有珠火山の噴火史の再検討：寛文噴火（1663年）と明和噴火（1769年）に挟まれた17世紀末の先明和噴火の発見

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(6) . . ,**. + +. ,**/ + 2 !. Re-investigation of Eruption History of Usu Volcano, Hokkaido, Japan : Finding of Pre-Meiwa Eruption (Late +1th Century) between Kanbun (+00-) and Meiwa (+103) Eruptions Mitsuhiro NAKAGAWA῍, Akiko MATSUMOTO῍, Jun TAJIKA῍῍, Wataru HIROSE῍῍ and Tadashi OHTSU῍῍ We newly identified tephra layers sandwiched by soil layers between AD +00- (Kanbun age) and +103 (Meiwa age) tephras at several sites on the northwestern flank of Usu volcano. The layers consist of pumice fall, ash fall and pyroclastic surge deposits. The presence of soil layers were confirmed by their carbon and nitrogen analysis. Petrographical and geochemical features of the pumice from newly recognized tephra largely di#er from those of +00- eruption, and are similar to those of +103 one. However, on the basis of modal volume and chemical compositions of phenocrystic minerals, it can be possible to distinguish the pumice from those of the +103 eruption. Considering the thickness of soil layers, the eruption would occur at around end of +1th century. We name the eruption as the pre-Meiwa eruption, because petrological features of the pumice is similar to those of +103 (Meiwa). The eruption was generated by distinct magma from other historic eruptive magmas. The sequence of the tephra suggests that the eruption started with phreatic eruptions followed by magmatic eruption to form plume. In the climactic stage, pyroclastic surge occurred with the formation of plume and its activity ended with phreatic eruptions. The finding of the pre-Meiwa eruption could provide important information to reveal not only relationship between eruption and dormancy but also evolution of magma systems. Key words : Usu volcano, pre-Meiwa eruption, historic eruptions, tephra, soil layer +ῌ ῐ ῎ ῑ ῏. +32+N O6 "#8$96E8W=>?6f. "#$%&'()*+,-./0123456723. DC+iP<N OP+/W +00- E867 +*0 . 458$9 (Fig. +), : +*** +;</=>?67 &. 6=>?Hj +103 / +IE8K<N O. @ +00- AB - CDE8FGHIJK< L$

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(10) u²³´¥wµ¶ Division of Earth & Planetary Sciences, Graduate School of Science, Hokkaido University, N+* W2, Kita-ku, Sapporo *0*ῌ*2+*, Japan. §*0*ῌ*2+3 ¨©ª(«( +3 ¬& +, ®. ()*·uv°±_ Geological Survey of Hokkaido, N+3 W+,, Kita-ku, Sapporo *0*ῌ*2+3, Japan. Corresponding author : Mitsuhiro Nakagawa e-mail : [email protected].

(11) ~®¯°¦±²³´¦µ¶ ·¦¸¹ ¦º» ¼. 40. Fig. +. Index map of Usu volcano, showing the locations of the trench site and outcrops where unknown tephra layer has been recognized. Basal map of + : +*,*** topographical map of Geographical Survey Institute..

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(49) BCDEFGHIJKFLM NFOP QF R S. 42. Fig. -. Historic tephra layers outcropped west of Konpira crater (Loc. + of Fig. +). (+) The outcrop at Loc. +, indicating tephra layers from Us-b surge deposits (AD +00-) to pumice fall deposit (AD +311). Dotted square shows the area of photograph ,. (,) Tephra layers between Us-b surge deposit of +00- eruption and +311 pumice fall layer. We recognized three or four layers on the basis of soil and weathered layers. Bar indicates +m thick. Dotted square shows the area of photograph -. (-) Photograph of the Unit C, which is considered as newly found tephra (Pre-Meiwa tephra) in this study..

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(90) ¯°±²{³´µ²¶ ·²¸¹ ²º» ¼. 46. Fig. 0. Whole-rock chemical compostions of pumice from three layers found at outcrops of northwestern flank of Usu volcano..

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(156) 52. 2345G ,¹º {» ¼. ῌ ῏ ῎ ῍ Amma-Miyasaka, M. and Nakagawa, M. (,**-) Evolution of deeper basaltic and shallower andesitic magmas during AD +.03ῌ+32- eruptions of Miyake-jima volcano, IzuMariana arc : inferences from temporal variations of mineral compositions in crystal-clots. J. Petrol., .., ,++ῌ,+-2. (+32,)

(157) ,1 -,/. !"#"$%&' ()*+ ,- ./01 2345 6789 :; 8- <=> ? @5 ABῌCDE F GH+G?I (,**,) J ,*** K : LMNOPBQR .1 ,13ῌ,22. Nakagawa, M., Wada, K., Thordarson, T., Wood, C. P. and Gamble, J. A. (+333) Petrologic investigations of the +33/ and +330 eruptions of Ruapehu volcano, New Zealand : formation of discrete and small magma pockets and their intermittent discharge. Bull. Volcanol., 0+, +/ῌ -+. Oba, Y., Katsui, Y., Kurasawa, H., Ikeda, Y. and Uda, T. (+32-) Petrology of historic rhyolite and dacite from Usu volcano, north Japan. J. Fac. Sci. Hokkaido Univ., Ser. IV, ,*, ,1/ῌ,3*. Okumura, K., Soya, T., Ono, K. and Satoh, H. (+32+). Petrology of rhyolite and dacite erupted in last -** years from Usu volcano, Japan. +32+ IAVCEI Symposium, ,10 ῌ,11. ST

(158) UG?IVWX5 (+32+) JYZ[\]^_ ` YZabc :; ?"$% (,**.) def +3., Kghihjklmno .3 +,3ῌ+.,. pGq+rs8Ctu+ v-Cwx yz{|}~X (,**,) J ,*** K .1 0./ῌ00+. C (+33/) Jg 0/ 03,ῌ031. C Ctu+ (,**,) J ,*** K - -+ Nhih .1 00-ῌ01.. Tomiya, A. and Takahashi, E. (+33/) Reconstruction of an evolving magma chamber beneath Usu volcano since the +00- eruption. J. Petrol., -0, 0+1ῌ0-0. {<'* (+31-) JYZ\] ¡¢£ ¡¤¥ ,/. p. ¦§ (+331) ¨© ª X«-¬ ®¯¨© ° ±²`³ .-ῌ/-. ´µ¶· Cwx¸.

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