Seafloor Spreading

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Seafloor Spreading

• (still continuing ch. 4.1)

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• Underwater mountain range called the Mid-Atlantic
  Ridge discovered in 1947.
• Part of an 80,000-km-long mid-ocean ridge system
  around the whole earth.
• Rocks from the ocean floor were young compared to
  most continental rocks.

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• Princeton geologist Harry Hess hypothesized that
  the mid-ocean ridges were breaks in the earths
  crust, where magma was rising up and spreading
  away from the ridge in both directions, forming
  new rock.
• If the ocean floor was moving, then maybe the
  continents were also moving !?!

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Paleomagnetism of the Ocean Floor

• Earth acts as a giant magnet.
• Compass needles align with the magnetic lines of
  force that extend from pole to pole.
• When magma cools, iron-rich minerals line up with
  the magnetic field.

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• When rock hardens, magnetic orientation of the
  minerals become permanent and point to north.
• Geologists have discovered some rocks with
  magnetic orientations pointed south
• All north-pointing rocks (normal polarity) were
  from similar time periods.

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• All south-pointing rocks (reverse polarity) were
  also from similar time periods.
• This led to the discovery that Earths magnetic
  field has reversed itself many times throughout
  history.
• Magnetic patterns on the ocean floor show
  alternating bands of normal and reverse polarity.

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• Identical magnetic striped patterns on either
  side of the mid-ocean ridges.