Experimental analysis of bubble-driven magma motion in the conduit, for persistently active, open-vent volcanoes

Abstract

This study uses analog experiments to understand the role of bubbles in inducing conduit convection, for persistently degassing lava lake systems. To do so, air was fluxed through an initially stagnant column of liquid and the resulting return flow was measured. The dynamics suggested by the experimental results is compared with that of quiescent, persistently active volcanoes, with a focus on eight volcanoes that exhibit summit lava lakes. We find that magma flux is a function of combined gas flux, conduit size, and magma rheology. Experiments with high gas fluxes through low viscosity liquid took on turbulent characteristics, which correspond to a high degree of return flow, whereas lower gas fluxes through high viscosity liquids yielded slug flow, which corresponded to less return flow. We model the magma flux due to bubble ascent and find that gas-driven liquid flow can yield faster flow rates than other mechanisms at work in volcanic conduits. This can explain the discrepancy between previous estimates of magma flow in conduits and relatively fast, lava lake surface velocity observations. We show how bubble-driven convective flow can work alongside density-driven convection and discuss the depths in the conduit where each are likely to dominate the system.