On the 27 September 2014, a large explosive eruption, with fine ash, lapilli and larger bombs were thrown out as well as blocks and ash flows flowed down the slopes with over 100 km/h.
At the time of the outbreak, there were about 200 to 300 walkers and pilgrims on the volcano. So far, 30 deaths have been confirmed, but still many people are missing.
The outbreak happened without any visible sign. This is typical for so-called phreatic eruptions, where water strongly heated leads to steam-gas explosion. Volcanic rock is torn and with thrown out while overheated water can suddenly expand. Such explosions are not necessarily linked to the rise of new magma from depth, and can occur, for example, very close to the surface. Due to missing signals from the depths, phreatic eruptions are difficult to predict.
Ontake-san is a large and complex volcano, 3063 m high, and is considered a sacred mountain of Japan since centuries. The first historically documented eruption of Ontake-san was in 1979. At that time only with very small magnitude, which reached only level 1 on the Volcanic Eruptivity Index (VEI). Numerous eruptions occurred since then, but usually quite small, or as pure water vapor clouds, which caused neither victims nor damages. Especially since the early 1990s many tremors were repeatedly measured - signs of the geological activity of the Ontake volcano.
The reason for active volcanic activity and frequent earthquakes in Japan is caused by the geographic location or better the plate tectonic situation. Japan is located on the so-called "ring of fire". This stretches from New Zealand via New Guinea to the Philippines after Japan to America's West Coast.
Around Japan, various continental plates meet each other. Both the Eurasian, the North America, the Philippines and the Pacific plate collide in a relatively small area. Active volcanism and frequent earthquakes are the consequence.