VESICULATION OF MAY 18, 1980, MOUNT ST-HELENS MAGMA

The May 18, 1980, eruption of Mount St. Helens, Washington, Produced b oth white and gray pumice, similar in composition but varying in pheno cryst, microlite, and vesicle content. The white pumice has fewer phen ocrysts, no microlites, and higher vesicularity, and is thus less dens e than the gray. In addition, vesicles in the white pumice are larger and more interconnected than those in the gray. Both white and gray ha ve effective (crystal-free) vesicularities (85.7% and 72.2%, respectiv ely), close to the traditionally accepted ''fragmentation vesicularity '' of 75%-77%. Given that the two pumice types were erupted concurrent ly for several phases of the eruption and therefore probably had simil ar eruption paths, we suggest that differences in vesicularity and ves icle size distributions result from the presence or absence of microli tes. The presence of microlites in the gray pumice not only increased the magma viscosity and effective vesicularity, but appears to have ai ded bubble nucleation and hindered bubble expansion and coalescence. T hus, magmas with microlites may fragment at a lower bulk vesicularity than those without microlites. Fragmented microlite-bearing clasts are also likely to expand less after fragmentation and therefore more clo sely preserve the bubble distribution and structure at the time of mag ma fragmentation.