There have been warnings that volcanic debris may settle within 4km (2.5 miles) of the peak. Efforts to recover the bodies of at least 24 climbers have been suspended again after the eruption on Japan's Mount Ontake intensified.
Despite Japan's advanced volcano monitoring system, this eruption came with virtually no warning. Scientists suspect this is because it was a phreatic eruption which was caused by steam rather than magma. When magma moves under a volcano, it can be picked up by seismic monitoring systems. In a phreatic eruption, ground water is super-heated rapidly by the heat from magma. Steam is formed which takes up more volume than liquid water.
If there is enough steam formed by super-heated ground water quickly enough with no way to escape, it can blast through the overlying rocks, pulverizing it into ash.
Japan is situated on the edge of two colliding tectonic plates. The Pacific plate is being forced beneath other plates all along its border, this region is also known as the Rim of Fire. The continuous collision generates earthquakes. The more the Pacific plate is pushed into the Earth’s mantle, the more surface water and hydrated minerals heat up, which melts the mantle and creates the magma that causes volcanic eruptions. Japan has more than 100 active volcanoes.
Picture courtesy: imaggeo.egu.eu