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Plate Tectonics
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Tectonics refer to the deformation of Earth’s crust and results in the formation of structural features such as mountains
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Alfred Wegener• Proposed hypothesis in 1915 • Published The Origin of
Continents and Oceans
Continental drift hypothesis • Supercontinent Pangaea (meaning “all
lands”) began breaking apart into smaller continents about 200 million years ago
Continental drift: An idea before its time
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Pangaea approximately 200 million years ago
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Ocean opens between rigid plates
As the plates moves, the distance between two cities on the same Plate, New York and Denver, for example, remains relatively constant while the distance between New York and London iscontinually changing
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Continental Drift EvidenceGeometry of continental margins
Similarity of rocks and structures
Correlation of glacial deposits
Distribution of fossils
Paleomagnetism and “polar wander”
Topography and other features of the seafloor
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South America and Africa fit almost perfectly.
Not at the shoreline, but at the true edge of the continental margin (at the bottom of the continental slope).
South America and Africa were joined in the past and split apart when the Atlantic Ocean opened
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Paleozoic glacial deposits and features are common among the now separate southern continents…
But they are now near the equator! Huh?
Also, the direction of ice movement is from what is now the sea towards the interiors! Huh?
These observations only make sense if the southern continents were once joined and were at the south pole!
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The great debate
Objections to drift hypothesis • Inability to provide a mechanism capable
of moving continents across globe • Wegner suggested that continents broke
through the ocean crust, much like ice breakers cut through ice
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The great debate
Continental drift and the scientific method
• Wegner’s hypothesis was correct in principle, but contained incorrect details
• For any scientific viewpoint to gain wide acceptance, supporting evidence required
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Plate tectonics: The new paradigm
More encompassing theory than continental drift
Mix of ideas that explained motion of Earth’s lithosphere by subduction and seafloor spreading
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Plate tectonics: The new paradigm
Earth’s major plates • Associated with Earth's strong, rigid
outer layer– Known as the lithosphere
– Consists of uppermost mantle and overlying crust
– Overlies a weaker region in the mantle called the asthenosphere
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Basal tractions drive plate motions