Yoshihisa Kawanabe

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

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.

Volcanic activity of the Satsuma-Iwojima area during the past 6500 years

Satsuma-Iwojima is a small volcano island located on the northern rim of Kikai caldera to the south of Kyushu, southwest Japan. Observations of new outcrops and 14C dating of the tephra layers have revealed post-caldera activity in the Satsuma-Iwojima area. After the large-scale ignimbrite eruption in 6500 y.B.P., volcanic activity was resumed with rhyolitic activity. At the foot of Iwodake, post-caldera tephra layers are divided into eight units by the development of humic soils. K-In-1 and -2 were formed by basaltic activity with phreatomagmatic eruption around 3900 y.B.P. and had ended by 2200 y.B.P. Other tephra layers (K-Sk-l and K-Sk-u) are rhyolitic ejecta with an increasing proportion of silicified fragments in the younger tephras. On the slope of Iwodake, there are also some pumice fall deposits and pyroclastic flow deposits (K-Iw). From the 14C data of K-Iw, the most recent magmatic activity of Iwodake was around 600−500 y.B.P.

Mafic-felsic magma interaction at Satsuma-Iwojima volcano, Japan: Evidence from mafic inclusions in rhyolites

Geochemical and petrographic studies of the rhyolites and mafic inclusions from Satsuma-Iwojima volcano were carried out in order to investigate evolution of a silicic, bimodal magma system during the post-caldera stage. Abundant mafic inclusions, which are fine-grained with vesicles in their cores, are present in the Showa-Iwojima rhyolitic lava. Inclusions with similar textures are found in Iwodake volcanic bombs but are less common than in the Showa-Iwojima lava. The major and trace element compositions of the inclusions plot along mixing lines connecting the host rhyolites with spatially and temporally associated basaltic to basaltic andesite magmas. Plagioclase phenocrysts in the inclusions have a large variation in core compositions (An42 to An96), and exhibit various zoning profiles and reaction textures, indicating they coexisted with melts ranging from basaltic to rhyolitic composition. Pyroxenes also exhibit a wide range in composition and a variety of zoning patterns consistent with multiple sources. These results suggest that a stratified magma chamber exists beneath the volcano, consisting of a lower basaltic layer, an upper rhyolitic layer and an episodically-present, thin middle layer of andesite. Variations in the chemistry of the Iwodake and Showa-Iwojima mafic inclusions suggest that multiple injections of very similar basaltic magma have occurred since the growth of the Iwodake dome. More extensive textural disequilibrium shows that the Showa-Iwojima rhyolites formed through more extensive interaction with mafic magma. The mafic-felsic interaction is consistent with degassing model of a magma chamber estimated by other researchers, which consists of degassing of upper rhyolitic magma by convection in a conduit and supply of a CO2-rich volatile phase from underlying basaltic magma to the rhyolitic magma.