有珠2000年噴火の噴出物 : 構成物とその時間変化(＜特集＞;2000年有珠山噴火 (2))

抄録

During the 2000 Usu eruption, a series of eruptions and/or explosions had intermittently continued since March 31. The initial eruption on March 31 had been the largest one and produced tephra fall with small pyroclastic flow. A scale of each eruption and explosion has decreased since early April. Since the middle of April, eruptions have spreaded trace amount of ash fall mainly around active craters. Considerable amount of juvenile materials have been recognized only in March 31 eruptives. Their volume percent in total eruptives had decreased abruptly from several tens % (March 31) to less than 1% (April 2). It had been hard to find juvenile materials in eruptive ash since then. Therefore, although the initial and following eruptions were phreatomagmatic, almost all the following eruptions since early April have been phreatic. Most characteristic feature in these phreatic eruptions was a discharge of mud bombs, because crater floors had been filled with mud. Juvenile materials of March 31 are dacitic pumice (SiO₂=69.0～69.9%) which are slightly less differentiated than those of 1977～78 eruptions. This is consistent with temporal variations in historic magma since AD 1663, suggesting that magma plumbing system has evolved since 1663 to 2000. However, compared with the former 1977～78 eruption, the 2000 juvenile materials show distinct, parallel chemical variation for example in SiO₂-TiO₂ diagram. This suggests that distinct and/or modified magma system has been active in the 2000 eruptions.