A number of opinions have been framed regarding the origin of island- arcs, which are conveniently grouped into three categories as follows:
1. Views explaining the origin of island arcs by processes of organic deformations. These views have been propounded by Suess, Tokuda,Wegener and Hobbs etc. But, since these views have left much to be explained for many of the features associated with island arcs, they are not commonly accepted.
2. Venning Meinesz, Bijlaard, Hess, Umbgroove, De Sitter have formulated their theories to explain the interrelationship between morphologic-cum-other structural features and the inner features like infrastructure, gravity and seismicity anomaly etc. But their views failed to be compatible with physico- mathematical models and as such did not receive much acceptance.
3. The modern theories of Plate-tectonics have been able to explain the formation of island-arcs and is widely accepted at present.
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The plate-tectonics interpretation of island arcs is that they are sites of under-thrusting on a large scale, along which the lithosphere of one plate disappears or is consumed as it passes beneath the edge of another plate.
During the movement of the plates sometimes thin, dense oceanic plate margins run into a thick, low-density continental margin.
The rocks of the oceanic plate are always denser than the rocks of continents and so the oceanic plate, is flexed downward and forced under the continental crust where it may be wholly or partially melted. The ocean floor is buckled downward, forming deep oceanic trenches or foredeeps.
The earthquake foci associated with any given trench are found to lie in a narrow zone that intersects the surface of the earth in the general vicinity of the trench, and which dips beneath the adjacent continental margin or arc.
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This downward-sloping seismic zone which may reach upto 600 to 700 kms is termed as Benioff-zones after Hugo Benioff, the geologist who first described their geometry.
The fact that there are no earthquakes at depths greater than 700 kms supports the idea that the oceanic plates are melted or sottened by the time they reach this depth. Along the Benioff Zone the descending plate disappears or is consumed by complete or partial melting.
Geologists refer to zones of oceanic plate consumption as subduction zones. The vuicanicity and high heat flow associated with trenches are thought to be associated with the partial melting of the descending oceanic plate including entrained sediments at its upper surface, which return to the surface as volcanoes.
Subduction angle determines the arc-trench spacing. If the subduction angle is gentle the arc trench distance is greater, which may be illustrated as follows:
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So far as the lithologic features are concerned, there occurs a complex mixture of deep-water sediments, sedimentary material derived from the oceanic landslides from the continental rise and ocean floor basalts scrapped off the advancing margin of the subducted plate, seaward from the island arcs.
This litholoeical mix is called the melange. Besides, it also explains the occurrence of biuescnisis as well as ophiolites and that of andesitic lava.
A modern example of subduction of oceanic plate directly beneath a continental one is furnished by the collision of Nazca plate and South American plate. The Nazca plate is being forced under the western margin of South America at a rate of 5 cms per year.
Apart from the above, there can also be collision of two plates capped by oceanic crust, in which case also, one plate dives (subducts) under the other. (For details, please see the chapter Plate-tectonics).