Towada volcano, which is located in the northeastern part of Honshu, is a typical double caldera of Krakatau type, about 10km in diameter and 500m deep. It is formed on the basement composed of the Tertiary rocks and the Pleistocene volcanic rocks. The volcanic activity of Towada volcano can be divided into two stages by the formation of calderas. At the first stage, the activity is represented by mafic to intermediate andesite. Then, this activity was followed by the eruption of the tremendous amounts of dacitic ash falls and pumice flows. Following this the outer caldera was formed by the collapse of the upper part of the stratovolcanic cone. At the second stage, the activity, which is much smaller than the first cone, is also represented by mafic to intermediate andesite. After this activity much andesitic pumice fall erupted in the Holocene age, resulting in a smaller caldera, called “Nakanoumi”. The rocks of each stage are divided into two rock series, namely pigeonitic rock series and hypersthenic rock series. The earlier eruption is pigeonitic series and the later is hypersthenic. In the first stage activity, the change from pigeonitic series to hypersthenic series is continuous by petrographicaly. A magma of hypersthenic series would be produced by a magma of pigeonitic series by fractional crystallization under higher oxygen partial pressure and lower temperature conditions.
