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 aircraft and XXI Bomber Command B-29s also occasionally participated in these attacks. The Seventh Air Force also provided air defence for the Mariana Islands; this task was assigned to the 318th Fighter Group, which was equipped with P-47 Thunderbolt fighters, and the 6th Night Fighter Squadron which flew Northrop P-61 Black Widows. The Thunderbolts were also responsible for suppressing the Japanese-held Mariana Islands, though it proved difficult to stop the 3,600-man garrison from repairing the airfield on Pagan. In addition, the US Army stationed elements of eight anti-aircraft gun battalions and two searchlight battalions on Saipan between July 1944 and