Earth Planets Space, Vol. 54 (No. 3), pp. 175-185, 2002
Hiroshi Shinohara, Kohei Kazahaya, Genji Saito, Nobuo Matsushima, and Yoshihisa Kawanabe
Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba 305-8567, Japan
(Received June 14, 2000; Revised August 15, 2001; Accepted September 17, 2001)
Abstract: Large changes in the surface manifestation of degassing activity were observed from 1990 to 1999 at the summit crater of Iwodake cone of Satsuma-Iwojima volcano. During this period, a new high-temperature fumarolic area formed in the center of the crater floor and became a degassing vent with a diameter of 40 m. Altered volcanic rocks were ejected during the course of vent formation. Although glass fragments were observed in the ejected ash, the glass comes from altered Iwodake rhyolite that covers the crater floor. The highest fumarolic temperature and equilibrium temperatures of volcanic gases had a maximum of about 900°C at the beginning of the vent formation. The flux of SO2, measured by COSPEC, varied from 300 to 700 ton/day and correlated directly with maximum fumarole temperature. During this period, open fractures formed along the southern rim of the crater almost contemporaneously with the vent formation and changes in the nature of fumarolic discharges. The continuous and intense degassing at Satsuma-Iwojima is likely caused by volatile transport from a deep magma chamber through a convecting magma column. An increase in the magma convection rate might have caused these large changes in surface manifestations, including increase in the SO2 flux and fumarolic temperatures, ground deformation, and the vent formation.