The Slug and the Plug: The Evolution of the Strombolian Paradigm

It is generally accepted that Strombolian eruptions result from the burst of large gas slugs decoupling from a low-viscosity magma. However, recent studies at Stromboli are starting to elucidate some of the complexities in these explosions, with high speed imaging identifying multiple ejection pulses within single explosions. Furthermore, petrological data suggests the presence of a highly crystalline magma in the upper portion of the conduit, which could act as a plug, and indicates a potential source for complexities not seen in existing first order (single viscosity fluid) models. Here we present a detailed laboratory investigation to represent a more complex fluid dynamic scenario in which a viscous liquid (plug) overlays a less viscous fluid within the conduit.

Scaled experiments were designed to reproduce the effects of a plug at the top of the conduit on the dynamics of slug expansion, burst and the resulting geophysical signals. Single slugs ascended silicon oil capped with castor oil in a vertical tube; pressure was measured in the liquid and within the gas 'atmosphere’ above the liquid, and correlated with high-speed imagery of the experiments. Experiments were carried out with different slug gas volumes, thicknesses of the plug and experimental ambient pressure. The results were compared with CFD simulations, carried out both at laboratory and volcanic scale, to further explore the complex pressure distributions and forces exerted on the conduit.

Our results highlight complex fluid dynamic processes, related to the intrusion of slugs in the viscous plug leading to different flow configurations and liquid mixing. Furthermore, the presence of a viscous plug strongly controls variations in the magnitude of the associated pressure transients, and the slug-plug interaction post burst favours a more impulsive pressure release.