Flow- and Fracture-Driven Bubble Throat Growth Rates and Dynamic Permeability in Crystallizing Magma

Abstract

Pyroclasts typically exhibit coalesced vesicle textures, which are the evidence of bubble coalescence and the incomplete bubble wall retraction in magma during volcanic eruptions. The sizes of bubble throats or inter-bubble apertures in permeable networks control the extent of magma outgassing, and therefore, quantifying the growth rates of the bubble throats is important but has remained poorly constrained. Using dynamically similar experiments with spontaneous bursting of a single bubble in rheologically well-characterized particulate suspensions, we investigate the growth rate of bubble throats for a range of particle volume fractions. For suspensions with $\lesssim$0.50 particle volume fraction, a circular hole (bubble throat) forms following bubble bursting, which after an initial fast growth starts plateauing at a throat-bubble size ratio of $\gtrsim$0.20. The throat growth time scale overall increases with increasing particle volume fraction due to the increase in suspension viscosity. On the other hand, bubbles in suspensions with particle volume fraction near the maximum packing fraction ($\sim$0.64) exhibit a fracture-like opening. Thus, our experimental results suggest that the plateauing of the bubble throat growth in crystal-poor to crystal-rich magma likely contributes to the wide occurrence of the incompletely retracted vesicle walls in pyroclasts. The implications of the flow- to fracture-like growth of bubble throats on the development of dynamic permeability in magma are discussed.