Geology
Aogashima is a complex Quaternary volcanic island 3.5 km in length with a maximum width of 2.5 km, formed by the overlapping remnants of at least four submarine calderas. The island is surrounded by very steep rugged cliffs of layered volcanic deposits. The southern coast also rises to a sharp ridge forming one edge of a caldera named Ikenosawa (池之沢?) with a diameter of 1.5 km. The caldera dominates the island, with one point on its southern ridge, Otonbu (大凸部?) with a height of 423 metres (1,388 ft), as the island’s highest point. The caldera is occupied by a secondary cone named Maruyama (丸山?).
Still considered a Class-C active volcano by the Japan Meteorological Agency, the last eruption of Aogashima was during a four-year period from 1781-1785.
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