Abstract
The Middle Pleistocene Onoko volcano is situated at the southernmost part of the Northeast Japan volcanic arc. The volcanic history of the Onoko volcano can be divided into the following three stages.
Stage 1: Junigatake stratovolcano, a conical-shape volcano with total volume of 13km3 and 1800m high above sea level, was formed. It is composed of lava flows and volcaniclastic rocks of basaltic and andesitic composition. The crater was probably located at about 300m to the west of the summit of Mt. Onoko. Junigatake-minami explosion crater was formed at the western flank of this stratovolcano, and also the Nakanotake mass and the andesitic radial dikes intruded in this stage.
Stage 2: Collapse of the Junigatake stratovolcano took place, and a relatively small-scale caldera (about 2km in diameter) was formed at the summit of the stratovolcano, and small U-shaped valleys (about 100m deep and 1km wide) were also formed both at the southeastern and the northeastern franks of the volcano. In relation to this collapse, a debris avalanche occurred and the deposits are distributed on the foot of Onoko volcano.
Stage 3: Voluminous andesitic lava flows (Miyazawa lava and Onokoyama-minami lava) extruded and the Onokoyama mass intruded in the final stage.
Andesitic magmas most probably rose through E-W trending fault. This is suggested by the difference of the height of the Miocene basement rocks beneath the volcano.
