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,**.  2  -+  ,**/  0  +1 The July +*, ,**- and the January +., ,**. Ash Emissions from a Hot Water Poolof the Nakadake Crater, Aso Volcano, Japan
Yasuo MIYABUCHI῍, Shin-ichiro IKEBE῍῍ and Kazunori WATANABE῍῍῍Ash emissions occurred from a hot water pool of one of the craters of Nakadake, the only active central coneof Aso Volcano, southwestern Japan, on July +*, ,**- and January +., ,**.. The former ash was dispersed
throughout an area extending +. km east-northeast of the vent, whereas the latter ash extended 2 km to theeast-southeast. Based on isopleth maps, the total weight of ash discharged by each event was .+ tons and -, tons,
respectively. Both ash-fall deposits were composed of ﬁne-grained (+ mm) glass shards, crystals and lithic
fragments, and contained neither lapilli nor blocks. Some glass shards appear fresh and are probably juvenile
material. We therefore believe that these phenomena represent non-explosive gas-and-ash emissions through the
hot water pool. The ash particles emitted from the hot water pool of the Nakadake crater were aggregated at asize of +ῌ- mm and deposited because of a high content of water. The ash-fall deposits in proximal areas (+./km from the crater) were ﬁner grained and more poorly sorted than in distal areas (/ km). Moreover, thedeposit volume obtained from actual data in the proximal area was much larger than the volume extrapolated only
from the isopleth data of the distal deposits. These phenomena suggest that the proximal deposits were emplaced
mainly by another ﬁne-grain-rich column di#erent from the main ash plume that extended to distal areas, or thatsome parts of the column detached from the main plume that emplaced the proximal deposits.
Key words : ash emission, hot water pool, Nakadake, Aso Volcano+ῌ ῐ ῎ ῑ ῏}-.BC \(~ ,**- a ; YZ[\(]^_`  ,***  ++ ! + "#$%

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Kyushu Research Center, Forestry and Forest Products Research Institute, Kurokami .ῌ++ῌ+0, Kumamoto
20*ῌ*20,, Japan.
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Aso Volcano Museum, +3-*ῌ+ Akamizu, Aso, Kumamoto

203ῌ,,-,, Japan.
 ¦20*ῌ2/// §¨©ª ,ῌ.*ῌ+§º»¼»
Faculty of Education, Kumamoto University, Kurokami,ῌ.*ῌ+, Kumamoto 20*ῌ2///, Japan.
Corresponding author : Yasuo Miyabuchi
e-mail : yasuo@a#rc.go.jp

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`C !zw! -_ ,**- 6 1 A +*   ,**. 6 + A +. ¼M,CD78 

    `M 7 +  
./

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>?@

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+ A +/ +B,C

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B=C 0 cm `M "&Da+*,[bEc [*

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233

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p,* cmqA 2 km rWXQ tY ,0/ 1Zu¨©M=ªND J ^«H£q7s< s§ [v\ +./ kmr  + m, uD qr¬­ I8 ]%`M fu Ec j. <T (Fig. 1) zy®¯kl + m, `D .*

\7M sH  B=/7M @?A +.- km

w^FG6_Ig ,* g +* g + g *./ g *.- g

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² Fig. .

.ῌ- 
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Fig. .

  PB MB¸qr ¹P  h? *.-ube h?e0

ube`-_I *./ km
1

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 ,**- 6 1 A +*  [r` .+ ¯) D 66
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234

 

Fig. 0. Photographs of the January +., ,**. ash-fall deposit. (A) Western view of the Nakadake crater and the+. January ,**. ash-fall deposit on snowpack. (B) Aggregated volcanic ash deposited on snowpack at site D(about *./ km E of the crater). Scale segments are +* cm. (C) A wooden fence to which volcanic ash
adhered at site D. (D) A cap to which spot-like volcanic ash adheres at site G (about 0.- km ESE). (E)The loading platform of a track to which volcanic ash adhered at site G. Scale in centimeters.

¡ ¢£# ,**- ( 1 ) +* " ,**. ( + ) +. ¤B¥¦WH[\



235

Fig. 1. Distribution of the +. January ,**. ash from the Nakadake crater. Base map is a + : /*,***-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 Tokyo Datum.
/ ,**-  1  +*  ,**.  +  +. { |}~#/B .f QRi_ w




/ῌ+



_! TUbcd#* /*ῌ rqd#* .+ῌ #    A 
 B  

*.3 km! "... km! #$%&' ,**- ( 1 ) +* "  D  * *./ km! +,- E  .

* +./ km!  F ,**. ( + ) +. .

* / km! #$%&'#/0 1 23/B ,**. ( + ) +. 
23"M1*f(+ mm) QRZ#

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_

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 sf  ,.0 #  4 5 6 5 7 8 9    : ; < = > (Malvern

tu5vw]:G

Mastersizer S)  ?9@A,B CB D/B pq rqd#/0 F 23MstEF Fig. 2 G"+H# IJKL"M

u5vw:GoY +e-f(+/,e+/2 mm) fb

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 *f (+ mm) QRZ[\]H ^_1`aW'] bcd A 23 +e,f (+/,e+/.

mm) fbg5hY/H i]jklTUYmn

o0:

g5hY/H sf  +.3 "yzY,  .f QRi_
TU bcd#* /3ῌ #/0 rqd#00 pq rqd B  -e.f (+/2e+/+0 mm! g5hM stu5

* ,0ῌ #/B'\EFSUo0" "M

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/ῌ,

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

 _~\[o0 

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 A # -.,  B # ,./ #/H bcdqYyzY  A 

*.3 km! #$% ,**- ( 1·¸¹Hº}fH»f¼

236

Fig. 2. Grain-size histograms of the +* July ,**- and the +. January ,**. ash-fall deposits. See Figs. , and 1 forlocations of the sites.

 +* 

D*./ km,**.  +  +.   +*

;- 77kZ *.+=*./ mm m[ +* ῌ t+- ,*ῌ ,;S8 Y!"#$%&'()(*+,**-  1  +* ,8!;- NOP,- ./ 012 34 ! 5&'(6 EPMA ,kA

,78!+9: g!8 H- ,**-  Z;- NO"#$%&'(6;- +,<=><

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NOP G+=G- 7k<NOP? @ A B - CD EFGHIGHJKK

 G.  . ,y!8 G+ ;,5

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TUV; W + mm X6YA- Z[+@\%\NOPh- G, 7k G- ;+; B?A]JY NOP,;><^_ - ><`A,ab8; R!

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- ¢£¤J G+ ABB¥¦§NOPAab*+Aij !8A¨©y!8 H- ,**-  ª«- ,**.  + 

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NOPArve!8 (Fig.

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8*- ,**-  1  +*  ,**.  +  +. ,

!8S8+Ah- @xy;cKdA *./ mm± ;- +,n 8z{z,*B

X6

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0ῌ ῍

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237

Fig. 3. Polarizing microscope photographs of glass shards included in the July +*, ,**- (A, B) and the January+., ,**. (C, D) ash-fall deposits. Width of each photo is about + mm.

0ῌ+,**-  1  +*  ,**.  +  +. 
 ,**-  1  +* ,**.  +  +.  t 

+./ km 78

DE

uvwxyz{I /3ῌ ?@T |8 9/ km?@< 

,0ῌ ($")X)F}!

78~I~0 ;T  UV   !" #$ % &$"  FF   ;")

' ($") *+$  ,- .

)W12 9 

/012345 6)" 78

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9: *./ km ;7< => - mm ?@>3A 78 9BC

"DE") F   G!-H!F
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0.- kmDET,-  $.WX YZ")  FWF+./ km <)"  BC-H~3;.W

XHDEF I")   .
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(Fig.

/)  .* gῌm,  ¡¢ 9 D)W[ \]^$ F$F  E< £¤'I_`(aBC-H"YZDE(Fig. 1) X© ,**.  +  +. ª«ow  ,**-  1  +*  6 ¬­®¯°   !"

F¥¦§~¨X

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KT,- UV$");iOjkdfglmfgnopq ,**.< ,**.  +  +. @rsµ ·¸ #$  A$ X©" ¹~
3;.WX78 DE$X»\¼*+js+j

238

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1 234 56! 7+"#b@AcdeB +33/ Gt3Qu ,**-  1 8 +*  ,**.  + 8 +. 13

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a>/b?b0J/7Gt&R`Z/ 0>?$%@ABC ,**. a
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3 8R

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0 8 -* D1 8 + X 3 8 ,3 3Y/0

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1/234 Z!5[6 T\7]#. +)Y%&z#!CBY/{!0 T"#898V +./ km :% + m, M0|_ a  Fig. . T,$!3-.W&)/+"#

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 a>/b?b@AcdeB +33- +!#nAK %&$!Ku3  ;f7]#! P y3 Y/{ +ῌ+* TG +33.  3 8 +/ 3 4CDEF

w + mm P  +**ῌ G JmGHIgJKhLO &X+t&xG[ ¡Mi ; T99NO /./ km;< +Pj%k$!  a>/b?
b@AcdeB +33.
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 1 8 +*  ,**.  + 8 +. "  ,**-

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Tgº¬ GW,"EhiGJj_ ! .0*+,- ,**-  1 T +* ) ,**.  + T +. )¼ ;K$+,L 0239~/( &

0R1 + gῌm  *./ gῌm  *.-gῌm, $ - 

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( $6 +5e%f$g"!7 0
0 $, +ῌ0 9K K$i_89:(  ;<7+$ 0 c?=7

 ( Fierstein and Nathenson (+33,) $.=M   e%f$g"!tY c ,**.  +Fig. +*. Dispersal-fragmentation diagram (Walker,+31-, +32* ; Wright et al., +32*). The +* July,**- and +. January ,**. ash-fall deposits froma hot water pool of the Nakadake crater areplotted in the area of Surtseyan eruption. Data
for ash eruption of Nakadake are from Ono et al.
(+33/).

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Cas, R. A. F. and Wright, J. V. (+321) Volcanic successions,modern and ancient. Chapman and Hall, London, /,2 p.
Fierstein, J. and Nathenson, M. (+33,) Another look at thecalculation of fallout tephra volumes. Bull. Volcanol.,/., +/0ῌ+01.
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Inman, D. L. (+3/,) Measures of describing the size distribution of sediments. J. Sediment. Petrol., ,,, +,/ῌ+./.
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Ono, K., Watanabe, K., Hoshizumi, H. and Ikebe, S.
(+33/) Ash eruption of the Naka-dake crater, Aso volcano, southwestern Japan. J. Volcanol. Geotherm. Res., 00,+-1ῌ+.2.
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 y£Y  ¤K¥  bEFG  z¤z  ¤p=m2£K¦4y=§¨v/©^ (+331 a) ª«¬| +330 Q - r., +.+ῌ+/+.
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Walker, G. P. L. (+31-) Explosive volcanic eruptions - a
new classiﬁcation scheme. Geologische Rundschau, 0,,.-+ῌ..0.
Walker, G. P. L. (+32*) The Taupo pumice : products ofthe most powerful known (ultra-plinian) eruption? J.
Volcanol. Geotherm. Res., 2, 03ῌ3..
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Wright, J. V., Smith, A. L. and Self, S. (+32*) A working
terminology of pyroclastic deposits. J. Volcanol. Geotherm. Res., 2, -+/ῌ--0.

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