Characteristics of plate tectonics

One of the most important recent scientific discoveries involves the development of the theory of Plate Tectonics. According to this theory, Earth's outer layer, or crust, consists of a series of plates made up of lithospheric material which floats on the denser asthenosphere of the upper mantle within Earth's interior. Spreading centers at the bottom of the world's oceans bring up liquid rock from the asthenosphere as the plates move apart. When the liquid rock (or magma) comes in contact with the cold, deep-ocean water, it cools rapidly to form brand-new lithospheric crust. Gradually, the freshly made material settles into a hard, dense rock called basalt. As new magma rises into the spreading center, it repeats this process and pushes the freshly made rock to the side. Two famous spreading centers in the ocean are the Mid-Atlantic Ridge in the Atlantic Ocean, and the East Pacific Rise in the Pacific Ocean. Spreading centers can also exist on land. The East African Rift Valley represents a boundary where a large portion of lithosphere is breaking off of the African continent. One day, perhaps millions of years from now, water from the Indian Ocean will rush into the Rift Valley in a catastrophic flood, and East Africa will instantly become a large island. Over time, Earth's plates collide, and some plates can be forced underneath others. These "subduction zones" are like large recycling centers; lithospheric material (as it is subducted under another plate) melts in the extreme heat of Earth's interior to become magma once again. This hot, molten, less-dense rock, however, likes to rise toward the surface, and can be discharged back onto Earth's surface through a volcanic eruption, or to help build high mountain ranges. The most extreme example is the Peruvian Trench along the western edge of South America. Here, the Nazca Plate is being forced beneath the South American Plate. The Andes Mountains (which run parallel to the trench) are a direct result of this dramatic, ongoing collision.

Credit: Dylan Prentiss, Department of Geography, University of California, Santa Barbara

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