Underground Magmatic Activities Associated with the 1986 Eruption of Izu-Oshima Volcano

Abstract

Various features of the eruption that began on November 15, 1986 in Izu-oshima volcano are examined to infer the underground magmatic activities and the mechanism of the eruption. A massive dike intrustion that is assumed to have happened at the same time as the fissure eruption cannot explain the seismicity, deformation and other evidence consistently. Another preferable model that gives a systematic explanation of the available data is proposed as follows. A magma reservoir was situated at a depth of about 5 km below a NW (north-western) flank of Izu-Oshima volcano. This magma reservoir supplied magma to the summit crater through a well-developed vent, and caused the first summit eruption under an enhanced NW-SE compressive component of the tectonic stress. As the magma reservoir deflated with the discharge of magma in this summit eruption, a compressive stress increased in the neighboring area centered at the northern rim of the caldera. The stress finally fractured this area with intense seismicity, and produced fissures. Along these fissures, the magma that had penetrated into interstital space between rocks effused explosively with bubbling of steam. The magma had been more or less cooled and chemically differentiated in the interstitial space so that the ejecta from this fissure eruption was more felsic. The deflation of the magma reservoir due to the summit and fissure eruption resulted in a significant subsidence of the NE part of the island. Strain associated with opening of the volcanic fissures was transmitted through a strike-slip fault to the SE part of the island, and caused a local extensional stress and graben there.