Plate TectonicsPlate Tectonics

Chapter 9Chapter 9

Rigid Earth TheoryRigid Earth Theory

It was once believed that Earth’s crust was hard and brittle and could not bend

PlasticityWe now know that Earth’s crust can bend (like a tough plastic) before breaking

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IsostacyIsostacy• Maintaining equilibrium in the crust• Addition or removal of crustal material causes a sinking or

rebounding of crust– A glacier growing or remelting, crust eroding off the

surface, sediment deposits, water bodies on land, esp. those created by dams, etc.

Alfred Wegener andAlfred Wegener andHis Continental Drift TheoryHis Continental Drift Theory

German meteorologist, 1920s--Pangaea (Gk. “whole land”)“The present continents were originally connected as one enormous landmass that has broken up and drifted apart over the last few 100 million years. The drifting continues….”

What evidence led him to this theory?

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Wegener’s Lines of EvidenceWegener’s Lines of EvidenceSimilar geology (rocks and rock structures)……petrology (rock chemistry),…matching glacial features (U-shaped valleys, glacial deposits, 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

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……but no one bought it.but no one bought it.

What do you mean, “The

continents are floating???”

So why don’t we see the

crust ripping apart right

now?And hey, what’s the power source driving

these movements of all the land masses,

anyway???

The crust is too rigid!

What a knucklehead.

Then along came Then along came OceanographerOceanographerHarry Hess in the 1960s…Harry Hess in the 1960s…

The evidence continued to The evidence continued to mount…mount…•Military seafloor mapping: Seafloor geology—Military seafloor mapping: Seafloor geology—

structure, chemistry, and agestructure, chemistry, 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 Rigid Earth folks retired—paradigm shift to

plasticityplasticity•Geologists, geophysicists, seismologists, Geologists, geophysicists, seismologists,

oceanographers, physicists, and paleontologists oceanographers, physicists, and paleontologists all agree the theory fits the evidence gathered all agree the theory fits the evidence gathered within their respective fieldswithin their respective fields

•Convection currents as mechanism/power sourceConvection currents as mechanism/power source

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Convection CurrentsConvection Currents

Slow-moving convection currents within the mantle transfer heat from the outer core to the upper mantle

Finally, there was a mechanism and a process for moving all those pieces of crust!

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The Theory of Plate TectonicsThe Theory of Plate Tectonics

Tectonic (crustal) platesPulling apart (diverging)

= Spreading centersSlamming together, sinking (converging)

= Subduction zonesSliding laterally (sideways)

= Transform fault boundaries

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African Rift ValleyAfrican Rift Valley

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Divergent Plate BoundariesDivergent Plate BoundariesSpreading centers

–Crust pulling apart, magma rising to the surface

Convergent Plate BoundariesConvergent Plate Boundaries

Subduction zones–Crust being forced together–Lightest material rises (mountain-building) while the

heaviest stuff sinks (pushed back into the mantle)–Remelting (mostly from friction) creates volcanoes–Intense, deep-focus earthquakes

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Three Types of Subduction ZonesThree Types of Subduction Zones

1. Continental crust meets oceanic crustOceanic crust sinksBig trench offshoreVolcanoes on the continental marginBig earthquakes (potential for tsunamis)

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Continental-Oceanic SubductionContinental-Oceanic Subduction

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Three Types of Subduction ZonesThree Types of Subduction Zones

2. Oceanic crust meets oceanic crustThe older and colder crust will probably sinkBig earthquakes and volcanic islands (“island arcs”)Deep ocean trenchPotential for tsunamis

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Oceanic-Oceanic SubductionOceanic-Oceanic Subduction

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Three Types of Subduction ZonesThree Types of Subduction Zones

3. Continental crust meets continental crustToo light to subductMountain-buildingBig earthquakesLittle if any volcanism (mostly intrusive)

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Continental-Continental SubductionContinental-Continental Subduction

Transform Fault BoundariesTransform Fault BoundariesTectonic plates slide past one another

Earthquakes are less intense than subductionNo volcanoesLittle or no mountain-building

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““Hot spots”Hot spots”

• Also called magma plumes• Generally occur some distance from any other

type of plate boundary• Unrelated to convergent, divergent, or

transform boundaries• Anomalous (odd) “balloons” of rising magma

Hot spot stays in one position as the moving, island-covered crustal plate rides away from it

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Accreted TerranesAccreted TerranesA moving continent may pick up new land material as lighter (felsic) material scrapes off of a subducting plate

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CratonCraton• These terranes were added to the original

material first formed from magma that rose out of Earth’s earliest crust–Craton--the name given to these ancient proto-

continents

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cratons

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Continental ShieldsContinental ShieldsMore magma material was added to the cratons, forming continents.

Continental shields: Where the earliest continental material still exists intact and is exposed at the surface.

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TopographyTopography

• Right from the very beginning, the crust was affected by stresses and strains that caused crustal deformations

• Over time, the crust has continued to be folded, faulted, broken, eroded and further built upon, creating the topography, the ups and downs of land relief, that we see today