Plate Tectonic Theory

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Contracting Earth Theory
The molten Earth cools and contracts The crust
wrinkles and crumples Mountains form
What are some problems with this
hypothesis? What observations do you know of
that may contradict this idea?
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The Expanding Earth
20th century - radioactive decay Land masses are
ripped apart Explains the continents
What are some problems with this
hypothesis? What observations do you know of
that may contradict this idea?
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Discussion Questions

• 1. How did Wegners Hypothesis challenge the
  contracting Earth model?
• 2. How does the distribution of ages of the
  ocean floor support Continental Drift and
  challenge the expanding Earth model?

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Plate Tectonic Theory
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Continental Drift

• Proposed by Alfred Wegner in 1915
• Continents are a giant jigsaw puzzle
• Put together get Pangea (all land) a
  supercontinent

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Evidence for Continental Drift

• Jigsaw Pattern Fit
• Using the Continental Shelves continents fit
  nicely
• Continents began breakup ? 200 million years ago

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Evidence Cont.

• Fossils
• Mesosaurus
• Fresh water reptile
• Could swim, but not an ocean
• Found in South America and Africa

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Evidence Cont.

• Fossils
• Lystrosaurus
• Land dwelling reptile
• Could not swim
• Found in Africa, India, and Antarctica
• Cynognathus
• Land dwelling reptile
• Could not swim
• Found in Africa, South America.

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Evidence Cont.

• Fossils
• Glossopteris
• Fern
• Habitation area dictated by ability of plant to
  spread.
• Seeds to big to be transported by wind.
• Found in Africa, India, South America, Australia,
  and Antarctica.

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Evidence Cont.

• Rocks
• Similar composition, age, and geologic structures
  on both sides of the Atlantic
• When continents are together have continuous
  pattern

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Evidence Cont.

• Mountains
• See continuation of mountains on both sides of
  Atlantic Ocean
• North America and Europe together have a
  continuous mountain chain, rock is similar
  composition and age

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Evidence Cont.

• Paleoclimatic Data
• Evidence of Tropical Climates in non tropical
  areas.
• Evidence of Low-latitude deserts in non desert
  areas.
• Evidence of Glaciers in areas of tropical
  climates today.

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Glaciated Areas
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Question for Wegner

• What is the mechanism of the movement?
• Wegner proposed that the tidal influence of the
  moon was strong enough to move continents.
• Continents plowed through ocean crust.

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Refining a Hypothesis

• Paleomagnetism
• Earth has a magnetic field creating a north and
  south pole similar to a small magnet.
• Magnetic Field has reversed over Earths history.

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Magnetic Signature

• Iron-rich (ferromagnesium) minerals become weakly
  magnetized in the direction parallel to the
  existing magnetic field as they cool below 580
  oC.
• The rock retains the signature unless heated up
  past 580 oC.
• If the rock moves or if the magnetic pole
  changes, the rock retains the original magnetic
  signature.

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Polar Wandering

• Study of lava flows showed that magnetic
  signature of lava flows of different ages varied
  widely.

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• Magnetic signatures indicate that the magnetic
  poles have moved.
• Taken separately with no continental movement,
  the data indicate different pathways for the pole
  movement.

• If continental movement is incorporated, the
  polar wandering for the different continents are
  identical.

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Magnetic Reversals and Ocean Floor Data

• Earths Magnetic field has reversed
• In addition to magnetic signature, polarity is
  recorded in rocks either normal (present day) or
  reversed.
• At mid-ocean ridges, new oceanic crust (basalt
  mafic composition) is generated and records the
  magnetic signature and polarity.

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Magnetic Reversals
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Magnetism Patterns at Mid-Ocean Ridges
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World-wide Seafloor Distribution