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.