The Plate Tectonics

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The Plate Tectonics
Revolution
A paradigm shift in the Earth Sciences
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• Topics
• Wegeners Theory of continental drift
• Seafloor spreading the Vine-Matthews-Morley
  hypothesis
• The plate-tectonics revolution
• What is a plate? How many are there?
• J. Tuzo Wilson and his cycles
• Triple junctions

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• Promoted continental drift with publication of
  The Origin of Continents and Oceans in 1915
• Alternative to land bridges as theory to explain
  similar fossils on different continents

Alfred Wegener (1880-1930)
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• Supporting evidence Appalachian mountains match
  Scottish Highlands similar rocks in South Africa
  and Brazil
• Coined term Pangea (Greek, meaning all the
  Earth)

Alfred Wegener
Scottish Highlands
Appalachians (eastern N.A.)
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In 1962, U.S. Navy marine geologist Harry Hess
proposed that ocean crust is much younger than
continental crust, and forms by seafloor
spreading
Harry Hess
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• Magnetic surveys revealed a distinctive striped
  pattern in the oceans

Raff, A. D. and R. G. Mason. 1961. Magnetic
survey off the west coast of North America, 40º
N. latitude to 52º N. latitude. Geological
Society of America Bulletin 721267-1270.
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In the meantime, scientists in U.S.A. and
Australia discovered that the Earths magnetic
field has reversed many times
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The Vine-Matthews Hypothesis

• Magnetic Stripes Seafloor Spreading Magnetic
  Field Reversals
• Basalt extruded at mid-ocean ridges is
  magnetized in the direction of the Earths
  magnetic field

Vine, F. J. and D. H. Matthews. 1963. Magnetic
anomalies over oceanic ridges. Nature
199947-949.
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• but did they discover this first?
• several months before Vines and Matthews, Larry
  Morley (Geological Survey of Canada) had
  submitted a paper to Nature with the same
  conclusion
• His paper was rejected as too speculative
• Revisionist history
• Vines-Matthews hypothesis is now known as the

Larry Morley
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The new global tectonics of the 1970s Earths
ultimate recycling program

• New oceanic lithosphere is created at mid-ocean
  ridges
• Oceanic lithosphere is destroyed at deep-sea
  trenches (subduction zones)
• This process largely explains the global
  distribution of earthquakes and volcanoes

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What is the lithosphere?

• Outer part of the Earth, 100-200 km thick
• Includes the crust and part of the upper mantle
• Behaves rigidly
• Forms plates

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Tectonic plates
Large plates (61) North American, South
American, Eurasian, African, Australian,
Antarctic Pacific Small plates (2 6) Arabian,
Indian Scotia, Caribbean, Nazca, Cocos, Juan de
Fuca, Philippine Sea
Contain continental lithosphere Purely oceanic
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J. Tuzo Wilson

• Proposed hotspot hypothesis in 1963, in support
  of Hess seafloor spreading model
• Had difficulty getting this work published!

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J. Tuzo Wilson

• Proposed transform plate boundary hypothesis in
  1965 to explain linear fracture zones in oceans
• Realized that motion on an oceanic transform
  fault is opposite to apparent offset of ridges

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The Wilson Cycle
1. Continental breakup (rifting) 2. Opening of
ocean basin 3. Subduction zones form 4. Ocean
basin closes 5. Continents collide and form high
mountains 6. Crust thins by erosion
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Types of plate boundaries