In what year did Mount Fuji last experience a major eruption, and what impacts did this eruption cause?

Okay, let's talk about Mount Fuji's "most explosive" eruption.

Fuji's Last Major Eruption: The Hōei Eruption (1707)

To talk about Mount Fuji's last major eruption, we have to go back over three hundred years to Japan's Edo period. Specifically, it occurred in 1707, known as the "Hōei" era in the Japanese calendar system. That's why this eruption is historically known as the "Hōei Eruption" (宝永大噴火, Hōei dai funka).

The eruption began on December 16th, 1707, and lasted for about two weeks on and off. It's worth noting that this eruption was unique—no lava flowed out. Instead, it was like a giant who'd held in a sneeze for a long time finally letting loose, ejecting enormous amounts of volcanic ash, scoria, and pumice.

What Were the Impacts of This Eruption?

Despite the lack of lava, the impact of this eruption was massive and far-reaching. Key effects can be broken down into two main phases:

1. The Immediate Effects During the Eruption

Sky-Blanketing Volcanic Ash Ash was carried by westerly winds over 100 kilometers, reaching as far as Edo (modern-day Tokyo). Records indicate that several centimeters of ash accumulated in Edo, plunging the city into day-evening-like darkness where people had to light candles inside their homes. Imagine opening your door or window—instead of rain or snow, it would have been a hazy "black snow" falling, with the air thick with choking dust.
Life Descended into Chaos:
- Health Problems: Inhaling the volcanic ash caused respiratory illnesses, leading to widespread coughing.
- Building Damage: Heavy buildup of ash on roofs caused many wooden houses to collapse under the weight.
- Agricultural Destruction: Farmland surrounding the volcano was completely smothered under thick ash, leading to total crop failure.

2. The Lasting Secondary Disasters (The Real Nightmare Began Here)

Although the eruption stopped, the true nightmare was just beginning.

Decades of Flooding This proved to be the most severe consequence of the Hōei Eruption. The vast amounts of ash deposited on mountainsides and plains were washed down by rain, mixing with sediment and flowing into rivers. This rapidly raised and clogged riverbeds.

The most famous example is the Sakawa River. Its bed became significantly elevated by the volcanic deposits, drastically reducing its drainage capacity. Consequently, for decades afterwards, whenever heavy rain fell, the Sakawa River flooded catastrophically, inundating vast tracts of farmland and villages downstream. Local residents endured an arduous and prolonged struggle to build flood control measures, a fight against nature that lasted for generations.
Long-term Famine and Social Unrest The destruction of farmland and constant flooding directly caused prolonged food shortages and famines. Although the Shogunate (government) invested heavily in disaster relief and reconstruction, life for people in the affected areas remained extremely difficult.

In summary, while the 1707 Hōei Eruption didn't involve dramatic molten lava flows, the truly immense volume of volcanic ash it unleashed inflicted years of profound, complex disaster on Japanese society at the time. It also serves as a reminder that the seemingly serene and beautiful Mount Fuji is a dormant giant, who has now been sleeping quietly for over three centuries.