Correlations between SO2 flux, seismicity, and outgassing activity at the open vent of Villarrica volcano, Chile

The characteristics of the open vent activity of Villarrica volcano, Chile, were studied in detail by integrating visual observations of the lava lake, analysis of the seismic tremor, and measurements of SO2 flux. The outgassing activity comprises a persistent gas plume emission from the bottom of the crater as well as frequent explosive events. Three main styles of bubble bursting were identified at the surface of the active lava lake: seething magma, small short-lived lava fountains, and Strombolian explosions. Seething magma consists of continual burst of relatively small bubbles (a few meters 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 have durations of 20–120 s and reach 10–40 m high above the lava lake. Correlations between seismicity and visual observations indicate that the seismic tremor is mostly caused by the explosive outgassing activity. Furthermore, for different periods between 2000 and 2006, during which the activity remained comparable, the real-time seismic amplitude measurement system (RSAM) and SO2 emission rates show a very good correlation. Higher SO2 emissions appeared 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 emissions lower than 600 Mg/d

Proceedings Of The 18th Annual Meeting Of The Asia Oceania Geosciences Society (Aogs 2021)

The 18th Annual Meeting of the Asia Oceania Geosciences Society (AOGS 2021) was held from 1st to 6th August 2021. This proceedings volume includes selected extended abstracts from a challenging array of presentations at this conference. The AOGS Annual Meeting is a leading venue for professional interaction among researchers and practitioners, covering diverse disciplines of geosciences

Degassing of open-vent low-silica volcanoes

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