Continental Drift

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Continental Drift

• Proposed by Alfred Wegener in 1915
• Land masses were once joined into a
  supercontinent known as Pangaea which split into
  the present day continents
• Major evidence is the fit of the coasts (with
  continental shelves) of Africa and South America

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Evidence for Continent Movement

• Sea-floor spreading
• Paleomagnetism
• Similarity of fossils and rock formations on
  distant continents
• Actual measurement of continental separation

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Theory of Plate Tectonics
The upper section of the Earth is divided into
sections called plates. These plates move and
change size over time. Plate boundaries are
defined by seismic and volcanic activity.
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Recent World Seismic Activity
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World Volcanic Activity in the last 1 million
years
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Earths Plates
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A small divergence to talk about the layers of
the Earth...
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Layers of the Earth
http//scign.jpl.nasa.gov/learn/plate1.htm
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P and S Waves
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P and S Waves
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Earths Layers (based on Seismology) CRUST

• Two forms - continental and oceanic
• Continental crust is composed of less dense rock
  rich in silicates thicker than oceanic
• Oceanic crust is basaltic and denser than the
  continental crust rich in silicates

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Earths Layers (based on Seismology) - MANTLE

• Composed of iron-rich silicates
• Has an upper layer that is plastic or
  semi-fluid
• Has a higher temperature than the crust

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Earths Layers (based on Seismology) - CORE

• At the center of Earth
• Has two sections, outer and inner cores composed
  mainly of iron and nickel
• Outer core is molten while inner core is solid
• Fluidity of outer core may account for Earths
  magnetic field

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Another Way to View Earth Layers

• Lithosphere - composed of crust and uppermost
  section of the mantle rigid layer that composes
  the plates
• Asthenosphere - plastic region in the upper part
  of the mantle under the lithosphere the plates
  ride on the asthenosphere

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Lithosphere and Asthenosphere
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For Some Perspective
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Plate Motion

• Plates can move away from each other at divergent
  plate boundaries or spreading centers like the
  mid-ocean ridge in the Atlantic Ocean
• Plates can move together in convergent plate
  boundaries
• Plates can slide past each other in transform
  fault boundaries

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Divergent Plate Motion

• Tensional forces stretch the lithosphere
• New Earth materials are formed between plates
  where hot mantle material rises into stretched
  area

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Divergent Plate Motion

• Mid-Ocean Ridge in Atlantic Ocean has been active
  for about 160 MY
• Rift valleys may result from divergent motion as
  seen in East African Great Rift Valley

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Plate Motion Animations

• http//www.pbs.org/wgbh/aso/tryit/tectonics/

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Convergent Plate Motion

• Causes collision of plates with compressional
  forces. Convergence leads to subduction of one
  plate under another
• Oceanic crust may go under continental crust
  generating a trench and causing mountain building

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Convergent Plate Motion

• Continental crust may go under other continental
  crust with mountain building

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Convergent Plate Motion

• The Indian plate collided with the Eurasian Plate
  40-50 million years ago to form the Himalayan
  Mountains. These mountains are still growing
  about 1 cm/year because of the push from the
  Indian Plate.

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Convergent Plate Motion

• Ocean crust may go under ocean crust in a trench
  causing oceanic volcanoes

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Transform Fault Boundary

• Plates may slide past each other laterally with
  no subduction
• Seismic activity is high along these slip
  boundaries

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Transform Fault Boundary

• The San Andreas Fault in California is an example
  of this boundary

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Can you identify the types of plate boundaries
and geologic features?
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Boundaries and Features
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Plate Motion Over Time
http//www.uky.edu/ArtsSciences/Geology/webdogs/pl
ates/pangaea-1.html