Abstract
Recent activity of Usu Volcano began on December 28, 1943 with severe earthquakes at the northern foot of the volcano, and ended in September, 1945, with the formation of Syowa Sinzan, a new parasitic volcano on the eastern foot. This activity was characterized by a series of severe earthquakes of various kinds, marked topographical deformation, paroxysmal eruption, and protrusion of a mass of juvenile magma. Syowa Sinzan is composed of a dome of new lava, protruding from circular, platform‐shaped “Roof Mountain,” which is an upheaved part of the ground, and consists mainly of Somma lava of Usu Volcano. The new lava is a hypersthene dacite, similar to the lavas of the other domes of Usu, and the Somma lava is an olivine‐augite‐hypersthene andesite, sometimes including large crystals of calcic plagioclase. The petrographic significance of the new lava and its intimate relation to the mode of activity of the volcano is discussed.
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