Capturing the collapse of Bardarbunga

Abstract

Large volcanic eruptions are commonly associated with a collapse of the roof of a crustal magma reservoir, forming a caldera. Only a few such collapses occur per century, and the lack of detailed observations prior to the Bardarbunga collapse meant we knew little of the mechanical interplay between collapse and eruption. The collapse of Bardarbunga Caldera 2014-­‐2015 provided a unique opportunity to observe these processes in action. We borrowed four GNSS receivers from late 2014 to summer 2015, to assist in monitoring the caldera collapse, which occurred during the largest eruption in Iceland for 230 years, and the subsequent re-­‐inflation once the eruption ended in late February 2015. The GEF receivers joined a pool of other receivers to provide good overall coverage. Because of particularly high snowfall over winter 2014-­‐2015, we were not in the end able to deploy all receivers in the pool, including the GEF receivers, but this report is based on the success of the overall project. GNSS monitoring of the deformation around the caldera played a key role in constraining the depth of the magma reservoir associated with the collapse. Taken together the data can be explained by collapse initiated through withdrawal of magma, and lateral migration for 48 km from a 8-­‐12 km deep reservoir. The collapse rate, which followed a near-­‐exponential decline, and the eruption rate, which followed a virtually identical decline during the 180-­‐ day long eruption, can both be explained by the interaction between the pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow path.