Saturdays eruption was completly unexpected which is what made it so fatal. So far 47 bodies have been accounted for and at least 16 hikers are still missing. Autopsies on those brought down from the summit have concluded most died from crushed skulls or ribs from the super heated rocks that were catapulted by the blast, other suffered internal burns from inhaling the ash. This has been coined as Japans worst volcanic disaster in living memory but why so in a country so prepared for such disasters.
Mount Ontake is one of 47 Japanese volcanoes under constant monitoring. Seismicity, ground deformation and gas emissions are just some of the ways volcanologists can see if magma is migrating towards the surface so why was the 27th such a surprise? The answer is actually quiet simple; it was not a magmatic eruption.
Last weeks eruption was not caused by magma breaking through to the surface as most peoples ideas of volcanoes would expect. Instead it is what is know as a phreatic eruption/explosion. When water, whether it be on the surface or ground water, meets the super heated rock of a geothermal system it instantly turns to steam. It operates on a similar principle as geysers such as those in Iceland or the famed Old Faithful in the Yellowstone system. Steam does not react well to be confined in the high pressure environments underground and will find any way out it can including smashing rock to smithereens.
Warning signs for such an eruption are minimal. There was some increased seismicity at Mount Ontake in the week leading up to the 27th but not enough to be classed more than “normal background levels”. In some instances the run up to a phreatic eruption may see slight temperature rises in bodies of water near to the system such as crater lakes, but other than this there are no real indicators for when a phreatic eruption will take place which can make them one of the more fatal styles of eruption.
A week on chances of finding any more survivors is slim, but rescue workers are still scaling the flanks in the area of the eruption as thoroughly as they can before Typhoon Phanfone takes hold of the region.