北海道洞爺カルデラ中島火山の火山地質と形成史 : 空中レーザーマッピングと地表地質調査による推定 Geology and Evolution of the Nakajima Islands (Toya Caldera, Hokkaido, Japan) Inferred from Aerial Laser Mapping and Geological Field Surveys

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Abstract

我々は,北海道洞爺カルデラ中島火山の火山地質を明らかにするため,ヘリコプター搭載型の高分解能レーザースキャナを用いたレーザーマッピングと,地表地質調査を行った.レーザーマッピングは中島の全域を含む3×3kmで行い,計測データから3次元のデジタル地形図を作成した.地表地質調査は3次元デジタル地形図を用いて中島の全域で行い,詳細な産状記載と岩石記載を行った.デジタル地形図と地表地質調査から,中島の詳細な火山地質と形成史が明らかになった.中島は,デイサイトおよび安山岩質マグマが噴出して形成した8個の溶岩ドーム(東山ドーム,西山ドーム,北西ドーム,北山ドーム,南西ドーム,観音島ドーム,弁天島ドーム,饅頭島ドーム),デイサイトマグマが湖底堆積物を押し上げて形成した潜在ドーム(北東岬ドーム),およびデイサイト質のマグマ水蒸気噴火により形成したタフコーン(東山火砕丘)からなる.中島の北東部と南西部には,泥岩と砂岩からなる湖底堆積物が分布しており,中島の火山活動がカルデラ底の隆起を伴ったことを示す.中島は,洞爺カルデラ中央部のリサージェントドームの形成と,それに伴うデイサイト〜安山岩質マグマの噴出により形成されたと考えられる.高分解能レーザースキャナによる地形計測と3次元デジタル地形図を用いた地質調査は,火山地質の解明に極めて有効である.

Aerial high-resolution laser-scanner mapping and geological field surveys were performed over the Nakajima Islands within Toya caldera, southwestern Hokkaido, Japan, to study the evolutional history of the islands. The laser-scanner mapping survey covered the entirety of the Nakajima Islands, an area of 3×3 km. A three-dimensional digital map produced from the laser-scanning data revealed detailed topographic features of the islands. Geological field surveys were carried out over the whole area of the Nakajima Islands, to determine how their topography relates to the geological and lithological features. These surveys suggest that the islands consist of a tuff cone, eight lava domes, and a cryptodome. The tuff cone has a low profile and wide crater and consists of dacitic pyroclastic deposits, suggesting that the cone was produced by explosive eruptions resulting from the interaction of dacitic magma and groundwater. The lava domes are conical or pancake-shaped and composed of dacitic to andesitic lavas, suggesting that the domes formed by extrusions of high-viscosity, dacitic to andesitic magmas. The cryptodome consists of coherent dacite overlain by mudstone and sandstone, suggesting that the dome formed by the uplift of caldera-floor deposits following the intrusion of high-viscosity dacitic magma. Mudstones and sandstones are present along the northeastern and southwestern parts of the islands, implying that the caldera floor was uplifted during or prior to the volcanism, forming a bulge(small resurgent dome) at the center of the caldera. We infer that the Nakajima Islands have evolved from caldera resurgence related to the ascent of voluminous dacitic to andesitic magma, followed by subsequent formation of multiple dacitic to andesitic domes and phreatomagmatic eruptions on the resurgent dome.

Journal

  • BULLETIN OF THE VOLCANOLOGICAL SOCIETY OF JAPAN

    BULLETIN OF THE VOLCANOLOGICAL SOCIETY OF JAPAN 60(1), 17-33, 2015

    The Volcanological Society of Japan

Codes

  • NII Article ID (NAID)
    110009919295
  • NII NACSIS-CAT ID (NCID)
    AN10512786
  • Text Lang
    ENG
  • ISSN
    0453-4360
  • NDL Article ID
    026262594
  • NDL Call No.
    Z15-101
  • Data Source
    NDL  NII-ELS  J-STAGE 
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