Sedimentation from strong volcanic plumes

Abstract

[1] We develop descriptions of the key processes influencing tephra dispersal from strong volcanic plumes. These are characterized by the development of a subvertical eruption column in the atmosphere that forms a spreading current at a level of neutral buoyancy. We describe the propagation of the spreading current due to both gravity and wind advection using scaling arguments and a simplified geometry to model particle sedimentation in a windy as well as a wind-free environment. New parameterizations are used to describe the wind field below the spreading current, particle aggregation, and particle density variations. We conducted a broad study to investigate the effects of these processes and made comparisons with field observations. The greatest variations resulted from wind advection below the spreading current, which shifts the plumecorner mass accumulation and the position of transitions in fallout regimes downwind. Particle aggregation strongly depends on the initial grain-size distribution and significantly affects deposit thinning, depending on the aggregate size and density, and on the relative amount of aggregating particles. Small aggregates can reproduce secondary maxima of mass accumulation when sedimenting in a wind field. Variation of particle density also affects the resulting thinning trend. The model provides acceptable reproduction of observations of the propagation of the spreading current (tested with the Plinian phase of the 18 May 1980 eruption of Mount St. Helens) and of observed thinning of tephra fallout deposits (tested with deposits from Mount St.

A review of volcanic ash aggregation

“…Fine ash particle aggregation results in greater fallout of fine ash close to source and has the effect of reducing distal (100s-1000s km) atmospheric ash concentrations. Even though ash-poor deposits can still be described without accounting for particle aggregation (Bonadonna and Phillips, 2003), models that do not account for aggregation have proven unable to reproduce tephra deposits rich in fine ash (Bonadonna et al, 2002a;Carey and Sigurdsson, 1982;Macedonio et al, 1988;Folch et al, 2010). Enhanced sedimentation of volcanic ash is important both in proximal areas and in medial-distal areas (e.g.…”

“…Finally, Bonadonna et al (2002a) and Bonadonna and Phillips (2003) have used a combination of the Cornell et al (1983) model, the parameterization of Gilbert and Lane (1994), and field observations from the Montserrat eruption (Vulcanian explosions and dome collapses of August-October, 1997; Bonadonna et al, 2002b) to compile the hazard assessment of Montserrat (West Indies) and to investigate the effects of aggregation on the thinning of tephra deposits.…”

“…Observations of recent eruptions indicate that the type of aggregate falling from ash clouds changes with distance from the volcano (Rosenbaum and Waitt, 1981;Hobbs et al, 1981;Sorem, 1982): proximal aggregates are larger and can contain water on deposition (liquid or frozen); distal aggregates are much smaller, fragile, and often reach the surface without direct evidence for the involvement of water in the process (we loosely use the term proximal for regions within the plume corner, i.e. <15km depending on plume height; Bonadonna and Phillips, 2003, and distal for regions beyond the plume corner). We support improved observation and documentation of ash aggregates both during eruptions and within deposits in order to advance knowledge of this important topic.…”

An example of enhanced tephra deposition driven by topographically induced atmospheric turbulence

“…Thickness decay rates in proximal to medial portions of pyroclastic fall deposits (i.e. those dominated by deposition of particles with a Reynolds number >500 (fine lapilli to coarse ash); Bonadonna et al 1998;Bonadonna and Phillips 2003) are close to exponential, even when wind-advected (Pyle 1989;Sparks et al 1992Sparks et al , 1997 departures from a single exponential trend occur in the most proximal and in more distal regions (Bonadonna et al 1998)). Given the axial trend of our measurement sites, thickness measurements should therefore define a straight line on a log (thickness) versus distance plot.…”

“…If aggregation is generally more pervasive across a deposit, or if other processes such as convective instabilities enhance fine-particle deposition, then the result may not be a well-developed secondary thickness maximum but a stretching or distortion of the isopachs, altering the rate at which the deposit thins with distance (whether this rate is described by multi-segment exponential, power-law or Weibull thickness decay patterns; cf. Pyle 1989;Bonadonna et al 1998;Bonadonna and Phillips 2003;Bonadonna and Costa 2012). The grain-size distribution of any bulk sample from a fall deposit affected by aggregation is expected to be a composite of multiple grain-size populations (spanning multiple phi intervals, and potentially forming multimodal distributions in relatively proximal deposits) and enriched in fine grains (i.e.…”