Comparative Study on Eruption Sequences during the Caldera-forming Eruptions of Aira and Irosin Calderas

Abstract

 The eruption sequences of two caldera volcanoes, Aira in Japan and Irosin in the Philippines, were examined, and an eruption sequence for caldera volcanoes is constructed as follows: first, eruptions of felsic magma as a precursory event; second, a plinian eruption associated with intra-plinian flows followed; third, a fine-grained ignimbrite eruption occurred, which is followed by a catastrophic caldera-forming eruption; fourth, voluminous co-ignimbrite ash was generated and dispersed over a wide area; and, finally, post-caldera volcanoes were formed. In addition, intense earthquakes occurred either at the late stage of the plinian phase of the Irosin caldera or shortly before the final caldera-forming eruption of the Aira caldera. A similar eruption sequence was observed for the 7.3 cal ka BP caldera-forming eruption at Kikai caldera in Japan. Therefore, the eruption sequences observed at caldera volcanoes in Japan and the Philippines are considered to form the most fundamental processes of a caldera-forming eruption.