GradeswillbecountedfromTODAY!!

Clickers???

Platetectonics:Theplatesarethelithospherethestrong,cool,quitebrittle,fracturedoutershell

StrengthorRheologyDefinestheplates

Composition

3,000(Oceaniccrust)

alcrust)

Figure2.6

PlateTectonicsII:HowandWhy?

LearningGoals

Describetechniquesforobservingplatemovements(Hotspotstracking,GPS,faultdisplacement,magneticstriping,lithologicalcorrelation)

Applyknowledgeofplatemovementmeasurementstoidentifydirectionandrelativevelocitiesofplatemovementsthroughtime

Describemantleandlithospherepropertiesthatleadtothemovementofplatesandcreationofrocks.

Distinguishbetweenridgepushandslabpullandnotetheareaswheretheseprocessesoccur.

PlateTectonicsasaTheory:Support

Wegener,1912:Thecontinentsaremoving

Evidence?Problems?

https://www.youtube.com/watch?v=Np9ADpKnBSY

Wegeners Evidence: Matching geological, fossil, climate, and rock signatures on different continents. Why do they match?

fromP.Hammer

Mountain Ranges

Appalachian Mountains present on several continents

Caledonian Mountains

fromwww.searchanddiscovery.com

Mountain Ranges

Appalachian Mountains present on several continents

fromwww.capewrathtrail.co.uk

Fossil Distributions

Fossil ranges of organisms restricted by dispersal barriers are good indicators of connection of landmasses

TextFigure19.7

Evidence of Glaciation

GlacialstriationsinMtRainierNationalPark(wikimedia commns)

GlacialstriationsonbasaltnearWhistler,BC

Evidence of Glaciation

TextFigure19.4

GlacialStriationsinthePermian(~250Ma)ofAustralia

Evidence of Glaciation

Evidence of Glaciation

TextFigure19.4

Evidence of Glaciation

ImageofTillite courtesyofBritishGeologicalSurvey

Evidence of Glaciation

TextFigure19.4

Evidence of Glaciation

TextFigure19.4

TextFigure19.7

Wegeners Hypothesis

Lessdensecontinentsplowedthroughhighdensityrocksoftheoceaniccrust

PropelledbythecentrifugalforceoftheEarthsrotation Interactionbetweenoceanandcontinentleadtocrumpling

mountainranges Centrifugalforcenothighenough!

Figure 19.11

TextFigure19.11

Putting it together Strongevidence

fossildistributions continentaledges rockdistributions glaciation

Continentsshiftovertime.

Pangea asupercontinentexistentfromabout300200Ma

https://www.youtube.com/watch?v=uGcDed4xVD4

Figure 19.11 Decades Later . . .DuringWWII,extensivesonarmappingoftheseafloorrevealedtopographicfeaturesoftheoceaniccrustpreviouslyunobservable.HarryHess(right)proposedthatseafloorwasspreading,generatedattheanomalousridgesSpreadingCenters knownasmidoceanicridges.

Divergent boundaries(plates pull apart)

Paleomagnetism

Rocks can lock in the orientation and polarity of the magnetic field at the time that they form.

TheEarthsmagneticfieldisgeneratedby?

A) theoutercoreB) theinnercoreC)OceantidesD)MagneticmineralsinthecrustE)TheorbitoftheEartharoundthesun

ClickerQuestion???

Magnetic Striping

Changes in polarity are recorded in rocks that crystalize from magma.

Polarity recorded on the sea floor can be correlated to dated rocks on land.

TextFigure19.16

https://www.youtube.com/watch?v=YIAXiE8RedA

Magnetic Striping

Ingroupslookatthismapanddiscusstheagedistributionoftheoceanfloor.Whereistheoldest?Whereistheyoungest?

Bycorrelatingthepatternofmagneticstripinginrocksontheseafloortothatofrocksonland,wecancomeupwithanagemapoftheseafloorshowingyounglithospherebeinggeneratedatdivergentmargins/midoceanicridgesandolderseafloortowardsthesubducted edgesorpassivemarginsofaplate.

GPS Tracking of Plate Movements

NRCAN/GSC

GPS Tracking of Plate Movements

NRCAN/GSC

GPS Tracking shows absolute movement in three dimensions

Plate Velocities & Vectors

Exercise:Platemovementrates

Supposetwocontinentsareonacollisioncourse.

Theyareseparatedby6000kmandconvergingatarateof3cm/yr.

Howmanyyearswillittakefortheirmarginstomeet?

Subduction Zones

Seismicprofilesthroughconvergentmarginsrevealbandsofearthquakes revealthegeometryofsubduction zones.

Subduction ZonesHesssModel:Newcrustgeneratedatdivergentmargins/ridges,carriedbytheforceofmantleconvectiontoconvergentmargins,whereitbeginstosinkunderthelessdenseoverridingplate.

Problem Notenoughforce.

ContinentsaremovingWegenersevidenceModernGPSdata

NewlithospherebeinggeneratedatridgesMagneticstripingSeaflooragesTopographyofridges

Lithospherebeingconsumedatsubduction zoneSeismicdataMissingSeaFloor

Whatismovingtheplates?CentrifugalForceXMantleConvectionXMoonX

Evidence Thus Far

TextFigure19.2

Bringalltheselinesofevidencetogetherandwecanreconstructplatemovementsoverhundredsofmillionsofyears

Driving Forces: 3D Mantle Structure from Seismic Data

A.Dziewonski/IRISConsortium

3D Mantle Structure from Seismic Data

A.Dziewonski/IRISConsortium

Drivers? Heat and density

ContinentsOceanbasins

Seismicimagingofthemantlerevealsoldslabsoflithospherecontinuousdeepintothelowermantle.Convectionwithinthemantleleadstoanomalouslyhotregions.Theseregionsfeedvolcanicactivityatbothdivergentmargins(whenextensiveinlateralduration)andhotspots(whenconfinedtosmallerareasofthemantle).

TextFigure19.29

RidgePush:Movementofplatesatridgesispropelledbyslidingalongtheelevatedlithosphereasthenosphere boundary.

SlabPull:Dense,subducting lithospherepullstheplatealongwithit.

More Mechanisms

MantlePlumes(Hotspots)

Figure 19.31

Hotspots

TextFigure19.31

NextClass:Activity

PlateTectonics Whatdoesitallmean???

Whattobring:ForWednesdaysactivityyoushouldbringpens/colouredpencilsYourplatetectonicsnotesAnInternetenableddevice(checkoutthemapsinthelecturenotessection)Bepreparedtoworkwithotherpeople