Abstract
Increase in magma flux as the result of conduit enlargement is one of the key processes that triggered caldera collapse and eruption of the Ito ignimbrite from Aira Caldera at ~29 ka. We examine the total volume of the pumice fall deposit, vertical variations in grain size of pumice, and the lithic content in the Osumi pumice deposit to investigate the trigger for caldera collapse. Wider distribution of the later-stage unit and the upward coarsening of grain size throughout the Osumi pumice fall deposit indicate an increase in magma discharge toward the onset of collapse. The total volume of lithic fragments in the Osumi pumice fall deposit is estimated as ~1.6 km3, based on the lithic content in several representative outcrops and the total volume of the Osumi pumice fall deposit. The lithic fragments in the Osumi pumice fall deposit indicate intense mechanical erosion of the conduit during the Plinian eruption prior to caldera collapse. Caldera collapse requires decompression of the magma chamber by withdrawal of magma; effective enlargement of the conduit diameter during precursory eruptive phases is one of the important processes that subsequently allow the rapid discharge of a large volume of magma, which in turn facilitates decompression of the reservoir and induces caldera collapse.
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Acknowledgments
The authors appreciate Tetsuo Kobayashi and Hideto Naruo for their advices and discussions during our work. They also supported our fieldworks. We would like to thank Takahiro Yamamoto, Hideo Hoshizumi, and Kazuhiko Kano for discussion. We also appreciate Kazuhiko Kano for providing the data and samples of the boreholes. Raffaello Cioni, Guido Giordano, James D.L. White, and anonymous reviewer provide critical comments and suggestions. The fieldworks of NG were supported by JSPS Kakenhi 24510251.
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Geshi, N., Miyabuchi, Y. Conduit enlargement during the precursory Plinian eruption of Aira Caldera, Japan. Bull Volcanol 78, 63 (2016). https://doi.org/10.1007/s00445-016-1057-9
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DOI: https://doi.org/10.1007/s00445-016-1057-9