Hōei eruption

The Hōei eruption began with a violent explosive phase on December 16, 1707, after a series of earthquakes, including the 1707 Hōei earthquake (magnitude 8.6) that struck 49 days earlier. The eruption was a plinian event, ejecting a column of ash and pumice that reached heights of over 20 kilometers. It produced a series of subplinian to plinian pulses, with the most intense activity on December 16–17. The eruption deposited tephra over a wide area, with the main dispersal axis extending east-northeast toward the Kanto Plain. The total volume of tephra is estimated at about 800 million cubic meters, making it one of the largest eruptions in Japan during the historical period.¹

The ashfall severely affected agriculture, with crops buried under volcanic ash across the eastern foothills of Mount Fuji. In Edo, approximately 5 centimeters of ash accumulated, disrupting daily life and causing respiratory problems. The eruption also triggered lahars and secondary mudflows that damaged villages along the slopes. The Tokugawa shogunate organized relief efforts, but the eruption exacerbated a famine that had already begun due to poor harvests. The ash layer, known as the Hōei tephra, remains a key stratigraphic marker for geological studies in the region.

The eruption created a new crater on the southeastern flank of Mount Fuji, now known as Hōei-zan, a parasitic vent that formed a prominent bulge on the mountain's side. The eruption also altered the summit crater, and the ash deposits changed the local drainage patterns, leading to the formation of new streams and wetlands. The Hōei eruption is considered a benchmark for understanding the eruptive history of Mount Fuji, as it was the last major explosive event. Geologists have used the tephra layers to reconstruct past eruption cycles and assess future volcanic hazards.²

One lesser-known fact is that the eruption was preceded by the 1707 Hōei earthquake, which may have triggered the volcanic activity by stressing the magma system. Another is that the eruption occurred during a period of intense volcanic activity in Japan, with several other volcanoes erupting in the same decade. The ash from the eruption was so fine that it was carried by winds to the Pacific coast, and some reports suggest it even reached the island of Hachijō-jima, over 200 kilometers away. Additionally, the eruption had a cultural impact: it inspired the famous ukiyo-e print series 'Thirty-six Views of Mount Fuji' by Hokusai, though the prints were made decades later. The Hōei eruption also led to the development of early volcanic hazard mitigation measures in Japan, including the establishment of ashfall monitoring in Edo.³

Plinian eruption

A violent volcanic eruption characterized by a sustained column of gas and ash rising high into the atmosphere.

Tephra

Fragmented volcanic material ejected during an eruption, including ash, pumice, and rock fragments.

Lahar

A destructive mudflow composed of volcanic debris and water that flows down a volcano's slopes.

VEI

Volcanic Explosivity Index, a scale from 0 to 8 that measures the magnitude of volcanic eruptions.