection對流.Convectioncanhappeningases,inliquids,or,givenenoughtime,inductilesolids.Aprerequisiteconditionantleconvectionisthethermalexpansion熱膨脹ofhotrock.Convectiveheatistransportedwiththemotionofductilerock.,MantleConvection2,Rockexpandsasitstemperatureincrease,anditsdensitytherebydecreasesslightly.Thehotrockisbuoyantrelativetocoolerrockinitsimmediateneighborhood.A1percentvolumeexpansionrequiresanincreaseof300-400oCandleadstoa1percentdecreaseindensity.Viscosity黏滯係數representsthetendencyofrocktoductileflow延展性流動.UnitNewton.second/meter2,RockDeationElasticversusViscous,Foranelasticsolid,stressislinearlyproportionaltostrain,Ingeneral,300-1000atmospherepressure1atm1bar105PascalN/m2isrequiredtocompressarockby1/1000.Viscositymmeasurestheresistanceofasolidorfluidtoductileflow.Ductiledeationbecomesimportantatlargerdepth,whererocksarehotandlessrigid.100atmisestimatedtocausethemantlerocksbeneaththeplatestodeatasteadyrateof1/106peryear.,MantleConvection3,Rockdoesnotneedtomeltbeforeitcanflow.ThepresenceofH2Oencouragesflowinsolidrock.ConvectioncurrentsbringhotrocksupwardfromEarth’sinterior.,GeothermalGradient地溫梯度oftheLithosphere,Heatmovesthroughthelithosphereprimarilybyconduction.Thelithosphere-asthenosphereboundaryis1300-1350oC,dependingondepth.Oceaniclithosphereisabout100kmthick.Theaveragegeothermalgradientinoceaniclithosphereisabout13oC/km.Averagecontinentallithosphereis200km.Theaveragegeothermalgradientincontinentallithosphereisabout6.70C/km.,,,Fig.2.7,AdiabaticExpansion絕熱膨脹ofRock,Adiabaticexpansionmeans“expansionwithoutlossorgainofenergy.”Rockiscompressedandreducedinvolumebyincreasingpressurewithdepth;itisalsoheatedbytheworkdonebythepressureforceduringthecompression.Theassociatedtemperaturerisecausesadiabaticexpansion.,AdiabaticExpansion絕熱膨脹ofRock2,Inconvectivemantle,themeantemperatureincreaseswithdepthalonganadiabat絕熱線.Theadiabaticthermalgradient絕熱溫度梯度inthemantleistherateofincreaseoftemperaturewithdepthasaresultofcompressionoftherockbytheweightoftheoverlyingrock;itisapproximately0.5oC/km.,Earth’sConvectionDrivenFromtheTop,Belowthelithosphere,rockmassesinthedeepermantleriseandfallaccordingtodifferencesintemperatureandbuoyancy.Earth’sconvectionisdrivenmainlybycoldermaterialsinkingfromthetop.,Earth’sConvectionDrivenFromtheTop2,Thedensestlithosphereismostlikelytosinkbackintotheasthenosphereandthedeepermantlewhilelightercontinentallithosheredriftsacrosstheearth’ssurface.Oceanfloorandthecontinentsareslowlymovingupto12cm/yr.,Fig.2.8,PlatesandMantleConvection,Whencontinentssplitapart,anewoceanbasins.TheRedSeawasedthisway30millionyearsago.Subductiontheoldlithospheresinksbeneaththeedgeofanadjacentplate.,GlobalPositioningSystem,Inthe1960s,theU.S.DepartmentofDefenseestablishedanetworkofsatelliteswithorbitsthatcouldbeusedforreferenceinpreciselydetermininglocation.TheGlobalPositioningSystemGPSdetectssmallmovementsoftheEarth’ssurface.,Fig.2.9C.surfacemotionfromGPSmeasurement,GlobalPositioningSystem2,Itisaccuratewithinafewmillimeters.TwomeasurementsAGPScampaignresearchersestablishanetworkoffixedreferencepointsonEarth’ssurface,oftenattachedtobedrock.Thepositionisre-measuredeveryfewmonthsoryears.ContinuousGPSmeasurementthereceiversareattachedpermanentlytomonuments,andpositionisestimatedatfixedintervalsofafewsecondsorminutes.,FourTypesofPlateMarginsandHowTheyMove,Thelithospherecurrentlyconsistsof12largeplates.ThesevenlargestplatesareNorthAmericanPlate.SouthAmericanPlate.AfricanPlate.PacificPlate.EurasianPlate.Australian-IndianPlate.AntarcticPlate.,Fig.2.10,Fig.2.9,Fig.2.9A.Present-dayplatmotionbasedonmanygeologicaldata,includinglineationofmagneticanomalyonseafloor,relativemotionalongthestrikeoftransfaults,earthquakeslipdirectionanddisplacement,etc..Reddotsmarkthelocationofsignificantearthquakessince1965.Fig.2.9B.SurfacemotionsfromcontinuousGPSmeasurements.,Plateshavefourkindsofboundariesormargins板塊邊界,Divergentmargin/spreadingcenter分離板塊邊界/擴張中心e.g.EastPacificRise,Mid-AtlanticRidge.Convergentmargin/subductionzone聚合板塊邊界/隱沒帶e.g.JapanTrench,AleutianTrench.Convergentmargin/collisionzone聚合板塊邊界/碰撞帶e.g.Indo-Himalayacollisionzone.Transfaultmargin轉型斷層邊界,SeismologyandPlateMargin,Earthquakesoccurinportionsofthelithospherethatarestiffandbrittle.Earthquakesusuallyoccuronpre-existingfracturesurfaces,orfaults.Therearedistinctivetypesofearthquakesthatcorrelatenicelywithmotionatplateboundaries.,Fig.2.12threetypesoffaults,Fig.2.11Fourtypesofplatemargines,,,,TypeIDivergentMargin,Wheretwoplatesspreadapartatadivergentboundary,hotasthenosphererisestofillthegap.Asitascends,therockexperiencesadecreaseinpressureandpartiallymelts.Themoltenrockfromsuchpressure-releasepartialmeltingiscalledmagma岩漿.,TypeIDivergentMargin2,Oceaniccrustisedatmid-oceanridgeswithin1-2kmoftheridgeaxes.Foundineveryocean.acontinuouschainthatcirclestheglobe.Oceaniccrustisabout6-8kmthickworldwide.,AnimationofSeafloorSpreadingSourceCDofthetextbook,Pressure-releasepartialmelting,Seafloorspreadingandmagneticchron,BirthoftheAtlanticOcean1,WhenaspreadingcentersplitscontinentalcrustAgreatrift裂谷s,suchastheAfricanRiftValley東非裂谷.AsthetwopiecesofcontinentalcrustspreadapartThelithospherethins.Theunderlyingasthenosphererises.Volcanismcommences.Theriftwidensanddeepens,eventuallydroppingbelowsealevel.Thentheseaenterstoalong,narrowwaterbodyliketheRedSea.,,BirthoftheAtlanticOcean2,ThecontinentsthatnowborderitwerejoinedintoasinglevastcontinentthatWegenernamedPangaeawhichmeansalllands.About200millionyearsago,newspreadingcenterssplitthehugecontinent.TheAtlanticcontinuestowidentodayat2-4cm/yr.,Fig.2.13,CharacteristicsofSpreadingCenters1,Earthquakesatmidoceanridgesoccuronlyinthefirst10kmbeneaththeseafloorandtendtobesmall.Volcanicactivityoccursatmidoceanridgesandcontinentalrift.Themidoceanridgesrise2kmormoreabovesurroundingseafloor.Theprincipleofisostasyapplieslower-densityrockrisestoahigherelevationatridgesandthecoolingresultsthesubsidenceofseafloor,CharacteristicsofSpreadingCenters2,IfthespreadingrateisfastAlargeramountofyoungwarmoceaniclithosphereisproduced,andtheridgewillbewider.Aslow-spreadingridgewillbenarrower.TheAtlanticOceanspreadsslowly2-4cm/yr.ThePacificspreadingcenterEastPacificRiseisfastbycomparison6-20cm/yr.ThePacificOceanbasinisshrinkingbecause,,SeafloorSpreadingandAgeMaphttp//www.windows.ucar.edu/tour/link/earth/interior/seafloor_spreading.html,AnimationofTopographyandSubductionAnglesatFastandSlowMovingPlatesfromCDoftextbook,RoleofSeawateratSpreadingCenters,Seawatercirculatesthroughcracksbeneaththeoceanfloor.Coldwaterpercolates滲透throughthesecracks,warmsincontactwithsubsurfacerock,andrisesconvectivelytounderseahotsprings.Asmallfractionoftheseawaterremainsintherock,chemicallyboundwithinhydrouswater-bearingmineralslikeserpentineandclays.,TheCO2Connection,Asoceaniclithosphereages,itaccumulatesathicklayerofsedimentssuchasclayandcalciumcarbonateCaCO3fromtheshellsandinternalskeletonsofmarineorganisms.TheationofcalciumcarbonateconsumescarbondioxideCO2thatisdissolvedinseawater.SeafloorsedimentsremoveCO2fromtheatmosphere,andthushavealong-terminfluenceonthegreenhouseeffectandEarth’sclimate.,TypeIIConvergentMargin/SubductionZone,Astheplatecools,itgrowsdenserandtheprincipleofisostasydemandsthattheplatesubsides.Theprocessbywhichlithospheresinksintotheasthenosphereiscalledsubduction.Themarginsalongwhichplatesaresubductedarecalledsubductionzones.Thesinkingslabwarms,softens,andexchangesmaterialwiththesurroundingmantle.,TypeIIConvergentMargin/SubductionZone2,Underelevatedtemperatureandpressure,thecrustexpelsanumberofchemicalcompounds.WaterH2O,CarbondioxideCO2,andSulfurcompoundsS.AsmalladditionofthesevolatilesubstancescanlowerthemeltingpointofrockbyseveralhundreddegreesCelsiusThehotmantlerockimmediatelyabovethesinkingslabstartstomelt.Magmarisestothesurfacetovolcanoes.Subductionzonesaremarkedbyanarcofvolcanoesparalleltotheedgeoftheplate.,TheCO2Connection,Again,Water,CO2,andsulfuricgaseslikeSO2andH2Sreturntotheatmosphere.Subductionzonevolcanicactivityraisesthecarbondioxidelevelintheatmosphere,rtingastronginfluenceonthegreenhouseeffectandEarth’sclimate.VolcanismtendstoreplacetheCO2thatislostfromtheatmosphereintotheoceanandstoredintheseafloor..,VolcanoesAtSubductionZones,Ataplateboundary,theplungingplatedrawstheseafloordownintoanoceantrench.Whentheslabgetsdowntoabout100km,watersqueezedoutofthesubductedmaterialsbeginstoreactwiththeambientmantlerockandcausessomeofthemantletomelt.Moltenrockthatmakesitallthewaytothesurfaceeruptstoalineofvolcanoesspacedabout70kmapartfromthetrench.,VolcanoesAtSubductionZones2,Iftheoverridingplateisoceaniclithosphere,volcanoesaseriesofislandscalledavolcanicislandarc火山島弧,e.g.,MarianaIslands,AleutianIslands.Iftheoverridingplateiscontinentallithosphere,acontinentalvolcanicarcs.Sedimentwashedfromthecontinenttendstofilltheoffshoretrench,e.g.,CascadeRangeofthePacificNorthwest,theAndesofSouthAmerica.,EarthquakesinSubductionZones,Thelargestandthedeepestearthquakesoccurinsubductionzones.Thelocationofmostearthquakesdefinethetopsurfaceofaslabasitslidesintothemantlethesurfacetoasdeepas670km.Quakesdeeperthan100kmaremorelikelyassociatedwithfaultscausedbystresseswithintheslab.,Distributionofearthquakeepicentersfrom1975to1995.Depthoftheearthquakefocusisindicatedbycolor.,Fig.2.16Wadati-BenioffZone,AnimationofSubductionProcessfromCDoftextbook,TypeIIIConvergentMargin/CollisionZone,Continentalcrustisnotrecycledintothemantle.Continentalcrustislighterlessdenseandthickerthanoceaniccrust.Whentwofragmentsofcontinentallithosphereconverge,thesurfacerockscrumple擠壓摺皺togethertoacollisionzone.,TypeIIIConvergentMargin/CollisionZone2,Collisionzonesthat