Cyclic Variation in Discharge Rate during Volcanic Eruptions: Experimental, Theoretical and Field Observations

Large variations in discharge rate are common in andesitic dome-forming eruptions. We present an experimental and theoretical investigation of viscous flow through a heated elastic chamber connected to a cooled rigid conduit. Sugar syrup was pumped through the system using a peristaltic pump, and the discharge rate, temperatures and pressures were measured. The discharge rate showed a marked initial fluctuation before reaching a steady-state value where the input to the chamber is equal to the output. The hot syrup is initially strongly cooled in the conduit; as a consequence the conduit fluid has high viscosity and conduit resistance, so that the initial discharge rate is much lower than the eventual steady-state discharge rate. The pressure increases in the chamber and the fluid in the conduit progressively decreases in viscosity as it warms the conduit walls. Consequently the discharge rate sharply increases, reaches a maximum well above the steady-state discharge rate and then declines back to steady-state conditions. These observations have been reproduced in a mathematical model of this system. The concept is applied to lava dome growth at the Soufriere Hills Volcano, Montserrat, where the discharge rate increased in the period November 1995 to March 1998. We suggest that rising magma warmed the conduit walls, so that the effects of heat loss diminished and allowed progressively less viscous magma to erupt. This would lead to an eruption rate higher than the long-term average discharge rate, as was observed from May to December 1997. This presentation will include a critical evaluation of the performance of the mathematical model and laboratory experiment, and their application to a complex volcanological process.