Volcanic-gas monitoring

The environmental impact of volcanoes is closely related to the rates, style and chemistry of their gas emissions (Delmelle, 2003). Monitoring the composition and mass flux of volcanic gases is therefore central to understanding how volcanism impacts our planet, on both global and local scales. There are two main modes of volcanic-gas release on Earth (Chapter 14): (i) the impulsive emission of large quantities of gases during episodic, large-scale volcanic eruptions, and (ii) the far more sluggish, but persistent, passive gas release from quiescent or mildly erupting volcanoes. Characterising the chemical composition of impulsive emissions has remained a challenge, and direct measurements have remained limited to satellite-based SO2 mass estimates (Carn et al., 2003). These measurements have been combined with indirect estimates of gas budgets for other gases based on petrological models (e.g. Gerlach et al., 1996). However, it is the persistent type of emissions that dominates the global volcanic-gas budget over long-term (> decadal) timescales (Chapter 14), and this chapter focuses on the chemical composition of such gas emissions. It has been known for more than a century that the analysis and interpretation of compositions and fluxes of passive volcanic-gas emissions can provide profound insights into how active volcanoes work. Such insight can contribute to understanding and possibly even forecasting the transition from quiescence to an eruption. The basic concept is that magmatic volatiles have finite solubilities in silicate melts, and hence they inexorably degas to a vapour phase as magmas are decompressed and cooled as they ascend to the surface (Giggenbach, 1996).