HR: 0830h
AN: V51E-0329    [PDF]
TI: Effects Of Conduit Flow Parameters On Vulcanian Explosion Characteristics
AU: * Diller, K
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Clarke, A
EM: amanda.clarke@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404 United States
AU: Neri, A
AF: CNR-IGG, Dept. of Earth Sciences, University of Pisa, Pisa, PSA Italy
AU: Macedonio, G
AF: Osservatorio Vesuviano, Napoli, Napoli, NAP Italy
AU: Voight, B
AF: Dept. of Geosciences, Penn State University, University Park, PA 16802 United States
AB: Previous work produced numerical models of Vulcanian explosions which occurred in 1997 at SoufriŠre Hills Volcano, Montserrat. Plume evolution and velocities were calculated for the well-documented and typical explosions of August 6th and 7th 1997, and these data and other observations were compared to transient, axisymmetric, multi-phase flow simulations of coupled conduit evacuation and pyroclastic dispersal. Pre-explosion conduit conditions were estimated from Montserrat data, using a simple gas solubility law and assuming that conduit magma flow had stagnated with a constant overpressure prior to the explosions. The numerical simulations of the explosions resolved highly unsteady vent exit conditions such as velocity, pressure and mass flux. Our simulations produced behavior which generally mimicked the observed explosions. The best match to the observed and measured explosion parameters employed an intermediate conduit permeability value. Results suggest that a modest amount of volatile depletion in the conduit was an important factor in influencing the strength and style of the Vulcanian explosions on Montserrat. Ongoing work, which is presented here, uses a dynamic model of the conduit, rather than assuming that flow had stagnated. The new conduit model allows for magma viscosity and density changes due to degassing, crystallization and the presence of bubbles as the magma rises in the conduit, creating a non-linear overpressure gradient with depth. The first aim of this project is to examine conduit model parameters to determine which significantly affect the steady-state conduit pressure and gas volume fraction profile. Important parameters presented here are permeability (both vertical and horizontal), chamber crystal volume fraction, crystal growth function (curve fits of experimental results), magma flux rate and bubble-bearing rheology parameters, such as bubble size. The second aim is to determine to what degree such added complexity in the conduit model controls the resulting explosion simulation results. The first round of comparisons suggests that added rheological complexity does not significantly affect the energy of the explosion. For example, plume ascent velocities and collapse timing and height of a simulation with initial conditions determined by a rheologically complex model, but without permeability, were quite similar to those derived from our original, stagnated, non-permeable conduit. However, plume shape and collapse style were affected by the new conduit formulation. These results suggest that the strength of the explosion is particularly sensitive to the permeability of the conduit system, but is not sensitive to vertical pressure profile, while, on the other hand, the style of the explosion plume and fountain collapse is sensitive to both the conduit permeability and pressure profile. Simulated Vulcanian explosions resulting from the more complex dynamic conduit model, including effects of permeability, are therefore presented and compared against simulations derived from the simple stagnated, yet also permeable, conduit profiles. Again, results suggest that water vapor volume fraction is more important than complex rheology in predicting explosion strength.
DE: 8414 Eruption mechanisms
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting