What Are The Volcanic Island Arc, Accretionary Wedge, And Fore Arc Basins?


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Volcanic Island Arcs: When an oceanic plate sub duct under another oceanic plate, as the descending plate reaches depths of at least 100 Kms, ,magma is generated by the partial melting of asthenosphere. The magma moves upward to erupt as an island arc, a curved line of volcano that forms a string of Islands parallel to the oceanic trench. The rocks that occur in these island arcs are basaltic, andesitic and dacitic.

Accretionary prism: as the down going slab moves under the overriding plate the sediment in the trench may be taken down into the subduction zone or it may be scrapped off the overriding plate. The trench deposits which are scraped off are accredited onto the font of the overriding plate in a series of slices. Repeated addition of slices causes a build up of material known as an accretionaryaccretionary prism or accretionary wedge. It consists of thrust faulted and folded deep marine sediments and pieces of oceanic crust deposited in trench settings.

Fore-Arc Basin: The area between the volcanic arc and an accretionary prism is called fore-arc basin. It depends on the arc trench gap and the size of the accretionary prism, it nay vary between 50 to 100 kms in width.

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