Plate Tectonics Theory

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Plate Tectonics(Theory)

• The outer layer of the earth (the lithosphere) is
  divided into 7 major plates and lots of smaller
  plates which move relative to each other as a
  result of convection in the Asthenosphere.
• The plates are rigid and deformation is
  concentrated at their boundaries
• There are 3 types of boundariesconvergent,
  divergent, and transform

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Fig. 1-10, p. 17
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Background Observations

• Jigsaw-like fit of continents across the Atlantic
  Ocean--Required global maps before this
  observation could be made

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• Similarity of fossils and rock types across the
  Atlantic
• Paleoclimate indicators across several continents
  suggest glacial conditions on areas now near the
  equator

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Possible explanations (hypotheses)

• Pure coincidence
• The earth is expanding (or contracting?)
• The continents are moving relative to each other
• Plates (including both continents and oceans) are
  moving relative to each other

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Pure coincidence hypothesis

• Difficult to justify in view of large amounts of
  data we now have
• No good scientific test available so rely on
  negative testi.e. is it the only acceptable
  hypothesis? If not, assume something else is the
  explanation.

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The earth is expanding or contracting hypothesis

• This hypothesis was widely accepted until about
  50 years ago
• Expanding earth could explain fit of Africa and
  South America but not climate record
• No known mechanism for expansionis new material
  being added to the earth somehow?
• Gravity would change on the surface over time and
  there should be a record of this.
• A contracting earth might seem feasible in view
  of our ideas that the earth is cooling from some
  initial molten state. Most things contract when
  they cool but this would not explain any of our
  basic observations

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The continents are moving relative to each other
(Continental Drift) hypothesis

• Proposed by Alfred Wegner in 1915
• Fits basic observationsin fact, Wegner compiled
  an impressive number of observations in support
  of his hypothesis.
• Did not provide a mechanism for how the
  continents moved and did not discuss the role of
  oceans.
• The hypothesis was widely accepted, particularly
  in Europe, but not well received in the US where
  an expanding earth hypothesis was preferred by
  most scientists.

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Plates (including both continents and oceans) are
moving relative to each other Plate Tectonic
Theory

• Incorporated Continental Drift but added a
  mechanism (asthenospheric convection)
• Basically came about as a direct consequence of
  WWII submarine technology which gave us
  information on the sea floor.
• Fits basic observations and we have now measured
  the motions of plates with modern technology.
• Considered to be a Theory because it has survived
  many tests during the past 50 years.

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Quiz 3 hand in at end of class

• 1. World War II (WWII) helped to support which
  of the following hypotheses
• Continental Drift
• Pure Coincidence
• Expanding Earth
• Contracting Earth
• e. Plate Tectonics

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Lets take a closer look at the evidence

• Start with Alfred Wegners ideas

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Pangaea

• All of the continents were together in a super
  continent, termed Pangaea, until 200 million
  years ago (there is probably a figure in your
  text) when they broke apart and moved relative to
  each other.
• Supported by rock similarities, fossils, and
  Paleoclimate.

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Modern alignment of continents forming Pangaea is
based on the fit at a depth of 900 meters
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Fossils

• Mesosaurusaquatic reptile fossil found in
  western Africa and eastern South America.
• If it could swim the distance between the two
  continents, it would also be found in other,
  nearer, places but it is not.
• Glossopteriscold climate fern fossil found in
  Africa, Australia, South America, Antarctica, and
  India
• Suggests those continents were close together in
  a cold region of the earth.

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Paleoclimate
Alfred Wegner identified evidence of rocks
deposited by glaciers approximately 300 my ago.
Some of the areas, like India and Africa, are now
near the equator. In reconstructing Pangaea, he
brought all of the glaciated areas back together
near the south pole.
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Similar rock types on different plates and
evidence for ancient oceans

• There are a variety of rock types that appear to
  match on different platesfor example, the deep
  rocks in Florida match rocks in north Africa of
  the same age.
• More importantly, rocks found at the tops of
  mountains often contain fossils that suggest the
  rocks were deposited in deep oceans. These
  oceans are now gone but appear to have existed
  between continents at an earlier time.
• Mountain belts in Africa can be traced into South
  America and Mountain belts appear to mark earlier
  plate boundaries.

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More recent observations that support Plate
Tectonic theory

• Most earthquakes and volcanoes are located along
  narrow bands near plate boundaries (availability
  of global maps)
• Satellite images and land geodetic measurements
  record motion across Plate boundaries
• Many sea floor studies have given new information
  on critical pieces of the puzzle
• Better models for the thermal character of the
  earth have provided a mechanism.

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Most earthquakes and volcanoes are located along
narrow bands near plate boundaries (availability
of global maps)

• Beginning in the 1950s, much of the world was
  covered by a seismic network designed to detect
  nuclear blasts in the Soviet Union. It also
  greatly improved our knowledge of where
  earthquakes occurred and lead to the first global
  seismicity maps. The actual plate boundaries are
  defined to follow the major zones of earthquakes.
• In areas that often include volcanoes, the
  earthquakes were shown to originate at greater
  depths than elsewhere Wadati-Benioff zones.
  These deep earthquakes defined parts of the
  lithosphere that were plunging deep into the
  earth along convergent plate boundaries.
• Volcanoes appear to cluster above places where
  the Wadati-Benioff zones reach a depth of 100
  km. This suggests there is some mechanism
  related to this depth of lithosphere that
  produces volcanoes.

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Satellite images and land geodetic measurements
record motion across Plate boundaries

• Space technology has given us new tools for
  surveying (i.e. GPS) and imaging the earths
  surface.
• Detailed GPS surveys are mapping short term
  displacements across plate boundaries.
• Satellite images give a detailed view of
  displaced markers across fault boundaries.

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More on Plate Tectonic Theory
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Plates

• The lithosphere is the outer shell that is strong
  when subjected to tectonic stresses over geologic
  timescales.
• Contains both upper mantle and the entire crust.
• About 100 km thick, but can vary in thickness.
• Two kinds of crust
• continental crust (thick and light)
• oceanic crust (thin and heavy).
• Density differences are due to different
  compositions affect plate behavior.
• Lithospheric plates contain either oceanic or
  continental crust, or both.

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Fig. 1-9, p. 17