Outgassing and eruption of basaltic magmas: The effect of conduit geometry

The flow dynamics of magmas is controlled by several parameters, including magma rheology, density, surface tension, gas and liquid flow rate, and the geometry of the flow field, which is mainly regulated by the conduit shape. For this reason, magmatic flows along fissures and dikes are significantly different from those along axisymmetric conduits. Basaltic eruptions are typically fed by fractures or dike systems that reach the surface, giving rise to fissure eruptions. Scaled experiments show that in these low-viscosity systems, gas transport and segregation (i.e., the outgassing dynamics) are deeply controlled by the fracture geometry. Rapid bubble clustering and formation of bubble plumes determine the formation of lateral magma convection cells involving up to 70 vol% of the melt. In analogy with outgassing in cylindrical conduits, the average vesicularity and size of bubbles increase with increasing gas flow rate and melt viscosity and density, which also control the lateral extent of the bubble plume and convection cells.