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Daisetsuzan NP, Hokkaido
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At nearby Tenninkyo Gorge—not inside the caldera—geologists found river-laid gravel between 2 Ohachidaira pyroclastic-flow deposits. That quiet layer suggests 2 huge eruptions, at least 100 years apart.
A likely Ohachidaira ash layer has been identified about 137 km east, near Kiyosato in Shari. The caldera-forming eruption left traces much farther away than the mountain valleys.
Ohachidaira’s roughly 2-km-wide basin formed in an eruption that unleashed about 8 km³ of pyroclastic flows into the Ishikari and Chubetsu valleys. Deposits up to 200 m thick later hardened into column-jointed cliffs.
The map name 有毒温泉 (Yudoku Onsen) means “poison hot spring.” Toxic gas still vents from the caldera floor, which park guidance says to avoid; hydrogen-sulfide accidents killed 2 students in 1958 and 2 in 1961.
Lake-bottom sediments on Ohachidaira’s floor show that this apparently dry volcanic bowl once held a lake—a quiet, watery chapter tucked between its explosive birth and its present-day gas vents.
Microscopic crystals in the pumice and scoria suggest that dark, mafic magma mixed with silica-rich melt in the chamber before and during the eruption—a surprising recipe behind this caldera-forming blast.
The caldera wasn’t the end of the story: Asahidake grew on its southwest edge afterward, and its present summit formed about 5,000 years ago—much younger than Ohachidaira itself.
