Magma chamber and eruptive processes at Izu-Oshima volcano, Japan: buoyancy control of magma migration

Abstract

Underground magmatic processes are modeled for a sequence of eruptions that started on November 15, 1986 at Izu-Oshima volcano. It is predicted from the seismic gap, the seismic velocity structure, the deformation center and the gravity minimum that there should be a magma chamber at a depth of about 4 km beneath the northwestern flank of the volcano. The effusion of basalt that began at the summit crater established a well-connected conduit system between the magma chamber and the summit crater. The following eruption from new fissures a few days later produced more felsic and less homogeneous compositions, basaltic andesite to andesite, because of cooling and fractional crystallization of ascending magma. Subsequently, four small eruptions were caused by gravitational sliding of sluggish, semisolid magma into openings in and along the sides of the shallow conduit as magma was returning to the magma chamber. Systematic tilt changes due to transmission of pressure pulses from the summit crater to the magma chamber were observed during a small explosion on November 18, 1987 and for many periods of episodic volcanic tremor. The migration of magma is interpreted to have been driven by a buoyancy force associated with bubbles in the magma. In this regard, dominant ascent of magma beneath the summit crater and new fissures in November 1986 was triggered by vesiculation of volatiles in the magma, and the subsequent return flow of magma resulted from the release of bubbles through degassing. The observed response of volcanic processes following earthquake swarms northwest of the island is also explained by buoyancy-controlled magma migration that could intensify the effect of small fluctuations in the crustal stress.