伊豆大島 1986 年噴火の溶岩流

抄録

During the 1986 Izu-Oshima eruption, various types of lave flows were formed. Those are classified into four : coherent lava flows, overspill lava flows, clastogenic (rootless) lava flows and rheomorphic lava flows. Characteristics of each type are summarized as follows : a) Coherent lava flows. In the later stage of LCI lava flow, coherent lava was observed to be effused from C6 crater. b) Overspill lava flows. The summit eruptions formed a lava lake, from where LA I～IV lava flows spilt over and flowed down the slope of the cone. c) Clastogenic lava flows. Two clastogenic lava flows, LB I and LB III, were originated from high lava fountains from B fissure vents. Those show relict clast structures, covered by blocks of collapsed scoria cone. d) Rheomorphic lava flow. A rheomorphic lava flow, LB II, was observed to flow down from the scoria cone beside B7 crater two days after the eruption of B fissure vents. Chemical composition of LB II is uniform as SiO₂=57wt%, though LBI, LBIII and most scoria falls from B craters show SiO₂=54wt%, except for the earliest scoria fall showing SiO₂=57wt%. This suggests that the lowest part of the scoria cone was welded and deformed, and squeezed up to the surface.