The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 117 , Issue 12
Showing 1-6 articles out of 6 articles from the selected issue
SPECIAL ISSUE: Stratigraphy, chronology and eruption analysis for high-resolution volcanic geology
  • Mitsuru Okuno
    2011 Volume 117 Issue 12 Pages 654-662
    Published: December 15, 2011
    Released: March 26, 2012
    This paper reviews the use of high-resolution tephro-stratigraphy and radiocarbon chronology to establish a high-resolution tephrochronology. Tephra formation is distinguished by loam or humic soil layer which presents repose period. Background level and resolution of tephra layers depend on the type of deposits (aeolian, marsh, or lacustrine sediments). Varved sediment from lake bottom is the most reliable recorder. Accurate recgnition of tephra layer in the field should be observed at multiple sites to avoid over counting the numbers of layers. It is recommended to use combination of source volcano and type locality for the tephra name. If numbers are used in the name, they should be given in descending order from the surface.
    In the last decade, the development of techniques for 14C measurement provides high-precision dates. However, high-accurate date requires strict interpretation of geological relationship between dated sample and tephra deposition. Systematic sampling of soil can present high-resolution chronology. On the other hand, 14C wiigle-matching method is the most reliable technique to determine the eruption age.
    An empirical formula, V = 12.2TA (Hayakawa, 1985), is useful to calculate volume of fallout tephra. The case study of the 2000 eruption of Usu volcano allows application of the formula for small-scale eruption. For best results, the most constrained contour should be used for calculation.
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  • Yasuharu Hoshino, Teruki Oikawa
    2011 Volume 117 Issue 12 Pages 663-670
    Published: December 15, 2011
    Released: March 26, 2012
    In this paper, we review recent progress in the dendrochronological analysis of volcanic events. Tree rings provide useful information on past environmental changes. A number of studies have revealed that volcanic eruptions and their subsequent aftermaths are recorded in ring widths, wood density and so on with annually-resolved calendar dates. However, our review work suggests that methods for detecting variations in the characteristics of tree rings could be improved for trees that have been directly affected by, but have survived, volcanic events. Future efforts should concentrate not only on developing and extending site chronologies, but also on examining new analytical techniques for tree-ring time series.
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  • Masayuki Sakagami, Hisashi Sasaki, Yasuyuki Miyake, Sakae Mukoyama
    2011 Volume 117 Issue 12 Pages 671-685
    Published: December 15, 2011
    Released: March 26, 2012
    The Asama volcano erupted on 1 September 2004, followed by several more eruptions during the period from September to December 2004. In the proximal area, particularly on the western side of the crater, the distribution and details of ejecta from the September 1 eruption have not been well studied. The purpose of this study is to investigate the ejecta from this area, and to obtain information concerning the juvenile magma of one eruption. Sasaki and Mukoyama (2006) presented a distribution map of the impact craters of the September 1 eruption, using IKONOS high-resolution satellite imagery; this imagery is partially effective for determining the distribution of ejecta from the eruption. However, ejecta in deep forests are not detectable by IKONOS imagery, and the rock types of the ejecta cannot be ascertained from imagery alone. Therefore, we collected field observations on the distribution of ejecta, to evaluate the effectiveness of IKONOS imagery data. After examining 130 impact craters identified by Sasaki and Mukoyama (2006), we determined that more than 100 of the impact craters (of various sizes) had been formed by impacts of massive and/or altered accessory blocks. At the other 30 spots, we discovered three juvenile breadcrust bombs. Moreover, we discovered 24 bombs at sites not detected by the IKONOS imagery. These results indicate that a combination of satellite imagery and field surveys is required to obtain precise distributions and details of volcanic ejecta. The water contents in the glass rinds of the 22 breadcrust bombs were from 0.49 to 0.86 wt.%. Calculations of magma depth on the basis of these water contents indicate that the magma column was at least 180 m high.
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  • Takeshi Hasegawa, Mitsuhiro Nakagawa, Jun'ichi Itoh, Takahiro Yamamoto
    2011 Volume 117 Issue 12 Pages 686-699
    Published: December 15, 2011
    Released: March 26, 2012
    We investigated the correlation between pyroclastic materials in terrigenous marine deposits of the Kushiro region and pyroclastic deposits in the Akan volcanic area, eastern Hokkaido, with respect to Quaternary stratigraphy, glass chemistry, contents and assemblages of phenocrysts, and age determinations using tephro-chronology. A high-resolution Pleistocene tephro-stratigraphy of the Akan volcano was previously established for the Ak1–Ak17 sequence (youngest–oldest). The Akan eruptive deposits are intercalated with time-marker tephras derived from the adjacent Kutcharo volcano (KpI–KpVIII, youngest to oldest), central Hokkaido, and from unknown sources. In this study, pyroclastic materials in marine deposits in the Quaternary Otanoshike Formation, Kushiro region, were sampled at four type localities.
    The Otanoshike Formation, covered by two pyroclastic flow deposits, contains two volcanic ashes, one in the lower part and one in the upper part of the formation. The lower and upper ashes correlate with the LowK-1 (source unknown) and Ak5 tephras, respectively. The overlying lower and upper pyroclastic flow deposits correlate with the KpVI and KpIV, respectively. The ages of the LowK-1 and KpIV tephras were estimated at ca. 0.8 and 0.1 Ma, respectively. These ages indicate that deposition of the Otanoshike Formation started by at least 0.8 Ma and finished before 0.1 Ma.
    The Kushiro Formation, composed of the lower Takkobu and the upper Toro members, contains many reworked tephras, all originating from the Ak13–Ak17 tephras, implying that the Kushiro Formation formed before the Ak12 eruption. The Ak14 and Ak10 eruption ages are estimated at ca. 1.5 and 1.0 Ma, respectively, suggesting that deposition of the Kushiro Formation spanned the 1.5–1.0 Ma time period.
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