A Pyroclastic Flow Deposit Occurring at the Northeastern Foot of Nakadake, Aso Volcano (Japan) and its Stratigraphic Significance

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1 /+ (,**0).,-+,.-,**0. /,**0 0,+ A Pyroclastic Flow Deposit Occurring at the Northeastern Foot of Nakadake, Aso Volcano (Japan) and its Stratigraphic Significance Yasuo MIYABUCHI, Kazunori WATANABE and Yoshitaka EGAWA A basaltic pyroclastic flow deposit, the Izumikawa pyroclastic flow deposit, occurs at the northeastern foot of Nakadake Volcano, which is the only active central cone of Aso caldera, southwestern Japan. The pyroclastic flow deposit covers a fan-shaped area of about * km,, and the bulk volume is estimated at * 0 m -. The deposit is poorly sorted, and consists of subangular faceted clasts and spherical cauliflower bombs set in a sandy non-cohesive matrix. The deposit forms two di#erent facies : a black reversely graded lower unit and a reddish-gray reversely graded upper unit. The cauliflower bombs, which have slightly vesiculated crusts and denser interiors, are more abundant in the lower unit than in the upper unit. The presence of the cauliflower bombs suggests that the pyroclastic flow was generated by an explosion at the source lava lake or conduit, which was filled with mixture of solidified and molten lavas. The age of the deposit was estimated at ca. +3 cal ka, based on +. C ages obtained from charred wood fragments in the deposit. Recent tephrochronological studies reveal that Nakadake Volcano became active from ca.,,,+ cal ka and that violent scoria and ash eruptions of Nakadake were concentrated in two periods of ca.,,,+ cal ka and cal ka. The age of the Izumikawa pyroclastic flow corresponds an intermediate period between the two violent eruption periods. A similar pyroclastic flow deposit and a basaltic lava flow were also identified. They cover immediately the Izumikawa pyroclastic flow deposit. These facts indicate that multiple violent eruptions producing pyroclastic flows and lava flows occurred in a short period at ca. +3 cal ka. Recent activity of Nakadake has been characterized by ash eruptions, strombolian eruptions and phreatomagmatic explosions. However, the presence of the Izumikawa pyroclastic flow deposit emphasizes the potential hazard induced by bomb-rich pyroclastic flows that may rush down the flanks of Nakadake Volcano. Key words : Nakadake Volcano, pyroclastic flow deposit, cauliflower bomb, eruptive history + //- +33* +330,,,+ cal ka +. C cal ka,**. 20* *20, Kyushu Research Center, Forestry and Forest Products Research Institute, Kurokami , Kumamoto 20* *20,, Japan. 20* 2///,.* + Faculty of Education, Kumamoto University, Kurokami - Fig /,,,+ cal ka cal ka,**. +333,.* +, Kumamoto 20* 2///, Japan. 20+,,-- 3** Mashiki Junior High School, Souryo 3**, Mashiki, Kumamoto 200,,--, Japan. Corresponding author: Yasuo Miyabuchi yasuo@a#rc.go.jp

2 232 Fig. +. Diagram showing stratigraphic relation between edifices of Nakadake Volcano, the Izumikawa pyroclastic flow deposit and key widespread tephra layers, based on previous studies (e.g. Ono and Watanabe, +32/ Miyabuchi et al.,,**.). All ages (cal ka) in this figure are calibrated +. C ages. The ages of K-Ah and AT tephras are from Okuno (,**,). Fig.,A +32/ +33/ Ono et al., +33/. km Fig.,B A ,/,*,+* yrs BP +. C , (+33/) (Miyabuchi et al.,,**0) Miyabuchi et al. (,**0), +,/*0 m +,/3, m (Fig.,) 3** m 1** m +32/ 1** m +32/ /,***,* m +33, (+33+, +33,) Miyabuchi et al. (,**0) Izumikawa pyroclastic flow deposit IzPF IzPF 0.* 12* m 0+* m (Fig.,B) +** m 12* m (Fig.,B) 0+* m A*,., *.3 km, Fig.,B //* /0* m IzPF, m IzPF +.3 km, Fig.,B, / m IzPF * 0 m - +* 0 m -

3 ±²KL_³ µ 3 ) K ¹ABC X E º»¼ 233 Fig.,. Distribution maps of lava flows from Nakadake (A) and the Izumikawa pyroclastic flow deposit (B) modified from + :,/,*** topographic map Asosan and Bochu published by Geographical Survey Institute. Solid circles show localities of measured stratigraphic sections. The distribution of lavas and agglutinate is taken from Ono and Watanabe (+32/) and Baba (+333). Latitudes and longitudes are shown by Tokyo Datum. b#cdefe i IzPF D1 z{kl0e -ῌ, ῒῐΐῌῑ ABC']h@ IzPF N,ῌ* BC4ghi! "#$%& '() #*+,-. /0 (,./ YR./,) '() 1*+,-. 23) 4 5 'q) E A <h y K ABC i? IzPF ']^y KABC [P (Figs. -,.A) IzPF 67489:< A*,.. =>? CD# EF, cm AB IJKL0E M6 NOPQRNST IzPF $U& 4VW) X K -ῌ- ῒῐΐῌ IzPF KL[? K L0E D#Y EF *./ cm ZJ[\']^_I Lῌ Xi'rh 6J i GH ABC `aij b#cdef4ghi X #Y jk < ABC #*+,-. /0 lmn opn'q)rs tu E EF 2 cm 1*+,-. i e ª «K vw& x@ y IzPF 1Y z{ <} L (Fisher and Schmincke, +32., p. 3, Alvarado and EKL0E EF,2`0* cm& Soto,,**,) ' ]^ bomb-and-ash flow deposit y* IzPF z{kl0e 4 (Miyabuchi et al.,,**0) ABC4 F: 3/ cm (Fig..B) #Y IzPF 67 i 1* 89 5 A*,.. =>

4 234 Fig. -. Schematic stratigraphic section (locality A*,..) of the Izumikawa pyroclastic flow deposit and related deposits. (Fig. -) IzPF,2* cm +-* cm (Figs..A,.B) /* cm +-*,-* cm ML : - -, cm -* cm /* cm ML./ cm,,2 cm.. + IzPF (Fig..C) + +./ cm + mm (Fig..D) - mm IzPF cored bomb (Macdonald, +31,, p.+,1 +,2) (Miyabuchi et al.,,**0), IzPF blocky texture (Miyabuchi et al.,,**0)., IzPF A*,.. A*-*+ / f (-, mm)

5 235 Fig... Photographs of the Izumikawa pyroclastic flow deposit. (A) The Izumikawa pyroclastic flow deposit showing two di#erent facies : a black reversely graded lower unit and a reddish-gray reversely graded upper unit at locality A*,... (B) The Izumikawa pyroclastic flow deposit overlain by a lahar deposit and ash-fall deposits at locality A*,... (C) A cauliflower bomb (left) and a subangular faceted block (right) sampled from the lower unit of IzPF at locality A*-*+. (D) Cross section of a cauliflower bomb. +*/,. / f / f +f Fig. / IzPF +, f. - f +, f -, f * - f. f - +* (Miyabuchi et al.,,**0). - IzPF -/.* IzPF IzPF., -../* +./ (Yoshida, +321) + : +* X -*1*

6 IzPF SiO,.3.0 /*.+ wt. SiO,.3.3 /*./ wt. SiO, /*.+ /+.. wt. (Table +, Fig. 0) IzPF (,/ ) SiO, //.0 wt. +32/ SiO,.2.3 /-., wt. IzPF FeO* 3.2 +*.. wt. CaO +*.. +*.1 wt. K,O *.1 *.2 wt. (Fig. 0) / (Figs.,, 1) A*-++ IzPF../ m Fig. /. Grain size histograms of Izumikawa pyroclastic flow deposit sampled at localities A*,.. and A*-*+. Table +. Whole rock chemical compositions of blocks and bombs in the Izumikawa pyroclastic flow deposit and of lavas from Nakadake Volcano. 236

7 / Tn AT , Fig.,A A**+* +333 A**+* AT (Fig. 1) AT,3 cal ka,**, +333 (+32/) A*-*+ A**+* SiO,.3.2 wt. SiO, /-., wt. (Table +) +32/ AT m,* +/ YmS,* YmS+/,,,+ cal ka,**. YmS,* YmS +/,,,+ cal ka (Fig. 1) IzPF +*,* cm *./ cm 2 +, cm IzPF (Fig. 1), A*-++ /.+ m (Fig. 1) -.- m +.2 m IzPF A*,., (Fig. Fig. 0. SiO, variation diagrams for major elements of whole rock chemistry of the Izumikawa pyroclastic flow deposit (IzPF) and the Nakadake lavas.,b) IzPF, m (Figs. 1, 2). m +.. m (SiO, /,.0 wt. ). mm +32/ (Table +)

8 238 Fig. 1. Stratigraphic correlation of the Izumikawa pyroclastic flow deposit (IzPF), lava flows and tephra layers. See Fig., for localities of stratigraphic sections. All ages in this figure are +. C ages. The +. C age of IzPF at locality A3.+2 is from Watanabe (+33+, +33,). Schematic stratigraphic section of tephra layers at the east of caldera is from Miyabuchi et al. (,**.).

9 239 Miyabuchi et al. (,**0) b (AMS) +. C Table, - +. C +s +3,31* +2,,-* cal yrs BP (, s) +. C (+33+, +33,) +.,/,*,+* yrs BP +,.** +,2** (+33+, +33,) +3 cal ka IzPF (Fig. 1) A*,.. /./ m (Fig. -) *./ cm +33, IzPF IzPF, IzPF Arenal (Alvarado and Soto,,**, Cole et al.,,**/) Arenal IzPF blocky (Heiken and Wohletz, +32/, p.2/ +*1) Fig. 2. Photograph showing stratigraphic relation of Izumikawa pyroclastic flow deposit (IzPF) and Shirinashigawa lava at locality A*,.,. (Miyabuchi et al.,,**0) (Miyabuchi et al.,,**0) IzPF,2 0* cm

10 240 Table,. Results of +. C age determinations for charcoal in the lower part of the Izumikawa pyroclastic flow deposit (Miyabuchi et al.,,**0). 0, +32/ cal ka (Fig. 3),3 cal ka,**, AT +333,,,+ cal ka,**. +32/ A**+* AT +333,,,+ cal ka YmS,* YmS+/ *.2 km - DRE,** cal ka +* + YmS+* YmS+ *./ km -,,,+ cal ka cal ka +3 cal ka,+ +2 cal ka -,*** +* 0 m - (Fig. 1) +3 cal ka (Figs. 1, 2) A*-*1 +32/,, +3 cal ka -,***

11 241 Fig. 3. Stratigraphic relation between the Izumikawa pyroclastic flow deposit, lava flows and fallout tephra layers from Nakadake. Vertical tielines show relationships confirmed at outcrops. All ages (cal ka) in this figure are calibrated +. C ages. (Fig. 3),**-,**., IzPF +33/ IzPF +,-/* +,/** m / YmS/ cal ka,** cal ka / Onoet al., +33/ / /3 +32, + +./ km +33/. km * 0 m - +33* +33/ (Nakada et al., +333 Fujii and Nakada, +333) 1

12 242 (IzPF) /0* 12* m / m * 0 m - SiO,.3.0 /+.. wt. +. C (+0,-+* 3* +/,3-*,,* yrs BP) +3 cal ka (,,,+ cal ka) (+2 +0 cal ka) +3 cal ka +* 0 m -. km X +. C Beta Analytic Darden Hood Alvarado, G.E. and Soto, G. J. (,**,) Pyroclastic flow generated by crater-wall collapse and outpouring of the lava pool of Arenal Volcano, Costa Rica. Bull. Volcanol., 0-, //1 /02. (+33/) (+333) +,*, 2. Cole, P.D., Fernandez, E., Duarte, E. and Duncan, A.M. (,**/) Explosive activity and generation mechanisms of pyroclastic flows at Arenal volcano, Costa Rica between +321 and,**+. Bull. Volcanol., 01, 03/ 1+0. Fisher, R.V. and Schminke, H.U. (+32.) Pyroclastic Rocks. Springer-Verlag, Berlin,.1+ p. Fujii, T. and Nakada, S. (+333) The +/ September +33+ pyroclastic flows at Unzen volcano (Japan) : a flow model for associated ash-cloud surges. J. Volcanol. Geotherm. Res., 23, +/3 +1,. (+33*)./ +.0. Heiken, G. and Wohletz, K. (+32/) Volcanic Ash. University of California Press, Berkeley,,.0 p. (+330), /** p. Macdonald, G.A. (+31,) Volcanoes. Prentice-Hall, Englewood Cli#s, New Jersey, /+* p. (+310) Tn (+33,),10 p. (+331) +. C.,.*-.*2. (,**-) /,+.. (,**.) AT.3 /+ 0.. Miyabuchi, Y., Watanabe, K. and Egawa, Y. (,**0) Bombrich basaltic pyroclastic flow deposit from Nakadake, Aso Volcano, southwestern Japan. J. Volcanol. Geotherm. Res., +//, 3* +*-. Nakada, S., Shimizu, H. and Ohta, K. (+333) Overview of the +33* +33/ eruption of Unzen volcano. J. Volcanol. Geotherm. Res., 23, +,,. (,**,) -.+,,/,-0. (+32/) / +. (+32,) / (+33/).* +-- +/+. Ono, K., Watanabe, K., Hoshizumi, H. and Ikebe, S. (+33/) Ash eruption of the Naka-dake crater, Aso volcano, southwestern Japan. J. Volcanol. Geotherm. Res., 00,

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