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,**2 , 1 - + .O ,**2 , ++ - ,+ .OThe February ,**2 Ash Deposit from the Nakadake Crater, Aso Volcano, Japan
Yasuo MIYABUCHI and Shin-ichiro IKEBE
A small ash deposit was recognized on snowpack around the Nakadake crater, Aso Volcano, SW Japan, on+2 February ,**2. The ash deposit was distributed around the ﬁrst crater of Nakadake with the NE- andSE-trending axes. Based on the isopleth map, the total weight of ash was estimated at about , tons. The ash
deposit was composed of ﬁne-grained (*./ mm) crystallized or altered glass shards, altered lithics, crystals andapparently clear glass shards. The clear glass shards were transparent to pale-brown in color, and the proportionof these clear glass shards in the deposit was about +3ῌ. 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 ,**2 ash were related to newly
ascending magma and that they were partially hydrated or altered due to hydrothermal water after fragmentationof magma, and erupted due to an increased gas ﬂux probably in the evening of +1 February ,**2.
Key words : ash deposit, clear glass shards, Nakadake, Aso Volcano+    '(;?PLQR 2:STUV/W@01&+ +322 ,XY +33/ ,/XZ@:
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20*῍*20,  .῍++῍+0 !
Kyushu Research Center, Forestry and Forest Products Research Institute, Kurokami .῍++῍+0, Kumamoto 20*῍*20,, Japan203῍,,-, "#$ +3-*῍+
Aso Volcano Museum, +3-*῍+ Akamizu, Aso, Kumamoto 203῍,,-,, Japan.
Corresponding author : Yasuo Miyabuchi
e-mail : yasuo@a#rc.go.jp

202

Fig. ,. Relation between area (km,) and weight(g/m,) of the Mid-February ,**2 ash aroundthe Nakadake crater.
Fig. +. Distribution of the Mid-February ,**2 asharound the Nakadake crater. C+ : +st crater, C, :,nd crater. Base map is + : ,/,***-scale topographic map of the Asosan district published by
the Geographical Survey Institute. Broken lines
indicate isopleths (g/m,). Longitude and latitude
are shown as WGS2..

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203

Fig. -. Grain-size histogram of the Mid-February,**2 ash sampled at site A (SW crater rim).

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Fig. .. Polarizing microscope photographs of clear
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204

Fig. /. SEM image of a typical clear glass shard
included in the Mid-February ,**2 ash aroundthe Nakadake crater. The surfaces of blocky
glass grain are covered by a branching networkof cracks (arrows).

Fig. 0. Records of snowfall and daily maximum snow
depth from ,1 January to ,3 February ,**2
observed at the Asosan Weather Station (AWS),
located at about + km west of the Nakadakecrater. Data are from the Japan Meteorological Agency (JMA).

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