Pressure changes in the magmatic system during the December 2008/January 2009 extrusion event at Soufrière Hills Volcano, Montserrat (W.I.), derived from strain data analysis

In December 2008/January 2009 a 4-week episode of lava extrusion characterised by rapid dome growth and ground deflation occurred at Soufrière Hills Volcano (SHV), Montserrat W.I. Recorded strain dilatometer data show strain changes up to − 3500 ns during this event. Modelling the collected data assuming existing structural models of the magmatic system, best-fit solutions are obtained for a simultaneous decompression of the shallow and the mid-crustal magma chambers together with a dilation of the shallow dyke-conduit. We interpret our model results as magma ascending from the two chambers into the eruption feeding dyke-conduit and partly extruding at the surface. Dome growth volume estimations from visual observations at SHV fit the extrusion volume inferred from our best-fit model and support our results. The reported data is the first set of geodetic data that documents the dynamic coupling within the entire crustal magmatic system of SHV.

► We analyse new strain data from an extrusion event at Soufrière Hills Volcano. ► Results indicate decompression of the two magma chambers together with dilation of a shallow NW–SE trending dyke. ► The data document for the first time simultaneous dynamic processes within the entire magmatic system. ► Results allow us to constrain the volume ratio of the shallow to the deep reservoir to 1:3. ► During the extrusion event magma ascended from both reservoirs into the shallow dyke and partially up to the surface.