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The rates of passive degassing from volcanoes are investigated by modelling the convective overturn of dense degassed and less dense gas-rich magmas in a vertical conduit linking a shallow degassing zone with a deep magma chamber. Laboratory experiments are used to constrain our theoretical model of the overturn rate and to elaborate on the model of this process presented by Kazahaya et al. (1994). We also introduce the effects of a CO 2 -saturated deep chamber and adiabatic cooling of ascending magma. We find that overturn occurs by concentric flow of the magmas along the conduit, although the details of the flow depend on the magmas' viscosity ratio. Where convective overturn limits the supply of gas-rich magma, then the gas emission rate is proportional to the flow rate of the overturning magmas (proportional to the density difference driving convection, the conduit radius to the fourth power, and inversely proportional to the degassed magma viscosity) and the mass fraction of water that is degassed. Efficient degassing enhances the density difference but increases the magma viscosity, and this dampens convection. Two degassing volcanoes were modelled. At Stromboli, assuming a 2 km deep, 30% crystalline basaltic chamber, containing 0.5 wt.% dissolved water, the F700 kg s -1 magmatic water flux can be modelled with a 4-10 m radius conduit, degassing 20-100% of the available water and all of the 1 to 4 vol.% CO 2 chamber gas. At Mount St. Helens in June 1980, assuming a 7 km deep, 39% crystalline dacitic chamber, containing 4.6 wt.% dissolved water, the F500 kg s -1 magmatic water flux can be modelled with a 22-60 m radius conduit, degassing F2-90% of the available water and all of the 0.1 to 3 vol.% CO 2 chamber gas. The range of these results is consistent with previous models and observations. Convection driven by degassing provides a plausible mechanism for transferring volatiles from deep magma chambers to the atmosphere, and it can explain the gas fluxes measured at many persistently active volcanoes.

Abstract. Between December 1991 and March 1993 a continuous effusive eruption at MountEtna built a 7.6 km 2 lava flow field. Flows extended to within 1 km of the town of Zafferana before a successful artificial diversion was carried out higher up the volcano. During this eruption the spaceborne advanced very high resolution radiometer (AVHRR) acquired 308 images on which the activity could be detected. Since these data can be freely and directly available, such coverage potentially allows regular, real-time monitoring. Ground observations and a flow map that we produced using a SPOT image and electronic distance measurement allowed us to develop and test data extraction techniques. AVHRR radiance maps were consistent with known locations of surface activity. These documented the transition from channel to tube fed phases, the changing threat to Zafferana, and flow diversion. Quantitative analysis of the AVHRR data enabled estimation of active lava area, thermal flux, effusion rates, and total flow field volume. Our estimates for eruption rate and total flow field volume, 5.6 to 7.6 m3/s and 220 x 106 to 300 x 106 m 3, respectively, are in agreement with published ground-based estimates of 5.8 m3/s and 235 x 106 m 3. These correlations demonstrate the high degree of confidence that can now be placed in interpretations of AVHRR time series for eruptions where ground-based data are scanty.

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