Thermal Activity of Volcano

Abstract

Volcanic eruption is a discharge process of energies from the earth’s interior in the form of thermal energy of hot erupted products, kinetic energy of explosive movements, seismic energy, etc. Since the thermal energy constitutes a major part of the total energy discharged by eruptions, the studies on volume of volcanic products have called our attention, and lead the keynoting results on a spatial distribution of the volume of volcanic products (i.e. thermal energy discharge) and contribution of volcanic eruption to the global energy discharge rate, etc. It has not been well recognized, however, that volcanoes discharge significant amount of thermal energy even at a non-eruptive stage comparable with those energies in eruptions. In recent years, some remote measurement methods of heat discharge rate have been developed for inaccessible areas such as crater bottom or intense fumarolic zones, to which the earlier estimation methods were not applicable. Those methods were applied to the major active volcanoes mainly in Japan and heat discharge rates were estimated there. Non-eruptive heat discharge rates are compared with the eruptive energy discharge rates or with the volume of volcanoes to derive the eruptive energy discharge, and the result is summarized as follows. 1) Non-eruptive heat discharge rates around Northeast Japan Arc and Southwest Japan Arc are estimated to be 79 MW/100 km and 120 MW/100 km, respectively. These values are of the same order of magnitude as the thermal energy discharge rate at the eruptive stage estimated from the volume production rate of volcanoes. 2) Spatial distributions of non-eruptive heat discharge are similar to that of eruptive energy discharge, which has already been established. Namely the discharge is highest just behind the volcanic front, and rapidly decreases toward the inner side. 3) The ratio of eruptive and non-eruptive energy discharge reflects the regional stress. The heat source to maintain the non-eruptive heat discharge may be stored more easily under a tensile stress field but rather hard under a compressive stress field. Viscosity of magma is also expected to be another important factor, even though positive evidence is not presented in this review. Our knowledge about the heat source of non-eruptive thermal activities and the chronology of volcanic activities is insufficient to a better understanding on an energy discharge process from volcanoes, including the relations of intrusive and extrusive magmatism and their quantitative geological meanings.