Mount Kusatsu-Shirane was added to Your Favorites!
Joshin’etsukogen NP, Gunma
Claim this place to tell your story your way.
During the 1897 eruption sequence, a blast sent a 150 kg boulder 900 m; another burst wrecked a nearby sulfur mine. JMA’s record makes the volcano’s past feel startlingly close.
Yugama’s emerald is chemistry, not dye: the lake is about pH 1.2, and microscopic sulfur particles scatter light while dissolved iron helps shape its green color.
On 23 January 2018, the eruption came not from famous Yugama but from Motoshirane, a separate cone about 2 km south—the first eruption there in recorded history.
Miners once worked inside Yugama’s crater: sulfur was hauled out by trolley and lift until the mine closed around 1960, when refineries began recovering cheaper sulfur from petroleum.
Wooden sasatōba prayer markers linked with Shugendō were found at Yugama. Their estimated medieval date suggests this volcanic summit was a sacred mountain long before modern sightseeing.
Researchers examined 69 dated Kusatsu panoramas and found one showing a plume from Motoshirane in 1818–1830—possibly a brief eruption or a spell of vigorous fumaroles.
A 2013 survey found three gas vents beneath Yugama and bubbling plumes; researchers estimated diffuse CO₂ release at roughly 30 tonnes a day. The lake is active from below, too.
One of the summit’s crater lakes, Mizugama, lost all its water during a 1976 steam explosion. By 2003 it had refilled, but remained a shallow pool less than a metre deep.
When water drops at Kagami Pond on Motoshirane, its floor reveals tortoiseshell-like polygons. Repeated freezing and thawing sorts the stones into this rare patterned ground in Honshu.
The volcano’s acidic waters create a river-treatment challenge downstream: Kusatsu’s neutralization plant adds about 55 tonnes of limestone powder each day, helping rivers below support fish.
