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

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.