Tephras Outcropping along the So-called "Chisoh-setudanmen" Roadcuts, Sembazaki, Izu-Oshima Volcano, Central Japan. Standard Tephra Columns.

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  • 伊豆大島火山千波崎の地層切断面露頭群のテフラ　標準柱状図
  • イズ オオシマ カザン センバザキ ノ チソウ セツダンメン ロトウグン ノ
  • Standard Tephra Columns
  • 標準柱状図

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Abstract

Latest Pleistocene and Holocene standard tephra columns for the Izu-Oshima volcano and its adjacent areas are presented. The type locality is along the famous sightseeing roadcuts situated 3.5km southwest of the summit crater, where fresh tephra sections can be observed at any time.<br>More than 160 tephra layers deposited since about 25, 000yrs BP provide a detailed time scale to the worldwidely famous triple junction areas of the Philippine Sea, North American, and Pacific tectonic plates. These tephra layers are unconformably subdivided in ascending order from the Sembazaki fifth to the first formation. Their ages are approximately assigned as: older than 20, 500yrs BP, 20, 500-13, 700yrs BP, 13, 700-5, 600yrs BP, 5, 600yrs BP-1, 700yrs BP, and 1, 720-present.<br>The uppermost surface layers of the fifth formation are perfectly eroded, and only 29 layers (Osb127→Osb100) have been described. The surface soil of each tephra is pale yellow brown. The fourth formation is composed of 44 layers (Osb100-O64); the soil of each tephra surface is brown. The third formation is composed of 40 tephra layers (O63-Osb36-1), and the surface soil of each tephra is dark brown or black humic. O58 and O55 which are dated by their black humic soil as 11, 580±140yrs BP (GaK-16143A) and 10, 440±380yrs BP (GaK-15049), respcctively contain white rhyolite pumice. O37-3 contains pale reddish bubble wall glass shards which are correlated to the Kikai-Akahoya ash [Ah] ejected at about 6, 300yrs BP from Kikai caldera, 1, 000km WSW of Izu-Oshima Volcano. The second formation is composed of 36 tephra layers (O35-Osb1), whose surface soils are densely black humic. Sembazaki, the first formation (S, N, Y), is composed of more than 12 tephra layers which have been identified as the ejectas of syn and post caldera stages. The surface soils are brown and less humic.

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