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 0  .//ῌ.1- ῐ ῏ +00-  
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,**/  .  .  ,**/  ++  / Re-examination of the Magma Plumbing System beneath Usu Volcano,
Hokkaido, Japan, during the +00- Eruption
Akiko MATSUMOTOῌ, Mitsuhiro NAKAGAWAῌ and Yugo NAKAMURAῌῌ
The +00- Usu eruption was the ﬁrst and largest of all its historic eruptive activities after ca. 1*** years’
dormancy. Our recent study divided the +00- eruption into three stages in following ascending order : Stage I,
small scale of plinian and phreatomagmatic eruptions ; Stage II, climactic plinian eruption (pumice fall as so-called
Us-b fall) ; and Stage III, vigorous phreatomagmatic eruptions. Previous petrological studies mainly focused on
the Stage II event.
The +00- juvenile materials are composed of three types, white (from all the stage), banded pumices (Stage I
and II) and gray essential lithic fragment (Stage I). Major phenocrystic minerals (i.e. plagioclase andorthopyroxene) show nearly bimodal compositional distributions, and can be divided into two types : Type A,
iron-rich orthopyroxene (Mg#.0) and sodic plagioclase (An.,) ; and Type B, magnesian orthopyroxene (Mg#1*) and calcic plagioclase (An21). This observation suggests that the juvenile materials were mixing
products between maﬁc and felsic magmas. This observation is also consistent with linear trends in all the oxide
variation diagrams for whole-rock chemistry. Based on Wo (Ca/(CaMgFe)) content in orthopyroxene, An
and FeO* contents in plagioclase, however, the Type A phenocrysts can be further subdivided into two types :Type A+ (lower Wo, An and FeO* contents) and Type A, (higher Wo, An and FeO* contents). Moreover, theType A, phenocrysts are common in the juvenile materials of Stage I (gray essential lithic fragment) as well as in
the Stage-II. Based on phenocryst size, composition and whole-rock chemistry, it can be concluded that the TypeA, phenocrysts crystallized from the mixed magma between the maﬁc and felsic ones, and had grown for several
years before the +00- eruption.
Considering the eruption sequence and the types of erupted magma, the mixed magma was erupted during theinitial, weak eruption (Stage I), as well as the climactic, explosive eruption (Stage II). This indicates that themixed magma of Stage I event would not stagnate between maﬁc and felsic magmas, as is common in a normal
zoned magma chamber, but the top of the chamber. This could be explained by convective entrainment asfollows. The injection of the high temperature (+***) maﬁc magma into the lower temperature ( 2**)felsic magma could cause thermal convection to entrain the former into the latter. This entrainment would form
the mixed magma, which could rise to the top of the chamber. In e#ect, the upper part of the chamber would be
gravitationally stable until the eruption, because the mixed magma could be lighter than the felsic one. Our
petrological analysis concludes that the +00- eruption was derived from a compositionally reverse zoned chamberthat the mixed magma had existed above the normal zoned magma, and that maﬁc injection had occurred not justbefore the eruption.
Key words : Usu volcano, the +00- eruption, zoned magma chamber, eruption sequence!*0*ῌ*2+* "#$%&% +* '( 2 )*%+,-.-./.01234562.78
Department of Earth & Planetary Sciences, Graduate
School of Sciences, Hokkaido University, N+* W2,
Kita-ku, Sapporo *0*ῌ*2+*, Japan.
!*0*ῌ*2+* "#$%&% +* '( / )*%+,-.-./349:2.01/34;2.<=

Division of Geoscience, Graduate School of Environmental Earth Science, Hokkaido University, N+* W/,
Kita-ku, Sapporo *0*ῌ*2+*, Japan.
Corresponding author : Akiko Matsumoto
e-mail : am13*+*3@ep.sci.hokudai.ac.jp

;y
;y;y
;y

¶·¸¹;9:º»;9¼

456

Fig. +. Index map of Usu volcano showing thelocations of the outcrops (see text).

+      

  (Fig. +)   1*** !" $%&'()*+ +00- $12' 3456 3 70#

,-./
'()

89:;< ,**/= +00-   >? ,-./

0  8Us-b @ABC= +DE FGHGIJK

LM(>N/OP/Q (Us-b+-b0) +R'*
8S;< +31-=+00- T)%&UVWXYZ

[\]^ [ GHG_`a 345bcXYZ GHG_`aWUde\fgFig. ,. Columnar section of Nishi-Sekinai outcrop
and component analysis of the 3 units, including
the coarse ejecta, Stages I and II, from
Nishi-Sekinai outcrop ; Stage III, from Frankoutcrop (see text).

^h XYZ $ijk$*^[\+Ulmh F*n +00-  o)(pC

GHG_`a\ ,Oo)(qrstcmh() 8Okumura et al., +32+ ;uv +33/ ; Tomiya and Takahashi, +33/=']'+^ [h^wrsx

R>$yn

Us-b @ABC$z{|M(}{UN/OP/, +00-  +00- 

RAL{ ῌ῎ 6 - O/Q O

/Q ῎ o)(m^ / o.

'* ¡O/Q¢ \ 0£)

Q~$n* "o)(+mh()(Fig. ,)

)L¤ ¡. ¢ \p¥}L¦pCq§¨L  +00-  $q1'
Us-b @ABC'*?  U$^]E* m^[h5$

¤ ©A ª« +00-  R$AL{O/Qῌ 8.A= O/Q῍ 8.

B= O/Q῎ 8.C-G= \¬­ F0®¯° ±o)(²E()]M*N/OP/Q$'*o)(

89;< 9= rs 1'* +00- ³ o)( 9;< 89= $´µmj)*"pCq$)

h*)O +00- «± ¶·¶¸¹º K»¼

Fig. -.

457

Photographs of the +00- ejecta, (+) white pumice of Stage II, (,) gray essential lithic fragment of Stage I.

ῌ A : (basal(+33+)

part)  !"#$% &'()*+,% -./0123456
- )789/:/; +* cm, ; 1./ cm <=῍ B : >?

Us-b &'( @A456BC% / D !E *+, 9 F)78; 0* cm <=῎ C-G : >?

Us-b+-b0 

6% !E 8GHIJ6KL4MN0 <=


C >?

Us-b0

F)78; ,/ cm <% MNO) P)% OQ)% R
MNO)*+, OSTUV-W8% X
O)YZ[$6\]+/ 
Us-b/  8)7; / cm

 

E >?

D >?

^([_`)*+,

Us-b/ a 8F)7; 3 cm <%MNO) P)*+, ?

Us-b, -b.

F >8F)76; ,. cm <% bO)P)*+, a8cdO)Yde; - cm'(6\]+/% fg6@Ah- 9MWG >?

Us-b+ 8F)7; 1* cm <% &'()% XMNO) P)% ([_`)% X

Fig. .. SiO, vs. K,O diagram of the gray essential
lithic fragment of Stage I and Toya ignimbrite
pumice. Toya ignimbrite pumice data is fromFeebrey (+33/).

O) P)*+, OSTU<0% ij )k6\]+/
/W-,*N _`8 6 <- 
-ῌ+

,0 6C% ῌ[_`<=O_{%r+/O_`+*<= 

Z[$8lm'(<=n\]+/

<8% _`8_[<=% _ ¡O

6% 9 oῌ8pq'(rstu h-_*+,O_{% ¡O_ ¢£!£ *+_`6% ῍8pq'(6\]+/ (Fig. -)

,_r8¤L4~/ ¥Ni¦_[<v/+Z[$ wx% _`8(+33+)% Tomiya

and Takahashi (+33/) V-W ystM,-Z[_`z{ ylapilliz rMW|}~/W-N6% lr,῏8=§¨  '( ; Feebrey, +33/

r08©ªM,- (Fig. .) _`8% +00- «V¬9 ­῍®¯°±²$rr0% ©῎³´µ SiO,-K,

· ¸¹º"vw»"w5¼

458
O      




+00-

-ῌ, 

g_B0",ijg_B0   !"

#"$%&'()*-./ῌ   0+ cm : 1

 * 1* wt.ῌ 4 (Fig. ,) 5

89* .* wt.ῌ

67



! - cm, "  ! , cm -./῍

JEOL 22** PQ9

EVW| +/ kV, WXY+* nA 0}Q"ij3 +/,* nA 0}/"[}"Fe-Ti g

3  t\ -*῍+** ] I ij9:k.< , mm ! +* mm,  DE" *5):) +/ wt.ῌ  ; <

vw X M9zy EPMA  ZAF RST?>DE 9:U{
nA 0~Z3+),%23 _`e) #g_B0:N7 0Philips xO PW-+.*. y3 ?9>DE r

  
+00-&'ijs4 >h09: 9:#>KuL/ 
+00-

  Us-b 1 ^ 0 _-./῍3

1" #$ 2* wt.ῌ *   Fmr g_B01 , =Fm  / wt.ῌ %4) (Fig. ,) 

*)>9 (Okumura et al., +32+ ; Tomiya and &'% "=)>./ wt.ῌ  (B%4)C )"D?@A* /* wt.ῌ  *E "  ; <Takahashi, +33/) 5)

(a) Type A Fm 0Q`a : Na b}Q Fe b}/ Mg c9dr I3 (b) Type B Fm 0d

! 0῍1 cm `a : Ca b}Q Mg b}/ Mg

-./῎ +FGHI C, F, G =9> ,$Jbdr [} %3  ?e,-./*K0 %LM( % ,$a> fB0Fm*%L)>J,-.=9>K0 &'N 1OP

9* gpO&'_ )>9%a(Fig. ,)



#FGHI&9>
)"D&'Q

FGHI C 

1"&9>)"D* 3/ wt.ῌ  4*"  * /* wt.ῌ R?)

6 FGHI F  * +* wt.ῌ 1

* FGHI G .* wt.ῌ  R?) ,$J,&S  ; <

FGHI C  ! ,cm, F! 0 cm, G! . cm  1T2"3U4

&S



<

F

;

! - cm, G

FGHI C ! *., cm,

! , cm 

VW h}Q An Y +**Caῌ0CaNaK3 ¡}/ Mg# Mg#¢+**Mgῌ0MgFe3 £¤% ¥a

G Type A

B  , ¦%L }/ Wo Y +**Caῌ0CaMgFe3&'}Q An"FeO* &$§M Type A,=p9M( i% (67jAn"FeO* }Q&'j Wo }/¥a Type

A Fm ¨ An"FeO* }Q&'¨ Wo }/B0Fmkl ©) 1mno-./ῌ

¥aVWpqa "  9S>`6
. 

VW





/ῌ+  !"#$

5X,6/Y73U42Z&

',$J, (Fig. +) &9> ()67< + cm

 "   Fm&
 (Table +) Fmr/./ wt.ῌ r^ª}Q"}/"d   *%4)[ 3 FGHI%\]r% A&I"[}"~Z ^OP_89:`aR?) $ ij«¬!­® -¯H°\]b;67 < . cm b; + cC= < + cm =9>=9><=c

 ?  -./ῌ"῍-./῎/*°­jIb±²* 9C)

def> >&'#g_9:?@Ab;h0*9*

 #g_B09:+ : , ijk.lCDE-./῍ "&9>

Tomiya and Takahashi (+33/) Fmn.lDE>&'@Ab; Goa> ³micro-clots´s)dt5 *%4Cp9

q&DE 5)@+"HIr s4 r 9J`9 tr q&
DE 5&'Fmn.l0",ijB09:

 r

g_B

Fmn.lDE`ar

"9 ¡¡*89 ? 

) : Fm&}/*! *.,῍,.- wt.ῌ [}*! *.0 wt.ῌ 1 

5):);

<

+./mm, *.0mm µ¶«uµ¶ Fe-Ti gs +00-   Table +. Whole-rock and glass chemistry and modal compositions of phenocrystic
minerals of the +00- ejecta.

459

stuvw"xyz"{|}

460

  

Wo  Type A+ G Wo  Type A,   ῌ

Mg#'.+ῌ/2 ( Type A! "#$%&Mg#'1*ῌ1. ( Type B )*PQ R!

῍ = Type A, (ST&G (Fig.

0) UVWXY Type A+  !Z+, "-%&./+01234+%56[\%& Mg#'./ῌ/*Wo'+.*ῌ+., !]^78 (Fig. /ῌ+) Type A ( Mg#'..ῌ.0 &9:(

Z, _$`UVWX78 (Fig. 1) _ Type A, ;<= , ῍( ! Type B   Type A+  ab[\%&> cdUVWX78 (&NO*L & "( Wo ?!@A8

 Wo'+./ BCῌ῍!Zef+UVWX7g+ (&Sh(9:&D+E Type A  Wo ?(Fi>E./8  Type B jL( G (( , H(I4J!5K*L

7g+

(Fig. 0)F Wo  Mg#'./ῌ/* G Wo  Mg#'./ῌ// = , Mg# = M&NO*L 

L*Fm("#$%


klUVWX 

an!

!op*j ῌ= Mg#'1+ῌ

Fig. /ῌ+. Core composition of the dominant phenocrystic minerals of the +00- ejecta. Thick horizontal gray lines
indicate samples within the “micro-clots”. Data presented in this ﬁgure consist of multiple samples. WP :white pumice ; BP : banded pumice and GL : gray essential lithic fragment. Arrows show mineralcompositions represented by few numbers. An : +**qCa/(CarNarK) ; Mg# : +**qMg/(MgrFe).

+00- ((2. ῍ Mg#1,῍11 ῎ Mg#1+῍
2/  Mg-Fe  (Brey and Kohler, +33*) 

9:;<9=(NOPQL ]
MgῌMn+., ^_INM" (Fig. /῍,)# Type B  Type A+  Type A, 

!" (Fig. /῍,)# $% &'()*+,- .#Fe-Ti /01 : Fe-Ti /01(2

3 *.,῍,.- wt.ῌ461`a : `a*.- wt.ῌ bc BCDE *./mm (de.fKgVfK"# Leake (+312) (hij`a"#  ῌ 

Mg#0/῍03 nom klm

Mg#0*῍0/   Mg p 456 7861  9:;<9=qrUklmsXA'" (Fig. /῍,)# >61 ?$'"# @'A(BCDE 3 *.-῍)klmmm  CF.)(0-  ῌtu(vwUxA'"# GHIJ

KLM"# 456(NOPQL  ῌR

Mg#/2῍03 nomMg#0*῍῎  yz{= F  Z2 |?$'.U

῍R῎&') MgῌMn+.* ST(UCVJWX"

yz{= CRG U MgῌMnY-.* ()Z2[\M" (Fig. /῍+)#Mg#0/  Mg#// ^_J)#Mg#/*῍01 QL}U~U

Fig. /῍,. Core composition of the minor phenocrystic minerals of the +00- ejecta.
Fig. /῍+.

Explanation is the same as in

462







Fig. 0. Mg# vs. Wo Wo+**Ca/(CaMgFe) variation diagrams for orthopyroxene and An content vs.
FeO* (wt.ῌ) variation diagrams for plagioclase. Deﬁnition of the three types of phenocrysts is shown in thelowest diagrams.

 +00-  

 

463

Fig. 1. Core and rim compositions of orthopyroxene and plagioclase for the three phenocryst types in the +00ejecta. The region above the diagonal line in each ﬁgure represents reverse zoning, while the lower region
does normal zoning.

´.µ¶·47¸¹º47»1¼464

 :  *.0῍..- wt.ῌ   +./ mm  

  



!

"# Fe-Ti $%&'()*+

,-./01*+23453&678 9)

An:.*῍..  (Fig. /῍+) ;< => Type A ? 9) &*+()*+;& !b
Type A  (Mg/Mn:+.*)  2453

&6 Mg 6a|} (Mg/Mn:/./῍+2./ ; Fig. /῍+)

GJGm789) MgῌMn:+.*῍,.*& (Fig. /῍,) :  *.0 wt.ῌ

 *.. mm 

An:,0῍/2  @A6 An:1.῍3.  Type B w(xw(? Leake (+312)  #$1

B  7C9)6DEFGH/IJ9   %&N)  (Fig. /῍+) KL -./0῍ Type B

&'()*+;& 

78 9) Mg#:0-῍02  Mg#:0.῍00 6g/M ;N6O

& Type A PQ9)

An:.*῍., 6RS*+T? Type A UV An:./ W?M An X
N Y An Z(Fig. 1)

[N

Type B XN

jV -./0ῌ4῍23&Z (Fig. /῍,) :  *.3 wt.ῌ



 +.* mm &%&N /ῌ, 

}*+\2]  -.* wt.ῌ ^_? 23

()*+;& 78 9)53&`+ ##a (Table2 4 53&6 Type A  (An:-,῍/,) &b2453&  c

+)dGefgP.hij? -./0ῌ4῍24
53W?##kGjflGm6nop  

Type B  (An:1,῍3,)r : r *.1 wt.ῌ sr  r

tu6&*+ B 

 +.*mm, sr *.0 mm  v

B An:/,῍02

7C9)|} (Fig. /῍+)%&N
? ;

q

! Fe-Ti $%&''

./ &`+ FeO* (Fig. 0) W+ < An:0* ()N An
Mop

B



Type A & +!L An

w(x:-,῍./ Y FeO*  Type A+ An:./῍02 w(? Fe-Ti $%&'()*+;&M FeO*  Type A, & 2453

 r 23TWy53&&6 Type B 6 Type A+  (Mg#:.+῍/*) & Type B  (Mg#

(Fig. 1):03῍1/) 6 ! (Fig. /῍+)*+T? ZKLM Wo z{E  Type A, |}  23TWy

Type A, PQ9)

 XN (Fig. 1)

Mg#:/*῍0* 

An:.*῍./ 6RS[N*+

53&`+ Mg#:./῍0* &9)~ 6

XN

Type A+ PQ9)



An:.*῍0* & ~ @

&U{ (Fig. 0)

Mg#4Wo z6T+ 2453&6 TypeB N (Fig. 1)

 ,-./06T+PQ9)

Type A+ 

Mg#:./῍/*4Wo0 
+00- *+  SiO,:1-./῍10.+ wt.ῌ ¡G¢Gmx,-]? .26W()£ B:+.*῍+., 6RS*+T? Z [N Type (Fig. 2) -./0¤&6¥& -./0ῌ SiO,

A,  Type A+ &PQ9)VXN

:1/.*῍10.+ wt.ῌ ## SiO, 6n |}  @a N

:1-./῍1/.2 wt.ῌ &¦!6?



./0῎ SiO,:1/.*῍1/./ wt.ῌ &§y9) '7B-./0῍ SiO,
9)

~ -

sr78 9) Mg#:1,῍2+  Mg-Fe

 KLd/¨/©6T+ ,ª6& «¬­ (Brey and Kohler, +33*) 6 *+ Type B r

Z.®¯m°±/+ (Fig. 3)

&  (Fig. /῍,)KL 2453&6 N Fe-Ti $% :  *.0 wt.ῌ  !b

 GJGmtu6K  *.- mm  "&6#r&'

1
1ῌ+



< => +00- *+ 760  , G²
 (Type A4B)³|*+;&u +00- *+ +00-  

Fig. 2.

Fig. 3.

 

Temporal variations of SiO, content of whole-rock chemistry of the +00- ejecta.

SiO, variation diagrams for major elements of whole-rock chemistry of the +00- ejecta.

465µ¶·¸c ¹º»c K ¼

466

 , 

 Okumura *)Q Type A,  

et al., +32+ ;  +33/ ; Tomiya and Takahashi, +33/ A//m G 1"34/ 23

 Tomiya and Takahashi (+33/)  Us-b 

456Ql< Type A, 9/ /Q0  !"῍ #$(micro-clots)

% &'()*  +,-.

/ 01 2)34567 8+9>?

/ :;<* =(@ ;  A;< Wa*)(1"34/ Type A, bS() |,}~ m 
:X  MgῌMn/῍-*

MgῌMn 7

;< Tomiya and Takahashi (+33/)
8

I/
% &

micro-clots 0|,}~



m



@  B&C% & ;; MgῌMn+,῍,+ H 6a MgῌMn 7

ADE F2) (hybrid layer ; Bacon,I/% & bS() ! Type+320) 3 4  5 6 7

8+9/*A, /7hybrid layer <G0H +00- I!" ῌ 9567

!"῍: ; MgῌMn+* @#=(I$$ % &

7 I **  MgῌMn+* |,}~ m

J#K (Blake and Ivey, +320) ALM % Type A, < H* Ql<

&

N2)



1ῌ.

1ῌ,  Type A, TOP50  !"῍#&%'()     !"ῌ ῎QRS&T U  LM0 9!V 9W"@+1X Type A

B  , ,-./'()  *)(6@ +00- 
2( Type A /
Type A, /]

 T ^`%2)X=a
+cd( 6@e]XfghijkQ,l< 

!! #=¡X¢}£Q>¤ (Fig. 3) ** =( $ ?@[ , ¥#

1ῌ/

-

X

bS() 

+00- 5 , 

* =( §

- ,-.+567d(u6G 2)
(+) +

x0% &X+ +¢}£¨¡#;< bS() A

)(G % &A/&H ©

9!V 9/+=( - ,-.

Type B 9/+

1vwQl<0Q EPPL/m +nopQqr bS() stver. ,.* (Sugawara, ,**+) QBª'+uvwX bS()

 SiO,  /,.3

x0IdH%*}£#l2)+Qq % &S()  * Type A, /*+<*-! - ,-. +Ql<+ $B&6aF¦1H@ A)( )H b

1ῌ-

+

$YZ[\

8+/&'< bS() Type A, / Type A+ 567 Type B /

Type A, /m

* Q#(=Type A, / A+6@ W

OP5_2)

^

 % & B& 8X0Ql< r@ Type A,  !! #=¡¢ * #(=H
Type A+

Type A,  &X Type B 9!Vc9/Qyn 2(A9!V 1H
-9V0/ ;< x0.dHB

Xa*« % &

(Table ,) *0

Tomiya and Takahashi (+33/)  2) 0 (SiO,¬/.)%CH

! x0B&+ $

&X Type A+ 9!V5679QzX

D­/

Type A+ /H* QbE< /  A/ MgῌMn{,./ |,}~ mQ/

x0B&X®F (SiO,10 wt.ῌ) GH2) bS

y *~ m-k- 1vwH()   % & I@ EPPL ver. ,.* (Bacon and Hirschmann, +322) -k-0/(Sugawara, ,**+) QB [¯°+ 1X9y) OPT ^Type A,

9!V 9QzX/ < 9An 0 9!V Mg#

Wo 0QbS  # , 8X+567d(Qqr+Q±²0  Type A+ 9 ³´J<(Table ,)
Type A, +0 %@ <*& I<H +Q <¡¢ +00-  £¤¥ ¦§¨

467

Table ,. Comparison between the plagioclase compositions and magma temperature calculated
from EPPL ver. ,.* (Sugawara, ,**+) and those measured/estimated in this study.

 Type A, Type A,+,+**k m+Y j 13*k F (Table ,)

 Type A,   !"#  Fig. 3-$%&'() *+,+ . / 01 Type A, 2+3

Type A,  wx3

sRIHn@

QR1yQ zV{V&Y]4567(Holtz et al., ,**/ ; Rutherford et al., +32/ ; Scaillet and"#456789:;&"# <= 0 "#

Evans, +333) |n9 j 2.*῍3/*k bcR>?@"# A"#B C$%&'() *+ deY[E\m+Y} EPPL,-DE<F  EPPL ver. ,.* (Sugawara,ver. ,.* (Sugawara, ,**+) GHI Type A, 2+3"#,**+) GHIJK 0 LM Type A,  

:;&"# 4567ESTOP j 2/*῍!"#

SiO,N00῍1- wt.ῌ OPQR

33*k F~l9 (Table ,)

(,) S T1ῌ0 ῐ῎ῐ῔ΐῌ῏῍ῑῒ῎- UVW  ST  XYSTZ[E\]45^_LME`a E<bcRdeYF45F Type B 2fg3[E\Type A, C$%EI1yQ Type A,$%#+n@QRI Type A,{V $%yQs

h2g3 4i"#GHST (Wells, +311)

 j

[[E0

+,*1*k  lf m+YFn@+00-  2fg3[E\2+3{V

B "#678

QR45FoU(peq (Mg/MnN+.*) V;:r
V&GH

ST j 11*k  (Spencer and LM Type A,

Lindsley, +32+) s tuvHI EPPL ver. ,.* Type A+

GHI Type B  Type A+2+3"#:;&"#

4567ESTOP deYCj

89 Fig. +* 
Mg#  An {VA

j *.+῍*./ mm Aj *.+῍*.3 mmB Type B

Amicro-clots

 ; j *.+῍+.* mmB Qy SiO, N1*.0 wt.ῌzV{V&Y³´µ¶·v¸¹º·v» ¼

468

Fig. +*. Mg# of orthopyroxene and An content of plagioclase vs. crystal size diagrams for phenocrysts of three
types and in micro-clots. Note that Type A, phenocrysts are smaller than Type A+ and B ones.

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

  ..0῍3.*+*+* cmῌs ; Larsen, ,**/   !" #./. 01./." q EcdK#b7 RS*D./.

]^ hybrid.

Type A, $%layer '#XYU./.@qE& '#( +῍- )*&+ ,- ./.01

EH& (Tomiya and Takahashi, +33/)+ ? ijkl

2345678,9:&; (Nakagawa et al.,ῌ mQIft:& Type. A, ./.q,**, ; Venezky and Rutherford, +331) <=&>?

37 !qI@&+ ,#b* RS7@+

*D./.

AB?> +00- )3 , C./.?>*

B?>q [\ Bishop

Tu# : Hildreth, +313 ; Topopah Spring Tu# : Schuraytz et&D./. ?>3E*! 367(->

al., +323 ; Rattlesnake Tu# : Streck and Grunder, +331" E01./. FG - C./.?>*&D./E*@+ 9I +00- )367FGI@. ?>3EH&+ (I (D./.

./.  RS7_[>-I@&#b** 34)67JK-I@,7*&+

D./. E*?9_[>-&+

1ῌ1 

1ῌ2

D./.LM./. N7OPK-&+ (Snyder and Tait (+330)  XYU./.2Q RS7TU./.VW7XYU./.2

TU./.7r72 ¡ (QX

Z2

YU./.R2TU./.¢B£ ¤¥T [\ Bacon and Druitt, +322, Nairn et al., ,**."

+00-

(]^7 hybrid layer (Bacon, +320) J:&_[U./.01I 01./.2 ¦§7Z2`3a +00- )37'@I  (#b*L&,¨E©+ ,#b7 B7'@I>-&+

M2cdK-I@& (Tomiya and Takahashi, +33/)+,,E +00- )3./. LMefg:&Q7 hijklmQ3no7pbqr%sft:&+ Fig. ++ 7uijklqvu$wxy <  " zQ+ $wxyª*«¬LM©:./. ?>3:-\+00- )37'­&q./.r%

s

E&+
XYU./.7#9I ¡ ª7«¬D./. 

(-2 7®~*LMEH<2./.wxy <{|I@&}~:&-\ # ®~*DLM¯ s:&_[>-&+ 7~ Type. A, ./. 3i

7 XYU./.?>*! 4)°I3jklῌ '#

!q

_[>-& `aE ./. DLM2

,*῍-/ῌ" ijkl῎EI@&,2?&ijkl῍ E

36zE7 s:&±2H&+ (,E AVEª,*῍,/ῌ"+7«¬D./.  LMft:&+3  qijkl῍E Type. A,

Table - 7TU²./. XYU²./.':; +00- }:

;;;;
;;;;
;;;;

;;;;
;;;;

;;;;
;;;;

;;;;
;;;;

;;;
;;;
;;;
;;;;;;;;;;;;

;;;;
;;;;

;;;;
;;;;
;;;;
;;;;;;;;;;;;

;
;

469

;;;;;
;;;;;
;;;;;
;;;
;;;
;;;

;;;;;;;;;;;;

Fig. ++. Summary of the +00- eruption stages with corresponding phenocryst type ratios. Note that Type A,
phenocrysts are common in gray essential lithic fragments (GL) of Stage I and white and banded pumices(WBP) of Stage II.

Table -.

Estimated physical properties of the felsic, mixed and maﬁc magmas.

 Type A,   

OPR5S.TAB" OP Type A,   Type A, ABUVWXY0Z[\SN!"#$%&' (Holtz et al., ,**/ ; Rutherford

 (Fig. +,) Z]^ OP Typeet al., +32/ ; Scaillet and Evans, +333, )* +33/+

A, _!`a[ SiO, "

./0

,-

1213 ../῍0./ wt.ῌ 3 .῍0 wt.ῌ -4

510 (Table ,) 678 9:/῍+* wt.

ῌ b[Y0cd Type A, d[3 2*῍,**eTf HIMgghiDj"Y0;78 ,-./0 3 ,῍. wt.ῌZZS5 k.NElm/0.

<=>10 (Medicine Lake ; Eichelberger, +32* ; Kinzler

78[noEpXnq (Type A, et al., ,***)

+ ABr.stED0j ?@AB Lange and Carmichael (+33*)

CDE

FG >5.HIJ AB (Smyth and McCormick,+33/)

CDE AB KLMN
121 ,.,1῍,.-,Q+*- kg ῌm  ,./*῍,./1Q+* kgῌm  ,.,3῍,.-0Q+* kgῌm 78
-

-

-

-

}:6"~ 6660 [Y0d HI./0 .

OP 78OP
Type A, AB

uvlw.xyMABr.z{=.i5|-d.DE ῌ Type A, HIRi


>iDM[D (Fig. 1) Z1 nq.0nq£¤¥¦§o¨©ª«o¨¬D­

470

Fig. +,. Estimated temperature vs. density diagram of the end-member magmas. Gray area shows the density
range of the felsic magma.



 " IJ%&'&

(G\(

v" IJ% DO0  +00- !" #$%&'&()

+00- (d)opqrs῎ : v{G" &*+&'&,-." /0123 &'&4'&4\(#$%&'&

,-

 

v

2 

3

(567189" +00-  (&'&:;<DE F +00- v^ "  (wy w
ym YZ" G( =>?@. (Fig. +-)(a)+00- ! AB!C : DE FGH%#$%&'&IJ%&'&K
 

,-

#$%&'&" IJ%&'&" P(*&'&4

(B!IJ%&'&+&'& v #$%&'&4LM" 0N&'&4O,) #$%&'&IJ%&'&(*+ i

0  P(Q" LM IJ%&'&(R(ST (B!" 0N&'&4O0

#$%&'&UVWX  P(YZ" #$%&'&IJ%&'&([\]" *+-) *+&'& (wyv ^^&'&4(_`a  P*+&" P(0N&'&4#$%&'&'&(G)[b1

IJ%&'&(m i" &'&4(w

0N&'&,-.

c" /0123 0N&'&4O0 #$%&'&&'&4)(*+&'&(d(c))*+&'&,-.)

+00-  " yP(0N&'&4(\i" P(0N&'&4Bjk

v"  wxy&'&4(G\(opqrsῌ : 0N&'&4(/) /01 20N&'&4" LM IJ%&

\ *+&'&tu#$%&'&v '&(R1ST#$%&'&RUVW

efgh
lmn
(b)



+) v - (" P+00- 
+00- 

v{(

v



opqrs῍ : (wxymz" 0N&'&4(|\}v  (~"  ^" &'&4



X" IJ%&'& ¡(#$%&'&*+
].

O0

 &'&4\¢*+&'&" G) +00-  

 

471

Fig. +-. Magma plumbing system model for the +00- eruption. (a) Before the +00- eruption (several years ago),(b) Stage I : initial stage, (c) Stage II : plinian eruption stage and (d) Stage III : base surge stage.

M[[\ z472 

  !
"#$%&'( )*+,

-./012' +00- 345&67809:; 8)

<=>?@ A %   BC8D # E
F# $'(G45H6 DGI 45J36 $KL %῏῎MNO>PQ$  RST8U DVWXYZ [ \]^YZ _-`/78 [ ab`/7
8[5cdef78LgU DhijYZk?
l m %   nopqrst^uv wxy
zuvU'{| }~G

>JIA MDAI# % &I;%ῌ ῑ ῐ ῍
Bacon, C. R. (+320) Magmatic inclusions in silicic andintermediate volcanic rock. J. Geophys. Res., 3+, B0, 0*3+ῌ
0++,.
Bacon, C. R. and Druitt, T. H. (+322) Compositional evolution of the zoned calcalkaline magma chamber ofMount Mazama, Crater Lake, Oregon. Contrib. Mineral.
Petrol., 32, ,,.ῌ,/0.
Bacon, C. R. and Hirschmann, M. M. (+322) Mg/Mn
partitioning as a test for equilibrium between coexisting
Fe-Ti oxides. Amer. Mineral., 1-, /1ῌ0+.
Blake, S. and Ivey, G. N. (+320) Magma-mixing and dynamics of withdrawal from stratiﬁed reservoirs. J.
Volcanol. Geotherm. Res., ,1, +/-ῌ+12.
Brey, G. P. and Kohler, T. (+33*) Geothermobarometry infour-phase lherzolites II. New thermobarometers, andpractical assessment of existing thermobarometers. J.
Petrol., -+, +-/-ῌ+-12.
Eichelberger, J. C. (+32*) Vesiculation of maﬁc magma
during replenishment of silicic magma reservoirs. Nature,,22, ..0ῌ./*.
Feebrey, C. A. (+33/) Petrology and geochemistry of theToya ignimbrites and postcaldera volcanism, Toya caldera, SW Hokkaido, JapanImplications for island arc
caldera evolution. Ph-D thesis, Hokkaido Univ., ,-1 p.
Hildreth, W. (+313) The Bishop Tu# : evidence for theorigin of compositional zonation in silicic magma chambers. Geol. Soc. Amer. Spec. Pap., +2*, .-ῌ1/.
Holtz, F., Sato, H., Lewis, J., Behrens, H. and Nakada, S.
(,**/) Experimental petrology of the +33+ῌ+33/ Unzen
dacite, Japan. Part I : phase relations, phase compositionand pre-eruptive conditions. J. Petrol., .0, -+3ῌ--1.
Kinzler, R. J., Donnelly-Nolan, J. M. and Grove, T. L.
(,***) Late Holocene hydrous maﬁc magmatism at the

Paint Pot Crater and Callahan ﬂows, Medicine Lake
Volcano, N. California and the inﬂuence of H,O in thegeneration of silicic magmas. Contrib. Mineral. Petrol.,+-2, +ῌ+0.
Lange, R. A. and Carmichael I. S. E. (+33*) Thermodynamic properties of silicate liquids with emphasis ofdensity, thermal expansion and compressibility. Review in
Mineral., ,. (Modern methods of Igneous Petrology :Understanding Magmatic Processes), ,/ῌ0-.
Larsen, J. (,**/) Experimental study of plagioclase rim
growth around anorthite seed crystals in rhyodacitic
melt. Amer. Mineral., 3*, .+1ῌ.,1.
Leake, B. E. (+312) Nomenclature of amphiboles. Mineral.
Magazine, .,, /--ῌ/0-.
Nairn, I. A., Shane, P. R., Cole, J. W., Leonard, G. J., Self,
S. and Pearson, N. (,**.) Rhyolite magma processes ofthe AD +-+/ Kaharoa eruption episode, Tarawera
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