Mount Asama
浅間山あさまやまAsama-yamaThe smoking volcano above Karuizawa, witness of the 1783 catastrophe.
Mount Asama (浅間山), 2,568 m, is one of the most active volcanoes on Honshū, rising on the border of Nagano and Gunma prefectures above the resort town of Karuizawa. Its catastrophic eruption of 1783 buried villages, created the Onioshidashi lava field and contributed to the Tenmei famine.
Overview
Mount Asama stands alone on the watershed between the Chikuma river (Nagano) and the Agatsuma river (Gunma), a broad, usually steaming cone whose plume is visible from Karuizawa and the Kantō plain. It is one of the most closely watched volcanoes in Japan, with frequent small explosions throughout the twentieth century and eruptions in 2004 and 2009.
Asama shaped the lives of people below it. The great eruption of 1783 is among the most destructive in Japanese history; the preserved stone steps of Kanbara Kannon-dō and the Onioshidashi lava field are its monuments. At the same time the volcano gives the region its landscape: the light-grey plains of volcanic ash, the cool highland climate of Karuizawa and the hot springs of the area.
Landscape and formation
Asama is a complex stratovolcano. The older Kurofu volcano collapsed more than 20,000 years ago, leaving a horseshoe-shaped somma to the west (Kurofu-yama, Gyūdome-yama); inside it the younger Maekake cone and the central cone of Kama-yama grew. The summit crater, about 450 m across, is the source of the historical eruptions.
The 1783 eruption produced the Agatsuma pyroclastic flow, the Kanbara debris avalanche and, at its end, the Onioshidashi lava flow on the north flank, a field of black blocks now overgrown with pines and moss.
Highlights
Onioshidashi
鬼押出しOnioshidashiThe rugged lava field of 1783 on the Gunma side, with a park and a temple amid the rocks.
Kurofu-yama
黒斑山Kurofu-yama2,404 m, the highest point of the outer somma, with a classic view into the Yunohira basin and the active cone.
Maekake-yama
前掛山Maekake-yama2,524 m, the rim of the inner cone; the highest point hikers are permitted to reach in quieter periods.
Kanbara Kannon-dō
鎌原観音堂Kanbara Kannon-dōA small hall where 93 villagers survived 1783 by climbing the steps; excavations revealed the buried lower steps.
Ecology
The young volcanic surfaces show plant succession in action, from bare scoria near the summit to larch and pine forests on older lava. Karuizawa's woodlands at the foot are known for their birds.
Flora
Japanese larch
カラマツkaramatsuWidely planted around Karuizawa and native on the upper slopes.
Komakusa
コマクサkomakusaGrows on volcanic gravel of the somma.
Fauna
Copper pheasant
ヤマドリyamadoriLives in the forests at the foot.
Asian black bear
ツキノワグマtsukinowagumaPresent around Karuizawa, where bear management is a local issue.
Culture
Asama was feared and worshipped as a fire god; Asama shrines at its foot sought to pacify it. The Nakasendō highway ran along its southern flank through the post towns of Karuizawa, Kutsukake and Oiwake, and travellers' diaries record its plumes. From the late nineteenth century Karuizawa grew into a summer retreat, first for Western missionaries, and Asama became a motif for writers such as Hori Tatsuo.
Asama Volcano Museum
浅間火山博物館Asama Kazan HakubutsukanA museum on the Gunma side explaining the volcano and its eruptions.
Onioshidashi-en
鬼押出し園Onioshidashi-enA park in the lava field with a branch of Kan'ei-ji temple.
History
1108
Tennin eruption, one of the largest in Asama's record.
1783
Tenmei eruption: Kanbara village is buried and Onioshidashi lava forms.
1949
Jōshin'etsu Kōgen National Park is designated.
2004
Moderate eruption with ashfall reaching the Kantō plain.
2009
Small eruption with ashfall to the south-east.
Through the seasons
Spring
Snow remains on the cone into April; lower slopes green in May.
Summer
The hiking season on the somma; Karuizawa is a cool summer retreat.
Autumn
Larch forests turn gold in late October, framing the grey cone.
Winter
The cone is snow-capped, and the plume often stands out against clear winter skies.
Routes and trails
Kurumazaka-tōge – Kurofu-yama
From the pass at about 1,970 m along the somma ridge to Kurofu-yama, about 1.5–2 hours one way; moderate.
Asama-sansō – Maekake-yama
From the Tengu Onsen Asama-sansō on the Komoro side via the Kazan-kan area to Maekake-yama, about 4 hours up; only when access is permitted.
Yunohira and Kurofu loop
Descend from Kurofu-yama into the Yunohira basin and return via the Kusasuberi slope; 5–6 hours in total.
Practical notes
- Access restrictions around the summit change with volcanic activity; always check the Japan Meteorological Agency's Asama page and notices by Komoro city and the neighbouring towns.
- The Kurumazaka pass is reached by road from Komoro; Karuizawa is about one hour from Tokyo by Hokuriku Shinkansen.
- Carry a helmet on trails close to the active cone and note the positions of shelters.
Frequently asked questions
Can the summit of Asama be climbed?
The summit crater area is normally closed because of volcanic hazards. Depending on activity, hikers may be allowed to Maekake-yama on the inner cone, or only to viewpoints on the outer somma such as Kurofu-yama. Check the Japan Meteorological Agency and the local municipalities before going.
What happened in 1783?
After months of activity, Asama's Tenmei eruption culminated in early August 1783 with pyroclastic flows and a debris avalanche that buried the village of Kanbara and sent mudflows down the Agatsuma and Tone rivers. The eruption killed well over a thousand people and worsened the Tenmei famine.
Where is the best view of Asama?
From Karuizawa to the south-east, from the Onioshidashi Park on the Gunma side, and from the Takamine plateau to the west.
How long is the hike to Kurofu-yama?
From the Kurumazaka pass at about 1,970 m, roughly 1.5–2 hours one way; the route along the somma rim offers close views of the active cone.
Related
Sources
- 1.日本の主な山岳標高 (opens in a new tab) — 国土地理院 (Geospatial Information Authority of Japan)
- 2.火山登山者向けの情報提供ページ（浅間山） (opens in a new tab) — 気象庁 (Japan Meteorological Agency)
- 3.上信越高原国立公園 (opens in a new tab) — 環境省 (Ministry of the Environment)
- 4.群馬県ホームページ (opens in a new tab) — 群馬県 (Gunma Prefecture)