In plate tectonics the outermost part of the earth, the crust and uppermost mantle, act as a single mechanical layer, the lithosphere. The lithosphere is divided into separate 'plates' that move relative to each other on the underlying, relatively weak asthenosphere in a process ultimately driven by the continuous loss of heat from the earth's interior. There are three main types of plate boundary: divergent where plates move apart from each other and new lithosphere is formed in the process of sea-floor spreading; transform where plates slide past each other and convergent where plates converge and lithosphere is 'consumed' by the process of subduction. Convergent and transform boundaries form the largest structural discontinuities in the lithosphere and are responsible for most of the world's major (Mw > 7) earthquakes. Convergent and divergent boundaries are also the site of most of the world's volcanoes, such as around the Pacific Ring of Fire. Most of the deformation in the lithosphere is related to the interaction between plates, either directly or indirectly.
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