Degassing activity from Iwodake rhyolitic cone, Satsuma-Iwojima volcano, Japan: Formation of a new degassing vent, 1990–1999

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 SO 2 , 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 SO 2 flux and fumarolic temperatures, ground deformation, and the vent formation.

High SiF4/HF ratio detected in Satsuma-Iwojima volcano’s plume by remote FT-IR observation

“…(3) Addition of SO 2 from burning sulfur crust. The high-temperature fumaroles in the Ohachi-oku area have molar SO 2 /HCl ratio of less than 2.2 in 1996 (Shinohara et al, 2002). We do not have any evidence of high SO 2 /HCl fumaroles with a large flux in 1996.…”

“…* 2 The SO 2 fluxes are measured by COSPEC (Satsuma-Iwojima, Shinohara et al (2002) and Shinohara (Personal Communication); Aso, Ohta et al (1988); Vulcano, Allard et al (1993); Etna, Caltabiano et al (1994); Popocatépetl, Love et al (1998)). …”

“…They also suggest that typical sulfur flow combustion at Vulcano volcano would generate SO 2 flux of 0.6-44 t/hr (Harris et al, 2000). Shinohara et al (2002) report that the crater floor of Mt. Iwodake was heated up in the mid-1990's, and large area of the crater floor became altered zone whose temperature exceeded 500 • C at 5 cm depth in October 1996.…”

Evaluation of recent activity at Satsuma-Iwojima —Felt earthquake on June 8, 1996—

“…The seismicity at the volcano has been stable for the last 20 years. The stable seismicity at Satsuma-Iwojima volcano coincides with stable volcanic activity estimated by analysis volcanic gas and temperature measurement of fumarolic gas (Shinohara et al, 2002).…”

“…9. Shinohara et al (2002) interpreted an increase in SO 2 flux, and maximum and equilibrium temperatures of volcanic gas from the summit crater in mid-1990's as caused by ascent of a magma column. Pressure beneath the crater temporarily increased as a result of ascent of the magma column, however, the pressure may have decreased after a large amount of volcanic gas was emitted.…”