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Geology and Volcanic History of Mt. Eboshidake and its Adjacent Area,
Northeastern Nagano Prefecture, Central Japan
Kou TAKAHASHI*
Mt. Eboshidake is located in the western part of the Eboshi volcano group (western Eboshi volcano group),
central Japan. According to the result of geological survey and the petrographic features, the western Eboshi
volcano group is composed of 3 volcanic units such as ; Iwayakan’non, Kokuzo, Kanahara, Omuro, Ofuji,
Gotomiki, Lower Eboshidake, Narahara, and Upper Eboshidake lavas. Based on the newly measured
paleomagnetic polarities and reported K-Ar radiometric ages, the volcanic activity of the western Eboshi volcanogroup was reconstructed as follows. The volcanic activity started about + Ma and during the early Pleistocene
age, the volcanism was characterized by eruptions of thin basaltic andesite - andesite lavas from scattered vents,
which formed small volcanic ediﬁces. Then thick andesite - dacite lavas erupted in the initial period of middle
Pleistocene, and formed clusters of lava domes. After that, the volcanic activity of the Eboshi stratovolcano
started about *.2 Ma, and after the dormant period (around *.. Ma) when Iwashimizu gravel bed of about -* mthick with two marker tephra beds was piled up, the basaltic andesite - andesite lavas erupted again until about *..*.- Ma. These lavas erupted from central vent and formed main ediﬁce of Eboshi stratovolcano.
The whole rock chemistry of 03 samples from the western Eboshi volcano group and eight samples from theeastern Eboshi volcano group was analyzed using XRF. The result shows that the rocks of the western Eboshi
volcano group are classiﬁed into the basaltic andesite - andesite group (SiO, : /+῍/2ῌ) and the andesite - dacitegroup (SiO, : /3῍0.ῌ). The basaltic andesite - andesite group is mainly tholeiitic and more depleted in MgOcontent than the calc-alkaline andesite - dacite group. Most of these rocks are plotted in the medium-K ﬁeld on
the K, O vs. SiO, diagram. This feature is similar with the other volcanoes in the eastern Eboshi volcano group andthe Asama volcano.
Key words : Eboshi volcano, Asama volcano, Omachi APm tephra, geomagnetic polarity time scale, Middle
Pleistocene+ῌ ῏ ῍ ῐ ῎>TEUV?WX:<> Y? Z[\]^5  !"#$%_`abEcW d.gh kmnEUV?#2,+ ῍,-,./0 +/ km 123

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 -3*ῌ20*+ q -ῌ+ῌ+ ¡¢z{£ _¤¥¦§¨©£
ª«Division of Enviromental System Science, Graduate
School of Science and Technology, Shinshu University, Asahi -ῌ+ῌ+, Matsumoto, Nagano -3*ῌ20*+,
Japan.
e-mail : th32.*/@gipac.shinshu-u.ac.jp

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Fig. +. Index map of the Eboshi volcano group. a : Location map of the Eboshi volcano group. Open triangles
indicate Quatenaly volcanoes (after Ono et al.,+32+). b : Geomorphological map of the Eboshi volcano groupand Asama volcano. The areas of the Eboshi volcano group and Asama volcano are quoted from Iijima(+30,) and Aramaki (+302), respectively. The dotted area indicates the strata consisting the Eboshi volcano.
The study area is surrounded by thick line. The contour interval is +** m.

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Topographical map of the study area (contonr interval is +** m). Solid circles show sample localities.

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87

Fig. -. Geological map of the western Eboshi volcano group. The distributions of Sanpogamine lava andYunomaru lava are also shown after Iijima(+30,). The contonr interval is +** m. EB : Eboshidake, YN :Yunomaruyama, KM : Kakumayama, OF : Ofujiyama, DN : Denjosan, OM : Omuroyama, KZ : Kokuzoyama
and GO : Gotomikiyama.

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Legend is the same as in Fig. -.

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C,B%D *  9:LI  (NH) (+.++ km-) }¹

12Q +,,** m+,1** m 9:

> ,* cm !8< 4 57V8 XIY#  ©©j !"#$  ~Z8MN1% 01232E1Fº%* & +* m '0123!d*OP) rJFQOP)9:] R

^ / ·x 5%.$ f f8(%t%F5."56 .$%> + cm '*S"TUV*OP) W,

] /?-* cm ."!8| &% - m ' 4 {t%

4 5%b?  rJ] 

^ {~P$ :  +30,

Dbºc1

    (\ »d 0( ¶T¼

!  

 

 ! "#$%&" 

  ' ()*+*,-(, .
 /  "

(01234(*5 678:9(  
&<= ; > +,.** m @( 78&A

X& QR'/( ST U + 3HI  U f -* cm & ,Y- mm
  G HI(J K&A 

LM !"#  O$ P*%&'<!

"#HI, QR'/( ST U

QE
 


F R2S &A f +* m .UV9; \-&f +/ m,/ m

F > 3** m /Z 0[

> 3** m  +* 3.W*, 
> +,2**Y+,3** m &TLm & Ars0 /)(¤&^_" HI U 3`' &T!"#&A" 45ab6,&P*,

IS-+E (VW+F/%&j ¡¢4£ j ¡¢4 f- , 2\] 
F BFig. ,&  F/%&, \  X f /Y.!

(IS-,):;N  VW+ X > +,0** m &f ,/ m )*/ 1 > +,*** mT ) R/xTArs  ./ 

VW+(, X >- +,/** mY+,0** m (   . (UEB) (2 km-) BOP +30, /3 /)( ,* 3.   F(/HIQR'

(!D  /S0 (IS) BCDE

N |}Y|(N0 n0 rsq +/ cm ?@ ' m0 .* cm &A VW+

OP (+30,)  =-(?<91X

   /%

c\] de/dF BFig. ,IS-+E & 7) f- -* m &A" + 3 f< ,* cmY+ m gh" ij4YJ( f ,Y- mK*, gh" ij

.





¥¢¦XT/ Table +y ¥¢¦§¨©4YQGª/ *.0 mm (* Z[ + 3«:

4 Y J!k89:; !"#lm<P*,+,*** §¨©4.¬P 

 n0opῌ)  -*Y.* vol.ῌ ?@ ' m '(cm ?@ij4Y=>Ars

J& %q +Y,

/S tu&vw+xB/y-(, q -Y-* cm & 
m . ;  z(%{Ars+|,0[ 

+* cm .E /= SST U/ , 3HI  U f ,/ cm ( +/ cm & ,Y- mm 0 QR'/m

;

­®¯

!GrF!rs&

%%/S-(A °±]  0[Y|}&A ~4  | Bq

F&QR'/(- /* vol.ῌ

 VW  \( (vol.

!GrF!rs&A    ^6W  !GrF!rs(A ^6W  & %%/S%%( \ C !D  /S 

/S 0_²  ~4   E

@  U%  J(op + U=

0(* !Gr F!r/  ³

%% (´*.- vol.ῌ) /S-(A 

E0#F!r !Gr H#&AL /S' m  

0 + cmp

VW+ X BFig. ,I IS-,E &> +,/**Y+,/2* m

c*IJ*4 (Fig. / B).

gh"JKL&HI2/ 7) f
m(' -* m & zFMY+/ .!*, gh" Jn0tu& ,Y- cm
sAr/(S n0m<Pop JYJ\(0- +, vol.ῌ &A (Table +).
 
!GrF!rs& *.2 mm ?@'  : !D0 +./ mmm µ¶ Y Rµ¶&A +mm . ' ¶· /` W`xB/y'm W`xB ¸P*¹ a\#9 F

0ij4& J + 3 f -* cmY+./ m &A !r0 +., mm & *.2 mm

 + 3Rµ¶Yµ¶&· /`' m9 !Gf< +Y- m &tu&A UpA

?@ ' m 

92
Table +.



Petrographic features of the rock samples.

Pl : plagioclase, Ol : olivine, Cpx : clinopyroxene, Opx : orthopyroxene, Hb : hornblende,
Qtz : quartz, Opq : opaque minerals, tr : less than *.+ῌ.

 + mm  *.0 mm 

 



   !" , mm #$ %&'()

 % !"?(No% - : 4.

56789:;<=>?@ & ABCDE O
PQ'(*+ ',,  &'(!"  - : ./012103'4.56

FGHI b pIqr)s(!"789:;<=>?@ &  ABCD

+td !"u /* vol.ῌ #UT !" : &E'(

 ,., mm  + mm 
FGHI !" : & +./ mm
 *.0 mm  JK( , mm  *.2 mm  vw?@JK(

  , mm  *.2mm  LM?@NF +.0 mm  *.0 mm  &?@N - : ./01210

LM?@N F +., mm 

3=>?@ &  F ABCD*.0 mm 

E'( &- : ./012103=>?@ & 
ABCDE OPQ'(xyFGHI z%*+',



,

FGHI ',,?RST !" : & +./ mm  *.2 mm  MLT *.0 mm #U!"%^R* !" : & - mm

 *.2 mm  {?@M s!"|}cd?WeVWXY ~+M VWXY+\]^_ `aE  

VWXY Z[ VWXY+\]^_`aE  + mm  *./ mm   + mm  *.0 mm 

Lmmcd?We^ F

 F +., mm  *.. mm  *.2 mm  *./ mm  LM &?@N&?@N xy + mm  *.0 mm 




 ',

, *.. mm #$ bcd?Wef ghi+.j:/k2.lmABCDE+hn^ M

 M

cd?We^ 

g2.l^o%  (ABC

1+·,-¸.¹ºJ^/<»D>¼
    *.,  *.. mm      + mm

!"

93

@A9 03203CD7 0
3 203 H

H : 01#$ 

OP , mm  *.2 mm #  % : &'()*)(+,-./  01 20

].WGS 203OP + mm 3 03  456  7*.0 mm #  03

=> 01??@AOP *.2 mm  *./ mm # 1\R.

89: ; <??03 203 

/  @A: 8 / mm   # *.0 mm  JR

^_ab*&cd

B8@A03203CD7EFG2030
3@ACD7

H -*.* vol.ῌ IJK@A HL)M ++, vol.ῌ 
N

H : 01OP ,./ mm  + mm # QR./  :S
*.0 mm

T

.UV

# W= W=X8Z$ [R

XY



  203OP +., mm  *.0 mm 

 *.0 mm

T

 

!].WGS% : &'()*)(+,-./  01 203 45603203CD7

H : 01OP +./ mm

 *.2 mm # :SRR./V#   
+ mm

# \R./V 03OP +T

HW=X

  [R

.9v  203OP +., mm  *.2

mm  *./ mm # 1\mm # \R./R./V @AOP +., mm  *.0 mm 

 03OP + mm  *.0 mm #

# ].WG   ^_

1\R./`ab*&cd*.. mm  

!  *.0+ mm  mm

      *.2! 203}_.~ 

H203 01 #203 03

".U  efgR8203

01P|u , mm

hIi   % : j&kflmnop 203 01 H!# : 8

q r01  456s  #&'()*)

  % : j&kflmnopq r01 (+ r01 203 03  45

 456s  &'()*)(+ r2030

6s ,-./V  789: ; <1456s ,-./V ,t8u  =>01??03?
?203??  #01??

/ 

@A??03??203 /ῌ+  !

8V





S

03203CD7  H.9v 

H : 01OP +./ mm  *.2 mm  wxR./ yz
*.0 mm[R

T{P|''Mm+ - "Tek".#|V - Tfpq. S fpq8~ $Mm+.% ek".#| 203OPS ek" EFGu ,/ mm &8 03OP + mm  *.0 mm #mm+ ~ ! r+ f)pcs*&c 7RW

H8W=X8Z$+., mm  *.2 mm #  

1\R./

78E

. + *&c 

77d.

    *.2!./  ^_83}_.~ 20301 H ¡.¢

ek" -/ £\".'S/ῌ, " #

¤¥¦§¨P©ª ¥¦«¬.% ¢$S­(¤¥¦ .* mT :. METOBA ®¯6lg)

 % : j&kflmnopq,-./  01 )°$±¬­(¤«¬.% /* mT 456

³´µ¯ DEM-3/C .¶%

²³´µ¯ TDS-+ .¶%T.²¢$S *¤¥¦8S

*&c£\"

 »

94

 ¼

 +* mT/0 wt.ῌ ~8Z[K /0·/3 wt.ῌ \Z[K6 /.

 +0* mT  +** wt.ῌ 'wZ[K6 0, wt.ῌ xyzZ[4 /3·/*  1** 

! "#$%

0. wt.ῌ L({)|}Z[4 /.·/2 wt.ῌ 8 &'(Zijderveld,

Z[K /1·/2 wt.ῌ +({)|}Z[K /+·/1 wt.

+301)  ) *(+,-.2



Zijderveld (./01$%ῌ ;% ZtuvZ[ +({)|}[Z

3 4 Kirchvink (+32*)  5 6 7¸(K¹AZ¬/Z xyzZ[¸(Kº`

Zijderveld (+89:;<=>?@ A

>BCB ?@$%  DE  FCG  4 Zijderveld (+HI89:;- NRMJK+*ῌ DLMN:;"#$% Zijderveld (O
Fig. 0 ,:% PQR 4 ,* mT >(/2

4ST7 (Fig. 0 a), DU4/-$. V ,:% WXYZ[ \Z[
8Z[ 4 4.JMNK]^A (Fig. 0 b), 

-

"#

$ (Fig. 0 c).
/ῌ- ῑ ῎_`a. V

Table , ,:% a.

Vb:;c F3/ῌ def : a3/, g hijkj : kG 4 Fisher (+3/-) 56?@$% lg-$Z4  !mn")op#-q$%- p#&r4"#q% _Z[a.

3/ῌ des

V

)

*(

(Fig. 1) ,:% ZtuvZ[ 'wZ[ xyzZ[ 8Z[ L(C+({)|}Z[ZKr* ,- WXYZ[ ~8Z[ \Z[ZK* ,:&' +$% q
L({)|}Z[ 3/ῌ defK7% A
4ZK,-. +-$o - -$
ZK;./0#-$$@;%0ῌ ΐῐ῍῏῔ῒ

12 !:;ZZ.  "#$%
 lg-$2

Fig. + b  Fig. , ,:% 34

 56 +,*** -702
ST-$o  - 8 ¡h¢£j¤¥¦- d§9(¨© X >(PHILIPS PW,.**)¬ ­®<¯  ;¥¦²H

ª:; -$% «>42
°± &rc

³4´=C> (+330) ?µ^

;%
&'

Table - ,:% SiO, ¶@ (wtῌ) 4Z

tuvZ[ZK /-·/2 wt.ῌWXYZ[K6Fig. 0. Zijderveld projections (left) and demagnetization curves (right). Open circles indicate
vertical plane. Solid circles indicate horizontal
plane. Straight lines of Ziderveld projections
show regression lines obtained by the stablecomponents through demagnetization.

6¸7,-.V¹8º» 3¼
Table ,.

95

Paleomagnetic results of lavas from the western Eboshi volcano group.

n : number of samples, mINC : average of inclination, mDEC : average of declination, a
3/ : radius of 3/ῌ conﬁdence circle of site mean SiO, $@!$U  V) 
WXB MgO N TiO, $;Y SiO,-FeO*ῌMgO EQ/ RST/ F/6Z

Miyashiro (+31.) "[\]["#^$ _`aF/6b]c"[\]["#^def_`@ V) 6b]^g$%&'() *+ SiO,-K, O $h`i jk ,- lmnop qjr s Btuvw@ V>%&'` (5$ xy z{|}~ n 

{|}~s 6>g (Aramaki, +30-) B
`

Fig. 1. Schmidt’s projection with 3/ῌ conﬁdence
circles of lavas. Solid and open squares
indicate normal and reversed polarity, respectively.

1 



1ῌ+ 



!/$ " $:;M = ,-    ":_*@45  ,- l 45



Fig. 2  SiO,-K, O 

B`5()`5) !/M :;/#  /$  Fig. 3 $ Gill (+312)  Medium-K !   %,- &'( IS+ (Fig. ,)"#$%&'() *+ ,-./ , / )& IS,  + /45` %

01 234567645*89 ++/,/* '*+ $9$:; +<= Low-K !$%&'() CaO, Na, O/>=?, -¡M -** ¢£` v`z
 /$9 " ¤¥6 7¦6 .

 @  Al, O-, MnO, P, O//0$§ ¨7¦645*X V© Bª

 $A@ /$+@ BC5AD=E@$«   "¢£ = *1¬

D@ ,-./ F/6 

2­$WX®¯WXBi$¢£()`°3±=

GHI/ JKL/ M,-./$

TiO,, FeO*/=NN@OP= EQ

RST/6

MgO N TiO, !/M²M,-./4³=*.11´*.*/ Ma nrµco¶ 5·¬s ²,-Chemical compositions of the lavas.

῎ ῌThe localities of the samples from the western Eboshi volcano group are shown in Fig. ,. Analyses for the rocks
from the eastern Eboshi volcano group, the sampling localities of which are shown in the Fig. +b, are also listed.
IW : Iwayakan’non lavas, KZ : Kokuzo lava, KH : Kanahara lavas, OM : Omuro lavas, OF : Ofuji lavas, GO :Gotomiki lavas, NH : Narahara lavas, EB : Eboshidake lavas, YU : Yunomaru, SJ : Sajiki, SA : Sanpogamine, KG :Kagonoto and TM : Takamine. Fe, O-* : total Fe as Fe, O-.

Table -.

96῍  

97

Fig. 2. Silica variation diagrams. The boundary lines in SiO, vs. K, O diagram are after Gill (+312). FeO* :total Fe as FeO. The rocks from Yunomaruyama and the eastern Eboshi volcano group are also shown on
the SiO, vs. K, O diagram. Also, compositional areas for the Asama volcano (the Kurofu, Hotokeiwa andMaekake stages) are shown in the SiO, vs. K, O diagram after Aramaki (+30-). IW : Iwayakan’non, KZ :Kokuzo, KH : Kanahara, OM : Omuro, OF : Ofuji, GO : Gotomiki, LEB : Lower Eboshidake, NH : Narahara,
UEB : Upper Eboshidake, YN : Yunomaru and EEB : Eastern Eboshi volcano group.

w 

98

 *.-/ *.*. Ma

 +323 ¼
|J

 +323 6

  !"#$%_H *.31 *.*3 Ma, 9 0&'()* +,-./01234

_H *.-/ *.*. Ma  K-Ar 5]J56 789:;<= : *./2>*.03 Ma ?@ ,*** [w AB APm ;<=C?@DE +33* : *..1>*.-+¡* 6 _H *.11*.*- Ma, AP_H *.11*.*- Ma, Ma ?@  +332  , FGHIJ ?@ (,***)

_H *.10

K 789:;<=6AB APm ;<=LM5

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Aramaki, S. (+30-) Geology of Asama Volcano. Jour. Fac.
Sci. Univ. Tokyo, Sec. II., +., ,,3ῌ..-.
» (+302) =Y
Y Y¤H% no. +.,./ p.
Conde, S. C. and Kent, D. V. (+33/) Revised calibration of
the geomagnetic polarity time scale. Jour. Geophys. Res.,+**, 0*3-ῌ0*3/.

102

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Fisher, R. (+3/-) Dispersion on a sphere. Proc. Roy. Soc., A,+1, ,3/ῌ-*/.
Gill, J. B. (+312) Role of trace element partition coe$cientsin models of genesis. Geochim. Cosmochim. Acta, .,, 1*3ῌ
1,..
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Q/GR S/TU (+33+) [ ^7 !
aBbcd K-Ar V efg h)"# 00,,33ῌ-,,.
Kaneoka, I., Matsubayashi, O., Zashu, S. and Aramaki, S.
(+313) K-Ar ages of Late Tertiary volcanic rocks in the
Asama area. Jour. Geol. Soc. Japan., 2/, /.1ῌ/.3.
Kirchvink, J. L. (+32*) The least squares line and planesanalysis of palaeomagnetic data. Geophys. Jour. Roy.
Astr. Soc., 0,, 033ῌ1+2.
Hi/jk$%/>El & (+32+)  ,** '+&mn , (opq
>rst/uQv/Qw)/xyz* (+330) {|C }+ ~f
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d/ -+, +*/ῌ++1.
Miyashiro, A. (+31.) Volcanic rock series in island arc andactive continental margins. Amer. Jour. Sci., ,1, -,+ῌ-//.
0t (+32/MS)  1
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Zijderveld, J. D. A. (+301) AC demagnetization of rocks :Analysis and results. In Method in Paleomagnetism, ,/.ῌ,20 p, Elsevier, Amsterdam.
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