抄録
Small ash emissions were frequently observed from a crater lake, in which the amount of water decreased considerably, at Nakadake, Aso Volcano, southwestern Japan, between April and August 2005. During this period, major ash emitting events, which produced mostly white aggregated ash, occurred on April 14, June 10-12, June 21 (4.2 tons) and July 25 (1.2 tons). Ash emissions at the Nakadake crater in 2005 were classified as three types: gentle release of white aggregated ash from fumaroles inside the crater lake (e.g. daytime of April 14, June 10-12 and July 25); short (ca. 20seconds) gas-and-ash emission through the crater lake (April 14 20: 41); and emission of black ash from an almost dry vent (June 21). All products from these ash emissions were composed of fine-grained ( ＜ 1mm) glass shards, crystals and lithic fragments, and contained neither lapilli nor blocks. Although the glass shards show varying degrees of crystallization and alteration, clear glass shards, which appear fresh and are not altered, may be juvenile materials.
