Mount Usu is a volcanic mountain located south of Toyako and towers 737 meters high.
On the top of the mountain is Sobetsu-cho, and the mountain straddles Toyako-cho and Date-shi. Over the past 100 years, there have been four recorded eruptions, and Mount Usu is one of the few active volcanoes in Japan.
It is a double volcano, and among the mountains forming the outer ring, which is around 1.8 kilo meters across, there is Dai-Usu, Ko-Usu, Mount Ogari, and Usu-Shinzan, which form a group of lava domes. At the foot of the mountain, there is another group of lava domes, including Showa-Shinzan, Konpira-yama, and Meiji-Shinzan.
Volcanic activity since 1663 has been due to highly viscous magma that includes SiO2, and before an eruption, deformations in the ground appear and earthquakes occur in a series. New mountains are formed through latent domes and lava domes when eruptions occur.
(1) Monstrous pyroclastic flows
Around 110 thousand years ago, there was violent volcanic activity around the present area of Toyako, generating a monstrous pyroclastic flow (fast-moving currents of hot gas, ash, and rock). This flow stretched to the Sea of Japan, and volcanic ash fell as far away as central Tohoku region.
(2) Birth of Toyako
Between 110 and 50 thousand years ago, the area was buried by a pyroclastic flow generated by a major eruption, turning the whole area into a flat plain. Water accumulated in the caldera formed by the eruption, and this was the prototype of Toyako.
(3) Appearance of Nakajima
Around 50 to 40 thousand years ago, volcanic activity increased at the center of the Toya caldera, and there were repeated eruptions of viscous magma, forming several lava domes. This is the present Nakajima.
(4) Activity of the stratovolcano of Mount Usu
Around 20 thousand years ago, Mount Usu formed on the southern wall of the Toyako caldera. This steadily grew due to repeated eruptions of viscous magma, forming a composite volcano with a gentle plain that is very similar to Mount Yotei.
(5) Collapse of the Mount Usu shape
Around 8 to 7 thousand years ago, there was a major collapse of the south-west face of Mount Usu as a result of volcanic activity. The landslide of rocks generated by the collapse flowed into the Volcano Bay triggering a tsunami, which left a mountain formed from a mudflow on the south-west foot of Mount Usu.
(6) Reactivation of Mount Usu
In 1663, Mount Usu once again started to erupt, and since the magma was extremely viscous, it formed several lava domes, such as Dai-Usu and Showa Shinzan. The pyroclastic flow washed down the side of the mountain.