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We report here the first measurements of the SO 2 flux from Erta 'Ale volcano (Ethiopia); the measurements were obtained in March 2003 with a portable ultraviolet spectrometer. Emission rates averaged 0.7 kg.s - 1 from the active lava lake and 1.3 kg.s - 1 from the whole volcano (including fumarolic emissions in the north part of the caldera). This modest output combined with an estimate of the melt sulfur content indicates a magma-supply rate of 350-650 kg.s - 1 (∼0.13-0.25 m 3 .s - 1 ). Radiant heat power from the lava lake, measured by infrared thermography, was found to vary between ∼5 and 30 MW according to activity of the lake surface and time elapsed since resurfacing events. We show that 8%-14% crystallization and/or 30-60 °C cooling of the upwelling magma reaching the lake, as well as degassing, are sufficient to increase magma density in the upper conduit and drive convection between the surface and a feeding reservoir. Fluctuations in the system, such as degree of vesiculation and magma supply rate, can be buffered by ascent or descent of the magma level within a flared vent region whose geometry directly controls lake surface area and hence heat loss.