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 population on the islands was repatriated back to Japan. People of mixed Japanese–Marshallese heritage generally stayed behind, although a few were granted an option if they were above 16 years old. The Japanese–Marshallese quickly assimilated with the Marshallese in the years after the war. Many of them became politicians and businessmen, and in the 1970s people of mixed Japanese–Marshallese heritage controlled most of the private enterprises in the state. Japanese–Marshallese politicians generally held critical opinions on nuclear weapon tests carried out by the United States in Operation Crossroads. At least one ethnologist, Greg Dvorak suggested that the shared Japanese and