Coupled conduit flow and shape in explosive volcanic eruptions

Volcanic conduit flow models generally utilise one of two conduit profile modes, which are referred to as parallel-sided (typically surface choked) or lithostatically pressure-balanced. Their limitations in application to supersonically erupted basaltic and rhyolitic explosive eruptions are investigated. Likely changes in conduit profile over time, due to catastrophic failure resulting from high wall stresses, and abrasive erosion, are investigated using a semi-analytical model. Although time-independent, this model nonetheless reveals likely trends in shape from a simple parallel-sided geometry. It is found that, for rhyolites, early wall failure will occur in all but unusual circumstances, resulting in expansion of the conduit, a reduction in wall stresses, and a trend towards a lithostatically pressure-balanced solution at depth. In both rhyolitic and basaltic eruptions, abrasive erosion results in conduit flaring near the surface, allowing the choking point to descent from the surface into the vent, substantially changing exit conditions and resulting in a trend towards the a lithostatically pressure-balanced solution at the surface. Although a truly lithostatically pressure-balanced system can never be attained, due to the effects of compressibility at supersonic velocities, it is no less valid than the more commonly utilised parallel-sided system for a variety of realistic eruption scenarios and can be a useful guide for potential steady states.