Settling-driven gravitational instabilities associated with volcanic clouds: new insights from experimental investigations

Abstract

Downward propagating instabilities are often observed at the bottom of volcanic plumes and clouds. These instabilities generate fingers that enhance the sedimentation of fine ash. Despite their potential influence on tephra dispersal and deposition, their dynamics is not entirely understood, undermining the accuracy of volcanic ash transport and dispersal models. Here, we present new laboratory experiments that investigate the effects of particle size, composition and concentration on finger generation and dynamics. The experimental setup consists of a Plexiglas tank equipped with a removable plastic sheet that separates two different layers. The lower layer is a solution of water and sugar, initially denser than the upper layer, which consists of water and particles. Particles in the experiments include glass beads as well as andesitic, rhyolitic, and basaltic volcanic ash. During the experiments, we removed the horizontal plastic sheet separating the two fluids. Particles were illuminated with a laser and filmed with a HD camera; the Particle Image Velocimetry (PIV) is used to analyse finger dynamics. Results show that both the number and the downward advance speed of fingers increase with particle concentration in the upper layer, while finger speed increases with particle size but is independent of particle composition. An increase in particle concentration and turbulence is estimated to take place inside the fingers, which could promote aggregation in subaerial fallout events. Finally, finger number, finger speed and particle concentration were observed to decrease with time after the formation of fingers. A similar pattern Manuscript Click here to download Manuscript Paper_Revised.docx Click here to view linked References 2 could occur in volcanic clouds when the mass supply from the eruptive vent is reduced. Observed evolution of the experiments through time also indicates that there must be a threshold of fine ash concentration and mass eruption rate below which fingers do not form; this is also confirmed by field observations.

Statistical analysis of dispersal and deposition patterns of volcanic emissions from Mt. Sakurajima, Japan

“…Proximal to the vent, ash fallout is also heavily affected by volcanological effects such as changes in the eruption dynamics and sedimentation regime (Bonadonna and Costa, 2013), ash aggregation (i.e. the joining of airborne ash particles) within the volcanic plume that affects the total GSD of the ash (Cornell et al, 1983;Bonadonna and Houghton, 2005;Sulpizio et al, 2012) and can both enhance the sedimentation of light ash (Brown et al, 2012) and impede the sedimentation of heavier ash via rafting (Bagheri et al, 2016), as well as downwards propagating instabilities that arise from local differences in ash concentration (Manzella et al, 2015;Scollo et al, 2017). These effects act in tandem with the meteorological fields and topography, with the final ash fallout patterns decided by the complex interplay of all these factors.…”

“…These effects act in tandem with the meteorological fields and topography, with the final ash fallout patterns decided by the complex interplay of all these factors. At greater distances from the vent the impact of volcanological effects is decreased due to lower concentrations of ash (Gilbert and Lane, 1994;Scollo et al, 2017) and deposition patterns can be assumed to be more readily affected by preferential dispersal and the effects of the topography. Topographic heights located along the dispersal path have been implicated in enhancing deposition of ash in their lee side (Watt et al, 2015;Eychenne et al, 2017).…”

“…Other than that mountain waves and points of flow stagnation (i.e. points of flow splitting) could introduce localised maxima in the concentration of airborne ash, and enhancing both aggregation (Gilbert and Lane, 1994) and deposition via gravitational instabilities (Manzella et al, 2015;Scollo et al, 2017). Despite an increase of research in the field in recent years (Watt et al, 2015;Eychenne et al, 2017), the interactions between volcanic ash and local orographic phenomena have currently not been comprehensively studied and analysed, mainly due to the lack of concrete observational data.…”

The contribution of experimental volcanology to the study of the physics of eruptive processes, and related scaling issues: A review

“…The authors derived a simple criterion for the onset of these settling-driven gravitational instabilities based on the Stokes' settling velocity and the density gradient in the environment. Further works on this type of instability were done by Carazzo and Jellinek (2012), Manzella et al (2015), and Scollo et al (2017) who ensured the suitability of the experiments to reproduce natural eruption plumes by matching the Reynolds, Richardson, Grashof, Stokes, and stability numbers. Similar observations were made by Carazzo and Jellinek (2012) in their experiments producing turbulent particle-laden jets and subsequent intrusive gravity currents into a stratified environment.…”

“…For natural eruptions, this settling-driven gravitational instability was expected to occur depending on the grain-size distribution of the particles, particle concentration, and to a lesser extent plume height. Manzella et al (2015) and Scollo et al (2017) refined this work by performing laboratory experiments in which an aqueous suspension of glass beads or volcanic material (dp = 32 -180 m, p = 2550 kg m -3 ) was emplaced above a layer of sugar solution. Instabilities formed at the boundary of the two layers propagated downward.…”