Origin of New Islets (An-ei islets) Formed During the An-ei Eruption ( , of Sakurajima Volcano, Southern Kyushu, Japan

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1 /. (,**3) ,,**1 / +.,**2 +, 3 Origin of New Islets (An-ei islets) Formed During the An-ei Eruption ( , ) of Sakurajima Volcano, Southern Kyushu, Japan Tetsuo K OBAYASHI I reconstructed the eruptive sequence of the An-ei eruption ( , ) of Sakurajima volcano based on documentary records. The important results are as follows; first, submarine eruption started earlier than the entrance of terrestrial lava flow into the sea ; second, a larger volume of magma intruded into the sediments at the bottom of the sea, and generated a large submarine cryptodome. When magma was ejected, floated islets consisting of giant pumice were formed for a while, which were subsequently followed by violent phreatomagmatic eruptions, and finally massive lava appeared on the sea. However, when the sandy islets were uplifted, they were not accompanied by any eruptions. All the islets were formed nearly within a year, then followed by submarine explosions that generated tsunami. The explosions were produced by the interaction between seawater and the intruded magma of the An-ei cryptodome. Some tsunamis might be triggered by landslides on the slope of the submarine cryptodome. I found similar explosion vent structure at Yuwandake, an early Pleistocene intrusive body into wet sediments, to the northwest of the Aira caldera. Funnel-shaped vent breccia penetrated the central part of the jointed Yuwandake lava, suggesting that the submarine explosion occurred at the final stage of the consolidation of magma. Similar vent structures may exist in the An-ei submarine cryptodome. Key words : +, Omori ( +3+0 ) +3+2a, b: Omori ( +3+0 ). / + ( +3+2a, b) ( +3+2 a, p. +3, +30) ( +3+/ ) Koto ( +3+0) - ( +3-/ ) ( +32, ) ( +33, ) 23* **0/ +,+ -/ +,+ -/, Korimoto, Kagoshima 23* **0/, Japan Department of Earth and Environmental Sciences, Fac- Corresponding author: Tetsuo Kobayashi ulty of Science, Kagoshima University koba@sci.kagoshima-u.ac.jp

2 Fig. +. Index map of Sakurajima volcano (solid triangle) and distribution of lava flows and tu# ring in historical time. An-ei eruption in +113 occur- Fig.,.

+ : submerged first-islet,,: Inokojima, -: broken lines in the sea denotes the lava flow Nakanoshima,.: Iojima, /: Shinjima, 0: Doroplunging into the sea and submarine cryptodome.

2 2 Fig. +. Index map of Sakurajima volcano (solid triangle) and distribution of lava flows and tu# ring in historical time. An-ei eruption in +113 occur- Fig.,. Newly-formed topography of the north-eastern red from fissure vents that opened on the NE and sector of Sakurajima volcano during the An-ei S sectors of the volcano. NE area enclosed by eruption. + : submerged first-islet,,: Inokojima, -: broken lines in the sea denotes the lava flow Nakanoshima,.: Iojima, /: Shinjima, 0: Doroplunging into the sea and submarine cryptodome. jima (now submerged). Submarine craters are represented +. by hachures. Bathymetric map is from +113 Japan Coast Guard ( +33* ).,. + :,: -:.: /: 0: ( +33* ) ( +3+2a, b), ( +31* ) ( +31* ), ( +3+2 a, b) +.* m ( +311) ( +3+2 a, b). ( +332) -* +** m ( +31* ) ( +311) -. 0* +** m ( +31/ )

3 , 3 Fig. -. North-south geologic cross section of Shinjima, the largest among the An-ei islets (Fig.. in Yamaguchi, +3+/ ). Pumiceous deposits are overlain by a fossil layer / 1 ( +31* ) ; +31*. ( +310) +** m - ( +3+/ ) +3+. ( +310)

4 4 ( +332) Fig... Map drawn about / years after the formation of the An-ei islets (In Sangoku-meishozue)... / ),) -) / - + ),. 0*

5 , 5 + ),,),, -),),,.) ) /)., / + + +

6 6 + /.) + / +.) /) ,1. + ), ) -),. ) +023 / ) 0) + ) 0) ( +31* )., +12* + ),,), -), /) 0 ) ( +31* ).) +, / ( +3+/ ) 0) + ),) + ) -).) /).) /).) /

7 , 7 : : ( +31* ) : : ( +31* ) : : : : : :, : + : : : : ( +31* ) : / : :

8 8 ( +31* ), +, +,, Fig. /. Formation sequence of Showa-Iojima (Fig. +- ( +31* ) in Tanakadate, +3-/ ). a) Dec.,/, +3-/ ; b) early January, They were probably drawn from the summit of Satsuma-Iodake. /. +3-/ +- a) +3-/ +, / b) / ** m, ( +3-/ ) ( +3-0) +* km /, -* m +* m 1 +* m, + +* km 1 km Matumoto ( +3.-) (Plates XXI and XXII) +3+.

+113 +12, 9 Fig. 0. Giant pumice erupted in the submarine eruption of Showa-Iojima. a) drifted giant pumice as large as 1 m on the coast of Iojima (Fig.

than +* m on the submerged caldera floor near Showa-Iojima (Photo. a) - in Nakamura et al., +320).

9 , 9 Fig. 0. Giant pumice erupted in the submarine eruption of Showa-Iojima. a) drifted giant pumice as large as 1 m on the coast of Iojima (Fig. +, in Fig. 1. Occurrence of giant pumice on the An-ei Tanakadate, +3-/ ); b) sunken giant pumice larger islets. a) Nakanoshima, b) Shinjima. than +* m on the submerged caldera floor near Showa-Iojima (Photo. a) - in Nakamura et al., +320). 1 b) Sharp bread-crust surface is observed on the surface of giant pumices. 0. a) 1 m +3-/ +, b) +* m ( +320) 1 ( +3-/ ) (Clough et al., +32+, Wilson and Walker, +32/ ; Stewart, and McPhie,,**.) ( +3+/ ) Koto ( +3+0, p. /- /. )

10 10,* dome or tholoide +12, + +2 ( +3+2b, p ) - +, - + ( +310) - ( +311) ( +311) ( +332) ( +311a, b) ( +332) Koto ( +3+0 ) ( +3+2b) Koto ( +3+0) ( +3+/ ) Aramaki, +32. ;,**/ ; Kano et al., +330 ; Kobayashi, * m ( +330) 0,

11 , Koto ( +3+0, p. /,) ( +30- ) ( +303 ) ( +31* ) ( +31* ) +023 Fig. 2. Sketch of intrusive rock ; Yuwandake (Figure in Kobayashi et al., +32* ). The central part of the body is penetrated by funnel-shaped vent ( +311a, b) breccia. Vertical scale is exaggerated twice the ( +320) lateral scale * ( +32, ), +. C (Okuno et al., +332 ) ,,0 m ( +310) 0. ( +3+2b, p. 122* ) +3+2b,*** ;. / +32* 2 +* m

12 , + +12* * , + Fig. 3. Estimated elevation change at the rim of Aira , +, - caldera through historical time (modified Fig. 2 in Kamo and Ishihara, +32* ). H represents the scale of elevation change at Kagoshima city * 2 H + ) ( +32* ),) 3 -) ( +/. km ) (,*. km ) ( +/. km ).) + + /) -* m - km 0/ * +12/ / * Koto ( +3+0, p. // /0) Omori ( +3+0, p. 01, 1,, and 1. ) after-eruptions +12/ *3 +133,* +2*3 -* ( +311a)

13 , 13 ( +303) no ( +31* ) +2,2. (,) ( +311b) no. 1 0/ 20., ( +3-0) +*/ *. Aramaki, S. ( +32. ) Formation of the Aira Caldera, south- Matumoto, T. ( +3.-) The four gigantic caldera volcanoes ern Kyushu,,,,*** years ago. J. Geophys. Res., 23, 2.2/ of Kyushu. Jap. J. Geol. Geogr., +3, /1p. 2/*+, ( +333) ( +320 ) ,,. / ( +310) ++/ +,3. // 12. Clough, B.J., Wright, J.V. and Walker, G.P.L. ( +32+ ) An ( +320) unusual bed of giant pumice in Mexico. Nature,,23,.3 giant punice /*. ( +3-/ ) + +,,***,,,0, //. ( +332 ) Okuno, M., Nakamura, T. and Kobayashi, T. ( +332) AMS C dating of historic eruptions of the Kirishima, Sakura- ( +33, ) jima and Kaimondake volcanoes, southern Kyushu, Japan.,++ P. Radiocarbon,.,, 2,/ 2-,. (,***) Omori, F. ( +3+0) The Sakurajima eruptions and earthquakes. II. Bull. Imp. Earthq. Invest. Committee, 2, -/ +*0 10, 12, I ( +33* ) + : /*,*** 0-/+ ( +3+2a) no. 20,-0p. ( +3+2b) no. 21 (,**/) ++0p. ( +311).. +/,3.,, +*0. ( +32* ) ( +32* ) no ,2. -/ 10. Kano, K., Yamamoto, T., and Ono, K. ( +330 ) Subaqueous ( +30-) eruption and emplacement of the Shinjima Pumice, no.,,,/+,10. Shinjima (Moeshima) Island, Kagoshima Bay, SW Japan. Stewart, A.L. and McPhie, J.,,**., An Upper Pliocene J. Volcanol. Geotherm. Res., 1+, +21,*0. coarse pumice breccia generated by a shallow submarine explosive eruption, Milos, Greece. Bull. Volcanol., 00, +/ ( +310 ),2.,3 ( +3-/ ) /.. +, +22,*3. ( +32, ) : ( +330),1,11,3,. No ,. Kobayashi, T. ( +322) Tectonic setting and major pyro- Wilson, C. J.N. and Walker, G.P.L., +32/, The Taupo clastic eruptions in southern Kyushu. A guide book for eruption, New Zealand I. General aspects. Phil. Trans. R. Sakurajima Volcano (Kagoshima International Confer- Soc. London, A., -+., +33,,2. ence on Volcanoes, ed), Kagoshima Prefectural Govern- ( +3+/ ),1 ment, ( +32* ) ( +31/ ) +1 +,2p Koto, B. ( +3+0 ) The great eruption of Sakura-jima in J. Coll. Sci. Imp. Univ. Tokyo, -2, +,-1.

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