ῒΐ  //  (,*+*)
23ῌ+*1,῕ ῔ ¸ ¹º!

,**3 S - T ,1 UVW ,*+* S - T ,- UVX

Eruption Age and Sequence of Ogurayama Lava Domeat Towada Volcano, Northeast Japan Arc
Takashi KUDO῍
Towada volcano is an active volcano located in the northern part of the Northeast Japan arc. Ogurayama
Lava Dome (OLD), which is a dacitic lava dome located near the center of Towada volcano, has been regarded
as a product of the latest eruptive episode A. In this paper, the author reports that the OLD is older thanpreviously thought and that it was formed at the end of eruptive episode D῍.
The OLD overlies pyroclastic deposits of the eruptive episode E and is overlain by pyroclastic deposits of theeruptive episode C. These stratigraphic relations restrict the eruption age of the OLD to 3.,ῌ0., cal kyr BP.
Within this time interval, two eruptive episodes (D῍ and D) are recognized as tephra fall deposits in the distal area.
The distribution of Herai Ash from the eruptive episode D῍ shows that the source vent is located in the vicinity of
the OLD. Furthermore, the petrological features of the OLD closely resemble those of the Herai Ash. Theseobservations indicate that the OLD is the product of the eruptive episode D῍ (1./ cal kyr BP).
The probable eruption sequence of the eruptive episode D῍ is as follows. Intermittent phreatomagmatic
eruptions occurred in the earliest stage. These eruptions produced the lower part of the Herai Ash. Subsequentlava eruptions formed the OLD and accompanied intermittent vulcanian eruptions produced the main part of the
Herai Ash.
The source vent of the eruptive episode A is not the Ogurayama, because the Ogurayama was formed before
this episode. Since the only crater topography currently recognized in the Towada volcano is the Nakanoumi
crater (NC), the source vent of the eruptive episode A is considered to be the NC.
Since the NC has been the main crater throughout the post-caldera stage, future eruptions will probably occur
in the NC. There is the current NC at the bottom of the lake of -,* m in depth. A detailed examination ofprobable eruption style in the future will be required for predicting volcanic hazard of Towada volcano.
Key words : Towada volcano, Ogurayama Lava Dome, stratigraphy, eruption age, eruption sequence, eruptive
episode D῍, eruptive episode A+ῌ ῐ ῎ ῑ ῏YZJ[\]Y^I_` >?abcQP defg

hij?<*kUDlk:emn_ opq ++ km

rs tmu A, B, C =vewxyz{|Y}~`=YJK@?<FK +a, b  hij?

<=>?@  Hayakawa (+32/) A<BC (+320)
+ (+320) e` =uers`~K s=34es( hij?<=>?@

Xrs`~K (Hayakawa, +32/)
 

DEFGHIJKLM<NOPQR (Hayakawa, +32/)
 hij+W=LM:YJK
|7=NOPQRFK +a, c  hij?<=  // ka    (//ῌ

6734sF' s`¡¢ =£+/./ ka) ¤ +/./ ka¥¦ = - &=§¨Q

I©LM<NOPQRYhij?<ª«=>¬­F®efgrsK ¢ ¯ =>?z°|>?±²

KI³´`~K Hayakawa, +32/ ; µt¶· +30, ;! "-*/ῌ2/01 #$%&'()* +ῌ+ῌ+ +, 1-./012345678934:;
AIST, Institute of Geology and Geoinformation, Geological Survey of Japan, Central 1 , Higashi +ῌ+ῌ+,

Tsukuba, Ibaraki -*/ῌ2/01, Japan.
Corresponding author : Takashi Kudo
e-mail : kudo-taka@aist.go.jp

90

12

3

Fig. +. (a) Topographical map of Towada volcano showing the locality of outcrops. (b)Location map of Towada volcano. (c) Topographical map of Ogurayama Lava Dome
showing a survey route and the locality of outcrops. A part of + : ,/,*** topographic map“Towadako-toubu” published by the Geographical Survey Institute is used.
 + (a) 
  (b)   (c)
   !" #$%& , ' /() + *+,-. /0
2 +3-3 ; Kuno et al., +30. ;  ) *+,-./0 191 +320 ; 34

 +31* ;  +320 ;  +310 ; 5
+32. ; & +31, '(  (+310)  Hayakawa (+32/)  ) *+,-67 3+/  89:,+ A  ;.<=>0>
      +331 ; ,**- ;?@ +333 ;ABC<= DEFG!"HIJKL# $(+3,3)  (+31,) ) *+,-M7 N%OPK0>6 Q&RP
STU<=JKPKV'WX6) *+,-M7 N%0I>
ST *+,-Y(;Q=ZAIK

N%6[\]^P]_>0;`L 5 +32. ;Hayakawa, +32/ I]I )a,bYOIK

N%6>c>c[\]^P]_>d*+e ,-;Zfg=D.IJKPKd*+A`L 'PYh
/0ij0&R+;k ) *+,-0lh//mij;\KJA QnoMp12;3qI>
=cO.EIP]_> rY0]^ )
*+,-M7045sL>tuT06O`_>0vgwLM7PK
  08xsL0 9:;<

=KA O>? >@ rl
yzO VEI  .  / 8x2{A|PB92

M}ZCIJKL Hayakawa, +32/ ; " ,**2 ; "#! ,**1 0]^  ~' +MDs|  + \O`L0vL~' MEFsL>t;   GHMO|

Fig. ,. Eruptive history during the post-caldera stageof Towada volcano. Ages : *Machida et al. (+32+)
and Hayakawa and Koyama (+332) ; **Kudo andSasaki (,**1) ; ***Kudo et al. (,**-) ; ****Kudo(,**2) ; *****Horiuchi et al. (,**1).
    , 

(+332) ; : *   (+32+) ** "  #!  (,**1) ; *** "    (,**-) ;**** " (,**2) ; *****Horiuchi et al. (,**1).

LIR;JsLKL6`L ) *+,-./M&sL0  ZMNP

9OMPsL>tOP Q2P

, 

 , ;

  LMUs

 EFeRS>t;ALO`L

rlOxI ; M $cal kyr BP% 0T

QOUWXO ) *+,-./M&

BsL 

 O +/./ cal kyr BP (Horiuchi et al.,^];sL>t rl'PVMP_> c,**1) ;9:I> 89:,+ L ;Z W9) *+,-XnoPY)aMI *:0ZM[K +,-0QYb/mijM&R;I<=> (Hayakawa, +32/)   Q.6.

> c> ) *+,-;i\sL ]^0

_]^¡¢I> (Hayakawa, +32/) £ 

Q6`M)L>t  ;\KJ¤;¥a*¦ *§¨§;L<PL©ª«¬/m­2®VMP_> P

N%RP/m¯ N b©ª«¬ *WWN;

S8>t; cdO7^=>/m*e°3;kL/ .Ofg^=L Hayakawa, +32/ ; U%*h­2°3MijsL06]±O`

Hunter and Blake, +33/ ; ²k³  ,**- ,**. =L QOUWXO U%,+lm^ \  Q ++.1 cal kyr BP cOn -2***H­l6oMPK Q=^,´M/mS8

pK>0µ^=L " ,**2

;jI> Q=^¶·M; ) *+,-./0Q 1;\KJºMP_>

y++.1 cal kyr BP r §¨§l6¸¹¸b¤1;qHI Hunter and Blake, +33/ ; ²k³  +333 ; ²k& ,**1 »«,¼ e§¨§rs½ 6<=92
Table +.

>

Modal compositions of eruptive rocks from the eruptive episodes A to G in Towada volcano.
 +.  AG 

?@A3BC (Hayakawa, +32/) DEFG

H +c IC JK %L3M

FNO/" ++ cal kyr BP <= ,**2  G 

PQ'RS CT UV4AQWX

Y Z[X 3+/  \] ^ +332 ; _ `

,2 km!a ,, km bc3d5FNOefghi+32+  A I/ j2P1 2 kl'mBC H +a Rn"oKEpqrAs1tu

9vwx5ez'{|P C

(Hayakawa, +32/) Dy  E 1 2" C 345" 6789:;
5}UP ~ 5U9   - km 6 U
H +a, c 9wxCPwx5

Hayakawa,-ῌ, 

 G 3d5'   (vol.ῌ) 'mBC

% E+32/ ;  ; +320 ; ; +310 IC ,O

pqr &13d5

3"J& %L (Hayakawa, +32/) PJK" #$%&' ()*+,-*./ + 0%L9wx5 9 DxY

3d -***.*** ,-*+ 5TC  A " "; 9U63 CT!"/s 
1¢ CA3!"£/  #&3d5"t¡/"9¤¥/ CT ¦§'C  B ¨w
+*-* g ©ª + «¬«&'z$  ­®w¯5

- 
-ῌ+

¨w' C QxY&"°%'5&l C ' 5±OE '²5 ~³´µ'¶ C ~JK %LefM·'"Y3%LV4AºLt(b

CT

Rn'»¼

³"­®w¯5¨w's ¸¨wY~³'¹C

5±OE ¨w'½ C ~³")'5 ;<= >?@A/ BC

93

Table ,. Representative whole-rock major element compositions for eruptive rocks from theeruptive episodes A to G in Towada volcano.
 ,. 



 AG

*./ mm $DEFG *./῍*.-// mm )HIJ KLMLῌ 5NOPQRST,++U/VW ! +" XYZ[)*+,\]56+,^_ `a5b9: KLWc+)FPQdefghijP/kOWLP`a56+, lmnopqnr iKLML( 2**-ῌ- ῐ῎ῌ῍ῑ῏+/** spqno9 0t34-.589: uv

 G $%&''( 3. )*+,

w/123xyd Lipman (+301) )FPuvw/1

z{5|9: |)d}~

23BY56+,34)89 KLW34



-./01234-.56+,7 589:+.*. 569: |)d + : +* uvw 5

 r5 + )

6+:  , )|15 $i

E Z[ A /Z[ B c+)FPXYQ)*

 X | PHILIPS  PW

d,g +** wt.ῌ )(89:56+,#

+,d &id (¡¢5£¤,+¥5£¦ E  , )9:0z{§¨©Q (,s)+

d }~

9V9 , *ª[56+,«_: 

B EFG DXY15®P¯I °±d + vol.

¬§¨ JB-+ a 5 +* ­²I³9XY9:/j|´Q5

7

94

8
0 (9:; F-F (< , m => ?@ABCD0)%+/ m =4 $(:;EF(*G - .

12 HIAJK:;L12KMN&

E (< ,* cm =>  OPQRSTUV*#$%& '() / m * F (+,- - WX* YZ[ <Z!\]^_O.K`0a>12K"%& E ( 4bc5^_d0&_6 #$eK()fg_h%
D (< -* cm =>!" /* cm => "A&*#$%& )%' +* m * E (+,- WX* YZ[

<Z!\]^_O

.K`0a>12K"%& !"ijkl ;mnoZpqR($(:;r @s @(*G

C ()% . m

6 D (+,- - t)( )% / cm =>*G u*vw(*Z+m0&.K`0Hxy12*GKz{0&|:;!"12*G6 C

B ()% 3 m =4

(+,- - >*'< +* cm!"}~Z+* |, O/K W
X*<Z!\]^_O.K`0a>12Kz{
0& 4e`^_|,Z: !" }~-. )% .* cm@s @(  
A ()% +* m =4$(:;/|r-

@  A*G < cm m

qUp

xB*#$%& A (K B ( 0 K ;m+*O -.ῌ+ῌ, B ( C (5 D (+,*G6 112/`0O 
Fig. -. A stratigraphic columnar section at the crater
wall of Nakanoumi (loc. +). Its location is shownin Fig. +c.
- 
 

  + 
+c 

Z^_ .&0 + - :;

p5 K¡0

& !\E( *2 a>!"¢£¤`0#$%& |¥Z¦: ijkl §3!" E 

!" (Hayakawa,+32/) Ku4. 56 @ ¨ nAl©ª$«  56 @ ¨. 

4*.ῌ+  !

3*G 

.ῌ+ῌ+ "#$%

@A ´ * @AK>(7«

ª$8¬:. .&0

 TiO, K MnO *§9:.K*­®¯°!56±²-.KZ³;*G  + *  + +c»¼Zµ¶%&- ..*/0&12 3456 A

µ¶%&; BD ( ·_½<¨:_6$*¸¹¯º^ , 

+c *)% 2 m=4 /| a>!"12K  +!"!" 9:" ; <=>(?@A95

Fig. .. Whole-rock TiO,-SiO, and MnO-SiO, diagrams for eruptive rocks from the eruptive
episodes C, E and G. The other eruptive rocks during the post-caldera stage of Towada
volcano are plotted in the outside of these frames.
 .  C, E   G   TiO,-SiO,   MnO-SiO, "#$%&'()*+ ,-./  0123456.*

BCDEFG*H IJ , K /.LM , NZ[ -./ LM,\$ GF
 E CYOb=5cd

eQfgBh23456.*  . a + ?
,b=CaxyG  . z{| }?~
F *? BD , E

LM

6.*  -O*PQRS.* GT. Q UVWXYR,

C BK^*-?v/ y?Q 
 F ^*G, : (Hayakawa, +32/)a|O*? /.*.ῌ, 

] 2*ῌ K^*_,`a_&

iji5K kl HmnopqrO*
LG 
1 cm

&iji5s  t, u v/w6=%577  K  w6 ,1 cm

%577w6 / mma E GF,MEyFGyG

!

v/y*

G I  ; u%5 ew +* cmcd %577  H  w6 -, cmu"  F  C6.*  / K  I 
 H , w h lC_C&iji5s  L, J, G, E  ?K^*-?v/ &ij

i5s ? ?@¡6.* -.

.ῌ,ῌ+ / ,v/¢£ +,+28K¤¥O*

9:" ; ¦§¨©ª IJ - K, /

« &iji5s  u¬ 

F

u"

,

v/ a  h2 C6.*  / --K­/.*®¯456.*  / " ,°s<  y 

+,   /

&iji5?! "# Gpq&iji5LMKW

L  J  G   E ,&iji5s K^=6.* ®¯45 a d f h ,PK±pqrO*

AL* w,h lO*C ].².eK 0* cm ³
 ,* cm, -- cm, 1/ cm K^*  / GT.QPK $GK g´ . cm ³u " KW=6.*®¯45 a ,

E 

Fµµ¶

·¸¹)

&iji5s ?ºB  ji

FG*  / ®¯45 h , G  »%&H* ¼

 &iji5'KW=6.* L ,g´ + m ½

F  w lCG  / ®¯45 bg´ .*+* cm



96




!"# h $%&'() G *&+,-(.

./ ,-012 ( %345(.6 789
E *&+$ *4 +- cm &:3 ;&<)=>?@A# B C * D *  *4 0/ cm &:3 ;&<)=>D?@A# B CEF* C * 8//  )GFHIJKL&MN OP Q

RS3 ;8<) TFE$UKV&W,-
39XYZF C *&+$4Z +- cm &=>D?:*B*

89 TF4Z - m [+&:&\]^U,- A * 8/  \]^U,-$ _ ,* cm []&?^U`ab&c
3deZF fgh&ijK^U$ k8lmn
53opq?TX89
.ῌ,ῌ, ῍ῌ῎῏ - 3rsZF.tgfg#N **  \]u ,-F*E *`a L`a\]^U,-

A * v)w xyz{e`a|c5}{e)

w*~K.
xyz{e& 0  )GF&! H"(wU #U U $%
X(wO8 !uoyz C, D `a F &$EUbOTX89 & +  %$!uoyz' 9)$ !"#'K89
0  TFEX - &,-X tg

fg#N *`a\]u ,- &tgfg#$ % {P(czyX)*
*0  # +&\]^U,- A&^U$ EUOK)H&&!uoyz C &^UX)*

0 

1 (czy tgfg#N * \]u

 ,-&|c5}{e"#(.y y  xyz{eX+, |c5}{e)wH¡¢&{e$)*

1  (.8w TFE$(czy£¤Sv,-39X-ZF
 tgfg#N *$ cL3 O./8l
Fig. /. A stratigraphic columnar section at the southeastof Ogurayama (loc. -). Its location is shown in
Fig. +c.
/.    +c 

|wN 3deZFTXE (czy¥¦0§¨©,-XPEF tgfg#N *ª«W,-&¬ H#E&,-&­®¯.  # \]u ,-$

(czy&1°(.203!u`±
XYZF

$u XC4uD&C²*39 |³X(wD?
!"# c e g $)GFH*4 + cm []&´>?C4uD*39 u µ¶ )GF& !"#H ;8)

!"# h ·)w¸p#A»"t¼p(w)TXE \],-X½¶EF

+&\]^U,- A * $ |c5}{e)wH!uoyz C X)*.  4Z

8 - m [+X34)TXE !uoyz C &

5^U ¹º +32- ZFX-ZF



Fig. 0.

 ;< !"#=>?6

97

Modal compositions of eruptive rocks from the eruptive episodes A to G in Towada volcano.
 0. 

.ῌ- 

 AG!"#$I. #LMNCDOPQ
UVFWXa:ST .  +c ) !"#LMN

BCDEF.GCDHIJ:6789-: EKYZ[L\*]4 Z[^_L +c `a Z[
bc ;<@A

CD+ijk-4 ST . I. mn .* cm opqr+stk-4
 2 ,qr.X6~L5:6 q

IJ]4  2 9.X+])46

GCDHR6E ST . .;<UVFWX

 +c !"#$%&'(

de9Afg4E6.78h) 4+] ,-9lB.;< !"#$CD4

C

DuvLwx6yzIg: lB{"|}+ C 6Fa:,6e9
 . ,-9.

 C uvIJ:6yzk

9-:S+a:  2 ,qrX-: 6,ST . %&I*lB.jxe]

wk-:e9 lB+ijk-4  2)*)+ ,-./01 23-4456789-:

lBn.I 2 cm IJ: lB.¡a:,6e9 ¢X£¤L¥¦|4l.ῌ. 

ST + H§¨©. ;<!"#+

56

98

Fig. 1.

8

Whole-rock TiO,-SiO, and MnO-SiO, diagrams for the Ogurayama Lava Dome and related rocks.
 1. 

῍TiO,-SiO, / Dῌ

MnO-SiO, D

9:;<= >?@ABCDEF14 2., cal kyr BP 56G' ,**1 HI
JK0

D L 4 1./ cal kyr BP 56G' ,**1

HIJK0D ῌ LMNO%PQLRSOTU

(Hayakawa, +32/) VP!W 

LP$#HI!9:X$%YZ[Y V$Q2P$#Q=\]^HIJK0!9:X$%YZ[Y.

Fig. 2. A sketch of a fallen tree near the summit of
Ogurayama (loc. .). Its location is shown in Fig. +c.

_`abLc OYOdL# HIJK0 2.   . !"#$%&'()*+ ,-. +c /

UTP$p!qrdstuLvQwZdX$TUd

Q DῌefghUT= D ijklm*n.o
U VP!xj!= _`abdyz{|QOT H
IJK0

0
 

E ~k12,!cPQ./ - UT= 

.Q

efgQijI ῏efgL HIJK0CD Q Dῌ>?@hUTqOUtu.}[  3 >?@   +* HIJK0
D Q Dῌ   ++ A.

/῎k12j, P$=HIJK0/ῌ+ 

C 12



 X# 

LOTU%PQ./. != 9:LHIJK0C 12!cPQ.

D/OTU xPQY# 

D

 

P$p!HIJK0

D   D >?@QOT= ijkefgk (Hayakawa, +32/) L^#$T%  #$%A¡ ¢Q Hayakawa (+32/) 

9:;<= 4 3., cal kyr BP 56 ,**2 HIJKA¡ ¢.£¤¥T¦U%A. ++ /0k=§¨©.ªk῏!«:X$ M¬­E 123! 4 0.+ cal kyr BP 567 ,**- ;56G' ,**1 HIJK0²!

C 12®

U¯©ks°.±[ =X *./mm xj³UrῌcU=´ῌI ¯A.µ¶ ·¸

¹!=P$.º , »ijklm*n¼½X$

 !" #$"%&'()*+,-./!01

Fig. 3.

Correlative stratigraphic columns for representative sections.
 3. 

99

Locations of each section are shown in Fig. +a.

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



I

100

J

Fig. +*. Stratigraphic columns for Nakanosawa Ash and Herai Ash at each type locality.
Locations of each section are shown in Fig. +a.
+*.

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

+*  K <;LMNO,-./0;2



PQ#RS2TU2V+32/)& RSNM (Hayakawa,  !"#+a $%

,-./0123& 45672

3 

#WX% ;& YZ5[\]^_`a^b#cde%f& g1hi\'j\#%kl-m++  89:; *.-. km- <=3& DRE (Dense

'n4ompqY& rskl-m\'t

Rock Equivalent) GH9:; *.*2 km- <= (Hayakawa,\uvwv.x& xy <=+32/)+,  `zMO,-.{LMN|e#}<`1&   ῕῎῔῏ῑῐ ῌΐῒ῍I;O,-.~ O,-.;YZ5[]() (,**1) ;& ]fO,-.#rU8`TU ;7

f1{hi& `<kl-m'n4ompqY2rsuvwv.x1hi}MO,-.

D:71 NM`L2N& ;!

;kl-m'n4ompqY1{hi& `<oD: 2" M #$<;Lrsuvwv.x1hi}M O,-.;

O,-.# *

rsuvwv.x1{h`+ 2%&%
 #&   '( ++ 2%



;& )#92

T& kl-m'n4ompqY#hi}+a ;& x¡N&O,-.¤M

&  - ¢+*  `zMO,-.{¥+, & D:C*#hi}£+>#$% LMN+>;& ?@?ABCD:

EF& Houghton

iX7TU)'¦)7<& O,-.;j§et al., ,*** ; Self and Sparks, +312 ; Walker, +32+ 2¨\&% TU1de&

O,-.;©ª\& O,-.;\#%  O,-.;)#92%1& O,-.;¦)«¬1hi& /mm ¯]f#TFTF} -°1±` `zMO,-.{¶· / mm ¯¸'¹º¸C*#hi}£;K<;TFTFX7W½O

;?@?ABCD: <=2V M
,7­® N" %2&RS;

]f2!² PQ2³Ee.´;µ²`++ ;RSN »#c¼& ]f2;» ^1~++ 

3456 7869:;<=>?@AB

Fig. ++. Isopach maps for tephras from the eruptive episode D (upper) and Dῌ (lower). Numerals show
the thickness of tephra in centimeters. A part of thickness data for Oguni Pumice (marked by *) is
quoted from Hayakawa (+32/).
 ++.   Dῌ    (cm)D!" #$%&'() * * +,-."  Hayakawa (+32/) /012

101

102

mn

o

Fig. +,. Photomicrographs of grains in tephra samples. (a) The middle unit of Nakanosawa Ash. It contains
blocky and ﬂaky glass fragments, spongy pumices and fresh dacite blocks. (b) Thin section of the middle unitof Nakanosawa Ash (open nicol). (c) Herai Ash. Most grains are fresh dacite blocks. It contains small
amount of spongy pumices. (d) Thin section of Herai Ash (open nicol). All grains are fresh dacite blocks.
 +,.  (a) !"#$%&!'()*+ ,-*./01.2345 3
678.9:5 ;<=>?@?+A4BCD (b) !"#$%&!'()*+E4 FG1H7)IJK(c) LM$%& N OPQ;<=>?@?+A4RS TUV3678.9:BCD (d) L
M$%&E4 FG1H7)IJK WXYZ;<=>?@?+A4RS

Fig. +-. Relation between the area enclosed by isopachs(S) and the thickness (T), for Nakanosawa Ashand Herai Ash. Broken lines are iso-volume lines
assuming a relation V+,., TS presented by Hayakawa (+32/) (see text in detail).
 +-. !"#$%&[\]LM$%&^?_`*
(S)(T)  
Q Hayakawa (+32/) \ V+,., TS B
abcd efg BhW FijklK

%&'
 

! F( )G>=HI*+J! ,K103 Hayakawa (+32/) () V

*+ "! +33, ; Houghton et al., ,*** ;+,.,TS V : L5MNO! T : ! S :  P

Self and Sparks, +312 ; Walker, +32+ Q# R-! . ST UVW# XYZ/

 +-^12_`a`9G3bc  +,# D0d

 # ῍  Haya-

kawa (+32/)  ,- . # , . P! [*/\0 ]A845e3$6e37)GP8fg+]hC 9 *.+- km bc#

!

 

o"=! ,**- ; Morrissey and Mastin, ,*** R>

-

"#

"/! +32+

! jOkl=m< n%01# 234! 5 6789

0"# :i*! /;! +33/ ; ! +320 ;# p?D ! ?$q5 5 rs +.. gῌcm-!  6789 +.1 gῌcm-! ? 6789

r@At! ! . ]hC !

+.. gῌcm !

, g+#-9 +./ gῌcm W# GB-gῌcm-  !



 ,./

 DRE 

 *.*2 km- #

CDC Euv! E-w xy

/ῌ, 



D῎

 

?$

  ++#    !

z{|}01V~~CF,! *JK

# ! + LM

 Nbc1! OZ! GP! [*#RSbcW# D

D+bTUP

G+ WP! [bD+ &?$'\T # ?$]^! J_%&'%&' D-a +.- km

b ^c +,    +a#L5"#bc1! 0q5 ! 5 !

?$ Hayakawa (+32/) ()

UVW# ?$῍.   +-# VWX YZ + +* cm  / cm?$ 9 *.+0 km- bc#.

, `"#< n%"/! +32+

jOkl=m01# 234! 2++ +./ gῌcm-! +.0 gῌcm-!  9 +.0 gῌcm-  ,./ gῌcm-  !

W# GB5 ! Ebg+# q5 d DRE  *.+ km- #Ke fWZ/



bc# g! ]^Gb +./$, cm h bc# 5 Z/ =Msg+B
g+ + jdKZ/

 +/ cm  Q! Hayakawa (+32/)  ,-.  - GP b  +*# q5 E0y

Bg+! F( )G>=HGbcHIP  ++#Hayakawa (+32/) ! ! :;<=> D῎"#

?$?$

0"#bci! 2+    +*#0 

!

 !

:;<=> D῎?$

! z{bPF(

G , g+  ++! ! kVR P]hC

i! lmdK$KB! BnG

! bcWi+D ! F( )G>=

3E0y2+=Ms ¡op d

HI*! qqr¢sbc# DDb!t#

KZ/

GB£W45g¤x.d#b[*g+! uvP# B"G3w¥q 0 b / cm
# B

y

P! 0z{  , cm p5bc

i? b/¥P / mm p51!

G3~PPW § # . dJK$JKe *Z/$/bc1! K$Ke gZ/

TDPc +.

! :;<=> G pLn#|G¦£}*

@A89B=C= # D+ E!
F(

)G>=H!

 B:;<=> D῎?$ ¨ }*  >=Hb *./ wt.ῌ h +.# F (SiO, ~P©w

5)GcP! =

  +*# . [*#nSK$ªKe ! :;<=>}*«¬8@~! :;<=>

]A8^12_`a`9G3.!

~}* ! ­2+,Q®b

Kk¯°B! nG3E0y±#3d 

! R}*²[* Ug+  +.# +,# .

±#}´µ¶VR>   +# ~Fw¥ / mm p5³$³tm

z·T# ¸-b*[PRS1! **( )G>=HP +* vol.ῌ ¹! ?$ºm  3#E02 2 vol.ῌ b»­P+V! ! :?0d ! ?$q5 E0y5 Z/

.! fW

E0yZ/#

=Msg+Bi (armored lapilli) d#D+45rsr@At!

0"# *BPE;<=> A ~ C 0d ¼! :;<=>bV .$,vol.ῌ h­P +D ! 2+,Q®­b  0# R-! F( )G>=H!
:;<=> D ½BV! GB£



104



Fig. +.. Whole-rock TiO,-SiO, and MnO-SiO, diagrams for the eruptive rocks fromthe post-caldera stage of Towada volcano.
 +.. 

 TiO,-SiO,  MnO-SiO,  !"#$%&  0 '( () *+!

,-"#  +. ./01234567%89:

TiO, ; MnO < =>?@ABC37%  +.D  ++ ! EFAGH8923&I367IJK!LMN(OP Q?%RST

UV$ WI XYZ[\ ]W7%^_? `abUV$  +. J7!LMN(OP Q?%,`cd  +, ! efgRhAijkO(l-U=$%mnmAo+*%JKUV$ ijkK"#o+p GH8923qrs%JKUVt

O(lKXYZuffv,-!"#o+$% wx ijkO(l)y !z{|!Bp /} &EF!G~ r 4567%89:D
*+&ABC37?* )JK23

C37?*JK23 ?K&ijkO(lALMN(O DῌSrs%K+@or!ijkO(l!LMN(O Dῌr|S6?* )UW LMN(O Dῌ37%

7IK$%

4U +/V$

wxijkO(l ~zAxoI
1 Dῌ 

XYZ\ufKijkO(lfv

K5o kO(lw{|U$%K z
:D!wxy23 */* m 6K4567%

A   )* %+B,CD 

105

E FG7@HI#$ ;JKL>MNO8 %+,-P7 + QRSTU
7!VE./0723456 WXL FG7,Y
L> Y  9 ;#$Z[>\]NO8 

+/ C ^_ - `=NO8a#$/ @  9b, WcLEd8e

fN 9 ghi + j7ik8lmW\]
NO8 eON./0123456 7!nB,Z
O> g@ op/q %+,-7!VE $
r8d@BLEsB;td uvw71V>x
ZO8 

+/D

)*

%+y$@ *.,- km- `rn DRE z{y$`@ *.+3 km- 18 (Hayakawa, +32/)  ;<=>  D ῌ ?  &DRE z{`@ *.,3 km 18 &@ *.-3 km-

fN|8

- F } B ~`rV>MNO8
2  3+/  A  

 (+310) Hayakawa (+32/)   (+320)
  (+320) @  A Kg;)*`r8\]> eO@)* %+W& ZOEp>e7!8 LfL 7!n)*
Fig. +/. Schematic diagrams showing the eruption sequence of the eruptive episode Dῌ. (A) Initial
phreatomagmatic eruptions and sedimentation of
the lowermost and lower parts of the Herai Ash.
(B) Initiation of lava e#usion. (C) Growth of the
Ogurayama Lava Dome and intermittent vulcanianeruptions that produced the main part of the Herai
Ash. (D) After the eruption.
 D ῌ    (A) +/.    !"
 # $ (B) %   & ' ( (C) ) *  %+,-./0123456 7!8
 9#$ (D)  :%+@

 Dῌ`B,ZOEd>eWL> L>WVE )*@ A K



g7@1n1d   (+32+)  (+33/)   (+332)@ 

 A K ; g \]>  + `r8 A `@

%&;

VEd1deWL> jn A @0 `(L>e718 uv A `=NO8  g^B@`r8 L>WV
E N\]!7  A K @ g`rV>\]8WP`r8
 g@A  :43D7 8P9"
1 g`r8 C07)# %+& B,
CD$C%WuvLEd>&@1dW 1q!" Dῌ`@'L>P D ῌa`@"PW g7RL}kd`rV>x8 e!1)*fN

ZO8 TULEp>@¢£L 8lmW¤d\]NO8

 ;¡eL>\]NO8 +/A gW

g7(VEd P g`¡puv ¥@ -,* m `rn ¦+§L ¨©ª@,«0¬`rn -daE./1W­]NOEd8 + Wuv!

 Y   ;#$Z[> ®:1¥d¯71V>@  E C 

C07 °W±²L a³ Y   +320 ;  +310 r8d@ ;#$Z[>\]NO8 C C (Hayakawa, +32/) xZOEp> LfL ®

´µ7!VE+/A

0F1W¶1718%+W,-;'(L +/B ®O7V

:7XL>

 B  A @ ·¸NP$¹01º2|NO1d»¼½6 `'(LE



106

   A  !
     

 A "4(56789:);3<"#$ X%"&"'() *+ +2 ,-./*+0123) =>'(5 ?@A)B  C

:5 ;"D5EFG@'H3

 ,**1

N OA":P Q$%8  RA)B  &'()S*T+U 0,3()G

NAV - .' PW A)/LXU 0 YLU" 13Z2N#

4W'-  Kano et al. (+330)

:)4W" 678 79:5Ba: bc(5;d:>gN)? h"?:';B  $%8 RKij"D5 EF@:H I 3ῌ ῎ ῍ ῏

*+"4rst1 @A)
+ u^o_vw  E JKR

3`  C JKR"`xA Hb{|4 u^o_vw}~
 E r C ri (3.,0.+ cal kyrBP) "mGA,  D῍pqm %o   Z

 l

u^osQ" ()3N !)

u^o_vwvt-8uv 
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wy 1./ cal kyr BP  D῍}GA)@A- D῍ !Oz"{ 79: %o s"&4s" sJ
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\ ] o ,2 ,0-ῌ,1-.
Hayakawa, Y. (+32/) Pyroclastic geology of Towada volcano. Bull. Earthq. Res. Inst. 0*, /*1ῌ/3,.
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Horiuchi, K., Sonoda, S., Matsuzaki, H. and Ohyama, M.
(,**1) Radiocarbon analysis of tree rings from a +/./-cal
kyr BP pyroclastically buried forest : a pilot study. Radiocarbon, .3, ++,-ῌ++-,.
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J.S. (,***) Phreatoplinian eruptions. In Encyclopedia ofVolcanoes (Sigurdsson, H. ed.), /+-ῌ/,/, Academic Press.
Hunter, A.G. and Blake, S. (+33/) Petrogenetic evolutionof a transitional tholeiitic - calc-alkaline series : Towada
volcano, Japan. J. Petrol., -0, +/13ῌ0*/.
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Kano, K., Yamamoto, T. and Ono, K. (+330) Subaqueous
eruption and emplacement of the Shinjima Pumice, Shinjima (Moeshima) Island, Kagoshima Bay, SW Japan. J.
Volcanol. Geotherm. Res., 1+, +21ῌ,*0.
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Kuno, H., Ishikawa, T., Katsui, Y., Yagi, K., Yamasaki,    M. and Taneda, S. (+30.) Sorting of pumice and lithic
fragments as a key to eruptive and emplacement mechanism. Japan J. Geol. Geogr., -/, ,,-ῌ,-2.
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Lipman, P.W. (+301) Mineral and chemical variations
within an ash-ﬂow sheet from Aso caldera, SouthwesternJapan. Contrib. Mineral. Petrol., +0, -**ῌ-,1.
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107

Morrissey, M.M. and Mastin, L.G. (,***) Vulcanian eruptions. In Encyclopedia of Volcanoes (Sigurdsson, H. ed.),Academic Press, .0-ῌ.1/.
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Self, S. and Sparks, R.S. J. (+312) Characteristics of widespread pyroclastic deposits formed by the interaction ofsilicic magma and water. Bull. Volcanol., .+, +30ῌ,+,.
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Walker, G.P.L. (+32+) Characteristics of two phreatoplinianashes, and their water-ﬂushed origin. J. Volcanol. Geotherm. Res., 3, -3/ῌ.*1.
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