Ontake, Japan, erupts

Ontake erupts. Photo by andreijejune as cited above.
The eruption started around 11:53 a.m. Saturday, local
time (in spite of the setting in the caption above)

UPDATE SUNDAY: The Japan Times is reporting that 31 people were found unconscious near the peak of Ontake, and that four have been pronounced dead. The BBC is reporting that there were a total of 45 missing climbers. Japanese officials only announce deaths after a formal examination by a doctor. I extend condolences to the families of the victims and missing.

A few hours ago (Saturday), Mount Ontake 155 miles west of Tokyo, erupted, sending a steamy ash plume high into the sky. It last erupted in 2007. News is conflicting about the casualties, but at least one person has been killed,  thirty people have been injured and the Japanese are organizing to rescue an unknown number (reports vary between 41 and 200?) people who were climbing on the mountain. As much as 20" of ash has been reported on the ground near the summit, and Japanese authorities are issuing an alert to stay at least 4 miles away from the summit. The alert level is "3" meaning "do not approach the volcano." Ash is reported to have gone 3 kilometers down the mountain in a pyroclastic flow. There are a number of YouTube videos showing the eruption through cameras held by hikers. Here's one.

From the Kinja Space site cited
in the text and the twitter
user identified above.

Ontake is the second highest volcano in Japan, at 3,067 meters, second to Mount Fuji. There is a nice description of the tectonic setting of Ontake, as well as a collection of eyewitness accounts, at Kinja Space, authored by Mika McKinnon, from which I take much of the following discussion. The author of this blog nicely states that because of geochemical differences in the magmas, volcanoes over oceanic tectonic plates typically have a relatively low abundance of silica (SiO2) and are fairly fluid allowing their gases to escape rather gently (think Iceland, Hawaii). When the eruptions do turn explosive, it is usually because the magma has interacted with groundwater or ice.  Volcanoes that are rich in silica are viscous and gases don't escape easily, leading to conditions that produce explosive eruptions. Such volcanoes usually are found where oceanic and continental plates intersect. The Pacific Ring of fire that stretches up from South America, through western North America and around to Japan is such a setting and eruptions here can be very dangerous. Eruptions of these volcanoes produce flying rocks, volcanic bombs, and hot pyroclastic flows. The movies of the survivors are lucky to be alive.

Ontake had a minor eruption involving water (phreatic) in 2007, but the last major eruption stretched from October 1979-April 1980. In spite of claims that it had erupted in 1892 and 774 AD, detailed examination of the records suggest that this is not true and that it had not erupted prior to the 1979-1980 sequence in recorded history, which is a long time in Japan. Local volcanologists/seismologists Koshun Yamaoka and Shigeo Aramaki are suggesting that the billowing white clouds seen in the eyewitness photos suggest that this is a phreatic eruption.  The possibility that phreatic eruptions are signaling heating of groundwater by rising magma leaves open the scenario of a major magmatic eruption like that of Mount St. Helens that began about 4 hours after the 1980 March-April lateral blast.