Mount Asama, 1783: The Summer the Sky Went Dark
The lava field at Onioshidashi does not look like something that moved. It looks like the ground broke apart, heaved itself into towers and ridges and sharp black confusion, and stopped. Eighteen square kilometres of it, fractured and still and silent. In places you can see the moulds of trees in the cooled rock, the shapes of things that were standing when the flow reached them. The name means the demon’s push. In 1783, the people who lived in the valley below described the mountain as though something inside it was forcing its way violently out. Standing at the edge of the field, it is not a difficult description to accept. I was there a few months ago.
What happened at Mount Asama in the summer of 1783 was not a single event. It was nearly three months of escalating activity that culminated in one of the most destructive sequences in Japan’s recorded volcanic history, playing out across a landscape people were living and working in as the mountain above them gave every warning and not quite enough time.
The eruptions began on 9 May 1783. Moderate at first, sporadic, the kind of activity Asama had produced before. Ash fell across the Kanto region. Through June and into July the columns grew larger, shock waves strong enough to shake houses in surrounding villages. A major phase on 17 July sent pyroclastic surges down the flanks. Then a period of relative quiet, and the people in the valley below made what use of it they could.
Kanbara was a post town on the Shinshū highway, roughly 12 kilometres north of the summit, population around 570. Farmers mostly, growing wheat and millet on rocky volcanic soil, with inns and tea houses serving the travellers who moved along the highway. Through May, June, and July the people of Kanbara had watched the mountain, endured the ashfall, and continued.
At around ten o’clock on the morning of 5 August, a giant explosion tore from the summit. It was heard 300 kilometres away. The blast ejected incandescent lava blocks up to 35 metres in diameter onto the steep northern flank. On impact they mobilised a massive debris avalanche that swept north toward the valley. Most of the villagers were resting at home. The months of eruption had exhausted them, the recent quiet had eased the tension. They were not in the fields. They were indoors, directly in the path of what was coming.
The debris flow buried Kanbara in minutes. Of the approximately 570 people living there, around 477 died. The survivors had reached the Kannon hall, a small Buddhist temple on a hill above the village, approached by a stone stairway of 112 steps. The flow rose to within ten steps of the top before stopping. Excavations from 1979 onward found the remains of two women at the base of that stairway, killed while trying to climb it. The temple still stands. The lower steps remain buried.
After Kanbara the debris flow crashed into the Agatsuma River and became a lahar that tore downstream, destroying more than 1,200 houses along the Agatsuma and Tone rivers and killing hundreds more in the valley settlements below. Four villages were completely destroyed. The torrent continued 200 kilometres to the Pacific Ocean, raising riverbeds and causing flooding that persisted for years. The Shin-Ohashi and Eitai-bashi bridges on the Sumida River in Edo were swept away. Debris from a mountain in Gunma Prefecture reached the sea.
Total direct deaths from the eruption are estimated at approximately 1,400. The Onioshidashi lava flow, 170 million cubic metres of it, ran 5.5 kilometres down the northern slope. The agricultural land of Shinano and Kōzuke provinces remained fallow or underproductive for the following four or five years.
The wider catastrophe was the Tenmei Famine, 1782 to 1788, the most severe agricultural disaster in Japan’s early modern history. Asama’s contribution was compounded by the Laki fissure eruption in Iceland, which began in June 1783 and released approximately 120 megatons of sulphur dioxide into the atmosphere. Research by Zielinski and colleagues in Geophysical Research Letters in 1994, drawing on GISP2 Greenland ice core data, established that Asama’s stratospheric sulphur loading was approximately 0.3 megatons, around 0.2 percent of Laki’s output. The global cooling of the mid-1780s was driven by Iceland, not Japan. What Asama contributed was local and devastating: blocked sunlight, failed harvests, an estimated 20,000 additional famine deaths concentrated in Tohoku. Farmland abandoned. Villages depopulated. Food riots in Edo. The political instability that followed contributed to the Kansei Reforms of 1787.
The account that reached Europe came from Isaac Titsingh, a Dutch East India Company diplomat stationed in Japan during the eruption. His account was published posthumously in French in Paris in 1820 and in English in London in 1822, the first detailed description of a major Japanese volcanic disaster to reach the West.
By then the survivors of Kanbara had rebuilt their village on top of the debris that buried the original one. The Kannon hall still stands on its hill. The stairway is still there, its lower steps buried where they have been since August 1783.
The lava field at Onioshidashi is managed now, with information boards and a temple and tourists with cameras. What is harder to hold in mind, walking through it, is that this frozen landscape was moving at speed on the morning of 5 August 1783, and that the people in the valley below had roughly the time it takes to climb 112 steps to decide whether they were going to survive.
Most of them did not make it.
Copyright © Tales from the Horizon, 2026
