+3.,. +3., , /,* +3, (Takeuchi and Nakamura,,+) +3., , -,,* / /,/ ,3 +31/ Katsui and Komuro, +32. ; +3.

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1 .3 (,**.) - +,3 +., +3.,,**- +, +*,**.. +* The +3., Eruption of Hokkaido-Komagatake Volcano Was Phreatomagmatic Ryo TAKAHASHI, Mitsuhiro NAKAGAWA, Kiyoshi NAKANISHI and Mitsuhiro YOSHIMOTO During the,* th century, two major eruptions of Hokkaido-Komagatake volcano occurred in +3,3 and +3.,. The +3,3 eruption was one of the largest magmatic eruptions in Japan in the last +** years, and has been investigated in detail. On the other hand, no systematic volcanological research has ever been done on the +3., eruption. Since it has not been clarified whether the +3., eruption was magmatic or phreatic, we decided to investigate the sequence and eruption mode of this event. At the summit, the +3., eruptives overlie the +3,3 pyroclastic fall deposits and are covered by ash-fall deposits of recent phreatic eruptions. Based on the photographs, documentary descriptions and sequence of the eruptive deposits, we were able to divide the eruption into three phases. Phase-+ : Start of the eruption on November +0 at 2 : ** (JST) that formed NW-SE trending fissure +.0 km in length. This was followed by eruptions at the main crater (+3,3 crater) that produced the pyroclastic surges ( 2 : +*). Phase-, : Eruption column had grown up from 2 : +2 2 :,* and ash-fall started to spread towards the southeast portion of the +3,3 crater. Phase-- : Decrease of the eruption activity at 2 :.* and continuous ash emission until the evening. We newly found fresh pumice and its fragments in the deposits associated with the Phase-+ event. This fresh pumice exhibits no alteration features and has similar petrological features to other historic pumice since +0.*. This fresh pumice, however, could be distinguished from other historic pumice by the most silicic nature of its matrix glasses. Therefore, we concluded that the pumice was derived from the juvenile magma that caused the +3., eruption. Considering the temporal evolution of the component of the eruptive materials, the +3., eruption began with a phreatomagmatic eruption (Phase-+) followed by phreatic eruptions (Phase-, and --). Before the two major eruptions in +3,3 and +3.,, small eruptions occurred intermittently from ,. and , respectively. Based on these observations, the preceding eruptions could be possible precursor activities of subsequent magmatic eruptions. Since +330, small phreatic eruptions have intermittently occurred in Hokkaido-Komagatake until,***. The present state of the volcano, as we would like to emphasize, has been quite similar to the conditions prior to the two previous magmatic eruptions. Key words : Hokkaido-Komagatake volcano, phreatomagmatic eruption, juvenile ejecta + +0.*. + (Fig. +). +330,*** *0* *2+* +* 2 Department of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N +*W2 Kita-ku, Sapporo *0* *2+*, Japan. *.3, Mori Town O$ce, Miyuki, Mori, Kayabe, +331 a;,**+,*, +3,3 *./, km - Hokkaido *.3,-3-, Japan. ++- **-, Earthquake Research Institute, University of Tokyo, Yayoi, Bunkyo-ku, Tokyo ++- **-,, Japan. Corresponding author : Ryo Takahashi takaryo@ep.sci.hokudai.ac.jp

2 , , ,*** +3, (Takeuchi and Nakamura,,**+) +3., +3.- Katsui and Komuro, +32. ; ,3 +3., +3., +3., +3., +3.,, -,,*** / /,/** /,*** * /0 +3,3. +31/ ; +323 ; +331 ; / +3,3,* ,. +3,3 *./, km , ,. - +* 0 t b;,**+ /* a +332,***,*** 0 Cl S,**+ Fig. +. Location of Hokkaido-Komagatake volcano. Active volcanoes are also shown , +3., (+3.-) (+3..) Table + +3.,. ++ +,

3 +3., * - (Fig., a). 2 +2,* 2,*** m, +** m 2,*,- (Fig., b), / 0 cm (Fig., c). +, km +* 2.* 2 /* /** +,*** m, cm (Fig. -) km (Katsui and Komuro, +32.)., ,3 - (Phase) (Phase-+), (Phase-,), (Phase--). +3., +3., +3,3 +3., +3., (Figs.., /). +3., +3,3 +** m +31/ +3,3,. cm +3., (+3-3) Table +. Sequence of the +3., eruption of Hokkaido-Komagatake volcano.

$/1 cm 9 /0 :; <= +>? @ 78ABC I II III - D EFGH#IJ 4 (Fig. 0). Fig.. # KLM#?N KEFGH 567 8OP1 $Q#KEFGH RSO T. f *+ #D + f U #+VJWOX YZ [#\] (Fig. 1).$

4 132 } ~ Fig.,. Photographs of the +3., Hokkaido-Komagatake eruption (Mori Meteorological Observatory, +3..). (a) Eruption cloud of about / minutes after beginning of eruption from Shikabe town (about +, km east-southeast from the vent). The arrow indicates pyroclastic surge. (b) Eruption cloud at +* :,* from Onuma (about 2 km south from the vent). (c) Eruptives of the +3., eruption at Shikabe town ,!"# +330 $% &'( )# *+ ),-./ , ) / /1 cm 9 /0 :; <= 78ABC I II III - D EFGH#IJ 4 (Fig. 0). Fig.. # KLM#?N KEFGH 567 8OP1 $Q#KEFGH RSO T. f *+ #D + f U #+VJWOX YZ [#\] (Fig. 1). 5Q^ EFGHῌ +> 2 mm $Q _` +a, f #bcdoedfgh<= ij: )k C Fig. -. Isopach map of the +3., volcanic ash (Mori Meteorological Observatory, +3..). Contour lines are,** m intervals. l /0 +3,3 )m noc po#cn q`jr1. 78 / cm st u` vwh 78 AB x (Fig..). y z ;{LM

5 +3., 133 Fig... Maps showing thickness of each unit in centimeters at each locality. Symbols A to F indicate the localities of stratigraphic columns in Fig. /. Contour lines are +* m intervals. II III II I II +3,3 (Fig..). -2 cm +3,3 Fig. /. Correlative stratigraphic columns for representative sections. Locations of each section are shown in Fig... cm II +3,3 I +3,3,* cm (Fig..). I II +* cm II / +3., +f PW-+.*. XRF + :, JEOL22** EPMA ZAF +3.,

6 134 Fig. 0. Generalized columnar section ofthe +3., tephra A, B (a) A A Fe-Ti -..* vol. (b) B B Fe Ti A (c) Fig. 1. Grain size distribution at representative section ofthe +3., ejecta. Fe-Ti, + intersertal + hyalo-ophitic (d) Fe-Ti eutaxitic (e) 0 +, A, B (a) A

7 +3., 135 A (b) B B (c) Fe-Ti 0, +3., +323 intersertal B 0 - +* wt. Table, -, f I 3/ wt. 2/ wt. A B II 2* wt. A 0* wt. B,* wt. B A B III -* wt. A A B , A, B Fe-Ti A B -..* vol. 1, SiO, /3.2 0,.. wt. Table - +3., B SiO, 0* wt. (Fig. 2). Table -. Representative whole rock chemical compositions of white pumice of the +3., and historic eruptions. Total Fe as Fe,O -. Table,. Temporal variations in component of the +3., ejecta.

8 136 Fig. 2. Harker diagrams for white pumice of the +3., and historic eruptions. Table.. Representative matrix glass chemical compositions of white pumice of the +3., and historic eruptions. Na, O Na,O B Na, O A B SiO,, Al,O -, FeO* CaO 1 - A B (Table.). SiO, 1/.* 11.* wt. SiO, Al, O - FeO*, CaO (Fig. 3). +3., A A SiO, 1/.2 2*., wt. SiO, Al,O - CaO FeO* (Fig. 3). B B SiO, 1/ wt. B +2/0 +3,3 SiO, Al, O -

9 +3., 137 Fig. 3. Harker diagrams of matrix glass chemical compositions of white pumice of the +3., and historic eruptions * +33/ (Nakada et al., +33/ ; Watanabe et al., +333),,***,**+ ;,**, ;,**,,***,**+ ;,**+ ;,**, +, - +,,, +,*** - -+

10 138,**+ ;,**, +, - Sakuyama, Ruapehu +33/ 3 +* (Nakagawa et al., +333),,* Arenal (Reagan et al., +321 ; Streck et al.,,**,).,*** +311,**,,***,**+ ;,**+ ;,**,,*** /** (Amma-Miyasaka and Nakagawa,,**-),*** /** Amma-Miyasaka et al., - 2, +3., , A, B A SiO, Al, O - FeO* (Fig. 3). FeO* SiO, Al,O ; +32+ ;,**- A B (Figs. 2, 3). B (Fig. +*). B SiO, K,O Al,O - (Figs. 3, ++). B B , , +3., +3., +3,3 +.0km (Katsui and Komuro, +32.). II III +3,3 (Fig..), +3,3

11 +3., 139 Fig. +*. Histograms of core compositions of phenocrystic minerals in white pumice of historic eruptions and Type-Bpumice of the +3., eruption. Fig. ++. Histograms for K,O content of matrix glass of white pumice from historic eruptions and Type-Bpumice of the +3., eruption. +3,3 I I II III +3,3 I +3., +3., (Fig..), +3., I II I II III (Figs. -,.). III III

12 140 I II Phase-+ III Phase-, +3., Phase-+ +3,3 Phase-+ (Phase-,), (Phase--) , +3., (,**+) Table /,*, (Table /).,*, ,*** 0,***,**+ +3,3 +3.,, +330,*** +3,3 +3.,,***,**- +3,3 +3., +3,3 +3., 3 +3., +3., Table /. Summary of eruptions of Hokkaido-Komagatake from AD +2/0 to,***.

13 +3., ,3 +3.,,, Amma-Miyasaka, M. and Nakagawa, M. (,**-) Evolution of deeper basaltic and shallower andesitic magmas during the AD eruptions of Miyake-jima volcano, Izu-Mariana arc : inferences from temporal variations of mineral compositions in crystal-clots. J. Petrol.,..,,++-, ,,03, ,3 0/ 2* +** ++, ,13, / p Katsui, Y. and Komuro, H. (+32.) Formation of fractures in Komagatake volcano, Hokkaido. J. Fac. Sci. Hokkaido Univ. Ser.,,+, /. +323,**,,***.1, Nakada, S., Motomura, Y. and Shimizu, H. (+33/) Manner of magma ascent at Unzen Volcano (Japan). Geophys. Res. Lett.,,,, /01 /1*.,**+,*** ++* *. 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+, +/ -+.,**+,*** :.0,3/ -*..,**,,*** :.1,13, /, +0* / 13.,** / S,,-,-2. Reagan, M. K., Gill, J. B., Malavassi, E. and Garcia, M. O. (+321) Changes in magma composition at Arenal volcano, Costa Rica, / : real-time monitoring of open-system di#erentiation. Bull. Volcanol.,.3,.+/.-.. Sakuyama, M. (+32-) Petrology of arc volcanic rocks and their origin by mantle diapers. J. Volcanol. Geotherm. Res., +2,,31 -,*. Streck, M. J., Dungan, M. A., Malavassi, E., Reagan, M. K. and Bussy, F. (,**,) The role of basalt replenishment in the generation of basaltic andesites of the ongoing activity at Arenal volcano, Costa Rica : evidence from clinopyroxene and spinel. Bull. Volcanol., 0., -+0 -,1. Takeuchi, S. and Nakamura, M. (,**+) Role of precursory less-viscous mixed magma in the eruption of phenocrystrich magma: evidence from the Hokkaido-Komagatake +3,3 eruption. Bull. Volcanol., 0-, -0/ -10.,**,,*** ,**+,*** - -+ /,,+/,, a , +.+ +/ b ,.,3.-+.,**+,*** 2 +2

14 142 SO, ++*,/1,1*. Watanabe, K., Danhara, T., Watanabe, K., Terai, K. and Yamashita, T. (+333) Juvenile volcanic glass erupted before the appearance of the +33+ lava dome, Unzen volcano, Kyusyu, Japan. J. Volcanol. Geotherm. Res., 23, ++- +, *

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