伊豆大島 1986 年 11 月 21∿23 日の 2B 溶岩とその噴火

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The phase 2 eruption of the Izu Oshima volcano, November 1986, was remarkably observed not only from the ground but from many TV helicopters in the air. The eruption began at 16 : 15 on the 21st from a 1-km long fissure vent, B, on the summit caldera floor, which are composed of northwestern B_N fissure (560m long) and southeastern B_S fissure (460m long). By 16 : 55, more than 10-km high subplinian eruption column was established on the B fissure vent, while the B_N fissure fed 2B_N lava flow (1.3×10^<10>kg) from the northwestern lowest point and the B_S fissure also fed 2B_S lava flow (6.6×10^9kg) from the northwestern lowest point. These lava flows never originated as a so-called rootless lava flow or rheological flowage of agglutinate. They were formed by simple overflowing of magma to earth surface. Another lava flow 2C (4.6×10^8kg) from C fissure on the northwestern flank is also an overflowing lava. The phase 2 eruption is interpreted as the surface expression of an intrusive event of three sheets of dikes from the magma chamber. A possible rootless lava flow was formed during the first 30 minutes of the eruption along the B_N fissure when the fountain height was still low. It was buried by the succeeding subplinian scoria. However, on the afternoon of the 23rd it again appeared on the scoria cone to form 2B_C lava flow. During several days after the eruption, a part of the scoria cone moved up to 30 m toward the down slope. This is due to rheological flowage of agglutinate layer which was a result of the oblique fountain from a vent on the B_N fissure.
特定非営利活動法人日本火山学会の論文
1988-06-30

伊豆大島 1986 年 11 月 21∿23 日の 2B 溶岩とその噴火

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