Degassing of open-vent low-silica volcanoes

Palma Lizana, José Luis (2009). Degassing of open-vent low-silica volcanoes. PhD thesis The Open University.



Open-vent activity at volcanoes of low-silica composition, such as Stromboli (Italy), Villarrica (Chile), Mt. Erebus (Antarctica), is characterised by persistent passive gas emission and recurrent mild explosive outgassing. Four styles of bubble bursting activity have been recognised in such volcanoes: seething magma, small short-lived lava fountains, strombolian explosions and gas puffing. At Villarrica, one of the two case study volcanoes, seething magma consists of continual bursts of bubbles up to a few metres in diameter, with varying strength over the entire surface of the lava lake. Small lava fountains, seen as a vigorous extension of seething magma, commonly last 20-120 s and reach 10-40 m above the lava free-surface. Strombolian explosions can last for less than a second in a single bubble burst that erupts mainly bombs, as seen at the lava lake of Mt. Erebus and Villarrica volcanoes, or for more than 30 seconds accompanied by large amounts of ash, as seen at Stromboli and Mt. Etna volcanoes. At Stromboli, the second case study volcano, gas puffing consists of small but repetitive bubble bursts with a generally stable eruption frequency in the range 0.2-1.2 s-1. More vigorous explosive phenomena, such as hundreds-metres high lava fountains or very strong (paroxysmal) explosions, may occur during eruptions or episodes of elevated activity.

Correlations between seismicity and visual observations at Villarrica volcano indicate that the seismic tremor is mostly caused by explosive outgassing. Real-time Seismic Amplitude Measurements (RSAM) and SO2 emission rates (measured by FLYSPEC) show a very good positive linear correlation between periods of background and elevated activity. Higher SO2 emissions appear to be related to higher levels of the lava lake, stronger bubble bursting activity and changes in the morphology and texture of the crater floor. Background (low) levels of activity correspond to a lava lake located >80 m below the crater rim, small and/or blocky morphology of the roof, seismic amplitude (RSAM) lower than 25 units, few volcano-tectonic earthquakes, and daily averages of SO2 emission below 600 Mg d-1.

Convection of magma in the narrow conduits of the plumbing system can explain the sustained degassing with negligible effusion of lava, while supporting the variable outgassing styles at open-vent volcanoes. Theoretical analysis and laboratory experiments carried out with immiscible fluids in vertical and inclined pipes, constrain the convection in terms of a 'flux coefficient' that depends on the viscosity ratio between the liquids, flow regime, angle of inclination of the pipe, and position of the interface between the fluids. Prediction of the flux coefficient is possible within an acceptable range of error. Application of this model to Villarrica and Stromboli volcanoes, along with the analysis of the physical properties of the magma and gas data collated from the literature, allow the estimation of two parameters that constrain the dimensions of the convection: the magma flow rate and equivalent radius. Magma degassing at Villarrica is characterised by the ascent of a relatively degassed magma. Most of the gas exsolves at shallow levels in the system, leading to continuous bubble bursting activity at the lava lake. At Stromboli. magma degassing takes place in an inclined dyke (or dykes). Within this geometry. magma convection adopts a stratified regime of the gas-rich magma overlying the degassed melt, which favours coalescence of bubbles and an efficient convection. Interconnected conduits at the uppermost part of the system constrain the release of the large gas slugs observed during strombolian explosions.

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