Hopefully you remember this

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Hopefully you remember this..

• Convection currents under Earths surface cause
  the plates sitting/floating on top of them to
  move.

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Sea-Floor Spreading

• Molten material rises from the mantle, spreads
  out, pushes older rock to both sides.

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Mid-Ocean Ridges form where the magma comes up at
these divergent boundaries.
Mid-Ocean Ridges (in the ocean) form the longest
chain of mountains in the world.
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How do we know about sea-floor spreading?

• Age of ocean floor New rock is near mid-ocean
  ridge and older rock is farther away from ridge.
• Magnetic stripes Earths poles reverse every so
  often.

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Magnetic Reversals

• 780 thousand years ago-the last time the Earths
  polarity switched.

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Subduction

• Subduction happens at the other end of the
  oceanic plate.
• Ocean floor sinks back into the mantle.
• It often sinks beneath less dense continental
  plates.
• Takes millions of years

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Subduction
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What changes are occurring?

• The size and shape of the oceans
• The Pacific Ocean is shrinking
• The Atlantic Ocean is growing