抄録
Izu-Oshima volcano is an active insular volcano mainly of basalt and located 120 km to the south of Tokyo. There are two connected calderas, 3-4km across, on the summit, and Miharayama, a post-caldera cinder cone, occupies the southern part of the caldera. The eruption of 1986 began with splended fire fountaining on Nov. 15 at the pit crater in the summit crater of Miharayama (A vent), after 12 years of dormancy. Well vesiculated scoria and ash including Pele's hair were prduced from the fountain. Lava filled up the Miharayama crater and flowed down to the caldera floor on Nov. 19. fountaining activity stopped on the night of 19th. Intermittent explosive eruption began on 20th and bombs became the dominant ejecta. A violent eruption started at 16 : 15, Nov. 21 with opening of fissure vents (B) trending NNW on the caldera floor. Eruption column rose up to 16, 000m. 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 (LCI) had almost reached to Motomachi, the largest town in the island. Until next morning most vents ceased their activity except A and B3 vents. A vent continued intermittent explosions until 23th, and B3 vent ejected fine ash until 22th. A large spatter rampart was formed along B fissure vent and two lave flows (LBI, LBIII) spread out on the caldera floor. Another small lava flow flowed out from unsolidified interior of the spatter rampart on Nov. 23. These lava flows are aa and transitive lavas to block lava. They are 5 to 10 meters thick and their surfaces were covered by reddish brown blocks of agglutinate from the collapsed spatter rampart. Thick deposit of air-fall scoria was distributed mainly on the east of the vents. This air-fall scoria is comparable in volume to the scoria ejected in the past large eruptions, but has different texture and vesicularity. Another small Strombolian eruption at A vent occurred late in the evening of Dec. 18 and lasted about three hours. The activity of 1986 erupted 6-8×107 tons of magma. This quantity is nearly comparable to that of 1950-1951 activity. Change in the mode of eruption (continuous fountaining to intermittent explosive activity) at A vent indicates the viscosity increase of the magma. Microscope and SEM observations have revealed that the groundmass crystallinity of the scoria erupted from A vent markedly increased with time, less than 3% to about 15%. This increase in crystallinity raised the viscosity of the magma ten times or more. Temperature decrease and degassing during the early eruption phase caused the increase in crystallinity.
