9614753 Cashman It is proposed to quantify the effect of magma ascent rate on both volatile loss and crystallization through a detailed study of the products of several eruptions representing a range of eruptive styles, magnitudes, and magma composition. The central focus will be to re-examine the 1980-1986 explosive and effusive products of Mount St. Helens, with the specific goals of determining quantitative relations between eruption rate and pyroclast textures, and examining the conditions that led to the 1980 transition from pulsatory explosions to purely effusive eruptions. Additional studies of deposits from other well characterized volcanic eruptions (i.e. Taupo in New Zealand, and Vesuvius in Italy) will permit generalizations of this work to different magma compositions and eruptive conditions. With the results of these studies, it will be possible to evaluate the importance, respectively, of factors such as interaction with external water and compositional dependence of crystallization in the generation of variably vesicular and micro-crystalline pyroclasts. The larger goal of this investigation will be to determine the physical processes that control eruptive styles in intermediate-to-highly siliceous magmatic systems.
