火山性流体

抄録

Volcanic fluids are H₂O-based fluids containing volatile components degassed from magma, and are valuable media that carry information on degassing magma and hydrothermal fluids to the surface. Magma and hydrothermal fluids are the driving forces behind magmatic and phreatic eruptions, respectively, and observations of volcanic fluids are useful for evaluating volcanic activity. Volcanic gases are particularly suitable for volcanic activity monitoring because they are gaseous and have a large velocity of movement within the crust. The volume of degassed magma can be estimated from remote observation of SO₂ contained in the plume directly degassed from the magma. An increase in the CO₂/SO₂ ratio of the plume is understood to be a precursor to a magmatic eruption. The CO₂/SO₂ ratio of magmatic gas varies with degassing depth, but it may also increase with sulfur sequestration even at a constant degassing depth, so care must be taken in interpreting the observation results. The mixing of magmatic gas and groundwater and the subsequent formation process of vapor and hydrothermal phases can be understood from the sampling and analysis of fumaroles. The ratio of magmatic to hydrothermal components in fumaroles can be a useful indicator of activity in volcanoes that produce phreatic eruptions.