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 Pagan. The Japanese were unable to use most of these bases, however, as they had been isolated and heavily damaged by United States air attacks, and the Imperial Japanese Army Air Force (IJAAF) and Imperial Japanese Navy (IJN) had relatively few aircraft and pilots available. Despite the limited threat of attack, the United States military took steps to protect the B-29 bases in the Mariana Islands. The USAAF's Seventh Air Force played the most important role, and continuously attacked the Japanese-held islands until just before the end of the war. United States Navy (USN) and United States Marine Corps (USMC)