Volcanic Gas Study of the 2000 Miyakejima Volcanic Activity: Degassing Environment Deduced from Adhered Gas Component on Ash and SO2 Emission Rate

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.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.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.