This is the science behind Mount Ontake's violent eruption in Japan on Saturday morning. Unbelievably, many hikers survived, documenting their perspective of the pyroclastic flow and trekking out of the volcanic wasteland.

Left: Hikers evacuating after the eruption. Credit: @mori____mori. Right: Aerial view of the Mount Ontake eruption. Credit: NHK

The eruption occurred mid-morning on a beautiful Saturday, when hikers were out on the volcano enjoying the start to their weekend. Injuries were reported in the immediate aftermath, with people missing at near a shrine and potentially elsewhere, and likely fatalities in the hours ahead. This is one of those times where I feel utterly helpless as a disaster scientist: I can explain what happened and why, but that doesn't help any of the people who are going to be grieving lost loved ones before today is over.

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The volcano is a popular destination for religious pilgrimages, and many hikers were out enjoying the beautiful start to the weekend and the autumn hiking season when the eruption hit. This photo of Ninoike, a pond just short of the volcano's summit, was posted shortly before the eruption:

Twitter user enari hadn't posted any messages since the eruption. While I optimistically hope survived in the pond-side hut, were posting a photo long after their trip to the summit, or otherwise this-isn't-what-it-looks-like, I'm not feeling optimistic. (Update: A similarly-named account claiming to be the enari says they're fine, lost their phone in the chaos so had to make a new account, and that the photo is from 30 min prior to the eruption. I have no way to verify this claim.) (Update: The original account posted a note saying she was fine and deleting the account. The second account appears to be a fake. Thank you to a lovely collection of folks for passing along the link.)

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:

Geoscience of the Ontake Eruption

Japan is at a triple-plate subduction boundary between the Eurasian continental plate and the Philippine and Pacific oceanic plates.

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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.

Plate tectonic diagram. Image credit: 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.

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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.

Volcanic bomb at Craters of the Moon National Park in Idaho. Image credit: National Park Service

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.

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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:

Hikers sheltered within the hut during the eruption:

Some of the survivors hiked out with ash still raining down on them.

The heat of magma rising within the volcano melted surface snow, adding to existing water runoff. Ash is so thick that local streams are opaque, running white with suspended sediment:

Once reaching the roads, they were collected by emergency responders and bused out of the region:

Another Twitter user, あべるん, had a similar experience. Their photograph of a building near the volcano makes it clear how ash from eruptions can quickly overloads structures and cause roof collapses. Upon evacuating the region, they returned to their car to find it coated in ash:

Still others in the region report the stench of sulfur accompanied the eruption. The nearest village is 12 kilometers from the summit, and is reporting no substantial damage.

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Over 200 survivors have made it down the mountain; Prime Minister Shinzo Abe ordered the military to run search and rescue operations. Eighty assistant climbers went up the volcano to assist upwards of 30 survivors who are seriously injured, buried, or otherwise unable to come down on their own. Not all were able to be rescued before nightfall, and with the eruption ongoing, will likely continue sheltering in place within tourist villas overnight. Police have set up a hotline for information on potential climbers that may have been unaccounted for thus far.

Ashfall in the Surrounding Region

The ashfall is being observed by people far away from the eruption in a rain of tiny, sharp shards of volcanic glass. The owner of a mountain lodge near the volcano's summit is reporting half a meter of ash so far. In major eruptions with substantial ash output, the ash can overload structures, collapsing roofs.

Fine volcanic ash is highly abrasive. The jagged nature of the glass shards is readily apparent under magnification.

Magnified view of volcanic ash produced by Yellowstone eruptions. Image credit: USGS

The airspace around the volcano has been marked off-limits to aircraft. A pair of Jetstar flights diverted to an alternate airport to avoid potential ash.

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Projected ash cloud. Image credit: Japanese Meteorological Agency

Although flight rules involving ash clouds have changed since the infamous Icelandic eruption disrupted European flights, pilots still need to avoid ash to avoid splattering delicate engines with molten glass.

One Twitter user spotted the eruption from their flight:

Ash can also lead to an increase in landslide risk. The ash forms a slick layer of sediments prone to landslides in the future. It can also smother plants, killing stabilizing vegetation.

Eruption History of Mount Ontake

Mount Ontake is located between Nagano and Gifu prefectures in central Japan. At 3,067 meters tall, the is the second tallest volcano in the country second only to Mount Fuji. Mount Ontake is also known as Ontakesan and Kiso-ontake. Despite the similar name, it is not related to Ontake, an offshore volcano with a vigorous eruption history.

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Area map of the centrally-located Mount Ontake volcano. Image credit: Japan Meteorological Agency

The volcano was not under any alerts or warnings at the time of the eruption, although it was in a period of heightened seismic activity (animation; seismograms). It seems like the volcano failed to exhibit clear signs of an imminent eruption, but there could have been a breakdown in the monitoring process. Even if monitoring was ideal, being close to a known active volcano is risky with a whole mess of potential hazards.

Mount Ontake's last major eruption was a major series of eruptions stretching from October 1979 to April 1980. The volcano had not erupted prior to that in recent history. Earlier claims that the volcano erupted in 1892 or 774AD did not stand up to reexamination of the historical records.

1979 eruption at Mount Ontake. Image credit: Tokiko Tiba/National Science Museum

Although Mount Ontake hasn't had a volcanic eruption since 1979, it had a minor phreatic eruption in 2007, and an unconfirmed similar eruption in 1991. Phreatic eruptions are steam-driven explosions caused by magma heating groundwater. Many news agencies are reporting this as the most recent eruption.

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Local university professor Koshun Yamaoka suspects the pyroclastic flows are low temperature, indicative of a hydrovolcanic eruption, where interaction between lava or magma and groundwater result in steam enhancing the eruption's explosive potential. Volcanologist Shigeo Aramaki concurs, suggesting that the white clouds are characteristic of water steam. However, until he can collect and analyze ash samples, Aramaki is not ruling out the possibility of a more troublesome lava eruption.

I'm not yet convinced this is a phreatic eruption. Without access to samples for analysis, the long-distance geogossip is that these eruptions look hot, light, and decidedly not wet. Getting massive pyroclastic flows out of phreatic eruptions is unusual, but not impossible. We aren't seeing lava in any of the footage or photographs so we also can't rule out that it's phreatic. Either way, it'll be interesting to see which way the evidence leads in the future. (Update: Here's more on what phreatic eruptions are, and how they can happen without warning.)

The volcano also had a major landslide failed in 1984, sending a debris avalanche tumbling 13 kilometers along river valleys and killing 29 people. The landslide appeared to be triggered by an earthquake independent of volcanic activity, although ash mixed in the sediment likely increased the landslide's mobility.

The Japanese Meteorological Agency has since placed the volcano under a Level 3 alert, warning everyone to stay away. A handwritten sign at the entry to the volcano warns people to stay away, reading, "The volcano has erupted. Climbing forbidden." (Thank you for the translation, Keshav Kini!)

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As always, I'll be active in the discussion section to help with your questions.

Errata: An early version of this article featured images from the Sakurajima eruption (thank you to @Cristoforou for the correction!), and recycled a tectonic map from Chris Rowan with a potentially confusing epicenter marker from the 2011 Tohoku earthquake and tsunami. Thank you to @holasmitha for translation assistance when the auto-translating robots failed me.

Update: Here's a follow-up on things we learned from this eruption.