ῒΐ  /.  (,**3)
 0  ,/-῍,0, ῕ ῔
 .,1** 
 


 ,

]

e

-^

,**3 L . M - NOP ,**3 L +* M +- NO

Newly Found ..1 ka Pyroclastic Flow Deposits on the Northwestern
Foot of Tokachi-dake Volcano, Central Hokkaido, Japan andReexamination of the Eruptive Activity During Holocene
῍῍Shinya FUJIWARA῍,῍῍, Yoshihiro ISHIZUKA῍῍῍, Toshitsugu YAMAZAKI῍
and Mitsuhiro NAKAGAWA῍
Tokachi-dake volcano restarted eruptive activity at the Ground crater in middle Holocene after longdormancy of nearly +* ky. The activity at the crater (Stage +) was the most explosive and voluminous one in the
volcano during Holocene. We newly found a pyroclastic ﬂow deposit beneath already recognized pyroclastic ﬂow
deposit (Gﬂ-+ : Ground Crater pyroclastic ﬂow deposit +) of the Stage-I, intercalated with lahar deposits and thinsoils. The deposit is composed of gravel-sized blackish blocks, pumices, banded pumices and altered blocks withyellowish to reddish brown colored ﬁne sand matrix. Progressive thermal demagnetization experiments for theblackish blocks and the pumices show that the direction of the remnant magnetization of the deposits is close tothat of the geocentric axial dipole and did not change up to blocking temperature of about /2* , indicating thatthese blocks and pumices are essential magmatic materials. Thus, the deposit can be considered as pyroclastic
ﬂow one. However, this is characterized not only by the heterogeneous distribution of these essential materials
in the deposit but also by heterogeneous matrix. These features and abundant accidental altered blocks suggestthat generation of the pyroclastic ﬂow might be related with sector collapse of the volcanic ediﬁce. This isconsistent with the topographic feature of the Ground crater. The petrography and the whole-rock chemicalcompositions of these essential materials are similar to those of the Stage ῌ. This suggests that the newly foundpyroclastic ﬂow must be related to the initial eruptive activity of the Stage ῌ. New +.C ages for the deposit are
in ..1 ka, indicating that the activity of the Stage ῌ began much earlier than previously recognized age (-.- ka).
Finding of the pyroclstic ﬂow deposit, named as Gﬂ-*, suggests that the Stage ῌ had continued and been derived
from the same magma system for more than +.** years. However, considering a long interval between Gﬂ-* andGﬂ-+, the Stage ῌ might be divided into two sub-stages, Stage ῌ-* and Stage ῌ-+.
Key words : Tokachi-dake volcano, pyroclastic ﬂow, sector collapse, thermal demagnetization experiments+ῌ ῐ ῎ ῑ ῏

QRS -STUVWSX/YZ[T\ ]^ _`abcdefQ -.;g NE-SW hiXjV+31+ WSg-klmWngopQ abcgqr

st abdeugv;Xwxys defgop

kz{|Z[}s (Fig. +) ~kQ*l -,-**

*0*ῌ*2+2  +*  2  !"#$%&'(/0Department of Earth and Planetary System Sciences,
Graduate School of Science, Hokkaido University,
N+* W2 Kita-ku, Sapporo *0*ῌ*2+*, Japan.
=7> +2-ῌ**/1 ,?@A-BCD ,ῌ,.ῌ+ E
FG2HIJK
Present address, Kokusai Kogyo Co., ,ῌ,.ῌ+, Harumi-

chou, Fuchu +2-ῌ**/1, Japan.
 -*/ῌ2/01 )*+, +  +ῌ+ )*+-. 1
1234567 89:;<Institute of Geology and Geoinformation, Geological
Survey of Japan, AIST, Central 1, Higashi +ῌ+ῌ+,
Tsukuba, Ibaraki -*/ῌ2/01, Japan.
Corresponding author : Mitsuhiro Nakagawa
e-mail : mnakagawa@mail.sci.hokudai.ac.jp

4 56789:;<=>?@A

254

Fig. ,. Location of craters formed on the northwestern
ﬂank of Tokachi-dake during Holocene.
Fig. +. (a) Index maps of Tokachi-dake Volcano. (b)Map showing the area studied in this paper. Location of Ground Crater is also shown. (c) The
localities (A and B) where pyroclastic ﬂow deposits
derived from the Ground Crater have been found.
The topographicmap (+ : ,/,***) by the Geographical Survey Institute is used.

BCDEFGGH . IJ KL% M>/NOI*+P,- .QRI
(Stage ῌ)-,-** ()STU% TV

WXYZE[\ T12 !ZB3]\% T%M ! ^\_`1aB3
]\%&'Zb)Lc K)&L) W

 ()d>efKg1Z)hiT/jk ^ l mZd\% &mdp )L1Z Mm6qWrZ M

W 'mdn\o&\c4   ,**1 ; 6?
(,**1)-,-**  +31+ ; st +30- 4 . 

Luv\% W d'w/xc&'()Stage ῌ'yz)L% 1 {|}~() &

J \% &>/ NOI*+P,- .QRI

md/ N_`( W1 Z]\%)(1Uc 4   ,**1

&'Z)(1U% \(\N   :

012' +*** &'

hi*+P,- . QW 

0** m -,-**   !"#$%

 .' M

d\% M NOI*+P,- .I\ VLl.\%QUc,** m  ,  .ZGZ]w%  /(Fig. ,) &&'()*+,- ./ 012' , 3

 M !"#r!p J KLc"ZxcZ M 

\ NOI*+P,- .QRIv

)(/ 1(U% #/ s/ E  -,-**, 



 +* cm CD¥¦§¨{©ª/«¬­ %&s :®/ :¢+"#Kgc1` 1 $³W´µ1FIZ + )Ej%^-,-**  ()I*U% 4 ()I¡$KLc¢+ (Tk  ,) £¤¯c'yz)L%°'¦ :±({ ²K/ cmZb)L% (Tk-+) \(\1Z) M¶·p 1¸+/x 4 ¹( ,**1º,

*+P,- .Qy-¯c' *+P,- .Q,**1 &I/s

B .QR\W 'mdn\c M\¶· . dm6qW¼½ M\

RI (Stage ῌ)  »1cZ/xc  .,1** 



255

Fig. -. Photos of the locality A. (a) The whole sequence can be observed at site A. We newly foundunknown deposit beneath Sm (Shirogane mud ﬂow deposit) which can be divided into three layers, A+,
A, and A- in ascending order. Note the captured deformed fragments of A, in A- (arrows) and theirregular boundary between A, and A-. There are little blocks in the A- at this site. (b) Photo ofanother site about +* m south of locality A. A lot of blocks are concentrated in A-. Abbreviations :Gﬂ-+, Ground Crater pyroclastic ﬂow + ; Gﬂ-,, Ground Crater pyroclastic ﬂow ,. (c) The degassed
pipe structure (dashed lines) observed at site B.

-ῌ ῔ῑ῕῎ῒῐ῍῏ΐ! "#$%# &' () *+, -  A, B (Fig. +)  ./012

34 5**6789

:5;<=>? @=&  A H +,*./ m  B H 3-* m

A -'BCD-( 4EFG IJKLM$#'B 1*ῌ NOPQM' O?48RS (,**1) T9./012:UVPQW9 IJKLM$#'B N> $#' , (Gﬂ-,) X>./012:

B (YZ[ +** cm \\ & (A]%# + (Gﬂ-+ ; -,-** cal yBP) X>? ^!_@`%#Ma9! "b(c#W9$#@$a d

e&f%g& (Sm) 8h8>?

Ki/jkVlm &@no$#'B*pq^!_r@e&fpqa9!'s8 Sm T

a9 PQ(YZ[ ,* cm %#`%#@$& t>?pqa9@ XO?48G A- A,((^ UVPQ($#'BPQ!uMvw&

A+ ra9 (Figs. -, .) N>( A @"x X

yV@za h8$#'B\PQ({M"#$%8|}@~# ' )&ft *Z$#'B+A - r(r 2*῍+/* cm

M{Mn,(('B48-E!.a9 &!fm 6)-'BCD-@ /0;rh8(H MM1!H^ (Fig. -c) &4

\('s8^ {M(F`%#`&!8 $#'B PQTDUVPQ({M2u%#@$a9! YO]%#@$a9 {MM34 567 m /IF¢7£¤¥( O?CD>?^* ;' 

4&f^9¦w!6^ § h8!3¨©ª)X89(3«¬8 ­'^3*pqa9 (Fig. -a, b) M' (; <=: /* volῌk@¡f3

"#$#>?®-' "#$%# ¯°±

  M&®-'M'²³ ]#M' W ´& >?8

9 (]#%#@$& !m ) 5

Wt YZ[( -* cm W

9­' (/%#@$a9µ& O?X8

V@za

r48(¶@A !ya9

(A `%#]%#@$&f^9 ª >?{A , r( A - r>!_B@epq& C&

M;' (`%# O?X8(`%#]%mM&"%#·¸¹M{Mn,(¬9m

#ºD 0f N> !EF&f^9 ­'  "#YZ[ / cm $"#%#M' @» r( ,῍ $#>?®-',* cm W9 O? A- r(¼GV½&ft H ]#M'()f^9 !"#$%&'(256

Fig. ..

)

Correlative columns of locality A and B.

A-  *+,-./0123/ (Fig. -a)

A+ 

A, 

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

Q4RSTRUTVW XWYZ;[\2]^ _4 5.678 + : ,9:;<=4>?@

ABCDE FGHII0@JKLM@NOP' X `1abc;de4

fTfT g2]hiWj kHl ,* cm mn ;  _Tjo pqr  stuv  w

x y

H}hr

0* cm z{;3/ iWj | . {0 -* cm ;iWj w2

~6  

Fe-Ti v;3/ _Tjo+ vol. ῌ ?st26./ st _Tjo2. 

WvWq6./ A- -* vol. ῌ p2

m-./_Tjo;l,/ vol. ῌ ;3/ stkH; -.2 mm ;3/ p n  ¡¢E1£jI¤E1a¥¦2§¨ ©6. Qtª04 £jI¤E  ®¯;°W_Tjop

/r 2±h  st ²;l k«¬ ­³ + ´ µ¶· ¸h¹ + ¸ 8º»H; +./ mm ;3/ ²







¼½

5678 9: % ;.#.,1**</=> ?@ A567/B:C/D/EFG

257

HIJKLMNOPL QRSTUVW2X2/ῌ ῎ῑ῏῍ῐῒ

9Y,VZ[9 +.0 mm 9PL \]^_I`

A- a%bc#LdefghijkT%l! mn#LO/OopqL Fe-Ti rs/tAuvVX29



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

P Y,VZ[9 *.2 mm 9PL wxTUV

yz{|}%i ~ / /* Y, ,*῍

W2X29 Y,VZ[9 *./ mm 9PL T-* cm   ! "# $%&'()*VdefgI UA hyalopilitic /^_!

 kTVe/xhyalopilitic 9PL !  "# /* Magnetic Measurements+, ,/ mm -. ,, mm /012*34

defgV SiO,/-.1῍//.3 wt. ῌ 9PL kTV

A ,G ~

SiO,/1..῍/3 wt. ῌ 9PL (Table +) /s!  Vy!  ¡w¢9V A- a%bc#LdefgAkTV}

N£¤¥/$9 c¦W§s (NRM) x¨y=©2ª« (Stage ῌ) /¬fs­ª®At¯

! °/± +**῍-** c9 +** ²³ -**῍0/* c´!h °#µ¶/D« (Stage ῍-῎) AV

9 /* ²³9mn

%¹9ºL (Fig. /)

! Zijderveld ¢Table +.

Whole-rock chemical compositions of rocks in A- layer.

 "#

/·¸%l

sª/+»¼A½¼

s : scoriaous block, p : pumice.

Fig. /. Whole-rock chemical compositions of scoriaous blocks (solid square) and pumices (open square) in
A- layer. Small open and solid circles show the products erupted in Stage I and II-IV (Fujiwara et al.,,**1), respectively.

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



258

  0

! " # $ % & ' " (  ) * + ' A )*+' A- ),-./0 *1

Zijderveld 23)456780

9:;. >? @ (MAD) 9 +/AB"C 0
K'L#KM $ ++ NOPQR . S"NOPQR5; ,

9:;. V"("NOPQRD +3] ,+.

< A- 2"=),-./0" +.C   DE!F!FT%

GHI (IAA) )J" E& (AMS) #+'  (D'U) K  ' WῌX Y Z [ῌW * \    

+.

C D

:;.NOPQR"6^ +Libby ",_ //02  #+'`a (" d +-C - bC."

#+'/c0

C."

+C

/ +

"NOPQRD%&' 0/* -305;.'+/"3bC

bC

K%
9

Khi/ (Table ,) Fig. 0 )678pRqY" Zijderveld K%&'"()S+'"+CC   (Kirschvink, +32*)

 bC

"t80

9:;<u)6 -xyz{>"|5;}:~?

1d$23 

efD ,s 3]/  #$"4g3)D Intcal*..*.c (Reimer etal., ,**.) "jklk)mno OxCal ver.-.+* (Ramsey,+33/, ,**+) r#  #$ Table - )s%A- ),-.'+0D v / cm =w +1
cm 3$0)0'a " +.C D .,+2* @? * A? 0..0 ] 's "-* yBP (IAAA-1++*0) 3] - A- B6=#$ +CD C"0"9o7 D *./ cm ^ D + mm AB "09,-E4sw8+ E bC

DFE4 B4M)1./ "0" +.C D -,..* .* yBP (IAAA-+0"9;. V"GE4D@? ,0.2

/+./*) s .; , "-DKHI

A? 03.2 (a3/ +*) K8 ~?K+-

73]/ A- ),-./0 (.,+2* -* yBP)s +C"g34D .,2.* .,0+* cal. yBP (,s) )JK A-)S+'D Fig. 0 "NOPQR E- "L);./16) +C. (RT-,**) 3

D .,1** '"MN;Khi;./

D0"_O9Pg784)]/ Ei' +C"0E4"9+KD 3hi+

1 

C

D7 0

3D8o Q7)5w.

1ῌ+

A+A-

RSt80 (VRM) 3]/TS9b+Khi;.

A+

A- D+.U cm A2"VKV.

/ E bC



)S+'"2"#$D ¡ 

¢PY£ ¤p¥¦"mT5;8/mT§¨"©W

pRqY9 Xª3 «¬[k (/2*) A2^ - 8+ "K5;­YZN;

"bC['+Ks®'+/

K\3M/A- )DAB)s% bC¯]3N;K^w./ . "°±9aw./ %8^ (+) mT"²
H92_³´`²a</ (,) µb5Scd
Table ,. Characteristic directions obtained from progressive thermal demagnetization. The average directions of each sample are also shown.

¶·9:;./ (-) 09,-./ (.) µb 8¶  "{) e M f ¸¹¥q© W 9  :;./
(+) D b+CSN;9Xº)6)+2_ 5;bCW0)1' ` ²0Ks®%/(,) DbC"»¼»T gT 9MaKs®%/ - (-) Dg9½C1b+C3N;
Ks% (.) " e M f ¸¹¥qD hZ (hotlahar) "168Pg7+C"N;3:;./K

Table -.

Results of AMS +.C dating for charcoals of Tokachi-dake Volcano.

 .,1**   !"#$%259

Fig. 0. (a) Orthogonal plot of typical progressive thermal demagnetization data. Solid and open circles show
horizontal projection and north-south vertical projections, respectively. Broken lines and chain lines
mean the characteristic lower and higher temperature directions, respectively. Variations of normalized
intensity are also shown. (b) and (c) show equal-area projection of characteristic directions of lower andhigher temperature components, respectively. Open square and oval show the mean direction and 3/ῌ
conﬁdence limit, respectively. The diamond shows the geocentric axial dipole direction (Dec.*, Inc.
0..0) around Tokachi-dake volcano.

&'() * &*+,-.(/012- 345/6789:;<=(>?@&AB

CD.(EF. GHIJKLMNOPQP RSTUF.>V@ '(EF. *W(,-.( . A- 3X45/6789:;

YIZ&>V@[\]^IZ /2* 8_`HIabcd>ef

ghiQ>jkglXZ)'m,-.( noEF. A- 3X45/

.(E& A- 3)E./;pq[\]^6 :; rJ:;&*++sW h A- 3&

IZ8_`HIJK WYOELMOPQ( A,-(E) h(ABCDEFGH260"" (a) A- IJ.KKLJ-MN OP
A, !NVW-

 (b) JJ9LJ-;  Y9MN !"#.Z[& (a) 1Y;e(. A- 9Of 3I

12Q@9LJ3RS"TU;N 
. Gfl-+   ,**1  X"&\3$]^%S_`2&abc%'. Gfl-+ dgh'i (Stage ῌ)  4%')* 3 + < ,- j . 31[&OP A ,  kl.9#$% "&

 '()*+ -,-**, -.

" A , 9 A - m&3RnNMY/3o091p

qr0i 4.s2&abc%'3ac4

"tu>?@1Y (b) 1Y;JJL2" Q@v@.w5-l 963;[& ,**1  A+A- J7. x8C9K:J%* y.;3!
NVWz<=a>9?< J 49'LJ3R

". Stage ῌ{|i

>?

9@

n;[g"@;  1Y;.C9

A-M  1[&} . A- B3

C~3DYa;z@92f  EF +33+ * (Gﬂ-*) GHf  Gfl-* 9.IJ; Ui et al., ,*** 3 Xf  "";9 A- .

9@ gK;N.L-M 9 KCa >?

`2CM 1*ῌ N9JJ-M

O a

U-MN IJ3JJ OO Gfl-+ `2 Gfl-, 1N.P-M

T"a> 9dy@   A- .a;

Gfl-* 94 U P V a  c 4 ; W  Stagez@31 >? 1N  f;

ῌ3w-l .? Gfl-*  Gfl-+  +,.** 31 . !"-MN XRS;

 (Neuendorf et al., ,**/) f 9 \-.@A>? %] A + K A , 3.

J>?@Ba>e'\@  @. -",,. Stage ῌr0i,, Y +** 1NZ  Q- ,,>[+,***"& Stage ῌf " .,1**^;a>KIJJ.8¡_¢ _ ;RS3`J"

Q79< +*, km- RSf 9@A-MY

4Uf r0 Stage ῌ- * -,-**/ Gfl-* /U

f r0 Stage ῌ- + GHf  (,**1) .'()aCb- 

£f¤¥.cg;  "9[&@.

 + (Gﬂ-+) 3de\@ ¦§ (Tk-,) O"y.Q@3¨ S-"678

P3 Tk-+ ¦§4" Q¦§. © - mm Oa>fghª«t]¬iOjkJX\@ N A +  A - "%l;N J.cg@¢mn­ >?

-. y¢6789" Q3DY@  Q

®4; \p .,,**.,.** yBP 3 ¯">?@  Q67. Tk-+ ,obC"&  

83. mn­ D[Q79M  A-,**1 /012 Tk-+ . Stage ῌ- * 3

.°Ja>3q± e("&a;z@3 X

de-l Q79M(Fig. 1) /"1Y3 "2f  \3.rst

 Gfl-*  435"&6789"&

3²"uvhw" (Fig. ,) C9C~£³f 

Tk-+ 

 A -  

-.C 9C ~"& "@; 

 p .,1**4"



D[Q79 1ῌ,

9

9678´lµ¶tu '()mBx .Z ·piA- 

z¹-{ A- |b}>9ºK~ - .,2.*
.,0+* cal. yBP (,s) £" p .,1**?@  A- NP3A

4"mn­/ 4>h'"/¡y-y< r0i9²¸"963;[ h'"r
0-M

Stage ῌ. µ¶31N - »i3w-l y¢ Gfl-* 9  " p .,1**/ Gfl-+ 012

 + (Gﬂ-+)  !"Gfl-, 9" -,-**/ Q"&

Y|b}>. ºK~

a4"i-M

 Y .,1**

9{@&-p -,-** cal. yBP  ,**1 N A- 

. Gfl-+ :;< +,.**= >?@ / -,-**/ 934-MN Q@v@-C9C~
D[&

>?@  ¡ <h43.

"9[& A+A- " . 

. , »9S"h"&0N (Fig. ,)

h'i (Stage ῌ) .5r0"&w5f¼l

.4 i9 , ½M[4

"@; 

-M  f;<h43 -"& .,1**"";9 A- JJ.Oa U 0

1Nt9 Q"& --**

/

/ 3

-.rt9 .,1** 

  !

261

Fig. 1. Schematic columns showing the correlation between (+) northwestern foot (column (+)), near the
Ground crater (column (,)) and eastern foot (column (-)). Revised sequence of eruptive deposits of
the Stage I is summarized as column (.). * Fujiwara et al. (,**1) ; ** this study.

"#$%&'()'*+ *+ ,- . /& 012
34(56789.:)

2ῌ ῎ ῍ ῏;09<=>?.
HIJKLM * (Gﬂ-*) . 
NOP.QRST& UVW X
1`a /2* bcP$>%d.<9/Sklm &/N)nbopnq*
/x0` (N;01y>2m )nNHIJKLM"#~(?6.qE789>:.
<0`   <= > .,1**  l&HIJKLM"#~ (Stage ῌ) g~(?$%&'  Stage ῌNYA (,**1) $%1`~~` ,  (  B q  ?>m/S4&l=mΐ ῒ
e&"E)NC.<9 D.<99 ;E"  ¡F¢£¤!¥G¦§1`)¨N"6©ª H*«#012¬q­#.®NI¯JICK012_)°±§0L=m )*+²'NQ³´µM¶*+·;nz«#²¸
.<k :e&½O*+ e&;ῐ ῕ ῔ ῑ@ABCD (+330)  EF G,,3 p.
YZ[\] [^!B_"# (,**1) e&f'
;gh -,-** i(j; /, ,/-ῌ,1+.
rsOtuv+w, (+30-) / -. + nz3  012{4| e&}5 .1p.
[u!;  rsY ; (+31+) ;n(j 012?@e&? +-0p.
Kirschvink, J.L. (+32*) The least-squares line and plane
and the analysis of paleomagnetic data. Geophys. J. Roy.
Astron. Soc., 0,, 033ῌ1+3.
Neuendorf, K.K.E., Mehl, J.P. Jr. and Jackson, A. J. (,**/)
Glossary of Geology, Fifth Edition. American Geological
Institute, 113p.
Raimer P. J. et al. (,**.) IntCal*. Terrestrial radiocarbon
age calibration, ,0ῌ* ka BP. Radiocarbon, .0, +*,3ῌ+*/2.
Ramsey, B.C. (+33/) Radiocarbon calibration and analysisof stratigraphy : the OxCal program. Radiocarbon, -1,.,/ῌ.-*.
Ramsey, B.C. (,**+) Development of the radiocarbon
program OxCal. Radiocarbon, .-, -//ῌ-0-.
NG¹º (+33+) q» 4¼ G<;

262







  +2 +/-ῌ+02.
Ui, T., Takarada, S. and Yoshimoto, M. (,***) Debris
avalanche. In Encyclopedia of Volcanoes (Sigurdsson, H.,

Houghton, B., McNutt, S.R., Rymer, H. and Stix, J.
eds.), 0+1ῌ0,0, Academic Press, San Diego.
!"#$ %&'(