2002 Fiscal Year Final Research Report Summary
Petrological research on magma behavior during volcanic eruptions
| Project/Area Number |
12304033
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| Research Category |
Grant-in-Aid for Scientific Research (A)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Petrology/Mineralogy/Science of ore deposit
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| Research Institution | The University of Tokyo |
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Principal Investigator |
NAKADA Setsuya Earthquake Research Institute, Professor, 地震研究所, 教授 (60128056)
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| Co-Investigator(Kenkyū-buntansha) |
YASUDA Atsushi Earthquake Research Institute, Associate Professor, 地震研究所, 助教授 (70222354)
FUJII Toshitsugu Earthquake Research Institute, Professor, 地震研究所, 教授 (00092320)
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| Project Period (FY) |
2000 – 2002
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| Keywords | Volcanic eruption / Degassing / Miyakejima volcano / Usu volcano / Unzen volcano / Petrology / Eruption products / Magma behavior |
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| Research Abstract |
Decompression and degassing process during volcanic eruptions magma becomes vesiculated and crystallization of melt occur. As a result, diversities of melt compositions and rock texture appear. In the research, we aimed at understanding of magmatic behaviors during eruptions, by reading the above petrological characteristics. Samples from Miyakejima, Usu, and Unzen volcanoes were investigated.
During the 2000 eruption at Miyakejima, basaltic andesite magma that generated eruption was replaced with phyric basalt magma in the early August. Melt inclusions from these magmas with different bulk compositions have the chemistry not distinct largely to each other, including volatile contents. The method to calculate oxygen fugacity on volcanic glass was developed, and the pressure was estimated to be near NNO for both magmas. Grain size analysis of the tephra showed large fragmentation and small dispersion, characterizing phreatomagmatic eruption rather than magmatic eruption.
The magma ascent speed was estimated roughly, based on crystallinity and vesicularity of the 2000 eruption products at Usu ; where.two-stage vesiculation events were proposed. As fragmentation of lavas occurred before and after quenching near surface, petrological distinguishing of one from the another becomes important for discussion.
The extents of crystallinity in groundmass and degassing, the latter of which was estimated from porosity of lavas, are linked intimately to depths of low-frequency earthquakes during the 1991-95 eruption at Unzen. We expected the genetic relationship of low-frequency earthquakes with fragmentation of foamy magma and the associated flowage of volatiles. We introduced a model in which magma more enriched in volatile can be broken by larger strain rate, accompanied by high decompression rate.
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