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The Izu-Bonin-Mariana (IBM) arc system is a tectonic-plate convergent boundary. IBM extends over 2800 km south from Tokyo, Japan, to beyond Guam, and includes the Izu Islands, Bonin Islands, and Mariana Islands; much more of the IBM arc system is submerged below sealevel. The IBM arc system lies along the eastern margin of the Philippine Sea Plate in the Western Pacific Ocean. It is most famous for being the site of the deepest gash in Earth's solid surface, the Challenger Deep in the Mariana Trench. The IBM arc system formed as a result of subduction of the western Pacific plate. The IBM arc system now subducts mid-Jurassic to Early Cretaceous lithosphere, with younger lithosphere in the north and older lithosphere in the south, including the oldest (~170 million years old, or Ma) oceanic crust. Subduction rates vary from ~2 cm (1 inch) per year in the south to 6 cm (~2.5 inches) in the north. The volcanic islands that comprise these island arcs are thought to have been formed from the release of volatiles (steam from trapped water, and other gases) being released from the subducted plate, as it reached sufficient depth for the temperature to cause release of these materials. The associated trenches are formed as the oldest (most western) part of the Pacific plate crust increases in density with age, and because of this process finally reaches its lowest point just as it subducts under the crust to the west of it. The IBM arc system is an excellent example of an intra-oceanic convergent margin (IOCM). IOCMs are built on oceanic crust and contrast fundamentally with island arc built on continental crust, such as Japan or the Andes. Because IOCM crust is thinner, denser, and more refractory than that beneath Andean-type margins, study of IOCM melts and fluids allows more confident assessment of mantle-to-crust fluxes and processes than is possible for Andean-type convergent margins. Because IOCMs are far removed from continents they are not affected by the large volume of alluvial and glacial sediments. The consequent thin sedimentary cover makes it much easier to study arc infrastructure and determine the mass and composition of subducted sediments. Active hydrothermal systems found on the submarine parts of IOCMs give us a chance to study how many of earth's important ore deposits formed. ==Historical importance of IBM arc system== Guam in the southern IBM arc system is where Magellan first landed after his epic crossing of the Pacific Ocean in 1521. The Bonin Islands were a significant stop for water and supplies for New England whaling during the early 19th century. At that time they were known as the Peel Islands. Terrible battles were fought on the islands of Saipan and Iwo Jima in 1944 and 1945; many young Japanese and American soldiers died in these battles. George H. W. Bush was shot down in 1945 near Chichijima in the Bonin Islands. Twelve Japanese seamen were stranded in June 1944 on volcanic Anatahan for seven years, along with the overseer of the abandoned plantation and an attractive young Japanese woman. The novel and 1953 movie Anatahan is based on these events. The B-29 bomber Enola Gay flew from Tinian to drop the first atomic bomb on Hiroshima in 1945. Sergeant Shoichi Yokoi hid out in the wilds of Guam for 28 years before coming out of hiding in 1972. The Brown tree snake was accidentally introduced during World War II and has since devastated native birds on Guam. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Izu-Bonin-Mariana Arc」の詳細全文を読む スポンサード リンク
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