Formation and Maintenance of a Viscous Plug in a Strombolian Volcanic Conduit

Abstract:

Textural studies of pyroclasts have shown that both degassed, crystal-rich magma, and gas-rich, crystal-poor magma may be ejected by a single strombolian explosion. Furthermore, some pyroclasts contain both magma types, intermingled. These pyroclasts have been interpreted as evidence of the presence of a plug of degassed, crystal-rich – and therefore high-viscosity – magma at the top of the volcanic conduit, overlying gas-rich, crystal-poor, low-viscosity magma. Analogue experiments have indicated that the presence of a viscous plug increases the vigour of strombolian explosions by enhancing the build-up of overpressure in the gas slug that drives them.

The purported plug material has a higher density, as well as a higher viscosity, than the magma that underlies it. Consequently, the configuration is expected to be unstable, and the plug and underlying magma should convectively overturn. Does this mean that a viscous plug is an ephemeral feature? Or can a plug persist for long periods as a dynamic feature, which is continually created at the same rate at which it is convectively consumed? We conduct laboratory analogue experiments to investigate the formation and maintenance of a viscous plug. We drive thermal convection of a range of Newtonian and non-Newtonian magma analogue fluids in a 20cm diameter vertical pipe by heating the bottom, and cooling the top. The fluids have strongly temperature dependent rheology; both rheology and driving buoyancy are scaled to the volcanic scenario. We present preliminary results which characterize the spatial distribution of viscosity, density and velocity. We find that the hotter, lower-viscosity fluid ascends the core of the pipe, whilst the cooler, higher-viscosity fluid descends in an outer annulus. The upper region, in which the flow reverses direction, constitutes a dynamic plug.