Geochemical Study on Ground Water, Thermal Water and Ash Fall Collected Immediately after the 1977 Eruption of the Usu Volcano, Japan

Abstract

Emergent studies on some water and ash samples collected immediately after the August 7,1977 eruption of Usu volcano have been made by a joint group of geochemists. This paper reports a part of the results obtained for the samples collected in August and September. The items measured were temperature, pH, major and minor dissolved chemical components including radon, hydrogen and oxygen isotope ratios for ground water, thermal water around the Usu volcano and fumarolic condensates of the Showashinzan, the adjacent newer volcano to the Usu, chemical and mineral composition of ash including its soluble components. Through a short period of observation, we could not find any significant change in chemical and isotopic compositions of a variety of waters after the eruption. However, a rough idea of hydrothermal and hydrological situation around the Usu volcano has been obtained. In this comection, we set standing stations for water sampling and the observation through these stations has been still continued. Ash fall contained montmorillonite and chlorite. This fact indicates that clay minerals formed beneath the crater during the dormant period was expelled together with pumice originated from the fresh magma pertinent to this eruption. When the chemical composition of the water-soluble components of the ash is also taken into consideration, temperature of the coexisting gas phase with ash is suggested to be fairly high.