Bonin Islands Pacific Plate is subducting under the Philippine Sea Plate, which creates an oceanic trench to the east of the islands: the Bonin Trench. The crust of the Ogasawara Islands was formed by volcanic activity when subduction began 45–50 million years ago, and is composed mostly of an andesitic volcanic rock called boninite, which is rich in magnesium oxide, chromium, and silicon dioxide. The Ogasawara Islands may represent the exposed parts of an ophiolite that has not yet been emplaced on oceanic crust. The rocks of the Volcano Islands are much younger; Iwo Jima is a dormant volcano characterized by rapid
Islands of Four Mountains Islands of Four Mountains Islands of Four Mountains is an island grouping of the Aleutian Islands in Alaska, United States. The chain includes, from west to east, Amukta, Chagulak, Yunaska, Herbert, Carlisle, Chuginadak, Uliaga, and Kagamil islands. This island chain is located between Amukta Pass and the Andreanof Islands to the west, and Samalga Pass and the Fox Islands to the east. These islands have a total land area of 210.656 sq mi (545.596 km²) and have no permanent population. The two largest islands are Yunaska and Chuginadak. Chuginadak is mainly made up of the active volcano Mount Cleveland. The