The volcanoes are stratovolcanos that are 1 million to 200,000 years old. The rock is mainly basalt, dacite, and andesite. Mount Yoko has shown the most recent activity and is now considered an active volcano. It last erupted about 800 years ago. In 888 a debris avalanche on the East side of the volcanic massif caused Lake Matsubara to form. The Tateshina-kogen plateau was formed by a large lava flow.