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OBAYASHI

( )

Department of Earth and Environmental Sciences, Fac-

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.

:

Omori ( ) a, b :

Omori ( )

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( ) Koto ( )

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, Korimoto, Kagoshima , Japan

Corresponding author : Tetsuo Kobayashi

ulty of Science, Kagoshima University e-mail : koba@sci.kagoshima-u.ac.jp

Tetsuo K

Origin of New Islets (An-ei islets) Formed During the An-ei Eruption ( )

of Sakurajima Volcano, Southern Kyushu, Japan

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Fig. . Index map of Sakurajima volcano (solid tri-

angle) and distribution of lava flows and tu ring

in historical time. An-ei eruption in 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

broken lines in the sea denotes the lava flow Nakanoshima, : Iojima, : Shinjima, : Doro-

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9

Fig. . Giant pumice erupted in the submarine erup-

tion of Showa-Iojima. a) drifted giant pumice as

large as m on the coast of Iojima (Fig. inFig. . Occurrence of giant pumice on the An-ei

Tanakadate, ) ; b) sunken giant pumice largerislets. a) Nakanoshima, b) Shinjima.

than m on the submerged caldera floor near. a)

Showa-Iojima (Photo in Nakamura , ).b)

face of giant pumices.

. a)

m

b) m

( )

( )

(Clough , Wilson and Walker, ; Stewart

and McPhie, )

( )

Koto ( , p. )

et al.

et al.,

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0

1 +,1

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10

dome or tho-

loide

( b, p. )

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( )

( a, b)

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Koto

( ) ( b)

Koto ( )

( )

Kano ; Kobayashi, m

( )

et al.,

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Koto ( , p. )

( ) ( ) ( )

( )

Fig. . Sketch of intrusive rock ; Yuwandake (Figure

in Kobayashi , ). The central part of

the body is penetrated by funnel-shaped vent( a, b)

breccia. Vertical scale is exaggerated twice the( )

lateral scale.

.( )

C (Okuno

)

m

( )

( b, p. )

b

;

m

et al.

et al.,

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12

Fig. . Estimated elevation change at the rim of Aira

caldera through historical time (modified Fig. in

Kamo and Ishihara, ). H represents the

scale of elevation change at Kagoshima city.

.

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)

( ) )

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( . km ) ( . km ) ( . km )

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Omori ( , p. , , and )

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ern Kyushu, , years ago. ,

jima and Kaimondake volcanoes, southern Kyushu, Japan.

.

unusual bed of giant pumice in Mexico. ,

C dating of historic eruptions of the Kirishima, Sakura-

( )

Shinjima (Moeshima) Island, Kagoshima Bay, SW Japan.

explosive eruption, Milos, Greece. ,

( ) : ,

Kobayashi, T. ( ) Tectonic setting and major pyro-

13

( )

no. .

( )

. ( )

( b) no. .

( )

. .

Aramaki, S. ( ) Formation of the Aira Caldera, south- Matumoto, T. ( ) The four gigantic caldera volcanoes

of Kyushu. , p.

, ( )

( ) .

( )

. .

Clough, B.J., Wright, J.V. and Walker, G.P.L. ( ) An ( )

giant punice

.

( )

. .

( ) Okuno, M., Nakamura, T. and Kobayashi, T. ( ) AMS

.

( )

P. , .

( ) Omori, F. ( ) The Sakurajima eruptions and earth-

quakes. II. ,

. .

( a) no.

p.

( b) no.

p.

( )

. .

( ) ( )

no.

. .

Kano, K., Yamamoto, T., and Ono, K. ( ) Subaqueous ( )

eruption and emplacement of the Shinjima Pumice, no. .

Stewart, A.L. and McPhie, J., , An Upper Pliocene

, . coarse pumice breccia generated by a shallow submarine

( ) .

( )

. .

( ) : ( )

. No. .

Wilson, C.J.N. and Walker, G.P.L., , The Taupo

clastic eruptions in southern Kyushu. A guide book for eruption, New Zealand I. General aspects.

Sakurajima Volcano (Kagoshima International Confer- , .

ence on Volcanoes, ed), Kagoshima Prefectural Govern- ( )

ment, . .

( ) ( )

p.

Koto, B. ( ) The great eruption of Sakura-jima in .

, .

J. Geophys. Res.,

Nature,

Bull. Volcanol.,

Jap. J. Geol. Geogr.,

Radiocarbon,

Bull. Imp. Earthq. Invest. Committee,

J. Volcanol. Geotherm. Res.,

Phil. Trans. R.

Soc. London, A.,

J. Coll. Sci. Imp. Univ. Tokyo,

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