HR: 10:20h
AN: V42A-01 INVITED     [Abstracts]
TI: Volatiles, Vesiculation, and Styles of Eruptive Activity
AU: * Cashman, K V
EM: cashman@uoregon.edu
AF: University of Oregon, Dept of Geological Science, Eugene, OR 97403 United States
AU: Rosi, M
EM: rosi@dst.unipi.it
AF: Universita di Pisa, V. Santa Maria 53, Pisa, 56126 Italy
AB: The exsolution, expansion and escape of volatiles from ascending magma provides much of the kinetic energy that drives volcanic eruptions. The past decade has seen important advances in our understanding of the physical processes by which vesiculation and degassing occur. For example, comparison of natural and experimental samples suggests that the numerous very small bubbles characteristic of silicic pumice probably formed under conditions of very large ΔP. This, in turn, suggests that highly energetic silicic eruptions are sustained by large pressure differentials between the vent and the magma reservoir, and that the efficiency of magma fragmentation depends on both strain rates in the conduit and the ability of gas to escape through permeable networks. Moreover, the need for large ΔP prior to substantial bubble nucleation and growth permits silicic magma to migrate to shallow levels prior to experiencing either large volatile losses or consequent extensive degassing-induced crystallization. Thus silicic eruptions are commonly preceded by only shallow seismicity (related to degassing?). Moreover, syn-eruptive degassing-induced crystallization is thereby restricted to eruptions of durations of at least hours. In contrast, basaltic tephra has fewer but larger vesicles than silicic pumice, with size distributions that suggest that bubble nucleation is typically followed by bubble growth (expansion) and coalescence. Moreover, in contrast to silicic systems, which show no discernible relationship between bubble number density and explosivity, basaltic tephras show a positive correlation between mass eruption rate and bubble number density. Although there are no experimental data available for comparison, these observations suggest that vesiculation in basaltic magmas occurs at near-equilibrium conditions (small ΔP). This suggests that the rate of magma ascent exerts a strong control on not only vesiculation but also the extent to which bubbles segregate from the magma (two-phase flow). Additionally, small barriers to volatile exsolution imply that hydrous mafic arc magmas are likely to experience extensive decompression-driven crystallization en route to the surface. Together these processes help to explain the large range of explosive activity exhibited by mafic systems.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005