Bubble coalescence in basaltic lava: Its impact on the evolution of bubble populations

Morphological properties (bubble number density, porosity, mean radius, specific surface area) of vesicular rocks provide quantitative information on the rates of bubble nucleation, growth and coalescence in magmas when measured as a function of time. Such data are useful in constraining the timing and style of gas release during volcanic eruptions. Volcanic rocks commonly show strong zonation with respect to bubble size and porosity, indicating a variation in the amount of bubble growth and coalescence preserved within a single sample. Morphological properties and bubble size distributions (BSD's) were measured in a suite of zoned alkali basalts using image analysis and the data were compared to theoretical predictions. Our data indicate that at porosities greater than 35%, extensive coalescence occurred during the growth of bubbles with restricted nucleation; at lower porosities, vesiculation is dominated by nucleation and diffusion with no coalescence. The interiors of many of our samples have undergone 4-7 binary coalescence events after eruption in a time of around 15 min. The Ostwald ripening effect has not significantly modified the BSD's.