Oshima is an island stratovolcano with a large caldera on its summit. A cone called Mihara-yama stands in the center of the caldera. The island is ~8 miles (13 km) across from NNW to SSE and ~5.5 miles (9 km) across from ENE to WSW. It is in the form of a cone with a relatively flat top. This cone slopes from 5-15 degrees in most areas. It slopes 25 degrees on the west side. The eastern slope is cut by a sea cliff. This cliff was a fault scarp that formed while the cone was growing. The base of Oshima is made of old stratovolcanoes of olivine basalt. The main cone is a stratovolcano made of basalt lava and pyroclastic rocks. The caldera on the summit of this cone is ~2-2.5 miles (3-4 km) across. Steep walls surround the caldera except in the northeast. This part of the wall is buried under lava flows from Mihara-yama.
Layers of lavas and pyroclastic materials in the caldera have been faulted and folded. Mihara-yama is a stratovolcano that rises 394 ft (120 m) from this lava field. It has an outer slope of about 20 degrees. Its summit crater is filled with lava from a 1950-51 eruption. A small cinder cone was formed by this eruption on the southern rim of the crater. This is the highest point on the volcano.
Almost 30 parasitic cones and craters can be found on the island. They are found on three rift zones. These rift zones extend from the center of the main cone to the north-northeast, northeast and south-southeast. They were probably formed by the upward pressure of rising magma.
Oshima has erupted 74 times. Strombolian eruptions with extrusions of aa and pahoehoe took place at the Mihara-yama crater and nearby vents during historic times. Bombs and scoria built up to form small cinder cones inside the Mihara-yama crater. The crater floor is usually deep during the dormant periods, but gets shallow before eruptions start. The last eruption of Oshima was in 1990.
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additional photos of Oshima and Mihara-yama.
Kuno, Hisashi, "Catalogue of the Active Volcanoes of the World Including Solfatara Fields: Part XI, Japan, Taiwan and Marianas," International Association of Volcanology, Rome Italy, 332 pp., 1962.
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