Temporal Variations of the Petrological Features of the Juvenile Materials during 2006 to 2010 from Showa Crater, Sakurajima Volcano, Kyushu, Japan(＜Special Section＞Sakurajima Special Issue)

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  • MATSUMOTO Akiko
    Department of Natural History Sciences, Graduate School of Science, Hokkaido University
  • NAKAGAWA Mitsuhiro
    Department of Natural History Sciences, Graduate School of Science, Hokkaido University
  • AMMA-MIYASAKA Mizuho
    Department of Natural History Sciences, Graduate School of Science, Hokkaido University
  • IGUCHI Masato
    Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University

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Other Title
  • 桜島火山,2006～2010年の昭和火口噴出物の岩石学的特徴の時間変化(＜特集＞桜島火山)
  • Temporal Variations of the Petrological Features of the Juvenile Materials during 2006 to 2010 from Showa Crater, Sakurajima Volcano, Kyushu, Japan

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Abstract

On June 2006, Sakurajima volcano in Japan resumed its eruptive activity at Showa crater in Minamidake. We investigated dated ash samples, including lapilli ejecta of several eruptions, to evaluate the temporal relationship between their petrological features and eruptive activity. The activity is characterized by low frequency of explosive eruptions accompanied with small, periodic inflation from June 2006 to August 2009. No juvenile materials existed in the ash during this period (1st period). Frequent explosive eruptions accompanied with large, continuous inflation had continued from September 2009 until March 2010 (2nd period). Juvenile materials such as unaltered scoria and pumice with fresh glass have been recognized since late September 2009. The whole-rock chemistry of these juvenile materials is similar to those from AD 1955 to AD 2000 but is the most mafic (SiO2=58.5-59.1 wt.%), indicating that the magma system, in which mafic magma was injected into silicic magma, has not changed. The matrix glass compositions of the juvenile materials are dacitic (SiO2=67.2-72.7 wt.%). During the 2nd period, the proportion of juvenile materials in eruptive ash increased and the SiO2 contents of glass decreased with time. These temporal changes suggest that the high level of eruptive activity during the 2nd period was caused by the increase of mafic components in eruptive magma. During April to May 2010 (3rd period), the number of eruption had become small without inflation, and the proportion of juvenile materials also decreased. From June 2010 (4th period), although the mode of crustal deformation has changed to deflation, eruptive activity has increased again and the juvenile materials have been mainly ejected. The glass compositions during this period have become slightly higher in SiO2 content. These temporal changes of eruptive style and glass compositions suggest that the eruptive activity since AD 2006 has not been directly affected by the addition of mafic components in the magma system, but has occurred because of the silicic (andesitic) magma already-supplied beneath the volcano. In this way, the monitoring of the petrological features of dated eruptive materials could provide us useful information to evaluate ongoing eruptive activity as well as geophysical monitoring.

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