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The Sequence and Style of the +322ῌ+33/ Eruptions ofNakadake Aso Volcano, Kyushu, Japan῍῍Shin-ichiro IKEBE῍, Kazunori WATANABE῍῍ and Yasuo MIYABUCHI῍
The latest eruptive activity of Nakadake, Aso Volcano, occurred in the period +322ῌ+33/. We observed thesurface activity and the ash-fall deposits to determine the sequence and style of eruption during this period. The+322ῌ+33/ Nakadake activity is divided into four stages : the early stage, the climactic stage, the post-climacticstage, and the ﬁnal stage. The ﬁnal stage means afterheat activity. In the early stage (March +322-October+323), the crater bottom, which had been occupied by water, gradually dried up. Thereafter, it emitted black ashfrom dried-up vents. The glass included in the ash fall deposits was composed mainly of blocky particles. In theclimactic stage (October-November +323), the activity alternated between strombolian and phreatomagmaticeruptions. During this stage, there was a rapid increase of brown and clear scoria glasses in the deposits, together
with increased activity of isolated volcanic tremors. From the post-climactic stage (December +323-February+33+) to the ﬁnal stage (March +33+-October +33/), surface unrest at the crater continued due to a remarkablechange in water level of the crater lake. Ash, strombolian and phreatomagmatic eruptions were observed. Inthe post-climactic stage, opaque blocky grains were predominant in the ash fall deposits. The largest explosive eruption of the +322ῌ+33/ activity occurred at the crater lake on ,* April +33*, generating scoria deposits.
Distinct fractures were identiﬁed on the surface of ash particles in the ,* April +33* deposit, which strongly
suggests that the ash grains were chilled rapidly by magma-water interaction (phreatomagmatic eruption). In theﬁnal stage, most of the blocky glass particles were surrounded by a brown skin, possibly a hydration skin. Thus,
the surface activity and characteristics of the ash fall deposits probably varied between stages during the activityof +322ῌ+33/. We believe that the presence of a large amount of water in and around the crater played animportant role on the eruptive activity at the Nakadake crater. For hazard mitigation purposes, it is importantto remember that phreatomagmatic eruptions can suddenly occur, not only in the climactic stage, but also in thepost-climactic and ﬁnal stages of an eruption.
Key words : +322ῌ+33/ activity, phreatomagmatic explosion, ash grain morphology, Nakadake, Aso Volcano+ῌ ῐ ῎ ῑ ῏789:;<=8>?:8@ABCDEF; GHI

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Aso Volcano Museum, +3-*ῌ+ Akamizu, Aso, Kumamoto 203ῌ,,-,, Japan.
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Faculty of Education, Kumamoto University, Kurokami ,ῌ.*ῌ+, Kumamoto 20*ῌ2///, Japan.

20*ῌ*20,  .ῌ++ῌ+0"#$%&'()*+(Kyushu Research Center, Forestry and Forest Products Research Institute, Kurokami .ῌ++ῌ+0, Kumamoto 20*ῌ*20,, Japan.
Corresponding author : Shinichiro Ikebe
e-mail : ikebe-shinichiro@asomuse.jp