National Aeronautics and Space Administration

Introduction. This chapter review several facets of the study of volcanic gases. The focus of the review is generally, but not exclusively on "high-temperature volcanic gases" - i.e., gases emitted at temperature over 500° C from various sources at active volcanoes (erupting magma, lava flows, lava lakes, lava domes, eruption vents, fumaroles, fractures, etc.) - since these gases contain a substantial proportion of volatiles released directly from shallow magmas. Shallow magmas beneath active volcanoes release volatiles during both passive degassing and volcanic eruption. Passively degassing volcanoes often permit direct sampling of volcanic gases from ground-level sources and from volcanic plumes. Investigations of volcanic gases during eruptive degassing generally require remote sensing methods, including satellite-based methods. Because of the close tie to magma degassing, investigations of high-temperature volcanic gases provide important data for constraining the compositions, amounts, and origins of volatiles in magma. Volcanic gas data provide insights into magma degassing processes and critical information for evaluating volcanic hazards. In recent years, volcanic gas emissions have also received attention because of the effects on the atmosphere and climate, and as benchmarks for comparison with anthropogenic gas emissions.