三宅島火山２０００年噴火における火山ガス　　火山灰の付着ガス成分およびＳＯ２放出量から推測される脱ガス環境

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書誌事項

タイトル別名
  • Volcanic Gas Study of the 2000 Miyakejima Volcanic Activity: Degassing Environment Deduced from Adhered Gas Component on Ash and SO2 Emission Rate.
  • ミヤケジマ カザン 2000ネン フンカ ニ オケル カザン ガス カザンバイ ノ フチャク ガス セイブン オヨビ SO2 ホウシュツリョウ カラ スイソク サレル ダツガス カンキョウ
  • Degassing Environment Deduced from Adhered Gas Component on Ash and SO<SUB>2</SUB> Emission Rate
  • 火山灰の付着ガス成分およびSO<SUB>2</SUB>放出量から推測される脱ガス環境

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抄録

To understand degassing processes during the 2000 Miyakejima volcanic activity, we applied the following two methods : 1) repeated analyses of adhered water-soluble gas component such as SO4and Cl ions on ashes produced at eruptions from July 8 and mid-September, and 2) SO2 flux measurements by COSPEC since August 26.<BR>The repetitive analyses of soluble component show remarkable changrd in Cl/S. Until August 18, the adhered SO4 concentrations are quite high and Cl concentrations are always low with Cl/S of 0.01-0.05, indicating that a certain mass of groundwater existed in aquifers beneath the summit crater and Cl component selectively dissolved in the groundwater. Since August 29, chlorine concentrations became greater and Cl/S ratios were determined as 0.1-0.14 on August 29 and 0.5-1.5 in September, which coincided with the strong volcanic gas emissions which started in mid-August. Groundwater boiling and establishment of gas conduit are likely to occur to prevent HCl from being absorbed in the groundwater.<BR>Sulfur dioxide emission rate has been monitored since August 26. The SO2flux increased in midSeptember from thousands to tens of thousands tons a day. The average SO2 flux after midSeptember to the present is 48 ktons/day. The highest flux was observed on December 7 to be 230 ktons/day. The mass rate of the magma degassing is estimated as 20 Mtons/day and the total volume of the degassed magma is calculated to be 1 km3 so far. The continuous magma degassing without eruptions occurs at a shallow environment by the convective transport of magma from a chamber to a magma head through conduits. A huge degassing rate is likely to be due to a large surface at the magma head, which would be made by a piston-cylinder type of collapse of the volcanic body.

収録刊行物

  • 地学雑誌

    地学雑誌 110 (2), 271-279, 2001

    公益社団法人 東京地学協会

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