Sea-Floor Spreading

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Sea-Floor Spreading
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Was Wegener Right?

• Recently, new technology has given us new clues
  into drifting continents.
• Wegeners theory of drifting continents is
  supported by this new evidence.

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Mapping the Mid-Ocean Ridge

• In the mid-1900s, scientists found the mid-ocean
  ridge underwater, which is the longest chain of
  mountains in the world.
• Scientists used sonar to map the ridge.

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Evidence for Sea-Floor Spreading

• Finding the mid-ocean ridge supported Wegeners
  theory of continental drift.
• Harry Hess was a scientist who suggested that the
  ocean floors move like conveyor belts.

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Evidence for Sea-Floor Spreading

• Remember, there are two types of crust.
• Continental Crust ( land)
• Oceanic Crust (the land under the oceans
• There is a big crack in the oceanic crust, and
  this is where the mid-ocean ridge forms.

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Evidence for Sea-Floor Spreading

• Hess suggested that along the mid-ocean ridge,
  magma rises from the mantle and erupts.
• The magma spreads out and pushes older rocks to
  both sides of the ridge.

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Evidence for Sea-Floor Spreading

• This process is called sea-floor spreading.

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Evidence for Sea-Floor Spreading

• There are three key areas of evidence that
  support the theory of sea-floor spreading.
• Evidence from molten material (magma)
• Evidence from magnetic stripes
• Evidence from drilling samples

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Evidence from Molten Material

• Using a submarine, scientists found rocks that
  can only be formed when magma hits water.
• This proved that molten material has erupted from
  the mid-ocean ridge.

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Evidence from Magnetic Stripes

• Remember how Earth is a big magnet?
• Scientists found out that Earths magnet has
  reversed itself in recent history.

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Evidence from Magnetic Stripes

• When Earths magnetic field pointed north, the
  iron in the magma that erupted into the ocean
  faced one direction as it hardened.
• When Earths magnetic field reversed, the iron
  faced the opposite direction. This formed
  stripes.

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Evidence form Drilling Samples

• Scientists sent a ship to drill deep into the
  ocean floor and collect many rock samples.
• They then tested the age of the rocks using
  carbon dating.

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Evidence from Drilling Samples

• Scientists found out that the farther away from
  the mid-ocean ridge the samples were taken, the
  older the oceanic crust.
• The oceanic crust near the mid-ocean is youngest.

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