HR: 0800h
AN: V31D-0652    [Abstracts]
TI: Experimental Investigation of Volcano-seismic Forces Generated During Gas-slug Expansion in Strombolian Eruptions
AU: Lane, S J
EM: s.lane@lancaster.ac.uk
AF: Lancaster University, Department of Environmental Science, Lancaster, LA1 4YQ United Kingdom
AU: * James, M R
EM: m.james@lancaster.ac.uk
AF: Lancaster University, Department of Environmental Science, Lancaster, LA1 4YQ United Kingdom
AU: Corder, S B
EM: s.corder@lancaster.ac.uk
AF: Lancaster University, Department of Environmental Science, Lancaster, LA1 4YQ United Kingdom
AB: Strombolian activity is attributed to the bursting of a large bubble, or slug, of water vapour at the top of a magma-filled conduit. The bursting may be accompanied by an audible detonation, followed by a jet-like escape of gas and pyroclasts, indicating that the bursting pressure was significantly greater than the local atmospheric pressure. This super-static pressure is generated by the inertia of the portion of liquid column above the gas slug, as the slug expands during rise from a high-pressure source region to lower pressure at the surface [Vergniolle, 1998]. The upward acceleration of this liquid piston has been identified as a candidate for the generation of a downward single force measured during Strombolian activity [Chouet et al., 2003]. We present the results of laboratory experiments designed to test this hypothesis. In our experiments, similarity was established on the basis of steady-state flow, with slug ascent controlled predominantly by the inertia of the liquid, but with a significant contribution from liquid viscosity. By exposing the liquid surface to reduced pressure, the ratio of the slug-generation to slug-burst pressures was varied from 1.1 to 10 000, spanning the expected volcanic values of 10s to 100s. Contrary to expectation, the experimental results indicate that the expansion of the gas slug and acceleration of the liquid piston do not result in a net downward force on the apparatus. Indeed, we observe a small upward force on the apparatus as the slug nose approaches the surface. We attribute this to changes in the liquid flow-field in front of the slug that accompany its rapid expansion in the near-surface region. Accepting that our experiments scale to the volcanic case, then expansion of a decompressing gas slug in a smooth-walled conduit cannot explain the single forces recorded during strombolian activity. This suggests that another mechanism must be responsible, such as the rapid liquid accelerations associated with slug flow through a conduit discontinuity of significant size [James et al., sub judice]. Chouet, B., P. Dawson, T. Ohminato, M. Martini, G. Saccorotti, F. Giudicepietro, G. De Luca, G. Milana, and R. Scarpa (2003), Source mechanisms of explosions at Stromboli Volcano, Italy, determined from moment-tensor inversions of very-long-period data, J. Geophys. Res., 108, 2019, doi:10.1029/2002JB001919. James MR, Lane SJ, Chouet BA (submitted). Gas slug ascent through changes in conduit diameter: laboratory insights into seismic source processes at basaltic volcanoes. J. Geophys. Res. - Solid Earth. Vergniolle, S. (1998), Modelling two-phase flow in a volcano, Proceedings of 13th Australasian Fluid Mechanics Conference, Melbourne, Aristoc. Offset, Monash Univ., Melbourne, Australia, 647-650.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8419 Volcano monitoring (7280)
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005