the plate tectonics revolution a paradigm shift in the earth sciences
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The Plate Tectonics
RevolutionA paradigm A paradigm shift in the shift in the
Earth SciencesEarth Sciences
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Topics: Wegener’s 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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Alfred Wegener (1880-1930)
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
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Alfred Wegener
Supporting evidence: Appalachian mountains match Scottish Highlands; similar rocks in South Africa and Brazil
Coined term Pangea (Greek, meaning “all the Earth”)
Appalachians (eastern N.A.) Scottish Highlands
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Harry Hess
In 1962, U.S. Navy marine geologist Harry Hess proposed that ocean crust is much younger than continental crust, and forms by “seafloor spreading”
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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 72:1267-1270.
Magnetic surveys revealed a distinctive striped pattern in the oceans
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In the meantime, scientists in U.S.A. and Australia discovered that the Earth’s magnetic field has reversed many times
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Vine, F. J. and D. H. Matthews. 1963. Magnetic anomalies over oceanic ridges. Nature 199:947-949.
The Vine-Matthews Hypothesis
Magnetic Stripes = Seafloor Spreading + Magnetic Field Reversals
Basalt extruded at mid-ocean ridges is magnetized in the direction of the Earth’s magnetic field
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Larry Morley
… 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
Vines-Matthews-Morley hypothesis
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The new global tectonics of the 1970’sEarth’s 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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Large plates (6+1):
North American, South American, Eurasian, African, Australian, Antarctic
Pacific
Small plates (2 + 6):
Arabian, Indian
Scotia, Caribbean, Nazca, Cocos, Juan de Fuca, Philippine Sea
Tectonic plates
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 Cycle1. 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