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Akan-Mashū NP, Hokkaido
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Roughly 1,500 fumaroles vent steam across Mount Io’s slopes, with sulfur crystals coloring the rocks vivid yellow. The steam can exceed 100°C—a reminder that this striking landscape is an active volcanic zone.
Mount Io’s geothermal activity warms the hot springs at nearby Kawayu Onsen, and that spring water is used to steam the area’s popular eggs. It’s a tasty connection between the volcano and everyday life in town.
Atosanupuri—also written Atosa-nupuri—means “bare mountain” in Ainu, describing its exposed, rocky face. The name is a vivid piece of local knowledge: the slopes really do look stripped of their green clothing.
Mount Io’s newest dome is young in geological terms: the Japan Meteorological Agency dates its formation to about 1,500–1,000 years ago. The latest known eruption in the volcanic group was a steam explosion a few centuries ago.
In 1887, a 41-kilometer railway was built from the sulfur mine toward Shibecha in just 8 months. It was Hokkaido’s second railway, and at first it carried mined sulfur—not passengers.
In the forest at Mount Io’s foot, an old rail from the sulfur-mining days was found caught in a tree as the trunk grew around and lifted it. A national-park ranger photographed this remarkable meeting of industry and forest.
Before commercial mining began, Ainu people used sulfur from Mount Io as tinder. The Kawayu Visitor Center identifies this practical use as the beginning of the mountain’s recorded human connection to sulfur.
Volcanic gases and acidic ground help create an unusual plant community around Mount Io: hardy highland species such as dwarf pine and Ezo isotsutsuji grow here at low elevation. The volcanic landscape shapes the vegetation as well as the view.
Mount Io’s sulfur became a major regional industry after mining began in 1877. It was exported to places including Qing Dynasty China and the United States, and used for products ranging from matches and gunpowder to insecticides and paper.
The sulfur boom also brought a grim chapter: the mine used prisoners from Kushiro Prison for labor. Teshikaga Town’s history records eye disease and blindness caused by sulfurous gas and dust among those who worked there.
