Mount Ontake

Japanese: 御嶽山 - おんたけさん

A composite stratovolcano towering over the border of Nagano and Gifu prefectures. It is also called Kiso Ontake. The central crater cone, Kengamine, is the highest peak at 3,067 meters above sea level. The outer rim (Marishiten Mountain, Mamahadake Mountain), parasitic volcanoes (Tsugikodake Mountain, Mikasa Mountain), and crater remains (Ichinoike Lake to Gonoike Lake) are connected from north to south, and are collectively called Mt. Ontake. Of the five lakes around the summit, Ninoike Lake and Sannoike Lake always contain water, but Ichinoike Lake is often dry (the crater wall on the northeast side is chipped, and water flows into Ninoike Lake). Ninoike Lake is the highest lake in Japan (2,908 meters), and Sannoike Lake is the third deepest alpine lake in Japan (13.3 meters). Up until the sixth station there are mostly miscellaneous trees and larch, but from around 2100m the area changes to a beautiful forest of natural cypress, hemlock and larch, and at the eighth station there are flower fields. The majestic base of the mountain has been developed into a ski resort, campsite and villa area, but the main areas are Kaida Plateau, Samuhara Plateau and Ontake Plateau, and northwest of Ontake Plateau there is Tanohara Natural Park (2190m) which is rich in alpine plants and has a walking trail. The breeding ground for the Japanese roller near Kurosawa is a nationally designated natural monument.

The mountain itself has also long been an object of worship as a sacred site for the Ontake sect. Until the mid-Edo period, the Owari clan prohibited the general public from climbing the mountain in the name of forest conservation, but around the Tenmei and Kansei eras (1781-1801), climbing was first permitted, and Ontake ko, a group of believers from the Chubu, Kansai, and Kanto regions, became popular. Each ko established a sacred place (cemetery) along the mountain trail up to the seventh station, where each ko erected a spirit monument as a place for their spirits to rest after death. There are approximately 20,000 of these monuments, and they are still being erected today. There are several trails to the mountain, including the Otakiguchi, Kurosawaguchi, Kaidaguchi, and Nigorigo Onsenguchi in Gifu Prefecture, and on the Nagano side, you can take a bus from Kiso-Fukushima Station on the JR Chuo Main Line. In addition, believers chant "Rokkon shojo, the mountain is clear and sunny, may our repentance be protected" in order to purify their body and mind and pray for the merit of being able to ascend the mountain safely.

[Hiroyoshi Kobayashi, August 21, 2018]

Volcanic activity

It is an active volcano in the Eastern Japan Volcanic Zone, composed of basalt to rhyolite. The central cone, Kengamine, is composed of andesite. It was first formed as a stratovolcano in the middle of the Pleistocene (Pleistocene), and after about 100,000 years of dormancy, it erupted violently about 80,000 years ago, forming a caldera. It continued to erupt repeatedly from the end of the Pleistocene to the beginning of the present age, and at the end of that period, a series of craters (lakes) known as the "Five Peaks and Five Lakes" were formed. At the time, it was thought to be an extinct volcano, but a small phreatic eruption suddenly occurred in 1979 (Showa 54). This eruption led to the abolition of the term "extinct volcano." Subsequent geological surveys revealed that there have been at least 15 eruptions in the last 7,500 years (excluding historical periods).

Small-scale fumarolic activity also occurred in 1991 and 2007, and in 2008, the Japan Meteorological Agency introduced eruption alert levels, designating it as level 1 (normal). In 2009, the Coordinating Committee for Prediction of Volcanic Eruptions selected it as one of the 47 volcanoes that require improved monitoring and observation systems for volcanic disaster prevention, and its activity was monitored 24 hours a day as a volcano under constant observation. In mid-September 2014, the number of volcanic earthquakes temporarily increased, but then decreased. As there was no crustal movement, the alert level was not raised. However, at 11:52 on September 27, a phreatic eruption occurred. At 12:36 on September 27, the eruption alert level was raised to 3 (mountain entry restrictions). The scale of the eruption was small, similar to the 1979 eruption, but large volcanic rocks were scattered near the summit, catching many climbers in the process. The eruption also produced low-temperature pyroclastic flows, and the plume of smoke temporarily reached a height of 7,000 meters. 58 people were killed, and 5 were missing (as of November 6, 2015). Since October 2014, there have been no eruptions, and although weak plume and seismic activity continues, volcanic activity has decreased, so in June 2015, the eruption alert level was lowered to 2 (restrictions on movement around the crater). Since then, plume activity and seismic activity around the summit have gradually decreased, and volcanic activity has calmed down, so in August 2017, the eruption alert level was lowered to 1 (note that it is an active volcano).

[Akira Suwa and Setsya Nakata, August 21, 2018]

"Kiso Ontake" by Hitoshi Yokouchi (1955, Kiso Ontake Tourism Federation)" ▽ "Mount Ontake" by Yasutaro Shimada (1982, Senmura Shoten)"

[References] | Ontake sect | Kiso Valley

Mount Ontake
A stratovolcano also known as "Kiso Ontake." It is 3,067 meters above sea level. It is a symbol of the Kiso Valley and has been known as a sacred mountain since ancient times, and is still worshipped by many people. The photo shows the view from Kaida Plateau. Kiso Town, Kiso County, Nagano Prefecture, Otaki Village / Gero City, Gifu Prefecture © Shinshu-Nagano Prefecture Tourism Association ">

Mount Ontake

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

長野・岐阜県境にそびえる複式の成層火山。木曽御嶽(きそおんたけ)ともいう。中央火口丘の剣ヶ峰(けんがみね)が最高峰で標高3067メートル。外輪山（摩利支天(まりしてん)山、継母(ままはは)岳）、寄生火山（継子岳、三笠(みかさ)山）、噴火口跡（一ノ池～五ノ池）などが南北に連なり、これらを総称して御嶽山とよぶ。頂上周辺の一ノ池から五ノ池のうち、二ノ池と三ノ池はつねに水をたたえているが、一の池は涸(か)れていることが多い（北東側の火口壁が欠けて水は二の池へ流出している）。二ノ池は日本最高位にある湖沼で（2908メートル）、三ノ池は高山湖としては日本で第3位の深さである（13.3メートル）。六合目あたりまでは雑木やカラマツが多いが、2100メートルあたりから天然のヒノキ、コメツガ、カラマツの美林になり、八合目にはお花畑がある。雄大な裾野(すその)はスキー場、キャンプ場、別荘地などに開発されているが、開田(かいだ)高原や寒原(さむはら)高原、御嶽高原などが中心で、御嶽高原から北西の山寄りには田ノ原天然公園（2190メートル）もあって高山植物が多く、遊歩道もある。黒沢付近のブッポウソウ繁殖地は国指定天然記念物。

　また、山体は御嶽教の霊地として古くから信仰の対象になっている。江戸中期までは尾張(おわり)藩が森林保護を名目に一般人の登山を禁止していたが、天明(てんめい)・寛政(かんせい)年間（1781～1801）ごろ初めて一般人の登山が認められ、中部、関西、関東方面から信者のグループである御嶽講の登山が盛んになった。登山道沿いに七合目あたりまで講ごとに霊場（墓地）を設け、そこへ各人が死後霊が納まる場所として霊神碑を建立している。その数はおよそ2万基といわれ現在も建てられている。登山道は王滝口、黒沢口、開田口、岐阜県の濁河(にごりご)温泉口などがあり、長野県側はJR中央本線木曽福島駅からバスを利用する。なお、信者が口々に「六根清浄(ろっこんしょうじょう)お山は晴天、懺悔(さんげ)懺悔守らせたまえ」と唱えるのは、心身を清め、その功徳によって無事に登山できるように祈るもの。

［小林寛義　2018年8月21日］

火山活動

東日本火山帯に属する活火山で、玄武岩～流紋岩からなる。中央火口丘の剣ヶ峰は安山岩。まず更新世（洪積世）中期に成層火山ができ、約10万年休眠後、いまから約8万年前に大噴火してカルデラを生じ、以後、更新世末ないし現世初頭まで噴火活動を反復し、その末期に「五峰五湖」と称される一連の火口（湖）を生成した。当時死火山と思われていたが、1979年（昭和54）に突然小規模な水蒸気噴火が発生した。この噴火は「死火山」という用語を廃止するきっかけとなった。その後の地質調査で、最近7500年間（有史を除く）に少なくとも15回もの噴火があることが明らかになった。

　1991年（平成3）、2007年（平成19）にも小規模な噴気活動が発生し、2008年には、気象庁により噴火警戒レベルが導入され、レベル1「平常」に指定された。また2009年には、火山噴火予知連絡会により「火山防災のために監視・観測体制の充実等の必要がある47火山」の一つとして選定され、常時観測火山として活動状況が24時間体制で監視されることとなった。2014年9月中旬、火山性地震が一時的に増加したが、その後減少。地殻変動を伴わなかったため、警戒レベルは引き上げられなかった。しかし、9月27日11時52分に水蒸気噴火が発生。9月27日12時36分噴火警戒レベルが3（入山規制）に引き上げられた。噴火の規模としては1979年の噴火と同様に小規模であったが、山頂付近では大きな噴石が飛散し、多数の登山者が巻き込まれた。噴火に伴い、低温の火砕流も発生し、噴煙は一時7000メートルの高さに達した。死者数58名、行方不明者数5名（2015年11月6日時点）。2014年10月以降は、噴火は発生しておらず、弱い噴煙活動や地震活動が続いてはいるものの火山活動は低下していることから、2015年6月、噴火警戒レベル2（火口周辺規制）に引き下げられた。その後も噴煙活動や山頂直下付近の地震活動は緩やかに低下し、火山活動が静穏化したため、2017年8月には噴火警戒レベル1（活火山であることに留意）に引き下げられている。

［諏訪　彰・中田節也　2018年8月21日］

『横内斉著『木曽御岳』（1955・木曽御嶽観光連盟）』▽『島田安太郎著『御嶽山』（1982・千村書店）』

[参照項目] | 御嶽教 | 木曽谷

御嶽山

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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