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Chubu-Sangaku NP, Toyama / Nagano
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Washiba Pond’s 250–300 m crater cuts the southeast flank of 2,924 m Washiba-dake, a mostly granitic mountain—not a volcanic cone. Lava from this flank volcano flowed as far as the Yumata River, near 1,850 m elevation.
Some Edo-period maps label what is now Washiba-dake 龍池ヶ岳, “Dragon-Pond Peak.” Historical gazetteers suggest the name honored this crater lake, then called 龍池—a hint of the reverence its high-alpine waters inspired.
Peat near Washiba’s crater preserves a thin layer of Kikai-Akahoya ash, fallout from a giant eruption off southern Kyushu about 7,300 years ago. This distant volcanic time marker lies beneath younger ash in the Northern Alps.
Two ancient ash layers near Washiba Pond were first attributed to its crater. Later analyses found neither held the volcanic-rock particles expected from that source; researchers instead traced them to steaming Iosawa, about 6 km away.
