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,**-  0  .  ,**.  .  / Rhyolite to Dacite Lava Flows Newly-discovered on the Western Slope of Aso Central Cones,
Southwestern Japan
Naoaki MASUDA, Kazunori WATANABE and Yasuo MIYABUCHI
Three lava ﬂows : Nakahono, Nagano, Otogase lava ﬂows and Otogase debris avalanche deposit were newly
discovered on the western slope of Aso central cones, southwestern Japan. The Nakahono lava is biotite rhyolite(SiO,1- wt.ῌ) and is distributed on only a ﬂoor of the Otogasegawa River. Occurrence age of the Nakahono
lava is older than that of ACP- or ACP/. The Nagano lava is pyroxene dacite (SiO,02 wt.ῌ) and comprises
a pyroclastic cone (ca. 0** m diameter). The Otogase lava is pyroxene rhyolite (SiO,1- wt.ῌ) and forms alava plateau (ca. + km long and ,**῍-** m wide). The lava is interbedded between the Aira-Tn ash (AT, ca. ,3cal ka) and the Kikai Akahoya ash (K-Ah, ca. 1.- cal ka). The Otogase debris avalanche deposit is possibly
generated by a collapse of lava ﬂow or dome on the vent of the Otogase lava. Based on the stratigraphic
relationship with tephra deposits, the eruption age was estimated at ca. ,* cal ka. In the period of younger stage
than approximately -* cal ka, intermittent eruptions producing three rhyolite to dacite lava ﬂows provide
important information about eruptive history and magma evolution in the western part of Aso central cones.
Key words : lava, rhyolite, dacite, debris avalanche, tephra+.   <2A 2J=  !"#$%&'() *+,-./0,12

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no.)  !"#$%p>IAPQRqr5
s>tuFvYZwx KL
3{ H O(+32/) 2y3VZ A 9{F  9%*mRQ= (Fig. +). NOT

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2 £¤@¥Q6,9:;<=>I2J V.)

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VRQZMª203῍,,,/ «¬­®¯° +,00
d±Aso Junior High School, Kurokawa +,00, Asomachi,
Kumamoto 203῍,,,/, Japan.
ª20*῍2/// «²°³ ,῍.*῍+«d´dp
Faculty of Education, Kumamoto University, Kurokami,῍.*῍+, Kumamoto 20*῍2///, Japan.

 +32/ A1~©.

ª20*῍*20, «²°³ .῍++῍+0µ¶·¸¹º»¼¹
Kyushu Research Center, Forestry and Forest
Products Research Institute, Kurokami .῍++῍+0,
Kumamoto 20*῍*20,, Japan.
Corresponding author : Naoaki Masudae-mail : msdn_*.@cocoa.ocn.ne.jp

·¸¹:;<>º:v»¼120

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+.

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Fig. +. Geologic map of the study area showing localities of the stratigraphic sections. Base map is referred from
+ : ,/,*** topographic map “Asosan” and “Tateno” published by the Geographical Survey Institute. Contour
interval is +* m.

,,»-ª¤¼UN6d 2q> I:;<;=/

 (+313) 

(+32+)    (+32+)

῍+ 

  W Y$ ,**-4 (X3

121

  W3)*

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'()*

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/0lm$?_` 

 

-ῌ+

Xno 11p 0**I0+* m4 (2q>
36rstu4,- vw ?$! @A$BC* DE FG

'5>  , "6x()^ Z H$ I:;<;=/J

Xno7y

$K L!"M#(Fig. +). 

!#NO" Fig. , 9*b

(Fig. +).

"'(3 (,**-)  cXe  Z
 *bz89{()+ 7y62|** ,- a+-. $(PQ- "" ,**-  , R+/ %%/STU%V&T 1'(3}> / m j) ~Y  kq$( '$}>():!

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DE SiO, ; 1- wt. ῌ 9
(Table +).

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C 1 ,**+4 D]K 
?$! ,- 2q> 7E

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HS-. ¡¢£) ¤4 ^ + km ZI-(
1pZp ¥JZ!?H$ ( p CH$ K$ -ῌ,  ¦3§ 1+3-24  ( ce vK¥L¨ MN© -ª*«^~Y ¥ 11p 0,* m4 Z"-¬¤>!6¯°OP-.®¥B
 ±²²H$

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?! 2­-.Q³!

-.QR´S()  LUIL

6 µTU¶>   12 3
 +32/4 H$
Fig. ,. Diagram showing stratigraphic relationshipsof lava ﬂows and tephra layers distributed on
the western slope of Aso central cones.
Vertical tielines show relationships conﬁrmed at
outcrops. Broken tielines show estimated relationship. All ages (cal ka) in this ﬁgure arecalibrated +.C ages except for Takanoobane lava
(K-Ar age). Thin frame : previous works. Bold
frame : this study. *Matsumoto et al. (+33+)**Miyabuchi et al. (,**- b)·®"M'5

! ^ ¸P ±?!Vy WX¥ (U¶>$WX -.^"! #Y>$ 

0¹! H| c
.vw
.g

-.Q*b ¥e *h*

-.´ 

(Fig. +).

 :  ,./ mm jk6 s 1;+*ῌ4  +., mm 36rs 1; +῍,ῌ4 *.0 mm jk

6-rs 1+ῌ jk4 Z! 1; -ῌ4 º¶·µuxy¸u¹º»¼122  *.- mm

 +  X!"# SiO, $ 02 wt.ῌ %&'&()*+,-. (Table +).   /012345
678

9:;< ?@ABC+,D
39 $ E

39 $ E-ῌ-  





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;678

 -.$@4

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IJ-4678 +D=>>j aB+-43=  ;O῍+4bcdf;=   R ¡ bcdf(Fig. +) ; ++ m ; -.<¢.£  ¤+D ¥; ¦§¨+F 678

GHIJF©9-4 +.1 mm  G /῍1ῌF 9ª«

678KLMNOOP GQR //*S0** m, T
,**S-** mF +P*D UVV+ ¦§®<[   ¦§ G ,.+ mm  :  1῍+*ῌ  +.* mm 

678 9WX
QR //*S0** m 678KYZ R[ +*S,*m \]^_+-43= `_aB;bcd

e-bcdf

G+ῌ F ; *., mm V UV9¬­¨©$Ga

F $ gh i-4

V :  +ῌ9 *.- mm UV : +ῌF =w[ bcdf;$ +D *.*, mm ¯°±§$²Dj9;  jbcdf  Ok; l GmnoF p³´h+P*D j

9Wq 4r

!"# SiO, $ 1- wt.ῌ 

4. -s-

tuv (+3-2) >w

uxy (+32/) z j {;Of|h}~;
[ 4s@. .-  (+313) 3 +  X)*+,-.

(Table +).

{. °D O῍+ 34678 µ

Table +. Whole-rock chemical compositions of lavas and pumice on the western slopeof Aso central cones.

º» =N_ACt@A>?E¼gE> 
-, .).   (Figs.
   

123

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 "uv (clast support) sw6T* (Fig. /) ! "#$% &' ()xyfg+zfguv (matrix support) sw6T

*+,- ./ ; OtP0

3 ",-{D E]"^@.|}!

12

3 "45 6+,- +  ./ : OtP+0  76~I@/0E EXe120E"O"

89 5 6+,- ,  ./ : OtP,0UV ?'@"&A3* = >c"3/ .ML ; 45b3E -=> < ?'@A3  - BC

/6h0  2* cm D 3/ .MP ; 7

3: ;  <

 SiO,  1- wt.ῌ D (Table +), +,-

83 -/6h0  ,/ cm, 9 *.1῍+,-E FG9HIJ KELMGN"OP

+.0 D:%  03 +,- +  E Q"RQQS

A3 fg ;<AE

6TUV ?'@" &':(

WXYZ*A; [\'@3,0 +,-E]"^@UV._/0+,-`&' a&b! c" d

6h +., D%

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qgE .>?E0  rr)p!qL*AE .
0 Y@Asw6@;%3#Xefg&'"hg"UV 6 +,-p!qgE* +,-E 7gD%3 )pE]"^@UV!qL*AE D=/F'@3

123

i$ -,/ jk%&῍. l / m E]UV

?'@ '(m6nN* (Fig. /). oUV c" p!qg+,-Ec" rr)

D F<* -./ mm =/ . 1῍+*ῌ0  +., mm = > . +῍,ῌ0  ,., mm

=> . -ῌ0<. +ῌ0 6z3

fKD  *.+ mm =/>AF *3 ¡;AF"O78
D; f*¢£p!qg"YK";%3
p!qgc" , BC SiO,  1*῍1+ wt.
ῌ D rr)p!qL*AE + BC SiO, 0, wt.ῌ D%A3 EgXeKELMGN&'
p!qgc" +,-E6¤¥  [\'@
3 A E]"^@UV @ + BC SiO,  1, wt.ῌ+,-Er+,- ¦§7¨6T* +,-EZ© Bª %+,-ECD*A* [\'@ (Table +). %&῍. sw6@
E + Bp!qg*C SiO,  1- wt.
ῌ rr)p!qL*A* SiO,  02 wt.ῌ D %&῍+* D78 .+ B0 p!qg*C
SiO,  1- wt.ῌ rr)p!qL*A* SiO, 
0. wt.ῌ D%A (Table +). F< XeKELMG

N&' p!qg* +,-E «P*3E]"^@EFGD%

t@¬­®¯sw.Ui, +32- ; Ui et al., +320 "O0 ° UV 45H± ";IA'"3 A E
²J³rJ´µ@K XeL¶·Y9 7
Fig. -. Photograph of a stratigraphic section at site+. OtP+ : Otogase pumice +, OtP, : Otogase
pumice ,, Kpfa : Kusasenrigahama pumice fall.

="O;'@"3 *&* 78E]"^@UVEF(°D%>@ %n d" ;3¸ . ,¹D?'@ (Figs. +, 0). 4ZM%&῍+* E]n3

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Fig. .. Stratigraphic sections of Nakahono, Nagano, Otogase lavas and Otogase debris avalanche deposit showing
stratigraphic relationships with Sawatsuno lava and tephra deposits. See Fig. + for localities.

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 0 m  

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 !"#$%&'()*+,'1258< +32/> 'a67& V 

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krs®k 8 < = +32, ; P

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 ./?'  V 6^ J"H !

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Fig. 0.

Fig. /. An outcrop of Otogase debris avalanche
deposit at site-..

Photograph of a hummocky hill (approximately - m high, -* m across) at site-++.

·5W%6¸A¹6º»¼&

126

lN<mn (+*YR.ῌ.) ab^O3 ab

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

()

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!"#$0

+./**ῌ+./*, %&'0 +.1,0ῌ+.1-+ 1,- ῌ,K-Ah

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R* +,- * o Beta Analytic p

 +.Cq rsteu (AMS)  ,UV- ./ -ab v +.C

R ++,//*w

BP (Beta-+0/,3,) 1,- 2

R Rx.*v (Stuiver et al., +332 ; Talma and Vogel, +33-) <
 +-./ cal ka  (Table ,).
yz ! {| } lN<~a^5678 9,**-: 0;<=> ?@AB<20C

D- (Fig. 1).

ka: 0S3 9!56 +331: 2 2 2

E FGHI AT 9 ,3 cal ka ; J,**, : ,0ῌ,3 cal ka ; 45678 ,**-:  K-Ah 9 1.-

K +33/ ; J  ,**,: L MN<20

cal ka ;3O10 D3 PQ RS-T

+.

C



 R* UV-
ῌ+
1 cm

FGHI WX YZ [ MP

FGH\ + ]'^ (OtP+)  E0  _`

Aabcdae fgMN<!h^i^jk FGH\ , ]'^ (OtP,) 0lN

R  ++,***

|BP 9xv R +- cal`< F 



CD

- 9 ,**-: FGHI ~a^ , m _` 1  S3- +.C~a^

R

R (+- cal ka) x< 

>0S3 D 2-ab0_ D-!
3
FGHIA"`#fg ATlN< AT  Kpfa L

-m

!hab^

^

 +m 10  #¡¢L L£ ,,***¤-,***<
FGH\ , ]' W_ 9FGHI _` ,.0 m:

¥ D

!heab^ fgs¦A §`¨©ª< OtP ,#¡R AT 

«¦3  #¡ R

0,***¤3,***+1¤,- cal ka ¬­®g20¯ D3

` + mOtP, OtP+ 01 ° ± OtP+R®g ,+ ka 7² +.¤,+ cal ka ¥!heab^

D

DO3¥ < ³¦$ FGHI OtP+#¡ R ,* cal ka ¬­¥ -
Fig. 1. Refractive index of volcanic glass andorthopyroxene (opx) of Kikai Akahoya ash(K-Ah) and Aira Tn ash (AT).

FGHI´µfg ῌ+*Ah ~a^ K¶d O I´Table ,. Result of AMS +.C dating for a buried soil just below a pumice bed
underlying the Otogase lava at site +.

¡L¢/y£¤#$\%P4K<O%<Ww-. K-Ah  

127



F< =>?PQR !"  ῌ. Z (Fig. +) B2 => 6HI 

!" #$% &$ + m '()* +,-./0*

12 ῌ+*  34
5  K-Ah 6789)*/0*. : ; /

%<F< #$BX4>zT<]() @ P 
cA9!?PQRBC 5 &D_( =>T<C{EP4 F
G-=>  /H5

0* ῌ,  12<=>?@AFqT 8T<

5 B2  =>

 F<>zI YB2 0I*>C DEF<GHDEI%<z #<]() ^ TBT ^5X<
F 124P2 D_JKL  (Fig. .).

0IJ754  #<K/. 



L

MNO%< - 3PQR
0. 

S34KT<+( 

K<O%< ¡L¢/y£¤#$\

34  U14V2%P¥?@AMN4<1 8S W2BX< T

Fig. , Q` O¦Z/ ^P§

BT MNY %<!"Z #$[ \%¨ QR STU>V/©X<&5B2 %2'5]() ^

-wW.PQR/©

12 &uv,( *

82 +32/ ; 82 ,**+( => Xª0W* +_T<Q`*,-*5 a*.CQX4 «0 (OtP+) /C¬/0b cT4 TBJ */0 deYT<KL2 8/Z7 ?@

B2./T</0IfT4 0!"B2

AB23 ,* cal ka D{[\% &D_( TBJ 

8S/05]() ^/C]E 9 >zT 

-( =>

T<]() ^=>g1d2 #$h< i3 /** m j

MN=>PQR=>«0O 3 -+12 (Fig. +). =>!"Z`kl

ka STU>V/ &­pv^ ,**- b( QqJ_®

J4^ ^mn =>5o4^ 0,* m 6H

KTD7 3 . cal ka &­pv82 +331( ¡L5 j'p QqD7 rst¢/y£` + «0 &^a +323 ; b¯ ACP+( }~J

 &uv82 +32/(

W./T4 2

B2 =>j'9Z &4^3 0,*m 6H( B2j:Q`#9T<K°X4<T4¡L/03 ,3 cal ka &± ,**,(  AT /0*}² ³cB2´0.µC =X

L 
F<I;< st!"Z#$\%

4KL24< &­pv^ ,**- b( TBT

5<4^ 0+*w0/* m 6H xx#$h<!" DE¡L¢/y£¤#9 W.

I

12 :%</yKL2 F<  8d`«0/

T4<I =6H>zQX4#<]()J Q`¢/y£¤=B2<5 &{_( !"B2|4 =>S4^

 Ww-./ ¶°X4

0+*w0/* m 6H}~KL <`·¸ ¹,/05¡L/0/.( =>

Cº PQReº» ¼fg

. T<Q` ?Q =>hi°T4=> !"  5KL2  0I

12x!"B2 => 5KL21.   

¡L¢/y£¤#$\%P4 K< - 3
 + 3 O%<X!\|B\n K

128 SiO,1- wt.ῌ      

 ACP-  ACP/    

! "#$#%!& '( #)
 #)*+ ,   -.)/ +

0 !0 12324 SiO,

02 wt.ῌ       "0 !
0  SiO,1- wt.ῌ  #$5 %&' 0,*60/* m (7))89 :;<)
=>?9



*

#$+, ,* cal ka -@A   #$.B CD  E-F/"0)DG  H?9IJ12K- km

34 -,/ 56(7LM

N9  7OPMG "$#%!& 8Q123248$##RS9DTNMG N)9?)-S N N)
# 7:;QUVU<W)8 =->?
A
῏

῎

XY X 6@AZAMGB[\C0]D
E[^F_`abcGdHB[_`efgh iIJ
KLjM-M  klNWm nOoP
L+,QAZRp?MG F 
STU VWqKL)XYZL jPr[\st u]^BL =-_. vwf4 Fx ,9 y`AS  Jz{AM |}a?9~b Dῌ ῑ ῐ ῍
2cdB (+32+) "1< 8e_`f // +g_`C_`h ,* p.
 ijk\lm (+3-2) "$#!&8bn
 # . +02῍+2+.
opqr (+33/) st . \ uv$ )DG w +x/"0 yz_` -. +-/῍+.3.
{|}B\]~K (,**+) NWl+ (+2+0)"  u N\ P1
7 # .0 +.1῍+0-.
X \K (+310) H# Tn # 7O),   .0 --3῍-.1.
X \K (+312) cG1<?9#$DH:;< # yz_` +1
+.-῍+0-.

X \K (,**-) [#4<¡), | ¢$£ --0 p.
X!\|B (,**-) "#$#%!& ¤ D ¤ G 8£
£ no. +,, +3.
B\¥¦l\§\|B (+33+) "# K-Ar +,Q# *) )#J ¨ #£ +33+ +g©£ª"`\ 1-.
+.
C +,n K\|B (+331) #«¬?9­®M"#:;< #$+, # .,.*-῍.*2.
n K\|B\ $ (,**- a) "$#%!&7OE/"0),9 +.C+, # .2 ,,3῍,-..
n K \ VWqK \ 'Xq¡ \ |B \ ¢d(,**- b) "#j¯°± 3 %+£L/"0 # .2 +3/῍,+..
¤ &k (+313) "1<' 8e_`f /- +g_`C_`h ,0 p.
()*\¥¦§+ (+311)   2²³ ´fµª«¨©-9r#$5¤ Ge_` y ,  ª«¬­ .2 +῍1.
 $ (,**-) , F ¶® klNW¯°Shy ,*3 \ klst£ --0 p.
· ± (,**,) cG²7 - %+:;< +,e_` yz_` .+ ,,/῍,-0.
§\|B (+32/) "#8 / %
+ #8 . 8°S³
Stuiver, M., Reimer, P. J., Bard, E., Beck, J. W., Burr, G.
S., Hughen, K. A., Kromer, B., McCormac, G., van derPlicht, J. and Spurk, M. (+332) INTCAL 32 radiocarbon
age calibration, ,.,***῍* cal BP. Radiocarbon, .*, +*.+῍+*2-.
'Xq¡ (+323) "$#%!& :;< k£­ 3* 2῍++.
Talma, A. S. and Vogel, J. C. (+33-) A simpliﬁed approachto calibration +.C dates. Radiocarbon, -/, -+1῍-,,.
Ui, T. (+32-) Volcanic dry avalanche deposits identiﬁcation and comparison with nonvolcanic debris stream
deposits. J. Volcanol. Geotherm. Res., +2, +-/῍+/*.
Ui, T., Kawachi, S. and Neall, V. E. (+320) Fragmentationof debris avalanche material during ﬂowage evidence
from the Pungarehu Formation, Mount Egmont, New
Zealand. J. Volcanol. Geotherm. Res., ,1, ,//῍,0..
|B (,**+) "# C -¸8¹
ºv B n;´t)NW"µ 1 B n
 ,.+ p.
|B\§\2cdB (+32,) ¶m. ·#
 ##¤ G # ,1 --1῍--2.
XB\¸ *\iIJK (+331) "»/¼
r, | «¬ C+,)¹C yz£ªº\ ,1 +/.῍+//.
[\»¼ u]^B