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The Sequence and Style of the +322ῌ+33/ Eruptions of
Nakadake 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-climactic
stage, 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 the
climactic stage (October-November +323), the activity alternated between strombolian and phreatomagmatic
eruptions. 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. In
the 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 an
important 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 the
post-climactic and ﬁnal stages of an eruption.
Key words : +322ῌ+33/ activity, phreatomagmatic explosion, ash grain morphology, Nakadake, Aso Volcano+ῌ ῐ ῎ ῑ ῏

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Aso Volcano Museum, +3-*ῌ+ Akamizu, Aso, Kumamoto 203ῌ,,-,, Japan.
20*ῌ2///  ,ῌ.*ῌ+,-./-0
Faculty of Education, Kumamoto University, Kurokami ,ῌ.*ῌ+, Kumamoto 20*ῌ2///, Japan.

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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 

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1812345624789:;
Table +. Summary of activities from +322 to +33/ at the Nakadake crater. C.L. : Crater Lake, M.
E. : Mud Eruption, Inc. : Incandescence, A.E. : Ash Eruption, Str. : Strombolian Eruption, Phr. :Phreatic Explosion or Phreatomagmatic Explosion.
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Table ,. Strombolian and phreatomagmatic eruptions at Nakadake between October and November +323. St. : Strombolian eruption, Ph. :Phreatomagmatic eruption.
 , +323  +*  ++  St. : Ph. : 

Fig. +. The bottom of the Nakadake ﬁrst crater
during the climactic stage. Arrow indicates wet
part near active vents (the photograph wastaken on +/ October +323).
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21

Fig. ,. Illustrations and photographs showing the variation of the vent size and location within the Nakadake ﬁrst
crater during the climactic stage. (+) Vents 23+ and 23, were combined almost in the same location (photograph no.+ was taken a few days before the combination). (,) Vent 23, was enlarged after a phreatic explosion at the end of October +323 (photograph no. , was taken on -* October). (-) A new vent opened at
the west of 23, vent on +1 November. (.) Vent 23, was buried by the deposits of the phreatic explosion
before dawn on ,0 November (photograph no. . was taken in the morning on ,0 November). (/) Vent 23,
opened again due to the phreatic explosion in the morning of ,0 November (photograph no. / was taken on
the same day). (0) The north western part of the crater wall collapsed on +1 February +33*.
 , 
 +  (+) 23+  23,  !"#!"$ %&'() (,) +323 * +* +,- . 23, /01234 56789: #+* + -* &;<) (-) ++ + +1 &. 23, => ?9@AB (.) 23, /. ++ + ,0 &CD 4E9:01234 567FGH: #++ + ,0 & 2 : -3 ;<) (/) ++ + ,0 &IJK. LM01234 4E
9. 23, /B9: #++ + ,0 & 2 : /, ;<) (0) +33* * , + +1 &. N=>O PQ9:

!"#$%!#&'()*22

Table -. The sequence of the phreatomagmatic explosions on ,* April +33*,
recorded by the AVM camera.
 -



 +33*  .  ,* +33.  /  ,  + ,-./ 012345678



29.:; 1 <=> ,+ <=+ ?@ABCDEFB

GHI J . KLM1N +34O8P+O5

Fig. 0

H;RI2 W  /  ,  XYLM,-./+Z[  #\ZLO,-./ ]^ _R;W 012`O]^Ua:I Fig. 03458Gc 2STdeI + V >42fg;

ῌ bKLM01 

2hij+kg lm262:;4;RV>2ndeI WH; +. o .1 pK/ Fig. 0012qr

῍LM ῎ _W +* pu vI:;01

6H;RIV>2 :;STde Fig. 0

Ua

st m 72

2345wexyI We zi:; +* <{|}LM+012 +* m {|῏ 2QH;R 2STU4;R V

῍ .I01 7/  ~U+  xN FBcNOGUaFC@G >XO[R;+  (+33+)  (+33/) Ono etNO PQR +322+33/ STU 

VWXY

23

Fig. -. Di#erent types of eruptions during the eruption on ,* April +33*. (A) Black smoke with white steam
and a large amount of cinders in the phreatic explosion. (B) Dark brown and dry smoke in magmatic
eruption (the bright object in the left part of this picture is a heated scoria).
 - +33*  .  ,* !!" #$%&'()
):;<=/0>?@ABFig. ..

 , * (B) +,+ (A)   -./0123$ * 456789

Sequential photos of a large-scale mud eruption from the crater lake on +/ November +33,.
 .

+33,  ++  +/ 

CDEFGHIJKL

M

DEFGHIEGJKLMN

24

al. (+33/) 6OPQ2R!S+TUV-WXY%!$

Z[\]123^-_`a23 bcde
+322  +33/ Y%f!gfhij1WXkl32 @Wmn!$ZS+TUV!$ZO!Po6
pq-r<[s WXk[<t/!uv
PQ2wx1 UV!yz[{Q/ῌ+ 

W|}~!WXk!A-e

Q +323  0  3

8 6 WXk[ij1 R! e W|Ede ![12  PQ  l32e  + W|!7d 8k[ij 6OFig. /. Floating materials, possibly scoria, on thecrater lake on +, October +33,.

PQ / +33,  +*  +, 

o¢ +**  l32£¤1 WXke¥¦§[¨

fh y -1/ !WXk -ijd¡ R!
©s ª«¬­®1= ¯° 1 ±Q¦²

Fig. 0. The activity in the crater lake on , May +33.. (+) Before the rise of water in the crater lake (normal
status). (,) Water level rose slowly by approximately +* m (+ second before the beginning of the water levelfall). (-) Water level fell down by about +* m (- seconds after the beginning of the water level fall). (.) A
mud eruption occurred (,+ seconds after the beginning of the water level fall).
 0 +33.  /  , !"#$%&' (+)!"#$%( )*+, (,) "#-. +* m /0123 )"#!456789: + ;(, (-) "#-. +* m 78 )"#!456789:< - ;=,(.) >?@ABC )"#!456789:< ,+ ;=,456789 +322+33/ :;<=>?@A6BCD

25

Fig. 1. Brown and clear scoria glass grains in the , November +323 ash deposit (photos + and ,) and brown andclear blocky glass grains in the ,3 May +33* ash deposits (photos - and .). Left photos : Photomicrographs
taken under crossed polars. Right photos : Scanning electron microscope (SEM) images. The particles in thephotographs are not identical.
 1 +323  ++  ,    + ,  +33*  /  ,3     - .  :  ! " : #$ %& ! ' ()*"+,-&.+/0

=E0FG !  ! #$ %& !=*)@

HIFJKLMIN

B :   1 2  .+ /OP*Q

/ῌ, ῍῏῎ῌῐῑ.+ R3S (+33/) Ono et al. (+33/) TU3
S (,**.) /^=H@_

`aLO= bcZde

V/OW.XYZ=[\]@(P* R3S(+33/) +fFghijN k.+ lP?mQ

nopP*qrs)N67tLuvHw= AF 

V/O=+x067t  jy z{:;u0|

67t }NO/^=_}N AF 67tde~Luv

=@
A:

W)@

L./0( / =E0F+ G=zA6L(}N s)N/67 =+ /O D.@(P* z)*L}F IN¡¢£¤+l*)/0(P* ¥¦/§   Fig. 1  ῎ ῏ +=¨©

}  s)NO.ª@ij*)@ T

U3S +33+ /^ zw¯ 1

2 Fig. 1 =«¬­/IN®c.Ar}NO. °±z²³± ´Z/µ=HIF¶ῌ ῍ + £· ¥¦/ H­0¸¹.

ºNOij*)@ R3S +33/

Ar}NO. ´µ=H*»  O=¼

C:¬WW½/IFAr}N.ª@ij

1

V&
2 w¯ QV/O=E0F

% &' 26

'()*+ ,-./0$12345678 59:F ;<0=>?@AB# @CD#$ E-'D#$

L@ Fig. 2 -M#$

FGHIJK=6

NO80=

PQ -+RSTJ-U4VW

D :   X@Y$ /8- 4ZU7[P-U4V\]\3^^_/0 `3abcd6N
O80=

-+Pi43K=5678

l/03E0m0noJK=6pq0= 50+efST-g=WXdhY$ EL12j^-+ Table . gk
Prs3t80$

8Z+"JK=u v3K=3 55J

wx@<6y=6 z6Q{JK=+ >-U4V+|4 }4+"#$

A  ~i3 +323 ! +*6"3##d$J+P4J Pd++ %:- # E+6bD#

6+"#V{Y$

V4P456 E0-[qYV +33* !w%

E : :-&YV78+ B   ~-+ "#$"@6= i3= - C   ~ii@4V 6bi '(3;<0=+%: +323 ! 3+* -4563jJK=

508-U4V+ %:)

' D    ^ E  ~i-

6#V4=5678 E6b3""JK

 % :-#$gkP $ P  +t80P4 F=6NO80= !"#$ +

F :  !"#$^%:-Zt80=

RSTJ* 3 $J ^+-tO=
JK= 123 Pd3Z

¡,K=4+.c<{ J"3abc <$+ /¥ ¨3©{80$d6NO80=D3<=9:/ῌ. :¢ k£ ¤Pd3 , -KY$ E08@w-¦§=
ª«0¬#$ +33* ! .  ,* 1-+ .

/ῌ- 

c<{78­®2P\]\¯°±3030²#$ 5

4³STJ 5´ + mm µ6¶@·# Q

6¸30J+6Z-7J+"JK=­

QgE¹3 *.+ mm wx-U4Vº» A

 ?¼ cm @½O= @-D#$ '

Fig. 2. The variation of ratio of each grain type in the ash. A : brown and clear scoria glass. B : brown andclear blocky glass. C : opaque scoria grain. D : opaque blocky grain. E : crystals. F : rock fragments andaltered materials.
 A :  B :   C : 2  D :    E :  F :  !"#$
Table ..

 +322+33/  

The component characteristics of ash-fall deposits during the +322 to +33/ activity.
 .







!

27

"#$

+322 +33/ 

%&'



(

)!*

%&'

+*ῌ A

"#%&' 7B( CD%&' 

3E/-FG:3H. ,4  ! "#$ %&'I@J(74-:K 89:%&' L;" /1ῌ '<=>#?@$

MNO *., mm PQ RLSKT T;" VW (XAFG. (Fig. 3)  Y

+,-. /. SEM 0123456789:!

T(U:HFG. (Fig. +*)Z[\]H.( ^_\`*Tab(6cT34

0 



Accd e:fN ghijklmnI7B

0ῌ+ 

5(E/-FG ooL   pYqr-os t5uv.0M o-/\w4-FG

FA/H.oLxyATz{( |3\R -XAFGoLxAFGXA.
3F5%&'  +33, +33- .}~x.KHF
FG A3A 

(AFG

LL}~:s(,-FG (d osGF

E/-! "#?@$ %&' 

xAFGI  (+33/) (AFG    I\d Ono et al. (+33/)(!3H. A3A:(4 os%&'  0DD¤?@x¥{. !" (¦§A#¨¢£(d h(©$ªx«¬oAF

F (Fig. +*) ^_\ ­®¯% °?
@$ 0DD·* (¸ ,-FGLQ\_0h m¡¢£:(±²,-³&xAFG ´µ¶,-. +¹:sºC»¼½  

@ A4 B 4CDEF

28

Fig. 3. Brown and clear blocky glass particles in the ,* April +33* ash deposit with many rapid-cooling cracks
caused by magma-water interaction. (A) Photo taken under crossed polars. (B) Scanning electron microscope (SEM) image. The particles in the photographs are not identical.
 3 +33*  .  ,*    !"#$% (A) &'()*+ (B) ,- ./'()*+ 0 1*+234/5267

Fig. +*. Photos of brown blocky glass grains in the ,/ February +33- ash with a dark brown “skin” caused byhydration. (A) Photo taken under crossed polars. (B) Scanning electron microscope (SEM) image. These
particles in the photographs are not identical.
 +* +33-  ,  ,/   !189()*+ (B) ,- ./'()*+ 0 1*+234/5267

GHIJKL MNOP1QROS
7^_UG]
0ῌ,  :

 (A) &'TUV GWXYZOS7[\5]
1!`6[\52abcdeOPfg
!"#$%&'

h6iS !jUklfm6h6 nOSV
GofZO pqr 
Gstu5]v`() *+,+323  +*  0 wxyz{q|  HG}~i+* Nw xyz{q|Hf

P xyz{q|H[\52 GH f!`6 HGG

IJh ; m N<= ~5GiSS7

iS7 +*  ,+ >,2 2G

 112GHw[MNQHIJh  ¡OPf¢£5

Zu*+¤¥M¦§6¨ww5]

 Owf 1©HGª? + «©¬¨w
°S±O ¬²³ + HI7i´7~5Gi

Gµ¶OP5 Z·¸xG¹h6 º

P1w nOP1v2w6»7¼½5w

H­®¦¯w2 12pqr

  +322+33/  










 !"/0)123,*

B/-

+33*

%   !"  #"

(JK&%3)L&) A%N/0)*O&'- +* I ,* $8
3 ,+ $8'-&)./

#$%&'()*%+,'- .44 567 +33* **83 9:9%

;<+=>?@ A  E  >FG  E&)H

29

AP0Q

2/ !"#" +33*

BCDC&'-)*%( 0' . I ,* $%&   9:9%'#  /-*"DF &,&)G/123,*  +1 @ *2 183R SD0)% P$ +- @TU833V 5*@ AYZ [/

>0'9:9 +M%&%

W4%X*0'-) +-@ ,- 1.\*"DF6783]8(9^:;_`a < =>Q?:;@b%(c-%/0)*>123, AW4% +1 @dO&'- Fig. 2  +323  +* I ,+ $ efg

383Bhij`323,'-

 F H

>k  E&) @ b cdlm

+1 @ *2 1839:9+M%& BC/0)0'-% *,DnoEp#-'E&)*"DFqFr AsHtu%vFG8,)w@b  D =>Q?:;@b xH%Iy&Gz +2 @ +* 1{83E !" Asa

J|80))} K~r' D =>Q?:;-&) +2 @ -* 1{JL

@b &'i &'-)}/M&)% P*83 BN+9^++ I +1 $ ,0 $O) ;Bs PQ)>?T +

BC6 am

9:9 +M%&%WXYFZ+,)*

-G3,'F RS0'  B"'# %UV&'('-*O&'-hE

Asa-[\

>G/ *@%&   9:9

G0'--% ]$^-[_ +** `3U DFf>G P+,'- 4a +33+ ; #baQ) +* I ,+ ${9:9(F 9:9+33*

f%N+0'-0)!9:9%#&)AYZ83 =>Q:;_`a %

*'# %cUV&'*>0'K~

P, k %odV0'-%ef 3

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r ¡%g3,)G123,
+33*  .  ,*  
* +322+33/ £]8  &8GR

Buttner et al. (+333)  9:9'# 

>

%&  -' ¢@ AqFThX*0)G

dry-type  wet-type  1  ¤¥wzg La
Fossa di Vulcano i¦&) P8/0) *%&  *"DF83§DF

wet g¨%&¤¥83¢@ A ©mª«Q

J&Q%& [jD§2) AO& P,¬2' k]B@b-' _`lm -& P /* 1­um-) P&'

an=k>0'o®, +3*, dryP¯ JL ¯paq-r9:¤¥YZ+,80)r°gf

9 ²0) D) £t%&@;u cock’s

vT&'O)w³+,)@b c-cvT

tail jet %>´-rµx> *¶y@b[ %Bij`323,'-G88®3s±y %&·Y+,'3y ¸0)6783%&vz/(¹qF+, P,3 La Fossa /0)*º®= *{ +323  +*++ I9 dry 9:9 +M%&>F wet 9::9 +M%&@GA/0) *>? v|

9 +M%&3, Q'-*"DF83%&   }& -®

+M%&/&' +33*  . I ,* +33*  . I ,* $ k &~&)_`a » Buttner et al. (+333) Q'-*$  *"DF6783SD0' %& 6 /F '#»r'# /-

7%@ _ /* 1 m( P¯R 9:9

*"DF  _`a@b% ,'r+,)*²-? /F *{83 £C&'(®}'&-W4/0)M4 Eefg83¹ +33*  . I§}{
83¼QE%¬&'F D) , I½ B

OrG/!" "#$%

30
0ῌ.

+33,+33.  

  !"#$%&'%()

 &'()*+,=>?@A BCD2EF1 AGH

ID'AJK' GHLMH NO
X  YZ[N/01

PQPR'S'0/00TUV+33,  0 dGe-.1!f"g#h,i$Li$jkID'lmNA JK2
rZ

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2 "2 3PQPH} .'IJbc4

G'5N$01   $A 6*'A +33, #9:('AzT0.1 ^ A7Ngh,'XN1 xylX '10 A . d ,* -  oH0

./01qN{9 'p`12`/0$- +* 8`x ~   gG+33-  i $A , dJbc2  ,`/':1 W'JK"; #h,L9( g A"m 'm69:(<,oTgq 'p`/01q`Ng0  ¡  '7N `` ¢G Az9£:¤¥ A/ @9A=

:($; >78'$¦N/ NAH?

 `

`g  glqx $AGJb9( §¨X 2q

@`/01qN4  _¨© 0A PQPª«¬";

6­*gi$D® g0 JK2(s`/0()*+LJbcA¯A'A°±`/0g0 A 

GH'o./ _ ¨z 02qN1 GH} 2²`1Jb9(~³0ῌ0 *+%,-.%/0'o./ JK2(s`/0()*+gB_

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0ῌ/

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A »MHNOX· gl JK G

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+* d'0A *'i$BCD'gFig. ++.

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W (Fig. +,) q 1 

Temporal change in size of the crater lake from +311 until ,**0 (observed by JMA).
 ++

+311  ,**0 

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31

1  

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  #A -C&'#D'(#E9FG,-/($

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/OPQRST
Fig. +,.

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Groundwater discharge from the crater wall.

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(hih!"#WX&Y,-/  =Z ) "@[ I[\P3 ^9_ ¡T`3 v *./+.*aT CbZ?- '   

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0ῌ1 

q;(3>u }IP;(XF]

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  4 Z` ;-opq ½slm7 4

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32

Fig. +-. (A) Temporal changes in the frequency of isolated volcanic tremors (observed by JMA) and (B)Proportion of brown scoria glass and clear, brown blocky glass.
 +- (A) 




 (B) 

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Buttner, R., Dellino, P. and Zimanowski, B. (+333)   +322+33/ 
Identifying magma-water interaction from the surface
features of ash particles. Nature, .*+, 022ῌ03*.
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)* (,**+) +,
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?@A>!D>" (+33+) SEM GHIJ
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 20 ++,ῌ++1.
!D>":."> (+323) 
/ 1 6 ef3g  -03.
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 .1 0+ῌ1*.
$%E H (,**/) 1 I  - J  0-/p.
$%E (,**/)   K¡¢£¤¥ 7¦§ .p.
©56 L ªkM«: (+33+)  +33*  . x ,* 1    3?+@
 .3 -3ῌ.,.
456N6 89¬­ (+33*)  
® +33*  . x ,* 1   3?+@
 .1 1+ῌ2-.
¯°±!D>"> (,**/)   I ,**-  1 x +* 1 ,**.  + x +. 1¶·¸4¹ºi-. E  /* ,,1ῌ,.+.
¯°±!D>"> (,**1)    ,**/     /1 +--ῌ



33

+.1.
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.
;<=>?@AB AC!D>"(+33/)   E  F  .* +--ῌ+/+.
5KL#M$%&'NO
PQ()R, -VWX Y
Z[\>.] /#(,**-)   2h 45 i   S 6 6 jk '(56 89l +, 0,ῌ0/.
Ors7Btu89:vw (+33*)+33*  . x ,* 1  !"#$%&89 y /+ 3.ῌ3/.
M$%& (,**-)   =>}~=> a|A +. B89l/,ῌ/0.
M$%& (,**.)  3=>%
=> a|A +/ B89l.3ῌ0*.
M$%& (,**1) 6 CD -+3p.
Tanaka, Y. (+33-) Eruption mechanism as inferred fromgeomagnetic changes with special attention to the +323ῌ
+33* activity of Aso Volcano. J. Volcanol. Geotherm.
Res., /0, -+3ῌ--2.
>!D>" (+33*)   +323 
f¨  S*?T6U+6 ^05 16_2`abcdef3g  23.
>!D>"?@A;<= (+33*)   +33*  . x ,* 1  1 6 ef
3g  +,0.
>!D>"¯°±?@A (,**-)    ,**-  1 x +* 15²OP³´ 1 6 ,**- wµ5 ef3g /+.

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