Lava flows are streams of molten rocks that flood the land surface like a river, instead of being broken pieces, when a volcanic eruption occurs. The form of lava flows depend on the magma’s quality and quantity. While smooth textured lava flow can form a volcanic dome, mildly textured lava flow can form volcanic sheets like at the foot of Mt. Fuji. The lava flow at Mt. Ontake represents the latter.

Mt. Ontake is formed by many layers of ejecta, such as a lava flow. The lava flow at Kusakidani Gorge streamed down from Marishitensan peak and covered the Hyouendani area about 60,000 years ago. It stretches about 15 kilometers up to around Gantate area now.