As volcanoes go, Ontake has led a quiet life. Apart from hydrothermal emissions and one unconfirmed eruption in 774 CE, the volcano has no Holocene activity prior to 1979, when it awoke without the usual explosivity one might expect. It started as a vapour column issuing from the summit, but soon changed over to ash and lapilli, which mostly fell to the northeast. After the initial day, activity declined to low level steam emission, and only some livestock had to be relocated due to ashfall in their pastures. The most lasting impression of the eruption was a new 500 m long row of fissure vents running northwest-to-southeast near the summit of the volcano.
The volcano quieted down for another decade, but in the spring of 1991, quake swarms and numerous episodes of volcanic tremor along with renewed emissions from the 1979 vents were cause for concern. It was becoming apparent the volcano was awake and would remain so. In 2007, inflation of the volcano and increased fumarolic activity along with volcanic tremor suggested magma was rising below the volcano and a phreatic eruption was likely. Later fieldwork discovered new ash deposited around the summit as well.
But still these were small and relatively recent events, and by no means were they what Ontakesan was known for. Being the 2nd tallest volcano in Japan behind Fuji, Ontake is more than just basaltic andesite with a bit of dacite and rhyolite. To those who practice Ontake-kyo Shinto, the volcano is their most sacred place, to which members of their sect make great pilgrimages. There are shrines along the ascent, but near the summit there is Ontake-jinja Okusha. White-robed pilgrims visit the shrine which was founded there 1,315 years ago to honour the three Kami (deities) Kunitokutachi no Mikoto, Onamuchi no Mikoto, and Sukunabikona no Mikoto, all of whom are associated with the mountain. The volcano is also a popular destination for hikers who are just seeking nature’s beauty. The ascent is considered suitable for novice hikers, and there are numerous trails and facilities on the volcano’s slopes to accommodate the people who come to see sights such as the five crater lakes, including Ni no Ike, Japan’s highest crater lake. (I previously stated Onamiike at Kirishima as being the highest, but that source appears to be in error.)Not surprisingly, the mountain’s slopes were covered with around 300 hikers just before midday on 27 September of 2014 when the volcano, with none of the usual warning signs, exploded in a phreatic eruption below its summit on the south flank. The plume from the eruption rose 7,000 m above the source vents and a pyroclastic densityh current travelled 3 km down the southern flank. Some rescue attempts began almost right away, and as the day wore on, the numbers of estimated missing persons rose to 32, believed to be buried by ash. There were also reports of many broken-bone injuries due to flying or falling rock. On the next day, the Japanese military and police put around 500 personnel on the search-and-rescue efforts, using metal and mine-detecting devices to look for victims buried in ash. In the days and weeks to come, the rescue would be hindered by nature again and again; first by increased tremor from the volcano and fear of renewed activity, then by the rains brought by Typhoon Phanfone which made helicopter missions to the muddy summit area too dangerous, and finally by the arrival of wintry weather. 27 October was declared a day of mourning for those lost in the eruption, the final official total being 57 dead and 6 missing. The 2014 eruption of Ontakesan is recorded as Japan’s most deadly volcanic eruption in the post World-War-2 era.
There is some astounding video footage captured by people who were on the mountain when it erupted.
The volcano today is still hydrothermally active, but otherwise remains calm.There is an Ontakesan webcam, but it appears to be offline at the moment I posted this entry.
Thank you Glenn Rivers for finding me, and thanks to all who guessed or followed along. Hopefully flu season at Villa Volcanohead has ended, and I’ll be on-time and ready this Monday to take you on new adventure at a new volcano.