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***Readingsaretobedoneinclass:doNOTtakehome NW8-3Power UnderstandingPlateMotions Scientistsnowhaveafairlygoodunderstandingofhowtheplatesmoveandhowsuch movementsrelatetoearthquakeactivity.Mostmovementoccursalongnarrowzones betweenplateswheretheresultsofplate-tectonicforcesaremostevident. Therearefourtypesofplateboundaries: • Divergentboundaries--wherenewcrustisgeneratedastheplatespullaway fromeachother. • Convergentboundaries--wherecrustisdestroyedasoneplatedivesunder another. • Transformboundaries--wherecrustisneitherproducednordestroyedasthe platesslidehorizontallypasteachother. • Plateboundaryzones--broadbeltsinwhichboundariesarenotwelldefined andtheeffectsofplateinteractionareunclear. Divergentboundaries Divergentboundariesoccuralongspreadingcenterswhereplatesaremovingapartand newcrustiscreatedbymagmapushingupfromthemantle.Picturetwogiantconveyor belts,facingeachotherbutslowlymovinginoppositedirectionsastheytransportnewly formedoceaniccrustawayfromtheridgecrest. PerhapsthebestknownofthedivergentboundariesistheMid-AtlanticRidge.This submergedmountainrange,whichextendsfromtheArcticOceantobeyondthe southerntipofAfrica,isbutonesegmentoftheglobalmid-oceanridgesystemthat encirclestheEarth.TherateofspreadingalongtheMid-AtlanticRidgeaveragesabout 2.5centimetersperyear(cm/yr),or25kminamillionyears.Thisratemayseemslow byhumanstandards,butbecausethisprocesshasbeengoingonformillionsofyears,it hasresultedinplatemovementofthousandsofkilometers.Seafloorspreadingoverthe past100to200millionyearshascausedtheAtlanticOceantogrowfromatinyinletof waterbetweenthecontinentsofEurope,Africa,andtheAmericasintothevastocean thatexiststoday. Mid-AtlanticRidge ThevolcaniccountryofIceland,whichstraddlestheMid-AtlanticRidge,offersscientists anaturallaboratoryforstudyingonlandtheprocessesalsooccurringalongthe submergedpartsofaspreadingridge.Icelandissplittingalongthespreadingcenter betweentheNorthAmericanandEurasianPlates,asNorthAmericamoveswestward relativetoEurasia. ***Readingsaretobedoneinclass:doNOTtakehome NW8-3Power MapshowingtheMid-AtlanticRidgesplittingIcelandandseparatingtheNorth AmericanandEurasianPlates.ThemapalsoshowsReykjavik,thecapitalofIceland,the Thingvellirarea,andthelocationsofsomeofIceland'sactivevolcanoes(redtriangles), includingKrafla. InEastAfrica,spreadingprocesseshavealreadytornSaudiArabiaawayfromtherestof theAfricancontinent,formingtheRedSea.ThisisanotherexampleofaDivergent boundary.ItislikelythisregioninEastAfricawillbethesiteoftheEarth’snextmajor ocean! HistoricallyActiveVolcanoes,EastAfrica OldoinyoLengai,EastAfricanRiftZone Convergentboundaries ThesizeoftheEarthhasnotchangedsignificantlyduringthepast600millionyears,and verylikelynotsinceshortlyafteritsformation4.6billionyearsago.TheEarth's unchangingsizeimpliesthatthecrustmustbedestroyedataboutthesamerateasitis beingcreated.Suchdestruction(recycling)ofcrusttakesplacealongconvergent boundarieswhereplatesaremovingtowardeachother,andsometimesoneplatesinks (issubducted)underanother.Thelocationwheresinkingofaplateoccursiscalled asubductionzone. ***Readingsaretobedoneinclass:doNOTtakehome NW8-3Power Thetypeofconvergence--calledbysomeaveryslow"collision"--thattakesplace betweenplatesdependsonthekindanddensityoflithosphereinvolved. Oceanic-continentalconvergence IfbymagicwecouldpullapluganddrainthePacificOcean,wewouldseeamost amazingsight--anumberoflongnarrow,curvingtrenchesthousandsofkilometerslong and8to10kmdeepcuttingintotheoceanfloor.Trenchesarethedeepestpartsofthe oceanfloorandarecreatedbysubduction. OffthecoastofSouthAmericaalongthePeru-Chiletrench,theoceanicNazcaPlateis pushingintoandbeingsubductedunderthecontinentalpartoftheSouthAmerican Plate.Inturn,theoverridingSouthAmericanPlateisbeingliftedup,creatingthe toweringAndesmountains,thebackboneofthecontinent.Strong,destructive earthquakesandtherapidupliftofmountainrangesarecommoninthisregion.Even thoughtheNazcaPlateasawholeissinkingsmoothlyandcontinuouslyintothetrench, thedeepestpartofthesubductingplatebreaksintosmallerpiecesthatbecomelocked inplaceforlongperiodsoftimebeforesuddenlymovingtogeneratelargeearthquakes. Suchearthquakesareoftenaccompaniedbyupliftofthelandbyasmuchasafew meters. RingofFire Oceanic-continentalconvergencealsosustainsmanyoftheEarth'sactivevolcanoes, suchasthoseintheAndesandtheCascadeRangeinthePacificNorthwest.Theeruptive activityisclearlyassociatedwithsubduction,butscientistsvigorouslydebatethe possiblesourcesofmagma:Ismagmageneratedbythepartialmeltingofthesubducted oceanicslab,ortheoverlyingcontinentallithosphere,orboth? Oceanic-oceanicconvergence ***Readingsaretobedoneinclass:doNOTtakehome NW8-3Power Aswithoceanic-continentalconvergence,whentwooceanicplatesconverge,oneis usuallysubductedundertheother,andintheprocessatrenchisformed.TheMarianas Trench(parallelingtheMarianaIslands),forexample,markswherethefast-moving PacificPlateconvergesagainsttheslowermovingPhilippinePlate.TheChallengerDeep, atthesouthernendoftheMarianasTrench,plungesdeeperintotheEarth'sinterior (nearly11,000m)thanMountEverest,theworld'stallestmountain,risesabovesea level(about8,854m). Subductionprocessesinoceanic-oceanicplateconvergencealsoresultintheformation ofvolcanoes.Thedescendingplatealsoprovidesasourceofstressasthetwoplates interact,leadingtofrequentmoderatetostrongearthquakes. Continental-continentalconvergence TheHimalayanmountainrangedramaticallydemonstratesoneofthemostvisibleand spectacularconsequencesofplatetectonics.Whentwocontinentsmeethead-on, neitherissubductedbecausethecontinentalrocksarerelativelylightand,liketwo collidingicebergs,resistdownwardmotion.Instead,thecrusttendstobuckleandbe pushedupwardorsideways.ThecollisionofIndiaintoAsia50millionyearsagocaused theIndianandEurasianPlatestocrumpleupalongthecollisionzone. ***Readingsaretobedoneinclass:doNOTtakehome NW8-3Power Above:ThecollisionbetweentheIndianandEurasianplateshaspushedupthe HimalayasandtheTibetanPlateau.Below:Cartooncrosssectionsshowingthemeeting ofthesetwoplatesbeforeandaftertheircollision.Thereferencepoints(smallsquares) showtheamountofupliftofanimaginarypointintheEarth'scrustduringthis mountain-buildingprocess. Transformboundaries Thezonebetweentwoplatesslidinghorizontallypastoneanotheriscalledatransformfaultboundary,orsimplyatransformboundary.Mosttransformfaultsarefoundonthe oceanfloor.Theycommonlyoffsettheactivespreadingridges,producingzig-zagplate margins,andaregenerallydefinedbyshallowearthquakes.However,afewoccuron land,forexampletheSanAndreasfaultzoneinCalifornia.Thistransformfaultconnects theEastPacificRise,adivergentboundarytothesouth,withtheSouthGorda--Juande Fuca--ExplorerRidge,anotherdivergentboundarytothenorth. ***Readingsaretobedoneinclass:doNOTtakehome NW8-3Power TheBlanco,Mendocino,Murray,andMolokaifracturezonesaresomeofthemany fracturezones(transformfaults)thatscartheoceanfloorandoffsetridges(seetext). TheSanAndreasisoneofthefewtransformfaultsexposedonland. TheSanAndreasfaultzone,whichisabout1,300kmlongandinplacestensof kilometerswide,slicesthroughtwothirdsofthelengthofCalifornia.Alongit,thePacific PlatehasbeengrindinghorizontallypasttheNorthAmericanPlatefor10millionyears, atanaveragerateofabout5cm/yr.Landonthewestsideofthefaultzone(onthe PacificPlate)ismovinginanorthwesterlydirectionrelativetothelandontheeastside ofthefaultzone(ontheNorthAmericanPlate).
