PETROLOGY AND LIQUIDUS TEMPERATURE OF THE MAGMA OF THE 1970 ERUPTION OF AKITA-KOMAGATAKE VOLCANO, NORTHEASTERN JAPAN

Abstract

The eruption of Akita-komagatake volcano in northern Honshu during the period September, 1970-January, 1971, was characterized by the steady “Strombolian” type of eruption and the outlet of lava flows. The new magma was augite-hypersthene andesite (Si02=58%, mineralogy Vd→c) and definitely more acidic than basalts composing the Medake cone at the top of which the eruption occurred. Chemical compositions of the whole rock, groundmass, and glass are given together with microprobe analyses of augite, hypersthene, Ca-poor clinopyroxenes, plagioclase, titanomagnetite, etc. Ca-poor pyroxenes are the only pyroxene phases crystallizing in the stage immediately before and after the eruption. The wide scatter in composition of these Ca-poor pyroxenes probably reflects metastable condition of crystallization. Controlled-atmosphere, high-temperature experiments on ejecta of 1970 eruption and artificial mixtures simulating groundmass glass composition indicate that the liquidus temperature of the magma at the time of eruption was close to 1100°C, which is near the highest observed temperature (1090°C) in the field. The plagioclase liquidus of the whole rock and groundmass composition is much higher than that of the pyroxenes while in the groundmass glass composition the difference is very small. The difference is attributed to the difference in water pressure of the magma in the reservoir and that at the time of eruption.