The Shikaribetsu Volcanic Group
Volcanoes surround much of Lake Shikaribetsu. Kita-Petoutoru (1,400 m) and Minami-Petoutoru (1,348 m) north and west of the lake are among the oldest. They formed over the course of 200,000 years from approximately 300,000 BP. The cluster of volcanoes stretching east-west along the southern side of the lake was formed more recently, between 60,000 and 10,000 BP. Prolonged periods of sustained volcanic activity gave rise to the Shikaribetsu volcanic group and changed the face of the Shikaribetsu area.
Stratovolcanoes and lava domes
The Shikaribetsu volcanic group is made up of stratovolcanoes and lava domes. Stratovolcanoes tend to be conical, with relatively steep sides. They are built up by many layers of hardened lava, ash, and rock from mostly explosive lava eruptions. Kita-Petoutoru (1,400 m) and Minami-Petoutoru (1,348 m) are stratovolcanoes.
The younger volcanoes along the southern side of the lake are lava domes. These are formed by relatively small, bulbous masses of lava too viscous to flow any significant distance. This thick lava piles up around the vent, and the dome grows gradually. Lava domes typically emit lava steadily onto the ground through effusive eruptions. Explosive eruptions can occur, however, when gas pressure builds up within the dome.
The Shikaribetsu Volcanic Group Observatory offers a panorama of the volcanoes, with the forms of Nishi-Nupukaushinupuri (1,251 m) and Higashi-Nupukaushinupuri (1,252 m) clearly visible.
Lava dome collapse
The sides of a growing lava dome are steep and unstable, and they sometimes collapse. A lava dome collapse can be triggered by earthquakes, the buildup of gas pressure, or other factors including new growth. When a dome collapses, it can cause pyroclastic flows and debris avalanches that can move several kilometers outward from the dome. Over time, a new dome forms from the partial remains of the collapsed dome. This process has occurred repeatedly among the lava domes of the Shikaribetsu volcanic group, and the debris from past collapses has formed many small hills across the northern part of the plain. The observatory at the Higashi-Urimaku Hummocky Hills geosite offers views of the undulating landscape with the volcanic group in the background.
Lava domes and Lake Shikaribetsu
Shikaribetsu’s lava domes have had a significant impact on the landscape. As they formed, they blocked the path of a river, resulting in the creation of Lake Shikaribetsu. The lake covers 3.4 square kilometers and has 13.8 kilometers of shoreline. The original lake, however, would have been larger and extended farther north and west. The summit of Minami-Petoutoru (1,348 m) rewards hikers with views of the lake, the lava domes to the south, and the Tokachi Plain in the distance.
