῎῏  /.  (,**3)
 +  +/῍-0 ῑ ῐ +  . 
L M N OKP Q

 
RKS

T UKV W X Y

,**2 F 0 G +- HIJ ,**3 F + G +- HI*Tephrostratigraphy and Eruption Style of Mashu Volcano,During the Last +.,*** years, Eastern Hokkaido, Japan
Hiroshi KISHIMOTOῌ῍, Takeshi HASEGAWAῌ, Mitsuhiro NAKAGAWAῌ and Keiji WADAῌῌThe eruptive history of Mashu volcano, located at the southeastern rim of Kutcharo caldera in eastern
Hokkaido, Japan, has been divided into three stages : stratovolcano building, caldera-forming and central conebuilding. Tephrostratigraphy and eruption styles of the latter two stages were reinvestigated in conjunction withpetrological analysis. We found new evidence of several eruptions from the volcano. These data, combined withrecent +.C ages and the presence of wide-spread tephras, allow us to evaluate the temporal evolution of eruptive
activity and styles of Mashu volcano during the last +.,*** years.
After the formation of the stratovolcano, activity of the caldera forming stage started with plinian eruption(Ma-l) about +. cal ka. Approximately 1./ cal ka, climactic caldera-forming activity began with a phreatomagmatic
eruption (Ma-j) followed by plinian falls (Ma-ig), and a catastrophic pyroclastic ﬂow (Ma-f) occurred resulting
to the formation of the summit caldera, 1.//./ km in diameter. Total volume of the climactic eruption deposit
is estimated to be +2.0 km-. Temporal variation of pumice/lithic fragment and white (silicic)/gray (maﬁc)pumice ratio with eruption sequence suggest withdrawal of a zoned magma chamber with more silicic magma
overlying more maﬁc one through the newly opened and enlarging vent. In the central cone building stage, atleast eight eruptions (Ma-e, Ma-e῎, Ma-d, Ma-c.-c+ and Ma-b, in ascending order) have occurred repeatedly
during the last 0,*** years. The latest eruption (Ma-b : ca. *.3 cal ka) was the largest one in this stage.
Although most of the tephra layers in this stage are composed of pyroclastic fall deposits, thin pyroclastic ﬂow
deposits can also be recognized from the two eruptions (Ma-e and -d).
Juvenile materials in the Mashu tephras are commonly pyroxene dacite (SiO,0.-1, wt.ῌ, K,O*./῍*.1 wt.
ῌ in whole-rock compositions). The pumice of the caldera-forming stage are nearly aphyric (+0 wt.ῌ),
whereas those of the central cone building stage are more porphyritic (+-,. wtῌ). They can be also
distinguished in SiO,-oxides diagrams.
Most of Mashu tephras consist of gray ﬁne ash layers including blocky lithic/pumice fragments andaccretionary lapilli, indicative of magma-water interactions. In the case of large eruptions, such as the climactic
caldera-forming eruption (Ma-jf) and Ma-b, eruptive styles changed from the wet to dry due to increase of themagma/water ratio. The long-term magma discharge rate was *.2 and *.- km- DRE/ky during the last +.,***
and 0,*** years, respectively. Mashu volcano can be interpreted as one of the most productive and active
volcanoes in Japan during Holocene.
Key words : Mashu volcano, tephrostratigraphy, zoned magma chamber, magma water interaction, active volcano*0*῍*2+*   +* !" 2 #$%&'('()*(+,)-./0(12Department of Natural History Sciences, Graduate
School of Science, Hokkaido University, N+* W2,
Kita-ku, Sapporo *0*῍*2+*, Japan *1*῍20,+ @  2A 3%&BC'(@ D=(+,EEarth Science Laboratory, Hokkaido University of
Education at Asahikawa. Hokumon-cho 3, Asahikawa*1*῍20,+, Japan.

 : ,+/῍***.
  +῍,῍,34356789:;<=>1;<=>?
Department of Disaster Prevention, Asia Air Survey
Company with Limited Liability, ,῍,, Manpukuji-+,
Asao-ku, Kawasaki ,+/῍***., Japan
Corresponding author : Takeshi Hasegawa
e-mail : hasegawatakeshi@mail.sci.hokudai.ac.jp

OP)Q R)QST)U

16

Fig. +. Index map of the studied area. Points and numbers indicate the localities of columnar sections andsamples. Lakes are shown by shaded pattern. Double circles indicate towns. Solid triangles show
summits of volcanoes. Contour interval is +** m (dashed line indicates /* m contour above sea level).
Longitude and latitude (WGS) of type localities are as follows ; Loc.+ : .-*-/ῌ1+N, +..*-1ῌ/.+W ; Loc. - :.-*--ῌ/0+N, +..* -2ῌ/,+W ; Loc. ,/ : .-*-,ῌ**+N, +..*/+ῌ,0+W ; Loc. .0 : .-*-2ῌ+-+N, +..*-*ῌ+1+W.

VW+    a'Eb- cd. e

- 
8B!"0fg(op)- fg'8B (,0,* km) C;)

XBY0Z[\;V56&]^, _0`h- i5 #j$%k&l- m%n/0
012- 3 -456 %$, #7%08

qri0 +.C *s - G+ ,t0<u" (Fig. +), vw (+3//, +3/2, +30+, +30,)-x<y-5z{8q5"|}..j3$ 9 ~

Katsui et al. (+31/) E-/ 0"HIBW [+1$%- a9 :;0< =>", ?@- +.C A>

0C; %$sE- i320 (cal ka) 

E3 + 4 1 

5>", 2E IntCal*..+. (Reimer, et al.,- _00675  ^ a8,**.) - 2 BE Calib/.+* (Stuiver et

B (1.//./ km) C;, 8BC;&D

al., ,**/) 8!,, #77- a8B"<  9- ri0E3 !", a<E- :|" ;E5!- ,**- 

( r6

, 

a

 
E- fg'"- fg'<#j M -.2  !"#$%& '(-$ vw) - +30- z= C;m7- z>,**/-  0¡¢?m7", $5%- v@£3 ,* km 0;

w%0¤¥?¦- §>"¨©5(N[+[+5¤¥E«Bm7 %¬- C4W

±0E²J! EF³ 0G!,_#<·/E- a

0 IJ0¡1- n/

0 12³ >"¸- <u" vw- +3//,A

Ea8Bu- _0"Eª ­® D¯ 2/2 m - 8BEª°
H ´A®µ¶>",Katsui et al. (+31/) Ea0- ;C

4K+5^;G- 8BC;G-  ­®C;G¹- º

8BC;G?¦LM (N[+) [+¤¥) (,**1) E- F , G0HIBJ! - KLMN

]^, #7»<]¼7 !5$^3$0½;$

VN AMS N " +.C  = (Fig. ,),MN +  . *$O$<=>P Q

17

?    (+30-) RS     (+31+) ) .T
Ma-c  UV

. W$"!,- "#,- Ma-b $3)

+ ka (*.3 cal ka)

$XYZ$%Y &I + km ?['. (Katsui et al., +31/)

 Katsui et al. (+31/)
 , () Ma-e +

Ma-b $* +,- . /0($+12)

34  5- 6



Ma-a

\]. -%) ^!(_T`AO:;3)a bc)*+$78 4 d789:;$<=>34  ?@ . AB

CD +30+-    -ῌ+ 

,ef$-g) Katsui et al. (+31/)hi>['2jk$XYZ['2$<=>34  .l/m0n)O:;$<=>?12a!(V O hi>N3 op4q34  ,ef3) 5r5Q6
783<=>$\]stuB v93w<=>$(x6:; T_ ,- y,A5Q78_T<PR(=jk( z{| $78 BA>?@u}- ~ $A!) Wentworth (+3,,)6}- ,B3) " (. mm)
  !C|B2

!"#$% &' ~ $D) E^F? ,_GB ?4  R YNH +* km 9 $^!( IJ3|_}- _T $ K
3# L6
Fig. ,. Schematic column and representative radiocarbon
dates of Mashu Tephras and Lavas after Katsui etal. (+31/, +320). * : New radiocarbon dates by Itohet al. (,**1).

D  

?3|

Hayakawa(+32/) $QMB- y$) 784N1?M#

2a @6OPQ$BE^FsM,- ~R
 $D)!(S T^UA-  O:;$<=>   O<=> $V
) WaX4q$<=>ef BAM. AB

 T  +  1 Y$'['23) ,A[( ($(~ ?+, $1$

¥¦§)\¨_T Ma-a  xhi>['23)iL¬­¬

.



]"6

 _T ,ef3# T$VZV  5-,B3) ¡W$  ¢£¤L
¢£¤L©'Vw=ªH$

&k
«¢

£¤L &' ®!^, $12$<=>) ¨_T +, ka +.

-ῌ,

cal ka $ Ma-l,  +* ka +, cal ka $ Ma-k, y,A, $_¯`) ab°c.-de3

0.0 ka 1./ cal ka $ Ma-jf 34  $±$ Ma-j

f~, ghf ) * ³5B¡j$ 34 ´>¥ k' ) l µ mC x n x o $ SEM-EDS /¥<

$3# Ma-f )MC¶5$~R34! ,O3(JEOL-JMS-T--*LINK ISIS) 3,- !pK)·aq y$¸$$XYZ['23#  ,AiE F +/ kv GHI / mm 1* +2* sL¬­¬¹HL ,.* kcps 3 stº» ) ZAF uMB- + 

~?r|}A_¯² !i,A- 

JKL$XYZ?['. - CD (+30,) ) $12$<$¢£¤L=>

½xk'!p ,A) I - cm \$_T) +¨_T Ma-e  Ma-b $ . 

 ! ,AB

| +* ¼KL\,- v(

* +

18,-./

Fig. -. Correlation diagrams between loss on ignition vs representative oxides (Al,O- and K,O). Regression
lines are also shown with each unit. Coe$cient of determination : (Al,O-) Ma-l : *.22, Ma-i : *.0-, Ma-h :*.03, Ma-g : *.12, Ma-d : *.-+ ; (K,O) Ma-l : *.2*, Ma-i : *.+1, Ma-h : *.//, Ma-g : *.0,, Ma-d : *.+3. The
errors (,s) are in the range of all plots.

0)123 4 / mm 56 - cm 5)&'()78"9:;%

1"9:!<0= + 9)1%

>? &'("@AB! >CDE!F: + : , "GHI(JKLM2N 5)OPQ!RSTBRB

3  U

\>R $ ] %T;)RB :^! V  V " ) XRF JWXYZJ[ #Magix Pro Q&'% _`) Ma-l )1! _;(ab"cdefg%BhBij )k:l*m+S $" l*m+",n Al,O-" B iopf!q)-r::

K,O ).m+_;st/Q 9u)v'8< 07.ῌ+ 

8<)1w _!Qxg% (Fig. -) %g Ma-lyz{(|3}2~3"9: 4  Fig. 0)1! 5b)6e;$"!:" =3)78 Fig. 1 "ST Katsui et al.

 u)9 :;<=% I(J>3"(+31/) )yz{(|3}!` Ma-l

9:! "?>": uq@AB$ Ma-l, Ma-k ?j Ma-j )"!u

Ma-f ) 1 ":;%<ST

CA qqQ Ma-j Ma-f ! + )2
DE3Qxq -2~+ Ma-l, k e; + 5. 6!yz{(|3"7g%F>G

"4 Fig. . "ST HI)JKq AN  yz{(|32  ! Fig. / " #$%&'()! Table +A%!2~3  io"AN% %>3 Ma-e  Ma-d )".ῌ+ῌ+  (Ma-eῌ) 

! LM M9)¡N¢":O   l (Ma-l)   (+30,) 

A%:9)£P)&'("?: Q¤T

TBR u)6`"=SDE3

=SDE3~ %>¥¦§¨©Y:ªgTT `)&'( Ma-l, 6`)=SDE% $"«¬9)&'()78 23

3 Ma-l+ ¥¦§¨©Y:T !U)% q)n"=Q! ­V#$>g%§¨©

HIQ $< cm )®RC¯3|3}

Y:°±: 0$STqQ :9)

(Katsui et al., +31/) )² b&'(Wn Ma-l, )³'´% XQ! 2YZ)$>¡:Oµ![\[]¶Q Loc. ,/ Q! +// cm Qx

&'(;P·$?e ^¸ _¹ +323  u º`""!»¼¥D½Q , - cm )R)* +  . +,-./012 3.4

19

5678 96 9:6 9:6 ;<  8 Ma-l+ =7  >?@A@>?B #C#C-DE FG
HGI"& "J2K@L =7MNOPQ 567R S Ma-l, T   0.0 km- UV.ῌ+ῌ, k (Ma-k)  (+30,)

 -./ ! .* km W"XEYV

Z#[HG\./]  ^_E`]$  +/ cm    ! Ma-l
%a Loc. ,*  ,, cm  (& `] b&Ecdef]gEhi j'k  EPldE]g./]@mn?L
 op 6"& =7 6"&]:6FGHGI@q@ (&E r/&st<"&@ &E (&@L ]FG

HGI  uvwJ2K"7" x)w*+EXW  7 96:6  8 6y z
G{|}1~0~zG{|0~M
 =7 >?@A@>?B #C#C-DE FGHGI"& "J2K@L RMNOPQ 96 9:6 9:6;<  8 g56 "fA E,-@ .#$/0d  y1&92563YVE4
Fig. .. The schematic tephrostratigraphy above Ma-lin the studied area.

)(&E ] @ ,

L, cm );#

 W$(56d wq_&.ῌ+ῌ-   ! Ma-k bE5#

/ E6  Ma-jf :  +30, 

2 cm  bZ#¡ <¡"fsB b&¢£gE (56 6"&

 ]:6FGHGIr/&@q_@ ^_@§gE"#$%&&'(  "& ¨ 7w+ n#wA T ,E  - cm #[HG\8./
 56 ^f856 :#*..0 km- 

YV 
d]@#C#CL   +2.0 km- B (. f !c69qj (Ma-j)  (+30,)

Ma-j

-./ !¤w¥E69

Z#[¦=HG\`](./] +/ cm 

+2 cm  #! Ma-k %a Loc. ,*>¡Et +*© W  Loc. +2  ,*   Ma-l, ?bE:ª@A@«¬ (Fig. 2) (&E#®./C6@L(Ma-l+) @n B/ mm '(]./]¯°± hiDB./] w£ n#wA 

6"& =7st<856

 6"&]:6FGHGI ^_E§



 

#



+ mm ­

6"&  =7st<856"./]d op  56"./]  &Z#w&6E L

N

 ;&6 6"& =7d w 56"./

W$ 8²<"& uvw(& y"]·E¸@¹ 566"& @ºF fsBl ^_E-.@¹A 7@L(&E

RMNOPQ 96 9:6 9:6;< Ma-l++ cm '(G¡H@Ind .3YVJK¬ Ma-l EW

K³´µ

8 6"&*./ mm 'b-.¶

L»O@B1¼½E*

fs

#$%&'( )*(+,-.

20

Table +. Summary of eruptive units. Facies ; Pfa : pumice fall deposit, Afa : Ash fall deposit, Pﬂ :pumice ﬂow deposit, Aﬂ : ash ﬂow deposit. Types of lithic fragments ; Gd : gray-colored dacite, Ba :blackish andesite, Al : altered rock, Ob : obsidian. Types of pumice ; Wp : white pumice, Bp : banded
pumice, Gp : gray pumice. Mineral assemblages of isolated crystals are shown in parenthesesῌ.
Grain-size divisions are those of Wentworth (+3,,) ; m. s. : medium sand, c. s. : coarse sand.

/ 012 34546 7189: ;1<1=>?@ABC Loc. ,* !DEFG -- cm 

HI: JK?C HI:L!MN!OP 013

MQ

4546UV>!W;101

hijak

XY4Z[\]^_`

lmno OP pnoq/RS" +./ cm T>!

aNUV>!bc Ma-i ;1ttuv!34546I:wIxyd J

Bz {|" *./ mm W }P

K?C iU Ma-i 

I90m

 F  *.1 km- Ma-i  i (Ma-i) 

K



(+30,)

N! ,- 
I:w!¡01a¢ Ma-j ¥¦  Loc. 0 !DPFG +,* cm  909

I:w~ !bCm 4!U?V&  2  2  

90F 9 abc >?  !;1<;
1

defg

! Wr s

FGB>? , cm +*   +.* km- g (Ma-g) (+30,)

Ma-g   Ma-h /q!a£¤F 01qbC ;1 /

bCP

I:w~ Ma-h  Ma-h  !">?|§z ="¨1©ªP!« Ma-h @

C?¬­MQak MN" -* cm

BC  ¥¦®¯° Loc. -+ FGa± ²N ³1´ 1/ cm  MQ 01cm  w~defg ;1

qJK?C

 Ma-j 

I:

!bC

cm  Ma-h RS" ,/qB?!W }P F

m &  2  2  ´µ!«³<1a¢¶ OP   ..* km  ·¸3yo¹

bCm º5»¼½`6*{|

2  2  

-


Ma-h h (Ma-h) (+30,)

 Ma-i 

km- !

~ !?V&     -.,,-. + / . 0123456789:;<5=>21

Fig. /. Selected columnar sections in the studied area. Locality of each section is shown in Fig. +. Ma-b
Ma-l are abbreviated as “bl”. Ap-pﬂ : Atosanupuri pyroclastic ﬂow deposit (Katsui et al, +320), Rs-pﬂ :Rishiri pyroclastic ﬂow deposit (Ui et al., ,**+).

f (Ma-f) (+3/2)

Ma-f  Ma-g GHIJKL5MNOPQ%

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

? @A# Loc. 3 :B +/ m C DEF
RSTGUV WX

YZ[\

56]

9^(X_T`aLbTc  '(@(de (+3/-)  fgG Ma-f- hgG Ma-f+ )

'(%& iocpg2qcrstuvwx#'(XYyR)z '({|}~# '(:G Ma-f , )  Ma-f , G+  Ma-f-  Ma-f+  !"#Ma-f-  3494NlBOP) sms9 G$ . cm %&
cr

j

c kl'(mn$ ,* cm '(&)*)*+

'(w  Ma-g { :, .*ῌ-(X_TGUq rc ¡(¢6_ £X_¤a¥_c  (X_T

22

2

3

4 5 46789:* 2 cm ;)*9 +./ cm 

Ma-f+ * *<=>?@ *A@ B*C

DEF@GH .  I

Ma-f- JKL :M  f- N!

 "OP #QRS$TUVWXYZ[\]*%^Q_ `aTb& Loc. - 03 Qc * Ma-f- d'L
Ma-f+  Ma-g 

(Fig. /) Ma-f+*eEFfghQ ()i_!  ?* +j,(klmM-.  !n* Loc. 3  Loc.
03 "#$%&' +/ km  0* km ,* cm  +/ cm

 &' Ma-f-  !

n,jo/_ gh9:* Loc. 3
03

+ cm

  Ma-f-/ cm Loc.

ghh()

.

LI peqrstu%v(EFf /w &0
12* Ma-f- 34   x5 Ey6K7z{/_  |}0 +320 ; Qc ~ (,**-) *ghEFf89L

!K

:

 Ma-f g*LI;.

<=h  >7* ghs A h
A&h ?Ah @A7   B*|}0 (+320)

/_Ma-f-  f+ C

$KD_ 3.1 km- _EF.ῌ+ῌ.  Ma-jf )0_CG;. ^7s,jCL Loc. ,/rXYZ[_

: ,.2  ,,.. mm H /** +*** g L Ma-i, h, g

* ?!0 Ie ¡H¢J £¤¥¦/§ K¨
ghh() l / gh*s
LL &'©ῌ ªL (Fig. 3)
91 Ma-j * I ( 32ῌ) h()  Q «I p- gh(EFf*.MM  Ma-i gh*MMgh¬  9I &* 0.ῌ   0I ­JMMgh®¯L h()*+ ,-.  / 01 Ma-h*&I MMgh +.ῌ°,L 0I *MMghQ_ N± MMfM²³ghO´L fMghfMµfM²³gh¶·.
 h()¸  Ma-h*  Ma-i

90I (-ῌ) ¶TTIQ 0I ­J®¯.
Fig. 0. The schematic column of pyroclastic deposits
during the caldera-forming stage. The horizontal
extents of beds are proportional to grain size.

(0+-ῌ) ¹ Ma-g

* 1 Ma-hO´LMMfM²³ghPIQ fMghQº^  h()* Ma-h 90ITT-Q_ Ma-f-* fMgh .3ῌ9¶_ fµfM²³gh¶»GI (+,ῌ) MMgh¶p- (,ῌ) ¼½.  &L h()* +  .  ! "#$ %&'( #)Fig. 1.

Isopach maps of pyroclastic fall deposits during the caldera-forming stage.

Ma-g *+,-./01 

23 45=>#?@ABCD +6E.

)FG H INO9 Fig. +* + P#QRST+UV

WX#Y239 Fig. ++ +_1.ῌ,ῌ+ 

WX#Y
Yyz



78L45+ +ZE.E[[\ ]K^  Ma-e+ 

op+ 1  qjbrLstu vwLxE yz{+|}(Fig. +,-a)

9u. Ma-f 9< 

9u[M

JK L (Fig. ++) M Ma-ee,e,, 6 789:6;<+

eghi #$jk^G L + cm l mn[n.[L   * +#Ma-e- e, e+ Ma-e-(+30,)

j~ feghi #$j+ +/ cm ,* cm `N ab cdef

e (Ma-e) "#$" +*,* km

Thickness is centimeters.

  +* cm -/ cm .ῌ, 

X

23

Gk^ 9Xk^

- 6+gQRST 1 Ma-e-  qjfeghi  ¤B1 W [^.EGEL ab #$'   rbL
 #Yyz 

M 91

L|E

Ma-e+  *., mm  

9 ¡ Ma-e+e- ¢£u.j ¥hghT ¦$r§¨~ 

X#$j #$©9¡ ª«h¬­®=>br9¯ Ma-e+ °±F+r9¡ ¥hgh#?@45

T²³ Ma-e, +´E µA¶µA ·cm 

Loc. -  +* cmLoc. /*  /

 Ma-e , Gfeghi

WX#$j jj~9»u  Loc. -Loc. /*

Ma-e+e- +¢£u.\^  ¸¯¹º ¼

 Ma-e, e-½D *.,1 km-

24  !!"#$%&'() *+ ,-./012345/6 78-)9:;)<= >?@ABC 1234DE FG
2H&I

JKL78-)MN=O P*.*+ km- )QR=O.ῌ,ῌ- d (Ma-d)  LSH / cm

TUV(+30,)

Ma-eῌ

W2XY (Fig. +,-b) >?@ABC 1234
H !Z[\ ]" ^/ _H&I L
S<= Ma-d , ) -d +,\`abcdeH

f=O

J g# +3/- hiJH Ma-d+SH , 2X$);OHjV:KL2klQPJ

mn Ma-d  LS<= Ma-d, Ma-d+ Ma-d*-fl,   Ma-d*-fa  /\

`abcdeHo

Ma-d*-fl+  J Ma-d*-fl, p' Ma-d*-

fa  Loc. + Y Ma-d *-fl +  Loc. + p' Loc. - YQPO LHq: Ma-d
O rI Loc. +#$ %&'(0)q

R Y

Ma-d , slU  ltU*:+,u  o-`@v KL2klY ,wxU. y/zH/ J{|0 1<= } +- cm Y ^/23
0lU ~g@`@e)4tU 5"!Y lU@`@e6,-."Y 789HO
J8 Y O:  g~g%&';<)YI g@87d
@C7dq: J Ma-d , H

2k=g0O Ma-d+ xx>5"
Fig. 2. Field occurrences of eruptive deposits during
the main caldera-forming stage. Overview (a),and close-up of Ma-j and the lower part of Ma-i(b) at Loc. ,/.

4

tU?'@U KLgY } +/ cm g/,w , cm Y+,: TU AB

k!C 2w + cm L3YD
Ma-d*-fl,   Ma-d+ Ma-d+ d, &IR}502 , cm2 +* cm

HE YFG0 2kl" H@UY.ῌ,ῌ,  eῌ (Ma-eῌ) g0V: g H¡f /,w + cm Y

 Loc. + p' Loc. - H%

 g"!CHTUk!)5"!0DOQPOJslU

ltU 2klY (Fig. +,-a, b) LSH}
- cm

TUj¢Y Ma-e / cm

TU  j¢Y *+Ma-d*-fa  !0 Ma-d+ H9:0 &Io,K

SH}

LgY } - cm. g@UY/,w / mm

Ma-d H  £OY Ma-d*-fl+  Ma-d*-fl, H!09:0

Loc. +, Loc. - )RH} / cm Y ¤¥e¦§d@Ug&I+, /,w - cm Y¥¨: FG0& +,uo-`@v ©ª«J } ,* cm Y LS

g"!C®5J®5 KLm¯M):0 °NY L3Ho,g"!C0O±: Ma-d

2w + cm*-fl, %&' fl+ 0²H³´Y«Jµ¶P=O@UgRO g Ma-e+&I:0xx2:RD0

O

 QR

S·Y g"!C£<H5"¡fJlU ~g@`@e023 U¹ @UtU 5"!RO KLm »¼£½R Ma-e )STY

+,:Ma-d*-fa ) ¬­I

0 "3 2k,-.mC)Og!"0¸TY;)<= 2X$Y
)MN=O HOg :ºVg   ~g W ~g p' ;<)   Y+.



25

Fig. 3. Stratigraphic variations of rock-type proportions (left), and whole-rock SiO, contents for juvenile
materials (right) during the caldera-forming stage. The geological column is that of Fig. /.

Ma-d+ d,  ! *.-, km- "#$.ῌ,ῌ. c (Ma-c) (+30,) 
Ma-c ! %&'()"! + *+,-./012.3$4 56  7 (+30-) 89  :  7(+31+) "! . ;<=> (Km-bKm-e) ?@A1

2.3$ B()"C Ma-d  Ma-b ? . *+,4DE'2F (Fig. +,-c) G2' . ?
! HIJK4LMN$GO' P+"! Ma-c Q .
;<=>-./RN$ +SO' Ma-c. Ma-c
- Ma-c, Ma-c+ 3T2C@UVWXP+YZ[\]^_`abcO'd$ Ma-c.c+ ?! ef-.,IgWVWhZ[\4ij2$4 G2'!kldm
nZ"#o pqdrs?tj2$ uc! vw
Y  xY]^_yz{|>"#o Y}!~W
Fig. +*. Detailed information of type localities (Loc.
+, - and .0) for eruptive units during the central
cone building stage.

~W QN jF`abc! 434C#o --
?!gWVWhZ[\Zmn4DE'2$ !ef-.vwY vxY vxY

 

26

Fig. ++.

 

Isopach maps of pyroclastic fall deposits of the central cone building stage.


.ῌ,ῌ/ c. (Ma-c.)  (+30-) (+31+)

!"# $%&'()*+*, -%./0 1%2%&34056-70 -%./089 $%)*+*,:9# ;<=>? @AB

CDE*FGHIJK +/ km, L#MNO#PQ*.,+ km- 

R S$%$2%&TUVW+*X&Y8Z/.ῌ,ῌ0 $C 8[#C\56] - cm &-%^_`abThickness is centimeters.

c- (Ma-c-)  (+30-) (+31+)

 Ma-d `cd Loc. + #5eC\ .* cm  89

CDE*FGHIJK ,* km #PQR S$%#f!ghi<=>?Z/j(klm? (40n

o$%&TUpqTU+*X&Y8Z/$C +  .   

Fig. +,. Field occurrences of eruptive units of the central cone building stage. Ma-e+e- and overlying
Ma-eῌ, separated by thin humic soil at Loc. - (a). Ma-d consists of alternate of pyroclastic fall and ﬂow
deposits, at Loc. + (b). Ma-c+c., in descending order, each separated by paleosols at Loc. + (c).
Ma-b is composed of three pumice fall and intervening two ash fall layers at Loc. .0 (d).

27

/ 01 21345

28

Table ,. Summary of widespread tephras in this studied
area. * : Soya (+31,) ; ** : Furukawa et al. (+331) ;*** : Hayakawa and Koyama (+332) ; **** : Machida
and Arai (,**-), Furukawa et al. (,**0).

6 78%9') 786:;<=<>?@78ABC Ma-b/b+  (+320)   ..0 km- 

DEF
=GHIJ>

Ma-b / KLM%N& 

OPQABRSATU& ! Loc. ..
 , mW ,* cm+ "V#W - cm K# W +* cm XY$%%?Z[   !"# $# % & ') -\L&'%]^#9_ Ma-b `a,-( / cm   Loc. + ,T -/ cm

Ma-c .

 Ma-c. ()`% *+e,PQ-%.dB)

 `b. Ma-b+  Loc. .0 ,T%()*) +* cm
,* cm

%% cd ()B"V#W

()*) , mm + cm+ Ma-b/ -b- ,-. -b+ % f6 % g6 h% f6/ 

0iR R1: 2R: b.345

78 j 6:;<=<> kQ8

l69_

@78 + m nkQ878`% 7q  *.,0 km.ῌ,ῌ1 +o`PQ6 + Ma-b0 Ma-b. b. Ma-b,  p#r8#r=<sg66@PQ6 + t

 c, (Ma-c,)  (+30-) (+31+)

 Ma-b0 % Loc. .1 , , cm Ma-b. %Loc. + , +/ cm (E Ma-b,  Loc. .0 ,uvw<xyz9:1 +* km ;{ !

 2 cm+ m)B|)L78<=}i #r8#r=<so`PQ6 + `,-.~7PQ6ABC\ 78 +a  ,-( / cm   

Ma-c -  6;<=<>>C?@{') m80 Ma-c. Ma-c- ()g66/E Loc. ,il6R:345l69 A

 0 cm+ 78B Ma-c- ( +  *.*, km.ῌ,ῌ2  c+ (Ma-c+)  (+30-) (+31+)

D  EE(O%;<=<>78
(7%.dB)

 Loc. + b. Loc. , ,.dB) 6@
 #r8#r=<so`PQ6 + `a / cm 

Ma-c,  Loc. + , - cm Loc. ,

, cm



<zJC <=}iR R1: 2R: b.345l6ABC\ 

+

 

+ 78.ῌ-  !"PQS!US E

(+323) (FE PQC GH

.EF (Fig. +-)S Ko-c, I

@78  kQ8% &¡  Loc. , b. Loc. -/ , Ma-b ha.

6;<=<>L9')  *.**/ km-¢)F£J + B-Tm  KQ¡  Loc. ,* ,¤¥L¦F Ma-b `aAB Ta-c, LMPQ¡  Loc. . .ῌ,ῌ3 #$%& b (Ma-b) '( (+30,) )*+,SO§P¨Q US8Ro`g6PQ6©ABC\ `a / cm 

Ma-c. `aABNF m)BPQC tU

Ma-c+  

Table , EF

 (+30,)-\`aAB Ma-b /  b + =GHIJ>

TB)/ -./012

,\ Ma-b/ b- b. b+ o` Ma-b. b. b,/ῌ+ 34567&89:;<

o`PQ6 + (Fig. +,-d) Ma-b/b+  !uªv«;¬­oO |)L0il

=GHIJ>-¦®C\ (Katsui et al., +31/) `6R R1: 2R:b.345l69_aABj 9: ¯ 9: ¯1° +;<=<> + R1:2R:H

±²

 Loc. .1 , Ma-b/ S`³¢|g´µ (LE&R1:1%¶¶&

 (Table

6PQ6  Ma-b 0 : · ¤EF m)B-) (PHIJ>T¸¹<y%9')´

Ma-b0b+ ºUE  'F7PQ

µ L ¹<y-0il6»¼½CNC

BC+. DEF G HIJKLMN O

29Table -. Phenocryst contents and modal compositions(wt.ῌ) of pumice in each eruption unit.

Fig. +-. Glass compositions (K,O-TiO, diagram) ofwidespread tephra layers. Compositional ranges
presented by shaded pattern are from referenced
data ; Ko-c, : Tokui (+323), B-Tm and Ta-c,
Machida and Arai (,**-), Ta-c, : Analysis ofproximal deposits of Ta-c, at Tarumai volcano.
All data is normalized to +**ῌ anhydrous.

PQRSFTURVWNXYZ[!3\]K^ SiO, * 01.2 YX 1/./ wt. ῌ _*  !"#$ (Fig. +/)`abcK"d!e PQRS* SiO,
* , wt.ῌ fg, h `abcK i j *klmPQRS* Ma-j ,B SiO, n Ma-i, h, g No4!ip!qrXs/! abcKN

e,t'uv SiO,  i/ (Table .) Bw

*+p!,qrXs abcK

Ma-f * ,xyi/, j>?

 Ma-l Ma-if *+, +.*0., wt.ῌ *z{$ (Fig 3) Ma-e῍N B| PQRS* 

 Ma-b , Ma-d  +,.3 wt.ῌ

Ma-b * +2.,,-./ wt.ῌ N | !}5+,!
/ῌ, 

S

abcKf0 G HIJK  SiO,

GIJK *B SiO, (wt.ῌ) ~ (SiO,1..,1/.3ῌ) N ' ao "/,X$, Ma-b )- Ma-e῍ MgO /5)!'IJ
KN,/0..-1+.1 wt.ῌ 8* +**ῌ  f1 clS* Gill (+32+) *a

0 



$ (Fig. +.) ao* SiO, 

0ῌ+   !"#

%& ' TiO, Al,O- FeO* ( MgO CaO )G HabcKf0IJKz{$

- MnO "#   Na,O )- K,O >?, _  Z[!IJK  @A¡"#$ `abcK"d!e ¢

Ma-k, -e, -e῍)- Ma-c  £¤¥01 H¤YX

PQRS SiO, * + wt.ῌ ¦§, h/ jsX3N ¨+,`* ©!ª `abcKN ' SiO, , 02.* Y

H«3 A A¬~xy!IJK  @A

GIJK *, !

Ma-l, Ma-jf )- Ma-b * ./012314

*nN K, O ª FeO* a* + ­  H¤,®¯$ jsX° H¤L/S "$, TiO, ª MnO * 1"*3n ±#/²$_s

PQRSF*%/TURV,qrXs

J³´²µ$"&$jN

`abcK*lmPQRSF*"/5 67689:; <=>?  @A Wohletz,

X 1+.0 wt.ῌ *H«3"¬~jN

/,XN ' + ­S "$ ¦+32- ; H¶ +33. "&$ Ma-k ª Ma-c  XY§ SiO, * e·¸ Ma-i, h , TiO, ª MnO /676 2323 H¤ qrXs/!,

* Ma-g, f ªª¹!º"#$, j» |¼/]K^'+*J³´clSª½

30



Fig. +.. SiO, variation diagrams of major elements for Ma-bMa-i (bulk samples).
to +**ῌ.

All data is normalized



+

.



31

Fig. +/. SiO, variation diagrams of major elements for matrix glass of Ma-bMa-l pumices. All data isnormalized to +**ῌ anhydrous.

 !"#$%&'() *)+,-,$.

/012345+6789:;8 <=>?;@A

BC?DEFGH (Morrissey et al., ,***) IJ$

KLMN$OP8QR ST@2 U,-,VW8

XYZR*[\ ]^H^_`\)R a^@bFGc)D/01$Qd <=MNePZRf`g hi5Gc)Dj@klmno 8QR+ pq8Srs@t^@b5uv

567 879:;32
Table ..

Representative major element compositions of volcanic glass and whole-rock samples.

<= > ?#@AB (CD

E2FG2 Ma-e+ HIJ?

/2KL@ !J? M N

O" PQRSTUVGL Ma-b H"# @

G# -.- !J?EW@KXTY

NB0 Z[#\, $]% ^_`ab2&Z[0 cSdedfg'#\h? @Pi`j

m(2Nn g`)op?,.- *+)o,   ,2qrstu@2 J? ,-.v# w/g .

xy,2J?

vz{=p0|" 12}U`jh?
>-.-/k V "1 Z[# ? @"# kl-~3@K4 + 5 .  6 l

 @"# 789-.-(CD:;"

< =Z[> M* :; (Table /)  " #$%&'()*+,

A,2`j, 0 <"=Z[

#3@K(?&@"-.-/01234   Heiken and Wohletz,>@(?&B()

CJ+33+ ; Houghton et al., ,***!  "1? ?QRSTUVBAB

Ma-k  --.-@(D,2<= -.-( =        Wilson,,**+! ?

"# (ῌ-.-*11>

 #G T`V 
Hh?#,,@/>

.-(CD :;Eh? M '
1F@" Ma-l "QRSTUV M
#QRST" @9

2C


0ῌ, 



¡Z[`

"#PQRSTUV@IJ, ¢VK*

L£ ¤¥ M¦N@K¤h? Ma-e L Ma-c.

 [§9St¨h? @ u ©x-.-qO)ª`{ ?#V)«0b¬­®Y`J¯ (Fig. +0) °£'± P PQRS

c - c ,% ^_`ab2 ~&`j (Fig. ++)?# 9-.-K´ ( DKTUV@# -.-(CD (Ma-j) h?²³,2
QyTµ¶R# ·RY9`¸S

11!  ¹ T@" 2©U"1,2`j °2VºHg' M, G  Hh?@/> »¼

 3QRSTWg XGY½?ῌ+.

῎

33

Table /. Summary of age, eruptive volume and styles for each eruptive unit from Mashu volcano during thelast +.,*** years. PDC : pyroclastic density current. * : Katsui et al. (+31/), ** : Itoh et al. (,**1).  :alternative between the two sides. Calibrated ages are calculated by the program of Stuiver et al. (,**/).

Fig. +0. Schematic illustrations showing eruptive sequence with magma systems during the main
caldera-forming eruption.

 (Katsui et al., +31/)   !"# $%

ῐ&'() *῏&+,)-./0 0

1 234)-5267)89:&;<=>/ ?

>@A&B4) CDE&67FG8HIG

Ma-i (..* km ) =J KL&'() MLJN" )/ 0GOῑ;P2Q)-R/ L%1STU)V.0 WX)85YZ[\ῐ& Ma-g ]=O^_V`abc28d/0e 1S-

 Pf)8HIG/ ? Ma-h (*.32 km-) =g&hijO=k/0 lm5=n0o8

J p"=L%1S qrW1$%1srEtu

HI-vwV.

tx &ee(yz/0 {|ῐ5}~&?  %=k/KL1L _

FG/& )-JKL (21 3. wt.ῌ) &z Y2Hz/ ] KL&/= WX)8YZ[\&)8HIG

Q TUz/0 1 7\C\ .

/ Ma-g (-., km ) = % tYJ$z/

- Ma-i f Jhi + ¡¢x£¤¥2¦0- L%1S2k]§¨©)V.YZ[\2, ª7«¬­2®X=>/ KLJ)8 °)-´µῐ (+.+

±²\³=k/ Ma-f (3.1 km-) Jº»& ~> L%1S W1$%1q῍1

tY ¯

0.+ wt.ῌ) ¯¶·R­¸¹&j

῎=k/& ¼r  sr½

SiO, ῐ&`az/

!" #$%&34(Fig. 3) '( )

*+(,- -

. /! 0-1/!2"34 5# 67
 *$% 1 89

5: 1

: SiO,03 wt.ῌ %0 ' #;  89<
5: : SiO,1+ wt.ῌ %0 7<=> &@,'2(?(

ABCDEFG Ma-j 

,I(
 K

'#- 

Ma-i L

)*H+
J.

M 42 Ma-h

-5N/! 2O.467   Ma-i  Ma-h '#-RSDTU

P<2L Q/*

'0-7

I( :- 1
1#; 

V 6 W
1 9XYGI(. /!Z ' 0LRSDTU*2[2 Ma-g \]' . X

3 1

7<I(

 '( 4^5 *_`Ta
Z6<2hR

Ma-f  bcde

' 78-74f( g9\ijW

Fig. +1. Cumulative volume of pyroclastic deposits
from Mashu volcano during the last +.,*** years.
The age of Ma-c+c. are approximated uniformly
as a mean of two ages in Table /. The age of
Ma-e῍ is estimated by the thickness ratio of paleosol
layers between Ma-de῍ and Ma-e῍e.

<k K l*mnZ- 'o:;
p<<Fqrst4

 (67

 (Fig. +.) @x*ybcdeZ0

0ῌ-

ABCz

2 
 +.

G 6 uv =K @x*y 



*DE7 }Fr cG4 + H . BCz'5#J CKL<M( \z N*w>u? @x*y{  |}*yG~

3

2A km- IG* 2 -  Ma-l, Ma-j  f k  Ma-b ' z + km- 5:*  * 6  (Fig. +1) AMS  }F-2 . G '+.0+.2 cal ka

CKL OP ,**1 2GQ]'RS

F- Ma-c - T5U\zzk( . }

5N V WXCKY @ + H . BC

*2Z o [ * *.+ km-

z _e \  -* km- 5#G BCG

5: G Ma-e῍L ] @ 0,*** Cz 3[]( 2  *.2 km DREῌky  (DRE)-*2,^(k*}( N:/!*.2 gῌcm- *_+.* gῌcm- !

F ,^`.< LI(LBC + G,./ gῌcm- / < 6  2:7o  u *y87abc<d }*y e*y @ + HCz b

Q] Ma-e῍+322 ¡f3g @<¢hiLG ' Q] *`2{ +,/** Cz (Nakamura, +30.)

9jG k£l}F5m%y

u<[

*4f(

(

@

L 6 u< *DE Ma-b 7 BC2

3,*** Cz *./ km- DREῌky : ¤P ,**0 ¡k£l¥¦(6 '( ,**- C*yibn2

@ , HC z *., km- DRE ῌ ky : ª   P,(6 Hop ,**/ <q4 §¨©

,**2 L3W 9+Bg«r4*yT-5 ¬? ­®-¯(K }F-°±4

Q] }F5*y²s©@¢hiq4fWG³u Kz-,/** C

2 *.*- km DRE ῌ ky  G  O ´  P-

*GI Ma-b*}F 30G67 µ¶5[,**1 @x*y t27{  G bcd

 *2u/ ·fu)*H*.- km- DRE ῌ ky 

*2,^4¢hi2{ 6 Q]

¸v ,w

eZ-0@ 0,*** CzO.42 'Lk£l§¨©<9j

¹  º»bxN12G¼½7L< 7+

 %&'

.

35

89 :9;<=>,9? )#=@A !!B"<=CD #E#F$,*G%,A9# *HIJKLM!N&#=8O

 )'P(Q"8$ )RO,*S
T+U -,V%WXYZ-##F6* T+U\7*]^_cd:0,>e ,C ;<*

cd,=8=\>?!@!8 fghi
op2I&9q'#JK) F()C^F
1ῌ ῎ ῍ ῏

cd,*4Zx'yOzHP+ !"#$ %&'() # .

\ *S>? }8$}R !"#$(8= ~*HJF!O
(+) T=9UV* Ma-e’ )8 Ma-l, e, d, b

!I#$ JDWX90 p/+) F9q -) = O=`!"#$ R \c> !8=(,)

d4Zx'yO,* R

>!

*

  <! *0,>efgGh  

yO!M=<= oo!"#$*
aeooeoo!Jgoo.ODij<=(-)

* ook O=*_k tu!Jookv345S!
$#=  * k!8]!C=
(.) R ῌ:Vc!J c;

*

+

.

  +, *.2 km- DREῌky   -

0,*-#)= *?9
S3S,C=>@ * #E#F$,>G%,A9#-,Cΐῒcd) F!C=D

A!I#$*` !

Bcd¢6sC£¤ 0D£¤! 
O+IJK¦'§O+!I#$*!
Bcd¢oFE£¤  R 5 ¨e¡£¤!I
LF!9= 6{|,*0G¢£©ª«¬!2z¤)¥8$#=@A ­®¦H¯§¨q.%
­!*!ILF!9= <!«¬´2µ®C­!*%&'!M¯I)°¶8$±#= ·hZ!¸FOD³!* Jb$9q)?
©=±#= , MµM|¶:-«¬,Cy!
N½0º©»!* ¼)9O½$X9+Oῐ ῕ ῔ ῑ.[/0+12345! (,**1) ` 0a 8(b7 - -**  9
 /, ,/-ῌ,1+.
A"Bj CD#EB$%FkGlmHn(+331) ` rsb* +03. !8=` !It +1+2 Lu'( ., ,03ῌ,13.
A"B0+1Av wj CD (,**0) MN
Q9R'-, {|  ,2 -*+ῌ-*1.
Gill, J. (+32+) What is “Typical calcalkaline Andesite” ? In
Orogenic Andesites and Plate Tectonics, +ῌ+,, SpringerVerlag, New York.
Hayakawa, Y. (+32/) Pyroclastic geology of Towada Volcano. Bull. Earth. Res. Inst., Univ. Tokyo, 0*, /*1ῌ/3,.
Y,uZ[ (+332)  `)*<_, +* Lu!\]#,=^"!{8%F_
ab  ., ,03ῌ,13.
Heiken, G. and Wohletz, K. (+33+) Fragmentation processes in explosive volcanic eruptions. In Sedimentationin Volcanic Settings (Fisher R.V. and Smith G.A. eds),+3ῌ,0, Society of Economic Paleontologists and Mineralogists Special Paper ./.
Houghton, B.F., Wilson, C. J.N., Smith, R.T. and Gilbert,
J.S. (,***) Phreatoplinian eruptions, In Encyclopedia ofVolcanoes (Sigurdsson H. ed), /+-ῌ/,/, Academic Press,
London.
l m/n0oFpq12r0s(+3/-) wx[6yz{|7} ` ~
yz{|7} , +.,p.
.     0+1 23   (,**1) `  4Zx'c*9'   < = ,**1 !> V+/1-P*-*.
(m0 (+3//)   0+ .2+ῌ.3/.
 -3 +3ῌ
(m0 (+3/2)     ,3.
(m0 (+30+) * =-,9
 // 2ῌ+0.
(m0 (+30,) / + +   _ IJK  ` ¡ .,p.
(m0q. (+30-) / + +  ¥ IJK  ` ¡ .,p.
Katsui, Y., Ando, S. and Inaba, K., (+31/) Formation andMagmatic evolution of Mashu Volcano, East Hokkaido,
Japan. J. Fac. Sci., Hokkaido Univ., +0, /--ῌ//,.
(m0° ©±F 1ªF«¬5 ²³
 (+320) ./0123 456123` !It!Mcd7} ² +* - 
` '#¹ +*.p.
´ (,**/)   -¦K ² - L ´- pp. ,/ῌ,3 !·¸º¹» ¼

36'()*+, -.,/012

3456 (+322) 7 8
9 4:; 3. .--ῌ./,.
Morrissey, M., Zimanowski, B., Wohletz, K. and Buettner,
R., ,***, Phreatomagmatic Fragmentation. In Encyclopedia of Volcanoes (Sigurdsson H. ed), .-+ῌ../, Academic Press, London.
Nakamura, K. (+30.) Volcano-stratigraphic study of Oshima
Volcano, Izu. Bull. Earthq. Res. Inst. Univ. Tokyo, .,,0.3ῌ1,2.
Reimer, P. J., Baille, M.G.L., Bard, E., Bayliss, A., Beck, J.
W., Bertrand, C. J.H., Blackwell, P.G., Buck, C.E., Burr,
G.S., Cutler, K.B., Damon, P.E., Edwards, R.L.,
Fairbanks, R.G., Friedrich, M., Guilderson, T.P., Hogg,
A.G., Hughen, K.A., Kromer, B., McCormack, F.G.,
Manning, S., Ramsey, C.B, Reimer, R.W., Remmele, S.,
Southon, J.R., Stuiver, M., Talamo, S., Taylor, F.W.,
van der Plicht J. and Weyhenmeyer, C.E. (,**.) IntCal*. terrestrial radiocarbon age calibration. *ῌ,0 cal kyr
BP, Radiocarbon, .0, +*,3ῌ+*/2.
~  0 ~ 

8 1  ,! (+31+) Y6
  !"# $%&"#+* ++1ῌ+,-.
"#  ~       
0% (+30-) KI>CopO*+<=> Y6¦§¨©ª«¬­W no.
+- +03ῌ+31.
µ¶· (+31,) ¸¹
GH Ta-a º Ta-b º +0 +/ῌ,1.
<=>
Stuiver, M., Reimer, P. J. and Reimer, R.W. (,**/) CALIB/.* (WWW program and documentation). http : //calib.
qub.ac.uk/calib/?1@A (,**-) BCDEFGHIJ
KL
MNOP H +,+. QR:"#ST U
"# (C) (,) "#HVWX Y6Z7
   :  ,.ῌ-,.
[ (+323) Y6 +1 &\]KL ^_`a bc d e4 - ,1ῌ--.
f ghi j (,**+) Y6klmnopOq 9Krstu 4v
wR: :x ,**+ yz{x|}
Wentworth, C.K. (+3,,) A scale of grade and class terms
for clastic sediments. Jour. Geol., -*, -11ῌ-3,.
Wilson, C. J.N. (,**+) The ,0./ ka Oruanui eruption, New
Zealand : an introduction and overview. J. Volcanol.
Geotherm. Res., ++,, +--ῌ+1..
Wohletz, K.H. (+32-) Mechanisms of hydrovolcanic pyroclastic formation : grain-size, scanning electron microscopy, and experimental studies. J. Volcanol. Geotherm.
Res., +1, -+ῌ0-.
 (+33.) e C 4:$} .- 0-ῌ1,.
¡jg  3¢£&  ¤¥ '  .,/0 ¡1()(,**2) Y6®¯
 °,9±9 ²³´ 4; ++. --0ῌ-.1.
}»¼ ½ #