Think of volcanoes, and you might picture an eruption column jetting from a summit crater, punching through the atmosphere with roiling coils of dark ash. Or perhaps you are of a more tranquil disposition and imagine a peaceful summit crater but brightly stained with orange and yellow minerals deposited from fumarolic clouds hissing through vents and fissures amidst the scree. It goes without saying that gases are behind both these manifestations, providing the violence needed to propel eruption plumes to altitudes of tens of kilometres, or leaking more slowly from unseen magma bodies to fuel hydro thermal systems (Fig. 1). The speciation and exsolution of these volatiles in magmas, the quantification of their emissions into the atmosphere, the application of such measurements for volcano monitoring purposes, and characterization of their impacts on the environment (in its broadest sense) are the subjects of this book.
‘Next to nothing is known about the sources of the volatile components of magmas or how they are distributed and transported between the mantle and shallow levels of the crust.’This is how Williams and McBirney began the chapter on volcanic gases in their influential 1979 textbook, Volcanology. In the following two decades, diverse investigations spanning experimental phase petrology, isotope geochemistry, and thermodynamical and fluid dynamical modelling, have replaced this pessimistic view by a much more encouraging outlook on our understanding of the origins, storage and transport of volcanic volatiles. In particular, laboratory analytical work on natural and synthetic melts has gone a long way