Plate Tectonics

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• Plate Tectonics
• Overview
• Historical Development
• Continental drift and paleomagnetism

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PLATE TECTONICS
1) Overview
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PLATE TECTONICS
1) Overview
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PLATE TECTONICS
1) Overview
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PLATE TECTONICS
2) Historical development
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PLATE TECTONICS
2) Historical development
1915 Alfred Wegener published hypothesis
of continental drift
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200 million years ago
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PLATE TECTONICS
2) Historical development
Fossil Evidence
Several fossil organisms have been found in
common on different continents
mesosaurus
lystrosaurus
How can the same species evolve on widely
separated continents???
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PLATE TECTONICS
2) Historical development
Rock evidence
Mountain belts on one continent match up with
another.
Similar rock structure and age
Appalachians (eastern US)
British Isles, Scandanavia
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PLATE TECTONICS
2) Historical development
Paleoclimate evidence
(ancient)
Ice sheets covered big areas of southern
hemisphere 220-300 million years ago
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PLATE TECTONICS
2) Historical development
Paleoclimate evidence
Ice sheets covered big areas of southern
hemisphere 220-300 million years ago
Glacial striations
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PLATE TECTONICS
2) Historical development
1924 Wegeners book translated to English met
with hostile criticism
Main objection no way to explain continental
drift.
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3) Continental drift and paleomagnetism
PLATE TECTONICS
Wegeners idea died until 1950s. Renewed
interest from rock magnetism
Paleomagnetism ancient magnetic field of Earth
recorded and frozen into rocks
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3) Continental drift and paleomagnetism
PLATE TECTONICS
Rock magnetism
-Certain minerals are magnetic (e.g., magnetite,
iron) -They loose magnetization when heated above
Curie point (580oC for iron) -When cooled below
Curie pt, magnetic grain aligns w/ Earths
magnetic field
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3) Continental drift and paleomagnetism
PLATE TECTONICS
Rock magnetism
-Certain minerals are magnetic (e.g., magnetite,
iron) -They loose magnetization when heated above
Curie point (580oC for iron) -When cooled below
Curie pt, magnetic grain aligns w/ Earths
magnetic field
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3) Continental drift and paleomagnetism
PLATE TECTONICS
Polar wandering

• discovery in 1950s.
• location of North Pole infered from rock
  magnetization
• appears to have moved in past
• Either N pole has wandered or continents have
  drifted

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3) Continental drift and paleomagnetism
PLATE TECTONICS
Sea floor spreading
Harry Hess, in the early 1960s proposed

• ocean ridges are above mantle upwellings, which
  cause seafloor to spread, like a conveyor belt
• magma replaces seafloor as it moves away,
  becoming new oceanic crust
• deep ocean trenches are locations where oceanic
  crust dives back into planet

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3) Continental drift and paleomagnetism
PLATE TECTONICS
Geomagnetic reversals

• Earths magnetic field reverses
• recorded in lava flows

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3) Continental drift and paleomagnetism
PLATE TECTONICS
Seafloor magnetic stripes

• 1963, Vine Matthews connected seafloor
  spreading continental drift, from magnetic
  field reversals recorded in cooling lavas of new
  seafloor
• symmetric patterns (stripes) on either side of
  spreading center (mid-ocean ridge)
• changes in width of a given stripe indicate
  changes in spreading rate.

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