Numerical Simulation of Magma-hydrothermal System at Iwodake Volcano, Satsuma- Iwojima, Japan

The thermal activity of Iwodake volcano, Satsuma-Iwojima, south of Kyusyu, Japan, is characterized by the predominant volcanic gas emission at the summit crater and the heat discharge from high ground temperature area, which is distributed widely from the summit crater to hillside of the mountain. The volcanic gas at the summit crater is of magmatic origin and its temperature is observed to be 880 °C in maximum. The total amount of the volcanic gas discharged from the summit area is estimated to be 200kg /sec from the SO2 measurement and the chemical composition of the volcanic gas. The heat discharge rate from the high ground temperature area at the hillside is estimated to be 80MW from the surface temperature measurement. These thermal activities of Iwodake volcano are thought to be continued for more than 800 years. The continuous active degassing causes the hydrothermal system within the volcano because the volcanic gas, ascending the conduit, is diffused to the surrounding formation. The development of such hydrothermal system is studied using the mathematical simulation. The simulator accounts for mass and heat convection of liquid water and/or vapor within a porous media. In this calculation, the degassing is considered as the source at the top of the columnar magma with constant high temperature. The results show that the overall thermal activity of Iwodake volcano such as the volcanic gas ejection at the summit crater, widely distributed ground temperature anomaly at hillside can be caused by the volcanic gas flow which is diffused from the vent. The important factors in order to induce the wide-ranging hydrothermal system are permeability of the volcanic edifice and the depth of the degassing. The simulation indicates that the permeability of 0.1 darcy and the degassing above sea level are necessary condition for the Iwodake thermal activity.