The caldera on which Lake Shikotsu sits was formed 40 to 50 thousand years ago. According to the Global Volcanism Program, the caldera was formed 31 to 34 thousand years ago by one of Hokkaidō's largest quaternary eruptions. The caldera consists mainly of dacite, rhyolite, and andesite. The volcanoes Mount Tarumae, Mount Eniwa, and Mount Fuppushi formed on the rim of this caldera.