The Eruption of Izu-Oshima Volcano since 1986
1988 Volume 38 Issue 208 Pages 131-138
Details
1988 Volume 38 Issue 208 Pages 131-138
Izu-Oshima Volcano erupted in 1986 after dormant 12 years. The first phase of eruption started on Nov. 15 at the crater A on the summit of Miharayama cone by splendid' fire fountaining which attained 540 m high. Lava extruded with high rate, filled up the summit crater and flowed down to the caldera floor on Nov. 19. The second phase of eruption started on Nov. 21 with opening of fissure vents B trending NNW on the caldera floor. Eruption column rose up to a height of 16, 000 m. Strong seismic activity began two hours before opening of the vent and continued during the eruption. One and a half hour later another fissure vent C opened on the upper slope outside the caldera rim. Eleven craterlets were opened and a lava flow LC had almost reached to Motomachi, the largest town in the island. Until next morning most vents ceased their activity. Main part of the B fissue was covered with a large spatter rampart from which two lava flows LB I and LB III spread out on the caldera floor. Another small lava flow LB II flowed out from unsolidified interior of the spatter rampart on Nov. 23. Thick deposit of air-fall scoria was distributed mainly eastward.
A small eruption occurred on Dec. 18, lasted about two hours, and ejected bombs from A crater.
The activity of 1986 erupted about 6×107 tons or more of magma. This quantity is nearly comparable to that of 1950-1951 activity and a tenth of 1777-1792 activity. The rocks erupted from A crater are basalt, very common in this volcano. The rocks from B and C vents are more differentiated basaltic andesite or andesite which has not commonly found in this volcano. Volcanic tremor, once stopped after the eruption of Dec. 18, recurred on Jan. 1, 1987. Activity of tremor and earthquake gradually increased and since Aug., 1987 remarkably. It culminated to the small eruptions of Nov. 16 and 18, 1987. The pit crater on the summit of Miharayama was reproduced at the Nov. 18 eruption.
No marked activity has occurred after the eruption of Nov., 1987. Activities as volcanic tremor, earthquake and gas emission, however, has continued since then. Prudent watching and monitoring of the activity and the study of the eruption mechanism are needed.
