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 infor mation on the rates of bubble nucleation, growth and coalescence in ma gmas when measured as a function of time. Such data are useful in cons training 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 grow th and coalescence preserved within a single sample. Morphological pro perties and bubble size distributions (BSD's) were measured in a suite of zoned alkali basalts using image analysis and the data were compar ed to theoretical predictions. Our data indicate that at porosities gr eater than 35%, extensive coalescence occurred during the growth of bu bbles with restricted nucleation; at lower porosities, vesiculation is dominated by nucleation and diffusion with no coalescence. The interi ors of many of our samples have undergone 4-7 binary coalescence event s after eruption in a time of around 15 min. The Ostwald ripening effe ct has not significantly modified the BSD's.