Oshima is an active volcano located on the northern tip of the Izu-Bonin volcanic arc. Oshima erupts very frequently, on average every 1-3 years, but has been quiet since 1990, which is an unusual long interval compared to its activity in historic times.
Eruptions at Oshima volcano occur from both summit and flank vents and its lavas are relatively fluid. The eruptions often produce lava fountains, strombolian activity, lava flows and the formation of lava lakes.
The volcano is not ble for its strombolian eruptions, but it also has experienced phreatomagmatic explosions. The eruptions between 1552 and
1974 occurred at the Miharayama crater or the bottom of the surrounding caldera, but the 1986 eruption ocurred inside the Miharayama crater and at the fissure craters in the caldera bottom and outsid of the calderarim. Precursors for the eruption, such as ground deformation, earth quakes and tremors, geomagnetism, resistivity, and gravity changes have been observed. The volcano, composed mainly of basalt, is between 49.5 and 58.0 wt%..
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