Lane, Stephen and Corder, Steven and James, Michael (2010) Source mechanism regimes for the acoustic signals generated during the expansion of rising and bursting gas slugs in low-viscosity magmas. In: AGU Fall Meeting 2010, 2010-12-13 - 2010-12-17.
Abstract
V53C-2281 Strombolian eruptive activity produces gas-rich, magma-poor ejecta suggesting the separation and concentration of volcanic gases within the plumbing system. These gases may then rise as relatively large bubble rafts or individual 'slug' bubbles, and cause detectable seismic activity on interaction with conduit geometry. Rising within a magma column, a gas bubble must expand appreciably in order to maintain magma-static pressure, for instance volume would increase by a factor of c. 200 for a 1 km rise to the magma-atmosphere interface. For a canonical gas slug this expansion is one-dimensional and increases in rate non-linearly on approach to the surface. Small gas slugs can expand rapidly enough to maintain approximate magma-static pressure, but large gas slugs become dynamically overpressured. In laboratory experiments, such unsteady flows of gas and liquid generate pressure changes measured in the ambient atmosphere above the upper liquid surface, driven by both slug expansion and burst. We present experimental evidence of a range of burst processes that depend on the degree of gas overpressure in the slug. These processes range from the quiescent formation of a relatively long-lived liquid film that bursts some time after the gas slug has reached the liquid surface, through complex transitional behaviour where the meniscus detaches from the tube walls to form a bubble, to wholesale meniscus disruption when overpressure becomes appreciable. The style of pressure signals measured in the atmosphere changes with the degree of overpressure, being predominantly driven by the two slug-depressurisation processes, namely the rise of the liquid-atmosphere interface accompanying expansion and burst of the overpressured slug. When slug overpressure is significant, then expansion and burst merge into one continuous process. We compare experimentally simulated results to existing acoustic measurements from a number of volcanic centres and discuss the insights this gives in understanding flow dynamics during strombolian eruptions. We also explore where laboratory, numerical and field approaches could be more closely related to provide increasingly accurate descriptions of the eruption process.