ῒΐ  /+  (,**0) .  ,-+ῌ,.- ῕ ῔,**0  .  /  ,**0  0  ,+ A Pyroclastic Flow Deposit Occurring at the Northeastern Foot of Nakadake,Aso Volcano (Japan) and its Stratigraphic SigniﬁcanceYasuo MIYABUCHI῍, Kazunori WATANABE῍῍ and Yoshitaka EGAWA῍῍῍A basaltic pyroclastic ﬂow deposit, the Izumikawa pyroclastic ﬂow deposit, occurs at the northeastern foot ofNakadake Volcano, which is the only active central cone of Aso caldera, southwestern Japan. The pyroclastic
ﬂow deposit covers a fan-shaped area of about *.3ῌ+.3 km,, and the bulk volume is estimated at ...3..+*0 m-.
The deposit is poorly sorted, and consists of subangular faceted clasts and spherical cauliﬂower bombs set in asandy non-cohesive matrix. The deposit forms two di#erent facies : a black reversely graded lower unit and areddish-gray reversely graded upper unit. The cauliﬂower bombs, which have slightly vesiculated crusts anddenser interiors, are more abundant in the lower unit than in the upper unit. The presence of the cauliﬂower
bombs suggests that the pyroclastic ﬂow was generated by an explosion at the source lava lake or conduit, which
was ﬁlled with mixture of solidiﬁed and molten lavas. The age of the deposit was estimated at ca. +3 cal ka, based
on +.C ages obtained from charred wood fragments in the deposit. Recent tephrochronological studies reveal thatNakadake Volcano became active from ca. ,,ῌ,+ cal ka and that violent scoria and ash eruptions of Nakadake
were concentrated in two periods of ca. ,,ῌ,+ cal ka and +2ῌ+0 cal ka. The age of the Izumikawa pyroclastic ﬂow
corresponds an intermediate period between the two violent eruption periods. A similar pyroclastic ﬂow deposit
and a basaltic lava ﬂow were also identiﬁed. They cover immediately the Izumikawa pyroclastic ﬂow deposit.
These facts indicate that multiple violent eruptions producing pyroclastic ﬂows and lava ﬂows occurred in a shortperiod at ca. +3 cal ka. Recent activity of Nakadake has been characterized by ash eruptions, strombolian
eruptions and phreatomagmatic explosions. However, the presence of the Izumikawa pyroclastic ﬂow deposit
emphasizes the potential hazard induced by bomb-rich pyroclastic ﬂows that may rush down the ﬂanks of
Nakadake Volcano.
Key words : Nakadake Volcano, pyroclastic ﬂow deposit, cauliﬂower bomb, eruptive history+ῌ ῐ ῎ ῑ ῏h!i

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20*ῌ*20,  .ῌ++ῌ+0 ¡¢£Kyushu Research Center, Forestry and Forest Products
Research Institute, Kurokami .ῌ++ῌ+0, Kumamoto
20*ῌ*20,, Japan.
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Faculty of Education, Kumamoto University, Kurokami,ῌ.*ῌ+, Kumamoto 20*ῌ2///, Japan.
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Mashiki Junior High School, Souryo 3**, Mashiki,Kumamoto 200ῌ,,--, Japan.
Corresponding author : Yasuo Miyabuchi
e-mail : yasuo@a#rc.go.jp