Hiruzen is a dormant, but not extinct, volcano located to the southeast of Mount Daisen. The three peaks of Hiruzen, along with a few smaller peaks, form the Hiruzen volcanic belt, a subset of the Daisen volcanic belt. An eruption of Daisen occurred approximately 350,000 years ago, and created a lake, known as Lake Hiruzenbara, above the Hiruzen Plateau. The lake supported an extensive population of diatoms for nearly 50,000 years, and Diatomaceous soil on the plain reaches deep in some areas. Lake Hiruzenbara was gradually eroded on its eastern perimeter by the Asahi River, and dried up and disappeared over time.