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22Rigid Earth TheoryRigid Earth Theory• It was once believed that Earth’s crust washard and brittle and could not bend• Plasticity–We now know that Earth’s crust can bend (like atough plastic) before breaking
33IsostacyIsostacy• Maintaining equilibrium in the crust• Addition or removal of crustal material causes a sinking orrebounding of crust–A glacier growing or remelting, crust eroding off thesurface, sediment deposits, water bodies on land, esp.those created by dams, etc.
44Alfred Wegener andAlfred Wegener andHis Continental Drift TheoryHis Continental Drift Theory• German meteorologist, 1920s–Pangaea (Gk. “whole land”)“The present continents were originallyconnected as one enormouslandmass that has broken up anddrifted apart over the last few 100million years. The driftingcontinues….”What evidence led him to this theory?
77Wegener’s Lines of EvidenceWegener’s Lines of Evidence• Similar geology (rocks and rock structures)…• …petrology (rock chemistry),• …matching glacial features (U-shaped valleys, glacialdeposits, etc.) on continents separated by oceans,• …continent shapes that seem to fit together,• …patterns in the locations of volcanoes,• …paleontology (fossilized plants and animals)...Ex.: S. America/Africa, Madagascar/India, Australia/Antarctica
88……but no one bought it.but no one bought it.What do you mean,“The continents arefloating???”So why don’t wesee the crustripping apartright now?And hey, what’s thepower source drivingthese movements of allthe land masses,anyway???Thecrust istoorigid!What aknucklehead.
Then along came OceanographerThen along came OceanographerHarry Hess in the 1960s…Harry Hess in the 1960s…
The evidence continued to mount…The evidence continued to mount…• Military seafloor mapping: Seafloor geology—structure,Military seafloor mapping: Seafloor geology—structure,chemistry, and agechemistry, and age– Oceanic crust: only 100 m.y.oOceanic crust: only 100 m.y.o– Continental crust: 4.1 b.y.o.Continental crust: 4.1 b.y.o.• Core samplingCore sampling• Seafloor sedimentSeafloor sediment• Rigid Earth folks retired—paradigm shift to plasticityRigid Earth folks retired—paradigm shift to plasticity• Geologists, geophysicists, seismologists,Geologists, geophysicists, seismologists,oceanographers, physicists, and paleontologists alloceanographers, physicists, and paleontologists allagree the theory fits the evidence gathered within theiragree the theory fits the evidence gathered within theirrespective fieldsrespective fields• Convection currents as mechanism/power sourceConvection currents as mechanism/power source
1313Convection CurrentsConvection Currents• Slow-movingconvection currentswithin the mantletransfer heat fromthe outer core to theupper mantle• Finally, there was a mechanism and aprocess for moving all those pieces of crust!
1717Divergent Plate BoundariesDivergent Plate Boundaries• Spreading centers–Crust pulling apart, magma rising to the surface
1818Convergent Plate BoundariesConvergent Plate Boundaries• Subduction zones–Crust being forced together–Lightest material rises (mountain-building) while theheaviest stuff sinks (pushed back into the mantle)–Remelting (mostly from friction) creates volcanoes–Intense, deep-focus earthquakes
2424Three Types of Subduction ZonesThree Types of Subduction Zones3. Continental crust meets continental crust–Too light to subduct–Mountain-building–Big earthquakes–Little if any volcanism (mostly intrusive)
2626Transform Fault BoundariesTransform Fault Boundaries• Tectonic plates slide past one another–Earthquakes are less intense than subduction–No volcanoes–Little or no mountain-building
2727““Hot spots”Hot spots”• Also called magma plumes• Generally occur some distance from any othertype of plate boundary• Unrelated to convergent, divergent, ortransform boundaries• Anomalous (odd) “balloons” of rising magma–Hot spot stays in one position as the moving, island-covered crustal plate rides away from it
3030Accreted TerranesAccreted Terranes• A moving continent maypick up new land materialas lighter (felsic) materialscrapes off of a subductingplate
3131Accreted TerranesAccreted Terranes• A moving continent maypick up new land materialas lighter (felsic) materialscrapes off of a subductingplate
3232CratonCraton• These terranes were added to the originalmaterial first formed from magma that rose outof Earth’s earliest crust–Craton--the name given to these ancient proto-continents3232cratons
3333Continental ShieldsContinental Shields• More magma material was added to thecratons, forming continents.–Continental shields: Where the earliestcontinental material still exists intact and isexposed at the surface.3333
3434TopographyTopography• Right from the very beginning, the crust wasaffected by stresses and strains that causedcrustal deformations• Over time, the crust has continued to befolded, faulted, broken, eroded and furtherbuilt upon, creating the topography, the upsand downs of land relief, that we see today3434