Arc volcanoes are most commonly defined in terms of their magmatic and eruptive histories but the sustained flux of high temperature reactive gases through them, at all stages of their eruptive cycles, equally defines them as large scale, gas phase, chemical reactors (Fig. 1). Inside them, reactive mass transfer occurs continuously as magmatic vapor, released from crystallizing silicate melt at lithostatic pressure, expands through a subsurface volcanic gas plume (Henley and McNabb 1978) to atmospheric pressure at the surface. Between eruptions, volcanic gas fluxes dominated by water and reactive gases, such as SO2(g) and HCl...

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