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,**1  /  +.  ! ,**2  +,  3  Origin of New Islets (An-ei islets) Formed During the An-ei Eruption (+113ῌ+12,)
of Sakurajima Volcano, Southern Kyushu, Japan
Tetsuo KOBAYASHI῍
I reconstructed the eruptive sequence of the An-ei eruption (+113ῌ+32,) of Sakurajima volcano based on
documentary records. The important results are as follows ; ﬁrst, submarine eruption started earlier than theentrance of terrestrial lava ﬂow into the sea ; second, a larger volume of magma intruded into the sediments at thebottom 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 thesubmarine 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 theconsolidation of magma. Similar vent structures may exist in the An-ei submarine cryptodome.
Key words :+ῌ ῐ ῎ ῑ ῏"#$ %&' , ()*+,-./0123456

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23*ῌ**0/  +ῌ,+ῌ-/
Department of Earth and Environmental Sciences, Faculty of Science, Kagoshima University) (+3-/) ¹ (+32,) Eº (+33,) 6 » +ῌ,+ῌ-/, Korimoto, Kagoshima 23*ῌ**0/, Japan
Corresponding author : Tetsuo Kobayashi
e-mail : koba@sci.kagoshima-u.ac.jp

H

2

Fig. +. Index map of Sakurajima volcano (solid triangle) and distribution of lava ﬂows and tu# ringin historical time. An-ei eruption in +113 occurred from ﬁssure vents that opened on the NE andS sectors of the volcano. NE area enclosed bybroken lines in the sea denotes the lava ﬂow
plunging into the sea and submarine cryptodome.
 +.

  
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Fig. ,. Newly-formed topography of the north-eastern
sector of Sakurajima volcano during the An-ei
eruption. + : submerged ﬁrst-islet, , : Inokojima, - :Nakanoshima, . : Iojima, / : Shinjima, 0 : Dorojima (now submerged). Submarine craters are represented by hachures. Bathymetric map is fromJapan Coast Guard (+33*).
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3

Fig. -. North-south geologic cross section of Shinjima, the largest among the An-ei islets (Fig. . in
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Fig. /. Formation sequence of Showa-Iojima (Fig. +in Tanakadate, +3-/). a) Dec. ,/, +3-/ ; b) early
January, +3-0. They were probably drawn fromthe summit of Satsuma-Iodake.
+ /.
      +3-/+,  /  b) +3-0    a) +3-/
+     
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AB" (Plates XXI and XXII)

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Fig. 0. Giant pumice erupted in the submarine eruption of Showa-Iojima. a) drifted giant pumice aslarge as 1 m on the coast of Iojima (Fig. +, inTanakadate, +3-/) ; b) sunken giant pumice larger
than +* m on the submerged caldera ﬂoor nearShowa-Iojima (Photo - in Nakamura et al., +320).
Sharp bread-crust surface is observed on the surface of giant pumices.
 0.    a)  " 1 m  #$ +3-/ % +, &' b) () *+, +* m  -./ +320 01 - 2' 34
5678 9:; <=>  DE   FG

Fig. 1. Occurrence of giant pumice on the An-ei
islets. a) Nakanoshima, b) Shinjima.
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and McPhie, ,**.)

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0$%1

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SDA3%1 {B!( J| }i 1#

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p

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Kano et al., +330 ; Kobayashi, +322 != (9./v9 -)l3) (©

oªb [\[kS_`aU [\[­ 
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$~

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Q<=> (+311b) ;+<=>?#R?H./0#@%  , ./0#DESF TG9H*+NOP./0SFN +*/ῌ++3.
Aramaki, S. (+32.) Formation of the Aira Caldera, southern Kyushu, ,,,*** years ago. J. Geophys. Res., 23, 2.2/ῌ2/*+,
Q<=>UVWX (+320) ;+<=>?./0#
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Clough, B. J., Wright, J.V. and Walker, G.P.L. (+32+) An
unusual bed of giant pumice in Mexico. Nature, ,23, .3ῌ
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\l (+32*) ?0)vxy4.#/07) .AcjZFcj*+, "E
c +3ῌ,2.
Kano, K., Yamamoto, T., and Ono, K. (+330) Subaqueous
eruption and emplacement of the Shinjima Pumice,
Shinjima (Moeshima) Island, Kagoshima Bay, SW Japan.
J. Volcanol. Geotherm. Res., 1+, +21ῌ,*0.
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Kobayashi, T. (+322) Tectonic setting and major pyroclastic eruptions in southern Kyushu. A guide book for
Sakurajima Volcano (Kagoshima International Conference on Volcanoes, ed), Kagoshima Prefectural Government, +ῌ0.
UVWXm¶ ¢ (+32*) ·l&£¸¤¥d¦1# §t4U»¨¼©ª#«  +1
 -1-ῌ-1..
Koto, B. (+3+0) The great eruption of Sakura-jima in +3+..
J. Coll. Sci. Imp. Univ. Tokyo, -2, +ῌ,-1.

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Matumoto, T. (+3.-) The four gigantic caldera volcanoesof Kyushu. Jap. J. Geol. Geogr., +3, /1p.
Q[9\ (+333) ]^_R`aSFT&8).#YZ# *[T /0 .. +++ῌ+,,.
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s*+,!  ,  tuvX ,*** *+ +-1ῌ+//.
Okuno, M., Nakamura, T. and Kobayashi, T. (+332) AMS+.
C dating of historic eruptions of the Kirishima, Sakurajima and Kaimondake volcanoes, southern Kyushu, Japan.
Radiocarbon, .,, 2,/ῌ2-,.
Omori, F. (+3+0) The Sakurajima eruptions and earthquakes. II. Bull. Imp. Earthq. Invest. Committee, 2, -/ῌ+13.
9 (+3+2a) HcjF, no. 20 MI/  ,-0p.
9 (+3+2b) HcjF, no. 21 MI/  ++0p.
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bU (+30-) ./0#w*&)/#1a 9'G2 no. ,, ,/+ῌ,10.
Stewart, A.L. and McPhie, J., ,**., An Upper Pliocene
coarse pumice breccia generated by a shallow submarine
explosive eruption, Milos, Greece. Bull. Volcanol., 00, +/ῌ
,2.
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fª (+330) «mZPYZg¬­®­g_ # !³ P No. .0 +.3ῌ+0,.
Wilson, C. J.N. and Walker, G.P.L., +32/, The Taupo
eruption, New Zealand I. General aspects. Phil. Trans. R.
Soc. London, A., -+., +33ῌ,,2.
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