Mount Haruna started to form more than 300,000 years ago and the last known eruption was c. 550. The volcano has a summit caldera containing the symmetrical pyroclastic cone of Mount Haruna-Fuji, along with a crater lake, Lake Haruna, along the western side. To the west of the lake is Mount Kamonga, the tallest of Mount Haruna's numerous peaks at 1,449 m high.