The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 117, Issue 6
Displaying 1-4 of 4 articles from this issue
SPECIAL ISSUE Development of high resolution volcanic geology using earth material science
Review
  • Information from detailed temporal variations of petrologic features of eruptive rocks
    Masao Ban
    2011 Volume 117 Issue 6 Pages 310-328
    Published: June 15, 2011
    Released on J-STAGE: November 03, 2011
    JOURNAL FREE ACCESS
    This paper reviews recent petrological studies on evolution of magma feeding system beneath active stratovolcanoes with high time resolution. Petrological studies, based on well-documented historical eruptions or detailed stratigraphy of eruptive rocks, are effective to reveal the evolution of the system. The results provide useful information for prediction of volcanic eruption.
    In terms of active basaltic volcanoes, these studies have revealed the structure of feeding system as well as the evolving nature of the magma chambers. The shallew chamber is replenished intermittently by more mafic magmas from the deeper part. The temporal variability of the deeper magmas was also detected. In the cases of andesitic to dacitic volcanoes, several patterns of magma evolution course have been recognized; e.g., the rapid change type within a single eruption, the cyclic change type over approximately 100-800 year intervals, and the type which remains stable over time.
    Most of volcanoes experience remarkable changes in the rate and explosivity of eruptions in their history. These sometimes correspond to the renewal of the magma feeding system either depth or shallow levels, or both. The renewal provides constraints on the longevity of shallow and deep magma chambers.
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  • Michihiko Nakamura
    2011 Volume 117 Issue 6 Pages 329-343
    Published: June 15, 2011
    Released on J-STAGE: November 03, 2011
    JOURNAL FREE ACCESS
    Recent progress in the science of the products of volcanic eruptions has made it possible to reconstruct past eruption processes on the basis of high-resolution volcanic stratigraphy. Case studies on changing eruption style and related conduit- and magma chamber-processes are helpful in establishing systematic, general patterns for the development of eruptions. They also contribute information towards an “eruption mechanism tree” that describes bifurcations in eruption processes according to the governing conditions. Methodological progress has been made on various aspects of eruptions, including our understanding of magma-ascent processes in relation to decompression-induced crystallization of groundmass microlites, precise determinations of volatile solubility in silicate melts, the development of analytical methods to measure volatile contents in glass and estimate magma pressure, the accumulation of permeability data, and advances in analytical techniques for rock microstructures (e.g., X-ray microtomography). Given these developments, we can retrieve information on the timescales of eruption dynamics and on the volume of erupted materials from the volcanic stratigraphy.
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  • Tsukasa Ohba
    2011 Volume 117 Issue 6 Pages 344-356
    Published: June 15, 2011
    Released on J-STAGE: November 03, 2011
    JOURNAL FREE ACCESS
    Hydrothermal mineral-bearing volcanic products are produced by phreatic eruptions and are widely recognized in high-resolution volcano stratigraphy, as well as in studies on recent individual eruptions. Recent eruptions have revealed the close relationship between eruptions and subvolcanic hydrothermal systems. At volcanoes where the frequency of magmatic eruptions is low, volcanic products are rich in alteration minerals derived from the siliceous and advanced argillic alteration zones; at volcanoes where magmatic eruptions occur frequently, sulphur-bearing minerals are dominant, occurring as fallout tephra as well as in flow deposits. Phreatic eruptions include landslide-related and landslide-unrelated types. The compositions and input rates of magmas are closely related to the style and repeating pattern of phreatic eruptions.
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Articles
  • Yoshiko Masubuchi, Yasuo Ishizaki
    2011 Volume 117 Issue 6 Pages 357-376
    Published: June 15, 2011
    Released on J-STAGE: November 03, 2011
    JOURNAL FREE ACCESS
    The 3400 BC caldera-forming eruption of Numazawa volcano (the Numazawako eruption), NE Japan, began with a cataclysmic pyroclastic flow (phase I) followed by a Plinian eruption, phreatomagmatic eruptions, and a second Plinian eruption (phases II-IV, respectively). Petrological examinations revealed that a variety of juvenile pyroclasts were ejected during the eruption. White pumice (WP) rich in euhedral phenocrysts (64.7–67.1 wt.% SiO2) dominated the juvenile material expelled during the two early eruption phases, suggesting that WP-forming dacitic magma (∼760–840 °C) constituted the main and upper portions of the pre-eruptive magma chamber. Gray pumice, rich in crystal fragments (65.2–66.0 wt.% SiO2), was a minor component of this eruption, produced by the mechanical breakdown of phenocrysts as the WP magma ascended in the conduit. Whole-rock compositions of scoria suggest that two different mafic magmas, a low-Ba type (LBa; < 58 wt.% SiO2, > 964 °C) and a high-Ba (HBa) type (< 56.3 wt.% SiO2, > 1027 °C), were injected separately into the magma chamber. Diffusion profiles of dacite-derived magnetite phenocrysts in black scoria (BS; 58–64.5 wt.% SiO2) show that the Numazawako eruption was triggered by the injection of LBa magma. LBa magma mixed with dacite magma in the chamber and erupted as BS during the two early eruption phases. In contrast, gray scoria (GS; 56.3–59.8 wt.% SiO2) has a distinct chemical composition found only during the two later eruption phases. The abundance and whole-rock composition of GS suggest that injection of HBa magma occurred immediately before or during phase III; the HBa magma mixed with BS-forming magma in the chamber to produce the GS magma. This second magma injection probably resulted in over-pressurization of the chamber, thereby triggering the second Plinian eruption.
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