Petrological studies of the Usu volcano

Abstract

Usu, an active volcano situated in southwestern Hokkaido, consists of a somma, central lava domes, parasitic lava and crypto-domes. In early Holocene, on the southern wall of the Toya caldera, Usu volcano was constructed as a stratocone by repeated eruptions of lava flows and scoriae of basalts and andesites. After a quiescence, its summit was broken by violent explosions which led to the formation of a small caldera. Then the composition of magma changed to dacitic, and consequently the type of activity was converted into formation of lava domes and explosive eruptions of pumice or volcanic ash. Such activities have taken place intermittently also in historic times. Showa-Shinzan is the latest lava dome which protruded on the eastern foot of this volcano in 1943-1945. In the present paper, petrology of the somma lavas is mainly concerned. Compositions of the somma lavas varied gradually from augite-hypersthene-olivine basalt to pigeonite-hypersthene andesite, in accordance with the sequence of eruption. Simultaneously the composition of olivine and Ca-poor pyroxene, contained in the later somma lavas, became richer in Fe, and finally pigeonite crystallized out as phenocryst. Mineralogical and chemical studies on the somma lavas show that the somma lavas were derived from tholeiitic magma by fractional crystal-lization. Dome lavas are all very similar, but have quite different characters from those of the somma lavas both in mineralogy and chemistry. Another interesting problem about Usu volcano, is what kind of process took place during the crystallization of magma at the stage of sudden change of character as above mentioned. The writer describes some petrographical observations on the dome lavas, and he considers that contamination of granitic rocks by mafic magma is most suitable to explain these observations and sudden change in the composition of magma.