草津白根火山周辺の火山ガスの成分的特徴とその由来について

抄録

The distribution and chemical compositions of the volcanic gases in Kusatsu-Shirane volcano district have been investigated since 1967. The volcanic gases in this district can be classified into fumarolic gases, hot and cold spring gases and “low temperature” fumarolic gases. The temperatures of most fumarolic gases lie in a range of 94-99°C. These gases contain H₂S, CO₂ and R gas (mainly N₂) as main components besides water vapor. In some cases SO₂ and HCl are detected as minor components. Those gases show the local characteristics and the chemical compositions of each gas lie in a certain range. The volcanic gases evolving from several places of the bottom of the crater lake Yugama were collected on the surface and analysed. There is a certain relationship between the chemical compositors of these collected gases and the depths of the gas-evolving sites and these facts may be explained by the differences of the solubility of each component into the lake water. The chemical variations of the hot spring gases can be explained also by considering the contacts of the ascending fumarolic gases and the subsurface water flow. The very high content of H₂S in the volcanic gases of Manza area are noteworthy and we should consider some secondary reasons. Starting from several initial chemical compositions of fumarolic gases we tried some calculations on the differentiations of these gases based on the differences of the solubility of each component and found fairly good agreement with the actually observed tendencies. Finally we offered the reason of the existence of the “low temperature” fumarolic gases.