Oshima-Oshima

Geography & Climate

Oshima-Oshima is located in Japan, within the Northeast Japan Volcanic Arc of the broader Northwestern Pacific Volcanic Regions. Situated at 41.51° N, 139.37° E in the Northern Hemisphere, the volcano lies within a temperate climate zone. With a summit elevation of 732 meters above sea level, Oshima-Oshima is a moderately sized peak that remains accessible to hikers and researchers for much of the year. The volcanic landform is characterized as a composite, which describes the physical shape and structure of the volcanic edifice as observed from the surface.

Geological Context

Oshima-Oshima sits in a subduction zone, where one tectonic plate dives beneath another, creating intense heat and pressure that generates magma. Subduction zones are responsible for many of the world's most explosive volcanoes and deadliest eruptions. For communities in Japan near Oshima-Oshima, this tectonic setting means the volcano is capable of producing powerful explosive eruptions, pyroclastic flows, and lahars that can threaten populated areas within tens of kilometers of the summit. The dominant rock type is basalt / picro-basalt, a dark, fine-grained volcanic rock that forms from rapidly cooling, low-viscosity lava. Basaltic eruptions tend to be less explosive and produce fluid lava flows that can travel long distances. While less immediately dangerous than explosive eruptions, basaltic lava flows can destroy structures and infrastructure in their path, and volcanic gases released during these eruptions can affect air quality over a wide area.

Eruption History Summary

Oshima-Oshima has 6 recorded eruptions in the geological database, spanning from 800 BCE to 1790 CE. The most powerful recorded event was a cataclysmic eruption with ash columns reaching the stratosphere in 1741 CE, reaching VEI 4 on the Volcanic Explosivity Index. This eruption frequency indicates a moderately active volcanic system with periodic reawakenings.

Significance

With a maximum recorded VEI of 4, Oshima-Oshima has demonstrated the capacity for significant explosive eruptions that can impact regional aviation, agriculture, and communities tens of kilometers from the summit. VEI 4 eruptions produce ash columns reaching 15-25 kilometers into the atmosphere. With 6 recorded eruptions on file, Oshima-Oshima has enough documented activity to help scientists identify patterns in its eruptive behavior.

GVP Reference Summary

Oshima-Oshima, a 10 km2 island about 105 km W offshore from the SW tip of Hokkaido, is the emergent summit of two coalescing basaltic-to-andesitic stratovolcanoes. Higashiyama, at the east end of the island, is cut by a 2-km-wide caldera covered on its west side by Nishiyama volcano. The western cone failed during an eruption in 1741 CE, producing a mostly submarine debris avalanche that traveled 16 km and leaving a scarp open to the north. A tsunami associated with the collapse swept the coasts of Hokkaido, western Honshu, and Korea, and caused nearly 1,500 fatalities. The 1741 eruption concluded with the construction of a basaltic pyroclastic cone at the head of the amphitheater. No eruptions have occurred since the late-18th century.

— Smithsonian Institution, Global Volcanism Program