Bubble Migration and Coalescence during the Solidification of Basaltic Lava Flows

Abstract

The initial size distribution of gas bubbles in lava flows is modified by bubble rise and coalescence during post eruptive cooling and crystallization. A simple model which computes the effects of rise and coalescence accounts for observed features and may provide a means whereby the initial bubble size distribution can be inferred. A model lava flow of assumed thickness, viscosity, and volume percentage of gas bubbles is given an initial bubble size distribution. Bubbles coalesce due to differences in their relative rise velocities. Using a gravitational collection kernel, the process may be modeled by numerical integration of the stochastic collection equation, which yields the change in the number density spectrum of the population from the number densities of all pairs of bubbles and their probabilities of collision. Bubbles rise and coalesce within a fluid interior sandwiched between fronts of crystallization that advance inward from top and bottom. Bubbles that are overtaken by the crystallization fronts cease to migrate. The model predicts the formation of upper and lower vesicle-rich zones separated by a vesicle-free interior. The upper zone is broader, more vesicular, and has larger bubbles than the lower zone. Increasing the initial average bubble size in the model results in a stratigraphically lower and thinner lower vesicular zone, a thicker vesicle-free central region, and a higher and more vesicular upper zone.