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PLATE TECTONICS

Plate tectonics is a theory that explains how the Earth's continents float slowly on the surface, changing both their position and size over time. The theory was proposed by Alfred Wegener in 1912, but wasn't widely accepted until the 1960's.

The theory argues that the Earth's crust (the lithosphere) is broken up into about 10 plates. These plates float on the Earth's upper mantle, which is a partially-molten layer. Major geologic activity occurs at the edges of these plates where they push into one another, pull farther apart, or move sideways with tremendous power. At these rims there are earthquakes, mountain building, and/or volcanic activity.

Earth's Crust

Under the sea (where the Earth's crust is relatively thin but dense) and the plates are moving away from one another, new crust is formed at spreading ridges as lava flows from the upper mantle. This process is called sea-floor spreading, and increases the size of plates. When the rims of the thin, undersea tectonic plates come toward one another, one edge goes under the other (is subducted). This is a subduction zone, where plates get smaller.


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