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What is Plate Tectonics, and How does it Work?


moving 'lithosphere'


Today, scientists believe that the earth is divided into numerous layers in its interiors and rigid plates on its surface; both of these ideas are part of the concept of Plate Tectonics. The Earth's outermost layer is made up of continental and oceanic crust. The crust and a small portion of the underlying mantle make up the lithosphere, which is divided into a series of rigid plates. These lithospheric plates sit on top of a very slowly flowing portion of the mantle called the asthenosphere. The plates move slowly in varying directions. Where two plates move away from one another it is called a spreading center. At spreading centers, such as the Mid-Atlantic Ridge and East Pacific Rise, new seafloor is created as magma, molten rock and gas, moves up from below in the mantle and is extruded into the deep, cold ocean. It cools quickly and becomes basalt, the basic building blocks of oceanic crust. Over time the plates continue to spread, and more and more crust is formed. 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.


Created for Earth Update by Dylan Prentiss, Department of Geography, University of California, Santa Barbara

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