It is said that Mount Usu's basic ground shape was formed at an eruption about 20,000 years ago, whereas its peak is said to have collapsed about 7-8000 years ago to its present day shape. The volcano had already 4 eruptions in the 20th century alone, and it is said its eruption cycle is approximately 30 years long. A recommended spot is the main crater from the 1977 eruption, which you can see once you take the Usu Ropeway up to Mount Usu and walk a couple of 100m to the observatory deck.