On the Essential Ejecta of the September,**. Eruptions of the Asama Volcano, Central Japan

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1 /* (,**/) / ,**. 3 *.,**/ - -+,**/ 1 +. On the Essential Ejecta of the September,**. Eruptions of the Asama Volcano, Central Japan Yasuyuki MIYAKE, Kou TAKAHASHI, Tatsuro TSUGANE, Kuniaki MAKINO, Hisakazu KAKUZEN, Kuniaki NISHIKI, Takashi FUKUI and Shinshu University Research Group for Asama *. Eruptions Asama volcano in central Japan became active on September +st,,**. with a vulcanian eruption which was the biggest eruption among a series of small eruptive events from September to December in,**.. The ejecta of the Sept. +st eruption were mostly polygonal fresh andesitic lithic fragments. Most of them were derived from the andesite body which played the role of cap rock for the rising new magma and escaping volcanic gas. Less amount of pumice fragments were also ejected and most of them are mantled by the black and dense andesitic crust, which is broken and expanded to form breadcrust structure. These breadcrust pumices are concluded to have been the fragments of the new magma. After they were broken into pieces by the explosion, the outer margin of them rapidly consolidated, and then degassing and inflation in the inner melt caused rupturing of the outer crust. The pumices are clearly discriminated from the andesitic lithic fragments by their whole rock chemistry. The next vulcanian eruption occurred on September,-rd. Majority of the ejecta were again polygonal lithic fragments of andesite, although their chemistry corresponds not to the lithic fragments, but to the pumices of the Sept. +st eruption. Small amount of scoria are found among the Sept.,-rd ejecta. Their whole rock chemistry and the assemblage and chemical composition of phenocrysts are quite similar to those of the Sept. +st pumice. The black appearance of the scoria is derived from the less crystalline, therefore less di#erentiated groundmass glass in the scoria than in the pumice. From these petrologic evidences the following magmatic processes are deduced. Before Sept. +st, a column of new magma had risen into the pre existing andesite body beneath the crater floor. On Sept. +st, the built up gas pressure surpassed the tensile strength of the andesite body to result in a vulcanian eruption. This eruption provided fragments of the pre-sept. +st andesite as polygonal lithic fragments, along with smaller amounts of the breadcrust pumices. In the course between Sept. +st and Sept.,-rd, the column of new magma uplifted because the cap rock had disappeared. The upper part of the column was cooler and more crystalline than the lower. Some portion of the upper part of the magma column e#used and made a dome inside the crater. On Sept.,-rd, next vulcanian eruption took place. The ejecta consisted of the lithic fragments which had been upper part of the consolidated magma column, and the scoria derived from the lower level of the same magma column. Key words : Asama volcano, vulcanian eruption, essential ejecta, breadcrust pumice -3* 20, Department of Geology, Faculty of Science, Shinshu University, Asahi - + +, Matsumoto -3* 20,+, Japan. -3* 20, Division of Environmental System Science, Graduate School of Science and Technology, Shinshu University, Asahi - + +, Matsumoto -3* 20,+, Japan. Corresponding author: Yasuyuki Miyake ymiyake@gipac.shinshu-u.ac.jp

2 334 *. +,**. 3 (,**.) 3 3 +,* *, 0 km - cm , km - cm 3,3 +, +1. km. cm 3 +0,**/ 3 + 3,- 3 +, (,**.) (,**.) 3 +, ,, , (Loc. +) (Loc.,) (Fig. +) Loc. + (Fig.,a) (Fig.,b) / +* mm Fig. +. Sampling localities. a: Index map. b-d: Detailed map of Loc Topographic map adopted from + :,/,*** quadrangle map Kitakaruizawa (Geographical Survey Institute of Japan, +323). The eruptions took place in the Kama-yama crater which is the youngest central cone formed on the top of Maekakeyama. AVO : Asama Volcano Observatory. (Fig.,c, d) +3* g,.1 g Loc.,, cm 0 cm 2.0 g + m,..10 mm Table wt.

$+d) " U ' *.2(,.1 gῌcm- 9:; <=",->?9@> 01 Z,. P[\ ],1 P^_`a bcd/ef g - Fig.,. Photographs of the ejecta. a-d : Ejecta of the Sept. +st eruption sampled at Loc. +. a : The occurrence of andesitic lithic fragment.$

3 QRU$,**. h 3 O U ij k Loc. +,, *./ mm Fig AB CD EF/GH IJ 4",ῌ, 3,- ῌῑ ῐ!" #$%& ' +.1(,.1 gῌcm )*+,-. KLMN 3 O,- P '+ 3 O,1 P QR / 01' +.* gῌcm- 23 )45+ 67,801 STU$V WXY (Loc. - ; Fig. +d) " U ' *.2(,.1 gῌcm- 9:; <=",->?9@> 01 Z,. P[\ ],1 P^_`a bcd/ef g - Fig.,. Photographs of the ejecta. a-d : Ejecta of the Sept. +st eruption sampled at Loc. +. a : The occurrence of andesitic lithic fragment. Grasses beneath the fragment were heated and scorched. b : Andesitic lithic fragment with a polygonal outer shape. The length of the scale is +* cm. c : Breadcrust pumice. The length of the scale is +* cm. d : Cross section of a breadcrust pumice. The dark colored outer part is the non-vesicular crust, which was ruptured by the expansion as a result of the vesiculation in the inner magma, which is represented by the pale colored inner pumiceous part. The lower part of this sample is broken. e and f : Ejecta of the Sept.,-rd eruption sampled at Loc. -. e : Andesitic lithic fragments. f : Scoria.

-. Bulk density of the ejecta of the Sept. +st eruption.

4 336 *. Table +. Size distribution and the constituent rock types of the Sept. +st ejecta sampled at the Loc.,. Table,. Constituent rock types of the Sept.,-rd ejecta sampled at Loc. -. Fig. -. Bulk density of the ejecta of the Sept. +st eruption., cm + m Table, +/ wt. *.+ mm. +.- g cm - *. (,**.) *.*+ mm Table - Fig.. +*** - -** 3 + 3,-,.* mm *.. mm +.0 mm *.3 mm 3 +, ,- -, Fig. / Fig ,- Fig. 0 3,-

,**. 3 337 Table -. Modal compositions of the ejecta. The No. 2 and No.

tr : Exist but less than *.+ wt.. V, Cr Rh (,**+) 3,-., Table. Fig. 1 3 + Fig.

5 ,** Table -. Modal compositions of the ejecta. The No. 2 and No. 3 samples are the outer crust and inner pumice of a breadcrust pumice, respectively. tr : Exist but less than *.+ wt.. V, Cr Rh (,**+) 3,-., Table. Fig Fig... Modal compositions of the ejecta. Fig X (PHILIPS PW,.**), g. g (+330) SiO, Fig. 1g, h (Rb/Zr) (Fig. 1h) 3, , ,**- SiO, SiO, Fe, O -, MgO, CaO Fig. 1g, h MgO, CaO 3 +

$(EDS) E //{1* }F~Du E ῌ /0123 456 789:; <=>?@ TGcD (Fig. 2)3 >y u (,**.) 6AB/CDE FC21G> H An F Yc 6E c3 8 77 >^ G IJKLM NOPQ RS2TFU:.$

6 338 t ῑ k ª«ῒ ±²³ µ *. ¹ º»¼I: /ῌ ῐ ΐῒ hi>jk/cde 3 l + & m 6 3 l,- & nkh/ Mg o pqrso tcu Table!" #$ SEM / F]^3 vw > x y FbcGzD6 An %&'()* JSM-/-+*E+ῌEDAX DX., -. (EDS) E //{1* }F~Du E ῌ / :; <=>?@ TGcD (Fig. 2)3 >y u (,**.) 6AB/CDE FC21G> H An F Yc 6E c >^ G IJKLM NOPQ RS2TFU:.V WXG / Mg o 0*{1* acf ^D ub Y23 E F Mg o 12 Fc2D / Mg F u uc /ῌ+ ῑῐ D3 y 6 F c>c Z Z [\ Table / 6 Fig. 2 F]^3 c3 : > Fa 6` AB _`a` FbcG> Y2 deezc2tfg ῐ F D (Fig. 2)3 c Fig. /. Microphotographs of the ejecta. a : Outer rim of the Sept. +st breadcrust pumice. b : Outer rim of the Sept.,-rd lithic fragment. c : The Sept.,-rd scoria. Fig. 0. Sketches of the habits of phenocrysts by tracing the microphotographs (Fig. /). Vesicles and groundmass are not shown. For the arrows see the text.

7 ,** Table.. Representative chemical analyses of the ejecta. Fe,O -* Total Fe as Fe,O -. /, 3 + 3,- Fig ,- +, 2 (Table 0),**/ - EDS *.+ -* +. Fig. +* Table 1 SiO, ,./ 1..2 (Fe, O -) Fig. +* (Fig. 0)

8 340 *. Fig. 1. Whole rock chemistry of the ejecta. The products of the Tennin and Tenmei eruptions are from Takahashi et al. (,**-). Fe,O -* total Fe. All the data are recalculated on the basis that the total +**. See the text for the interpretation of the ellipses in the figures g and h.

9 ,** Table /. Representative chemical compositions ofthe phenocrysts. Walker (+303) SiO,-K, O SiO,-Rb Zr SiO, Fig. 1g, h 3 + pre3 + pre3 + Table. 3 + No. / 0 +, 3,- 3,- Figs. /, ,- 3,- 3,- 3, (Fig. 1) 3, , 3,- 3 + (Figs.., 1) 3 +,- 3, ,- 3 + SiO, Fe (Fig. +*) -* +. (Table 0) 3,- 3 +

10 342 *. Fig. 2. Compositions of pyroxene and plagioclase phenocrysts ,- 3, Fig pre3 + pre3 + (Fig. ++a) 3 + FT-IR *.0 wt. 2/* /* bar (Silver et al., +33*) (Self et al., +313) Self et al. (+313) +**,** bar (+33/)., bar /* bar (Fig. ++b) (,**.) +0 (,**.) SiO, Fe

$,,**+) K"LMN #$ 3 & +0 ' O 3 &,- ' ()*" +, $ ()* - PQ RSTUV/ WX YZ[#!" \I]P./0 1 23/ 4/ 5# 67 894/ :; < 67= Table 1.$ The chemistry of the microlites and glass in the groundmass of the Sept. +st pumice and the Sept.,-rd scoria.

The chemistry of the microlites and glass in the groundmass of the Sept. +st pumice and the Sept.,-rd scoria.

density of constituent minerals. Fig. 3. Back-scattered electron images of the groundmass. a : The crust of Sept.

Modal compositions of the groundmass from the crust of the Sept. +st pumice and the Sept.,-rd scoria. Fig. +*.

11 ^_O,**. ` 3 &ao bca,d e5 343 >?@AB0CD EFG0-H/I J!!" #$ % (Smith et al.,,**+) K"LMN #$ 3 & +0 ' O 3 &,- ' ()*" +, $ ()* - PQ RSTUV/ WX YZ[#!" \I]P./0 1 23/ 4/ 5# / :; < 67= Table 1. The chemistry of the microlites and glass in the groundmass of the Sept. +st pumice and the Sept.,-rd scoria. The average composition of microlite is calculated using the data of chemical compositions, modal compositions and density of constituent minerals. Fig. 3. Back-scattered electron images of the groundmass. a : The crust of Sept. +st pumice. b : Sept.,-rd scoria. Mg : magnetite, Pl : plagioclase, Px : pyroxene, Qtz : quartz. Table 0. Modal compositions of the groundmass from the crust of the Sept. +st pumice and the Sept.,-rd scoria. Fig. +*. Chemistry of the glass and microlites. WR : The whole rock compositions. The plots of the microlites are the average compositions of all constituent minerals as shown in Table 1.

$ῌ 5 3 + G"4Z<"[ \ D>]^0_`^&aW) )V2 33b cde 3,- f> D pre 3ῌ+ g?;&hij ;kd43 < l g?;d\ mn D:; op0q5rst3) <Z23 u_ vwz x 3 + 5,- yf>54z { <}~ f> c235 e\5# 8 V 3 yf>5 +. +2 f> j Dl Fig. ++.$

12 344 i >? *. f> ~,**/ 3 +!"#$%&'() *+,-). / #:;5<2 =&>?@A#BC (Fig. ++c) 3,- EF&G2,H)3 I;0J8KL) &M2 4ANOP5) 43 35QR (Fig. ++d) ANOP S@TA&UV W XDY *.+ wt.ῌ G"4Z<"[ \ D>]^0_`^&aW) )V2 33b cde 3,- f> D pre 3ῌ+ g?;&hij ;kd43 < l g?;d\ mn D:; op0q5rst3) <Z23 u_ vwz x 3 + 5,- yf>54z { <}~ f> c235 e\5# 8 V 3 yf> f> j Dl Fig. ++. The schematic illustrations of the eruption process. The size of the magma column and its depth are not in scale. a : Before Sept. +st, a column of new magma had risen to some shallow level beneath the crater ﬂoor and the decrease in pressure caused degassing of volcanic gas which was built up beneath the pre-sept. +st andesite body. b : On Sept. +st, the built up gas pressure surpassed the tensile strength of the pre-sept. +st andesite body to result in a vulcanian eruption. This eruption provided fragments of pre-sept. +st andesite, altered older rocks and magma. The magma fragments were originally polygonalshaped. While they were transported in air, they inﬂated as the result of the delay vesiculation inside the magma and formed into breadcrust pumices. c : In the course between Sept. +st and Sept.,rd, the column of new magma uplifted because the cap rock had disappeared. The upper part of the column was cooler and more crystalline than the lower part in which the residual magma was poorer in SiO, and more enriched in Fe therefore it formed darker glass than the pumice of Sept. +st eruption. Some portion of the upper part of the column e#used and made a lava dome. d : On Sept.,-rd, next vulcanian eruption took place. The ejecta consist of lithic fragments of already consolidated upper part in the magma column and the scoria derived from the lower level of the same column.

13 ,** , 3 + 3,- 3,- 3,- 3,3 +* +* ++ +.,**/ ,-,, ,- 3,,**. B (,**--B-*+) (C) : +1/.*.-/ (+33/) +++,/,3. (,**.),**. 3,**. +21 p. (,**.) p. (+330) X -+, +*/ ++1.,**/ (SAR),**. /*.*+.+* Self, S., Wilson, L. and Nairn, I. A. (+313) Vulcanian eruption mechanisms. Nature,,11,..*..-. (,**.),**.,**. +20 p. Silver, L. A., Ihinger, P. D. and Stolper, E. (+33*) The influence of bulk composition on the speciation of water in silicate glasses. Contrib. Mineral. Petrol., +*., +., +0,. Smith, J. V., Miyake, Y. and Oikawa, T. (,**+) Interpretation of porosity in dacite lava domes as ductile-brittle failure textures. Jour. Volcanol. Geotherm. Res., ++,,,/ -/. *. (,**.),**. 3 + :,**. +3, p. (,**-) -2 0/ 22. (,**.) ++* +/2 +1. p. (,**+) X ++ +, (C)(,) ++0.*..1,- -/ p.,**/,**.

14 346 *. Walker, G. P. L. (+303) The breaking of magma. Geol. Mag., +*0,

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