Mount Mihara

The central cone (758m) of the double-shaped Oshima Volcano, which occupies most of Oshima Island (Oshima Town, Oshima Subprefecture, Tokyo), is also called Mihara Shinzan or Uchi-yama. Oshima Volcano is an active basaltic volcano that has experienced repeated major eruptions at average intervals of about 150 years for the past several thousand years. The caldera, which opens to the east and has a diameter of 3-4 km, was formed by two major eruptions around the 5th and 6th centuries, and the current central cone, Miharayama, was formed by the last major eruption in 1777-1779 (An'ei 6-8). Subsequent eruptions have occurred within the Mihara crater, but in November 1986 (Showa 61), fissure eruptions occurred not only at the crater and caldera floor, but also at the northern outer rim. Lava flows approached the villages, strong earthquakes occurred one after another, and various abnormal phenomena that could be attributed to volcanic activity were observed in various places, so that almost the entire island population (approximately 10,000 people) evacuated off the island for about a month. This was the first eruption outside the caldera since the great eruption in 1421 (Oei 28). This volcano often erupts in the Strombolian style (small explosions repeated intermittently) or Hawaiian style (lava flows out in fountains), but major eruptions are characterized by violent scoria (rock scaly) eruptions, lava flows, and long-term intermittent volcanic ash eruptions, and lava flows often reach the sea. Medium eruptions occur on average every 30 years or so, and lava flows usually remain within the caldera. The total volume of erupted material in the 1986 eruption was about 60 million tons, which was about the same as the medium eruptions in 1950-1951, but the progress of volcanic activity will require particular vigilance for the time being. Small eruptions also occurred in 1987, 1988, and 1990. The 400-meter diameter crater inside the 800-meter diameter Mihara Crater is approximately 500 meters deep during dormant periods, but becomes shallower during active periods, which is useful for long-term forecasting of volcanic activity. The highest peak on the island is a cinder cone that formed inside the Mihara Crater in 1986, but the topography inside the crater changes due to its collapse and subsequent eruptions.

The eruption of Mt. Mihara is revered as a sacred fire, and the never-ending smoke is a symbol of Oshima, attracting many tourists, but in 1957, volcanic rocks killed one person and injured 53. The Izu Oshima Volcano Disaster Prevention Liaison Office constantly monitors the volcano, and there is also a volcano observatory of the Earthquake Research Institute of the University of Tokyo. It belongs to Fuji-Hakone-Izu National Park, and there is a bus service to the edge of the caldera. In February 1933 (Showa 8), a female student from Tokyo committed suicide by jumping into the crater, which was widely reported in the newspapers. In the six months following, 129 people (12 of them women) committed suicide, most of whom were young men and women between the ages of 18 and 25. As a result, the area became a popular suicide spot, and the number of suicides continued to increase, but it was only during the period from World War II to the postwar period that the number of suicides finally became rare.

[Akira Suwa]

"Special Geological Map 26: 1986 Eruption of Izu Oshima Volcano" (1987), published by the Geological Survey of Japan

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

伊豆諸島の大島(おおしま)（東京都大島支庁大島町）の大部分を占める二重式の大島火山の中央火口丘（758メートル）。三原新山、内輪山ともいう。大島火山は玄武岩質の活火山で、過去一万数千年間は、平均約150年間隔で大噴火を反復してきたが、直径3～4キロメートルの東方に開くカルデラは、5～6世紀ごろの2回の大噴火に伴って形成され、現在の中央火口丘・三原山の山体は、最後の1777～1779年（安永6～8）大噴火で生じた。以後の諸噴火は三原火口内でおきたが、1986年（昭和61）11月には、同火口やカルデラ床のほか、北側外輪山でも割れ目噴火した。溶岩流が集落に迫り、強震が続発し、諸所で火山性らしい種々の異常現象が認められたため、ほぼ全島民約1万人が、約1か月間、島外に避難した。カルデラ外での噴火は1421年（応永28）大噴火以来である。この火山はストロンボリ式（小爆発を間欠的に反復）ないしハワイ式（溶岩が噴泉をなして流出）の噴火をよくするが、大噴火は激烈なスコリア（岩滓(がんさい)）噴出、溶岩流出、長期にわたる断続的火山灰噴出が順次展開され、溶岩流が海に達したことが多い。中噴火は平均三十数年おきにおき、通例、溶岩流はカルデラ内にとどまった。1986年噴火の噴出物総量は約6000万トンで、1950～1951年の中噴火程度だったが、当分、火山活動の推移はとくに警戒を要する。1987年、1988年、1990年にも小噴出した。径約800メートルの三原火口内の径約400メートルの火孔は、静穏期には深さ約500メートルにもなるが、活動期には浅くなり、火山活動長期予測に役だつ。島の最高峰は、1986年に三原火口内にできた噴石丘であるが、その崩壊や次の噴火などで、火口内の地形は変動する。

　三原山の噴火は御神火(ごじんか)と仰がれ、絶えることのない噴煙は大島のシンボルで、観光客が多いが、1957年には噴石で死者1、傷者53を出した。伊豆大島火山防災連絡事務所が常時火山観測をしており、東大地震研究所の火山観測所もある。富士箱根伊豆国立公園に属し、カルデラ縁までバスの便がある。なお、1933年（昭和8）2月、東京の一女学生の前記火孔への投身自殺が新聞で大々的に報じられたのをきっかけに、半年間に129人（うち女性12人）もの自殺者があり、その大半が18歳から25歳までの青年男女だったため、一躍、自殺の名所にされ、自殺者が後を絶たなかったが、第二次世界大戦から戦後にかけてようやく自殺者はまれになった。

［諏訪　彰］

『地質調査所発行『特殊地質図26　伊豆大島火山1986年の噴火』（1987）』

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