伊豆大島火山,カルデラ形成期の火砕物密度流堆積物 : 差木地層S<sub>2</sub>部層の層序・岩相・年代の再検討 Pyroclastic Density Current from the Caldera-forming Eruption of Izu-Oshima Volcano, Japan : Restudy of the Sashikiji 2 Member Based on Stratigraphy, Lithofacies, and Eruption Age

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著者

    • 山元 孝広 YAMAMOTO Takahiro
    • 産業技術総合研究所深部地質環境研究センター Research Center for Deep Geological Environments, AIST, Geological Survey of Japan

抄録

The Sashikiji 2 (S<sub>2</sub>) Member in the products of Izu-Oshima volcano was formed by an explosive eruption accompanied with caldera depression. This member is characterized by breccia called as a "low-temperature pyroclastic flow deposit". In this paper, the S<sub>2</sub> breccia is re-examined based on stratigraphy, grain fabrics, grain-size distributions and modal compositions. The S<sub>2</sub> Member is divided into six units from S<sub>2</sub>-a to S<sub>2</sub>-f in ascending order. The S<sub>2</sub>-a unit consists of scoria, bomb and aa lava flows from flank fissures. The S<sub>2</sub>-b unit is made up of well-bedded ash and fine-lapilli from the summit. The S<sub>2</sub>-c unit is composed of matrix-supported breccia, locally filling valley bottoms and containing abundant deformed soil fragments and woods. The S<sub>2</sub>-d unit consists of reverse to normal grading, clast-supported breccia with ash matrix, covering topographic relief in the whole island. The S<sub>2</sub>-e unit is composed of dune- to parallel-bedded lapilli and ash in the proximal facies. The S<sub>2</sub>-f unit is clast-supported breccia with and without ash matrix. New <sup>14</sup>C ages of wood fragments in the S<sub>2</sub> Member have been determined as about Cal AD 340. Although the S<sub>2</sub>-c and -d units are previously interpreted to the low-temperature pyroclastic flow deposit, these units are quite different in sedimentological features as follows. The grain fabric measurements have revealed that the S<sub>2</sub>-d unit has a-type imbrication showing the longest axis of grains parallel to the flow direction. On the other hand, the S<sub>2</sub>-c has random fabric of grains. The grain size distribution of the S<sub>2</sub>-d unit shows a bimodal nature having subpopulations at phi -1.0 to 1.0 and coarser than phi -2.5. The bimodal nature and a-type imbrication suggest that the two transport processes overlap; the load of a turbulent suspension is not all in true suspension as the coarser population may travel in a cast-dispersion mass flow. The S<sub>2</sub>-c unit shows a polymodal grain size distribution with multi subpopulations from coarse to fine. The poor sorting, massive appearance, valley-confined distribution, and random grain fabric of the S<sub>2</sub>-c unit are characteristic of deposition from a cohesive flow without formation of traction-related bedforms or sorting of different grain sizes by turbulence. The modal composition measurements have indicated that the S<sub>2</sub>-c and -d units lack essential scoriceous or glassy fragments. This evidence indicates that both units are derived from steam explosions due to outburst of highly-pressurized geothermal fluid within the edifice. The S<sub>2</sub>-c unit was plausibly generated by remobilization of phreatic debris around the summit caused by ejection of condensed water from a plume or heavy rainfall. The S<sub>2</sub>-d unit was a pyroclastic density current deposit resulted from collapse of a highly-discharged phreatic plume. Estimated velocities of the current are 150 to 30m/s based on suspended grain sizes.

収録刊行物

  • 火山

    火山 51(4), 257-271, 2006

    特定非営利活動法人 日本火山学会

参考文献:  43件中 1-43件 を表示

被引用文献:  3件中 1-3件 を表示

各種コード

  • NII論文ID(NAID)
    110004798427
  • NII書誌ID(NCID)
    AN10512786
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    0453-4360
  • NDL 記事登録ID
    8092608
  • NDL 雑誌分類
    ZM44(科学技術--地球科学--海洋・陸水・火山)
  • NDL 請求記号
    Z15-101
  • データ提供元
    CJP書誌  CJP引用  NDL  NII-ELS  J-STAGE 
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