Physicochemical models of effusive rhyolitic eruptions constrained with InSAR and DEM data: A case study of the 2011-2012 Cordón Caulle eruption

Abstract

The 9 month long 2011-2012 eruption of Cordón Caulle (Southern Andes, Chile) is the best instrumentally recorded rhyolitic eruption to date and the first time that the effusion of a rhyolitic flow has been observed in detail. We use Interferometric Synthetic Aperture Radar (InSAR), with time-lapse digital elevation models (DEMs) and numerical models to study the dynamics of coupled magma reservoir deflation and lava effusion. InSAR recorded 2.2-2.5 m of subsidence after the first three days of the eruption, which can be modeled using a spheroidal magma reservoir at a depth of ∼5 km, ∼20 km long, and with a pressure drop of 20-30 MPa. The source is elongated in the NW-SE direction and its large dimensions imply a large plumbing system active throughout the eruption and spanning neighboring volcanoes, with a slight change in the geometry halfway through the effusive phase. TanDEM-X and Pléiades DEMs record the extrusion of both the rhyolitic lava flow and the intrusion of a shallow laccolith around the eruptive vent after the third day of the eruption, with a total volume of ∼1.45 km3 DRE. The laccolith was emplaced during the first month of the eruption, during both the explosive and effusive stages of the eruption. Both the reservoir pressure drop and the extruded volume time series follow quasi-exponential trends, and can be explained by a model that couples the reservoir pressure decrease, time- and pressure-dependent variations in the magma properties inside of the reservoir, and conduit flow. This model predicts both the temporal evolution and amplitude of both time series during the effusive phase, and a magma compressibility of ∼10−10 Pa−1, half the reported compressibility of the magma of the sub-Plinian explosive phase. Further, we estimate that the reservoir contained 1-3 wt.% dissolved H2O at the onset of lava effusion, with no exsolved CO2 and H2O in the reservoir throughout the effusive phase. This implies that the magma was significantly degassed after the explosive phase. The remaining volatiles in the magma after the explosive stage might have caused magma fragmentation, consistent with the hybrid explosive-effusive style observed during the waning of the eruption.