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Accretion, in geology, is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts, oceanic crust or other igneous features.
Accretion involves the addition of material to a tectonic plate. When two tectonic plates collide, one of the plates may slide under the other, a process known as subduction. The plate which is being subducted, is floating on the asthenosphere and is pushed against the other, over-riding plate. Sediment on the ocean floor will often be scraped from the subducting plate. This causes the sediment to accumulate as a mass of material called an accretionary wedge (accretionary prism), which attaches itself to the upper plate. Volcanic island arcs or seamounts may collide with the continent, and as they are of relatively light material (i.e. low density) they will often not be subducted, but are added to the side of the continent.
Continental plates are formed of rocks that are very noticeably different from the rocks that form the ocean floor. The ocean floor is usually composed of basaltic rocks which have a higher density than continental plates. In places where plate accretion has occurred, land masses may contain the dense, basaltic rocks that are usually indicative of oceanic lithosphere.
This process occurs wherever plates collide. Many examples occur around the Pacific Rim, including the western coast of North America, the eastern coasts of New Zealand and Asia. The Nankai accretionary complex of Japan, the Barbados Ridge in the Caribbean, the Mediterranean Ridge, and the Franciscan Assemblage of California are specific examples of accretionary wedges.
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