Bi-Directional Flow Experiments and Implications for Degassing Processes at Basaltic Volcanoes

Gas flux data at basaltic volcanoes such as Stromboli (Aeolian Archipelago), Etna (Italy), Masaya (Nicaragua) and Izu-Oshima (Japan) indicate that far more gas is being released during persistent degassing activity than can be accounted for by magma which is erupted during more active phases. For persistent degassing to occur over the very long time periods observed, fresh magma must be continuously supplied from depth. This can be sustained by an exchange flow of magma, driven by the density difference between dense degassed magma at shallow levels and buoyant gas rich magma at depth within the conduit. To interpret gas emissions and understand changes in activity at these volcanoes requires knowledge of the geometry of the exchange flow and how the flux of rising magma depends on its physical properties and conduit size. Experiments have been performed to observe the buoyancy driven, low Reynolds number (Re < 1.1) exchange of viscous fluids in a vertical pipe (length 1 m diameter 38.4 mm) between two reservoirs. Viscosity ratios (β, more viscous fluid divided by less viscous fluid) of the pairs of fluids in the experiments ranged from 2 to 1900. Two distinct flow regimes were observed; axisymmetric core annular flow in which the less viscous fluid occupies a cylindrical core and the denser fluid flows downwards in an annulus, and side by side flow where both fluids are in contact with the pipe and a single interface exists between them. Core annular flow formed at high viscosity ratios (≥100) and side by side flow formed at lower viscosity ratios (≤100). The flows do not form a configuration that maximises exchange flux, instead volumetric exchange flux is found to be independent of the flow regime with the flux and viscosity ratio forming a single power law trend. Using this empirical Q(β) power-law relationship we are able calculate the radius of the conduit required to account for observed gas flux data at persistently degassing basaltic volcanoes.