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.

Japan's biggest volcanic eruption in 50 years

Mount Kirishima/Shinmoedake eruption Jan. 26, 2011
From FNN-News.com

Mount Kirishima consists of several volcanoes covering an area about 20x30 km with more than 20 eruptive centers.  Shinmoedake, which last erupted in 1959, is approximately in the center. Eruptions have been recorded since 742, with strong eruptions in 788, 1716 and 1717 (Wiki). R. Imura published an "Eruptive history of the Kirishima volcano during the past 22,000 years" in Geographical Reports of Tokyo Metropolitan University, 1992. Geologic history older than 22,000 years is hidden by a very thick ignimbrite (ash) deposit from the eruption of the Aira caldera at that time.

Shinmoedake as seen from Takaharu Town Office
Early a.m. Jan. 27, 2011
Reuters/Takaharu Town Office/Handout
copyright status unknown

From 22-15 thousand years ago, five stratocones and two smaller volcanoes were formed.  The main products of the eruptions were lava flows.  Kirishima was fairly quiet between 15-7 thousand years ago, when eruptions then resumed.

Eruptions from this volcano appear to have become more explosive with the passage of time.  The eruption in 1959 was phreatic, that is, involving water.  It resulted in the formation of a 500 m long fissure on the west slope of Shinmoedake.  In 1991-1992 vigorous enough phreatic activity (steam explosions? not specified in the Imura article) caused authorities to ban climbing on it for about one and a half years.

Other posts about volcanoes on this blog are: Klyuchevskaya, Merapi, Merapi and map of major volcanoes of Indonesia, Santorini, Matua (volcanoes and atom bombs!), and  Etna.  A good blog on volcanoes is at the Smithsonian/USGS site.

Mount Merapi and "ash hurricanes"

This image from NASA's ASTER instrument on the Terra satellite shows
a signature of hot ash and gases flowing down the flank of Merapi volcano
as well as a hot spot on the central dome.  Image taken on November 1.

Since the eruption of Mount Merapi started on October 26, 2010,at least 156 people have died (reports are not consistent at this time).  Press reports have told of people dying from burns and ash suffocation, but no evidence had surfaced regarding the cause of the burns and suffocation because bad weather had prohibited direct observation of the volcano.  Now, the ASTER instrument on NASA's Terra satellite has documented that hot pyroclastic flows ("ash hurricanes") have been sweeping down the flanks of the volcano. (A video of pyroclastic flows on Merapi from previous eruptions can be found here by scrolling down the post to the video.) Temperatures of 450-600 degrees Celsius (842-1112 Fahrenheit have been reported by the Indonesian Volcanology Technology Development and Assessment Agency.  I have found a good blog on the Merapi eruption by Erik Klemetti here.

Merapi in eruption; images from
Clara Prima/AFP/Getty Images posted
on Washingtonpost.com.

What drives pyroclastic flows? In the early frames of the video, large boulders can be seen bouncing down the hill side-by-side with the pyroclastic flow.  This indicates that the flow, like the bouncing boulders, is being driven downhill by gravitational forces.  When gravity is the main driving force, hot material generally is confined to a valley--it's running downhill.  It can "run up" and over hills because of momentum--just like a car that loses its brakes at the top of a hill may race downhill and up the other side of a valley--but generally flows given by gravity follow the terrain. In later frames of this video, including a spectacular segment taken at night, you can see that the pyroclastic flows are being generated by collapse of individual pieces of the steep lava dome.  Section after section is peeling off and falling down and, as the sections impact the ground or other collapsing pieces, they fragment to release fragments and gasses that combine to give form the pyroclastic flow itself.  You do not see "blowouts" of gas-rich material in this video.

A more dangerous condition may arise if pressurized gases inside the collapsing dome decompress rapidly, forming blowouts. Jon Fink and I considered some of these conditions in a paper in Nature, vol. 363, pp. 612-615, 1993.  This paper was motivated by the fact that a pyroclastic flow on Mount Unzen on June 3, 1991, killed three volcanologists who were on high ground that had escaped damage from pyroclastic flows earlier in the eruption.  What happened?  We speculated that a section of the dome containing slightly more volatiles than earlier collapsed sections produced a pyroclastic flow that had higher velocities than had been produced in earlier volatile-poor eruptions.  We concluded that velocity excesses of over 100 m/sec could be caused by decompression of gases in such a section.  These flows would not be confined to valleys, such as the flows to date appear to have been at Merapi, but can have broader extent and longer runout distances. The lateral blast at Mount St.Helens was such a blast.

In spite of the inconveniences and economic consequences, it is necessary to evacuate people from areas around the volcanic summit that appear safe based on the trajectories of small, gravity-driven pyroclastic flows, but that may be devastated by more gas-rich eruptions. Reportedly, about 200,000 people have been displaced by the current eruption.