The July +*,,**- and the January +.,,**. Ash Emissions from a Hot Water Pool of the Nakadake Crater, Aso Volcano, Japan

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1 /* (,**/).,,1,.+,**- 1 +*,**. + +.,**. 2 -+,**/ 0 +1 The July +*,,**- and the January +.,,**. Ash Emissions from a Hot Water Pool of the Nakadake Crater, Aso Volcano, Japan Yasuo MIYABUCHI, Shin-ichiro IKEBE and Kazunori WATANABE Ash emissions occurred from a hot water pool of one of the craters of Nakadake, the only active central cone of Aso Volcano, southwestern Japan, on July +*,,**- and January +.,,**.. The former ash was dispersed throughout an area extending +. km east-northeast of the vent, whereas the latter ash extended 2 km to the east-southeast. Based on isopleth maps, the total weight of ash discharged by each event was.+ tons and -, tons, respectively. Both ash-fall deposits were composed of fine-grained ( + mm) glass shards, crystals and lithic fragments, and contained neither lapilli nor blocks. Some glass shards appear fresh and are probably juvenile material. We therefore believe that these phenomena represent non-explosive gas-and-ash emissions through the hot water pool. The ash particles emitted from the hot water pool of the Nakadake crater were aggregated at a size of + - mm and deposited because of a high content of water. The ash-fall deposits in proximal areas ( +./ km from the crater) were finer grained and more poorly sorted than in distal areas ( / km). Moreover, the deposit volume obtained from actual data in the proximal area was much larger than the volume extrapolated only from the isopleth data of the distal deposits. These phenomena suggest that the proximal deposits were emplaced mainly by another fine-grain-rich column di#erent from the main ash plume that extended to distal areas, or that some parts of the column detached from the main plume that emplaced the proximal deposits. Key words : ash emission, hot water pool, Nakadake, Aso Volcano +,*** ++ +,**- 1 +*,**- ;,**- a;,**-,** * *20, Kyushu Research Center, Forestry and Forest Products Research Institute, Kurokami , Kumamoto 20* *20,, Japan. 203,,-, +3-* + Aso Volcano Museum, +3-* + Akamizu, Aso, Kumamoto,**- a;,**. a * ; +32, +33*.,* 203,,-,, Japan. 20* 2///,.* + Faculty of Education, Kumamoto University, Kurokami,.* +, Kumamoto 20* 2///, Japan. Corresponding author: Yasuo Miyabuchi yasuo@a#rc.go.jp

2 ,**- 1 +*,**. + +.,,**- 1,**. +, Ono et al., +33/ ; +33/ +33/ 3 + (,**,,,**- b,,**. a,,**. b) +33/,**+ *,**, 2,**,.,**, +,,**-, +/*.**,**- 3 +/* 0** +332,**-,** ;,** ,*** ++,**. 3,**- / /-*,**. +,2,.+,,** ,**- /,- +* ;,**. c ++ /,**. +.,**- /,**- 1 +*,** * ++ +,,**. + m + +., -,**. a / m, +- -,,**. b - - -,**- 1 +* - +,**- 1 +*, 0 Fig. + + /** m // +3,/./

3 ,**- 1 +*,** Fig. +. Records of observations by video cameras and a microphone of the Aso Volcano Museum (AVM) and seismographs of the Japan Meteorological Agency (JMA) on July +*,,**-. The velocity seismogram records the E-W component observed /** m west of the Nakadake crater. Hatched area indicates the moment of the ash emission ,*,** ,2,*,3 cock s tail jet +3,* +1,* +** m +1 +3,*./,** mm * +,+., m /.* 0.- m s 1 +* +/ +,/** m +0 m s,,*** m,* m s 0 km +1 -* /* * ++ +,**- a -, ++,** *.3 km A; Fig., +** m,/* m

4 230 Fig.,. Distribution of the +* July,**- ash from the Nakadake crater. Base map is a + : /*,***-scale topographic map of the Asosan district published by the Geographical Survey Institute. Broken lines indicate isopleths (g/m, ). Longitude and latitude are shown as Tokyo Datum.. km,**- 1 +* Fig.,,**- 1 +*,**- a +. km +, km - -,** *.3 km A (Fig. -A) - (Fig. -B) + mm,**.. km (Fig. -C) mm -.,**- 1 +* (,**- a) *.3 km

5 ,**- 1 +*,** Fig... Relation between area (km, ) and weight (g/m, )ofthe+* July,**- and the +. January,**. ash-fall deposits from the Nakadake crater. Fig. -. Photographs of the July +*,,**- ash-fall deposit. (A) Muddy volcanic ash deposited on a paved trail at site A (about *.3 km NE of the crater). Pen is about +/ cm long. (B) A pole to which volcanic ash adhered at site A. (C) Plant leaves to which volcanic ash adhered at site C (about 0 km ENE). Scale in centimeters. A... km B, A 3 0/ cm B,..- cm + m, -1.0 g m, /.2 g m,, + m, -* g,* g +* g - (Fig.,) B 0 km C, - mm (Fig. -C),** G(Fig. 1) (Figs. 0D, 0E) G *./ g m, C,**- 1 +* *./ g m, *./ g m, (Fig.,). Fig... -* g m, Fig.. -* g m,,* g m, + *.*+ km, *./ g m,,**- 1 +*.+ (,**.) -** m 0* + mm 1* 3* + mm,**+ / 00 3./ cm +0 cm,**- b A B 1 ++ A 3-.+ g m, B ++.- g m, 1 ++,**

6 232 Fig. /. Records of observations by video cameras and a microphone of the Aso Volcano Museum (AVM) and a remote camera and seismographs of the Japan Meteorological Agency (JMA) on January +.,,**.. T he velocity seismogram records the E-W component observed /** m west of the Nakadake crater. Hatched area indicates the moment of the ash emission...,** ,**. + +.,**- 1 +*, - km Fig. / +/.+ +-,/,3 /0 +/.,,, /* +/.+ -* +/.- ** 2** m,**. a, +/., *- 0 +/.+., + +/.+ -,., +/.+ //,3.1 mm s,**. a,** /.+,3 +-,/..,..- m s 2** m + +,,1* m,,*** m /,,*** m +, m s +,/** m 2 m s

7 ,**- 1 +*,** ,**. a +/ /1 +, - + +/.,,**. + +-,** / km 0 cm + +/ +./ km + (Fig. 0A),** (Fig. 1) km,0/ +./ km +.- km *.3 km. -,** *./ km Fig. 1 D (Fig. 0B) - mm cm (Fig. 0C) (Fig. 0A).* g m, (Fig. 1) G; 0. -km G + mm / , mm (Fig. 0D) - mm (Fig. 0E),** mm.. +,,*,* cm + m, (Fig. 1) + m,.* g,* g +* g + g *./ g *.- g 0 Fig.. 0.* g m, Fig..,* g m,.* g m, *.- g m, -,,**- 1 +*.+

8 234 Fig. 0. Photographs of the January +.,,**. ash-fall deposit. (A) Western view of the Nakadake crater and the +. January,**. ash-fall deposit on snowpack. (B) Aggregated volcanic ash deposited on snowpack at site D (about *./ km E of the crater). Scale segments are +* cm. (C) A wooden fence to which volcanic ash adhered at site D. (D) A cap to which spot-like volcanic ash adheres at site G (about 0.- km ESE). (E) The loading platform of a track to which volcanic ash adhered at site G. Scale in centimeters.

9 ,**- 1 +*,** Fig. 1. Distribution of the +. January,**. ash from the Nakadake crater. Base map is a + : /*,***-scale topographic map of the Asosan district published by the Geographical Survey Institute. Broken lines indicate isopleths (g/m, ). Longitude and latitude are shown as Tokyo Datum. /,**- 1 +*,** / + A *.3 km B... km,**- 1 +* D *./ km E +./ km F / km,** (Malvern Mastersizer S) Fig. 2 +*f (+/+*,. mm),**- 1 +*, *f (+ mm) A +,f (+/, +/. mm) B -.f (+/2 +/+0 mm s f (Inman, +3/,) A -., B,./.f /*.+,** *f(+ mm) D E. /f(+/+0 +/-, mm) s f,.0 F + -f(+/, +/2 mm) s f +.3.f /3,0 /, A *.3 km,**- 1

10 236 Fig. 2. Grain-size histograms of the +* July,**- and the +. January,**. ash-fall deposits. See Figs., and 1 for locations of the sites. +* D *./ km,** * + mm *./ mm (Fig. 3) (Figs. 3A, 3C) *./ mm (Fig. 3B) (Fig. 2D) *.+ *./ mm +*,*,**- 1 +* EPMA,**- G+ G- G.. G+ G, G- G+,**-,** G+ G- - EPMA (,**-),**- 1 +*,**

11 ,**- 1 +*,** Fig. 3. Polarizing microscope photographs of glass shards included in the July +*,,**- (A, B) and the January +.,,**. (C, D) ash-fall deposits. Width of each photo is about + mm. 0 +,**- 1 +*,**. + +.,**- 1 +*,** *./ km - mm 0.- km + - mm,**- 1 +*,**.,** / km /3 / km,0,**. a +./ km (Fig. /).* g m, (Fig. 1),**. + +.,,*** m

12 238 0,,**- 1 +*,**. + +.,**- a;,**. a / , 0 +* 0 -* 1 + 3,3 +./ km / ; /./ km ,,**- 1 +*,**.,** km,,**.,**- 1 +*,**. + +.,**-,**- a,**-,**. +33, 0 +* / - +33/ +33/ (Walker, +31-, +32* ; Wright et al., +32*) (Fig. +*) + m, Fig.. + mm + +* + mm +** (Walker, +31-) (Cas and Wright, +321, p.+/0 +/2) +*,

13 ,**- 1 +*,** Fig. +*. Dispersal-fragmentation diagram (Walker, +31-, +32* ; Wright et al., +32*). The +* July,**- and +. January,**. ash-fall deposits from a hot water pool of the Nakadake crater are plotted in the area of Surtseyan eruption. Data for ash eruption of Nakadake are from Ono et al. (+33/). m 0 -,**- 1,**. + +./ km,** Fig.. -, / km + g m, *./ g m, *.- g m, - / + 0 Fierstein and Nathenson (+33,),**. + +.,1 (+ g/m,, *./ g/m,, *.- g/m, ) 3,** Fierstein and Nathenson (+33,) a +331 b,***,**, 1,**- 1 +*,** km 2 km.+ -, + mm + - mm

14 240 / km +./ km USGS Cascades Volcano Observatory Kevin M. Scott (+32*) ,*. Cas, R. A. F. and Wright, J. V. (+321) Volcanic successions, modern and ancient. Chapman and Hall, London, /,2 p. Fierstein, J. and Nathenson, M. (+33,) Another look at the calculation of fallout tephra volumes. Bull. Volcanol., /., +/ (,**. a) , p. (,**. b) +0, +- 2 p. (,**-),**- 1 +*,**- ++*. (,**-),**- 1 +*,**- +,.. (+333) +33* +,+ +, +0. (,**.),**- 1 +* 20 ++* +++. Inman, D. L. (+3/,) Measures of describing the size distribution of sediments. J. Sediment. Petrol.,,,, +,/ +./. (,**,),**, 3 p. (,**- a) +/ 1 0 p. (,**- b),**- +/ p. (,**. a) p. (,**. b) +0-0 p. (,**. c),**- ++ +,.3./ /*.,**.,**. +,. p. (,**.),**- / +* 20 +*, +*0. (+33-) (+33, ,) //,0.+. (+33.) +33. / +* 0*.. /.. (+33/) +33/, / 0, (+32,) / 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, (+33/).* +-- +/+. (,**,),***.1 0./ 00+. (+331 a) , +.+ +/+. (+331 b) ,.,3.-+. Walker, G. P. L. (+31-) Explosive volcanic eruptions - a new classification scheme. Geologische Rundschau, 0,, Walker, G. P. L. (+32*) The Taupo pumice: products of the most powerful known (ultra-plinian) eruption? J. Volcanol. Geotherm. Res., 2, (+33+) 03 p. (,**- a)

15 ,**- 1 +*,** ,**- 1 +*,**- /+. (,**- b) +-- +, +1. Wright, J. V., Smith, A. L. and Self, S. (+32*) A working terminology of pyroclastic deposits. J. Volcanol. Geotherm. Res., 2, -+/ --0.,**/ / +*,**/,**- 1 +*,**. + +.,**/ (,**/) p. (,**/). p.