Over the weekend, the Mount Ontake volcano—a popular hiking spot in Japan—erupted without warning, killing at least 36 people. The eruption sent billowing clouds of hot gas and ash streaming down the hillside; some scientists are speculating that this was a “phreatic eruption," triggered when water and magma mix deep underground.
A phreatic eruption, says volcanologist Erik Klemetti, “occurs when water seeps into the cracks in the crater area of a volcano and gets hot enough to flash to steam. This rapid boiling causes fracturing of the rock and explosively ejects material out of the crater as the pressure inside the crater or conduit goes up exponentially.”
Often, pending volcanic activity is preceded by signals like earthquakes or a swelling of the ground. But the nature of this eruption meant it occured without warning, says Klemetti
For those hiking the volcano's slopes, this must have been a truly terrifying ordeal, based on this first-hand video (via Phil Plait):
Rescue operations mounted over the weekend helped more than 200 people make it down from the volcano, says Mike McKinnon for io9. But those operations have been put on hold because of fears over toxic gas, says the Guardian.