The eruption started at 11:53 am Saturday local time. A live webcam installed at Takigoshi captured a pyroclastic flow chasing down the south face of the volcano. Japan Broadcasting Corporation NHK released aerial video of the eruption available here, another video including people on a ridge with the glowering ash tower, and a different vantage point before aerial footage here:

Japan is at a triple-plate subduction boundary between the Eurasian continental plate and the Philippine and Pacific oceanic plates. As the denser oceanic plates dive below the low-density continental plate, water from the saturated sediments lowers the melting point of surrounding rock. That magma feeds a range of volcanoes mirroring the plate boundary.
© Volcano World

Volcanoes can produce gentle, effusive eruptions like those in Hawaii, or landscape-scarring nightmares like Mount Saint Helens, Krakatoa, Vesuvius, and Mount Pinatubo. The science of predicting which volcanoes will produce what type of eruption is in their geochemistry. Volcanoes with silica-rich lavas trap gas, erupting violently. Volcanoes with silica-poor lavas allow the gas to escape, erupting gently. While details and exceptions can get complicated, the general rule of thumb is that volcanoes over oceanic tectonic plates are gentle, and volcanoes along oceanic-continental boundaries are violent.

Along the tectonic boundary in Japan, the melted magma from the subducting oceanic plate starts out silica poor. But, as it rises through the continental crust, it picks up silica. By the time it erupts, the is classified as an intermediate lava, with a high silica that produces spectacularly violent eruptions. These eruptions shape the very landscape of Japan with iconic conical stratovolcanoes like Mount Fuji and Mount Ontake.

The geochemistry of stratovolcanoes directly drives all other characteristics about them. Thick, high-viscosity lava flows produces steep, conical volcanoes compared to the gently rounded shield volcanoes made by runny low-silica lava. Silica-rich magma produces lighter-coloured rocks, slowly crystallizing into pink and grey granites with plenty of quartz and feldspar instead of dark basalts and gabbros. But the most painfully apparent difference is in the violent, hazardous eruptions produced but silica-rich lava.

Trapped gas in silica-rich lava drives explosive eruptions with towering clouds of abrasive ash and extremely rapid pyroclastic flows.

It even produces volcanic bombs, globs of lava ejected from the crater to land as aerodynamic missiles anywhere within kilometers of the eruption.

Heavy, poisonous gases seep out of the rock, sinking into depressions and valleys to silently and invisibly smothering anyone nearby. The eruption can be amplified by lahars: watery, rapid landslides. Boosted by rapidly melting snow, lahars are extremely destructive, washing away and burying anything unlucky enough to be downstream. Eruptions can also trigger more mundane landslides with tremors, emptying magma chambers, and redistributed of stress destabilizing slopes.

This horrible collection of hazards including pyroclastic flows, ash, and pyroclastic bombs is exactly what is happening at Mount Ontake.

First-Hand Accounts of the Eruption

Unbelievably, some hikers in the area not only witnessed the eruption and survived, but recorded it.

Pyroclastic flows frighten me. Everything about them is terrifying: they're incredibly fast superheated volcanic flows with billowing clouds of poisonous gas that can travel mindbogglingly long distances at outrageous speeds. My only survival advice if you see one coming towards you is, "Be somewhere else." These people are so, so lucky to be alive.

Several hikers managed to take refuge in a hut. Twitter user Mori Shota was hiking near the volcano at the time of the eruption. They described the eruption as sudden darkness, the eruption blocking all light to leave them in pitch darkness. They shared the following series of photographs while sheltering in a hut with other hikers. See images here.