Plate Tectonics Notes

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Plate Tectonics Notes
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Plate Tectonic Theory

• In the mid 1900s scientists proved that tectonic
  plates in Earths Lithosphere move.
• Explanations about how the plates moved
  developed.
• The Theory of Plate Tectonics evolved it was
  built on Alfred Wegeners ideas about Continental
  Drift.

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Theory of Plate Tectonics Based on Evidence
from Sea Floor

• Mid-Ocean Ridge
• Sea Floor Spreading
• Age of the Sea Floor
• Ocean Trenches
• Magnetic Reversal

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Mid-Ocean Ridges

• 1950s-scientists began mapping the sea floor.
• Huge underwater mountain ranges, called Mid-Ocean
  ridges were discovered.
• Ridges appeared in every ocean, circling Earth.
  like the seams on a baseball.

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Sea Floor Spreading

• Ridges form cracks in the Oceanic Crust.
• Molten rock rises through these cracks.
• Molten material cools and forms new crust.
• Old ocean floor crust is pulled away-to make room
  for newly formed floor.
• Sea floor slowly spreads apart.

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Sea Floor Spreading
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Age of Rock in Oceanic Crust

• Glomar Challenger-research ship took core samples
  of ocean floor.
• Samples close to ridge are the youngest.
• Samples furthest from ridge are the oldest.
• Oldest ocean floor only 160 - 180 million years
  old. Compared to the older Continental crust that
  is, 4.6 billion years old.

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Ocean Trenches

• Why doesnt the Earth get bigger-if the sea floor
  has been spreading for millions of years?
• Scientists discovered trenches on ocean floor.
• At these trenches, old ocean floor sinks back
  down into the Asthenosphere.
• Old crust is destroyed at same rate it is made.

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Old Ocean Floor Sinks Down into Ocean Trenches
Back to the Asthenosphere Where it Melts
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Earths Magnetic Field
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Evidence of Earths Magnetic Properties

• Earth has a magnetic north and south pole.
• Throughout Earths history, these magnetic poles
  have reversed.
• Evidence of this reversal has been found in the
  rock on the ocean floor.
• Last happened 780,000 years ago.

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How Magnetic Stripes Form

• Rock of ocean floor, contains iron particles.
• When molten material cooled, iron particles
  line-up in the direction of the Earths magnet
  field.
• Magnetic stripes that formed in the rock of the
  ocean floor show the direction of Earths
  magnetic field at the time the rock hardened.

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Formation of Magnetic Stripes on Ocean Floor