小野子火山の地質とその基盤の構造 Geology of Onoko volcano and its basement structures, central Japan

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小野子火山は,中期更新世に活動した複成火山で,体積約13km<sup>3</sup>の円錐火山からなる十二ヶ岳成層火山形成期,山頂部にカルデラ状地形を,北北東および南東斜面に溝状地形を形成して山麓部に岩屑なだれ堆積物を堆積させた山体崩壊期,カルデラ状凹地から宮沢溶岩や小野子山南溶岩が流出した山頂溶岩類流出期の3期に区分される.溶岩流などの火山岩は,複輝石安山岩を主体とするが,初期には玄武岩やかんらん石含有複輝石安山岩も認められる.十二ヶ岳成層火山形成期の後半には,火山体の西斜面で漏斗状の形をとる十二ヶ岳南爆裂火口が発生し,現在その内部構造の一部が露出している.山頂部付近には,中ノ岳貫入岩,放射状岩脈群,小野子山貫入岩などの貫入岩体が存在し,これらは東西方向に直線状に並ぶ.この方向に本火山活動と密接な関係をもち,かつ,中新統の相対落差が1100m以上に達する断層が推定される.

The Middle Pleistocene Onoko volcano is situated at the southernmost part of the Northeast Japan volcanic arc. The volcanic history of the Onoko volcano can be divided into the following three stages.Stage 1: Junigatake stratovolcano, a conical-shape volcano with total volume of 13km<sup>3</sup> and 1800m high above sea level, was formed. It is composed of lava flows and volcaniclastic rocks of basaltic and andesitic composition. The crater was probably located at about 300m to the west of the summit of Mt. Onoko. Junigatake-minami explosion crater was formed at the western flank of this stratovolcano, and also the Nakanotake mass and the andesitic radial dikes intruded in this stage.Stage 2: Collapse of the Junigatake stratovolcano took place, and a relatively small-scale caldera (about 2km in diameter) was formed at the summit of the stratovolcano, and small U-shaped valleys (about 100m deep and 1km wide) were also formed both at the southeastern and the northeastern franks of the volcano. In relation to this collapse, a debris avalanche occurred and the deposits are distributed on the foot of Onoko volcano.Stage 3: Voluminous andesitic lava flows (Miyazawa lava and Onokoyama-minami lava) extruded and the Onokoyama mass intruded in the final stage.Andesitic magmas most probably rose through E-W trending fault. This is suggested by the difference of the height of the Miocene basement rocks beneath the volcano.

収録刊行物

  • 地球科学

    地球科学 51(5), 346-360, 1997

    地学団体研究会

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各種コード

  • NII論文ID(NAID)
    110004862367
  • NII書誌ID(NCID)
    AN00141269
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    0366-6611
  • NDL 記事登録ID
    4302515
  • NDL 雑誌分類
    ZM41(科学技術--地球科学)
  • NDL 請求記号
    Z15-171
  • データ提供元
    CJP書誌  CJP引用  NDL  NII-ELS  J-STAGE 
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