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
Japanese occupation of the Andaman Islands objective on the islands was the city of Port Blair. The garrison consisted of a 300-man Sikh militia with 23 British officers, augmented in January 1942 by a Gurkha detachment of 4/12th Frontier Force Regiment of the 16th Indian Infantry Brigade. Following the fall of Rangoon on March 8, however, the British recognized that Port Blair had become impossible to defend, and on March 10 the Gurkhas were withdrawn to the Arakan peninsula. The Japanese, seeking to secure their seaward flank, dispatched a force to seize the islands. The force was composed of the following units and ships: Distant Cover<br>