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 too small to be effective. As it was not possible to allocate additional aircraft to bolster these missions, the GDC directed that the offensive cease and that the 6th Air Army be held in reserve to be used against any Allied forces which landed in the Japanese home islands. Overall, more than 80 Japanese aircraft were dispatched against Saipan and Tinian between November 1944 and the end of the offensive in February 1945. The Japanese destroyed 11 B-29s, caused major damage to eight and minor damage to another 35. American casualties were 45 dead and over 200 wounded. USAAF fighters