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Learn how Tectonic Plates really work!
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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