Magmatic Processes of The 2000 Miyakejima Volcanic Activity, Japan:

A Stoping Model

KAZAHAYA, K.1), UTO, K.1), SHINOHARA, H.1), SAITO, G.1),MORI, H.2), HIRABAYASHI, J.3),

MATSUSHIMA, N.1) and TAKADA, A.1)

1) Geol. Surv. Japan, 2) Japan Meteorological Agency and

3) Tokyo Institute of Technology Kusatsu-Shirane Volcano Observatory

1. Outline of Miyakejima volcanic activity

Miyakejima is a volcanic island about 10 km across, located at 150 km south of Tokyo on the Izu-Mariana arc (Fig. 1). Miyakejima is a basaltic composite volcano causing eruptions every 15-20 years. Volcanic activity in 2000 started in late June with the dike intrusion toward west associated with submarine eruption. A collapse crater began to be formed on July 8, 2000 at the summit with a small eruption. Amount of product is far less than that of the disappeared volcanic body. The crater intermittently grew to the size of 1.6 km width and 500 m deep as of early September. After some small eruptive activities, a large eruption occurred on Aug. 18 with plume height over 12 km. Since then volcanic gases has been released intensively. The average SO2 emission rates monitored by airborne COSPEC is about 40,000 ton/day from mid September to the present.

The current volcanic activity is summarized as follows:

a) The Island continued to be depressed possibly by the magma loss from a magma chamber as lateral dike intrusions. b) Considerable amount of essential materials has been included in the erupted materials suggesting the magma ascent to a very shallow level.

c) Intensive emissions of SO2 also suggest that undegassed magma is supplied to shallow environments.

Plume from the summit crater of 1.6km diameter Miyakejima volcano;

photos taken by JMA COSPEC team

Plume from Miyakejima volcano;

photos taken by JMA COSPEC team