Implications of bubble breakup and their coalescence in volcanic conduits for Hawaiian and Strombolian eruptions

Magmas ascending through volcanic conduits generate bubbles in large volume fractions due to the decompression-induced degassing of volatile matters. It is recognized that the bubble-forming process can significantly influence dynamics of magma eruptions. However, interactions between densely-packed ascending bubbles require further investigation, particularly to elucidate transient eruption behaviors. Using scaled analogue experiments and computational fluid dynamics (CFD) simulations, employing a framework of the level-set method for multiphase flows, we address this critical issue and demonstrate the hydrodynamic interactions of vertically ascending bubbles and their effects on deformation, coalescence, and post-coalescence breakup processes. These phenomena are analyzed as a function of density ( ρ* ) and viscosity ( µ* ) ratios of bubbles relative to ambient magma. Our results show that two consecutive bubbles in a vertical arrangement undergo contrasting deformations before coalescing; the trailing bubble elongates along the ascent direction, while the leading bubble flattens horizontally. A series of simulations systematically examine the conditions for bubble ascent with and without post-coalescence breakup. The findings suggest that repeated breakup and coalescence events can induce an unsteady state of magma flow in the conduit. We provide an estimate of the threshold spatial separation ( d* = 1.5) required for coalescence. Based on this estimate, we propose that a bubble occupancy of 50% by volume can trigger a transition from Hawaiian (bubbly flows) to Strombolian (slug flows) eruptions. This study enhances our understanding of the interplay between bubble dynamics and magma flow behavior inside a conduit, offering insights into the mechanisms driving transient volcanic eruptions.