IntroductiontoPLATETECTONICS

part1:Earth’sStructureandFoundations

AlessandroGrippo,Ph.D.

Mountainsalongatransformfault:theLynnCanalintheAlaskapanhandleSkagway,Alaska©AlessandroGrippo

Earthissubjecttochange

•  Earthchangesintime•  Earthchangesinspace

•  Energyforchangecomes(asheat)from:– anexternalsource(theSun)– aninternalsource(radioactivedecayinEarth’scoreandmantle)

•  HeatfromtheSun:weathering&erosion– atmosphericandoceaniccirculation,watercycle,erosion,“smoothing”ofoutersurface

•  Heatfromradioactivity:platetectonics– buildingofoceans,mountains,continents;volcanoesandearthquakes,“roughing”oftheoutersurface

Heatvs.Temperature

•  Heat– energytransferfromabodytoanotherduetoadifferenceintemperature

•  Temperature– ameasureoftheaveragekineticenergyofthemoleculesthatmakeupasubstance

HowDoesHeatTravel?Heatcantravel(transfer)inthreedifferentways:

•  Radiation–  Heatenergyisfeltfromahotobjectatadistance(aroundafire,

baskinginthesun,etc.)

•  Conduction–  Heatenergyisfeltbytouchingahotobject(ahotpan,hot

water,etc.)

•  Convection–  Heatenergyistransferredinbulkmotionorflowofafluidmass

(alavalamp,theasthenosphere,etc.)

modelingconvection

Earth’sInteriorTwowaystolookatthings

•  Concentricspheresdifferentiatedbasedonchemicalcomposition(madeofdifferentrocks),frominsideoutward:–  Core–  Mantle–  Crust

•  OceanicCrust•  ContinentalCrust

•  Concentricspheresdifferentiatedbasedonphysicalbehavior(differentkindsofrockswhichbehaveindifferentways),frominsideoutward:–  InnerandOuterCore–  Mesosphere–  Asthenosphere–  Lithosphere

Earth’sInterior

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

HowdoweknowaboutEarth’slayers?

•  Notbydrilling:wedonothavethetechnologytoreachthecoreorthemantle)–  deepestwellisabout11km(11,000mor~7mi)–  thicknessofthecrust:

•  oceaniccrust:around5km–  cannotdrillthroughitbecausewehavetostartfromoceansurface:5kmofoceanaboveit

•  continentalcrust:upto80km–toothicktodrill

– wehaveneverseenthemantleorthecoreinplace

•  WeknowaboutEarth’sinteriorthroughthestudyofseismicwaves

•  Wavesarerefracted(changepathandspeed)wherematerialshavedifferentdensities

•  P(primary)andS(secondary)waves– PwavesarefasterthanSwaves– Swavesdonottravelthroughliquids– Swavesdonotreachpastthe“shadowzone”– Swavesarestopped(reflected)atthemantle/coreboundary•  Thisimpliesthattheoutercoreisliquid

– ThepatternofPandSwavesrefractiontellsuswherethesechangesoccur

•  Earth’scoreismadeofIron(Fe)andNickel(Ni)

•  Theinnercoreissolidwhiletheoutercoreisliquid

•  MetalslikeFearekepttogetherbyachemicalbondcalledmetallicbond

•  Inmetallicbonds,electronsarefreetoroam

•  Whenelectronsroam,theycreateanelectricalcurrent

•  Inablockofironthesecurrentscanceleachotherout

•  Iftheoutercoreisliquid,andEarthrotatesarounditsaxis,theironitselfisinmotion

•  Iftheironisinmotion,itwillthencreateanelectricalcurrent

•  Anelectricalcurrentwouldalsocreateamagneticfield

Earth’sMagneticField

•  thespinningofEarth:

– causesametallicliquidoutercoretomove

– createsanelectricalfield

–  thatgeneratesEarth’smagneticfield

WhydoweneedtoknowaboutEarth’sMagneticField?

•  Earth’sMagneticField:– protectsus(life)fromdamagingsolarradiation

– canberecordedincertainrocks,particularlythoseformingatthebottomoftheocean

–  itspropertieschangefromplacetoplaceonEarth,andcanbeidentified

–  Intheend,itisveryusefulingeologicalstudies

Physical(mechanical)differentiation

•  MechanicalLayers–  Lithosphere(~100kmthick)

•  Rigid/brittleoutershellofEarth

•  Composedofbothcrustanduppermostmantle

•  MakesupEarth’stectonic“plates”

–  Asthenosphere•  Plastic(capableofflow)zoneonwhichthelithosphere“floats”

definitionofPLATETECTONICS

•  ThesurfaceofEarthisbrokendowninaseriesofLITHOSPHERICPLATES,thatmovearoundpassively,draggedbyCONVECTIONINTHEASTHENOSPHERE

•  Therearesevenmajorplates(NorthAmerica,SouthAmerica,Pacific,Eurasia,Africa,Antarctica,India-Australia)andavarietyofsmallerones

•  ContinentalDriftHypothesis–  Originallyproposedinearly1910sinordertoexplainthe“fitof

continents”,commonrocktypesandfossilsacrossoceanbasins,etc.

–  Insufficientevidencefoundfordrivingmechanism;hypothesisrejected

•  PlateTectonicsTheory–  Originallyproposedinthelate1960s–  Includednewunderstandingoftheseafloorandagoodexplanationof

thedrivingforcebehindmotion

–  Describeslithosphereasbeingbrokenintoplatesthatareinmotion

–  Explainsoriginandlocationsofvolcanoes,faultzones,mountainbelts

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

Thethreekindsofplateboundaries

DivergentandTransformBoundaries

ConvergentBoundary

Theoceaniccrustformscontinuouslyatamid-oceanicridgeTheoceanexpandsandtheageofthecrustincreasesawayfromtheridge

PlateTectonicsendofpart1

TheAndes:amountainchainformedbycollisionbetweentwoplatesRoadfromSantiago,ChiletoMendoza,Argentina

©AlessandroGrippo