Lava dome growth and mass wasting measured by a time series of ground-based radar and seismicity observations

Exogenous growth of Peleean lava domes involves the addition of lava from a central summit vent and mass wasting on the flanks as rockfalls and pyroclastic flows. These processes were investigated at the Soufriere Hills Volcano, Montserrat, between 30 March and 10 April 2006, using a ground-based imaging millimeter-wave radar, AVTIS, to measure the shape of the dome and talus surface and rockfall seismicity combined with camera observations to infer pyroclastic flow deposit volumes. The topographic evolution of the lava dome was recorded in a time series of radar range and intensity measurements from a distance of 6 km, recording a southeastward shift in the locus of talus deposition with time, and an average height increase for the talus surface of about 2 m a day. The AVTIS measurements show an acceleration in lava extrusion rate on 5 April, with a 2-day lag in the equivalent change in the rockfall seismicity record. The dense rock equivalent volumetric budget of lava added and dispersed, including the respective proportions of the total for each component, was calculated using: (1) AVTIS range and intensity measurements of the change in summit lava (similar to 1.5 x 10(6) m(3), 22%), (2) AVTIS range measurements to measure the talus growth (similar to 3.9 x 10(6) m(3), 57%), and (3) rockfall seismicity to measure the pyroclastic flow deposit volumes (similar to 1.4 x 10(6) m(3), 21%), which gives an overall dense rock equivalent extrusion rate of about 7 m(3).s(-1). These figures demonstrate how efficient nonexplosive lava dome growth can be in generating large volumes of primary clastic deposits, a process that, by reducing the proportion of erupted lava stored in the summit region, will reduce the likelihood of large hazardous pyroclastic flows.