The hearts of all of us in Kumamoto are extended to the victims of the recent volcanic eruption on Whakaari Island in New Zealand.
The eruption was reportedly phreatic, not Plinian. In the latter case, magma extrudes from the chamber towards the surface slowly enough that monitors are able to detect bulges in the terrain, allowing time for warning and evacuation. The former case is caused by “flashing” (instantaneous conversion of liquid to vapor) of supercritical fluids - water which would normally turn to steam is kept by immense pressure at a liquid state, but if any event removes the “lid,” the pressure is released and steam, being 1,600 times less dense than water, bursts out, carrying with it tons of rock and toxic gasses.
Most eruptions of Aso are phreatic as the Philippine plate subducting under the Eurasian plate (we’re here!) carries with it much water. Compared with Plinian eruptions, those phreatic are impossible to predict - they happen in an instant, with no warning. If visiting Aso, glance around at the shelters to familiarize yourself in the unlikely event you’ll be unfortunate enough to be there should an eruption occur. - William