1982 年 4 月 26 日浅間火山噴火の降下火山灰

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書誌事項

タイトル別名
  • Ash-fall during the April 26, 1982 Eruption of Asama Volcano
  • 1982ネン 4ガツ 26ニチ アサマ カザン フンカ ノ コウカ カザンバイ

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説明

A small-scale explosive eruption of Asama volcano lasted from 02 h 25 m to ca. 06 h, April 26, 1982. It produced a thin but far-reaching ash fall bed and very minor pyroclastic flows on the upper slopes. The main axis of ash-fall deposit extended ESE passing Tokyo to reach the Boso Peninsula. Subordinate axis extended toward the southwest to reach Lake Suwa. The wind above 5, 000 m was mainly responsible for the main axis while the low altitude winds produced the subordinate axis as it blew south to southwestwards. The amount of ash fall ranged from more than 300 g/m2 to 100 g/m2 at the distance of ca. 10 km from the vent and the total mass erupted was estimated to be about 8 million tons. The ash traveled at an average speed of 12 m/s as far as 200 km from the vent along the main axis. Grain size of the ash regularly decreases clockwise at the distance of about 10 km from the crater reflecting the changing wind direction with altitude. No appreciable change in the medium diameter was found for samples taken in Tokyo (130 km away, Mdφ=3.32) and the one 10 km away (Mdφ=3.26). All samples show marked skewness toward fractions finer than 63 μm suggesting that such fine particles descended in aggregates. Field evidence that in some places ash was incorporated in mud droplets strongly supports this mechanism although it was reported that ash fell apparently in a “dry” state. Ash contained several percent of hydrothermally altered older volcanic materials as well as much water-soluble substances, gypsum, alunite, etc. No clay minerals were found by x-ray diffraction, a fact in strong contrast with the ejecta of 1977-1978 eruption of Usu and 1979 eruption of Ontake volcanoes although both materials had a very similar appearance and clayey physical properties as the present ash. No vesicular, juvenile matelials were identified and the bulk consisted of polyhedral grains of hyalopilitic pyroxene andesite very much similar to recent lavas of Maekawa-yama. No liquid magma but a high-temperature steam jet deep out of the vent may have been responsible for this explosive eruption. High temperature of the erupted material was clearly demonstrated by the glowing deposits observed on the upper slopes immediately after the first phase of eruption. These were mainly laid down by the very small scale pyroclastic flows which overflowed the crater rim and descended for a short distance over the northern and southern slopes.

収録刊行物

  • 火山.第２集

    火山.第２集 27 (3), 203-215, 1982

    特定非営利活動法人 日本火山学会

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