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38 9 : ;,**3 . - / 3 012 ,**3 . 0 / ,. 01!Eruption History of Oakan Volcano, Eastern Hokkaido, Japan
Jun-ichi TAMADAῌ,ῌῌ and Mitsuhiro NAKAGAWAῌ
Oakan volcano is one of the post-caldera volcanoes of Akan caldera, and its eruptive history has not yet beenclariﬁed well. In order to reveal the structure of volcanic ediﬁce and eruptive history with possible age data, we
carry out not only geological survey but also tephrochronological study around the volcano. We identify +*tephra units, nine of which are wide-spread tephras from other volcanoes in Hokkaido and Baitoushan volcano.
Only one tehpra unit (Oafa) from Oakan volcano has been recognized, but the other nine tephras can be used as
good time markers for understanding the activity of the Oakan volcano.
The volcanic activity can be divided into two main stages : the early stage (E stage) and late one (L stage).
Although most of the ediﬁce of the E stage is covered by eruptive materials of the L stage, several lava ﬂows are
distributed on the southern ﬂank. After the formation of the ediﬁce, sector collapse of its southern part occurredand formed a debris avalanche deposit on the ﬂank. The ediﬁce and the debris avalanche deposit of the E stage
is covered by Meakan tephra (NaPS : ca. +.- ka), which indicates that the activity of the E stage had terminatedbefore +.- ka. After a certain period of dormancy, the activity of the L stage started, which can be divided into
two sub-stages : L-+ and L-,. During the L-+ stage, lava e#used from four crater areas, and the Futatsudake cone
was formed at one of these areas. Based on petrological features of these lavas, the L-+ stage could be divided
into L-+-+ and L-+-, groups. Oafa tephra layer recognized at the ﬂank was derived from the cone (L-+-,) judging
from its isopach and isopleth maps, and also from its petrological features. The tephra layer is sandwitchedbetween Ma-f (ca. 0.0 ka) and Ta-c, (ca. ,./ ka) tephras. Considering thickness of soils between Oafa and these
two layers, we estimate that eruption age of Oafa is about / ka. Although Oafa can be correlated with the activityof L-+-, group, it seems that the activity of L-stage began around / ka, because there existed no obvious timeinterval between L-+-+ and L-+-, groups. Thus, the dormancy period between the early and late stages can be
estimated to be about 2*** years. In the L-, stage, eruption centers moved northeast to construct the summit(Oakandake) pyroclastic cone, in which four craters were formed. At the same time, lava ﬂows repeatedlye#used from the cone to widely cover the north to southeast ﬂank. Based on the location of eruption centers andthe time sequence of these lava ﬂows, the activity of L-, stage can be divided into four groups : L-,-+ to L-,-..
The youngest crater at the summit cone of L-, stage was formed before + ka, because it is covered by Ma-b tephra(ca. +.* ka). This suggests that the latest magmatic activity of L-, stage occurred before + ka. Although noeruptive activity was recorded, weak fumaloe activity at the north crater on the mid ﬂank was reported. Our
study reveals frequent eruptions of Oakan volcano during Holocene, and suggests that the volcano must be
considered as an active volcano.
Key words : Oakan volcano, Akan caldera, tephrochronology, eruption history, active volcano

 +*  2 
*0*ῌ*2+* ! "#$%&'Department of Natural History Sciences, Graduate
School of Science, Hokkaido University, N+* W2
Kitaku, Sapporo *0*ῌ*2+*, Japan.
()* //*ῌ**+- + ,- +  --ῌ,*
Mont Bell Co. Ltd., Shinmachi +, Nishiku, Osaka//*ῌ**+-, Japan.
Corresponding author : Mitsuhiro Nakagawa
e-mail : mnakagawa@mail.sci.hokudai.ac.jp 

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 !"#$%154
Table +.

Modal compositions of phenocrystic minerals for representative samples of each stage.

Abbreviations. Pl : plagioclase ; Opx : orthopyroxene ; Cpx : clinopyroxene ; Ol : olivine ; Qz : quartz ;Hb : hornblende ; Opq : opaque mineral&' ()*+,-./0 &@>AB .

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>EJKL)MNOPQ>/ , mm )LR

)@RT)OPU,V W XP

HE +..* cm   CD +* /E89 +, cm ) SYZ[OPQ:;<= f >? V\)&  ]WE^_ `^aghi j)0kl/^a mn/0 CD +/ /)>AE / cm

opqgrstg0 )Wn

/89 - cm ) S:;<= b >? Pl

E^_ ^a `^a uv,v mn

/Ewx Pyz + Op{/E|56} .*/0

cm I8)A~7

bcdefRE

(/)

f>

(3)

>E CD +* />A +* cm )KL[OPQb>

>E KL)MNOPQ>/ RT)OP:;" 7OPQ}r~ 

U,V= ^_ `^a ^a)]W>AEr/

 CD +. /)>AE . cm / 8 +ῌ, cm g[&' )s Fig. / /E:;")>}

)@OPU,V ^_`^a ^a LLg

}KL7 89 , cm ) S:;<= a >?

OPQ:;)89 / cm ) S:;<= e >

(+*)? >E!2)def"8r

>Es #CD +. +0(0)a>L)MNOPQ>/ RTiET)OPU,V= ^_ `^a ^a)]W >E>:"vre>

>EV\bcdefXP)& MNV\
bc56/0 xwl >AEPles CD +. /)>AEFG +./ cm / - cm )>:g

4&@ +/ cm IN& -* cm Ai 
V\bc)HEPydefPl$/FG +./ cm

1 

/0 CD / /)>AE +* cm / 89 1 cm ) S

1ῌ+ 

:;<= b >? Pl/Ew ¡)%

!2%E¢&XP£¤¥¤¦/k'/8):;)56§)@CD defPyz , - Op

0g def) ¡) ¨)©{/rs V\bcEj)0(ª«/gi W)E®®¯ @(1))")g¬ (Table +) ­¡©) E-+  L-+-+ %def°p± Z (TDA) )¢&XP²E W)g®®¯ i &YY

d>

>E

 

LOPQ>/ ³OPU,V&W´µg0 I¶)%def TDA )

XP&£¤¥¤¦²E W= ·+¸¹ºCD +/ /E>A + cm IN/ *»¼ghiv

W/EaW=&YYW+@ ½. / 0123-4

56 L-+ 7 89 TDA :3;<  !<KLM

>?@ABCDEFGH AIJ*N*J,OP Q(RS$RT$RT

U7V7W:3XANAY8

(+)

=

155

9 TDA W:3

$Z[\]^_`(-) RS$ abcb$ RT$ RT$ dJIef $g=h iKj = +,.*,3.-ῌ
 L-+-+ 7:389XANAY<

 W:3 

=[h iRS$ abcb$ RT$ RT$k<"#dJIefKl mMc=[no$p

1./,-..ῌ KL hv i=w? (Table +)

$=`cM

qr

=[RS$  )*+,-

 

sMtMhu L-+-+ 7M xac x'( yz -.* mm, {|}$~ =<{(|}

RT$RT$Z[\]^_`h mMc$~[ sM

,-

?`cM abcb$ )*+*.--.0ῌ KLM yz *.2 mm xacx= AIJ*N*J,OP  RS$    

1ῌ, 

RT$+....-ῌ KLM yz ,.* mm 

Table , <?fp,- Fig. 0 < RT$x

*.1,.*ῌ KLM yz +./ mm x῍x  RT$< LMmh  mMc
T$  RS$ =  L@**P

K

dJIef )*+,-i sM%U  7*.+*.2ῌ *.+ mm  /,./῍//.*῍/0.+ wt. ῌ h,-! P<
SiO,  //ῌ ac 0.῍ $g )*+,- *./ῌ %&  yz ,./ mm

0/ῌ XANAY,-L h,-!P 

P L-+-+@**q<r<KLM

!"#$  *., mm %&'(  )*+,- *.+ῌ %& SiO, K U7 vh E-+ 7 SiO,

/0./ wt.ῌ

 =<dJIefmh  (01  //.*῍0/.. wt.ῌ $~"£P U7¤ fp,-¥¦§ ui E-+ 7h L-+-+ 7$¨UHAIJ*N*J,OP Q(RS$R

7$h¥i <W:3#©

MgO <T$RT$Z[\]^_`$j Al,O- <% mh &<ª' « ¬c< E-(,)= >?@ABCDEFG

 U7$ ¡¢?,-

SiO, 

v

U77W:3XANAY8, 7$  u SiO,  vh ¨U7

9 TDA :3XANAY

$h  ¥i SiO,  0* wt. ῌ

RS$ RT$ RT$ abcb$ 89

 Na,O, MnO 89

TiO, <$j K,O <%  L L-+-+ 7$dJIef=h iKj $g=`cM

SiO,/3 wt.ῌmh  =

¨U7$hª' «-*.,.+.2ῌ h ­ (+) 



1ῌ- 

=[RS$  )*+,-

 TiO, 89 Na, O <%

mh 

 

,*.--+.2ῌ KL

E®_23^_,-  e ¯ :3 g ¯ :M xac x'( yz -.* mm, {|}

3XANAY() sM%¯ XANA {(|}hY  () ?±²i  `cM RT$

0.2ῌ KLM yz +.. mm  x-.0 

 RT$

 (Fig. 1) m°

XANAY()?23^_ ¯+<%&
5?  r  `cM fp,-

RT$<LMmh  mMcT$ @**q<8i d ¯

RS$ =  L =<
dJIefmh  abcb$ )*+,*.,ῌ %&KLM yz *.0 mm  xac¨ »?RT$.¼½`cM

 dJIef mm (

 )*+,-

 $g*./  +.3ῌ *.+)*+,-LMmh  yz *.2 mm*.,ῌ %&K P $~ @**q ¡¢?*f³´P L e, g ¯%,..0.*ῌ KLM yz +.2 mm x῍ x  



g¯

l¶

¨h & <ª '

 ()

¯89 j ¯ª ' µ ,CaO <%

K,O <$« i ¯· K,O  P¶ r¸¹cM a ¯ b ¯ f
SiO, ¨

-º

i

 a ¯h f ¯¨ , ¯h¥i  K,O <$l

156
Table ,.

Whole-rock major elements composition of representative samples.

Note. FeO* : total iron as FeO. TDA represents debris blocks in Takiguchi debris avalanche.
The rocks of TDA are derived from both E-+ and E-, stages.

2   i !"

2ῌ+ 

#$%&'()*+,- i-,

#$%&(+) .)/01+234'.5 + (NaPf-ῌ) i-+#$%&'67890:;<=>?@AB.)CDEFG'H (NaPS)  IJ e !

KLMNBO !PQRS34TU .)/01+

Me-b 34VWF XYZ[\.)

MCDJF AB]^_`.)/0  f !"ab6789 c, (Ta-c,) b ! Z

1+cJ[\lm c, (Ko-c,) a ! ab6789 a (Ta-a) defgM hi  j ,**3 ; kno ,**- ; po +323 ;   qrs ;  j

34VWF tiu(  d !" v,**+ ; wixo +332 ycJ )z{|}~)MABO)ῌ

 (B-Tm) 34VWF`-JF / cm ui.)/01+MnFJ4JF - (+30/) " NBOJF Oa-b 

TiO,-K,O MsOJ (Fig. 2) I g !

EF i-+ !  .)/01+2CD- M 3 !" u(.)MABOEO"¡]JF I3¢c£MsO!P

j (+31+) 

NB-EO"¤¥VWF ` (+30/) "¦§JF Oa-a ¨©ª«¬.)  !PORS_^ i !}­®.)¯°CDJF j !"©ª89±²³!34VWO´U^W ­®.)¯°³῍(Kp῍) h !"«¬89±² f (Ma-f) IJ! c !"«¬6789! b (Ma-b) 34VWF ·¸" `O¹º» ¥¼^W` V^ ½

(Table -, Fig. 3)

 -!µ"µ` E ¶+ 



157

Fig. 0. Harker diagrams of whole-rock major elements compositions of lava samples from Oakan volcano.
Localities of sampling points for analyzed samples are shown in Fig. ..

 (Fig. +*)  g  ! "#$%&'() g *
Oafa "12+, -. Oafa /0

3456*78#9 1,*** :; Ma-f*< 9 ,,/** :; Ta-c, <='> ?'
@AB Oafa C56DE*F 56
GHIJ9 1***,/** :;C#KLML
Oafa "9 /,*** :;NO.#$PL#Q
2ῌ, Oafa ῌ῍ῐ῎ῌ῏ῑ
Oafa DRS"T#"UV L-+-, W

NO. ! X#$YZ[ +Fig. 1. Selected Harker diagrams for chemical compositions of volcanic glass for tephra layers in
Fig. /.

X\>

]$ (Fig. +*) ^_J\> Oafa " L-+-, W`aE'bcdJe " Oafa J E-, W
NO.*<f ?^> L-, WNO.)#" Oafa J
gh'UFijk#$ E Oafa li-+ m4nopqrst*uv Oa-a @ABF

#"UV|

}!#$%&'(,)

owkd4*xWyzNO.a{_h RS ,cm () Oafa ~4.l2w*h_ *l Oafa l2w*a{
FeOῌMgO-K,O # Oafa " L-+-+ l"

+, -. 

i#E'_@ABf g " #Q E Oafa " L-+-, Wlp L-, Wlm

Bq# ! JL#Q '#?

)J FeOῌMgO p K,O #"J'>U@AB#$ g "

L-+-, Wl¡¢^> (Fig. ++) _;£^_

 #¤RU+, -.#$¥ ?D¦§¨RS"

Oafa ©ª«.¬­=®F L-+-, Wlm#$



158

  Oafa  L-+-, 56!7859!"#$%&'()*+,$-./01234!59 (Fig. +,):;<=>56 E-, ῏?@ABCDEF

2ῌ- ΐ῍῏῎ῐῌῒ῎ῑ
(+) GH%&'!῎IJ KL*+# NaPS (ca. +- ka) )M#N4!,$ <=῎IJOPQ R-P0ST!3./U>?@A<PQVW +-,*** .<@XYN!Z[\%&']^?X (Fig. /, Table -) PQ_`$# ,<=>abcdN<efgh59NENijk56 L-+ =>./* lmn@X Oafa !opq#
L-+-,  ῍ /,*** .<=>!r῔st$# L-++ dN "῏ῐ L-+-, ῏!uῌvw

x9Eyhi$#ENi ῒz{x9Ec{{|}~3!Z[N  L-+-+ i L-+ =>῍ /,*** .<=>!x
9{NEN56 44! L-+ ῏N;# Ta-c, (ca. ,./ ka) 0 )M# "   Ma-f (ca. 0.0 ka) hi$#E,4!!NEN L-, * ῏ῐ()*+,$ !Z[$# L-,. ῏ Ta-c, )M#?X v +0  L-, 
Fig. 2. TiO, versus K,O diagram for volcanic glass ofthe tephra layers in Fig. /. Compositional ranges
of well-known tephras are also shown. Most layers
except for Oafa can be correlated with the tephrafrom distal and Me-akan volcanoes. Data sourcesof each tephra layers from other volcanoes are as
follows ; Ta-a and Ko-c, after Furukawa et al.
(+330) and Yoshimoto and Ui (+332), Ma-b after
Wada et al. (,**+) and Kishimoto et al. (,**3),
Ta-c, and Ma-f after and Kishimoto et al. (,**3),
B-Tm after Machida and Arai (,**2) and Wada etal. (,**+), Me-b after Wada (unpublished data),
and KpIV after Hasegawa and Nakagawa (,**1).
Compositional ﬁelds of Nakamachineshiri somma
stage tephra from Meakan volcano is drawn basedon glass compositions of proximal samples of NaPS.
Tephra names and their source volcanoes are shownin Fig. 3.

Table -.

῏῍ ,/** .<:5ῐ!N[5P

 + -C-C Ma-b ]^ gh59
 "#@Xe%&'hi$#EN  P-C=>῏ῐ
6 "=> Ma-b ῍ +,*** .<
ῑ!N[(,) -

4#:5,$῎IJO=> e@:!i$#
<=>5 +-,*** .<:5 ¡῏ῐ@`K-Pw`q# E-+ ,¢$¢£¤¥¦§¨©ª©ῐ " E-, ῏/uq#«¬­¬®©ª©=>0¯X° ΐ3῏ ±w²,$12q#<=> ³´~P!µ¶·¸vF!Z[$# ¹ºwP
Q0»2 ΐ3῏¼½,$12!4

Result of correlation of tephra layers and their ages.

 



159

  L-+-+      !""#$%& '() ++ , Oafa   -.*'(/0 12 '( *345  . 67 89 :; L+ 7 <= >+ <=?@ ( *ABC L, 7

   DBE

FGH;

>45+ -

 I ,,/** 6%> *AIJKLM+ N OP

Q +,*** 6R6ISTPUV5TW 7
 X I Oafa

 X Q Y Z .  0./ km- DRE(Dense Rock Equivalent) P[XQ5T

\3+I +I`] ^ 2** m 9_  *L<=Z5T ab +30/ ; cd

+33. % Xe7I *3 DBE(

*A ]+fe.DC gT5 9h+
IiHBC^j klm2.GWPUV5TW
2ῌ. ῔῍ῐ῎ῌ῏ῒῌΐῑ( <= nI %&o<=7p.q -q+r;_+rW s 67P7 <= " 2,*** " tu7vwxWgPQn+rW
L 7+I -yz+I /0ST.G

WgP{O I|}~+  -;PUV5TW  :+ FH /XS Cf/,B 

IZ5T G g B 

 nI

/S W M  f




( <=!

,FH /X
QV.{!

;.

(  " +33. ; cd +33. ; " +322 ; " +331 ;   +310 PI
Fig. 3. Results of correlation of tephra layers. Tephra
names and their ages are also shown. Seven tephra
layers were derived from the volcanoes in Hokkaido, which are also shown in the ﬁgure. B-Tm
tephra was from Baitoushan volcano located at theboundary of China and North Korea. Data sourcesof eruption ages of each layer are listed in Table -.

O

WQ

67 I^ +,+** m, XI ..1 km +rW ]+ 8 , TPQ G« ¬T&  . 
w



'(



7<=+I -yzI

>45#DB +./ km

Hy -yz = 4;_Qn+rW
<=I 45}%+
{!





SiO,-K,O £+<=7+Q¤¥$:¦§¡¢ o}¨©PW ;.67P7+I 2 *** " tu7r;Q44ª5&  %I 4;B
+rW : SiO,-MgO £+[WP 67DBE7

67<= 2,*** RDB­tu7IS®
7<=?@ L-+ 7+I E ,(+I<=7

B;. -yz=S  TT+

IG&T-

+rW % 



8'3

*AST  /,***  6P

Oafa  - L-+ 7¯7I 4·5·¸(¹<=¯7

E + DBE L +-+ +^ Mg <=.  I° Mg  <=xW±p²³Z5TW gT5 ´µ¶ n º »¼½G.I)*++&W

160

Fig. +*. Isopach map (a) and isopleth map (b) of the Oafa tephra. Numbers are thickness (cm) of tephra layer(a) and maximum grain size (cm) of juvenile materials (b).

Fig. ++. FeO*/MgO-K,O diagrams for the rocks of the
L stage (L-+-+,L-+-, and L-,) and Oafa. Compositional variation of Oafa tephra is the same asthat of L-+-, rocks.

Fig. +,. Summary of eruptive history of Oakan volcano. Time markers by tephra are shown. Tephra
names are the same as those in Fig. 3. Squarebox shows lava ﬂows and debris avalanche deposit.
Hexagonal shows tephra layer.

3ῌ ῎ ῍ ῏ !
23+-456#78'9
+) >?+ J">?"K">?"

, "#$%&' ()*#+ 2 *** ,-./0"1:;<9 ='>?+@ABCD EF GHI
DLMN#%O&P956QR$STUVW!+  ,) *+ , -.,  

5 +.- 

6789:;<=>?$
-)5**, /,***

%+ @&'AG:7HI' L-+ *H J5@&9:7I' L-, *:N'.)@RSG;TUV4 (Oafa) , L-

+ *5.Y' L-+-, *B

.Y7 JB ,))E"#UV4 %6^FG
aN- /,***-)

 +,***HKLN'b.,G9.

,P& Q'(c.,)R d:/kYlZ6k.Y'/) *%6^5*., J^*1m4m

n MgO b JZJ5  MgO 
hi6{ZG,(:|74Z Z~<H - { Nj.!Y '<H- ,dkmL[6'.Y'H7.~'῏῎hi:qkY7 _`UC!\]DH<
r=s t>?u %6^IJ@v,M$],@!OS:?~y C:,-{Z? CF!?,F!FG}{  QI'OS|qNZ?~ V:{Z
GdHumwxmD:IZ? 
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