Compressional and shear wave properties of marine sediments ...
OCN 201: Continental Drift - SOEST · • Craton or shield: Stable part of a continental block that...
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Transcript of OCN 201: Continental Drift - SOEST · • Craton or shield: Stable part of a continental block that...
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OCN 201: Continental Drift: Prelude to Plate Tectonics
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Tectonics
A branch of geology that deals with the major structural (or deformational) features of the Earth and their origin, relationships, and history…
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Tectonic Structure of Continents• Craton or shield: Stable part
of a continental block that has not experienced deformation for a long time…
• Mobile Belt: a compressional mountain belt: long narrow region of crust that has been deformed recently and repeatedly
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Shrinking Earth TheoryPrevailing theory in the early 20th century (This theory was wrong and is a good example
of how ideas in science can and do change…)1) Earth was formed in a molten state. It is still solidifying
and shrinking as it cools. 2) Shrinkage causes the previously cooled crust to wrinkle:
Large wrinkles = continents and ocean basinsSmall wrinkles = mountain belts.
Motion is mainly vertical, not horizontal…3) Continents are drained as ocean basins warp down
and flooded as they fill up with sediment.
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Theory of Continental Drift (Alfred Wegener, 1912)
• Continents are moving laterally across the face of the Earth.
• Continents were joined together in the distant past as a single supercontinent.
• Supercontinent (Pangaea) was surrounded by a superocean (Panthalassa).
• Supercontinent split apart and its pieces are still moving apart today…
• Motion is mainly horizontal, not vertical…
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Wegener
ModernReconstruction
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Evidence for Continental Drift: I• Fit of continents (first map and
suggestion by Francis Bacon, 1620!)
• Mobile belts near continental margins, NOT randomly distributed.
• Hypsometry of Earth suggests two types of crust, isostatically supported.
• Earth is oblate spheroid:(6367 +/- 11 km radius
viscous liquid, permits isostatic compensation).
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Evidence for Continental Drift: II• Distribution of living organisms• Distribution of fossil organisms• Distribution of paleoclimatic indicators in rocks:
Glacial tillsDesert sandsTropical soilsCoal bedsCoral reefs
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Problems with Continental Drift• Cause (?): Wegener postulated that earth tides
and centrifugal forces or “polflucht” (“flight from the poles”) were the driving forces…
• Ideas were rejected by many geophysicists because :– Lack of a suitable mechanism: the forces Wegener suggested
were too weak to move continents.– How could continents “plow through” the ocean floor?– Too unconventional: hard to accept even if it made sense.– Wegener was a meteorologist so he was considered an outsider.– European and N. American geologists mostly rejected his ideas.
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Support for Continental Drift• South African and Australian geologists, however,
tended to accept Wegener’s ideas because the geology of those continents seems rather incomplete:
Major geologic structures in S. Africa & Australia end abruptly at coastlines and seem cleanly cut apart.
Missing parts of these geologic structures reappear on another continent across the ocean.
• Paleo-evidence mentioned before…• Paleomagnetic evidence uncovered in the 1950’s led to
a revival of Wegener’s ideas…
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Earth’s magnetic field is generated by:A. The SunB. The gravitational interaction of Earth, the Sun, and
Jupiter.C. The Geodynamo: convection of molten iron in Earth’s
outer core, coupled with Earth’s 24-hour rotation.D. A giant bar magnet inside the Earth.
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And now, on to Seafloor Spreading…