HR: 08:00h
AN: V11F-01 INVITED    [Abstracts]
TI: Sponge Cake or Champagne? Bubbles, Magmatic Degassing and Volcanic Eruptions
AU: * Cashman, K
EM: cashman@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403-1272, United States
AU: Pioli, L
EM: lpioli@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403-1272, United States
AU: Belien, I
EM: ibelien@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403-1272, United States
AU: Wright, H
EM: heather.wright@sci.monash.edu.au
AF: Monash University, School of Geosciences, Clayton, VIC 3800, Australia
AU: Rust, A
EM: Alison.Rust@bristol.ac.uk
AF: Bristol University, Department of Earth Sciences, Bristol, BS8 1RJ, United Kingdom
AB: Vesiculation is an unavoidable consequence of magma decompression; the extent to which bubbles travel with ascending magma or leave the system by separated or permeable flow will determine the nature of the ensuing eruption. Bubbles travel with the melt from which they exsolve if the rise time of bubbles through the melt (the ‘drift velocity') is much less than the rise rate of the magma (sponge cake). This condition is most likely to be met in viscous melts (where bubble rise velocities are low) and in melts that experience rapid decompression (high ascent velocities). Under these conditions, bubble expansion within the melt continues until sufficient bubble expansion causes coalescence and the development of a permeable network. Typical pumice vesicularities of 70-80% and permeabilities of 10^-12 m² constrain this limit under conditions appropriate for subplinian to plinian eruptions (mass fluxes > 10⁶ kg/s). Slower rise rates (and lower mass fluxes) that characterize effusive eruptions produce silicic lavas with a wider range of vesicularities. In general, permeability decreases with decreasing sample vesicularity as bubbles deform (as evidenced by anisotropy in permeability and electrical conductivity) and pore apertures diminish. Degassing efficiency (and resulting densification of magma within the conduit) under these conditions is determined by permeability and the time allowed for gas escape. Bubbles rise through the melt if the drift velocity exceeds the velocity of magma ascent (champagne). This condition is most easily met in volatile-rich, low viscosity (mafic) melts at low to moderate fluxes. At very low magma flux, magma eruption rate is determined by the extent to which magma is entrained and ejected by rising gases (strombolian eruptions); when bubbles are too small, or are rising too slowly, they may not break the surface at all, but instead may be concentrated in a near-surface layer (surface foam). As the magma flux increases, segregation of bubble-rich from bubble-poor melt requires both longer conduits and lateral transport of degassed magma, as seen in violent strombolian eruptions. Flow transitions require coalescence, which is a dynamic process where bubble-bubble interactions are controlled by shear and gravitational (i.e. buoyancy) processes, both of which are strongly dependent on magma rheology. Also common in basaltic magmas that exhibit separated flow are by the simultaneous eruption of crystal-rich (shallow) and crystal-poor (deep) magmas. Upward increases in crystal content within the magma transport system will create rheological changes that may both slow upward bubble migration and change the size and shape of the bubble network (through deformation, coalescence, or bubble splitting). Preliminary experiments further show that abrupt rheological boundaries may concentrate bubbles at the boundary, allowing them to coalesce and move laterally prior to rising through the mush.
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting