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,**/  -  -+  !" ,**/  +*  +  #$Using Aggregated Particles to Estimate a Cloud Height Sedimentation Process of the September ,-, ,**.,
Pyroclastic Fall at the Asama Volcano Eruption
Marekazu OHNOῌ, Shuji YAMAKAWAῌ, Masayuki O’ISHIῌῌ, Kou TAKAHASHIῌῌῌ,
Tatsuyuki UENOῌῌῌῌ and Takafumi IDAῌA cloud height generated by a volcanic eruption reﬂects the immensity and/or magnitude of the eruption ;thus a measuring of the height’s temporal variation during the event is very signiﬁcant in judging whether theactivity will become violent or decline. However, when a volcanic eruption occurs during bad weather, we musttake information about the cloud’s height by means of the pyroclastic deposits. In general, the total time taken
for pyroclastic materials to be ejected and deposited at a given distance from the source vent can be divided into
three parts as follows : the time for the eruption cloud to ascend and reach its neutral buoyancy level (T+) ; the time
for the pyroclastic materials to be transported laterally by the eruption cloud (T,) ; and the time for pyroclastic
materials to fall and be deposited on the ground (T-). Since T- can be calculated from the settling velocity of
pyroclastic materials, if the time that the pyroclastic materials fell at a given locality was observed and a given
value for T+ is assumed, the most suitable wind velocity to explain T, can be determined. Thus the height at which
pyroclastic materials separate from the eruption cloud can be determined by using the vertical proﬁle of windvelocity around the volcano. These ideas were applied to the eruption occurred at +3 : .. (JST) on September ,-,,**., at the Asama volcano, which produced a pyroclastic fall deposit with a minimum weight of 1.,%+*0 kg.
Because this eruption occurred in bad weather, the pyroclastic materials fell as mud raindrops that were aggregate
particles saturated by the rainwater. Based on the depositional mass, the number of impact marks of the mudraindrops in the unit area, and the apparent density and the equivalent diameter of these drops during their fall was
estimated to be ,.,ῌ-.+ mm, which is consistent with the grain-size distribution of pyroclastic materials. According to some experienced accounts, mud raindrops several millimeters in diameter fell at ,* : *&+/0ῌ2//* '()*+ -ῌ,/ῌ.*,-.#-/0123456-6
Department of Geosystem Sciences, College of Humanities and Sciences, Nihon University, -ῌ,/ῌ.*,Sakurajosui, Setagayaku, Tokyo +/0ῌ2//*, Japan.
&+3,ῌ*-31 789:;< +ῌ+=>?@--A#-BC60#6-DE
Department of Geography, Tokyo Metropolitan University, +ῌ+, Minamiosawa, Hachioji +3,ῌ*-31, Japan.
&-3*ῌ20*+ F,:G -ῌ+ῌ+HI--AJ-KBC601LM23456DE
Division of Environmental System Sciences, Graduate
School of Science and Technology, Shinshu University, -ῌ+ῌ+, Asahi, Matsumoto -3*ῌ20*+, Japan.

&0/1ῌ2/*+ NO:P)QRST +ῌ+NO--AUV6-BC601WX23456-DE
Department of Earth and Planetary System Sciences,
Graduate School of Science and Technology, Kobe
University, +ῌ+, Rokkodai-cho Nadaku, Kobe 0/1ῌ
2/*+, Japan.
YZ[ : &+/0ῌ2//* '()*+ -ῌ,/ῌ.*,-.#-/UV6-BCZ
Present : The Institute of Natural Sciences, Collegeof Humanities and Sciences, Nihon University, -ῌ,/ῌ.*, Sakurajosui, Setagayaku, Tokyo +/0ῌ2//*, Japan.
Corresponding author : Marekazu Ohnoe-mail : mare@chs.nihon-u.ac.jp)BµOsj¶·s)<¸ks¹ ºsBlksu[»¼536

in the Kitakaruizawa area (about 3 km north-northeast from the source). Assuming ,ῌ/ minutes for T+ and++./ῌ+,.* m/s of average lateral wind velocity, the height at which the mud raindrops separated from theeruption cloud can be estimated at -,.-*ῌ-,20* m (-,0+* m on average). From this conclusion, thetransportation and depositional process of the pyroclastic materials generated on September ,-, ,**., at the
Asama volcano can summarized as follows : the explosion occurred at +3 : .. and the eruption cloud rose to-,0+* m while blowing ,..3 km downwind from the source. The cloud moved laterally for ../+ km withgenerating raindrops. At +3 : /., mud raindrops separated from the cloud 1.* km north-northeast from thesource, then fell to the ground at ,* : *- after being blown ,.* km downwind by a lateral wind.
Key words : cloud height, pyroclastic fall deposit, mud raindrops, sedimentation process, the Asama volcano

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Asama volcano. The distance of each locality from the source is shown as follows : a, 0., km ; b, ++.* km(locality of “III” in Fig. . and Table +) ; c, 2.0 km (locality of “I” in Fig. . and Table +) ; and d, +*.- km. By
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C JKLDEFLM NFig. . : TtO 36N{ d |kW,:(}~C N¡OX HUKK* !2*YZV/!P

SALD--***SO WB +. f (*.*0 mm) ,:6}[ + 9!\O?] J + ../ f  , 2"+3 X!Q: ,^- mm _R,`J,:(*¢* !8 £W

9 9!

 8 }¤TOD 

UK (3 km) ab$[.

'kWG

\O&W% +3 X!'S)*,:( O¥4! *^,f (+^*.,/ mm) lD +,2 -^/f (+,2^-, mm) ,2! , 2¦sFig. .  !"# ! M  (2) c
NOMdT ,**, c3UeVO Wf ;῏+ ῌ῎῏ῌ
Mg ῎ῑ hSi · Nmiimii+OjihSn · NmnimmaxOjῑ (2), ῍῍ῐῌ
ῐ§12¨u¦s©ª"(m+ Fig. / !m,:(«o&¬­H! 0.* km ®)*t ,,`:!Q: + cm 645J]  (Fig. -a) ®)*&. + % 9
tX>,:( )!l,2!OD"¦s99 S  , X !"!k6* Y m §&201)* + ¯i°6p Fig. / l !"mZIY.7 !±  *^, f &C)!l,2!IC ¦s§mmax n+)*oO !  n HUKK* !2*²4!,2!

 2 gῌm, =>? 2¦s§ !8 £V/!" ,/0 gῌm, !"6[pX[. + (2) c Fig. .  !"#!\W!.7 

 -^/ f !IC ,2¦s§UKV!"][" 2 W%Z +0 gῌm,

³&ICaQ´V³"+ 

qrs qrstur"8!v^ 9µ l,2¦s§, /i!

!%(Y+ H 0 km %7_!!w`*"8

45&C_R

:!.

\l,¶ ^ 45! 6,2¦s§9\l,¶^457+,2 gῌm, qrs66"8!H6

·C%!( ¸^¸ ,

v^+ 6 2 gῌm, abqrsx'y/!*¢*¹ )* +=6.67!&ODC" 8 !f
cv+ )!odz !"!ef,/0 gῌm,

542

  

Fig. .. An isopleth map of the pyroclastic fall deposit that accompanied with the eruption which occurred in
September ,-, ,**. at the Asama volcano. This map also shows localities where the diameters of mud
raindrops were estimated (I, II, and III on this map). The pyroclastic fall deposit distributes in a north tonortheast direction and has a secondary thickening at about 3 km away from the source. The amount of thedeposit within the isopleth contour of 2 g/m, is estimated to about 1.,+*0 kg. The meanings of theabbreviations in this map are as follows, Hg : Higashine city (Yamagata Prefecture), Yg : Yamagata city
(Yamagata Pref.), Ym : Yamato town (Niigata Pref.), Sm : Shima Hot Spring (Gunma Pref.), Wj : Wajima
weather station (Ishikawa Pref.), Tt : Tateno, Kk : Kitakaruizawa, Sz : Suzuranzaka crossroad, Fj : Fujiwara,
On : Onioshidashien park.

µ¶'·+2QR¸¸ ABCD ,**.  3  ,- Cp¹Cmº»(¼ ¸¸Fig. /. Grain-size distribution of pyroclastic deposits
derived from the eruptions that occurred on
September ,-, ,**. at the Asama volcano. The
distribution is bimodal, showing one grain sizethat is of a coarse mode, *ῌ, phi (+ῌ*.,/ mm indiameter), and another of a ﬁner mode, -ῌ/ phi(+,2ῌ-, mm in diameter). With increasing
distance from the source, the coarser mode
migrates toward ﬁner fractions, whereas theﬁner one is ﬁxed.

Fig. 0. A surface synoptic chart at ,+ : ** (JST) on
September ,-, ,**.. A stationary front was
stagnating south of Japan, and an atmosphericridge expanded to the Kanto area from a highatmospheric pressure existing southeast of
Hokkaido. This atmospheric pattern indicatedthat, in the Kanto area, the surface winddirection was north-northeast and its velocity
was weak.

I4(t [-ῌ/ 
-ῌ/ῌ+

543

3  ,- C * #V' 0 2QR UFig. 0  ,**.  3  ,-  ,+ 

(t8@ +?



Fig. 1  ,**.  3  ,-  ,+ |[\][ 3  ,-  ,+ S@+2quvwx78#+    !"# $%&'(* J#_

)* %+ ,+ - ./0 12
3' 42356789 *: 3

 *

*78't+ 2( # |[\]

 2/* hPa 2 +,/./

 ; !"#+ 4 ;<= 8>  3 m 1** hPa 2 -,+0* m /** hPa 2 /,2.* m>?8@+4(78 ABCDEFG

J# -** hPa 2 3,0+* m   4@8.HI 423(J478K: 3

T2q 78#+ABCD2( ;78*7%LMNF<. O
P#+(QRS@ ,* VW CTU>I 3  ,-XYZ[\]*^_.^+.- mῌs, ,+`abcdeJ@fg^* mῌs 4(78 ABCDu  |[\]

8@+[¡(¢M&£= 4(=(t  Fig. 1 h ( ( 277'

¤8¥(¦§¨# 44(78 C¢ 78h(@ *)*(i#= #782 +* km I, (¦§©!"#7#= 8 4*. *jklC+4( 7  _I #ª  2q

m (Fig. .) (no#=4(78 4C§«&= )*  * 782 +01

pCm j(#2qr%sS@+(m , <9¬.  

t 

h(@

-ῌ/ῌ,



 *2 ®(_«?= 8'  2quvwx 
  yz{|}

(Fig. 0) 78­$(i#= 2 2,1 m ,u~|[\] (Fig.

9*¯°±  2 +,/./ m 78 1,/.* m , *78² +3*¬,,/  ,'.) [S@+vwx+ 4@2qu[

9789%³ ' R

 ABCD

% (#|[\] U>ABCDD´ &2 ,,/0* m U>I- -,+0* m 

I4³b´µI¶@· ¸¹4A@ºB»¼

544

Table +. The results of the equivalent diameter of
mud raindrops estimated by equation (-). On
the assumption of ,*῍,/ wt.ῌ of water content
and ,,,**῍,,,2* kg/m- of apparent density, themud raindrops ranged from ,., to -.+ mm indiameter.

Fig. 1. A vertical proﬁle of wind velocity, temperature, and dew-point temperature obtained at the
Wajima weather station at ,+ : ** (JST) on
September ,-, ,**.. The additional number
beside the wind velocity is the wind direction.
The temperature and dew-point temperatureshow no signiﬁcant di#erence regardless ofchanging height, indicating that the air was
saturated with water at the time of the eruption.

`GTa@ yb C$ -!>?9(z!{|(5 }/~$G "# Fig. 1

<=>?9(D 0 9( D9# D  0y1?( C <=!9(
C ,*7,/ wt.ῌ      +* mῌs  1,/.* m   !"# $%&'() *+

6!

 C${|9(s0 .9 +* cm,<= C iB20
 +- mῌs  HU#V7WXYD0_/W#*^ 0y9(3 iB20oj MI4 +33/N

9(*+ !B059 +!*6C09(

37( M,,7,/wt.ῌ ; Schumacher and Schmincke, +33/  -+ wt.ῌ ; $,-./. 01 +33-N !x ' D 89 B

' 3 2 ,-  *!3 4./5!9(C iB2 w 0 ,*7,/ wt.ῌ B : ,,/0* m  1,/.* m607

0 +,*** kgῌm- D9#C0¡|'*+!"#89( 

0 ,,0** kgῌm- P-ῌ0 

= ¤0G#9( ;-῍, :;(<=*+>?@A *+ B!9(C9#<=>?9(  D! MFig.

. " ῌ! ῍! ῎!N 0O 9 C <=" PQ#R0S)(XY Z60[\]7$\] W#*^$_6`
GTa@0E' b% *c 2.2 km!d_ rw B :  Wp C"!ik*+ C¢: 

iB2CK:

Table +  Fig. . "!E9( -!d_(e&!x9 ¤ (-) 0G#S)(C PQR,῍+

F0G#S)(C

mm

6

T.#

0 (-) ¤!§;

9 C <=r R 60S)(

0i` (Fig. /) jk

l+.#R0,m'1n

(3) ¤

K:  ,,,**7,,,2* kgῌm-  ?5(e& WXY'(!f)gG#R #h#R C$5j*

¢:
s 0£

(3)

2 D9# rp *+*+  Fig. -b, -c !E9(T! HU#V7W

K:s¥w · rw¦Wp · rp

 ,῍+ !E9(F0G# >? HI 6<C  0JK#G.GL -9# C(Fig. /)



PQR ,.,7-.+

*+

#R

¨5U#.#R©ª«¬­®<¯DR0ojC$ -!<=>?9(pq W##h

I-G  *+>?r st!"#C" Bu!g/v'@A =°9.G#R >?

$ U##hwx+!g/v9.G.Q

-ῌ1



jk HU#V7WXY Z60W#*^$_6-῍/῍, E9(T! >? ±² ( ,**.

·¸ῐ8\]B3¹'ºº 3  ,-  ,+ 

!"# ,**.  3  ,- 3"5»¥HI"EFs¼V ºº 

545=:lm  ~$  2/* hPa S_2 $   !"&wJJ&DB/#$%&'()&*+,-./ 01211-ῌ2 

2 3"4567 !"#$%-ῒ0 56]Z " O 3 km ~5sDB

(T-) 

8ῒ9:;<567=:>2?@A/ BCD "HI8 ,.,-.+ mm &DB/ 1ῐEFGHIDBJKLMNO +,+** m 2PB

 5 P  ῐ   ῍ ῏   T      Re  /** ;Q JKLM%R' ῒ9:;<2=:B

Bonadonna et al., +332U 8 ¡DhG%8HI.

2/* hPa S T' +,/./ mU & 3*/ hPa S T' +,*+- mU

BQ ῐ&¢£'&!5¤X¥h()G¡^῎ TB&|¦ Sparks et al., +331U ;

8 (.) V5?W.1&5XYZDB/Table , [ΐ2\]B3"4!"#$%

8&^_&QB`2P/ abcd8

e.& (.) V JKLMfg%R' +.*/.

+

ῌ῎. Ts¨aUgd῏ῑ,Vt§ῌ ·
῍
·
a
C
῎
ῑ
D
῍
ῐ

(+*a)

kgῌm- &h^ 51%R' iJK

112 a %R' g ©ῑª' 0DZ CD «LMej 230 hPa &h/ 1>JKLMX¬)2P/ ῍῏8 ¡DhGῌI.­ ῐ^ N  G k ] K L M : ; <    ,+   = : l m®¯ CD ῐῐ6X°%R'±²5(3*2.- hPa) &noDh]BQ pqrs`YBJKLM(³^hx66X0 *..1 ´'>8& TB&|¦

%R'> tu>&vwxyhz&µ +322U/ XYZ©ῑª'8 3.2+ mῌs &.¦ (+*h]&{|/ ,} ~@fg5]Z ~@a) V¶IX¥5_&Q1&G2w ;S54BN +,+** m 8 5XYZ * mῌs&D 2/* hPa S567=:lm&c>8&YB/ 5~$ (Fig. 0) KLM:;<+ῌ
῎Ts¨aU ῏ῑ,Vt§/.,2ῌ
· d῍
a
῎
ῑ
῍
ῐ

Table ,. The upper meteorological air data around the Asama volcano and the settling
velocities of the mud raindrops calculated from these data.

(+*b)

z®¯°±²³´±zEµ¶±· ¸±/®ῐ¶±¹º»¼546

Fig. 2. The relationship between the height at which the mud raindrops separated from the eruption cloud and thetime at which they fell on to the ground (T- ; Fig. +). In consideration of the height of the Asama volcano
summit and some experienced accounts, the estimated range of heights at which the mud raindrops separated isrepresented as the gray area in this diagram.

(+*b)  -ῌ0 

 
mm῏ ,.,-.+ !"# $%&'(  )*+, ( (/) -.%/0 ῏  123456 768 9Fig. +T-: *# ; <= ( 1 >?=@ (A . km

2BC

(D'<EF.%G $ >HI1=JKLMN

Fig. 2 G| ,.,-.+ mm

6v

" Fig. 2<1i῍B6 768 * u%ij =JM 458 9+3 >: =  (,,/0* m) j 

6;¥¦ ,,/0*.,-/* m6jklD'mῌ D- G 23nopqaG

II G 1῍F#

6G Fig. 2 ih%. (Fig. 1)# WX%  1i45

5

1. 23v6 81

RS%.TUV1.# WX% YZ [\]^<=`8 a bcGd ( % +3*e,,/e fg

1 ~#  1< (ῌ6

O >H I (P'%.& (  GQ;
(G_6#

 1< (23458 (T -)  

-ῌ3¡¢£gv'ῑ¤fg;#






-ῌ2 .% *D' \  145irrs 9A ,*/e: tc ( b῎D'

-.%<u% (0) -.%)*# v YZw

# &&GD(<1=JMkl6 7D'fgxyC

8 (T, : Fig. +) ¨!" $'©

kgῌm  | -.+ mm-

1z{;CG ,,,2*

  $%yC 1}D.C G ,,,** kgῌm- | ,., mm ~&

 ( j2345 G& fg6
C ;% T-fg#(ª«*D'

=JM

ῌ

23v

+) L*6&W fg§

458 (T - : Fig.

< ( ῌ

¬­A# ; &&C<1i῍B6 768 (T+ : Fig. +) G * u6#¶·ῐ¸¹T3 ,Sºº k.l ,**. ³ 3 ´ ,- µ»&',()8¼ ºº

547

Fig. 3. The relationship between the height at which the mud raindrops separated from the eruption cloud and thetime at which they hit the ground. Combining some experienced accounts and results obtained from Fig. 2,
we estimate the range of heights at which the mud raindrops separated to have been -,//*ῌ-,2/* m.

Fig. 3 

῎῍    

!"#$%&'() *+,-.

!/0 T,

 T- ,12"34,+56 Fig. 3 4 Fig. 2 78῍9!:;$%< ,ῌ -.=9!3> ?+ῑ@,.A,BCDE῎F6!  $%GH

. 





.ῌ+




I,I  3 ῎<
*+ῌ

-. (T +)  * S!   
-. +3 #T6$%&'() ,ῌῌ`!T1 I>3

IJKLMN,OOPQRS #T3> U  *+ῌ 3-.3e + +5"3,VTΐW (X,,.** m) +Y,BC!6 Fig. 2  #4 w,. I FFQn"#῏`78 Fig. 3 ,Z[,,&'-.C \]a Q'  Fig. + , D+6^_`a3,῍ ,bcC Fig. 2

3"# ,I-.d 31 -ῌ3 +S 3,\]de Fig. 2 Z[,,&'-.f,῍ qN9 56 n"#

3 gh῍ _ +,0** m _ -,//*i-,2/* m ,oo+ I  3  *+ῌ 

bc!6o

j@k.l,lmnEVT

 Z[,gh1/!T6 n"# ,()ῒ,pqrsEt,bc

3U -,//*i-,2/* m&' 3ueav w,&'-xyz-,{|gh}~!9+N6-. (T +) !bc+`3- ,῍;,n ,   nEZH
T,῍ ,bcS 6.ῌ+ῌ+!"#$%&'(  )*+,,-./ ¡ (,**/a, ,**/b)  {|¢£n¤P¥q¦$§¨©?k.lª«¬­ #T®¯°±²῔`a3 ,**- ³ , ´ 0 µ £n¤ ,**. ³ 3 ´ +/ i +2

sC²6³´µ6s¶·D6]¸ ¹6CED6º »¼

548  

+3/1)

 (Scorer, ! "# $%& '(! )*+, +ῌ, -

. /0123 4

 5 67 (,**/a, ,**/b) 8! %$

39  :;<=>! ? %&@@12( AB212+, ! ,**-  , C 0D<8 ,/ ! ,**.  3 C +/E+2 %&/ @D<8 /*  E+,* F3&

G&H



9I J! ,**-  , C 0 8K
L( .** MNOP Q5 67! ,**/aR 3S%&T
U! VWI

Q1,,** MNR  X82YZ%[

3Y$! ,**.  3 C ,- ( ,**-  ,C 0  X\U]^_"38,`[a[ _ ! ,**.  3 C +. E+2 bKcd( /3,*** MN Qe 6 7!
,**/R 3S%&[2(! &8f,ghi Q0aj3# ,,*+* W ; kl! ,**.aR m
nP

K

E+2 8q<o"p2 ,**.  3 C +.

rsta(u%&[a[ Qkl! ,**.aR 3Y$123! ,**.  3
C ,- 8! 5I

6 7 (,**/b) (vUw

xy ,**.  3 C +.  E+2 gz

 X! (\U]^_" }

/@

{|

~!(]

[ OY$! ""8( Y$#A12

Fig. +*. The relationship between an average lateralwind velocity and the height at which the mudraindrops separated from the eruption cloud.
Assuming ,ῌ/ minutes of T+ and +*ῌ,* m/s ofaverage lateral wind velocity, we ﬁnd that an
average lateral wind velocity of +-ῌ+. m/scorresponds to the real wind velocity describedin Fig. 1 (the gray area in this diagram). Considering +* m/s of wind velocity near the
Asama volcano summit, we ﬁnd that the actual
lateral wind velocity should average a value
ranging from the average lateral wind velocity(+-ῌ+. m/s) to the wind velocity near thesummit (+* m/s), or ++./ῌ+, m/s. The heightat which the mud raindrops separated from theeruption cloud is therefore estimated to range
from -,.-* to -,2** m. See the text for a
detailed discussion.

12+, (T+) P ! ,**.  3 C +/E+2 
$%"p2 , &312 '! Y$ +3 &6(7~!a] (1,/.* m)  '8 ,* mῌs

8K(nP [23! 12\! (,3a2 ""8! 12(2?

(Y$) ! *+nP1212+, (T-)
8! (/) \U +* &,3H%&2 (Fig. ++a) 3Y$! ( Y$K(Y$)12_"

3 'F.ῌ+ῌ,12+, (T+T,) 8%j# +* &,3$&2 T, \U T+ '(%ja238j[ B! 
"8 T + / &312 OY$! \mK('-%&! *+nP12_"
+, (T,T-) 8 +.E+1 &3a2
T+,E/ &F"? \m P aj) (D+ : Fig. +) #.
 @ 8OP\mH%&2 ;D+T+ · uave V(++)

" uave V 8! (/Y$0$12}12_"2?  ' 3

"p2 QOP!  ' . 3 ' 3R Fig. 1! 


Fig. +* 8?£($2?

3

' ! ¤£83 ' 9¥%> +H%&
2Y$( )] 3U! &$ ¡,E/ &! (D<! ($+* mῌs Y$ ,* mῌs 3

3 '3K)] ¡ ¢8

0

:¦

T+

,.,E-.+ mm, §Y,,,**E,,,2* kgῌm- F"oH

 4

p2 T+ (%ja2¨©! (-6cd%>2

12+,8;ja2 B! ( (Y$)1
2]8<ja28ª"p2 Fig. +*3
41

§23! ? '( +-./ m ῌ s =>?«"8@¬AU­®
Y

! ?

3

'( +-./ m ῌ s Oa2

3! T+ 3( Y$)] ¡8¯B12

;23! 

&8! T+ °±m D+ (%ja2&! D, 3 D- ,45T2 '8 +* mῌs, _ K&<o)(;ja2 B! ]^Y$K nP%>a

·¸-¹G47(* '$ºº p Cq ,**. 5 3 » ,- 6C7gCj'¼8 ºº

+.+1  

' %#'NO% W+! x Cj'("#')*  

! Fig. +* "#$%

-,-1*.,,+* m +  "&'("#')*

! Fig. 1 ,*-' .+! (/0 1
2 .3456789:7: ;< 7 =>?@A'/0.

! 5B  CD/

549



%#

+,% Fig. +* G +! &'- G  +!  ,., mm, i",,,** kgῌm- z{|! T+ ' 1 ] ' 2 .G.% :F! iF# &'("#')* '¡ 5

EF.! /0GH * I3J KL

:F ,/* m / +  "#! &'-

+79! M'. NOPQR@GST UV!

 *.3 mm ¢,**/a ; ,**/b WX!

\9:! /0. *-YZ[7 (Fig. 1)

(] \9:^ (D+)! ('

I'

a3 

bc! &("#)*_.\9:Y`! (/0 EFdeG1 .I7 f 
W'\gh Fig. 1 + Fig. +* Gi+! ("#'Cj')*-,-1*.,,+* m

'k

*

'k*-' .+ 2 .Y`l. +-+. mῌs 'k+  W'm
deG1' !

(/0! p Cqq

.'/noI' . +* mῌs (Fig. 1)  W+"#! <r'
2 .W' .'k"#5Ks tu 7 \9: ! ! p CqqI' . (+* m/s)
+/v'

.+

m/s) +'

2G+9

2.Y`l

. (+-+.

. ++./+, mῌs ! (/0 1 2 ] 'kGY`"w'¡ ! 57¤("#)* 'k-,.-*-,2** m +  }/"#! &

'~%'*

.ῌ+ῌ-

 (T+)  (T,) 
 (D-)(/0 1 ] ' 2 .G ++./+, mῌs++! Fig. +* "#$% &')* '/n !  -.+ mm, i",,,2* kgῌm- '&

. +, m ῌ s ' , ":3/0("#{|! 3'n !  ,.,mm, i",,,** kgῌm- '& / ":

. ++./ mῌs '

3/0("#{|+  \9:! (^/¨''k0

T,
Fig. ++a/¨'$%

'

f 
T, + T- '©ªG,F'  T+ G ,/ +{|! T, + T- '1+.+1 +  \9:! <r'+3 ! W'iF4x"#p Cq'q

< T, + T- '1 +1 + +'¬! 3

!

bc 21* 

W+ T, 'nm ! T+ G / +:«3! T, + T-

'1 +. +

! T + G * +z{|'$m (-,//*

-,2/* m) +Y`'sm+9 W ! /0 (+'¬

! : T, 'kW+ 1 ] ' 2 .G ++./+, mῌs +{|!
D + 'k+,-2*-,0** m +

 WX! ('~%'/0*

 "#! (

21*+,,.* m,*[' ]*4#/0  /0Y'

9:F! ' ( \9:(% _ a7!T+ '/nG / ++! °± ²

7 \9: Fig. ++a "#! T - 'k (% ! Cj'("#')*(++) "#)#f  W'

G­ {¤ ! T-  + '{¤+ +* '{¤+ ! &'®¯. G2G deGo +#  (/0:%

'[%'87\G2{|! T, '/nm ! T+ G , +:«

T+ G ,/ +z:iF9&'("#')* T34&! T + ](#$'&* (,,/0* m) G7*+,,.* m +$

/** m ¥[deG1 +7 y  W' 2 .'k+ Fig. +* "#! ('/0("#)* ! T34^*

4! ¦§&'i" ' '-'.

+x!

G ,/ +z{|! &'("#')£F! #'("#')*3.,+*.,..20.2 +³$

T- 'ka3 +! 'k0Cj Z'/^(% '"«  Fig. ++b

! /¨'«%

T- + D- '©ªG, /´'µ!a T- 'k
%

D -!

3.,+*.,  "#$bcCj



^(%

 _ ! Fig. ++b ' '¶>,k! T3
4 +,3,*,,+** m +$% 

! W'm + tuF  F T+ G /.ῌ+ῌ.  !" #$%}/{|! Fig. +* ,R&("#)*! 7b('#$Nµ¥k7O¶·k ¸Pk¹ ºkNQPkhR»¼

550

 U>B; 3 V ,-   /5  -,/**6/,/** m "WBG 3 V +X'Y ++ V +.   ,Z[ ,**.a ,**.b. \B]^ 5_"G4/  ,**.  3  ,- 






/5 !`!"a#$bc&%:>d& Hekla +31* ' c&a# (Thorarinsson
and Sigvaldasson, +31,) Mount St. Helens +32* 'c&a# (Carey and Sigurdsson, +32,) Mef +33+
' c&a# ,gh +33- ; ijk( +33,. T>
B Hudson +33+ ' c&a# (Scasso et al., +33.)(l3)m BG n*oXp%:+ `!"(, jqrGB Carey and Sigurdsson(+32,)  Mount St. Helens +32* ' d&rGB /5!s--tu.Z"(/'_Bvw> <(x01 y>"z{> >5>( 23|}~ ,**. ' 3 V ,- 
Fig. ++. The relationship among T,, D- and Tunder the conditions of average lateral windvelocity estimated from Fig. +*. Considering
the range of cloud height estimated from Fig.
+*, we estimated T, as the intersection of theline showing a relation between T+ and the lineof the sum of T, and T-. The range of T, cantherefore be estimated at ..2 to 0.2 minutes (a).
Furthermore, the range of D-, which transported
the pyroclasts, can be determined as +,3**ῌ,,***
m (b).

45>Z67>BG u.Z'-t vwU4bG 3 Me
f +33+ 'c&a#8%:+`!

"(,rGBgh (+33-) X'Yij k ( (+33,)
 %:+,>BG!o79 +8
"5 %:+ `!"(, jq 79
w6"(c&'; "U4 >5> Mef
d& a# vw T I@)S"(:BG ,gh +33-.
;

 T ;k%:)>B"l

3 BG(G"G < T23 U 



>$7XGB ,**. ' 3 V ,-  ;  a# d&">B c&$c&N ! Ht "





 Hb "$ %&'#

()* (Sparks,

 T>B a# vw  U>
'}+"(a# c& 3
>6!;/G! ,/5==+320) ;
Ht++.-,,Hb-2b*.

(+,)> /.. km!. ; ?(@AB>a#

C<"5 a# vw/5 b* /0 -ῌ+*ῌ, 12



34 

/5

21*6+,,.* m> Da# %:+,>BG!o+ C+/* m "" /5 8" U" Fig. -  Fig. . ¢#$7 /

0! 8 ,,1-*6-,,,* m ,!

!7>BG"$¡G(G >5>a%:#£AEF

DG¤H

 /,,3*6/,12* m. "9:;"<  ;PIXGBa#=> (+,) > 5rT¥ 5|}¦"U4(

?@A>BCDEF BG HEIJ '(KLEM CDE >NO J¥Zvwp((G!

§¨"vw-t ©}ª>«J¬­'" ®K PQ>B>3 RSG T>a# L>BM¯°" ±²³´W"FK nE¢£¹¹ NO ,**.  3 8 ,-  +#C[º» ¼ ¹¹
Schumacher&+AX and Schmincke, +33/ ;  +33-  ]R

  ! "#$%&'()*+,
67-./01 2+34 5

 ,**.  3 8 ,-  ,* +"9 :;<=>?@AB 3/ῌ 

#CDBEF-.

EGH IJK (Fig. 1) #JL ,**.  3 8 ,-551

n+uvH&KR+H&

n+rXLFd]e+ X

K %]e+ nf 

m'&Kp34

+#JLQD



K^_R+ nm'u'&Kl0&1 CHd  MNOP  QRS+TU

m' Fig. +, + ,**.  3 8 ,- V&&1 W3 XC YZ[+IJ

[xyz{6  K X .῍+῍,  .῍+῍- '&K \ 

K T,, T- ¡¢£¤I ¥JLF Ks

X ]+ QRS+abf

XK^_

cXd_e+W

FCghi*jkcl0

 +33/ XmXD'+qJLr

`+F Q ncc Mop

Ks taS+ nuvw

+#JL+f
l0+dF V'+

FCxyz{MNOP  :;<=>?|}~g{sLF_=
V&LF

m'

n#ucEG+ 'opD_ n_m

+fV ^_[V FCxyz{cl0+dF¡¢£¤S¦¤+§F¤XL nm'¨E -,0+* m  n©ª«E +*/* m T+  .  T,  0  T-  3 
D+  ,,.3* m, %XL D-  ,,*** m XL¬­+3  .. ®¯X (°±² ++./³+, mῌs[´+"#% ,..3 km V&D'XK®¯m' .  +3  .2 + E -,0+* m +µXK
e

n©ª

n% ]V¶¶  +-³+. mῌs ·+#JL

[´+! ../+ km V&KnV& 

n XK_e+¸f

+

Fig. +,. A schematic illustration showing the transportation and sedimentation process of the pyroclasts derived
from the eruption that occurred on September ,-, ,**., at the Asama volcano.

·m¸¸¹¸0l º¸O¸2»¼

552      #$% &') 1 km

 +*

(



,+i  ! , 34 ,.,--.+ mm+

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+

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+
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eU634D

9 lm nR3 ,.,--.+ mm 9K4%  o6U
0DW6 ,-/ ¢=rs569 &

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3 km

 ,-- mm

34+9VA6t£_¤9 +3  ..

uv+( 4 ,..3 km #

D 0E9+ , >)F1GH

$0 -,.-*--,2** m T2   634+ I  

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mmv+% I v9

CSD* T

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YZ[\]99^

H_6`I+

! N+JW9% abK4+ 3 c ,- L
dN06 (-,.-*--,2** m)  ef+ /8+  ,.,--.+

 , km #== 3

 3 km %%y%12&H

34+  ¦z6Q{+ 5'6(
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9^ j_634+

©6wG}ª+I9 G« ( , mm34DJ0+Ax qr]stuB"   566kPD9 ../+

*J D 0DW X59 MV

) 1 kmH

>09

+32.U FGD% + + * ghOiI9G" .

¥ wS Oc06

F1% &'D>09 OJ% 9P ?M ,., mm/ 

km !"dN+% !"dND 2

56=JK78L%9: ,* wt.ῌq¡9R

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I96wS+% + + Ax+3(34+I96`ID9 I qr]

9 v
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Oc~;7v

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40l (+321)

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8MJ% , 8  6D`a

b6+!+%  uBv OcW7 !"d06 µ* G\6kD  &'C iGH
, 34C/+HP % 
06 49* D¨©6`ND 2   569, 5'9 E

I+34, ~;D4D@AB

=  kP6>0i%¶6! a(!O§0ῌ ῏῍ῐ῍ῒῑ῎ΐ῔*d e9! 1.,+*0 kg

Auq6+>0i%,6

VAB

¼*+ x¸!hh  ,**. A 3 Z ,- X  £9xy hh       !"# $%&'()*+, -"./012 33

456 786 9:; !
<"#++=12 >? @$%ABC&


D' (

)*EF+,-G H?+-I./"JK0./0L1

2345/M6>N2 O78P9-Q:
;7R 2   Bonadonna, C., Ernst, G. G. J. and Sparks, R. S. J. (+332)
Thickness variations and volume estimates of tephra fall
deposits : the importance of particle Reynolds number. J.
Volcanol. Geotherm. Res., 2+, +1-ῌ+21.
Carey, S. N. and Sigurdsson, H. (+32,) Inﬂuence of particle aggregation on deposition of distal tephra from the
May +2, +32*, eruption of Mount St. Helens volcano. J.
Geophys. Res., 21, 1*0+ῌ1*1,.
Carey, S. N. and Sparks, R. S. J. (+320) Quantitativemodels of the fallout and dispersal of tephra from volcanic eruption columns. Bull. Volcanol., .2, +*3ῌ+,/.
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Scasso, R., Corbella, H. and Tiberi, P. (+33.) Sedimentological analysis of the tephra from the +,ῌ+/ August+33+ eruption of Hudson volcano. Bull. Volcanol., /0,+,+ῌ+-,.
Scorer, R. S. (+3/1) Experiments of convection of isolatedmasses of buoyant ﬂuid. J. Fluid. Mech., ,, /2-ῌ/3..
Schumacher, R. and Schmincke, H. U. (+33/) Models forthe origin of accretionary lapilli. Bull. Volcanol., /0, 0,0ῌ
0-3.
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Sparks, R. S. J. (+320) The dimensions and dynamics of553

volcanic eruption columns. Bull. Volcanol., .2, -ῌ+/.
Sparks, R. S. J., Bursik, M. I., Carey, S. N., Gilbert, J. S.,
Glaze, L. S., Sigurdsson, H. and Woods, A. W. (+331)
Volcanic Plumes. John Wiley & Sons, England, /1.p.
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Thorarinsson, S. and Sigvaldasson, G. E. (+31,) The Hekla
eruption of +31*. Bull. Volcanol., -0, ,03ῌ,22.
Wilson, L., Sparks, R. S. J., Huang, T. C. and Watkins, N.
D. (+312) The control of volcanic column heights byeruption energetic and dynamics. J. Geophys, Res., 2-,+2,3ῌ+2-0.
Woods, A. W. (+322) The dynamics and thermodynamics
of eruption columns. Bull. Volcanol., /*, +03ῌ+2+.
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