Background
At the beginning of the 20th Century, scientists realized that they could not explain many of the Earth's structures and processes with a single theory. Many scientific hypotheses were developed to try and support the conflicting observations. One hypotheses was continental drift, which was proposed by Alfred Wegener in a series of papers from 1910 to 1920.
The principal thought of continental drift theory is that the continents are situated on slabs of rock, or plates, and they have drifted across the surface of the Earth over time; however, originally, they were all joined together as a huge super-continent at one time. In the 1960's, the theory of continental drift was combined with the theory of sea-floor spreading to create the theory of plate tectonics.
The idea for Wegener's theory was sparked by his observation of the nearly perfect "fit" of the South American and African continents. Additional evidence supporting the continental drift theory are:
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Fossils of the same plant (Glossopteris) found in Australia, India, Antarctica and South America.
- Fossils of same reptile (Mesosaurus) found in Africa and South America. This animal could not have swum across the existing Atlantic Ocean!
- Glacial deposits found in current warm climates and warm climate plant fossils found in what is now the Arctic.
- Nearly identical rock formations found on the east coast of U.S. and the west coast of Europe and eastern South America and western Africa.
Plate Tectonics
Plate Tectonics is the theory supported by a wide range of evidence that considers the earth's crust and upper mantle to be composed of several large, thin, relatively rigid slabs of rock, or plates that the continents and oceans rest on and move relative to one another.
Tectonic plates are also called lithospheric plates because the crust and the upper-most mantle make up a sub-layer of the earth called the lithosphere. The plates can move because the uppermost mantle, or the asthenosphere, is partially molten and possesses a physical property called plasticity, allowing the strong, rigid plates of the crust to move over the weaker, softer asthenosphere. The word TECTONICS is of Greek origin and it means "to build." The word "tectonism" refers tot he deformation of the lithosphere. This deformation most notably includes mountain building.
Tectonic plates, or lithospheric plates, are constantly moving, being created, and consumed simultaneously. The motion sometimes results in earthquake, volcanoes, and mountain ranges at the plate boundaries.
Plate motion is driven by heat escaping from the mantle. The constant movement of heat in the mantle leads to circular convection currents. These hot convective cells are similar to the rolling boil that occurs when water is heated on a stovetop. The flowing mantle has also been compared to a "conveyor belt," moving the rigid plates in different directions. Fundamentally, convection occurs due to uneven heating and different densities within the liquid.
Source: USGS, Department of Interior
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