The late quaternary tephra layers from the caldera volcanoes in and around kagoshima bay, southern kyushu, japan

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  • The late Quaternary tephra layers from the caldera volcanoes in and around Kagoshima bay, southern Kyushu, Japan

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The Aira, Ata and Kikai calderas of Kagoshima Bay have erupted more than twenty silicic tehra formations including more than fifty tephra falls and flows (about 1,000 km^3 in total volume) in the last 150 ka. On the basis of the sequence of eruptive phases which have many kinds of eruption types, the eruptive cycles of these caldera volcanoes are grouped into three types: 1) a plinian cycle (composed of a single plinian phase, or a plinian phase and a moderate-scale pyroclastic flow phase); 2) a large-scale pyroclastic flow cycle (composed of a plinian phase followed by phreatomagmatic, phreatoplinian, moderate-scale pyroclastic flow phases, and then a large-scale pyroclastic flow phase); and 3) a moderate-scale pyroclastic flow cycle (composed of a single moderate-pyroclastic flow phase). A "multi-cycle" lasts for about 50 ka, and consists of several plinian cycles first preceded a large-scale pyroclastic flow cycle, which is followed by a couple of moderate-scale pyroclastic flow cycles. The multi-cycle suggests evolution of a large magma chamber; the plinian cycles resulted from pressure of magma filling the chamber, and the large-scale pyroclastic flow cycle caused the pressure to decrease rapidly. The low magma pressure generated a few moderate-scale pyroclastic flow cycles.

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