A caldera within a caldera
Aogashima is a complex Quaternary volcanic island, 3.5 kilometers long and 2.5 kilometers wide, located in the Philippine Sea 358 kilometers south of Tokyo. The island's structure is a rare double caldera. The entire island forms the outer crater walls of a large caldera named Ikenosawa, which has a diameter of about 1.5 kilometers. Rising from the center of this main caldera is a smaller, distinct inner cone named Maruyama.
The island is the exposed part of a much larger submarine volcano, rising over 1,100 meters from the sea floor. Its geology is primarily basaltic, with some andesite, consistent with its position along the Izu-Bonin-Mariana Arc. The main body of the island is made up of layered deposits from countless eruptions, forming steep cliffs that surround it. The Ikenosawa caldera itself formed between 3,000 and 2,400 years ago after a major debris avalanche and subsequent explosive eruptions. The central cone, Maruyama, is the product of the most recent eruptive period in the 18th century. The highest point on the island is Otonbu, on the southern ridge of the outer caldera, which reaches an elevation of 423 meters.