Understanding active volcanoes: the case of Usu Volcano, Japan, with emphasis on the 1977 summit eruption

Abstract

Understanding the behavior of an active volcano requires a multidisciplinary analysis of its past and present activities. The aim of the present study is to explain the eruption mechanisms of Usu Volcano, which is one of the smallest active volcanoes of the world, yet a hazardous one as it has produced various types of eruptive activity such as pumice, ash and pyroclastic ejections, and the formation of lava domes at the summit and at its base. The past activity of the volcano, particularly the pyroclastic eruptions, is briefly discussed on the basis of the published papers and geologic maps. Usu is a dacitic volcano that has produced summit and basal eruptions from parasitic vents, as those of 1910 and 1943. The former was phreatic and the latter resulted in a lava dome formation. The 1977 eruption began with a pumice eruption at the summit, and phreatic eruptions continued for over 5 years. The 2000 eruption was phreato-magmatic with the outbreak at the NW base. The eruptions of this volcano since 17th century show a 57-year mean recurrence time. The 1977 eruption is one of the largest and was a typical case of the summit eruptions. This compilation of the various geophysical observations of this eruption will contribute in the understanding of its internal structure and of the eruption mechanisms of this volcano.