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IntroductiontoPLATETECTONICS
part1:Earth’sStructureandFoundations
AlessandroGrippo,Ph.D.
Mountainsalongatransformfault:theLynnCanalintheAlaskapanhandleSkagway,Alaska©AlessandroGrippo
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Earthissubjecttochange
• Earthchangesintime• Earthchangesinspace
• Energyforchangecomes(asheat)from:– anexternalsource(theSun)– aninternalsource(radioactivedecayinEarth’scoreandmantle)
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• HeatfromtheSun:weathering&erosion– atmosphericandoceaniccirculation,watercycle,erosion,“smoothing”ofoutersurface
• Heatfromradioactivity:platetectonics– buildingofoceans,mountains,continents;volcanoesandearthquakes,“roughing”oftheoutersurface
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Heatvs.Temperature
• Heat– energytransferfromabodytoanotherduetoadifferenceintemperature
• Temperature– ameasureoftheaveragekineticenergyofthemoleculesthatmakeupasubstance
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HowDoesHeatTravel?Heatcantravel(transfer)inthreedifferentways:
• Radiation– Heatenergyisfeltfromahotobjectatadistance(aroundafire,
baskinginthesun,etc.)
• Conduction– Heatenergyisfeltbytouchingahotobject(ahotpan,hot
water,etc.)
• Convection– Heatenergyistransferredinbulkmotionorflowofafluidmass
(alavalamp,theasthenosphere,etc.)
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modelingconvection
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Earth’sInteriorTwowaystolookatthings
• Concentricspheresdifferentiatedbasedonchemicalcomposition(madeofdifferentrocks),frominsideoutward:– Core– Mantle– Crust
• OceanicCrust• ContinentalCrust
• Concentricspheresdifferentiatedbasedonphysicalbehavior(differentkindsofrockswhichbehaveindifferentways),frominsideoutward:– InnerandOuterCore– Mesosphere– Asthenosphere– Lithosphere
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Earth’sInterior
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Chemical(compositional)differentiation
– Crust(~3-70kmthick)
• VerythinouterrockyshellofEarth– Continentalcrust-thickerandlessdense– Oceaniccrust-thinnerandmoredense
– Mantle(~2900kmthick)
• Hotsolidthatflowsslowlyovertime;Fe-,Mg-,Si-richminerals
– Core(~3400kmradius)
• MostlyIron(Fe)• Outercore-metallicliquid
• Innercore-metallicsolid
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HowdoweknowaboutEarth’slayers?
• Notbydrilling:wedonothavethetechnologytoreachthecoreorthemantle)– deepestwellisabout11km(11,000mor~7mi)– thicknessofthecrust:
• oceaniccrust:around5km– cannotdrillthroughitbecausewehavetostartfromoceansurface:5kmofoceanaboveit
• continentalcrust:upto80km–toothicktodrill
– wehaveneverseenthemantleorthecoreinplace
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• WeknowaboutEarth’sinteriorthroughthestudyofseismicwaves
• Wavesarerefracted(changepathandspeed)wherematerialshavedifferentdensities
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• P(primary)andS(secondary)waves– PwavesarefasterthanSwaves– Swavesdonottravelthroughliquids– Swavesdonotreachpastthe“shadowzone”– Swavesarestopped(reflected)atthemantle/coreboundary• Thisimpliesthattheoutercoreisliquid
– ThepatternofPandSwavesrefractiontellsuswherethesechangesoccur
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• Earth’scoreismadeofIron(Fe)andNickel(Ni)
• Theinnercoreissolidwhiletheoutercoreisliquid
• MetalslikeFearekepttogetherbyachemicalbondcalledmetallicbond
• Inmetallicbonds,electronsarefreetoroam
• Whenelectronsroam,theycreateanelectricalcurrent
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• Inablockofironthesecurrentscanceleachotherout
• Iftheoutercoreisliquid,andEarthrotatesarounditsaxis,theironitselfisinmotion
• Iftheironisinmotion,itwillthencreateanelectricalcurrent
• Anelectricalcurrentwouldalsocreateamagneticfield
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Earth’sMagneticField
• thespinningofEarth:
– causesametallicliquidoutercoretomove
– createsanelectricalfield
– thatgeneratesEarth’smagneticfield
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WhydoweneedtoknowaboutEarth’sMagneticField?
• Earth’sMagneticField:– protectsus(life)fromdamagingsolarradiation
– canberecordedincertainrocks,particularlythoseformingatthebottomoftheocean
– itspropertieschangefromplacetoplaceonEarth,andcanbeidentified
– Intheend,itisveryusefulingeologicalstudies
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Physical(mechanical)differentiation
• MechanicalLayers– Lithosphere(~100kmthick)
• Rigid/brittleoutershellofEarth
• Composedofbothcrustanduppermostmantle
• MakesupEarth’stectonic“plates”
– Asthenosphere• Plastic(capableofflow)zoneonwhichthelithosphere“floats”
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definitionofPLATETECTONICS
• ThesurfaceofEarthisbrokendowninaseriesofLITHOSPHERICPLATES,thatmovearoundpassively,draggedbyCONVECTIONINTHEASTHENOSPHERE
• Therearesevenmajorplates(NorthAmerica,SouthAmerica,Pacific,Eurasia,Africa,Antarctica,India-Australia)andavarietyofsmallerones
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• ContinentalDriftHypothesis– Originallyproposedinearly1910sinordertoexplainthe“fitof
continents”,commonrocktypesandfossilsacrossoceanbasins,etc.
– Insufficientevidencefoundfordrivingmechanism;hypothesisrejected
• PlateTectonicsTheory– Originallyproposedinthelate1960s– Includednewunderstandingoftheseafloorandagoodexplanationof
thedrivingforcebehindmotion
– Describeslithosphereasbeingbrokenintoplatesthatareinmotion
– Explainsoriginandlocationsofvolcanoes,faultzones,mountainbelts
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PlateBoundaries
• Divergentboundaries– Platesmoveapart,awayfromeachother– Magma(moltenrock)rises,coolsintoarock,andformsnewlithosphere– Typicallyexpressedasmid-oceanicridges
• Transformboundaries– Platesslidepastoneanother– Faultzones(transformfaults)andearthquakesmarkboundary– Example:SanAndreasFaultinCalifornia
• Convergentboundaries– Platesmovetowardeachother,orcollide– Mountainbelts,powerfulearthquakesandvolcanoescommon– Oceanicplatesmaysinkbackintothemantlealongasubductionzone,
typicallymarkedbyadeepoceantrench
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Thethreekindsofplateboundaries
DivergentandTransformBoundaries
ConvergentBoundary
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Theoceaniccrustformscontinuouslyatamid-oceanicridgeTheoceanexpandsandtheageofthecrustincreasesawayfromtheridge
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PlateTectonicsendofpart1
TheAndes:amountainchainformedbycollisionbetweentwoplatesRoadfromSantiago,ChiletoMendoza,Argentina
©AlessandroGrippo