火山ガス観測から推定される火山活動推移過程

抄録

Volcanic gases are derived from a magma or a hydrothermal system, which are sources of magmatic or phreatic eruptions, respectively. Changes in a magma or a hydrothermal system, leading to eruptions, likely cause changes in volcanic gas emission rate and compositions. SO₂ emission rates correlate well with volcanic activities and precursory changes of the emission rates are also common. The SO₂ emission rate also tell us a volume and a degassing rate of the SO₂ sources, that can constrain processes of magma ascent or crystallization. CO₂/SO₂ ratios increases are observed prior to major eruptions of volcanoes with Strombolian activity by automatic monitoring of volcanic gas compositions, and are attributed to accumulation of CO₂-rich bubbles in a magma chamber. Common occurrence of excess degassing by explosive eruptions implies an eruption of a bubble-rich magma, suggesting that a bubble accumulation is a necessary condition of an eruption. Various precursory signals are observed prior to phreatic eruptions but quantitative modeling of the processes is not simple because the volcanic gas compositions are largely affected by various reactions at a shallow hydrothermal system. Combination of volcanic gas emission rate and composition monitoring data may help us for the quantitative evaluation of the volcanic gas changes associated with phreatic eruptions.