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Geologic History of the Western Part of Post-caldera Central Cones of Aso Volcano, Southwestern Japan, Based on Stratigraphic Relationships between Lava Flows and Airfall Tephra Layers

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  • 溶岩流とテフラとの層序関係からみた阿蘇火山中央火口丘群西部地域の発達史
  • ヨウガンリュウ ト テフラ ト ノ ソウ ジョ カンケイ カラ ミタ アソカザン チュウオウ カコウキュウグン セイブ チイキ ノ ハッタツシ

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Abstract

The western part of post-caldera central cones of Aso Volcano, southwestern Japan, is characterized by wide chemical variation of lava flows (SiO2=49-73 wt.%) and complicated topography. Geologic history of the area was reconstructed by stratigraphic relationships between lava flows and airfall tephra layers. Stratigraphy and chronology of lavas and tephra distributed on the western slope of the central cones are as follows: Ayugaerinotaki lava, Matsuhata pumice (MhP), Tochinoki lava (73±10 ka; K-Ar age), Aso central cone pumice 4 (ACP4), Tateno lava. Aso central cone pumice 3 (ACP3), Takanoobane lava (51±5ka), Eboshidake lava, Karisako lava, Akase lava (30 cal ka ; calibrated 14C age), Sawatsuno lava (27±6ka), Kusasenrigahama pumice (Kpfa; 31 cal ka), Otogase lava and Aso central cone pumice 1 (ACPI ; 4 cal ka), in ascending order. Between about 70,000 to 50,000 years ago Tochinoki lava, Tateno lava and Takanoobane lava accompanied precursor pumice-fall deposits. A catastrophic eruption occurred in and around the Kusasenrigahama crater at 31 cal ka. The eruption was initiated by the dacite Sawatsuno lava now and produced the largest plinian numice-fall deposit (Kpfa; 1.2 km3 in DRE volume). In the western part of post-caldera central cones of Aso Volcano, total discharge volumes of felsic and mafic magmas from about 70,000 to 30,000 years ago were estimated to be about 2.4 km3 and 0.47km3 respectively.

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