Harunasan volcano is truncated by a small summit caldera containing the symmetrical post-caldera pyroclastic cone of Haruna-Fuji. Harunasan volcano dates back prior to 300,000 years ago and had caldera-producing eruptions at about 200,000 and 40,000 years ago. Viscous lava flows and lava domes were subsequently extruded within and around the caldera, the western side of which is currently partially filled by Lake Haruna. The Futatsudake lava dome east of the caldera was the source of two large explosive eruptions during the 6th century CE.