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+  ,1  ,**00  ,0 Pyroclastic Density Current from the Caldera-forming Eruption of Izu-Oshima Volcano, Japan :Restudy of the Sashikiji , Member Based on Stratigraphy, Lithofacies, and Eruption Age
Takahiro YAMAMOTO῍
The Sashikiji , (S,) Member in the products of Izu-Oshima volcano was formed by an explosive eruption
accompanied with caldera depression. This member is characterized by breccia called as a “low-temperature
pyroclastic ﬂow deposit”. In this paper, the S, breccia is re-examined based on stratigraphy, grain fabrics,
grain-size distributions and modal compositions. The S, Member is divided into six units from S,-a to S,-f inascending order. The S,-a unit consists of scoria, bomb and aa lava ﬂows from ﬂank ﬁssures. The S,-b unit ismade up of well-bedded ash and ﬁne-lapilli from the summit. The S,-c unit is composed of matrix-supported
breccia, locally ﬁlling valley bottoms and containing abundant deformed soil fragments and woods. The S,-d unit
consists of reverse to normal grading, clast-supported breccia with ash matrix, covering topographic relief in thewhole island. The S,-e unit is composed of dune- to parallel-bedded lapilli and ash in the proximal facies. The
S,-f unit is clast-supported breccia with and without ash matrix. New +.C ages of wood fragments in the S,
Member have been determined as about Cal AD -.*. Although the S,-c and -d units are previously interpreted
to the low-temperature pyroclastic ﬂow deposit, these units are quite di#erent in sedimentological features asfollows. The grain fabric measurements have revealed that the S,-d unit has a-type imbrication showing thelongest axis of grains parallel to the ﬂow direction. On the other hand, the S,-c has random fabric of grains.
The grain size distribution of the S,-d unit shows a bimodal nature having subpopulations at phi +.* to +.* andcoarser than phi ,./. The bimodal nature and a-type imbrication suggest that the two transport processes
overlap ; the load of a turbulent suspension is not all in true suspension as the coarser population may travel in acast-dispersion mass ﬂow. The S,-c unit shows a polymodal grain size distribution with multi subpopulations
from coarse to ﬁne. The poor sorting, massive appearance, valley-conﬁned distribution, and random grain fabricof the S,-c unit are characteristic of deposition from a cohesive ﬂow without formation of traction-related
bedforms or sorting of di#erent grain sizes by turbulence. The modal composition measurements have indicatedthat the S,-c and -d units lack essential scoriceous or glassy fragments. This evidence indicates that both units arederived from steam explosions due to outburst of highly-pressurized geothermal ﬂuid within the ediﬁce. The S,-c
unit was plausibly generated by remobilization of phreatic debris around the summit caused by ejection ofcondensed water from a plume or heavy rainfall. The S,-d unit was a pyroclastic density current deposit resulted
from collapse of a highly-discharged phreatic plume. Estimated velocities of the current are +/* to -* m/s basedon suspended grain sizes.
Key words : Izu-Oshima, S,, pyroclastic density current, caldera, steam explosion+ῌ ῐ ῎ ῑ ῏ (Fisher and Schmincke, +32.)3 45678.

 (pyroclastic ﬂow)   9:;4<=)>?@A2, BCDEF!"#$"%&'()*!+,-../ 012GH)IAJKL3 A; (pyroclastic ma-

M-*/ῌ2/01 NOPQRSTU +ῌ+ῌ+ VW 1
XYZ[\]^_`abcdef^_ghij
Research Center for Deep Geological Environments,
AIST, Geological Survey of Japan, Higashi +ῌ+ῌ+ Cen-

tral 1, Tsukuba -*/ῌ2/01, Japan.
e-mail : t-yamamoto@aist.go.jp

E º » ¼

258
terials)  



 -



    XmabnY"EopqaKr
cd< S, 7<   !"# !

f"$%&%'()* #56*)  +!"e Nakamura (+30*, +30.)

\  ] Eg (+312)

!" h*94>i jk;

<l.m>EgJn  S, 7<.EoD%   ,- !"&% p;%&%qh Er.)* ./ 0 "# %&%# opqas1 2345   +3306   

JR h3" (Nakamura, +30.)

9:; <!",*** # 7M><!"  t<E #78

,*** # 2 = ,3 $> 2%&'\sopqaKJ7uvQ>w ,***6 .9?(@) !>  A*  #

,**,) Dxd

S|! (Geshi et al.,Egh3>nY"E

>  )* .B+C3

opqaKy.;z" {|}+"

4D; B,-.E +3** #~Q>  =EopqaK

#

M

 !"/.0!>F/" 21

_ j] <!" ,*** #  op

G234HI +3*+6 J* K56qasL"  M7856>N9

"

.56(pyroclastic ﬂow deposit) 9O :

Xm

S, 7<

i Eg S, 7<i<l

P;Q7856"<>

 `z{|  S. .RST56

DT(pyroclastic surge deposit) 9U

Ub L"hi D ¡=.V>? 234 Fisher and Schmincke,Egh*56+32.6 D>+ 56  QW;@X

LavSp56  ¢b£1 

M%cz{|b}.

 KQ56 56  56  , L  =AB 144"; C"< .n > > }#.YDZ"[\ E

v .> ¤ #.¥ > nY"

56 RST56 (]F KQ"

EEr7opqa¦.GH  'U(I^J Q

Y E^§ 2E  +3306 Xm 56_ St.

 ¡ .¨ Q  56 _

Helens E +32* # / = +2 $`abcnY" S, <!" ,*** #_ L")K

RST56

 EgopqasLM =

Nd"C©(Druitt, +33,) ef  RST.OP
(gravity ﬂow) J    2pyroclastic density
current ; Valentine and Fisher, +33- ; Druitt, +332 ; Branney,ῌ ῎῔ῒΐ῏ῐ῍῕῏ῑnY"E Er7 ª -«. km w¬opq

and Kokelaar, ,**, ; Q  ,**/6 JR"Sa

3 DT Ug.hi,-F opqaK ­opqaE V8M 2Fig. + ;LL pyroclastic    VW jk @X.l (Yamamoto et al., +333) Xm nY
"EopqaKr> Xm @X.@ ZsEt opqaK|opqaE

Nakamura, +30*, +30. ; J +32. ;  +3326|opqaE®T  -ῌ. # ¯®T
!> 2J +32. ;  +3326 i.F t °.

(volcaniclastic

!" |opqaE±rE^ 2<²6 K

avSp (lahar) .@Q> |opqaErE^ 2±r"<²6  @X jwE\ 2345 +3306®T  , # ±rE^56 .!

]x avSp

materials) Tu[A"\8.?;y"³;56" 2J +32. ;  +3326 rE^%cz{|b}:P;M cohesive ^ F °b´zµ56   Y

2~\6 Y  noncohesive _`)

 %&%¡¶ !>" (Smith, +320 ; Parmer and Neal, +33+)"¢£7¤¥· ¸d<¦H§¹ (Loc. 3)  

ºh 4! #$%&'iij : b)* S, » O3 T\¼k259

=  - NO=3PQRS7
9 TC:! O)U! 7V

. 7V)! WXW(!=3"YZ[\#=$]%&'N WXW" ^_`%&=3(Nakamura, +30.) Nakamura (+30*, +30.) (aKLb)*!

M=.c7V-+= .!

e=#$%&f, 1LM

d

L (Nakamura, +30.) -=! S, 7V
= g.hijklL/0i )K 1! +312"! 4-2'NO3m 4)Nm n5No ND #$%&f,p6qr ! +30-" s= -t+** km-uM (Nakamura, +30.)
- S, 
S, O3v=sw! 9xm S,-a m S,-f

C  0 yz{+=|KL (Fig. ,, Table +) }~
07KL*8 -+ 9: G

(Fig. +)! S, ; 69: M<KS,-a=yz{+! S, 9x=u&>?P
QRS79i ! E =3DSSO$D )@ AKmJJBCB
DO G ! Nakamura (+30*, +30.) ~(+32.)

AKLEFmGH=m/FIm! #$%&JK~LKF=M e umD Fig. - ; 
M! +332" N OP'QqP{mNi' 1 S, 

M! K=RKmPQRS T
Fig. +. Index map of Izu-Oshima volcano. Numerals
are locality numbers. M¹Mihara Yama, S¹Shiroishi Yama, F¹Futago Yama.

(Nakamura, +30.)! SeT=<

CDL LNmDLJKIU! S,-a yz{+};muVK ! f,=3D ¡L

1Lm]m3.mN JK~LKFIM! +320 (W +**   +/* 






 #$%&' (#$%& ! )'*+

./ +, 01 234 +**ῌ+/*  ! )  +3 56
7 8-  )C ]^'

N©_m! S,-a  ª`« a ! +32+",-

X¢Y£¤ ¥¦umDLM ! Z&u S, F C§U¨[\ +,69" 3:; (Nakamura, +30*, +30.) <=>I1L-¬\­b S,-b?.KL79|0®/0VmN M}
#$%&EJ¯°±c² (Loc. /)CmN:! C} + mm®+, cm

 ,* 6³

G 

?@A  BCD

B´DO^~q^ dµ ! &>KL¶&H I ?@BJ K

P^ PQRS^" MC  eC

EF G:!
L

MGDLK 8 ! +330" <· =¬N:!

 Cv ¸V®fg

ῐ ῍ ῎ ῏

260Fig. ,.

Stratigraphic columns through the S, Member.

Table +.

Lithofacies and interpretation of the S, Member.

PDCῌpyroclastic density current

±$m£"# 6789]²³,"ux´vwx : µ\) S, |),)¶~Y·~¸¹,º»¼Fig. -.

Distributions of the S,-a to -f units. Numerals are the thickness in centimeters.

     !"#$%!&'()* +,()-#./#.0123
 (),56789:;<  )=>1

4261!? @AB  )=CD(E01 FG/
!HI* !JK< LMNOP,ST}~7P!&d B !pῌC0 S,-d MNOP!OP4 y LMNOP,S|! y ,~"ux
1

 ~"ux]!>14  ST0R,^?1(Fig. -)S,-c ῏῎ῌ῍(Fig. .)LMNOP QR0STUV,WXY;< Z[\]^_`ab!c ,dCDB

S,-d ῏῎ῌ῍(Fig. -)LMNOP ῌ ¡^VyYUV,We,f<0*  gh;<i!;j ,/#XYῌ"#WY0 (Fig. /) ¢£!CDB  

0k!=1 lm0 / m n0*  /#0,folB ! STz¤}¥¦§,"#!&¨mp + m ^qK

st"u{©0 }¥¦§n,"#!ª« k¬*

vwx4 yz"ux,{|0*  +,vw

 )= -**+* cm 0 6789:0=1 ­®|!

x)

 r (+312) !

^

S,-c  S,-d MNOP!Cj¯; >1 °¯*(Fig. -) y« 678

 » ¼ V

262

Fig. .. Matrix-supported breccia containing abundant deformed soil fragments and woods in theS,-c unit, Motomachi (Loc. +).

Fig. 0. The thickness and the mean maximum
diameter of clasts of the S,-d unit vs. thedistance from the caldera rim. White circles
are date in the southeastern part of the volcano
from Loc. +- to Loc. ,*. Black circles are datein other parts.
Fig. /. Reverse to normal grading, clast-supported
breccia with ash matrix in the S,-d unit,
Gojinkachaya (Loc. /).

|) !} ~  1
6 J S,-c :;<=6H








 (Fig. 0) ῌ(Fig.

0) !"#$%&'()*+,-./ 01

23



4

5678569 S,-d :;<=>, *.+ km- 6?@A B 0@A - C

 ?4 DEFGH 0* cm 6@ AI9 DE
FH

J6KL5M9 N6

 DEF



B[2uZt=9   DEF../ km, J - km "I 9 S,-d :;<=J+,#w 9
$%) 1 31&#'y1\6
S,-e ῏῎ῌ῍ (Loc. +2) 9 _a

FZ()Uy)]¡ ¢ £¤¥¢ £¦u§y¨9 y*)"gh^_y¨9©()U ª £¤¢£¦u§«/+O^_y¨ @_aO (Fig. 0) 5PQ S,-d :;<=Dy¨  JFZ(yz¦¬+,

EF

G R3ST)UV1­® 9 ¯,°±J (Fig. -)

W :;<=X)YZ=E51

²9 \  ©()U ^_y¨[9\J]^_+,6`JI1

kI³m JO

3 39 _a)b5\69 c d5S,-f ῏῎ῌ῍\69 efJ1)gh\6

9  i´µy¶· (Loc. /) ¸ +320C y¹ (Loc. -*) 9j8k31\6l m ]19 no pS,-d :;<=6-&31ª £¤¢ £¦u§y¨

qrstuv[ J9 8kwxyz{n)U

ºUyῌy)

 3

º



 STUS» |P6 : v- S, ¡9F9ci¼h 


!

+-.. cm

 



(Loc. +2)

'G6RS!
6>E@Gh[M ij"#$  %&'(263)* +,<6kNO@% lmb
(Fig. 3) 

k

k^!

bF%mno[K' (Sheridan et al., +321)  *Tb@% NOFJ@p%q ,. 

M vrII@'M-./0$12  

@J34 56 , 789 ! +332 p.

S,-d "#$

 +.*+.* f ' ,./ f s@/ 9 bcdJ`br6>FEM

+-/ῌ+.0:;< =" #> (Fig. / ; Loc. /)  S,-d "#$'$? (Fig. . ; Loc. +)  S,-c "#$ @% S,-c "#$@K/9bcdF,b

 ..* f XYS

%&@  +** A a B 7'B: ' c B 7(B: )

6>Ft SuF)+[CD*+ 2 E@ Loc. +  S,-c "#$\Ov` (Fig. 2) '!'@ ,-F.GHI  - +** A a

 ,B 7'B: @JK2 -/L , ,M c B 2

NO 01P



0

6kv@Ut^! S,-d9S,-c "#$

2 QE (Fig. 1)
S,-d "#$@JR' =" #> STU

ij<i@J6w@SbV343C@,  a B STU5

Dx&^! ..* f XYZ%%&K

WG ++* 6)WX7@G8Y  a B@J

STU9Z [\])W@^K

Sb

F:_ (Fig. 1) I c BFQ[`/0ab

 }|F~\ (Fig. +*)cdK;e<G8 fD6+* 6&=

6wbFI y"#$,¡¢0£¤ ]Kz{!|F:_ F¥U¡  y"#$'K¦'

>? S,-d "#$ @g< a h§HI S¦NWF4`i`/0abcd 7Walker, +31/ ; A +321: F  +F|N`¨oK=EGJK'BCEM`¨5O , u (+32.)

E

S,-c "#$2

S, 

D

@z{}`FHI'©ª

3j&!C@, F a B@ :_F bk@ ¦fW  2@)CFEMk )W'Fl RGm

;eM «  ,*** ce  2 ¬ +2 %&(Fig. 1)c B@JKHI

cdEM'$?  F JK`/0abn c BFLoKM|bNO 7­9 ,**+: KQ (Fig.
+*)«  ,*** ceG@ f ¥U¡F

N@O pP5OM ,q3 ]D . ® 7N '@;: HI

QG' -./0$1  <@Urs ,'BC

Y S,-d9S,-c "#$'

E

!

nGSFx

1 S, 

/S,-d9S,-c "#$6RSJ@[M

Loc. 1 M¯ S,-c "#$@HI..* f oT+@J Ut@oVW°N+ +-cm X±°²c -..* f XY@Jc³ (OS-S,-*+)  +,1,*´.* y BP nci

Z%@o6^!

NO)+u[\Ov2@iw$['F (Table ,) I Loc. +  S,-c "#$@H

S,-d9S,-c "#$I°N+ ,- cm X±=µ³

]Gx^y@, _z)F`F (Fig. 2) ;eMab+222 cLd(OS-S,-*,) K +,1-*´0* y BP nciFfg |P6'U}b 7e~

(Table ,) y³'K@¶]@

%''K@fgK: K2@QEK  ·¸ ]GEM (Yamamoto et al., +333) S,-d "#$ I y³ci  ]K Cal AD -.* c¹{e|P6' S,-c "#$

+222 c+222 cU}b,   

264

Fig. 1. Clast a- andc-axis fabric diagrams for the
S,-d unit (Fig.
/ ; Loc. /) andS,-c unit (Fig.
. ; Loc. +). +**axes each lower
hemispere, equalarea projections.
Black circle is alithic fragment.
White circle is awood fragment.

S, 



8uvb;w W,-.ax^GOy  S,-a !"#$%& +,.0*'2/

z&: c {| ,**+ < CKy BP ()

A

S,-c

+32.

*+,-. /0&1

23!"#$%& +,/**'+0* y BP, +,-/*'+** y

WXYZ (+33.) 

S, ,-axb /F 3}~Aw

cG =>?@AO1(^c< S, BP, +,--*'3* y BP 456!789: ;[b; Nakamura (+30*)(Nakamura, +30*)< =>?@A B:&CDAEF



GH&%I1JK /F () +32.
XYZ (+33.)
\]/ LMNO9: ;PQRSTUJ&J< (V W

O+ [W\] ^J_` S, %& +,1+3'3* y BP 456!7ab c< de\]

,-.f_:\]gh.ijkWlm8nob pq:c!7rst

S, [W"\%&\\a 0a AD-** Oyz

;< ()(+32.) k Nakamura (+30.) cA8J1?

@9:;% 89J%K;% 
E1%&J< wS, ObM Cal AD -.*  a^c<.L  :  S,  !"#$265

Fig. 2. Grain size characteristics of the S,-c and -dunits. M and s are arithmetic mean diameterand standard deviation in phi units, respectively.
Data of the Bandai +222 deposit are taken from
Yamamoto et al. (+333).

Fig. +*. Grain size distributions and compositionalmodes of the S,-c and S,-d units. Crystals
consist mainly of plagioclase and olivine. The
diagram of the Miyakejima ,*** August +2
deposit is taken from Uto et al. (,**+).

Fig. 3. Grain size distributions of the S,-c and -dunits. Curve lines (GF) are Gaussian ﬁts forindividual subpopulations. Although the residualline (RS) still contains the coarsest subpopulation,
the Gaussian curve hardly ﬁts the residual
showing a coarse-tailed asymmetric shape.

v » ¼ ¸

266

New +.C ages for the S, Member.

Table ,.

2 
2ῌ+



S,-d  ῌ 

  ῎   ῍ 

,./ f !"#$ %&'+.* f )*%&'+,-

+.*(./%&01-

2 (Fig. 3) ῏ 345678 a- 9/:/;56<&=>;  (Fig. 1)? @A,8A
BCD EFGHI , JKF+AIJL
OPQN$ (Fig. ++)? N-RS 

AM N

Q a- 9/:/;56<&=>;TU8 A#BCVDWXPMY7=>;

-PI$M@\Walker, +31/ ; ]Z[PI$+321^? S,-d 8_`a 2 #BCD6:1

Qbc&;-def gN h2-$idjkF
PM2 (grain dispersion-dominated

A+,l memplacement ; Branney and Kokelaar, ,**,)? pqr 2sno,ABCa  tuvw)*8 AFPI$M2D Y
7=>;DA xPMyFY:LAM2
 z-{!? @!-|8*D}a~I2
 \}uvQ r ^ a~I2

Fig. ++. Flow models for the S,-d and -c units.
See text in details.

8!a ? P+P S,-d 8" - 2# PI"  L{M
A8 a $ % S&M@ Da? pAo*D}a~M@ 8a"& u' Ew

E w  Br Fr (w) 8BkUw, )k Fr (w)erfc (Uw/U*) *LA

AkL U/ m, *#L ks

ZPM2a,,/** kg ῌ m 

*..

¡

 +**(+/* mῌs kD./F-~I ?
'

BCTU+, -* mῌs ¯kDa~M

2 z,A ? @A,°+±+, S,-d 8²1bY³+,3§F´µ¶

S-D,
'

¢+N ? Fig.

(Fig. 0) ·F¸DN

Br Fr (w)"gLAM@ D´5!?

? Uw F$I8 Brissete*.,/ qr'B +**¤ }F¥N

¦k

M 1

bY+,¬20AM123­®2 +.* f F%&+mand Lajoie (+33*)  B, D *.+-N

U* 8 Uln (-* y/ ks) U*/k +, 8 @£AkL U



U*

(Delinger, +321)?

Valentine (+321) F$ 1;+ kL y 1bY§¨©

BCDN M F8 A B, F! 2( ?

F!~IN-$I$ ? Fig. +, +,(Nªu (Loc. /) «,A !- +.* f F%&' 2ῌ,4¹kuF!

S,-c 


6ºF7

 ῍ _` ῌ  ῎ )* ..* f¸| iUjk;¹$p : º89 S, 99:p7%p.»¼

267/hDEq$Frk;: !CMN!* 4st GuHv w!Ix Loc. +y  S, `;9%"#1{|.E_G.Jz7"} + m K~&3 7.[ =LT) C 3+C * +
!Ix S,-c KL0 %(b MG=
<N ῐ =O)

P=* 78.[

Q

R=S.8=<T ῌ U$ V$: 6I d__>GHWX) &W : 6I ῍  8k ' ῎ RXmEC
$

Y%l&CI5!=OwUi,+32- ; pZ ,**,y* + G=O 78.[ &)[3I@ (granular ﬂow) Cb6\b!C=C]'^_3+C& E`_Ge
Fig. +,. Fraction of the suspended ash sizes in aturbulent current as a function of ﬂow velocity.
Solid and dotted lines are the fractions forindividual ash sizes in phi units, when solid
volume concentration in the current is *.+ and*.,/, respectively. Density of the ash is ,/**
kg/m-, and the current is a mixture of +**¶
steam and the ash. Hatched part is a available
ﬁeld for the S,-d unit at Loc. / having thesuspended mode in ·+.* phi.

_G_ : 6I (Cambell et al., +33/)* S,-c KL0 a b'D[78.[c%Cdb6I+C_* b_bS,-cKL0  78.[.G(eZ $f);b6I!Hg1{|h ia"#$%i7 .E$) W b!j@ w7%y &kVG .I* ! 78.[

=O3. 8kdkV+C&4l.I*=.>?

S,-c KL0$FrCMN*e

.I* @pAB (+330) !Ix S,-d KL0

(Fig. 3) (Fig. 1)῏ῐ 

 !"#$% & '( ) (Fig. .)* +,-.

 &/0

CMN6I& e mI6d78.[!,- +KL0) n_GQo

+Cb6Y qk;: !p'$Fr.I*12&3' 42 ,5678&9:@pAB (+330) &r=,-C¡¢!; ,'<  (Middleton and Hampton, +31-)*

iUj wJ} -£. kmy &stkFruC==.>? +,-. @A%

C iUjC' S, 9,¤I$E+_

BCDEF+GH 2IJ&I6I

vpZ (+30-) &¥wa¦_Gb!iUjx

56 !KL0"# (+312) MN!,y w§z -+*£-.* my ,d{I&.

-.$CO$P%CQ.R.I* !KL0¨.G.I* MG |}~ !Ix Loc. ,+ῌ(Fig. ++)*

,-. (cohesive) STU (Crandell, +31+),, © /** m  Y-.* i7&§ª#5!

&&'=!V WX YI6%()

«¬b ­I¤I$4®ZD[$&\W: *+&]-* ,- +222§z ,.* m Cb6I* iUj¥w 78.

./,- /^_G`ab!cCCd "#b!

[$&STUC.5!_ (Yamamoto et al., +333)

r&5!C= ¨}~+¯I 6Fr

e 01b6I!$&"c ,2f3  4b

|°

!_ E?Gg5d<h6703.I*,.* m 86CiUj®6 w±y C

2ῌ- ῍ῐ῎῏ῌῒῑNakamura (+30.)  iUj8kC/49:l

 %(=d _56YGH $F4:6I*+&* +­I (H)  .3* m, !iUjC®²³(L)  /,3** m  H ῌ L´*.*2- C.*5

m_GiUjk; S, 9,n N. 9,<

6 78.[ &+86µN|° 4b3

+C=0b + &o>k;: !CMN!*!V+(H/L) . ¨.G.I* Nakamura (+30.) o>)?b @pAB (+330)  CiUj& S, *.*2- ,d_.@:.RXmE  » ¼

268(-+* km ) 
*\"t~32M**a$3234

Cambell et al., +33/ ; M S5x0 1*)&W (-῍  ,**,  *.*2# ! "m (Fig. 0) 6789F n$%$&WE$+5+PZQ (Fig. +-) '1*;

'(  (+330) )&*+

932!

nm$ 12M(, -. 

$  ,*** X\^6789<,/$0. 12345( 12 3*2 ;\^6)&*J. b,)^

326789:$ ;!

<"$=#'( 4* >$

3

6789<,¡¢£¤GGH ¥y02M(+30-) 6789!

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F )GH- I-J. / 01K# 6789*¨:© \^ª2 1 ¦ WtRtE¨:

23<45L5#

M( 67 «¬G ­y0 1 ¦ 2 W\^!

(+333)  67898NOPQRSTUV967

$W@6789x_z41* (Geshi et al., ,**,)(

89:(;-W23<45$<" =>?

1 ¦ +. 2 ¦ +2 2 ¦ ,3 $\p5®! 8@AXBC)$Y5Z[D!\!6

ZQy02M( K$ 2 ¦ +2 \p ¯ +/789,])&2M( ^EF G_9`

km -$3*¬G$HabcdIJK$LJM )& . e^323*( 1 ¦ +.°+/ 2 ¦ +2 v?

8@$!\!!<NK32MfOP'g

$ .*ῌ5( 33 W 67

tRt'YsZQ'1* Fig. +* ; ±  ,**+((+333) !\!QR# \ S2v?

Tb4

2! \^S, @)$S.hi32M M Tjk

2 ¦ ,3 \^ +* m ῌ s A^x0

l1* S, @)

1*   ,*** Tb /ῌ

mUW;

<;#n;# V M(

?'1* ?  ,**,( ; W\^$3*2ῌ. ῑῐ῎῏῍ῌtRt tRt­$ - !?¡@W¢*

Nakamura (+30.)  o9pqr^WQR3*$+13* X +3,. X'YsZQtRt'YsZQ[$'u S,-bv\^ S,-a \^$].tRt^w

u320*# ± ,**, 6789W}-*\

 ,**+ ; ? £x_$12\ 3*# M(

A?6789x_y0*'Ys\^z'²¤ S, \^ ,*** X\^s¥³MG)&*( T`{#|T"$;)&$12M( S, \^\ ³M S,-d 5 ^x06789

Ka S,-cS,-d }~_$S.Tb4

x_$M¦M"$QR M;932

M;' (Fig. +*)( 3*12 ; W'Y

M( QR$¬G 'YsZQy;$lm

0sZQ  ac- *Dde 32M*
c'6789x_$ZQ"$s32!f

3*c!?'§ (Fig. +-) 8@J´e

$Lfµ ¶M ;$ ,*** X\^¨8@$$\ 3*)&   W M (Fig. +-)( Yamamoto

1K32M 1*#)&W 5( ©\^s¥

et al. (+333) G (,**+) m53*5$ 'YsZ

³MQ\$?!b sgh-ijk$

2M( [& S, \^ªs$«¬^x0Q

lm c M* \8nb^.oR3*­[32o9pqr^ +13* X\^®6789x_\^$M2#M -M>L$ ij'3*\p<,- k' (Decker and Christiansen, +32.)( ;\^ t( \8nb^q&ijLr\Rt^w·¸ A?$!^|$!?'0¯32°

L$ ! ^EUR*'c\

$W@tRt'YsZQ¹!±- 2M*(

32345( S,-c }~_9O7 ;

tRt'YsZQ\J$lm c'*s \^tQR3*fOP'( -W$\8 \p<,- G.  ,*** X\^ªnb^q \ij$ uS.

s$\Bºv?3232Mv0 . > \8'YsEU;

( ; $²3 +13* X^x0tRt?!

.wDd+0 ^x0QR$y3*t

01*\^³¬G 'YsZQQR32!

 ( S,-d }~_z4!$6789W -x0$TbtRt4

km 4nb

et al., +33*)( ;5 ­[^x0QR$

{|} ZQy0* 2M

M (McPhie

J&'K 7vfv>?89 : »w5) S, ())L¼M*¼+,NO-

269         !"#$%&' ( )*+,-
./0

12!3 456789: ;<)' =>?
 @ABC85D
%E FG!F/"H#$%I&'
3ῌ ῎ ῍ ῏J&'K S, ())L M* +,NO-$F
PQ R.) $F' +) S, ()+,
Cal AD -.* +S  T/ / 0UVWXF+,Y%3"Z

' ,) S,-a

) S,-f  0 [C\]^1

' S,-a [C\]K23456 _ F`X789 aXbcK: ) 
89 !' T/ ;@?89deXF"< S,-c  S,-d [C\]*=' -)S,-c [C\] >5f?g@Gh*ijkl

mnoAMῌKAmnM )3 ..* f BC^DKnh*jp' qF MErst5\uvDw: !' [C\]  F/?G
Hvxy7$z $F" !' .) S,-d [C\] {5I> |  J} K L&' 89~ῌM)Nj MEij,./ f bMEBy

+.*+.* f h*By ) 9Byw7
DO"p' qF MErst5\u a- 89
9Dt5yDjp' [C\]P $FQ?89  7
Fig. +-. Schematic cross sections showing the the S,
eruption. +. The beginning of the eruption by
ﬂank magmatic eruption. The S,-a unit wasformed during this stage. ,. The transfer of alarge volume of magma out of the chambercaused deformation of the summit region andleakage of highly-pressurized geothermal ﬂuids.
Repeaing phreatic explosions produced the S,-b
and -c units. -. Caldera collapse and catastorophic outburst of highly-pressurized geothermal
ﬂuids toward northeast. Ejected mixture of
steam and abundant lithics was too dense to getbuoyancy by entrainment of air, and generated ahigh-speed pyroclastic density current of theS,-d unit. MC º magma chamber ; PDC ºpyroclastic density current ; SGFºself-sealed geothermal ﬂuid.

v Q +**+/* mῌs, 7v )"RX
F'ST(" -* mῌs bQ!F")X' /) S, )XQ?
 7vUKVW(XY5 WFG;)X'ΐῒ

J&' Zc[ -¡¢\2£¤¥]^_¦§`a¨W$F©  S, () lpªb$F©F' qF c¥Jd«§¬ef§g­7h$F©F' ijk®#¯°­±lm² !F' 1³3´µn$q'ῐ ῕ ῔ ῑ¶o·p (+330) Kq' 5r£s !t5¸¹u ?

270

 £ ª «

 ,-,ῌ,-,.
 (+330)     ,,3ῌ,,3.
Brissette, F.P. and Lajoie, J. (+33*) Depositional mechanics of turbulent nuees ardentes (surge) from their grain
sizes. Bull. Volcanol., /-, 0*ῌ00.
Branney, M. J. and Kokelaar, P. (,**,) Pyroclastic Density Currents and the Sedimentation of Ignimbrites. Geol.
Soc. Memoir, ,1, +.- p.
Cambell, C.S., Cleary, P.W. and Hopkins, M. (+33/)
Large-scale landslide simulations : global deformation,
velocities and basal friction. Jour. Geophy. Res., +** (B/), 2,01ῌ2,2-.
  (,***)  ,***     ++/ῌ+,..
Crandell, D.R. (+31+) Postglacial lahars from Mount
Rainier Volcano, Washington. U.S. Geol. Surv. Prof.
Pap., 011, 1/ p.
Decker, R.W. and Christiansen, R.L. (+32.) Explosive
eruptions of Kilauea volcano, Hawaii. In Explosive volcanism : inception, evolution, and hazard, +,,ῌ+-,. National Academy Press, Washington, D.C.
Denlinger, R.P. (+321) A model for generation of ash
clouds by pyroclastic ﬂows, with application to the +32*
eruptions at Mount St. Helens, Washington. Jour.
Geophys. Res., 3, (B+*), +*,2.ῌ+*,32.
Druitt, T.H. (+33,) Emplacement of the +2 May lateralblast deposit ENE of Mount St. Helens, Washington.
Bull. Volcanol., /., //.ῌ/1,.
Druitt, T.H. (+332) Pyroclastic density currents. In The
Physics of Explosive Volcanic Eruptions (Gilbert, J. andSparks, R.S. J. eds), +./ῌ+2,. Geol. Soc. London Spec.
Publ. +./.
Fisher, R.V. and Schmincke, H.-U. (+32.) Pyroclastic
rocks. Springer-Verlag, .1, p.
!"#$%&' (,**,)   ,***  ()*)+, .1 .+3ῌ.-..
Geshi, N., Shimano, T., Chiba, T. and Nakada, S. (,**,)
Caldera collapse during the ,*** eruption of Miyakejima
Volcano, Japan. Bull. Volcanol., 0., //ῌ02.
. (,**/) /0123!("#$45
 /* 6 /* 78%&'9 S,/-ῌ,1,.
:(); (+332) <*+ =>=> +*=,?@ +ῌ2
A#B:-C (+330) <*+ DEFG
HIJ( & K +*/ +--ῌ+0,.
L.M/N%O (+332)
P0( Q1RS ,3 T Q6 -33 p.
UV28 (+32.) +W(= W= Q3X Y/
45( + =>6Z =,?@ +ῌ+--.
UV28[U78\]9 (+32+) + +( ^:;7_ : +. C <`#ab3Hc,0 ,-/ῌ,-2.
UV28[U7B:d= 8 ef> g$ ?hij (+30-) klmcnopq<*+ (DEF5r
 2 0+ῌ+*0.
McPhie, J., Walk, G.P.L. and Christiansen, R.L. (+33*)
Phreatomagmatic and phreatic fall and surge deposits
from explosions at Kilauea volcano, Hawaii, +13* A.D. :Keanakakoi ash member. Bull. Volcanol., /,, --.ῌ-/..
Middleton, G.V. and Hamton, M.A. (+31-) Sediment gravity ﬂows : mechanics of ﬂow and deposition. In Turvidites
and Deep-water Sedimentation (Middleton, G.V. andBouma, A.H. eds), +ῌ-2. Soc. Econ. Paleont. Mineral.,
Paciﬁc Section.
'@(s$Aj (+333) tuv*wx
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