Plate tectonics

     Plate tectonics is a theory dealing with the dynamics of the Earth's outer shell, the lithosphere.
     The lithospheric plates are composed of the rocky crust and outermost rocky layer of the mantle which are fused together. Plate movements occur because the layer of Earth directly below the lithosphere, the asthenosphere, is hot enough, and under enough lithostatic pressure to prevent its melting, that it can deform without fracture. The theory that describes this movement is called plate tectonics.
     Plate tectonics is the outcome of thinking that began with an idea posed by German meteorologist Alfred Wegener in 1915.
     According to the theory, the lithosphere is broken into seven large and many small moving plates. These plates, each about 50 miles thick, move relative to one another an average of a few inches a year. I think for example of this type of movement, called subduction, occurs at the boundary between the oceanic Nazca Plate and the continental South American Plate. Where continental plates collide, they form major mountain systems such as the Himalayas.
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