火山豆石として降下堆積した十和田火山八戸火山灰

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タイトル別名
  • The Hachinohe Ash : An Example of an Accretionary Lapilli-Fall Deposit from Towada Volcano, Japan
  • カザン マメイシ ト シテ コウカ タイセキシタ トワダ カザン ハチノヘ カ

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抄録

The Hachinohe ash is a widespread pyroclastic-fall deposit erupted from the Towada caldera about 13, 000 years B. P. Along the dispersal axis, the thickness is 150cm 50km away from source, and the estimated volume is 14km3 It is composed of alternating beds of fine ash (65%) and pumice lapilli (35%). No erosional break is observed and the contacts between beds are gradational. The fine ash beds have two components : a dominate component of grain-supported accretionary lapilli and a subordinate component of fine ash-coated pumice; an ash matrix is lacking. The maximum grain size of accretionary lapilli does not decrease systematically away from source. The size population of constituent ash particles shows a small degree of fractionation with distance from source; the grain size class 1mm to 1/4mm increases while the class finer than 1/16mm decreases. Pumice beds are composed primarily of sub-angular to sub-rounded pumice fragments coated with fine ash and a subordinate amount of lithic fragments and accretionary lapilli. Maximum pumice size and maximum lithic size systematically decrease away from source. The beds show bimodal grain size distributions and contain more than 10 weight percent fine ash. An individual fine ash particle has too low a terminal velocity to fall out as a separate grain near the source area. It is certain that, throughout the Hachinohe ash eruption, fine ash continued to fall in the form of accretionary lapilli and/or attached to pumice fragments. The fine grained nature and wide dispersal indicate that the Hachinohe ash is representative of the phreatoplinian deposit formed by the interaction of water and silicic magma during explosive eruptions. At times when the proportion of erupted magma to lake water gaining access to the vent became sufficiently high, violet eruptions took place and deposited pumice fragments and accretionary lapilli simultaneously at the same place. Examples of phreatoplinian deposits are also reported from the Kutcharo and Hakone calderas, in addition to two other deposits from the Towada caldera. Such deposits are used as a possible indicator of source environment.

収録刊行物

  • 火山.第２集

    火山.第２集 28 (1), 25-40, 1983

    特定非営利活動法人 日本火山学会

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