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 air attacks on the Mariana Islands the Mariana and Palau Islands campaign. These islands were needed to provide bases for USAAF Twentieth Air Force B-29 Superfortress heavy bombers to attack the Japanese home islands. Construction of the large air bases needed to support B-29s began on Saipan in June 1944, before the end of the Battle of Saipan, and work on air fields on Tinian and Guam commenced in July and August respectively. The B-29s of the XXI Bomber Command began to arrive at Saipan starting on October 12, and the 73rd Bombardment Wing began flying warm-up missions against Japanese-held islands in the Pacific on October