Island Arcs


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Island Arcs

 

relief structures of contemporary geosynclinal systems occurring in the transitional zones between continents and oceans. Island arcs are linear mountains that divide the basins of the marginal seas from the deep-sea trenches. The foundations of island arcs are underwater mountain ranges from 40–50 to 200–400 km wide and up to 1,000 or more km long, composed primarily of volcanic strata of basalt, a mixture of andesite and basalt, or andesite. The top of the range rises above sea level in the form of islands. Island arcs frequently consist of two parallel ridges, one of which, usually the outer ridge facing the deep-sea trench, is an underwater mountain range. In such cases the ridges are separated from one another by a longitudinal depression up to 3–4.5 km deep filled with a stratum of sediment 2–3 km deep. Transverse depressions are found in fracture zones and usually form the deeper straits.

Most island arcs are located on the northern and western margins of the Pacific Ocean; the only exceptions are the Antilles, the Lesser Antilles, and the Sunda arcs. In their early stages of development island arcs are a thickening of the ocean crust with volcanic structures on the crest, for example, the Mariana and Kermadec island arcs. In later stages island arcs form large islands and peninsulas, for example, the Japanese Islands, the Kamchatka Peninsula, and New Guinea; here the structure of the crust is similar to that of continents. Island arcs have highly differentiated gravitational and magnetic fields, increased heat flows, and active volcanic and seismic activity. Between island arcs and the deep-water trenches lies the region of earthquake concentration, the Benioff zone, which passes under the island arcs.

REFERENCES

Ostrovnye dugi: Sb. st. Moscow, 1952. (Translated from English.)
Gorshkov, G. S. Vulkanizm Kuril’skoi ostrovnoi dugi. Moscow, 1967.
Pushcharovskii, Iu. M. Vvedenie ν tektoniku Tikhookeanskogo segmenta Zemli. Moscow, 1972.
Udintsev, G. B. Geomorfologiia i tektonika dna Tikhogo okeana. Moscow, 1972.
Mitchell, A. H., and H. G. Reading. “Evolution of Island Arcs.” Journal of Geology, 1971, vol. 79, no. 3.

O. K. LEONT’EV

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Miller, one of Jordan's colleagues at USC, reported in Geophysical Research Letters detecting boundaries in the keels that suggest that island arcs smashed into cratons, piling up against them over time.
The belts are composed of volcanic island arc, back-arc sedimentary basin, and (granitoid-rich) microcontinent-type of terrains, amalgamated in the course of the active margin of Svecofennian ocean around 1.
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Pinet and Tremblay (1995) proposed that emplacement of the Quebec ophiolites was caused by the collision of an island arc with the North American continent during the Ordovician.
Shoshonitic volcanic suites are common in island arc volcanism and have been identified in the New Guinea Highlands (Mackenzie and Chappell 1972) and in Eastern New Guinea, including the offshore D'Entrecasteaux Island group (Jakes and Smith 1970; Smith 1972; Johnson et al.
Shoshonite occurs in association with calc--alkaline volcanism in intra-arc rift settings, in intra-oceanic island arcs and backarc basins, (Sun and Stern 2001; Adams et al.
Collision of major crustal features such as continents and island arcs is considered to be a principal cause of orogenesis that normally results in building mountain chains and thickening the crust.
Hoffman proposes that mantle convection swept the microcontinents and island arcs toward an area where cold mantle material was sinking.
As the seafloor spreads and collides with continents and other ocean plates at island arcs, old crust is also continuously destroyed--overridden and mixed back into Earth's mantle at subduction zones.
Volcanism during the first 10 million years of these arcs had characteristics intermediate between modern spreading centers and island arcs.
The rising water induces the melting of rocks, which eventually results in volcanoes called island arcs.