Standardized model runs and sensitivity analysis using the “Bubbledrive-1” volcanic conduit flow model

Abstract We apply the “Bubbledrive-1” numerical volcanic conduit model to the standard protocol defined at the 2002 conduit flow modeling workshop. The model is capable of additional factors such as treating the transient case and various conduit geometries that are not part of the protocol, so we fixed certain parameters to conform to the protocol for the purpose of model intercomparison and assessment. The model does not require chemical equilibrium between dissolved gases and melt conditions; oversaturation is allowed and commonly observed. This is a fundamental aspect of the model that enables us to explore the details of volatile exsolution and bubble growth that drives the eruption. We ran the model in “sensitivity study mode” in which all parameters were fixed except one so that the effect of each could be isolated. This is a useful diagnostic approach in that it reveals details that would be otherwise obscured by co-variations of parameters. Naturally, this leads to unrealistic situations in which, for instance, diffusivity is allowed to change, while viscosity and other magma properties are not. We conducted five sets of model runs to explore sensitivity to conduit depth, diffusivity, viscosity, recharge rate, and magnitude of eruption trigger. Each set consisted of four runs about a “standard” case, from which parameters were varied one by one. The results show that exit velocity accelerates linearly until a cylindrical conduit is emptied and the eruption abruptly stops. The discharge rate (mass flux) remains constant, indicating that increasing velocity is offset by decreasing density from greater vesicularity. Oversaturation at the vent is triggered at an initial value of 1 wt.% and does not change significantly regardless of variation in the protocol parameters. However, when other triggers are applied, the oversaturation still tends toward 1 wt.%, suggesting that it is controlled by magma properties and conduit geometry rather that the nature of the trigger. Variation of each parameter revealed specific aspects of eruption behavior. Extending conduit depth increases both eruption duration and the velocity at the vent. Increasing diffusivity increases vent velocity, but reduces duration. In contrast, increasing viscosity reduces vent velocity and extends duration, but includes a critical threshold beyond which vent velocity dramatically decreases (leading to effusive eruption). Recharge of magma from below enables the system to reach a steady state. Greater recharge rates reach steady state sooner, but the maximum vent velocity is not significantly affected by recharge. The magnitude of the trigger provided by decompression (e.g. landslide on the edifice) affects the acceleration of magma at the vent, but does not significantly control the maximum vent velocity. The “Bubbledrive-1” model is useful for diagnostic investigation of conduit flow and eruption processes. It is not intended as a simulation of natural eruptions, so cannot be readily compared to natural eruption products. However, the insights gained through diagnostic modeling may serve to augment our understanding of natural processes and help guide subsequent theoretical, laboratory, and field studies of volcanic systems.