Melting aided by the addition of water or other fluid is called flux melting. It is somewhat more complicated than this, but metamorphic dewatering of suducting crust and flux melting of the mantle wedge appears to account for most of the magma at subduction zones. Magma formed above a subducting plate slowly rise into the overriding crust and finally to the surface forming a volcanic arc , a chain of active volcanoes which parallels the deep ocean trench.
Beneath the active volcanic arc lie intrusive igneous rocks formed from magma that didn't make it all the way to the surface before crystallizing.
The volcanic arcs may be volcanic island arcs e. The most abundant igneous rock formed at volcanic arcs is andesite or intrusive diorite , though volcanic arc rocks may range in composition from basalt to rhyolite mafic to felsic. Most of the rising magma will be emplaced in the overlying continental crust, where it will cool and crystallize at a depth of several kilometers. The remaining magma will eventually migrate to the surface where it can give rise to numerous and occasionally explosive volcanic eruptions along a volcanic arc.
A volcanic arc is usually formed by a chain of volcanoes. It parallels the trench approximately km inland. When two oceanic plates collide against each other, the older and therefore heavier of the two subducts beneath the other, initiating volcanic activity in a manner similar to that which occurs at an oceanic-continental convergent plate boundary and forming a volcanic island arc.
Where two continents approach each other, the intervening sea-floor is subducted, causing arc-type volcanism. The created volcano chain is arc-shaped and parallel to the boundary of the convergent plate and convex towards the subducting plate.
The narrow, deep oceanic trench, on the sinking part of the island, is the trace of the boundary on the surface of the earth between the overriding and the down-going plates. The gravitational pull of the denser down-going plate pulls the edge of the overriding plate downwards forming the oceanic trench.
Numerous earthquakes occur along this boundary, plus the seismic hypocenter is at an ever-increasing depth beneath the island arc. The ocean basins which are reduced by this process are referred to as remnant oceans since they tend to shrink slowly out of existence before being crushed into the orogenic collision.
The overriding plate is the North American plate while the down-riding plate is the Pacific plate. Japan is the home to two of the most significant island arcs on the earth, the Ryukyu Islands and the Japanese Archipelago. The overriding plate of the Ryukyu Islands is the Eurasian plate, and the down-riding plate is the Philippine Sea plate. The top plates of the Japanese Archipelago are the Eurasian and North American plates, while the down-riding plates are the Philippine Sea plate and the Pacific plate.
Other smaller island arcs in Japan include the Bonin Island and the Izu islands. Greece has two large island arcs, the South Aegean-volcanic arc and the Hellenic arcs which formed along the Aegean Sea. The overriding plate for these two islands is the Hellenic plate, and the down-riding plate is the African plate. One of the oldest island arcs on Earth is the insular islands arc where giant chains of volcanic islands formed in the Pacific Ocean during the Cretaceous era.
Talwani and W. Pitman, III, eds. Washington, D. Research 77, — Jakes, P. Karig, D. Research 81, — Kulm, L. Government Printing Office, p.
Lake, P. Meyerhoff, A. Moore, G. Development of sedimentary basins on the lower trench slope, Geology 4, — Moore, J.
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