Magma buoyancy drives rhyolitic eruptions: A tale from the VEI 5 2008-2009 Chaitén eruption (Chile) from seismological and geodetic data

Magma buoyancy is a mechanism usually neglected with respect to magma overpressure in eruption models constrained with geophysical data. In this study we provide evidence for a buoyancy-driven mechanism of magma ascent during the effusive phase of the 2008-2009 Chaitén rhyolitic eruption. We also constrain the volcano plumbing system with source models from InSAR and teleseismic data. InSAR data did not detect unambiguous evidence of pre-eruptive ground deformation up to 1 year before the eruption. Teleseismic data indicate that a dike opened twice during the first two days of the eruption, while InSAR data show that three sources deflated during the eruption. These include one dike and a sill during the onset of the eruption, and a spherical source halfway through it, all located at depths between 3 and 12 km. 75% of the total ground deformation of ∼0.5 m occurred during the first three weeks of the eruption. A Pléiades DEM show that the dome has a bulk volume of 1 km³, and was extruded with an exponential trend, but the lack of deformation during most of the effusive phase of the eruption implies that the dome extrusion did not result in depressurization of a magma reservoir. Instead, we show that the time series of extruded volume can be explained by magma ascending due its buoyancy instead of its overpressure. Further, the end of the first pulse of dome effusion in September 2008 can be explained by an increase in the dome surface load that equals the effect of the magma buoyancy. These results imply that in some cases ground deformation data alone cannot be used to forecast the temporal evolution of an eruption. They also call for the acquisition of denser time series of extruded volume, as a way to better constrain their evolution through time.