Eruption History in the Post-caldera Stage of Akita-Komagatake Volcano, Northeastern Japan Arc: Correlation between Eruptives Constituting Volcanic Edifices and Air-fall Tephra Layers

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  • 秋田駒ヶ岳火山,後カルデラ活動期における噴火史 : 火山体構成噴出物と降下テフラ層の対比
  • 秋田駒ケ岳火山,後カルデラ活動期における噴火史:火山体構成噴出物と降下テフラ層の対比
  • アキタ コマガタケ カザン ゴ カルデラ カツドウキ ニ オケル フンカシ カザンタイ コウセイ フンシュツブツ ト コウカ テフラソウ ノ タイヒ

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Abstract

Akita-Komagatake volcano is situated at about 30 km west of volcanic front in the northeastern Japan arc. Development history of the volcano is divided into 3 stages, main strato cone. syn-caldera formation, and post-caldera emotion stages, in descending order. In order to reveal the comprehensive emotion history m the post-caldera stage of the volcano, correlation between eruptives constituting the volcanic edifices and dispersed air-fall tephra are investigated, along with re-examining geology of the volcanic edifices formed m the concurring stage. Petrological affinities such as whole rock major-element chemistry and mineral assemblages including modal abundances of phenocrystic chases are utilized m identifying the contemporaneous eruptives of various modes of emplacement. To avoid the effect of phenocryst-matrix separation, coarse-grained tephra samples were selected to the whole rock chemical analysis. The revealed outline of eruption history is as follows: Tholentic magmatism has been dominant through the stage, with episodic calc-alkahc magmatism at the final activitiy m the northern region (ca. 4,000-3,100 y.B.P.). Relatively large eruptions occurred at the beginning of the stage (ca. 13,000 to 10,000 y.B.P.), resulting m caldera and probably large horse-shoe shaped crater as well, rather than forming volcanic edifices. Formation of the northern volcanic edifice began ca. 10,000 y.B.P. Five pyroclastic cones were built up successively, and subordinate lava flows or pyroclastic flows accompanied with them. Composition of magma was abruptly changed from andesitic to basaltic around 8,000 y.B.P., and the basaltic volcanism lasted to 7,000 y.B.P. After ca. 3,000 years of dormancy, phreatic explosions occurred successively, probably giving rise to horse-shoe shaped crater at the center of the northern area. Calc-alkali andesitic magma effused to build up a small pyroclastic cone immediately after the major explosion event. After ceasing the magmatism m the northern area, volcanism in the southern area was initiated by phreato-magmatic explosion m the floor of the south caldera about 2,500 y.B.P., followed by vulcanian eruptions, lasting until ca. 1,600 y.B.P About 1,500 y.B.P., eruption center was localized to two major vents at northern part of the caldera. Repeated eruptions from the vents have built up Medake and Kodake cones and their associated lava flows. The latest emotion occurred at the summit of Medake in 1970 AD

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