῎ῑ῏ῐ90 /0 

The Caldera Eruptions in Ryukyu Arc : as Inferred
the Thermal Anomaly in Kyushu
Chang-Hwa CHEN
Beppu Geothermal Research Laboratory, Kyoto University, Japan
Institute of Earth Sciences, Academia Sinica, Taiwan

Abstract
The volcanism results from the subduction of the Philippine Sea plate under theEurasian Plate at the Nankai Trough and the Ryukyu Trench, respectively formed -.
active volcanoes in Kyushu and Okinawa areas. Based on the tectonic provinces, thesevolcanoes can be divided into two groups, the South-West Japan Arc and the Ryukyu
Arc, respectively. The Quaternary volcanic front extends from Taiwan to Kyushu. The
extensive huge eruptions in the Quaternary formed the four major calderas in Kyushu
area. The names of caldera from north to south are as Aso, Aira, Ata, and Kikai,
respectively (Figure-+). As for the heat ﬂow pattern in Kyushu, the higher +,* mW/m,contours are closely reconciled with the caldera areas. Therefore, the purpose of thispaper will focus on the relationship between the huge volcanic eruptions and the thermalanomaly.
Four major huge eruptions in the Aso caldera occurred in ,1* ka, +.* ka, +,* ka and 3*ka, respectively. The total eruption volume from the Aso caldera was estimated at much
more than 2** km- (Machida, +333). The Aira caldera was formed by co-ignimbrite
eruption at ,. ka, which eruption volume was estimated as more than ./* km- (Machida,+333). The Ata caldera located at the southern tip of Kyushu formed in 3/ ka ago, which
supplied much abundant of dacitic pyroclastic fall and vitric ash. The total eruption
volume of the Ata caldera eruption was estimated as more than -** km- (Machida, +333).
There were two huge eruptions in the Kikai caldera, which were 0.- ka and 3/ ka,respectively. The total eruption volume of the Kikai caldera was estimated as -** km(Machida, +333). Cumulated the above eruption volumes from four calderas, the conserved estimation of eruption volume along the northern Ryukyu arc is about +/2* km- inlast *.- Ma period. The average eruption rate per year is calculated around /῍+*ῌ- km-,which is the similar range with that of the Yellowstone, USA and the Taupo, NZ, -.,῍+*ῌkm-/year and 2./῍+*ῌ- km- /year, respectively (Hochstein, +33/). There did not obviously
appear any anomaly in the average eruption rate. In general, the eruption rate could
reﬂect the magma production rate in the mantle wedge under the volcanic front.
Therefore, the surface higher heat ﬂow did not imply any higher magma production rate.
Various model has been proposed to explain that the phenomenon of anomalously
high heat ﬂow coincident with the caldera localities. The rifting and crustal rotation in
the northern Ryukyu arc due to the accelerated trench retreat along the Ryukyu trenchin last , Ma (Kamata and Kodama, +333). They o#ered the very suitable tectonic
environments for the growth of magma chamber. The voluminous felsic magma ofcaldera eruption was stored up at the high-level of the upper crust for the long time, interms of it caused the higher heat ﬂow on the surface naturally.

῏ /- ῎ ῌ,**-῍ The Caldera Eruptions in Ryukyu Arc : as Inferred the Thermal Anomaly in Kyushu

91

Fig. + Location map of the caldera and Sakurajima volcano on southern Kyushu. Japan Quaternary
volcanoes are indicated as ﬁlled triangles. Major calderas are also shown in the map.
(This map adapted from Arakawa et al., +332)

References
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pyroclastic eruption, southern Kyushu, Japan. J. Volcanol. Geotherm. Res., 2*, +13ῌ+3..
Hochstein M.P. (+33/) : Crustal heat transfer in the Taupo Volcanic Zone (New Zealand) :comparison with other volcanic arcs and explanatory heat source models. J. Volcanol.
Geotherm. Res., 02, ++1ῌ+/+.
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