Aogashima

Geography & Climate

Aogashima is located in Japan, within the Izu Volcanic Arc of the broader Northwestern Pacific Volcanic Regions. Situated at 32.46° N, 139.76° E in the Northern Hemisphere, the volcano lies within a subtropical climate zone. At just 423 meters above sea level, Aogashima is a relatively low-elevation volcanic feature. Despite its modest height, it remains an active geological formation with the same underlying magmatic processes as its taller counterparts. 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

Aogashima 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 Aogashima, 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

Aogashima has 7 recorded eruptions in the geological database, spanning from 1800 BCE to 1780 CE. The most powerful recorded event was a cataclysmic eruption with ash columns reaching the stratosphere in 600 BCE, reaching VEI 4 on the Volcanic Explosivity Index. Notable eruptions include 1780 CE (VEI 3), 1652 CE (VEI 3), 600 BCE (VEI 4). This eruption frequency indicates a moderately active volcanic system with periodic reawakenings.

Significance

With a maximum recorded VEI of 4, Aogashima 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 7 recorded eruptions on file, Aogashima has enough documented activity to help scientists identify patterns in its eruptive behavior.

GVP Reference Summary

The small 2.5 x 3.5 km dominantly basaltic island of Aogashima is surrounded by steep cliffs and contains a small 1.7 x 1.5-km-wide caldera. Two pyroclastic cones were formed inside the caldera during the latest eruption from 1780 to 1785. Growth of the volcano began with construction of the Kurosaki stratovolcano in the NW part of the island, after which the main stratovolcano began growing in the SE. Both summit and flank vents produced pyroclastic surges and lava flows. Late in the construction of the main cone a 1-1.5 km crater was formed on the SE flank. About 3000 years ago pyroclastic surges swept over the entire island. During about the next 600 years, lava flows and scoria deposits filled the SE crater, which also collapsed repeatedly. The current Ikenosawa crater was considered by Takada et al. (1992) to have formed by ring collapse at the time of a debris avalanche, after which the volcano was quiescent until the eruptions of historical time.

— Smithsonian Institution, Global Volcanism Program