The recent eruption of Mt Ontake, Japan tragically killed at least 50 hikers who were on the volcano at the time. Within hours of the eruption taking place, social media was flooded with first-hand video footage illustrating just how close many survivors came to perishing in an onrushing pyroclastic flow.
Despite having a sophisticated seismic and geodetic monitoring system, many news reports stated that Ontake erupted seemingly without warning. Based on the evidence available at this time, it is probable that the event at Ontake was phreatic.
Phreatic eruptions occur when water enters a magmatic system and is heated to form steam. As these volcanic events do not involve the movement of magma, they are not accompanied by any of the normal eruption precursors such as ground inflation, seismic swarms or increased gaseous emissions.
As the Ontake hikers found out, a volcano which can erupt without warning is a pretty dangerous prospect.