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.
The Tenmei Eruption of 1785
The last major eruption of Aogashima occurred between 1781 and 1785. This period of activity is known as the Tenmei eruption, a name associated with other major volcanic events in Japan during the same era. Activity began in July 1780 with an earthquake swarm and steam rising from the Ikenosawa Caldera. In May 1781, further earthquakes preceded a new eruption. By April 1783, lava flows from the newly forming Maruyama cone forced the evacuation of all 63 households on the island.
The climactic event occurred in 1785. A massive eruption created pyroclastic flows and ejected large amounts of volcanic material. The event was catastrophic for the island's population of 327 people. Approximately 130 to 140 of the islanders perished. Following the eruption, the island was completely uninhabited for fifty years. It was not until 1835 that people, likely descendants of the survivors, returned to permanently resettle Aogashima. The Japan Meteorological Agency still classifies Aogashima as a Class-C active volcano.