Chokai volcano is the largest and highest (2230m) among the volcanoes belonging to the Chokai volcanic zone, and situated from 135°50'E to 140°07'E in Long. and from 39°15'N to 39°00'N in Lat. This volcano is composed of the west and the east volcanoes, both of which are double stratovolcanoes, built up on the Tertiary formation throughout the six stages of activities. The historical activities of this volcano was recorded in 861, 871, 884, 915, 1740, and 1802.
The rocks found in this volcano are classified into two series. One is the high alumina rock series derived from high alumina basalt magma through the differentiation, and includes augite olivine basalt, hypersthene bearing olivine augite andesite, and hypersthene olivine augite andesite. The other is the talc-alkali rock series derived from high alumina rock series through the assimilation. Cale-alkali rock series is devided into two types, hornblende-bearing type and hornblende-free type. The former includes hornblende-bearing hypersthene olivine augite andesite, hornblende-bearing hypersthene augite andesite, and hornblende hypersthene augite andesite and the latter includes olivine hypersthene augite andesite, olivine-bearing hypersthene augite andesite, and hypersthene augite andesite. It is characteristic that most of the hornblende-bearing type are found in the west Chokai volcano except Hottai lava flow which belongs to the east Chokai volcano.