Mount Ontake

Mount Ontake is an enormous volcanic massif rising from the forests of the Kiso valley, with a broad summit ridge more than 3 km long carrying several peaks and five crater lakes. Its highest point, Kengamine (3,067 m), makes it the second-highest volcano in Japan after Fuji and the westernmost 3,000 m peak in the country.

For centuries Ontake was a sacred mountain reserved for ascetics who had undergone long fasts. In the late 18th century two monks opened it to ordinary believers, and it became the focus of the Ontake-kō, pilgrim associations from all over central Japan. Their stone monuments, shrines and huts still line the routes.

The phreatic eruption of 27 September 2014, which struck without effective warning while the summit was crowded with hikers, changed how Japan communicates volcanic risk to climbers. Memorials stand at the summit and in the villages below.

Ontake is a composite volcano built over several hundred thousand years on top of older sediments. Its summit area is a chain of craters aligned north–south, from Tsugumi-dake to Kengamine, with crater lakes including Ichi-no-ike, Ni-no-ike – one of the highest lakes in Japan – and San-no-ike.

Before 1979 Ontake was thought to be dormant; the phreatic eruption of that year opened new vents on the south-west flank of Kengamine. In 1984 an earthquake beneath the south flank triggered the Ontake collapse, a huge landslide that buried the valley below. The 2014 eruption came from the same crater row as in 1979.

The flanks carry one of the finest subalpine conifer forests in central Japan, with fir, hemlock and the Kiso hinoki cypress lower down. The summit ridge is volcanic desert with scattered alpine plants.

In the Middle Ages the ascent of Ontake was restricted to ascetics who first completed up to 100 days of purification. In 1785 the monk Kakumei opened the Kurosawa route from Kiso-Fukushima to ordinary believers, and in 1792 Fukan opened the Ōtaki route. The Ontake-kō pilgrim associations they inspired, and later the Ontake-kyō Shintō sect, spread across central Japan. Pilgrims practise spirit-possession rituals (oza-date) in which a medium channels the mountain's deities.