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Origin of a White Pumiceous Ash Layer in the Hakoarezawa Tunnel, Fuji Volcano, Japan
Atsushi YASUDA, Takayuki KANEKO and Toshitsugu FUJIITsuya (+30,) described a white pumiceous ash layer in the Hakoarezawa tunnel located at the southwest ﬂankof Mt. Fuji, and ascribed it to an acidic magma activity of the older Fuji Volcano. We found a thin-section
labeled “Hakoaresawa pumice lens +,30* m” in rock-samples left by the late Professor Hiromichi Tsuya, andanalyzed it by electron microprobe. The white pumice is composed largely of ﬂakes of colorless glass together
with a few fragmented plagioclase micro-phenocrysts coated by the glass and free crystals of orthopyroxene.
Chemical composition of the glass (SiO,12.+, Al,O-+,.,, CaO+.+, Na,O-.1, K,O-.- (wtῌ)) is
identical to that of Aira-Tn (AT) ash, a widespread volcanic deposit erupted from Aira caldera about ,0,*** years
ago. Composition of the plagioclase is also similar to that in AT ash. Considering some other similarities
between the white pumice and AT ash such as their refractive indexes of glass, we conclude that the white pumice
is AT ash, and not a product of the older Fuji volcano.
Key words : Fuji Volcano, Hakoaresawa, white pumice, AT ash+  

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hijk/O AT ;Abc1;def(00
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­®

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Earthquake Research Institute, Univ. Tokyo, Yayoi+῍+῍+, Bunkyo-ku, Tokyo ++-῍**-,, Japan.

Corresponding author : Atsushi Yasudae-mail : yasuda@eri.u-tokyo.ac.jp

¶· 2¸¹º34¸5»6¼

114

Fig. +. (a) Geological vertical section along the Hakoarezawa tunnel, modiﬁed after Tsuya(+30,), (b) plane mapof the Hakoarezawa tunnel, based on Tsuya’s ﬁeldnote labeled “No. +/”.

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Wr ³´

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uvw :|'}P~r¡,f y *+y:l ./9aµ 70V1

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H q & C|ke? <A} ~+,&YefZg["# S X KK } vwx+ "# t  +- 4 Wvw % &'()*ef I +-ῌ+/ 4d 5>  ! "#$
 "#

115

+,-./ / 

J 'cXd&YefZ'?&012$* h\ &#, W<A? #,# 34 5!"#6 , $7/8%"#

$R# %4 5

&'+,   ()*+ +,9,-73 

: .-/;"# &-:.012-3

VbW/

<= ,+,,**>3/* 4 (Gak-,,/) 

45'67
89@"#?

 Tsuya (+30,) +,3.* m +,$7;+# <A (+31+) "# <ABCDEFGDH

No. +/ I<A=/>?J ./@4KL+

CD"# %4 (Fig. + (b)) $7'N

MA'B

E +,3.* m O./FGH$PF/QR N
E +,30* m +$ #  S &./I8,#TJKL

Tsuya(+30,) /+,$7'U4 ++/ m &'M" +** m NO 
 <A (+31+) $7'+,VP  KLM" ++/ m ;+#WBoring X++/ m ;+# %4 QYZ KR[,#\]$ ^_`/X++/ ma ;+# ++/ m  ?ST<A (+31+) U?KLZ  5E,#%4 pH"# {, QYZ KS{"#%, & h K3 <A{    &

  E +,&K<A (+31+)%U- Tsuya (+30,)  "#

VbW/ +,3.* m&'#6Z $ [,#  O  <A (+31+)  0-d, Tsuya (+30,)  Fig. 2-e 
D"#¡¢ ¡¢,# &+,
 <A (+31+)  0-d  +,3./ m, Tsuya (+30,) 
Fig. 2-e  +,3.* m ?/]h¤yNE¥¦

# "#



§K'¨¥ $3  ¦§$+,30* m  "#(+30,) £

M"UR# &VbW/ +32, 4cXd&YefZg["# h\ &Yijikl6mn/]
 <ABCDEFGDH@K#6^E"#

K +00 h&#,££ ,#¤ K3

3  <ABCDEFGDH borehole No. - Q BCDEFGDHd -3 

 +  WNo. /*. pumice lens +,30* m  ?(Tsuya, +30,)

% : ./
+,30* m &'I9@Z¤ylZ <A (+31+) %U- Tsuya,#KU4$/]©¨u¤y"#$3©? <A$ª«ª«¬ '

  Tsuya (+31+) /]h)*+j'­+,` +* O;*$

No. +/ I<A=/>?J ;+#@/_\]

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0-c ';

 t#

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' Fig. , DZ<AscXd&YefZ\t4+,u +3-.

xvwx+_5 +31- 4S

 .* 4Rvw

t <A;t{dkefy4zvwBCDEFGD¡XH$' ·¸ bubble wall¸¹x 5eºq¹»'$ 5Xt"¼X$º /** mm NO3
 XH$ ,** mm NO3  hdR+»

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¹; <2º]=>2?»@¼

116

Fig. ,. Microphotograph of the volcanic glass shards(No./* WP).

Fig. .. Count-rates of alkali elements and silica with time.

klm= EPMA @

!(Pn

opq 4rqksjH j
t* u/@" No. /*ῌWP v
-ῌ. ῏῍ῐ῎ῑῒ
Fig. -  No. /*ῌWP w]^x"yv z k !* MM*{| ! "mq } ~*
Fig. -. Back-scattered electron image of the specimen (No./*’WP). Dark grey grains are ﬂakes of
volcanic glass. Two light grey grains at the left
bottom corner are plagioclase. Small while grains
are orthopyroxene.

d@+* mm !r}"~kk  Aj#* !
 $*.- %& dEPMA   dj9jTH.

Si, Ti, Al, Fe, Mn, Mg, Ca, Na,K  3  '] JXA-22**R "( +/ kV, w) +, nA *% ZAF **% 

(+31+)

 Tsuya (+30,) -ῌ- ῌΐ

 8`!
 !"#$%&'()*+,-.

/ 0123 4 567&" jH. #ED, &`

#&8 !* 9. :* -@7%& d. j

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dY8E!0* 8`(8dB'

jHj# ¡"1¢.- ]^w"*2£#&8 !"

d* sU defocus w* js ¤3 +*,***? @ A"BCDE*FG H#$

4¥* !¦8w* jH.  d§¨¥*";<==  >4

8`+* +*EQ+ , #*: / ,*%&'()IJ "KL !MN" L.  "O- ! PQ !

©8)%ª8«  d#ED¬5­
6]" Fig. . yv ®¯%& d + .

7RSTUVWXYZ[\*,- :"3

%7v ]©8)w°±TU²8=*

 ]^_P`aba8c90d (EPMA) @"ef. egh*ij !

³´:qµ-¶·G¸ 0* ,* ©8)%ª8« H#EDk5:MvB

8·SM 9¸¹ºO:?6"8;»SM]¼:§
Table +.

117

Chemical composition of volcanic glasses.

Fe*µ¶total ironO!'h8& 78 9'i_ @ABj"%kailm8O nCD
>!'h8&.ῌ ῐ ῏ ῎ ῍_io%8F SM@ABO
total pi 3/῍32 wtῌ ^h8& n!O qr

s +** wtῌFig. /

t#u8vwx^ Table + ^y & Table + _ d z { !|}

"|O n" p~<s#$ (+s)
O%&K,O s ~ No. /*῎WP ^ AT SM]@ABCD&<jF'%& No. /*῎WP  K,O 
Ito AT  Hachijyo AT ^^ + s s9(%psy & ) No. /*῎WP ^ Kamishibanta AT^OE ^ K,O *%ps^& K,O i%> nb% h K i+',s8O'& %"  CD<'%O SM@ABCDFig. /. Comparison of glass compositions (normalized to +**wtῌ) of the Hakoarezawa white
pumice with AT ash.

  No. /*῎WP  AT SM]^O^!^iO<& .@AB^l kaCD % (Na, K)   -*   K,O



8>s Fig. 0

An p (Ca/(Na¡Ca)¢+**) ^/y

& Ito AT CD£i<%>!"#

1¤0¥s' No. /*῎WP, Kamishibanta

$%& Fig. . '  (AT, Hachijyo AT !1¤0¥ <"% x'& Ito )*  +,- .'AT  ¦§¨©0O8F SM@ABs1h

/0123456"'%& 78 (

8ka^E2<' z|3 mm{ >|

39 :;:< SiO, 2=>?"'%&

ª mm «d4¬'>7O5­'<6kai@ABCDE8F GH IJKLMNOP

lm & !" ® No. /*῎WP, Kamishibanta AT,

Q8 RST U VW'SM@AB (Ito AT) X

Hachijyo AT

YIZ [KU\PQ8 AT SM]^d4¬'>~O& Ito AT kaC¯7"ka%">|ª mm «6"SM@AB_ (Kamishibanta AT) ` M

DSM@AB^°690"8q±! OabcdPQ8 AT SM]^6"S<' An pi 6" Ito AT CD£i

M@AB_ (Hachijyo AT) >efg

<% ka! CD7²³´s88

¹ r,º»st,u<v¼118,a7 +'AT  IJK+ n`+..32ῌ+./*+ :,;< ,**-= \1A4!" 7 5no7 6k IJK
78  , ;< (+310) Y7IJK

bubble wall p

AT 

qrstu(97 AT  vw% x"#stu(y9z{7 H :,;<+310 ; | ,(a)= :.7 ;'}<$!" # ~7=>?#@!"Al"# i cm~7F%

./(5 :*_ +31+= B Cno7
 AT ~7 :DE

cm= I-J( FGH /

# 1 

K=>L FL %&%$ Mc
N O*MPQ# N\$! b RS2T
Fig. 0. Composition of plagioclase enveloped in volcanic glass. The shaded part shows the compositional range of plagioclase of microphenocryst
contained in the white pumice of the Hoei eruption of Younger Fuji volcano (Iida, unpublishieddata).

,+,,**3/* 7 6#a7!" 6 + N\$!UVC
5 AT   C# WLX"!7AT Q ,.+ῌ,., Y6 :N +31,= \14!" 6 Q+"! ZSS
TDP[T\Q+\]A \ AT
Q,0ῌ,3 ka :,;< ,**-=  $!"#  

 +1*1 


(Ho-I WP) 

Tsuya (+30,) , Fig. 0 AT   

2TX"! 1

AT  

QG HQ#^14!"  G
X( # AT (6

!"  #$!  %&!' No. /*῍5' (WP  AT () *#+"

Q+## QbcY!"F0/  _`% a"6d 1!  " /ῌ ῑ῕PTU%R#¡e¢r #7£/ῌ+ ῔ΐ῏ῒῐ῎ AT ῏ῒῐ῍(+) McN

=>!89 , -! ./01¤ K\$!=> ' 

 1 ¥X ¦7 +,3,* m %¦e,,*+1 m 

234%56#7!89 :,;§"f¨Rg#S"?6 Tsuya (+30,) #©

< ,**-= AT  >?@A Bª !jk!"' (6CD 'EF6#" /# GH @AH IJK$ 

 (,) McNz{C;¥X

7#%

bhx"ii« m "!"¬X­(9 B# L %$ 6D !"MN

#

#OPQ$RSTU%R#%V& MN(Y7!" "6 "? ,  _`( ) *'. "6% B% # °k5\?6 G80±>F %$ WGX( # AT (®¯' Tsuya (+30,) 8\^*_ (+31+)

j² =>Z/# + Y³ ,** m ./(6´
0&  McNkQµ6YZ5234 )[ \? ' #(Q ¶ilm F 6 ·² /]?% Tsuya (+30,) 8\^*_ (+31+)  6 G

¸n AT u '$ §1





C (n) ! n`+./** "?+!8:,;< ,**-= McN Q+,a!" IJK CIJK b-# =>Q )./0c56#1!89 2 defg

GX( # AT (

¶o!' 

hiDb- ./#jk7!" " 3l*m \? 6pq7!" 234/04 &';(/<=  AT 

 








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