When I logged on the net this morning I found Ontakesan erupted in Japan in Austrian news.They had gotten the name not completely right cause it is Ontake-san. San meaning mountain in Japanese and it is also a title of respect. The name translates as “Honoured Peak” and Ontake is used for some more other mountains in Japan so one needs to specify … Kiso Ontake is meant in this post.

Mount Ontake (御嶽山 Ontake-san?), also referred to as Mount Kiso Ontake (木曽御嶽山 Kiso Ontake-san?), is the second highest volcano in Japan at 3,067 m (10,062 ft) located around 100 km (62 mi) northeast of Nagoya. ( Quote Wikipedia)

Ontake-san is a stratovolcano. It is an “independant” volcano, meaning it is not connected to any system but has a system of it´s own and it is situated in Nagano Prefecture Kiso-gunKiso-machi , Otaki Village, Gifu Gero – Takayama 35 ° 53 ’34 “N, 137 ° 28′ 49″ E 100 km north east of Nagano. The highest peak Kengamine (剣ヶ峰), is 3,067 m altitude.

Ontake lies in a caldera with a diameter of approximately 5 km which was created 70,000 – 90,000 years ago. Farther up there is a second caldera which is 2 km in diameter. It seems to date back 23,000 years. Until, 1979 the volcano was dormant until an unexpected eruption happened from October 1979 to April 1980. Smoke and ash rose from several craters of the southern peak. A landslide occurred after an earthquake in 1984 killing 28 people. Fumarolic activity continued but 2008 the alert level was lowered. In the end of September 2014 the alert level was raised to 3 for the southern part of the volcano.

The base of the volcano is 17 km2 and the composition is mostly rhyolite but the upper parts also contain pyroxene and andesite. The longest lava flow that had been produced run 17 km.

5 old craters are filled with lakes. The highest lies on 2,905 m – Ni-no-ike 二ノ池.

On September 27th 2014 it erupted leaving 250 climbers stranded and 7 unconscious. http://ajw.asahi.com/article/behind_news/social_affairs/AJ201409270035

The article above states: “Nagano prefectural officials said hikers reported to Kiso fire authorities that many climbers collapsed on the trekking trail.

Cameras installed by the land ministry’s Chubu Regional Development Bureau show volcanic plumes sweeping more than 3 kilometers down the mountain’s southern slope.” (Quote)

Note that they said sweeping DOWN the southern slope. This may (or may not) describe a pyroclastic flow. A warning of debris flying at a radius of 4 km from the crater is issued.

http://www.japantimes.co.jp/news/2014/09/27/national/central-japans-mt-ontake-erupts-hikers-reported-injured/ states there are 15 cm of ash on the ground.

From http://www.t-online.de/nachrichten/panorama/katastrophen/id_71181648/vulkan-ontakesan-in-japan-ausgebrochen.html (in German)

Huffington Post ( in Japanese) http://www.huffingtonpost.jp/2014/09/27/ontakesan-volcanic-eruptio_n_5892000.html

And another image showing more peaceful times at the volcano.

IPTV video on ORF: http://iptv.orf.at/#/stories/2247361/

Webcam : http://www.kaidakogen.jp/livecamera/index.html (Thanks Diana)

This image came in via Twitter. It looks more like Mount Unzen than Ontake. If the location is wrong (as it is very often with Twitter images) see it as an image describing what a pyroclastic flow looks like. From the images I saw so far, I think (no expert) an ash cloud moved down the slopes. A pyroclastic flow would have killed the hikers within instants. Spica

Western Pacific : http://www.westernpacificweather.com/2014/09/27/japans-second-highest-volcano-explodes-saturday/

The volcano is popular among hikers because it is really easy to climb. To members of Shugendō- und Shintō-group like Ontake-kyō it has a high religious status and various rituals are practiced on the volcano. So many of the hikers are climbing it not only because of spotivic activities but also because of the religious importance.

Spica

Update with contributions from Edward, Esoadrille, Karenz and Rushes which were posted while I was editing:

http://space.io9.com/violent-eruption-traps-hikers-at-mount-ontake-japan-1639813357/1639819074/+spacemika

A collection of images and live reports and even a geological setting.

Rushes: “The animation of quakes in the Mt. Ontake region can be seen here: Seems there was remarkably little warning time.” http://www.data.jma.go.jp/svd/eew/data/hypo/Anime.html

KarenZ: Current VAAC for Onetakesan: Ash at FL260

http://ds.data.jma.go.jp/svd/vaac/data/TextData/20140927_ONTA_0007_Text.html

Initially ash reached FL 370:

http://ds.data.jma.go.jp/svd/vaac/data/TextData/20140927_ONTA_0002_Text.html

Ontakesan has been relatively inactive. Her previous eruption was a VEI 1 but no confirmed Holocene eruption prior to that. From GVP:

http://www.volcano.si.edu/volcano.cfm?vn=283040

And also from GVP:

“The massive Ontakesan stratovolcano, the second highest volcano in Japan, lies at the southern end of the Northern Japan Alps. It is constructed within a largely buried 4 x 5 km caldera and occupies the southern end of the Norikura volcanic zone, which extends northward to Yakedake volcano. The older volcanic complex consisted of at least four major stratovolcanoes constructed from about 680,000 to about 420,000 years ago, after which Ontakesan was inactive for more than 300,000 years. The broad, elongated summit of the younger edifice is cut by a series of small explosion craters along a NNE-trending line. Several phreatic eruptions post-date the roughly 7300-year-old Akahoya tephra from Kikai caldera. Ontakesan’s first historical eruption took place in 1979. A non-eruptive landslide in 1984 produced a debris avalanche and lahar that swept down valleys south and east of the volcano. Ascending this volcano is one of the major objects of religious pilgrimage in central Japan.”

http://www.volcano.si.edu/volcano.cfm?vn=283040

Edward´s Google Fu contributions which reached VC via mail:

This looks like a pretty good assessment of the stuff so far

http://www.westernpacificweather.com/2014/09/27/japans-second-highest-volcano-explodes-saturday/

Here are the results of my google fu – which might contain nuggest of interest

http://www.sciencedirect.com/science/article/pii/S0012821X10004462

http://www.wovo.org/0803_12.html probably the nearest volcano observatory

and it does mention “Kiso-Ontake” amongst it’s other volcanoes but I don’t know what the prefix kiso means

http://www.volcanodiscovery.com/is/on-take-eruptions.html

http://volcanoes.usgs.gov/hazards/lahar/ontake.php

and this shows that on-take is also the vent name for another volcano in japan http://www.wired.com/2010/02/siusgs-weekly-volcano-report-23-92010-stymied-by-weather/

From Elgjda, a comprehensive overview of previous activity: http://www.data.jma.go.jp/svd/vois/data/tokyo/STOCK/souran_eng/volcanoes/053_ontakesan.pdf

Thanks for the contributions! Spica