A description of the dynamics, chemistry and energetics governing a volcanic system can be greatly
simplified if the expansion of magmatic gas can be assumed to be adiabatic as it rises towards the
surface. The conditions under which this assumption is valid are clarified by analysis of the transfer
of thermal energy into the low conductivity wallrocks traversed by fractures and vents from a gas
phase expanding over a range of mass flux rates. Adiabatic behavior is predicted to be approached
typically within a month after perturbations in the release of source gas have stabilized, this
timescale being dependent upon only the characteristic length scale on which the host rock is
fractured and the thermal diffusivity of the rock. This analysis then enables the thermal energy
transport due to gas release from volcanoes to be evaluated using observations of SO2 flux with
reference values for the H2O:SO2 ratio of volcanic gas mixtures discharging through high
temperature fumaroles in arc and mantle-related volcanic systems. Thermal power (MWH/s)
estimates for gas discharge are 101.8 to 104.1 MWH during quiescent, continuous degassing of arc
volcanoes and 103.7 to 107.3MWH for their eruptive stages, the higher value being the Plinean
Pinatubo eruption in 1991. Fewer data are available for quiescent stage mantle-related volcanoes
(Kilauea 102.1MWH) but for eruptive events power estimates range from 102.8 MWH to 105.5MWH.
These estimates of thermal power and mass of gas discharges are commensurate with power
estimates based on the total mass of gas ejected during eruptions. The sustained discharge of
volcanic gas during quiescent and short-lived eruptive stages can be related to the hydrodynamic
structure of volcanic systems with large scale gaseous mass transfer from deep in the crust coupled
with episodes of high level intrusive activity and gas release.