Discovery and its Significance of Pyroclastic Flow Deposits on the southern slope of Mt. Fuji at the Primary Stage of the Younger Fuji Volcano, Central Japan

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  • 富士山南麓における新富士火山初期の火砕流堆積物の発見とその意義
  • フジサン ナンロク ニ オケル シン フジ カザン ショキ ノ カサイリュウ タイセキブツ ノ ハッケン ト ソノ イギ

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Abstract

Fuji volcano is the largest stratovolcano in Japan whose edifice consists of the alternation of basaltic lava flows and pyroclastic deposits, but very few pyroclastic density current deposits have been reported. We found two deposits of basaltic composition that are interpreted to be formed by pyroclastic flow at localities farther than 10km from the summit on the southern foot of Mt. Fuji. Each deposit is stratified with dense massive layer, well-sorted lapilli-rich layer, and fine ash layer, fundamentally in ascending order. Distribution of the three types of layers shows that the lower two layers deposited only on the bottom of valley whereas the uppermost layer deposited also on the bank. These facts indicate that generation mechanism of the deposits is pyroclastic flow but different from those reported in previous studies at Mt. Fuji. The vesicular lithology without any bombs or agglutinate blocks may imply that these flows were closely related to explosive eruption like Plinian style rather than immediate collapse of pyroclastic cone or fire fountain such that observed in Stromblian eruptions. The results of radiocarbon age were around 8400 yBP for charcoal samples in layers just below and above one of the pyroclastic flow deposits. The other pyroclastic flow deposit is covered by a lava flow of 9 ka. These results show that the two pyroclastic flows occurred in similar age when the edifice of Mt. Fuji is thought to have grown most voluminously but moderately by effusive eruptions in its volcanic evolution history of 100 kyrs.

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