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Joshin’etsukogen NP, Nagano
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On nearby Asama’s summit-crater rim—not on Kurofu itself—a boulder is called Sen-ton-iwa, “Thousand-Ton Rock.” The 1950 eruption hurled it out; estimates put its mass at 2,000–3,000 tons.
Kurofu was once a classic cone estimated at 2,800–2,900 m. Around 27,000 calendar years ago, a catastrophic collapse removed its eastern half; today’s 2,404-m summit stands on the surviving western rim.
The broad floor inside Kurofu’s broken rim is Yunotaira, the old volcano’s collapse basin. Asama’s younger Maekake volcano later grew inside it, with the active Kama crater at its summit.
In Saku, far downstream, Kurofu’s collapse dammed the Chikuma and Fuse rivers, leaving lake clay beneath Gorobei rice paddies. The soil turns rock-hard when dry, yet so soft when wet that farm machinery can sink.
Kurofu’s ridge and north slopes carry bog bilberry, locally called Asama-budo (“Asama grapes”) and used for jam. The same shrub feeds rare highland butterflies, so berry picking can damage their nursery.
A 75.8-hectare native larch forest grows on Kurofu’s volcanic deposits at 1,860–2,050 m. Protected since 1973, it is kept uncut and renews naturally—a rare larch community valued as a stable local forest.
“Kurofu” is a comparatively recent name: a Genroku-era map labels the ridge “Mitsuone-no-mine,” and an 1884 Gunma map calls it “Mitsuone-yama.” The name Kurofu came into use in the Showa era.
Kurofu’s flower celebrity is hime-shajin: 20–30 cm plants that cluster on rocky ground, with nodding blue-violet bells in July and August. It is the flower associated with Kurofu in Japan’s “100 Famous Flower Mountains.”
Kurofu’s older slopes carry forest up to the ridge. Across the basin, younger, repeatedly active Maekake starts with bare ground; lichens and hardy grasses arrive first, followed by low shrubs and eventually trees.
