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 settlement in the Marshall Islands across all 33 atolls in 1940. Within the same year, the mandated islands had a total Japanese population of 77,000. The largest towns in the Marshall Islands, Jaluit and Jabor, had a thousand Japanese each, while in the most isolated atolls, the Japanese populace amounted to no more than a few individuals. Korean labourers were also counted recognised as Japanese in official statistics, and accounted for another 1,200 individuals which were brought into the Marshall Islands during the war. There was a sizeable minority of people of mixed Japanese and Marshallese heritage, which was more common in settlements with a