Coupled subdaily and multiweek cycles during the lava dome eruption of Soufrière Hills Volcano, Montserrat

Observations of volcanoes extruding andesitic lava to produce lava domes often reveal cyclic behavior. At Soufrière Hills Volcano, Montserrat, cycles with subdaily and multiweek periods have been recognized on many occasions. Observations clearly show that the period of subdaily cycles is modulated by the multiweek cycle. The subdaily and multiweek cycles have been modeled separately as stick‐slip magma flow at the junction between a dyke and an overlying cylindrical conduit and as the filling and discharge of magma through the elastic‐walled dyke, respectively. Here, we couple these two models to describe the behavior over a period of well‐observed multiweek cycles, with accompanying subdaily cycles, from 13 May to 21 September 1997. The coupled model captures well the asymmetrical first‐order behavior: the first 40% of the multiweek cycle consists of high rates of lava extrusion during short period/high amplitude subdaily cycles as the dyke reservoir discharges itself. The remainder of the cycle involves increasing pressurization as more magma is stored, and extrusion rate falls, followed by a gradual increase in the period of the subdaily cycles.