HR: 10:50h
AN: V12B-03    [Abstracts]
TI: Modeling the near-surface expansion of gas slugs in basaltic magma
AU: * James, M R
EM: m.james@lancaster.ac.uk
AF: Dept. Environmental Science, Lancaster University, Lancaster, LA1 4YQ, United Kingdom
AU: Lane, S J
EM: s.lane@lancaster.ac.uk
AF: Dept. Environmental Science, Lancaster University, Lancaster, LA1 4YQ, United Kingdom
AU: Corder, S B
EM: s.corder@lancaster.ac.uk
AF: Dept. Environmental Science, Lancaster University, Lancaster, LA1 4YQ, United Kingdom
AB: The ascent and burst of large gas bubbles (slugs) in low-viscosity magmas generates a variety of geophysical signals that can be used to investigate sub-surface processes. Many of these signals are continuously recorded at several volcanoes (e.g. seismic and infrasonic data at Stromboli and Erebus) and, in order to interpret them in terms of slug parameters, models of the fluid flow associated with slug ascent are required. Due to the difficulties involved with two-phase flow modeling in complex conduit geometries we present two initial slug ascent models, a 1D mathematical model and a 3D computational fluid dynamic (CFD), based on ascent within vertical tubes. The performance of the models is assessed by comparing the results with data from laboratory experiments. These models are anticipated to capture the first order processes and represent the foundation from which more complex models can be derived. In order to simulate near-surface expansions appropriate to volcanic scenarios, the laboratory experiments were carried out under reduced surface pressure conditions. Following the observations made, the 1D model defines a constant rise velocity for the base of the slug and calculates the gas expansion during ascent, allowing the slug nose to accelerate through the overlying fluid. The model reproduces the evolution of rapidly expanding gas slugs observed in the experiments well and, at volcano scales, indicates that at-surface overpressures of several bars can result from the ascent processes alone. The 1D model accounts for the viscous and inertial forces resulting from bulk fluid motions but it does not provide details of the flow dynamics. Consequently, pressure distributions and forces exerted on the conduit cannot be realistically represented unless full 3D CFD simulations are carried out. CFD models are much slower to run, but calculate pressures and shear forces exerted throughout the conduit. At volcano scales, the vertical single forces during slug ascent are ~10⁶ N, two orders of magnitude smaller than those associated with very-long- period seismic events at Stromboli. This supports a previous interpretation of these events in which they are generated by gas slugs flowing through changes in conduit geometry, rather than being the direct result of slug eruption processes.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8430 Volcanic gases
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting