三宅島火山2000年陥没カルデラ

概要

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The activity of Miyakejima Volcano, in the Izu Arc, commenced on June 26, 2000AD, resulting in the formation of the 1.6km-across "2000AD Caldera," or "New Hatchodaira Caldera." This summit subsidence caldera duplicated the position and size of the Hatchodaira Caldera of 2500y.B.P., probably utilizing the former caldera structure. In this paper we describe these two caldera-forming activities and discuss the mechanisms of caldera formation. The volume of magma discharged during the Hatchodaira eruption was 3.7×108m3 (DRE) comprising a new batch of basalt magma mixed with preexisting andesite magma. The 2000AD event, on the other hand, was accompanied only by 1.1×107m3 of ejecta in contrast to 6×108m3 of the missing volume. The major reason for the 2000AD subsidence is thought to be that the magma in the reservoir underneath Miyakejima Volcano intruded to form dykes in the adjacent area of Niijima and Kozushima, 30 km northwest of Miyakejima. The explosive events in 2000 were caused passively by decompression in the magma-evacuated reservoir, rather than active magma itself. This sequence of the 2000AD event had never been experienced by Miyakejima Volcano during the geologically described last 10,000 years.