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,**3 f . g - hij ,**3 f 0 g ,. hi-!

A Newly Found Pyroclastic Flow Deposit around +* ka
at Nikko Nantai Volcano, Northeast Japan
Yasuyuki MIYAKEῌ, Miyuki SAITOῌῌ, Yoshihiro TAKESHITAῌῌῌ,Teruki OIKAWAῌῌῌῌ and Takeshi SAITOῌῌῌῌῌ
Nantai Volcano is a symmetrical stratovolcano, situated in the southern part of the Northeast Japan arc.
Many geologic studies hitherto have suggested that the stratovolcano was formed during the Main stage, and theoverlying pyroclastic materials and a lava ﬂow were formed in the Later stage. Because no sedimentary gap isfound between any deposits of the Later stage, it is inferred that all of the activity in the Later stage took place
successively around +, ky BP (+/ῌ+. cal ka BP) and went dormant until now. However, we found a pyroclastic
ﬂow deposit named Bentengawara Pyroclastic Flow Deposit (BPFD) at the northeastern ﬂank of the Nantai
volcano about , km from the summit crater. This deposit overlies an 2* cm thick deposit of weathered ashy
sediments that in turn overlies the Arasawa Pumice Flow Deposit, a member of the Later stage.
The lower half of the BPFD consists of volcanic lapilli and ash that is remarkably ﬁne-depleted while theupper half contains abundant scoria of mainly lapilli-block sized clasts. The deposit also includes a small numberof breadcrust blocks and occasional accessory lava blocks and fragments of charred wood. The breadcrust blocks
consist of a dense outer crust that is signiﬁcantly fractured and a vesiculated interior. It is noteworthy that theedges of the cracks are sharp and never rounded, suggesting that the vaporization of the inner magma that
produced these cracks took place just before or immediately following the settlement of the blocks.
Paleomagnetic data from three breadcrust block samples indicate that the magnetic vectors of high temperaturecomponents are aligned with our present-day poles. Two pieces of charred wood were measured for their +.C ages
with results of +,ῌ++ cal ka BP. The whole rock chemistry of scoria and breadcrust blocks are determined to besigniﬁcantly di#erent from any of the rocks of the Later stage, but the accessory block in the BPFD has the similar
chemistry to the Osawa Lava, the last product of the Later stage.
We therefore suggest that the BPFD was deposited after the Later stage with a short ("- ka) dormant period
between them. Since the age is possibly around +* ka, the Nantai volcano should be counted as active volcano
based on the deﬁnition provided by the Meteorological Agency of Japan.
Key words : Nantai volcano, pyroclastic ﬂow, breadcrust block, active volcano
#-3*ῌ20,+ $%&' -ῌ+ῌ+- ./01 1
Department of Geology, Faculty of Science, Shinshu
University, Asahi -ῌ+ῌ+ Matsumoto, -3*ῌ20,+, Japan.
#+*0ῌ**-, 234567%8 + 9: 2ῌ1;<=> ??@AB?@ACDEF
am/pm Japan, Roppongi +ῌ2ῌ1, Tokyo, +*0ῌ**-,, Japan.
#-2+ῌ.+*. GHIGH&JKLM -.**
GH&NOPQRQ JK/0STOPQ
Togakushi Museum of Natural History, -.** Tochiwara,
Togakushi, Nagano -2+ῌ.+*., Japan.
 #-*/ῌ2/01 UVW&2 +ῌ+ῌ+XY 1 Z[\]^_`/0ab.cde()*+,
National Institute of Advanced Industrial Scienceand Technology (AIST), GSJ, Institute of Geology
and Geoinformation +ῌ+ῌ+ Higashi, Tsukuba, -*/ῌ
2/01, Japan.
 #-3*ῌ20,+ $%&' -ῌ+ῌ+
International Young Researchers Empowerment Center,
Shinshu University, Asahi -ῌ+ῌ+ Matsumoto,-3*ῌ20,+,
Japan.
Corresponding author : Yasuyuki Miyake
e-mail : ymiyake@shinshu-u.ac.jp

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

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#/

 

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(+3/1) $#2&(PM) QRAMST)AI*A+*AMU,2-./IVW* XY0 (+313)$) 12*A21#-./ 0I+, ka BP (23Z4O+/ῌ+. cal ka BP : Appendix + 2 +. C 356M7

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+, ka BP (+/ῌ+. cal ka BP)

Fig. +. Index map. Bentengawara is situated along
the junction of Nantai volcano and the older
Omanago volcano. The topographic map isdrawn using the software Kashmir -D based on
the digital data by Geographical Survey Institute(GSI) Digital Map +* m Grid (Elevation).
The square shows the extent of Fig. ,.

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?@ABCDE7FG: ?@A
BCHIABCJK  +33- ; > +33. ABCLMNOP Q  +33.
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Fig. ,. Locality map. The topographic map is
adapted from the + : ,/,*** quadrangle map
Nikkohokubu by GSI.

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Columnar sections.

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+

167

Fig. .. (a) Occurrences of the Bentengawara Pyroclastic Flow Deposit (BPFD) and other deposits at Loc. ,.
(b) Ditto at Loc. +. (c) The bottom of the BPFD. The arrow shows the surge layer which is overlain by
the lower part of the BPFD. The lower part is depleted in materials ﬁner than silt. (d) The top of the
BPFD (the broken line). The arrow shows the charred wood. (e) Fragments of scoria from the BPFD.
Their shape is characterized by the aggregation of small lumps. (f) A breadcrust block in the BPFD. The
edges of the cracks on the surface are very sharp and never rounded, suggesting the formation of cracks, andtherefore the expansion of the block resulted from internal vaporization took place just before or immediately
following the settlement of this block.

*+,-./0123456. 7

168

89 :;<=>?@A : BCDE@FGHI;JK
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no: :  +.. mm pqrCD
Fig. /. An equal-area projection showing directions of
the high-demagnetization-level component. Solidcircles denote the 3/ῌ conﬁdence range of the
projections on the lower hemisphere. The mean
direction (open square ; Dec+*.. , Inc/+./ )and its 3/ῌ conﬁdence circle (dashed circle ; a3/-,./ ) are also shown. The direction of the present
Earth’s magnetic ﬁeld is also shown (star ; Dec
1.-- , Inc/*.-- ).

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qrCDez:;{@{ ?[@9 :;<=>?@ YZZ  +./ mm qrqre o +.+mm |reA : BCDE@FGHI;JK
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n }~

: :  ,./ mm  1

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@FO

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{ ?[@9 VW;<=>?@

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YZZ[\]^%bcN 88}~@cNO +, vol.ῌ h;ij k : -- vol.ῌ

 o|re +.+ mm e:Vlm : [ YZ
Z
: :  ,./ mm pCDeA : BCDE@FGHI;JK
L@ MNOPQRSTSRUVW;X=/ῌ,stuCj<O  *.3 mm&'t^? ( )*;qrCDe [+ =DM8 ¡^¢£¤?$£¤'; *.2 mm qrCDe z!"##$% X &'() (PHILIPS PW,.**):;{@{ ?[@9 :;<;,ON¥¦D '^-(§. , g ?¨©ª=>?@j YZZ  +.- mm qr«¬­®¯° . g ;±lcN²cDJKL³S´8{qre YZZ@[??

ONµ¶cD ¢/£¤·&¸¹º»¼0½§ !

+  "#$%&'( $)'( *+,-!./01 2"# (+330) 34

169

Table +. Whole rock chemistry. Fe,O-*total iron
as Fe,O-%  5( $%6& 7'$%&'(()89:$)'%;%*$&'( <=>?
@:  V A Cr +'%B?@C9DEF *
GH6& Rh IEJ=EK,*L-.6&
L$M /!N *+A010O
2" (,**+) 1P34%  QR Table+ A Fig. 0 ST UE23VW?@X4 

7Y5A657%MZ[\%

SiO, % /3.20+.0 wt.ῌ 48$

] *

+'

%:I>^ SiO, % //./ wt.ῌ A9'   :(  ;_ <`X4$=ab (,**.)
cde'% Fig. 0 S'( =ab (,**.) cf ;_ g> - ]ChiJ5A
 ? + jklm:I>^An0@Q:I>^ ?
, oAplmq=ArBCDEq= *'%? -sB@g  tuv? , A? - ChiJ *w <C<xZc yz{EC9|E$}i~iF
GHA

=ab ,**. * w $ Fig. 0

*AI*JS'( tFGj'(KL "g=4$# AtA
H  ῌ M6&}i~iF % :I>^
AUE23VW?@*GJW?=

(Fig.

0a, c, d, e) * **S'%  ῍ TiO, A
K,O }i~iF

%KL

"g=

4 ;_ ? - ChiJ4N'% "GO5 %(   

῎ / Cr }i~iFG' ;_ ? ,

PQ"ARST$5U'

ChiJ4NT  (' >@gQ :( t, 7'H#]!

H( VW7I>^UE23VW?@H  %? - ChiJ

] NikN-*+ X'( 4$ST ῏ MgO }i~iFGKL

3  (Fig. .d) NikN-*, JZ['%

":I>^AUE23VW?@? , ChiJ

=EFA? - ChiJ=EF H
 (T.^JW?=
£GtA$5A$\

YP 

cm Y



cm]VW NikN-*+ U9% NikN-*, _`a X¡E¢iA KL "UE23%N (  ¤EJh ¥^h~>¥b

VW?@A:I>^<C<xZc%c\(d

e$N(¦ CO, 'C9§¨|=¢i©?=$f<C<ghT Aª« 'H' *<C<&' ]VW$N( VW¬B?@C9EFi

;_ <`

X4Gj'%

AA«  / >@gQ4sB@g ? - C
hiJ A ± 6&²m'% ­ IAEA C+ ()7$%N( 5( ®7 d +-C $¯k°l



N

]$-`

'( VWQR$ Table , ³T Table ,  ´* IntCal*. (Reimer et al., ,**.) ¯µ JW

0ῌ ῐ ῍ ῏ ῎

C9¶ CALIB/.*+ (Stuiver and Reimer, +33-) $)'"5, JZ'%



cm Y¸=$ Loc. + 7n : NikN-*+ A

Loc. , (NikN-*,) H(AMS) c

, I ¹º'%» o ¼ %!

p½q&]VW$N( '( ] ·%³T NikN-*+  +*,-.*-*yr BP, NikN-*,  310*  +0* yr BP d

c* +  (BP)     NikN-*+  +,,,0,῍+,,-11 cal yr BP, NikN-*,  +*,0/2῍++,1+/ cal yr BP  

170



Fig. 0. Whole-rock chemistry. The data of the rocks other than the BPFD are after Ishizaki and Kureyama(,**.). All the results are reculculated on the basis that the total+**ῌ without water. See the text for
descriptions of the lines in the ﬁgures.

Table ,.

Age determinations.

 !"#$%&'()" *+ ,-./012345
6-78#9:;<=>?@?A-BCDEFG

1 



H# (Fig. .c) I<- JKLMN*=O9 P

1ῌ+ 

QRS*+.RTUVWXY-Z()[\I=+]2^_`abcdeDEF. fUg,.hijklmn

iopqrfUg,-N*=O9 (Fig. . f) @sj

tuvKwxXyz)"O9j<{|eDEF}< e8~.t(+9:;<= OUDEF.

$%&'+  %()%( * +330 +,-./
3 $-0%4&1567'3

171 !"/0&12 + (BP) #   ( )8&*+ 9:';<!=,> !/0 123456#/#9@5!6A-.

*+ =&12 +,,.**῍+,,-** cal yr BP, NikN-*, =&12

 3 B5!6A/ 6 

++,1**῍+*,0** cal yr BP -



-!

"#689:D;E /FG - !"/0-?.0 NikN!" C7H< !# !"H NikN-*+ I#J#=K>?@LMN:D;EO#;PQRA5 F

cm BCDE0 (Fig. .d) NikN-*, 021"

F/ F ST= 2UVWXFYG<C-

ZZH[!IJ\K 0- DI\K3E

( F/L 6# :  - !":D;->M]0H[^=_!56#/`E] !"a [=)  2NO/ 6^# H; %(bEPQR 2cde ST >(UfgV=.5

DWchF/=.>%($eDW

F/ =a ^F/0. /a iXY ,**, j



(

&

6#

- 2*+>Z;E=kF/ !lm[H<no02p\]=^_3

NO=` 

29:/ a `%(a% ++,1**῍+*,0**1ῌ, 

cal yr BP 1"U#p\]0."9 d/>/ bc

bcqn%(cdrsWt/

qn%( d8 uV (+/῍+. cal ka BP) / 7SDve'&#6H`=f5wxyky BC)]&#6-[/gh3= 3 FF/  zi3/ji-

21"0U#/#9p\]0836# 3

3{e|I=k}~l-[!#0-.F// .!%(m> %(mnI`6 F =Ho|1567'3)p =4F/=q43!"ar-.p\]=;#F/# 6# SiO,

stu#6v!9wx=.

/0 wt. ῌ rsWtyz{
0* wt.ῌ

SiO,

F SiO,=Z!#|6  !"#/z{  d8 %S|#60,>6&" W  (,**.) =^_ | ;!L { hO WiX (,**2) }%2b%(!l$

1 ka cal BP 1"0U#~DeD

W 6# bcqn%(0c>6 d8 Y0-%uV= 6&" bcqn%(
19%

Z!^/0 , km d-%w/ { 1" #SS(0| /#9156p\-.

2| !%|=d:  bcqn%(rsWt/

1ῌ-

F/0.F/=

!5-   

 7SDve' d8 #k|bcqn%(j/FGbcqn#-

WX¡/0_!56# 3  bcqn%(5lN=,> 6#¢- %(!l%  N0L`-!#(70£56#!#F/ 7

/SJ=cd 27Se'. ¤¥ &#6:-!#

1¦yz{  §/S yz{

75xojm¨© O¢,+/+ vol.ῌ ; W ,**. /H< F 

6#p\]0.

 F%(!L F/2

YbF/=h3 bcqn

Y %-.H ª«O^ %( d8 0/5["/¬¡3

7=H ­ 7/!562®S=¯Z

0 d8 ar/3§/S¢chF/

 d8 (+/῍+. cal ka BP) 1"0¥ #u

!56# (Figs. +, ,) - %(=£¤Z #7/!56# ¦/ 6 §%V¨-./`  FF/bcqn%(

S%(0©-°ª=¤Z !5±/*²W/]«D%³P´¢=) /  (Yamamoto et al., ,**/)

hF//¬­5-.

=µ¶  `F/® ·=.5p\]¸¯>bcqn%(cd
~DIJ°±5 !5
eDW ¨0.
bcqn%(#62 %(=b$²"¹

6[!3=^ arºB#6ghrd 2³>~D 0/`  jm!#M -." `h-Yb£»¼´½9

, -.

172
Appendix +.

/ 012 34Radiocarbon data by Akutsu (+313).

5

We calculated the calendar age.

67 89:;<=>"?@A@BC(,**0) " D EFGH @A@$D6I

GJK @A@ HI)7 L@A@

M FGNO @A@$PHI) Q M

" SiO, R)I FG$D"#$%SHI '/00* wt.ῌ !+I)

'@A@$89:;<= H L>T(UV&&'H @A@$W>X IY7Z H >I (0[ \])^$*+%& _`a :UVdef.DgLh i 'jb ,&-."@A@/0c,&7>H1H)

23klDm4e" L

"56$7) 6 +nopnq7rs HIe

8>tX IM W>9uI :(T #vs

SiO,$;Y&'HI)7 'w<" 23klDm4

(+303) $x=HI)7rs

r>6? @AB

SiO,

!" Walker

Ey%& z 89:;<=>X
m4>X

L&("D

IFDT(G , km 23klb40

HI M)NO?"TU

P+H,&Q(e" CODDDm4 R
b&7 {|}~ :;<= (Aramaki, +30-)

I)7>)V7 zHIJKS]L9HI
LsH Dm4 ]L )IRS6 )

V6$Y 'EW$Dm

4P+KX Y>HI)%&7 XM :def2ῌ ῎ ῍ ῏.D Z&IDm4$&h7L>" +31- [ , (+) \]BDX^ M)I )7 _

COD`aXDm4$&H j 89D(+/῍+. cal ka BP) P+K?7Tb4P+K

"$cH >d e +31- X#T(+f

g[

%&7

4P+K&H 23klDm4P+K>klH:def.Dj JDmK>HI@mn+$ %&78" opD +322῍+323 Dj d(,)

2* cm hDiO Zj XDm

23klDm4P+K0" fine deplete H

DY>DT(X7 g"DYD

%&IM DX@A@FGLq )¡ ¢ 89:;<=+RZ)7£KI

89:;<=rAW¤&s¥<¦$§t%plastic bomb, vesiculated bomb, jointed block X#$ %& $ '&(")*&+ SiO,


/-῍/. wt.ῌ(Yamagishi and Freeby, +33.)

L8 zHI23klDm4{CJKS|
@-6

hXne (Fig. .e) >&I)X)L>¨ :;<=©uªv[$«¬w0­p'®sL> xyWRZ)7L>X#T( ¯Dm4>HI}~H L>$%&7(-)

23klDm4JK">89

89:;<= (Fig. . f) RZ)7L>" D:;<= L&("?@A@BC°±9²

#@A@BCLq ) ³´Yµ ¶)I"

SHI)7 HTH L&(µ" _ 56 ) HI)7b

K)*&>+?HX) ?v nK>HI _

23klDm4>- e

 ·zTU  UW$Rb&I)7  !Dm4¸$x=%& 8>HI¹

I23klDm4P+K" _ r+z p

Dm4$_`(&7 Miyabuchi et al. (,**0)>X%&7r&E

Dm4"eD">@ºSJWRZM )*&+

+ LDm4eD""#$$%
¼$@½7 LL>X#T( Miyabuchi et al.

(.)

23klDm4P+K

W +.C [ }H

Rb&7 , »xy[ "G +,῍++ cal ka BP >f}%& $ '&r)V6+©%&I)7   +  ῏῎   !!"!#))*+*+,,--.(/0123
4567/89:;<= ))*+0>=:F  !!9G :;H<IJ(<K=
R:SR: XRF ?=:TU@VABI%;2T: WX!C YI D ZI E[F
I\UR]R<^GH2_`FaIbc:  B. Zimmer IJd`FaIbc: eKf:g:hLM9Kijῌ ῑ ῐ ῍kl@N (+3/1) mOnPNop5q  r Q
(s(! -- +ῌ++.
kl@N (+313) , ('t(u  v% Kw 9 +U 9 + V  v /ῌ,3.
$`6 (+30,) oa(7b#(~9Gc 
k!!d(!c +* +ῌ13.
Aramaki, S. (+30-) Geology of Asama volcano. Journal of
the Fac. Sci., Univ. Tokyo., Sec. , +., +ῌ.0.
i]j (+31-) lO  +31-  , - & 
m9:n    +2 13ῌ2..
i]jn3Y (+330)   $o(!p Yq
 ,,3ῌ,,3.
Y3Yt2 (,**0)   Mu&v
z{ !|Zt}FDpE !"d(!*+ .+ +,-ῌ+/*.
E[Ft (,**.)   +, ka& t E (! -- +31ῌ,*1.
E[F?1@2 (,**2)   ¤*\c©tBCª«5¬ +.C - ('!H°±² +2*p.
Kirschvink, J.L. (+32*) The least-squares line and plane
and the analysis of paleomagnetic data. Geophys. J. Roy.
Astron. Soc., 0,, 033ῌ1+3.
0 (,**/) z  9 - o 0)µ¶·65 0-/p.
rB¬$E $`6 (,**-) $c ¸!!voH --0p.
Miyabuchi, Y., Watanabe,K. and Egawa, Y. (,**0) Bombrich basaltic pyroclastic ﬂow deposit from Nakadake,
Aso Volcano, southwestern Japan. Jour. Volcanol. Geothermal Res., +//, 3*ῌ+*-.
@VABV ¡¹¢ (+330) ¡¢E£ºZ¨ X ?£»¼¤¥$
E¦5§½Zt¨©?ª  !!"!#

 

173

 -+ +*/ῌ++1.
 (+33,) 
$%"#$
& % '&!!('(!*+. +2 /3ῌ3+.
?1@2A3 456 (,**,) 7BCDE 8
L MN - >O& % ('!PQ +*2
22ῌ+*,.
Reimer, P. J., Baillie, 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., McCormac, F.G.,
Manning, S.W., 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, ,0ῌ* ka BP.
Radiocarbon, .0, +*,3ῌ+*/2.
xRy (+33-)  zS% {|}Tt
WX YZ /  #[(!*+\]^_~*+ d{|eftHg{= -h : i]j /-ῌ/2.
x  Ry (+33.) E!  (s +0,,,ῌ,-*.
Stuiver, M. and Reimer, P. J. (+33-) Extended +.C databaseand revised CALIB radiocarbon calibration program.
Radiocarbon, -/, ,+/ῌ,-*.
r s[t (+32*)  u
& twxy&v  , 1/ῌ21.
~R (+33-) 7or3W"#$Q (!PQ +*, 1-ῌ3*.
~RA3 416 (+33.) "#$f\:& %  (s +0 ,+/ῌ,,+.
@VAB (,**+) ¡¢E£Z¥$E¦5§`¨ X ? YZ ++ +,(!*+\/V­*+ (C) (,) *+Z®.¯ ³´ ++0.*..1 ,-ῌ-/.
Walker, G.P.L. (+303) The breaking of magma. Geol. Mag.,+*0, +00ῌ+1-.
Yamagishi, H. and Feebrey, C. (+33.) Ballistic ejecta fromthe +322ῌ+323 andesitic Vulcanian eruptions of Tokachidake volcano, Japan : morphological features and genesis. Jour. Volcanol. Geothermal Res., /3, ,03ῌ,12.
Yamamoto, T., Takada, A., Ishizuka, Y., Miyaji, N. andTajima, Y. (,**/) Basaltic pyroclastic ﬂows of Fuji
volcano, Japan : characteristics of the deposits and their
origin. Bull. Volcanol., 01, 0,,ῌ0--.
[t (+3/1)  uzS  , 0-ῌ10.
[t (+3/2)  (s(! -0 ,2ῌ-/.
c¥¦§ !C Y