抄録
A small ash deposit was recognized on snowpack around the Nakadake crater, Aso Volcano, SW Japan, on 18 February 2008. The ash deposit was distributed around the first crater of Nakadake with the NE- and SE-trending axes. Based on the isopleth map, the total weight of ash was estimated at about 2 tons. The ash deposit was composed of fine-grained (＜0.5mm) crystallized or altered glass shards, altered lithics, crystals and apparently clear glass shards. The clear glass shards were transparent to pale-brown in color, and the proportion of these clear glass shards in the deposit was about 19%. Some clear glass shards presented dark-brown rims and/or were characterized by typical blocky textures with cracked surfaces. The surface morphology of clear glass shards strongly indicates that they resulted from brittle behavior by interaction of magma and water under wet condition. We believe that the clear glass shards included in the February 2008 ash were related to newly ascending magma and that they were partially hydrated or altered due to hydrothermal water after fragmentation of magma, and erupted due to an increased gas flux probably in the evening of 17 February 2008.
