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0,**/ ( - 1 -+ 2345 ,**/ ( 1 1 +. 2367

On the Essential Ejecta of the September ,**. Eruptionsof the Asama Volcano, Central Japan
Yasuyuki MIYAKEῌ, Kou TAKAHASHIῌῌ, Tatsuro TSUGANEῌῌ, Kuniaki MAKINOῌ,Hisakazu KAKUZENῌ, Kuniaki NISHIKIῌῌ, Takashi FUKUIῌ
and Shinshu University Research Group for Asama *. Eruptionsῌῌῌ
Asama volcano in central Japan became active on September +st, ,**. with a vulcanian eruption which was
the biggest eruption among a series of small eruptive events from September to December in ,**.. The ejecta of
the Sept. +st eruption were mostly polygonal fresh andesitic lithic fragments. Most of them were derived from theandesite body which played the role of cap rock for the rising new magma and escaping volcanic gas. Less
amount of pumice fragments were also ejected and most of them are mantled by the black and dense andesitic
crust, which is broken and expanded to form breadcrust structure. These breadcrust pumices are concluded tohave been the fragments of the new magma. After they were broken into pieces by the explosion, the outermargin of them rapidly consolidated, and then degassing and inﬂation in the inner melt caused rupturing of theouter crust. The pumices are clearly discriminated from the andesitic lithic fragments by their whole rockchemistry. The next vulcanian eruption occurred on September ,-rd. Majority of the ejecta were again
polygonal lithic fragments of andesite, although their chemistry corresponds not to the lithic fragments, but to thepumices of the Sept. +st eruption. Small amount of scoria are found among the Sept. ,-rd ejecta. Their wholerock chemistry and the assemblage and chemical composition of phenocrysts are quite similar to those of the Sept.
+st pumice. The black appearance of the scoria is derived from the less crystalline, therefore less di#erentiatedgroundmass glass in the scoria than in the pumice.
From these petrologic evidences the following magmatic processes are deduced. Before Sept. +st, a columnof new magma had risen into the pre existing andesite body beneath the crater ﬂoor. On Sept. +st, the built up
gas pressure surpassed the tensile strength of the andesite body to result in a vulcanian eruption. This eruption
provided fragments of the pre-Sept. +st andesite as polygonal lithic fragments, along with smaller amounts of thebreadcrust pumices. In the course between Sept. +st and Sept. ,-rd, the column of new magma uplifted because
the cap rock had disappeared. The upper part of the column was cooler and more crystalline than the lower.
Some portion of the upper part of the magma column e#used and made a dome inside the crater. On Sept. ,-rd,
next vulcanian eruption took place. The ejecta consisted of the lithic fragments which had been upper part of theconsolidated magma column, and the scoria derived from the lower level of the same magma column.
Key words : Asama volcano, vulcanian eruption, essential ejecta, breadcrust pumice
8-3*ῌ20,+ 9:;< -ῌ+ῌ+!"#6#=>?@#@
Department of Geology, Faculty of Science, Shinshu
University, Asahi -ῌ+ῌ+, Matsumoto -3*ῌ20,+, Japan.
8-3*ῌ20,+ 9:;< -ῌ+ῌ+!"#"#AB#CDE@>FGHIJ@#KL
Division of Environmental System Science, Graduate

School of Science and Technology, Shinshu University, Asahi -ῌ+ῌ+, Matsumoto -3*ῌ20,+, Japan.
MNO P6Q 'RS T>UV WX
 YZ[\O ]^ _ `Z a bcde
Corresponding author : Yasuyuki Miyake
e-mail : ymiyake@gipac.shinshu-u.ac.jp

334 ³x´µ¶· ´y¸z{UO|C`}®~F¹º|t *. »d¼+     ,**.  3   (,**.)  3  
3+

,* ! *," # $%&'

(")$*+, 0 km ' - - cm %.
+

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jklmZ#nC $opDq#E289'FG9 6B ?N6rs?H1 L: tH1'Cu\$9H?H#AB  6,#',+

cdcCCvwbx#fg  +e 3 +, ,-Fig. +. Sampling localities. a : Index map. b-d :Detailed map of Loc. +῍-. Topographic map
adopted from + : ,/,*** quadrangle map
Kitakaruizawa (Geographical Survey Instituteof Japan, +323). The eruptions took place inthe Kama-yama crater which is the youngest
central cone formed on the top of Maekakeyama. AVO : Asama Volcano Observatory.

5cdcyb $z:89 2

L {| 3  +.

+, +2

 LM8 (,**.) rNO2~J#K$\2\
8 (,**.) `aC 3  +u[2%, +.

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C ,-

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p[a#'7 aa6Ub

9 B,#a6E (Fig. ,c, d) CM EL}9cdc6Pe

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,ῌ+3+

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 0 % ,? 3  , QRS T (Loc. +)CUT (Loc. ,) (Fig. +)

Loc. +  S T;<L:6

VW

$\

?<_W?,WhBB& I)Ca6E-'6 +3* g C ,.1 gLoc. ,  CG? , cm jk


B

% -lV 0 cm 2.0 g E'

B¡?< + m ¢+,m~J#$\9 (Fig. ,a) BCr WC?Y £n&¤ p o\ } $ ,/..10 mm p.\3Z% $\9BC2G0o?

¥¡¦§#Rq¨r©D& 2%,Iªs#X%

9BC#D&

?5 [Nt6«w# Table + ¬8 2L p.

6I)9 (Fig. ,b) 6,\H2'C­ 3®5¯°$\9±uv%9 ²

H2'+,29 ]>E)'0 6

59m$E ][ae\ ''²^ £^5_[H2E#0& //+* mm `nF> m?F>w 1.. wt.ῌ 

QRU$ ,**. h 3 OU ijk
Loc. +, , 

 *./ mm Fig. - !" #$%&' +.1(,.1 gῌcm- )*+ ,-.

9 ABCD
,ῌ,

335

EF/GHIJ4"3 ῍ ,- ῏ῌῑ῎ῐ

KLMN 3 O ,- P '+ 3 O ,1 PQR

/01' +.* gῌcm- 23)45+ 67,801

STU$VWXY (Loc. - ; Fig. +d) " U' *.2(,.1 gῌcm- 9:;<=" ,->?9@> 01

Z ,. P[\ ] ,1 P^_`a bcd/ef gFig. ,. Photographs of the ejecta. a-d : Ejecta of the Sept. +st eruption sampled at Loc. +. a : The occurrence ofandesitic lithic fragment. Grasses beneath the fragment were heated and scorched. b : Andesitic lithic
fragment with a polygonal outer shape. The length of the scale is +* cm. c : Breadcrust pumice. The lengthof the scale is +* cm. d : Cross section of a breadcrust pumice. The dark colored outer part is thenon-vesicular crust, which was ruptured by the expansion as a result of the vesiculation in the inner magma,
which is represented by the pale colored inner pumiceous part. The lower part of this sample is broken. e
and f : Ejecta of the Sept. ,-rd eruption sampled at Loc. -. e : Andesitic lithic fragments. f : Scoria.

336 ´Jµ¶`_ µ`K·LM`NObC`5PQQ`¸RS¹`Tº»¼`abc"d *. #$efghi
Table +. Size distribution and the constituent rocktypes of the Sept. +st ejecta sampled at the Loc. ,.

Table ,. Constituent rock types of the Sept. ,-rd
ejecta sampled at Loc. -.

- 
-ῌ+ 

,yz {h| " Table - Y Fig. . 23 {h|/   }~~g@h"-pq+***  EF"/  @ABC ' 

V W - ' ."-/ -**  / " !  3  + /LS567
8+ j,Y>0 3  ,- /@ABC?
YP;P P,1,8+  ,yz  ?YP    > 

7+,"YW3,yFig. -. Bulk density of the ejecta of the Sept. +st
eruption.

, P 2zP, @ , h ,.* mm   34"(5 67
8y 94):"( ,;y"<= 
6h @ Y>3 ,,60y"& <3 P

 

 

P,   , cm  + m  *.. mm  ,?

¡"( hG +.0 mm @A7 @ ! "#$ %&' ()*G *.3 mm ¢@A76£6"23"/ 01" Table , 23 4¤¤, @  h ¥,+,-.

5678+ 9:;<= y¦ 

 >?@ABC +/ wt.ῌ ; @AB,§  3  + /Y ,- /8+ ^J 3

>EFG"(H + /j,Y>0 3mBC NO@";P7IJK"4;<"( LM 3ND 3  ,- /@ABC ¨SNO@"C D *.+ mm

O@P NO@Q RST;PY

 @ABC >U V W . X© ª«3

'"YW

"/YZ[ >

gῌcm-  >\ S!]++.-^ ,§)*z -ῌ, 



Fig. / Y Fig. 0 +£¡^¬,yz ^ _`abc"d *. #$efgh

7"23 3  + /­07j,0Y 3  + / j,%k3Y&l > 3^J ,- /5678+£¡  m  n\ S o pqr'3YG,0yEF ZY¤¤®¯ 

i (,**.)*.*+ mm ( ,s`,`)t,YNO@+,  !]+4 u+vwx*++

9Fig. 0 LSG°= j,HY ZI±+£¡F  >D 3  ,- /@ABC >7 ²  <[@ABC£64,60y¡³­®s ,**. ¼ 3 o®:qO
Table -.

337

Modal compositions of the ejecta.

The No. 2 and No. 3 samples are the outer crust and inner pumice of a breadcrust pumice,
respectively. tr : Exist but less than *.+ wt.ῌ.

XY)Z[2

D8 V, Cr O\/8]Z[^1+_5C @&` O8 Rh a_bY_cdefg dehV f ! ABIJ8
ijMKL (,**+) OPQORSTUV
klC 0123456 Om!nC 3 o ,- pOq/!rs t2a)u 8C vw#xyz{`TC |v .}~01234
56 /!.ῌ, 8 Table .  Fig. 1 O 3 o + pq
'C "8C!kC@/"kf O8TUk @UO\/rs 
8C " `T/C " n SiO,
O/ Fig. 1g, h Of ObZYzUV rs  O8C ¡R/!#¢kV
ObZYzUVmV £¤¥¦ 
§ (Rb/Zr) f¨VC "@UOgk 
rsFig. ..

Modal compositions of the ejecta.

C8&©tYª_5k/VO§S
fc/! rs 8Yª_5`T8«UV§/V (Fig. 1h)3 o ,- pqO¨VC 2a)u 8
Fig. 0 

3 o + p"&©¬/!ObZYz

UV tfC 3 o ,- prs  8 3 o + p. 

"  ObZYzUC 3 o + prs

.ῌ+  8O¢k V  

 X  (PHILIPS PW,.**) 

§³VC ´µ®"2a)u8C ¯$%&'()*+, . g q SiO, O/¶· SiO, xC Fe,-./0123456 /!78O-, MgO, CaO k¸x@UT¢kV Fig. 1g,

9:;<=>?#6/! @Ah Oft¹rs 8C ¯°C ¯q

DEF GHIJ8C KLMN (+330) OPQ§SC @UºU MgO, CaO x¢k V @!BC

"# , g ­®s¯°¯®q ±²MNC ,**-

ORSTUV W!C  O8C # 

N 3 o + prs O6 /!>?01276 /!»V8kf¯°W!8¯338¡¢£¤t¥ ¢¤¦§¨ῑ¤©῍k¤ª«ῒ¬¤­῎®¯¤°±²³¤´µ¶· *. ¸¹¶º»¼I:

/ῌ ῐ῕῍῎῏ΐῒhi>jk/CDE 3 l + &m6 3 l ,- &  

῏!"#$ SEM%&'()* JSM-/-+*E+ῌEDAX DX. ,/0123 456

nKH/ Mg opqrsOtcu Table/ F]^3 vw> xyFbcGzD6 An -. (EDS)

E //{1* |}F~DuEῌ

789:; <=>?@

TGcD (Fig. 2)3  >y u(,**.) 6AB/CDE FC21G> H

An FYc6Ec3 877>^GIJKLMNOPQRS2TFU:.VWXG

/ Mg o 0*{1* acF^Dub

Y23

E F Mg o 12 Fc2D/ Mg FuuC/ῌ+ ῔ῑῐ῕

D3   y6Fc>CZ

Z[\ Table / 6 Fig. 2 F]^3

c3 :>F a6`AB_`a`FbcG>Y2dEeZc2Tfg

ῐFD (Fig. 2)3 c

Fig. /. Microphotographs of the ejecta. a : Outerrim of the Sept. +st breadcrust pumice. b :Outer rim of the Sept. ,-rd lithic fragment. c :The Sept. ,-rd scoria.

Fig. 0. Sketches of the habits of phenocrysts bytracing the microphotographs (Fig. /). Vesicles
and groundmass are not shown. For thearrows see the text.

¹º»« ,**. ? 3 %» @¼%Y$
Table ..

339

Representative chemical analyses of the ejecta.

Fe, O-*¸Total Fe as Fe, O-.

 



0/ῌ, 

0ῌ+



   3  +   3  ,- 

0ῌ+ῌ+

3  +   !"#$  % &'()+ , Fig. 3 -. 3  +  /0)*

3  +  %Yn/0$  9 K, Z ns n %12345267  89::26 ;<=>2

6 ?=@A26 B  C   D:9&Z s   9 'E    FG C 3 

B
,- 

 H y%x%p  &(N$ n )&( *&'()

n+$ F #Z  U

* IJK +,   2 + ,

n,-&Zp U 9

L$MJN JO ?3PQR,ST U12

K,

,XYNZ (Table 0)  

34526 V<W[\]^ *,**/ _+ `a  b - ,NZ.

 ῌ 11 &(   ῍ &( .n/N$j ῎ 0.be 123

K 9 c,U defg  

¡4 ῏ |.  bp5¢6(F£hi 'O EDS NZjk,lZ

¤nU#Z  0!U 9

B7211(s#Z¥s  ¦Un7

D:9 mnop % *.+ῌ

qrs

 

-*ῌ 

IJKt1234526n+.ῌ a$u v5

2.jNZ(s#Z. § ῎
123nU pZTs  U¨, Fig. +* -N 

w!, Table 1 -.  9 23 89    IJKv5

SiO,  10῍11ῌ xy'© ª«(p¬N$b! 0Z"z{.

 |v5 1,./῍

1..2ῌ p00#Z!,-N  v5"z}#N t(Fe, O-) 

n$~!uUn Fig. +*9 K . :­®;¯(n#$°$ (Fig. 0) ῌ&(

±²&³n´  µ< 11=¶>n· ±²&³ FU340    !"# *. $%"&'()*+

Fig. 1. Whole rock chemistry of the ejecta. The products of the Tennin and Tenmei eruptions are from
Takahashi et al. (,**-). Fe, O-*total Fe. All the data are recalculated on the basis that the total+**ῌ.
See the text for the interpretation of the ellipses in the ﬁgures g and h.

º»  ,**. ¼ 3 D
Table /. Representative chemical compositions of thephenocrysts.

Y t

341

Z8[ \]0"2HI2J)
0P"^_0`) 0P*J a2J 
3M -0P*JZbZc0`de0
$3 af3Z$g2M"h
!

M"ij20ῌ+ῌ,

3  ,- 

3 D ,- EkY0lmnoX$2WX

pqrs3 t2J Figs. /, 0 i"[ pqr
s uv"wxy z{|" 3 D + E}1 + E" ,- E



!

"#$* uv*wxy3~"-":|,$ +" pqrs uv3wxy Mu*,M"3
F pqrs Ip"$*3wxy"#$)*J $d.N0P*JM"i) 0P )2)+
3N0P*J"U F 
 "yp"))~2
(J0P- * 3 D ,- E

pqrs)

YY0P"L
3 D ,- E0)

t -.)Z*J 0P*J 2Z2 3 D ,- E  af  3 D + E "#$*[ 3 D + E! '(52?@AB (Fig. 1) wxy h1 '() !"0P 0 3 D ,- E   )  3 D +E2`J32J!0P"KL
3 0P
0ῌ, 
3 D ,- E

3D+E



pqrs

3 D + E" ,- E"0 Walker (+303)  &'()* +,
SiO,-RbῌZr /0

-. SiO,-K, O /"!)11 SiO, 23!

t2J)0P*

J¡¢3£ 2Z2 pqrslmno ¤X !"#$%

wxy)%&'()")!"d,$ (Figs. ., 1) 2J3*

!"|Ze#$ 3 D ,- E t2Jpq

rs3o*¥  !¦p3§X0PJ¨0P +©2 3 D + E! !¦ p

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Fig. 2.

 

Compositions of pyroxene and plagioclase phenocrysts.

 

et al., +313) Self et al. (+313)  3  +  !
"#  3  ,- $%&' !"

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Table 1. The chemistry of the microlites and glass in
the groundmass of the Sept. +st pumice and theSept. ,-rd scoria.

The average composition of microlite is calculatedusing the data of chemical compositions, modal
compositions and density of constituent minerals.

Fig. 3. Back-scattered electron images of thegroundmass. a : The crust of Sept. +st pumice.
b : Sept. ,-rd scoria. Mg : magnetite, Pl :plagioclase, Px : pyroxene, Qtz : quartz.

Table 0. Modal compositions of the groundmass
from the crust of the Sept. +st pumice and theSept. ,-rd scoria.

Fig. +*. Chemistry of the glass and microlites.
WR : The whole rock compositions. The plotsof the microlites are the average compositions of
all constituent minerals as shown in Table 1.

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Dl

Fig. ++. The schematic illustrations of the eruption
process. The size of the magma column and its
depth are not in scale.
a : Before Sept. +st, a column of new magma hadrisen to some shallow level beneath the crater
ﬂoor and the decrease in pressure caused degassing of volcanic gas which was built up beneaththe pre-Sept. +st andesite body.
b : On Sept. +st, the built up gas pressure
surpassed the tensile strength of the pre-Sept. +standesite body to result in a vulcanian eruption.
This eruption provided fragments of pre-Sept.
+st andesite, altered older rocks and magma. The
magma fragments were originally polygonalshaped. While they were transported in air,
they inﬂated as the result of the delay
vesiculation inside the magma and formed into
breadcrust pumices.
c : In the course between Sept. +st and Sept. ,rd, the column of new magma uplifted because
the cap rock had disappeared. The upper partof the column was cooler and more crystalline
than the lower part in which the residual
magma was poorer in SiO, and more enriched in
Fe therefore it formed darker glass than the
pumice of Sept. +st eruption. Some portion of
the upper part of the column e#used and made alava dome.
d : On Sept. ,-rd, next vulcanian eruption took
place. The ejecta consist of lithic fragments ofalready consolidated upper part in the magmacolumn and the scoria derived from the lower
level of the same column.

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(qr B(,**--B-*+) +!"Kb+;/_`<ablm`>] ECAB%*k*ῐ ῕ ῔ ῑDuno E{Fp} (+33/) qrs
N ?tGu v4wx! +++! ,/ῌ,3.
yzHN{\H|mK}eI~Kj,   JC  KL  -M   FVLNlOPC (,**.) 3L+! ,**.  3 +: Q
.' +!c ,**. ;! +21 p.
6A+! R¡`¢£ (,**.) v- =.
K&4S&¤M+!&¥v% O7 +0 3 ] 3L! +0 p.
jklmUTuM (+330) ¦0§?>%%7U¨f X uU©V:ª£«7U; \]cc
¬W! -+! ,! +*/ῌ++1.
XW YZ\[­®7P\¡ ,**/] ¢¯i]£57°'±£²£ (SAR) jB"&3L+! ,**. +"+'^=2<X+!! /*! .*+ῌ.+*Self, S., Wilson, L. and Nairn, I. A. (+313) Vulcanianeruption mechanisms. Nature, ,11, ..*ῌ..-.
+,nH  /C¤ mEF  JC _  C)(,**.) 3L+! ,**. + ?c³1
+!c ,**. ;! +20 p.
Silver, L. A., Ihinger, P. D. and Stolper, E. (+33*) The
inﬂuence of bulk composition on the speciation of waterin silicate glasses. Contrib. Mineral. Petrol., +*., +.,ῌ+0,.
Smith, J. V., Miyake, Y. and Oikawa, T. (,**+) Interpretation of porosity in dacite lava domes as ductile-brittle
failure textures. Jour. Volcanol. Geotherm. Res., ++,, ,/ῌ-/.
9´ l  \]c3L+! *. +¥YU£(,**.) 3L+! ,**.  3  + +  : G
¦ Xc67 +!c ,**. ;
! +3, p.
9´ §¨e`]HJu©)3ª«N¬ab­csd (,**-) 3Lµ+!®¯+ 
t?Xc67®+ e¶ cfc°±cqrJqr¬W! -2! 0/ῌ22.
²N·F (,**.) ! ³´µ}Rg+!
+!&¶  ¸¹·cZ£?c"º*% =5¸! ++*! +/2ῌ+1. p.
UTjklm (,**+) ¦0§?>%hQ7U
:ª£«i"&¨f X uU O7 ++g+,
;cqr9?X @qr (C)(,) qr7OR¹º! »»j¼ ++0.*..1! ,-ῌ-/ p.
UTc,]jklm9´ l ,**/] 3L+! ,**. + *k¼Ul>346   !"#$%&'()*+,-./"0123 *. 456789:;  
Walker, G. P. L. (+303) The breaking of magma. Geol.

Mag., +*0, +00ῌ+1-.
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