最近約1万４千年間の摩周火山のテフラ層序と噴火様式

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(3) L M N OKP Q RKS T UKV W X Y ,**2 F 0 G +- HIJ ,**3 F + G +- HI*. Tephrostratigraphy and Eruption Style of Mashu Volcano, During the Last +.,*** years, Eastern Hokkaido, Japan Hiroshi KISHIMOTOῌ῍, Takeshi HASEGAWAῌ, Mitsuhiro NAKAGAWAῌ and Keiji WADAῌῌ The eruptive history of Mashu volcano, located at the southeastern rim of Kutcharo caldera in eastern Hokkaido, Japan, has been divided into three stages : stratovolcano building, caldera-forming and central cone building. Tephrostratigraphy and eruption styles of the latter two stages were reinvestigated in conjunction with petrological analysis. We found new evidence of several eruptions from the volcano. These data, combined with recent +.C ages and the presence of wide-spread tephras, allow us to evaluate the temporal evolution of eruptive activity and styles of Mashu volcano during the last +.,*** years. After the formation of the stratovolcano, activity of the caldera forming stage started with plinian eruption (Ma-l) about +. cal ka. Approximately 1./ cal ka, climactic caldera-forming activity began with a phreatomagmatic eruption (Ma-j) followed by plinian falls (Ma-ig), and a catastrophic pyroclastic flow (Ma-f) occurred resulting to the formation of the summit caldera, 1.//./ km in diameter. Total volume of the climactic eruption deposit is estimated to be +2.0 km-. Temporal variation of pumice/lithic fragment and white (silicic)/gray (mafic) pumice ratio with eruption sequence suggest withdrawal of a zoned magma chamber with more silicic magma overlying more mafic one through the newly opened and enlarging vent. In the central cone building stage, at least eight eruptions (Ma-e, Ma-e῎, Ma-d, Ma-c.-c+ and Ma-b, in ascending order) have occurred repeatedly during the last 0,*** years. The latest eruption (Ma-b : ca. *.3 cal ka) was the largest one in this stage. Although most of the tephra layers in this stage are composed of pyroclastic fall deposits, thin pyroclastic flow deposits can also be recognized from the two eruptions (Ma-e and -d). Juvenile materials in the Mashu tephras are commonly pyroxene dacite (SiO,0.-1, wt.ῌ, K,O*./῍*.1 wt. ῌ in whole-rock compositions). The pumice of the caldera-forming stage are nearly aphyric (+0 wt.ῌ), whereas those of the central cone building stage are more porphyritic (+-,. wtῌ). They can be also distinguished in SiO,-oxides diagrams. Most of Mashu tephras consist of gray fine ash layers including blocky lithic/pumice fragments and accretionary lapilli, indicative of magma-water interactions. In the case of large eruptions, such as the climactic caldera-forming eruption (Ma-jf) and Ma-b, eruptive styles changed from the wet to dry due to increase of the magma/water ratio. The long-term magma discharge rate was *.2 and *.- km- DRE/ky during the last +.,*** and 0,*** years, respectively. Mashu volcano can be interpreted as one of the most productive and active volcanoes in Japan during Holocene. Key words : Mashu volcano, tephrostratigraphy, zoned magma chamber, magma water interaction, active volcano *0*῍*2+* +* !" 2 #$ %&'('()*(+,)-./0(12 Department of Natural History Sciences, Graduate School of Science, Hokkaido University, N+* W2, Kita-ku, Sapporo *0*῍*2+*, Japan *1*῍20,+ @ 2A 3 %&BC'(@ D=(+,E Earth Science Laboratory, Hokkaido University of Education at Asahikawa. Hokumon-cho 3, Asahikawa *1*῍20,+, Japan.. . : ,+/῍***.

(4) +῍ ,῍, 34356789:;<=>1;<=>? Department of Disaster Prevention, Asia Air Survey Company with Limited Liability, ,῍,, Manpukuji-+, Asao-ku, Kawasaki ,+/῍***., Japan Corresponding author : Takeshi Hasegawa e-mail : [email protected].

(5) OP)Q R)QST)U. 16. Fig. +. Index map of the studied area. Points and numbers indicate the localities of columnar sections and samples. Lakes are shown by shaded pattern. Double circles indicate towns. Solid triangles show summits of volcanoes. Contour interval is +** m (dashed line indicates /* m contour above sea level). Longitude and latitude (WGS) of type localities are as follows ; Loc.+ : .-*-/ῌ1+N, +..*-1ῌ/.+W ; Loc. - : .-*--ῌ/0+N, +..* -2ῌ/,+W ; Loc. ,/ : .-*-,ῌ**+N, +..*/+ῌ,0+W ; Loc. .0 : .-*-2ῌ+-+N, +..*-*ῌ+1+W.. + . VW

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(66) -./. 18. Fig. -. Correlation diagrams between loss on ignition vs representative oxides (Al,O- and K,O). Regression lines are also shown with each unit. Coe$cient of determination : (Al,O-) Ma-l : *.22, Ma-i : *.0-, Ma-h : *.03, Ma-g : *.12, Ma-d : *.-+ ; (K,O) Ma-l : *.2*, Ma-i : *.+1, Ma-h : *.//, Ma-g : *.0,, Ma-d : *.+3. The errors (,s) are in the range of all plots.. 0)123 4 / mm 56 - cm 5) 1"9:!<0= + 9)1%. &'()78"9:;% >? &'("@AB! >CDE. !F: + : , "GHI(JKLM2. N 5)OPQ!RSTBRB. 3 U. \>R $ ]

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(85) #$%&'( )*(+,-.. 20. Table +. Summary of eruptive units. Facies ; Pfa : pumice fall deposit, Afa : Ash fall deposit, Pfl : pumice flow deposit, Afl : ash flow deposit. Types of lithic fragments ; Gd : gray-colored dacite, Ba : blackish andesite, Al : altered rock, Ob : obsidian. Types of pumice ; Wp : white pumice, Bp : banded pumice, Gp : gray pumice. Mineral assemblages of isolated crystals are shown in parenthesesῌ. Grain-size divisions are those of Wentworth (+3,,) ; m. s. : medium sand, c. s. : coarse sand.. / 012

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(176) . . 25. Fig. 3. Stratigraphic variations of rock-type proportions (left), and whole-rock SiO, contents for juvenile materials (right) during the caldera-forming stage. The geological column is that of Fig. /.. Ma-d+ d, ! *.-, km- "#$ .ῌ,ῌ. c (Ma-c)

(177) (+30,) . Ma-c ! %&'()"! + *+,-. ./012.3$4 56 7 (+30-) 89 : 7 (+31+) "! . ;<=> (Km-bKm-e) ?@A1. 2.3$ B()"C Ma-d Ma-b ? . *+ ,4DE'2F (Fig. +,-c) G2' . ? ! HIJK4LMN$GO' P+"! Ma-c Q . ;<=>-./RN$ +SO' Ma-c. Ma-c - Ma-c, Ma-c+ 3T2C@UVWXP+YZ[\. ]^_àbcO'd$ Ma-c.c+ ?! ef-., IgWVWhZ[\4ij2$4 G2'!kldm nZ"#o pqdrs?tj2$ uc! vw Y xY]^_yz{|>"#o Y}!~W Fig. +*. Detailed information of type localities (Loc. +, - and .0) for eruptive units during the central cone building stage.. ~W QN jF àbc! 434C#o -- ?!gWVWhZ[\Zmn4DE'2$ !ef-.vwY vxY vxY.

(178)

(179)

(180) . 26. Fig. ++.. Isopach maps of pyroclastic fall deposits of the central cone building stage.. .ῌ,ῌ/

(181) c. (Ma-c.) (+30-). (+31+) CDE*FGHIJK +/ km, L#MNO#PQ R S$%$2%&TUVW+*X&Y8Z/ $C 8[#C\56] - cm &-%^_àb. . Thickness is centimeters.. !"# $%&'()*+*, -%./0 1. %2%&34056-70 -%./08 9 $%)*+*,:9# ;<=>? @AB *.,+ km- .ῌ,ῌ0

(182) c- (Ma-c-) (+30-). (+31+). Ma-d `cd Loc. + #5eC\ .* cm 89. CDE*FGHIJK ,* km #PQR S$%. #f!ghi<=>?Z/j(klm? (40n. o$%&TUpqTU+*X&Y8Z/$C.

(183) + .

(184) . Fig. +,. Field occurrences of eruptive units of the central cone building stage. Ma-e+e- and overlying Ma-eῌ, separated by thin humic soil at Loc. - (a). Ma-d consists of alternate of pyroclastic fall and flow deposits, at Loc. + (b). Ma-c+c., in descending order, each separated by paleosols at Loc. + (c). Ma-b is composed of three pumice fall and intervening two ash fall layers at Loc. .0 (d).. 27.

(185) / 01 21345. 28. Table ,. Summary of widespread tephras in this studied area. * : Soya (+31,) ; ** : Furukawa et al. (+331) ; *** : Hayakawa and Koyama (+332) ; **** : Machida and Arai (,**-), Furukawa et al. (,**0).. 6 78%9') 786:;<= <>?@78ABC Ma-b/b+ (+320) ..0 km- DEF =GHIJ> Ma-b / KLM%N& OPQABRSATU& ! Loc. .. , m + "V#W - cm K# W ,* cm W +* cm XY$%%?Z[ !"# $# %

(186) & ' ) -\L&'%]^#9_ Ma-b-. à,-( / cm Ma-c .. b. Ma-b+ Loc. .0 ,T%()*) +* cm.

(187) Loc. + ,T -/ cm Ma-c. (). ,* cm %% cd ()B"V#W. `% *+e,PQ-%.dB) `. ()*) , mm + cm + Ma-b/ -b- ,-. -b+ . % f6 % g6 h% f6/ . 0iR R1: 2R: b.345. 78

(188) j 6:;<=<> kQ8. l69_. @78 + m

(189) nkQ878 `% 7q *.,0 km- +. o`PQ6 + Ma-b0 Ma-b. b. Ma-b, p #r8#r=<sg66@PQ6 + t. .ῌ,ῌ1

(190) c, (Ma-c,) (+30-). (+31+). Ma-b0 % Loc. .1 , , cm Ma-b. % Loc. + , +/ cm (E Ma-b, Loc. .0 ,. uvw<xyz9:1 +* km ;{ !. 2 cm + m)B|)L78<=}i. #r8#r=<so`PQ6 + `. ,-.~7PQ6ABC\ 78 +. a ,-( / cm Ma-c - . 6;<=<>>C?@{') m80.

(191) Ma-c. Ma-c- ()g66/E Loc. ,. il6R:345l69 A. 0 cm + 78B Ma-c- (. <zJC <=}iR. + *.*, km- +. R1: 2R: b.345l6ABC\ . .ῌ,ῌ2

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(193) !". PQS. !US E. / cm Ma-c,

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(195) &. ¡ Loc. , b. Loc. -/ , Ma-b ha.. 6;<=<>L9') *.**/ km. ¢)F£J + B-Tm KQ¡ Loc. ,* ,. ¤¥L¦F. Ma-b àAB Ta-c, LMPQ¡ Loc. . . -. .ῌ,ῌ3 #$%& b (Ma-b) '( (+30,) ). *+,. Ma-c. àABNF m)BPQC tU. SO§P¨Q US8R. o`g6PQ6©ABC\ à. Table , EF. / cm Ma-c+

(196) (+30,) -\àAB Ma-b / b + =GHIJ>. TB). / -./012. ,\ Ma-b/ b- b. b+ o` Ma-b. b. b,. /ῌ+ 34567&89:;<. o`PQ6 + (Fig. +,-d) Ma-b/b+ !uª. v«;¬oO |)L0il. =GHIJ>-¦®C\ (Katsui et al., +31/) `. 6R R1: 2R:b.345l69_. aABj 9: ¯ 9: ¯1° + ±². ;<=<> + R1:2R:H

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(198) (Table. 6PQ6 Ma-b 0 : · ¤EF m)B. -) (PHIJ>T¸¹<y%9')´. Ma-b0b+ ºUE 'F7PQ. µ L ¹<y-0il6»¼½CNC.

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(200) . DEF G

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(202) O. 29. Table -. Phenocryst contents and modal compositions (wt.ῌ) of pumice in each eruption unit.. Fig. +-. Glass compositions (K,O-TiO, diagram) of widespread tephra layers. Compositional ranges presented by shaded pattern are from referenced data ; Ko-c, : Tokui (+323), B-Tm and Ta-c, Machida and Arai (,**-), Ta-c, : Analysis of proximal deposits of Ta-c, at Tarumai volcano. All data is normalized to +**ῌ anhydrous.. PQRSFTURVWNXYZ[! 3\]K^ SiO, * 01.2 YX 1/./ wt. ῌ _* !"#$ (Fig. +/) àbcK"d!e

(203) PQRS* SiO, * , wt.ῌ fg, h àbcK i j *klmPQRS* Ma-j ,B SiO, n Ma-i, h, g No4!i. p!qrXs/! abcKN

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(205) PQRS*.

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(219) 30.

(220) . Fig. +.. SiO, variation diagrams of major elements for Ma-bMa-i (bulk samples). to +**ῌ.. All data is normalized.

(221) + .

(222) . 31. Fig. +/. SiO, variation diagrams of major elements for matrix glass of Ma-bMa-l pumices. All data is normalized to +**ῌ anhydrous.. !"#$%&'() *)+,-,$.. /012345+6789:;8 <=>?;@A. BC?DEFGH (Morrissey et al., ,***) IJ$. KLMN$OP8QR ST@2 U,-,VW8. XYZR*[\ ]^H^_`\)R a^@b. FGc)D/01$Qd <=MNePZR. f`g hi5Gc)Dj@klmno 8QR. + pq8Srs@t^@b5uv.

(223) 567 879:;. 32 Table ... Representative major element compositions of volcanic glass and whole-rock samples.. <= > ?#@AB (CD . E2FG2 Ma-e+ HIJ?. /2KL@ !J? M N. O" PQRSTUVGL Ma-b H"# @. G# -.- !J?EW@KXTY. NB0 Z[#\, $]% ^_àb2&. Z[0 cSdedfg'#\h? @. Pi`j ? @"# kl-. m(2Nn g`)op?,. .- *+)o,

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(235) . ῎. 33. Table /. Summary of age, eruptive volume and styles for each eruptive unit from Mashu volcano during the last +.,*** years. PDC : pyroclastic density current. * : Katsui et al. (+31/), ** : Itoh et al. (,**1). : alternative between the two sides. Calibrated ages are calculated by the program of Stuiver et al. (,**/).. Fig. +0. Schematic illustrations showing eruptive sequence with magma systems during the main caldera-forming eruption.. (Katsui et al., +31/) !"# $%. ῐ&'() *῏&+,)-./0 0. 1 234)-5267)89:&;<=>/ ?. >@A&B4) CDE&67FG8HIG. Ma-i (..* km ) =J KL&'() MLJN" ). / 0GOῑ;P2Q)-R/ L%1S. TU)V.0 WX)85YZ[\. ῐ& Ma-g ]=O^_Vàbc28d/0e 1S. -. Pf)8HIG/ ? Ma-h (*.32 km-) =. g&hijO=k/0 lm5=n0o8. J p"=L%1S qrW1$%1srEtu. HI-vwV.. tx &ee(yz/0 {|ῐ5}~&. ? %=k/KL1L _. FG/& )-JKL (21 3. wt.ῌ) &z. Y2Hz/ ] KL&. /= WX)8YZ[\&)8HIG. Q TUz/0 1 7\C\ .. / Ma-g (-., km ) = % tYJ$z/. - Ma-i f Jhi + ¡¢x£¤¥2¦0. -. L%1S2k]§¨©)V.YZ[\2. , ª7«¬2®X=>/ KLJ tY ¯. )8 °)- ±²\³=k/ Ma-f (3.1 km-) J. ´µῐ (+.+ 0.+ wt.ῌ) ¯¶·R¸¹&j. º»& ~> L%1S W1$%1q῍1. ῎=k/& ¼r sr½. SiO, ῐ&àz/.

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(325) `)*<_, +* Lu !\]#,=^"!{8%F_ ab ., ,03ῌ,13. Heiken, G. and Wohletz, K. (+33+) Fragmentation processes in explosive volcanic eruptions. In Sedimentation in Volcanic Settings (Fisher R.V. and Smith G.A. eds), +3ῌ,0, Society of Economic Paleontologists and Mineralogists Special Paper ./. Houghton, B.F., Wilson, C. J.N., Smith, R.T. and Gilbert, J.S. (,***) Phreatoplinian eruptions, In Encyclopedia of Volcanoes (Sigurdsson H. ed), /+-ῌ/,/, Academic Press, London. l m/n0oFpq12r0s (+3/-) wx[6yz{|7} `

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(334) ¡ .,p. Katsui, Y., Ando, S. and Inaba, K., (+31/) Formation and Magmatic evolution of Mashu Volcano, East Hokkaido, Japan. J. Fac. Sci., Hokkaido Univ., +0, /--ῌ//,. (m0° ©±F 1ªF«¬5

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(340) 9 4:; 3. .--ῌ ./,. Morrissey, M., Zimanowski, B., Wohletz, K. and Buettner, R., ,***, Phreatomagmatic Fragmentation. In Encyclopedia of Volcanoes (Sigurdsson H. ed), .-+ῌ../, Academic Press, London. Nakamura, K. (+30.) Volcano-stratigraphic study of Oshima Volcano, Izu. Bull. Earthq. Res. Inst. Univ. Tokyo, .,, 0.3ῌ1,2. Reimer, P. J., Baille, 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., McCormack, F.G., Manning, S., 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. *ῌ,0 cal kyr BP, Radiocarbon, .0, +*,3ῌ+*/2. ~

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(354) rstu 4v wR: :x ,**+ yz{x|} Wentworth, C.K. (+3,,) A scale of grade and class terms for clastic sediments. Jour. Geol., -*, -11ῌ-3,. Wilson, C. J.N. (,**+) The ,0./ ka Oruanui eruption, New Zealand : an introduction and overview. J. Volcanol. Geotherm. Res., ++,, +--ῌ+1.. Wohletz, K.H. (+32-) Mechanisms of hydrovolcanic pyroclastic formation : grain-size, scanning electron microscopy, and experimental studies. J. Volcanol. Geotherm. Res., +1, -+ῌ0-.

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