A review on volcanic gas compositions related to volcanic activities and non-volcanological effects

Abstract

Volcanic gases have been monitored to predict eruptions since their compositions are strongly affected by volcanic activities. Here, volcanic gas geochemistry is reviewed with the main focus placed on the potential use of volcanic gases as eruption precursors. First, volcanic activities are differentiated into four eruptive cycles: eruptive, post-eruptive, inter-eruptive, and pre-eruptive periods. Second, we discuss geochemical factors affecting volcanic gas compositions: magma origin and degassing, hydrothermal interactions, and meteorological variability. Third, volcanic gas data in previous studies are revisited to address the practical aspects in utilizing volcanic gas compositions to predict eruptions. As a magma is depressurized due to its uprising, volcanic gases become progressively exsolved. Generally, the fluxes of volcanic gases (e.g., CO2, SO2, H2S, etc.) increase considerably close to eruptions. Also, CO2/SO2, SO2/HCl, and SO2/HF ratios in volcanic gases tend to decrease at the imminence of eruptions. Nonetheless, such trends may not be encountered due to both kinetically limited degassing and secondary effects including hydrothermal interactions and meteorological variability. Due to the site specificity of the aforementioned effects, a comprehensive understanding of volcanic gas chemistry requires the knowledge of magma vents and reservoirs as well as hydrothermal systems below volcanoes. Accordingly, the integrated use of volcanic gas data with geophysical and geomorphological information can warrant a better prediction of eruptions.