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Rock-magnetic evidence for the low-temperature emplacement of the Habushiura pyroclastic density current, Niijima Island, Japan

By
Reina Nakaoka
Reina Nakaoka
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Keiko Suzuki-Kamata
Keiko Suzuki-Kamata
Graduate School of Science, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
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Published:
January 01, 2015

Abstract

The emplacement temperatures of accidental and juvenile fragments in the Habushiura pyroclastic density current (PDC) deposit were estimated using analysis of rock magnetism. The Habushiura PDC was generated in a costal or shallow offshore area during the early stage of the AD 886 phreatomagmatic eruption on Niijima Island, Japan. We collected 160 samples from 11 beds, which include three lithofacies types: a massive lapilli tuff (Facies A); a graded and/or diffusely stratified lapilli tuff (Facies B); and a swarm of large pumice (Facies C). Juvenile specimens from Facies A and C show a wide range of emplacement temperatures, from less than 150 °C up to 300 °C, in contrast with the emplacement temperatures lower than 150 °C of Facies B. Morphological features of the ash components and random changes in palaeomagnetic direction by stepwise thermal demagnetization indicate that low-temperature emplacement occurred by phreatomagmatic eruptions and ingestion of ambient air by the turbulent current. The emplacement temperatures of accidental fragments were up to 380 °C, which is higher than juvenile fragments. They are thought to have cooled more slowly than juvenile fragments due to the smaller surface area with respect to their volume.

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Geological Society, London, Special Publications

The Use of Palaeomagnetism and Rock Magnetism to Understand Volcanic Processes

M. H. Ort
M. H. Ort
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M. Porreca
M. Porreca
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J. W. Geissman
J. W. Geissman
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Geological Society of London
Volume
396
ISBN electronic:
9781862396722
Publication date:
January 01, 2015

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