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
Sangihe Islands main islands of the group are, north to south, Sangir Besar (or Sangir Island), Siau (or Siao), Tahulandang, and Biaro. The largest island is Sangir Besar and contains an active volcano, Mount Awu (). Tahuna is the chief town and port, also hosting the islands' sole airport, Naha Airport. The area came under Dutch control in 1677, and became part of Indonesia when it declared independence from the Netherlands in 1945. The Sangir language is spoken in the islands; this Austronesian language is also spoken in some islands in the Philippines, and on the extreme northern tip of Sulawesi. The