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Abstract
Vesicles in pumice produced by pyroclastic eruptions record important information about the vesiculation history of the magma. This dissertation consists of three studies. Together they report numerous vesicularity measurements, vesicle size distributions, permeability measurements, images, and textural observations collected from dacitic to rhyolitic pumice clasts from several volcanoes, and use these data, along with simple modeling, to demonstrate how vesiculation, bubble coalescence, and permeability development all influence the fragmentation of magmas during eruption.
Differences in the vesicularities of white and gray pumice clasts from the May 18, 1980, eruption of Mount St. Helens reflect the presence or absence of microlites. The prevalence of bubble coalescence features preserved in the white pumice, the scarcity of such features in the gray pumice, and the contrast in vesicle size distributions between white and gray pumice all indicate that the presence of microlites in a vesiculating magma retards bubble coalescence and results in magmatic fragmentation at comparatively low vesicularities.
Bubble coalescence also plays a role in creating a permeable network through which exsolving magmatic volatiles may escape. Simple modeling is used to show that the bubble wall thinning and rupture necessary for bubble coalescence can occur on eruptive timescales. Estimates of magma ascent velocities combined with measured permeabilities of dacitic to rhyolitic pumice clasts suggest that pumice is preserved when the permeability achieved prior to fragmentation is sufficient to keep pace with required degassing; ash is formed when insufficiently permeable regions cannot degas and instead fragment more fully.
Finally, the vesicularity, vesicle size distributions, permeability, and textural features of pumice clasts from the climactic eruption of Mount Mazama (Crater Lake), Oregon, are examined more closely and compared to inferred fluctuations in eruption velocity. Relatively high vesicularities, low vesicle number densities, and substantial permeabilities correspond to periods of relatively low magma ascent velocity, providing ample time for bubble coalescence. If magma fragmentation does depend on permeability, then low velocity eruptions are predicted to produce larger pumice clasts than high velocity eruptions. This prediction is consistent with the frequently-made observation that the degree of fragmentation during pyroclastic eruptions increases with increasing eruption intensity.
