Plate Tectonics

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Plate Tectonics

• Geological and geophysical evidence of plates and
  plate dynamics

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Earths lithosphere consist of relatively rigid
plates that move independently

• Plates consist of crust and lithospheric mantle
• Any individual plate contains oceanic and/or
  continental crust
• Each plate relatively rigid, little internal
  deformation
• Plate boundaries defined by active seismic and
  volcanic activity
• Dynamic new ones form, boundaries change
  character, etc.

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GPS data allow us to track plate motions
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How fast do plates move? Typically 50 mm/yr (2
in/year), but up to 240 mm/yr (10 in/yr).
Geologic evidence shows that rates and directions
change over time
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What is the geologic record of plate tectonics?

• Patterns of magnetic anomalies on seafloor
• Locations of earthquakes, many volcanoes, and
  mountains
• Evidence of continental drift

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Magnetic Field

• Electric current/field
• orientation of compasses
• 3 properties magnitude, inclination, declination
• Electrical dynamo - Dipolar field
  poles near geographic poles ( spin
  axis)
• Results from movement in Iron-rich core
• Rotation of the Earth affects orientation and
  strength
• Poles move (Polar wander)
• Magnetic Reversals

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A rich geologic record of plate motions over time

• Magnetic stripes

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Geologic evidence of past movement is in the
continents

• Paleomag record of movement of plates through
  different latitudes (inclination)
• Age and character (internal structure,
  metamorphic grade, etc.) of margins (mountain
  belts, subduction zones, etc.)
• Patterns of sedimentation, sed provenance
• Fossil record of times when different continents
  shared flora and fauna also of movement of
  plates through different latitudes (climate)

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Plate Tectonics

• All rocks owe their origin to plate tectonics
• Plate motions record inevitable and
  irreversible cooling of the planet

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PLATE DRIVING FORCES

• Pull of sinking lithospheric plate (slab) at
  subduction zones SLAB PULL. This is thought to
  be the dominant mechanism
• Push of intrusion, gravity at mid-oceanic ridges
  RIDGE PUSH.
• Traction caused by motion of asthenosphere below
  plate BASAL TRACTION