196 Aso-4 9 Table 1. List of pumice-fall deposits and their stratigraphic relations with widespread tephra layers. Accurate stratigraphic positions ca

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1 48 (2003) Pumice-fall Deposits from Aso Volcano during the Past 90,000 Years, Southwestern Japan Yasuo M>N67J8=>, Hideo HDH=>OJB>, Hideki T6@696, Kazunori W6I6C67: and Sheng XJ Aso central cones located within Aso caldera, central Kyushu, southwestern Japan, initiated their activity soon after the formation of the caldera (ca. 90 ka). The cones have produced voluminous airfall tephra layers and lava flows. Most of the tephra layers distributed in and around Aso caldera are andesite to basaltic-andesite scoria-fall and ash-fall deposits. Their stratigraphy is very complicated because it is di$cult to distinguish between scoria-fall layers in the field. However, dacite to rhyolite pumice-fall deposits from some central cones interbedded between the tephra layers are very useful to correlate stratigraphic units at separated localities. Therefore, we used the pumice-fall deposits in order to construct the tephrostratigraphy and eruptive history of Aso central cones during the past 90,000 years. Thirty-six pumice-fall deposits were identified including eleven major key beds. In ascending order they are Nojiri pumice (NjP), Ogashiwa pumice (OgP), Yamasaki pumice 5 (YmP5), Sasakura pumice 2 (SsP2) and 1 (SsP1), Aso central cone pumice 6 to 3 (ACP6-ACP3), Kusasenrigahama pumice (Kpfa) and Aso central cone pumice 1 (ACP1). Phenocrystic minerals of most pumice are plagioclase, ortho- and clinopyroxene and magnetite, but NjP, ACP5, ACP3 and ACP1 include biotite, and NjP and SsP2 contains hornblende phenocrysts. On the basis of several 14 Cages of buried soils just below pumice (above Kpfa) and stratigraphic position eruption ages for the eleven major pumice are estimated as follows: NjP (85 ka), OgP (80 ka), YmP5 (69 ka), SsP2 (57 ka), SsP1 (56 ka), ACP6 (52 ka), ACP5 (45 ka), ACP4 (40 ka), ACP3 (39 ka), Kpfa (31 ka) and ACP1 (4 ka). During the past 90,000 years Aso central cones produced pumice-fall deposits at an interval of about 2,500 years. Many of the sources of the pumice appear now buried under the present Aso central cones Aso-1 9 Aso ; Kyushu Research Center, Forestry and Forest Products Research Institute, Kurokami , Kumamoto , Japan Institute of Geoscience, Geological Survey of Japan, AIST, Tsukuba Central 7, Higashi 1 1 1, Tsukuba , Japan Cultural Division, Kumamoto Prefectural Govern- 25 km 18 km Aso Aso-4 Aso-4 ment, Suizenji , Kumamoto , Japan Faculty of Education, Kumamoto University, Kurokami , Kumamoto , Japan. Scottish Universities Environmental Research Centre, University of Glasgow, Scottish Enterprise Technology Park, East Kilbride, G75 0QF, UK. Corresponding author: Yasuo Miyabuchi yasuo@a#rc.go.jp

2 196 Aso-4 9 Table 1. List of pumice-fall deposits and their stratigraphic relations with widespread tephra layers. Accurate stratigraphic positions cannot be determined (1985) (2001) (1989) (1991) (1996) (1999) (1991) Aso-4 Aso-4 Tn 1995 Aso- 4 (1989) 6 (ACP1 ACP6) Aso-4 60 m 100 m (Table 1) 3 Kpfa ACP2 25,000 yrs BP 14 C ; 1995 Tn AT; 1976; ,300 yrs BP K-Ah; 1977; 1978; , ; 1992; 1997 Aso (Figs. 1 2)

3 9 197 Fig. 1. Site-location map of measured stratigraphic sections in and around Aso caldera. Open and solid triangles show peaks of major Pre-Aso volcanoes and post-caldera central cones, respectively. Aso-4 36 (Fig. 3, Table 2) Fierstein and Nathenson (1992) VEI (Volcanic Explosivity Index; Newhall and Self 1982) (Table 2) VEI NjP; NjP A0218 A0236 A0218 Aso m 132 cm (Fig. 2)

4 198 0 K-Ah A9806 Teno A9915 Makino Farm A0110 Doi Farm A9564 Koga K-Ah A0101 Sasakura KsS A0015 Ohmichi KsS A9820 Ikenotsuru N2S KsS AT Kpfa ACP4 ACP5 YmS3 YmS15 ash AT Kpfa AT YmS3 NbP2 YmS15 K-Ah ACP6 Hnd pfl 10 SsP2 Hnd pfl ACP4 ACP5 ACP6 ACP3 Kpfa AT YmS3 NbP1 YmS15 OgP Aso-4 pfl 20 OgP Aso-4 pfl well-stratified ash SsP1 ACP3 ACP4 ACP5 MzP3 ACP6 OmP1 SsP2 AT Kpfa MzP1 ACP3 ACP4 MzP3 30 EXPLANATION Blackish buried soil 40 Brownish buried soil & silty ash Sandy ash Vitric ash Scoria Pumice Pyroclastic-flow deposit 50 m Fig. 2. Stratigraphic relations of airfall tephra layers from Aso central cones at sections northeast to southeast of Aso caldera. See Fig. 1 for locations of the sites.

5 9 199 Fig. 2. continued.

6 200 0 (60 ka) OjS 3 ka KsS 3.7 ka ACP1 4 ka K-Ah 7.3 ka MzP2 YmP ka ACP3 (39 ka) well-stratified ash YmP2 YmP3 YmP4 YmS4 17 ka ACP4 (40 ka) MzP3 10 NbP1 NbP2 (19 ka) ACP5 (45 ka) YmP5 (68 ka) YmP6 YmP7 YmP8 YmP9 (70 ka) YmP10 YmP11 YmS15 21 ka 15 ACP6 (52 ka) MzP4 AT 29 ka Horizon of Handa pyroclastic-flow deposit from Kuju volcano (54 ka) YmP12 YmP13 20 Kpfa 31 ka MzP1 32 ka OmP1 SsP1 (56 ka) SsP2 (57 ka) KtP1 KtP2 MbP1 MbP2 MbP3 EXPLANATION NjP Blackish buried soil 25 m Brownish buried soil & silty ash Sandy ash Vitric ash Scoria Pumice Aso-4 pfl 89 ka Fig. 3. Generalized composite stratigraphy of tephra layers from Aso central cones during the past 90,000 years at the east of Aso caldera. Detail names of pumice-fall deposits are shown in Tables 1 and 2. Ages were determined by calibrated 14 Cdates and K-Ar dates and ages in parentheses were estimated by their stratigraphic positions.

7 9 201 (2.5Y4/6) 77 cm (2.5Y7/3) (2.5Y8/4) 4 MP; 3 4cm 10 cm 45 cm (2.5Y8/4) MP: 2.5 cm 8km A cm (1977) A0218 3km Aso cm 6km 130 cm Aso-4 VEI OgP; Aso m (7.5YR5/8) A0106 OgP 3 A 9806 A0106 A cm 2 3 cm MP cm (NjP) NjP Aso m Aso m (Fig. 2) NjP Aso YmP10-YmP6; A m A m 5 (Figs. 2, 4A) 10 YmP10 6 (YmP6) (2.5 Y7/3) 9 (YmP9) 8 (YmP8) (Fig. 4A) YmP5-YmP1; A9746 A m (Fig. 2) A m A m m 1 2 m YmP5 YmP m 1/3 2/ 3 (2.5Y3/1) cm 5 (YmP5 YmP 1) (10YR7/4) cm A9748 MP 3.5 cm A cm cm cm MP cm YmP4 YmP1

8 202 Table 2. Characteristics of pumice-fall deposits from Aso central cones during the past 90,000 years.

9 ?? 0 (A) YmP5-YmP10 ash including scoria (B) SsP1 and SsP2 (C) ACP6 and MzP4 1 m 1 m 1 m unit 7 unit 6 unit 7 YmP5 YmP6 A0104 Shimoyokobori YmP8 YmP9 YmP10 A9748 Ohki YmP7 A9903 Yamasaki A9746 Yamasaki YmP5 YmP6 YmP9 YmP10 A0006 Kamitamarai A0101 Sasakura A0011 Yamasaki A9903 Yamasaki SsP2 A9746 Yamasaki SsP1 A0006 Kamitamarai unit 7 unit 6 unit 3 unit 2 unit 1 Makino Farm A9915 Sasakura A0101 Mizunomoto A9831 Yamasaki A0011 Yamasaki A9903 Yamasaki A9746 MzP4 Kamitamarai A0006 unit 5 unit 4 unit 3 unit 2 unit 1 unit 6 unit 5 unit 4 unit 3 unit 2 unit 1 ACP6 (E) Kpfa and MzP1 unit 6 unit 5 unit 3 unit 2 unconformity A0101 Sasakura A0015 Ohmichi A0006 Kamitamarai A9820 Ikenotsuru unit 1 A0013 Mizunomoto A9831 Mizunomoto A9903 Yamasaki A9745 Ohata MzP1 unit 1 unit 2 unit 3 Kpfa unit 5 unit 4 EXPLANATION Pumice Dark-grayish ash Grayish ash Whitish ash Brownish buried soil Blackish buried soil GRAINSIZE SCALE Silt Fine sand Medium sand Coarse sand Gravel 3 m (D) ACP3, ACP4 and MzP3 2 m?? MzP3 ACP3 ACP4 A9915 Makino Farm A0101 Sasakura A0015 Ohmichi A9820 Ikenotsuru A9831 Mizunomoto A9903 Yamasaki A0006 Kamitamarai Fig. 4 Stratigraphic correlation of representative sections for several pumice-fall deposits. Note that explanation in this figure is di#erent from that in Figs. 2 and

10 204 1 (YmP 1) (2.5Y8/4) (Figs. 2, 3) YmP5 A cm MP 3.4 cm (SsP2) 2 SsP2 A m (Fig. 2) (10YR8/4) m A cm MP 2.7 cm (2.5Y6/4) (Fig. 4B) (SsP1) 1 SsP1 SsP cm 2002 A0101 (MP: 1.6 cm) A cm (Fig. 4B) MP 3.6 cm 2cm (2.5Y4/2) (Kj-Hnd) 1977; 1997; Kj-Hnd Hnd (A0101) cm (Fig. 2) (1989) km (ACP6) ; ACP m (Fig. 2) Aso-4 A m ACP6 7 (Fig. 4C) 1 (10YR7/4) A0011, A9903, A cm MP cm 2 5 (2.5Y5/4) (2.5Y6/4) 3 (2.5Y8/4) (MP cm) 6 (2.5Y8/4) A cm MP cm ACP6 2.7 cm A ACP6 1 6 (Fig. 5A) 0.11 km 3 (Table 2) (1989) ACP6 ACP ACP6

11 9 205 Fig. 5. Distribution of thickness and maximum pumice size for six pumice-fall deposits from Aso central cones. (A), (C), (E): Isopach maps. (B), (D), (F): Maximum size isopleth maps using the average long-axis diameter of the three largest pumice at each locality ACP6 ACP6 6m (Figs. 2, 3)

12 206 (2.5Y4/4) 6 10 cm ACP6 (2.5Y7/4) 4 MzP (ACP5) 5 ACP ACP6 1 5 m (Fig. 2) A cm MP 1.9 cm (Fig. 5B) ACP km 3 50 cm (2.5Y4/2) cm (Figs. 2, 3) ACP (ACP4) ; ACP4 (10YR5/6 3 4 Fig. 4D (2.5Y8/4) (2.5Y8/6) ACP4 A cm A cm (Fig. 5C) ACP km 3 (Table 2) (2.5 Y5/3) (10YR5/6) 2 8 cm ACP4 3 MzP (ACP3) (1989) 3 ACP3 ACP4 ACP4 (2.5Y5/3) (Fig. 4D) ACP4 A9820 A cm A0212 MP 2.9 cm A0001 (MP 1.2 cm) ACP3 (Fig. 5D) ACP km 3 30 cm 3 12 (Kpfa) ; Kpfa 5 7 m m m Tn AT (Fig. 2) (1989) 2 ACP2 Kpfa ACP6 ACP4 A (Fig. 4E)

13 9 207 (Fig. 5E) cm cm (MP cm) (5Y7/3) 2 (Fig. 4E) 2 (Fig. 4E) 1 (10YR5/4) 2cm 3 1 cm (Fig. 4E) A (2.5Y4/3) 4 (2.5Y7/3) 2 3 cm 5 Kpfa 10 cm 20 cm MP cm cm cm A0013 (135 cm) (Fig. 4E) 6 A cm (Fig. 5E) 2.39 km 3 VEI 5 (Table 2) Kpfa Aso- 4 Kpfa cm :2.5Y3/3 (Figs. 2, 3) 1990; cm 1 MzP 1 Kpfa NbP2; Kpfa m (Fig. 2) 1985 HP; HP 1999 MP (1999) (Fig. 5F) HP 2 NbP2 HP cm 1 NbP1 AT (ACP1) ; ACP1 K-Ah (Figs. 2, 3) A9458 N7 (3,780 3,650 yrs BP) 1992;

14 C (1997) 14 C ACP1 3,700 yrs BP 11,000 yrs BP 14 C 1997 (Kpfa) 26,600 yrs BP 1997 Aso ka K-Ar 1991 Kpfa (AMS) 14 C (Table 3) 10 Beta Analytic 14 NEC 15SDH C Libby 5,568 d 13 C 13 C/ 12 C (Table 3) 14 C 20,000 yrs BP Stuiver et al. (1998) 20,000 yrs BP Kitagawa and van der Plicht (1998) 14 C (cal yrs BP) ka Elementar vario EL Aso-4 A 9915 Fig. 6 (Kpfa) 1990; ka (1997) Kpfa 32 ka Table 3. Results of AMS 14 Cdating for buried soils around Aso caldera.

15 9 209 Fig 6. Relation between stratigraphic positions of major tephra layers and their eruption ages at site A9915. Solid squares show 14 Cages reconciled with stratigraphic positions of each tephra. Ages of K-Ah and AT are given in Machida and Arai (1978; 1992) and Ikeda et al. (1995), respectively. Open square marks K-Ar age of Aso-4 pyroclastic flow deposit (Matsumoto et al., 1991). Solid circles denote 14 C ages of tephra below Kusasenrigahama pumice-fall deposit (Kpfa); they are incongruous with stratigraphic relations. Eruption ages of tephra layers below Kpfa were assumed by average accumulation rate between Kpfa (31 ka) and Aso-4 pyroclastic flow deposit (89 ka). Broken line denotes average accumulation rate of tephra for about 90,000 years. (1997) 28,100 yrs BP 32 ka Fig. 6 Kpfa MzP3 33 ka 34 ka 2 ACP4 19 ka 31 ka ACP4 MzP3 11,000 ACP ka 1 Kpfa Kpfa 6.36 m; 32 ka Aso m; 89 ka Kpfa (Fig. 6) 2 9 Fig. 6 OgP 80 ka YmP9 70 ka YmP5 69 ka SsP2 57 ka SsP1 56 ka ACP6 52 ka ACP5 45 ka ACP4 40 ka ACP3 39 ka AT K-Ah NbP2 18 ka Aso-4 OgP NjP 85 ka ACP6 54 ka (1980) 1 AT Aso ka (1996) 14 C 45, yrs BP (1998) 14 C 40,000 yrs BP (1998) ka (1996) (1998) (1998) Aso-4 (Fig. 3) Aso-4 K-Ar 89 ka ka Aso-4 1

16 m (1994) 50 ka Aso-4 2, ACP Kpfa 1982 ACP3 (39 ka) ACP5 (45 ka) ; 1985 Kpfa ACP ka K-Ar 1991 ACP3 (39 ka) SsP2 (57 ka) 1985 K-Ar 46 9 ka 1991 SsP2 K-Ar (Watanabe and Katsui 1976) SsP2 SsP2 1985; 2001 ACP4 (40 ka) 30 6 ka; 1991 ACP5 1994; Aso-4 (Fig. 3) 30 9 (Fig. 7) Aso-4 70 ka 2 85 ka VEI 4 5 (NjP) 80 ka VEI 4 (OgP) 1994; ka 5 :YmP5 (Fig. 2) 4 1 :YmP4 YmP1 60 ka

17 9 211 Fig. 7. Stratigraphy and explosive magnitude of pumice-fall deposits from Aso central cones during the past 90,000 years. Detail names of pumice-fall deposits are shown in Tables 1 and 2. Accurate stratigraphic positions cannot be determined. 57 ka 2 (SsP 2) 56 ka 1 (SsP1) 52 ka 54 ka (Fig. 3) (52 ka) 6 (ACP6) ACP6 6m (Fig. 3) 45 ka 5 (ACP5) 40 ka Aso-4 (VEI 4) 4 (ACP4) 39 ka 3 (ACP3) ka ka 1990; ka Aso-4 (VEI 5) (Kpfa) AT 29 ka; ; ka ka 1995 Ono et al., 1995 K-Ah (7.3 ka) 4 ka (ACP1) ka 1995; VEI 5 2 NjP Kpfa VEI 4 8 VEI 4

18 VEI 3 16 VEI 3 10 (Fig. 7) 9 (Fig. 8) (Table 2) mm / ky g/cm (DRE; 2.5 g/cm 3 ) Fig. 8 Aso-4 (Kpfa) 9 (DRE) 18.1 km km 3 /ky km 3 /ky km 3 /ky 2001 Fig. 8. Cumulative erupted volume versus time for air-fall tephra layers after the Aso-4 eruption (89 ka). 2,500 1 Kpfa 1.1 km 3 (DRE) 0.1 km km m Aso ; km 800 m 127 km g/cm DRE 112 km km km 3 /ky km 3 /ky m (NjP; 85 ka) (OgP; 80 ka) 5 (YmP5; 69 ka) 2 (SsP2; 57 ka) 1 (SsP1; 56 ka) 6 (ACP6; 52 ka) 5 (ACP5; 45 ka) 4 (ACP4; 40 ka) 3 (ACP3; 39 ka) (Kpfa; 31 ka) 2 (NbP2; 18 ka) 1 (ACP1; 4 ka) NjP ACP5 ACP3 ACP1 NjP SsP2

19 (DRE) 18.1 km km 3 /ky km 3 Aso-4 2,500 1 VEI U. S. Geological Survey Cynthia A. Gardner 2 (1999) 120, 2 8. (1999) 48, (1999) CD-ROM Fierstein, J. and Nathenson, M. (1992) Another look at the calculation of fallout tephra volumes. Bull. Volcanol., (1991) (1997) (1995) 14 C 34, (1994) (1997) p. (1998) (1996) 14 C 66, Kitagawa, H. and van der Plicht, J. (1998) A 40,000-year varve chronology from lake Suigetsu, Japan: Extension of the 14 Ccalibration curve. Radiocarbon, 40, (1980) (1976) Tn 46, (1978) 17, (1992) 276 p. (1991) K-Ar (1997) 14 C 42, (2002) 129, (2002) , (2001) , (1977) (1995) 110, 2 5. Newhall, C. G. and Self, S. (1982) The Volcanic Explosivity Index (VEI): an estimate of explosive magnitude for historical volcanism. J. Geophys. Res. (Oceans & Atmospheres), 87, (2001) 62, (2002) 3 41, (1998) 14 C 43,

20 214 (1989) (1990) 34 S201 S214. (1983) 5, (1985) (1977) p. (1993) 15, (1995) 40, Ono, K., Watanabe, K., Hoshizumi, H. and Ikebe, S. (1995) Ash eruption of the Naka-dake cater, Aso volcano, southwestern Japan. J. Volcanol. Geotherm. Res., 66, (1967) Stuiver, M., Reimer, P. J., Bard, E., Beck, J. W., Burr, G. S., Hughen, K. A., Kromer, B., McCormac, G., van der Plicht, J. and Spurk, M. (1998) INTCAL 98 radiocarbon age calibration, 24,000-0 cal BP. Radiocarbon, 40, (1995) 40, (1989) 90, (1994) K-Ar (1991) 98, (1992) (2001) p. Watanabe, K. and Katsui, Y. (1976) Pseudo-pillow lavas in the Aso caldera, Kyushu, Japan. J. Japan. Assoc. Mineral. Petrol. Econ. Geol., 71, (1990) 39, (1992) 5 1) (1982) 27, (1991) (1996) 4 83, (1997) 27,

熊本大学学術リポジトリ Kumamoto University Repositor Title 阿蘇火山米塚の噴火年代 Author(s) 宮縁, 育夫 Citation 火山, 55(5): Issue date Type URL Right Journal

熊本大学学術リポジトリ Kumamoto University Repositor Title 阿蘇火山米塚の噴火年代 Author(s) 宮縁育夫 Citation 火山 55(5): 219-225 Issue date 2010-10-31 Type URL Right Journal Article http://hdl.handle.net/2298/37047 2010 特定非営利活動法人日本火山学会