HR: 08:00h
AN: V11A-01 INVITED     [PDF]
TI: Using textures to constrain the kinetics and dynamics of eruptive processes
AU: * Cashman, K V
EM: cashman@oregon.uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403-1272 United States
AB: Volcanic eruptions are dynamic events whose progress depends, in large part, on rheological changes resulting from syn-eruptive vesiculation and crystallization. Bubble and crystal textures preserved in quenched pyroclasts and lava record both conditions of these phase transformations and the flow dynamics of the resulting suspensions. Several examples are summarized below. Volatiles provide the primary driving force for volcanic eruptions. Eruption style (intensity) thus depends on the timing of bubble nucleation, the extent to which volatile exsolution occurs under closed- or open-system conditions, and the mechanism of fragmentation. Analysis of pumice textures shows that silicic pumice is dominated by a large population of small ($< 50 ~\mu$m) bubbles that most closely resembles experimental textures produced by homogeneous bubble nucleation under conditions of rapid decompression. Subsequent bubble expansion and coalescence modifies the bubble size distribution and creates permeable bubble networks. As a result, the relative rates of magma ascent, bubble expansion, and coalesence control the extent of closed- or open-system degassing within the conduit. Bubbles also show highly variable degrees of deformation and elongation that indicate large strains along conduit walls, and bubble shapes and orientations can be used to infer the strain history of magma at different locations in the conduit. Magmas crystallize as the result of H2O-saturated decompression in volcanic conduits and of cooling during flow on the Earth's surface. Crystal textures record both the duration of crystallization and the relative importance of nucleation and growth. Syn-eruptive groundmass crystallization produces plagioclase number densities that span several orders of magnitude - a testimony to the sensitivity of crystal nucleation rates to effective undercoolings produced by decompression and cooling on eruptive time scales. Recent decompression experiments help to constrain the kinetics of degassing-induced crystallization in silicic melts; rates of crystallization in basaltic lava flows may be measured directly by sampling along active lava channels. Also important are changes in crystal habit with changes in effective undercooling. Determining the evolution of crystal shapes is important not only for accurate calculation of CSDs, but also for predicting changes in melt rheology with increasing crystal contents of melts. In summary, the textures of volcanic rocks provide a rich repository of kinetic and dynamic information critical for understanding volcanic activity. Challenges for the future include improved techniques for 3D textural analysis at the micron scale and incorporation of textural data into models of magma ascent, eruption and flow on the Earth's surface.
DE: 8414 Eruption mechanisms
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting