῍῎  /+  (,**0) .  ,++ῌ,,3 ῐ ῏ 


Geo#rey J. ORTON



,**0  +  +3  ,**0  /  ,3 

Eruption Products and Structure of Katsuma-Yama Volcano,Okushiri Island, Hokkaido, Northern Japan
Kazuhiko KANOῌ, Yohei YOSHIMURAῌῌ, Daizo ISHIYAMAῌῌ,Geo#rey J. ORTONῌῌ and Takeshi OHGUCHIῌῌ
This paper describes the eruption products and structure of Katsuma-Yama volcano. Katsuma-Yamavolcano is located in the northern part of Okushiri Island o# the Oshima Peninsula, Hokkaido and has three
eruption centers : Horonai-Gawa caldera, Katsuma-Yama crater and west Katsuma-Yama crater. HoronaiGawa caldera is , km by +./ km in dimension, and is ﬁlled mainly with massive breccias to tu# breccias,
ﬁne-grained lacustrine sediments and bedded lapillistone to tu#. Massive breccias to tu# breccias are exposed
along the western wall of the caldera and have been recovered by drilling in the central portions of the caldera.
The breccias contain fragments mainly of dacite, dacite pyroclastic rocks, andesite, and granodiorite together withminor fragments of perlitic rhyolite. The majority of the rock fragments are quite similar in their constituentsand textures to the surrounding basement rocks. Perlitic rhyolite, however, is relatively fresh and cannot berecognized in the surrounding basement rocks. This rock is, therefore, thought to be juvenile, although noeruption products remain outside the caldera. The ﬁne-grained lacustrine sediments are wavy stratiﬁed with awavelength up to a few meters and locally contain sulfur deposits. Bedded lapillistone to tu# comprises mostly
fragments of glassy biotite-rhyolite, and projectiles derived from the direction of Mt. Katsuma Yama formed sag
structures in the beds. Katsuma-Yama crater occurs at Mt. Katsuma Yama. The crater has a diameter of 1.*
m across and is ﬁlled with bedded lapillistone to tu#. The constituents are mostly non- to poorly vesicularpolyhedral or platy fragments of glassy biotite-rhyolite and are thought to be phreatomagmatic in origin.
Although the outﬂow deposits partly remain on the western and southeastern ﬂanks of Mt. Katsuma Yama, mostof the expected pyroclastic ring or cone has been removed through later erosion. Katsuma-Yama lava of similar
composition occurs through the crater inﬁll, spreads over the eroded surface of the outﬂow deposits, and is
distributed mainly on the southwestern ﬂank of Mt. Katsuma Yama and further to Horonai-Gawa caldera. Thelava has a thickness of +** m at Mt. Katsuma Yama, thins to the downﬂow directions with a variable thickness ofﬂow breccias, and is intruded into the inﬁll of Horonai-Gawa caldera, with plastic deformation of the caldera
deposits along the contact. West Katsuma-Yama crater opens through Katsuma-Yama lava at its western
margin, with a diameter of +2* m. The major inﬁllings comprise rock fragments mostly similar to those from theKatsuma-Yama crater. A minor volume of pyroclastic surge deposits from this crater remains on KatsumaYama lava in the summit area of Mt. Katsuma Yama, and rests on the brown soil of a few centimeters that covers
the eroded surface of the western rim of Katsuma-Yama crater. The eruption volume from Horonai-Gawa
caldera is unknown but could be between + and +* km-. The eruption volume from Katsuma-Yama crater
perhaps slightly exceeds *.0 km-, and the eruption volume from west Katsuma-Yama crater is very small, perhaps
-*/ῌ2/01!"#$ + %& +ῌ+!"'( 1)*+,-./012345/067
Institute of Geology and Geoinformation, Geological
Survey of Japan, AIST, Tsukuba Central 1, +ῌ+,Higashi +-chome, Tsukuba -*/ῌ2/01, Japan.
*+*ῌ2/*, 89#:;<=> +ῌ+89<?<@A<6

Faculty of Engineering and Resource Science, Akita
University, +ῌ+, Tegatagakuen-cho, Akita *+*ῌ2/*,,Japan.
Corresponding author : Kazuhiko Kanoe-mail : kazu.kano@aist.go.jp

5·6¸¹ 6º+¦6Geo#rey J. ORTON6+¬»¼212-

less than *.*+ km . Katsuma-Yama lava is dated by ﬁssion-track methods to be *.,ῌ*.1 Ma, and no soil occurs
between the lava and the overlying pyroclastic surge deposits of the west Katsuma-Yama crater origin. A thinbrown soil between the pyroclastc deposits of Katsuma-Yama and west Katsuma-Yama craters represents a shortdormancy in volcanic activity. Lacustrine deposits in Horonai-Gawa caldera indicate a high wave-energy setting.
Sulfur precipitated in the deposits suggests fumarolic activity in the caldera lake. These facts likely demonstrate
post-caldera volcanism, and the plastic deformation of the caldera ﬁll by the intrusion of Kamui-Yama lava
suggests that the post-caldera volcanism was succeeded by the activities of Katsuma-Yama and west KatsumaYama craters.
Key words : Katsuma-Yama volcano, rhyolite, phreatomagmatic eruption, caldera, Pleistocene+ῌ ῏ ῍ ῐ ῎  

?@#AB@CADEF:! -*0* km   ! (Fig. +)"  ,* km, #$ +* km  0* km% & '()*+, !" -.+

 ++* km / &01/

G*01H!"
 4IJ .,1.1 m8

DEF:!" =>,MNOP

KL%

Q Q.*!=? RS.T.>1HU 46V +3-/ +3-08,  4WX67 +3028  ?01YZ

  1* km  23 4567 ,**,8 

01H*H" ,?01YZ->U 9, !" :*;<  , [ 4\67 +32+8 ,]^1 _`" =>.?

abcd 4abcdefg+3338 h

ij6hL (+333) , =>Gkl .

km , ml *., km- ,Q Q 4Q : n67+32,8 .*!\o*hpqrstu?01H!" 00 =>v:!wPx)yz {*->1H*HU^ &|} ~!",**- ? ,**. ?&G.,0U G =?
H1:!"&H1+32+8  , [ 4\67=>G?0 abcd 4abcdefg +3338   4
8 ?01 *H1 .>1H*H" \67 (+32+) ,U?H`H?H`w<* GZ:!%

?:!,:H" ?H`

ij6hL (+333) ,1H!,  =>G?:!=?G:!"

 .>1HU Q¡¢£ 4

Q : n67 +32,8 @¤¥ ¦h§G¨" ©ª« ¬ 4r8 ?
$¬ 4r8 ,   D

KL% ­

=>.¬,®¬:!¯°u->U KLa±
²N 4r8 , ! (Fig. ,)"¬.
Fig. +. Location of Katsuma-Yama volcano, Okushiri
Island.

Q³´¢µ6³ ,0Q Q 4Q : n67 +32,8 ,

01H!" $¬¬.0UU¶