Theory Of Continental Drift

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Theory Of Continental Drift

• Alfred Wegener
• It is this German man to whom we credit with the
  proposal of the theory of Continental Drift.

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• While pondering the similarities between the
  coastlines of South America and Africa, Wegener
  came up with an idea
• What if the continents were once all connected
  and just drifted over the years?

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Wegeners evidence for Continental drift

• Ferns and Reptiles in many different continents
• Glaciers that were then tropical forests
• Same types of rocks on continents close to each
  other (yet separated by an ocean)
• The coastlines of Africa and South America

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The Flaw in Wegeners Theory

• He thought that each of the continents were a
  separate plate--they were just drifting on a
  never-changing ocean.
• Like styro-foam floating on a pool of water!!

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Canadian Correction

• J. Tuzo Wilson
• the 1960s Canadian scientist who resurrected
  Wegeners theory after years of disbelief by the
  science community.
• Today, we know that that's false, thanks to the
  discovery of crustal plates.
• The plates of the earth are not composed of just
  land they're composed of ocean too.

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Canadian Correction contd

• In some cases, the plates are just land, in
  others they're just ocean, and, in still other
  cases, they consist of land and ocean.
• They each have different boundaries and move in
  all different directions.

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Plates of the earth p. 12
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Continental Drift

• refers to the movement of the more than 20 plates
  (9 major) due to convergent, divergent, and
  transform boundaries.
• The continents drift at a rate of 2 inches a
  year.
• Started 200 million years ago
• Pangea (land) Panthalasa (sea)

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More about Continental Drift

• our text p. 10
• watch it at
• http//www.ucmp.berkeley.edu/geology/tectonics.htm
  l

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Plate Tectonics

• Tectonic plates move or float on top of the upper
  mantle.
• However they do not float freely.
• The plates are forced in specific directions by
  the flow of magma beneath.

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Plate Tectonics contd

• Plates move with the flow of magma.
• The magma closer to the core heats and then rises
  towards the surface as its density decreases.
• Once the rising magma reaches the lithosphere it
  moves in opposite directions.
• The magma forms convectional currents.

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Plate Tectonics Convection Currents
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Divergent Boundaries

• Tensional Forces occur where two tectonic plates
  are pushed apart. The tension is created as the
  plates move away from each other.
• Ridge Zones sometimes occur where two plates move
  apart. The magma rises between the plates and
  forms a ridge.
• Again caused by convectional currents in the
  magma

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Tensional Forces Ridge Zones
This diagram above shows Sea Floor Spreading
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Wheres the TENSION?
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Convergent Boundaries

• Compressional Forces occur where two tectonic
  plates come together. They compress against each
  other.
• Subduction Zones sometimes occur where
  compressional forces result from two plates
  colliding and one plate slips under the other.
• Again caused by convectional currents in the magma

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Compressional Forces Subduction
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Wheres the Subduction / Compression?
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More about Plate Tectonics

• our text p. 10 - 12
• watch it at
• http//www.pbs.org/wgbh/aso/tryit/tectonics/intro.
  html