It takes three to tango: 2. Bubble dynamics in basaltic volcanoes and ramifications for modeling normal Strombolian activity

This is the second paper in a two-part series examining numerical simulations of buoyancy-driven flow in the presence of large viscosity contrasts. In the first paper, we demonstrated that a combination of three numerical tools, an extended ghostfluid-type method, the level set approach, and the extension velocity technique accurately simulate complex interface dynamics in the presence of large viscosity contrasts. In this paper, we use this three-fold numerical method to investigate bubble dynamics in the conduits of basaltic volcanos with a focus on normal Strombolian eruptions. Stromboliantype activity, named after the famously episodic eruptions at Stromboli volcano, is characterized by temporally discrete fountains of incandescent clasts. The mildly explosive nature of normal Strombolian activity, as compared to more effusive variants of basaltic volcanism, is related to the presence of dissolved gas in the magma, yielding a complex two-phase-flow problem. We present a detailed scaling analysis allowing identification of the pertinent regime for a given flow problem. The dynamic interactions between gas and magma can be classified into three non-dimensional regimes based on bubble sizes and magma viscosity. Resolving the fluid dynamics at the scale of individual bubbles is not equally important in all three regimes: As long as bubbles remain small enough to be spherical, their dynamic interactions are limited compared to the rich spectrum of coalescence and breakup processes observed for deformable bubbles, in particular once inertia ceases to be negligible. One key finding in our simulations is that both large gas bubbles and large conduitfilling gas pockets (’slugs’) are prone to dynamic instabilities that lead to their rapid breakup during buoyancy-driven ascent. We provide upper-bound estimates for the maximum stable bubble size in a given magmatic system and discuss the ramifications of our results for two commonly used models of normal Strombolian-type activity, the Rise-Speed-Dependent (RSD) model and the Collapsing-Foam (CF) model.