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Nikko NP, Tochigi / Gunma
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In 1649, Mount Nikko-Shirane’s summit eruption opened a roughly 200-m-wide crater and swallowed an old stone shrine. In 1652, monks carried a new cast-bronze shrine up the mountain and installed it.
After Mount Nikko-Shirane erupted in 1873, a boulder-and-mud flood rushed down its western valleys and flattened stream beds. Contemporary reports recorded muddy water and dead fish in the Tone River downstream, in Saitama.
Shirane-aoi is named for Mount Shirane, but it isn’t a hollyhock: its showy purple “petals” are sepals, and this Japanese endemic belongs to the buttercup family, not the mallow family.
Poaching and deer grazing sharply reduced Shirane-aoi at Mount Nikko-Shirane, so an electric fence was installed in 1995. Local volunteers and Oze High School students collect seeds, raise seedlings and replant them.
Between Mount Shirane and Maeshirane lies Goshiki-numa, a pond only about 5 m deep at its deepest point. Its surface shows a complex mix of colors, giving it the name “Five-Colored Pond.”
A 2022 geological survey used detailed elevation data to map at least 15 craters in the summit areas of Mount Shirane and neighboring Zazen-yama—a volcanic landscape with more than one opening.
Geologists date the lava covering Mount Shirane’s summit to about 3,000 years ago—and found that the eruption also produced a pyroclastic flow. The discovery helped reveal a more complex volcanic history.
In 1952, observers reported stronger fumes and rumbling around Mount Nikko-Shirane. A geological review classifies the activity as increased fumarolic activity—not an eruption.
Lava flows from the Mount Nikko-Shirane area blocked valleys and created the nearby dammed lakes Suga-numa, Maru-numa and Oojiri-numa. Their calm surfaces owe their origins to volcanic rock.
During the 1889 eruption, ash from Mount Nikko-Shirane fell around Senjogahara and reached Nikko and Imaichi. On the Gunma side, volcanic mud clouded rivers—one eruption affected areas on both sides of the prefectural border.
A survey around Mount Nikko-Shirane identified 62 lichen species from 178 samples. Researchers noted forests draped with hanging beard lichens and patches of ground carpeted in reindeer lichen.
