Description of the November 21, 1986 Fissure Eruption on the Caldera Floor of Izu-Oshima Volcano, Japan : Analysis of a Series of Photographs

1986年11月21日に伊豆大島のカルデラ床で発生した割れ目噴火について,外輪山北東端と元町郊外で撮影したスチール・フィルムなどをもとに外形上の発達の推移を調べた.三原山の山頂火口から北へ約1km離れたカルデラ床でF1が16時15分に開口し,白煙に続いて灰色の噴煙と赤熱物が噴出した.追いかけるようにF1の南東側にF2が開口した.続いて,16時27分にF3がF2の南東側に,16時40分にF4がF3の南東側にそれぞれ開口した.全体としてみれば,噴火割れ目は北西から南東に,低地から高地に向けて次々に開口した.各割れ目は開口直後に約25m/minの速さで両方向にそれぞれ拡大した.噴火割れ目からの赤熱噴出物は火のカーテンの観を呈した.F1とF2は一列の噴火割れ目になり,それとF3はミ型に雁行した.噴火の勢いは16時50分頃から著しく増大した.火柱の高さはそれまで平均15m/minで増していたが,16時50分過ぎから100m/minの速さに変わった.火柱の高さは17時10分頃に最高の1.6kmに達し,その後しだいに低くなった.火口列の全長はちょうど1.0kmに達した.On the evening of November 21, 1986, new fissure eruptions occurred on the caldera floor of Izu-Oshima Volcano, Japan. The sequence of their formation is studied on the basis of time-printed photographs which were taken in succession by the authors at close range during the eruption. Shortly after 2 p.m., earthquakes began to increase in number. At 4 : 15 p. m. (7 : 15 a. m. in UT), the first eruption began with the opening of a small fissure from which gas started to rise into the air and then red-hot material spurted up. This outburst was about 1 km north of the summit pit of Mt. Miharayama. At about the same time the second outburst broke out 150 m to the southeast. At 4: 27 p.m., en echelon fissure started to open 110 m farther southeast. During the early stage of eruptions, individual fissures extended, with an average speed of 25 m/ min, both to the northwest and to the southeast across the caldera floor. Red-hot scorias with enormous clouds squirted from many points along fissures in the form of a nearly continuous line of eruptive fountains, producing a curtain of fire. The fountains gradually grew in height with an average speed of 15m/min. By 4: 40 p.m., the curtain of fire reached a height of 440 m and a length of 870 m. During the eruption, deep rumblings emanating from fountains were heard, and numerous earthquakes were felt. At 4: 40 p.m., a new eruption occurred 70 m farther southeast at the flank of Mt. Mihara-yama. On the assumption that the eruptive activity migrated to the southeast due to a rise of local dyke, an average rate of the rise is estimated to be about 3 m/ min. Shortly after 4: 50 p.m., the eruptive phase markedly increased in violence and changed form from fire fountains to some red-hot pillars. The pillars grew rapidly higher with an average velocity of 100 m/min. They reached maximum heights of 1.6 km at 5: 10 p.m. By that time, the fissure zone extended as much as 10 km long and consisted of four individual fissures 140-440 m in length. Lava from fissures spread out to form fan-shaped flows to the north-northwest and to the northeast of the caldera floor. The activity started to decline around 6 p.m. and ceased at about 11p.m., having lasted 7 hours. Although no lives were lost, a series of violent eruptions on November 21 forced all 10,000 islanders to evacuate Izu-Oshima island temporarily