Continental Drift

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Continental Drift

• Alfred Wegener

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Continental Drift
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The theory

• In 1912, Alfred Wegener and Frank Taylor first
  proposed the theory that 200 million years ago
  the Earth had only one giant continent, from
  which today's continents broke apart and drifted
  into their current locations.
• To support their theoryThey used the fit of the
  continents, the distribution of fossils, a
  similar sequence of rocks at numerous locations,
  and ancient climates.

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The Evidence the jigsaw

• The bulge of Africa fits the shape of the coast
  of North America while Brazil fits along the
  coast of Africa beneath the bulge.
• Pangaea Original landmass

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The Evidence rocks match

• A geologic column from Africa and South America
  matched when the end of the continents joined.

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The Evidence Ice match

• Glacial striations on rocks in South America
  match up to glacial scratches in Africa.

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The Evidence Fossils

• The fossil of a fern (tropical plant) found in
  Antarctica.
• Wegener believed that the climate has not changed
  but the continents moved toward a different
  climate zone.

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Sea Floor Spreading

• Harry Hess

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The Theory

• In 1962 Hess thought it possible that the
  continents were in motion. He believed that
  molten magma from beneath the earth's crust could
  ooze up between the plates in the Great Global
  Rift. As this hot magma cooled in the ocean
  water, it would expand and push the plates on
  either side of it.http//www.pbs.org/wgbh/aso/data
  bank/entries/do53ri.html

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The Theory cont.

• Hess proved Wegener's basic idea right and
  clarified the mechanism that broke the
  once-joined continents into the seven with which
  we are familiar. The continents are attached to
  the plates and do not move independently of them.
  But the plates themselves shift and change shape,
  carrying the continents along.

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The Evidence

• Molten Material
• In the 1960s scientists found evidence of new
  material erupting along the mid ocean ridge.
• http//pubs.usgs.gov/publications/text/topomap.htm
  l
• Drilling Samples
• The farther away from the ridge older rocks were
  found. The youngest rocks were always closer to
  the ridge.

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Subduction

• Deep Ocean Trench
• Oceanic crust (basalt more dense) is forced back
  into the mantle.

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Plate Tectonics

• J. Tuzo Wilson

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The Theory

• Wilson combined what was known about sea floor
  spreading and continental drift into a single
  theory.
• Wilson stated that the pieces of Earths
  lithosphere are in constant, slow motion, driven
  by the convection currents in the asthenosphere.

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Convection currents in mantle
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Plate Boundaries

• Transform boundary two plates slip past each
  other, moving in opposite directions. Crust is
  neither created or destroyed.
• Divergent boundary two plates move apart.
• - mid ocean ridge
• - rift valley occurs on land

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• Convergent boundary two plates come together.
  When two plates collide the density of the plates
  determines which plate will sink under the other.