Genji Saito

Excessive degassing of Izu-Oshima volcano : magma convection in a conduit

Excess degassing of magmatic H2O and SO2 was observed at Izu-Oshima volcano during its latest degassing activity from January 1988 to March 1990. The minimum production rate for degassed magma was calculated to be about 1×104 kg/s using emission rates of magmatic H2O and SO2, and H2O and S contents of the magma. The minimum total volume of magma degassed during the 27-month period is estimated to be 2.6×108 m3. This volume is 20 times larger than that of the magma ejected during the 1986 summit eruption. Convective transport of magma through a conduit is proposed as the mechanism that causes degassing from a magma reservoir at several kilometers depth. The magma transport rate is quantitatively evaluated based on two fluid-dynamic models: Poiseuille flow in a concentric double-walled pipe, and ascent of non-degassed magma spheres through a conduit filled with degassed magma. This process is further tested for an andesitic volcano and is concluded to be a common process for volcanoes that discharge excess volatiles.

Degassing process of Satsuma-Iwojima volcano, Japan: Supply of volatile components from a deep magma chamber

Satsuma-Iwojima volcano continuously releases magmatic volatiles from the summit of Iwodake, a rhyolitic lava dome. The temperature of fumaroles is high, between 800° and 900°C, and the water-rich composition of volcanic gases has not changed essentially over the past 10 years. Sulfur dioxide flux measured by COSPEC is almost constant with an average of 550 t/d since 1975. The present volcanic gas is likely degassed from a rhyolitic magma whose composition is similar to that erupted in 1934, 2 km east of Satsuma-Iwojima. Comparison of silicate melt inclusions and volcanic gas compositions indicates that the magma degassing pressure is very low, implying magma-gas separation at a very shallow level. The mass rate of magma degassing is estimated at 10 m3/s using the volatile content of the magma and the fluxes of magmatic volatiles. The rhyolitic parental magma is volatile-undersaturated in the deep magma chamber, as suggested by melt inclusion studies. Magma convection in a conduit, driven by the density difference between higher density degassed and lower density non-degassed magmas, explains the high emission rate of magmatic volatiles released at shallow depth from such a magma chamber, that is gas-undersaturated at depth. Model calculations require the conduit diameter to be greater than 50 m as a necessary condition for convection of the rhyolitic magma. Long-term convective degassing has resulted in the rhyolitic magma in the deep chamber to become depleted in volatile components. The melt-inclusion studies indicate that the rhyolitic magma responsible for discharging the present volcanic gas has been degassed during the long degassing history of the volcano and is now supplied with CO2-rich volatile components from an underlying basaltic magma. The total volcanic gas flux over 800 years requires degassing of 80–120 km3 of basaltic magma.

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.

Mass and heat flux of volcanic gas discharging from the summit crater of Iwodake volcano, Satsuma-Iwojima, Japan, during 1996–1999

Abstract Volcanic gas of Iwodake has been discharged continuously from high-temperature (900°C maximum) fumaroles and a degassing vent which formed on the central floor of the summit crater after 1994. Although most of the high-temperature fumaroles located on the crater wall before 1991, many fumaroles appeared on the crater floor associated with the vent opening, suggesting a shift of thermal activity from the peripheral to the central crater. A large amount of volcanic gas has been discharged from the fumaroles and vent. Since heat has been transferred from ascending gas to the surrounding soil, a region showing a surface temperature anomaly has developed around the fumaroles and vent. To quantify Iwodake thermal activity, heat and volcanic gas mass fluxes were estimated during 1996–1999 using infrared thermal images, and plume velocity and temperature data which were observed by a pitot tube and thermocouple. The estimated gas mass flux was compared with the COSPEC data to investigate the accuracy of our estimation. The gas mass flux had been decreased from 230 kg/s in October 1996 to 30 kg/s in November 1999. Although the vent diameter had increased from 20 to 70 m during the same period, this mass flux variation had indicated the decline of degassing rate. The degassing depth was estimated from the volcanic gas mass flux and temperature. The depth showed a tendency of magma head migration to shallower depth during 1996–1999, which was consistent with the drastic change of the surface manifestation.

Degassing process of miyakejima volcano: implications of gas emission rate and melt inclusion data

Abstract Various techniques of volcanic plume studies were applied to monitor degassing activity of Miyakejima volcano, Japan, including SO 2 emission rate measurement with COSPEC, heliborne-CO 2 /SO 2 ratio measurements, Cl/S ratio measurements with alkaline-trap methods. The degassing activity of Miyakejima, that started mid-2000, was characterized by its larger emission rate (up to 40 kt/d SO 2 ), gradual decrease of the emission rate and constant composition even during the emission decrease. Composition of the volcanic gas was estimated by combination of these plume studies. Volatile contents of the magma were estimated by the analyses of melt inclusions in the Miyakejima 2000 basalt, and close similarity was found between the estimated melt composition and composition of the volcanic gas, indicating a shallow degassing of the magma. The results of volcanic plume and melt inclusion studies were integrated to evaluate the degassing process of the volcano, and the intense degassing was modeled based on the volatile-transport through a convecting magma conduit

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.

Volatile evolution of satsuma-iwojima volcano: Degassing process and mafic-felsic magma interaction

Abstract Petrological and geochemical studies of volcanic rocks and melt inclusions of Satsuma-Iwojima volcano, Japan, were carried out to investigate the volatile evolution of the magma chamber since its latest caldera-forming eruption. Petrological studies on basalts, rhyolites and mafic inclusions in the rhyolites from post-caldera eruptions suggest there is a stratified magma chamber beneath the volcano, which consists of a lower basaltic layer, upper rhyolitic layer and an episodically-present, thin middle-andesitic layer. Chemical analyses of 30 melt inclusions in plagioclase and pyroxene phenocrysts from the basaltic and rhyolitic eruptions showed large variations in the volatile concentrations (H 2 O, CO 2 , S and Cl) of the melts. This suggest the following volatile evolution processes in the magma chamber: 1) a gas-saturated condition due to pressure variation in the rhyolitic magma chamber just before the caldera-forming eruption; 2) low pressure degassing of the rhyolitic magma chamber by magma convection in the conduit during the active degassing period in the post-caldera stage up to the present; and 3) the addition of CO 2 -rich volatile from the underlying basaltic magma in the stratified magma chamber to the upper gas-undersaturated (degassed) rhyolitic magma.