Quantitative shape measurements of distal volcanic ash

Abstract

[1] Large-scale volcanic eruptions produce fine ash (<200 mm) which has a long atmospheric residence time (1 hour or more) and can be transported great distances from the volcanic source, thus, becoming a hazard to aircraft and public health. Ash particles have irregular shapes, so data on particle shape, size, and terminal velocities are needed to understand how the irregular-shaped particles affect transport processes and radiative transfer measurements. In this study, a methodology was developed to characterize particle shapes, sizes, and terminal velocities for three ash samples of different compositions. The shape and size of 2500 particles from (1) distal fallout ($100 km) of the 14 October 1974 Fuego eruption (basaltic), (2) the secondary maxima ($250 km) of the 18 August 1992 Spurr eruption (andesitic), and (3) the Miocene Ash Hollow member, Nebraska (rhyolitic) were measured using image analysis techniques. Samples were sorted into 10 to 19 terminal velocity groups (0.6-59.0 cm/s) using an air elutriation device. Grain-size distributions for the samples were measured using laser diffraction. Aspect ratio, feret diameter, and perimeter measurements were found to be the most useful descriptors of how particle shape affects terminal velocity. These measurement values show particle shape differs greatly from a sphere (commonly used in models and algorithms). The diameters of ash particles were 10-120% larger than ideal spheres at the same terminal velocity, indicating that irregular particle shape greatly increases drag. Gas-adsorption derived surface areas are 1 to 2 orders of magnitude higher than calculated surface areas based on measured dimensions and simple geometry, indicating that particle shapes are highly irregular. Correction factors for surface area were derived from the ash sample measurements so that surface areas calculated by assuming spherical particle shapes can be corrected to reflect more realistic values.

Monitoring presence and streaming patterns of Icelandic volcanic ash during its arrival to Slovenia

“…In previous studies of volcanic ash, in-situ devices and remote sensing techniques were demonstrated to be the most powerful tools for quantitative measurements of its presence (Pieri et al, 2002;Sassen et al, 2007;Delene et al, 1996). Furthermore, in the case of in-situ measurements, the morphology and chemical composition of the volcanic ash can simultaneously be analyzed (Riley et al, 2003;Taylor and Lichte, 1980).…”

High-resolution 3D analyses of the shape and internal constituents of small volcanic ash particles: The contribution of SEM micro-computed tomography (SEM micro-CT)

“…Eiríksson et al, 1994;Dellino and La Volpe, 1996;Carey, 2002, 2007;Riley et al, 2003;Shea et al, 2010). These methods, however, provide only incomplete shape descriptors, due to the inherent limitations of 2D techniques to fully retrieve 3D features (Baker et al, 2012).…”

“…In terms of aerodynamics, shape has a direct impact on the drag forces acting on rising and falling particles (Stringham et al, 1969;Bursik, 1989;Ganser, 1993;Mitchell, 1996;Chhabra et al, 1999;Mele et al, 2011;Dellino et al, 2012), which in turn influence the settling velocity (Wilson and Huang, 1979;Coltelli et al, 2008;Alfano et al, 2011), the atmospheric residence time (Riley et al, 2003), and the transport distance and dispersal of tephra (Sparks et al, 1997;Scollo et al, 2008). The aerial behavior of ash is also indirectly influenced by the surface-to-volume ratio, which affects the propensity of particles to form aggregates, both in dry and wet environments (e.g.…”

Mechanism of formation of volcanic bombs: insights from a pilot study of anisotropy of magnetic susceptibility and preliminary assessment of analytical models

“…From a microscopic perspective, the relation between particle morphology and eruptive and transport mechanisms has been abundantly explored (e.g., Dellino and Liotino 2002;Ersoy et al 2006Ersoy et al , 2008Heiken and Wohletz 1985;Leibrandt and Le Pennec 2015;Liu et al 2015;Rausch et al 2015;Riley et al 2003;Sheridan and Kortemeier 1987;Sheridan and Marshall 1983;Vonlanthen et al 2015). Heiken (1972) considered that the shape of particles in the ash size range is mostly dependent on the shape of magmatic gas bubbles at the moment of fragmentation: flat and platy shards arise from broken vesicle walls, and droplet shapes are produced from low viscosity magma where the effect of surface tension is likely to become more important.…”