Geologic Study on the Myoko Volcanoes, Central Japan : Part 1. Stratigraphy

Abstract

The Myoko volcanoes constructed from the Yakeyama, Myoko, Kurohime, Iizuna, Madarao, and Sadoyama volcanoes, are situated in the boundary area between Nagano and Niigata prefectures, central Japan. Most of the volcanoes are distributed along the anticlinal or synclinal axis of the basal Neogene strata. Except for the older volcanoes, Sadoyama and Madarao, the volcanic cones form a north-south trend, arranged at nearly equal intervals of about 8 km, and become younger in age toward the north. The migration of eruptive activity toward the north seems to be closely related with that of the movement of the underlying rock mass. "MYOKO" are the typical stratovolcano, except for the dome-shaped Yakeyama volcano. In particular, three volcanoes, the Myoko, Kurohime, and Iizuna, are "double" stratovolcano provided with a small summit caldera. They are similar to each other in shape, but are different from each other in growth history, petrography, and other characteristics. The Myoko volcano consists of an older and a younger volcanoes, and furthermore, the younger one stratigraphically consists of four groups, which correspond petrographically to a basalt-andesite series, respectively. The Iizuna volcano consists of two groups of two basalt-andesite series, and the Kurohime volcano of a single group belonging to a basalt-andesite series. The mode of eruption in each group tends to change as follows; eruption of scoria fall by basaltic magma→alternative eruption of lava and bomb by basic to intermediate andesite magma→eruption of a somewhat volumious pyroclastic flow associated with viscous lava flow by comparatively acidic hornblende andesite magma. Moreover, the volcanic mud flow deposit also tends to concentrate at the middle to upper part in each group.