Modelling the rapid near-surface expansion of gas slugs in low-viscosity magmas
Published:January 01, 2008
M. R. James, S. J. Lane, S. B. Corder, 2008. "Modelling the rapid near-surface expansion of gas slugs in low-viscosity magmas", Fluid Motions in Volcanic Conduits: A Source of Seismic and Acoustic Signals, S. J. Lane, J. S. Gilbert
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The ascent of large gas bubbles (slugs) in vertical cylindrical conduits and low-viscosity magmas is simulated using 1D mathematical and 3D computational fluid dynamic (CFD) models. Following laboratory evidence, the 1D model defines a constant rise velocity for the slug base and allows gas expansion to accelerate the slug nose through the overlying fluid during ascent. The evolution of rapidly expanding gas slugs observed in laboratory experiments is reproduced well and, at volcano scales, predicts at-surface overpressures of several atmospheres without requiring any initial overpressure at depth. The near-surface dynamics increase slug nose velocities through the overlying magma by a factor of c. 2.5 and the gas expansion results in pre-burst magma surface velocities of c. 35 m s−1. To examine pressure distributions and the forces exerted on a conduit, 3D CFD simulations were carried out. At volcano scales, the vertical single forces during final slug ascent to the surface are c. 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.
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Fluid Motions in Volcanic Conduits: A Source of Seismic and Acoustic Signals
Volcanoes become active when fluids are in motion, and erupt when these fluids escape into the atmosphere. Volcanic fluids are a mixture of solid, liquid and gas. These mixtures result in a complex range of flow behaviour, especially during interaction with conduit geometry. These processes are not directly observable and must be inferred from interpretations of field observation and measurement. One of the outcomes of this complexity is the generation of pressure and force transients as high-density phases accelerate and decelerate during unsteady flow. These transients are one means of flexing the conduit wall, a process that manifests itself as ground motion and is detectable as volcano seismic signals. On eruption, volcanic fluids interact with the atmosphere and generate acoustic and thermal signals. In this Special Publication we present a series of papers based on field, numerical and experimental approaches that seek to establish links between geophysical signals and fluid motion in volcanic conduits.