Eruption ages and whole-rock chemistries of scoria cones on the northern to western slope of Fuji Volcano based on trenching surveys.

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  • トレンチ調査から見た富士火山北‐西山腹におけるスコリア丘の噴火年代と全岩化学組成
  • トレンチ調査から見た富士火山北-西山腹におけるスコリア丘の噴火年代と全岩化学組成
  • トレンチ チョウサ カラ ミタ フジ カザン キタ ニシサンプク ニ オケル スコリアキュウ ノ フンカ ネンダイ ト ゼンガン カガク ソセイ

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Abstract

Eighteen unknown-aged scoria cones on the northern to western slope of Fuji Volcano were trenched to determine their stratigraphic positions. We recognized three key tephra layers derived from large-scale eruptions of Fuji Volcano from 1,400 B.C. to 300 B.C. and detected two widespread tephras of the Kikai-Akahoya at 5,300 B.C. and Amagi-Kawagodaira at 1,400 B.C. within volcanic ash soil layers. Based on the detected levels of the widespread tephras and thickness of volcanic ash soil layers, the eruption ages of the scoria cones could be estimated as follows. 1) Three (Nagayama,Futatsuyama, and Sawarayama) cones formed from 8,300 B.C. to 6,000 B.C. 2) Five (Yumiizuka,Nishiken, Hokusei-okuniwa, Maruyama, and Toyazuka) cones were constructed from 5,300 B.C. to 2,600 B.C. 3) Five (Hachiken’yama, Nishi-kosukemaru, Kosukemaru, Tomine, Usuyama, and Hokuseiusuyama) cones formed from 2,300 B.C. to 1,800 B.C. 4) Four (Hokusei-yumiizuka, Higashiken,Shikanokashira, and Katabutayama) cones were erupted from 1,600 B.C. to 1,300 B.C. The petrographic features and chemical compositions of the products from the scoria cones conform with those from contemporaneous lava flows. During the last 10,000 years, the K2O content and FeO*/MgO ratio of the products from the northern to western flank eruptions decreased during 2,300 B.C. to 1,800 B.C.

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