Even if the gas is immobile, just the existence of vaporsaturated
conditions in the shallow crust should affect the
manner in which any hydrothermal region transmits pressure
pulses between magma and the ground surface (Norton
1984). Surface displacements are commonly interpreted by
assuming a volume change in a discrete magmatic source
embedded in a deforming, elastic, isotropic crust; such
assumptions would likely lead to underestimating the size and
depth of any magma source beneath a gas-rich, compressible
hydrothermal system (Dzurisin 2007; Hurwitz et al. 2007b).
Clearly, any added insight on the abundance of volatiles and
their dynamics in the shallow crust will help us to assess
which signals of caldera unrest are most likely to foretell an
impending eruption (Newhall and Dzurisin 1988).