It is commonly accepted that shallow crystallization and degassing of silicic magmas contribute to the potential explosivity of a volcanic eruption, but the rates at which such processes operate in natural systems are unknown. The recent eruption of Mount St. Helens involved both explosive and non- explosive (dome-building) episodes that have followed a pattern of decreasing intensity and decreasing time-averaged magma production rates from 1980 through 1986; this trend has been interpreted to be the result of progressive of a single batch of dacite magma (e.g. Scandone and Malone, 1985). This project is for a detailed textural and chemical study of the products of the 1980 explosive eruptions of Mount St. Helens volcano with the goal of 1) using changes in glass chemistry and groundmass crystallinity together with stratigraphic work on deposits of May 18, 1980 (provided by Criswell, 1987) to assemble a pre-eruption stratigraphy of the Mount St. Helens magmatic system. 2) quantifying observed groundmass changes through the summer of 1980 to determine rates of crystallization in response to system degassing and depressurization, and 3) relating measured crystallinity and bulk chemistry to physical changes in eruptive style of the volcano; in particular, relating these parameters to observed patterns of degassing and changes in eruption rate and intensity. Proposed methods of study including not only detailed microprobe, BASEM and TEM analysis, but also quantification of textural parameters (e.g. the size and shape distributions of crystals and vesicles) made possible by recent advances in automated image analysis capabilities. This study differs from other approached to the "petrologic" inverse problem" not only in the extreme sample control based on timing, seismicity, and eruption style, but also in the application of textural as well as chemical criteria for sample classification.
