Study on Magma-hydrothermal System in Iwodake Volcano, Satsuma-Iwojima, by Numerical Simulation

Abstract

The thermal activity of Iwodake volcano is characterized by the predominant volcanic gas ejection 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 400kg/s from the SO₂ flux measurement and the chemical composition of the volcanic gas.The heat discharge rate from high ground temperature area is estimated to be 46MW at the summit and 83MW at the hillside of the mountain, respectively.These thermal activities of Iwodake volcano is thought to be continued for more than 800 years. A plausible model to explain the long degassing of large amount of volcanic gas is magma circulation in a conduit which is extended from the deep-seated magma reservoir. The continuous active degassing probably causes the hydrothermal system within the volcano because the volcanic gas ascending the vent from the top of the magma to the surface is diffused to the surrounding formation. The development of such hydrothermal system is studied by using the numerical simulation. The result shows 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 and hot springs along the coast can be caused by the degassing activity. 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 10^-13m² at the mountain edifice, the permeability of 6×10^-11m² at the vent, and the degassing above sea level are suitable conditions for the Iwodake thermal activity.