Ash aggregation in explosive volcanic eruptions

Abstract

[1] Eruption dynamics are sensitive to ash aggregation, and aggregates are commonly found in eruptive deposits. While ash dispersal and associated hazards are sensitive to aggregation, few experiments have been conducted on this phenomena using natural materials across the diverse range of conditions expected in volcanic flows. We have isolated two regimes, wet and dry, in which aggregation occurs due to two different forces, electrostatic and hydrodynamic. Using a closed chamber to create a controlled atmosphere, we found that relative humidity, residence time, and kinetic energy are the three variables necessary to define wet and dry flow regimes. A series of processbased equations defining the behavior of ash particles have been developed. We propose an aggregation model that can be used for ash dispersal forecasts across a range of conditions in an eruptive plume.

Atmospheric processes affecting the separation of volcanic ash and SO<sub>2</sub> in volcanic eruptions: inferences from the May 2011 Grímsvötn eruption

“…Improvements in dealing with the complex nature of the interaction of the atmosphere with the erupting column are needed, including better parameterizations of the aggregation process (e.g. Textor et al, 2006;Costa et al, 2010;Telling et al, 2013b;Folch et al, 2016), improved understanding of bentover plumes (Woodhouse et al, 2013), and improved modelling of the effects of partial or total column collapse. While perhaps less common, except in large eruptions (VEI > 4), column collapse can lead to the generation of pyroclastic density currents that can act as secondary sources for new column generation, so-called co-ignimbrite plumes (Self and Rampino, 1981).…”

“…These observations suggest the possibility that the column may have undergone partial collapse sometime during the evening of 21 May, causing an outflow of ash, not dissimilar to the outflow often observed from a collapsing thunderstorm. As large ash aggregates fall through the column, enhanced by the presence of copious amounts of water, for example see Telling et al (2013b) for a discussion of this process, ice would have formed on the ash, increasing the size and fall speed and effectively removing particles from the column. These ice-coated ash aggregates, sometimes termed volcanic hail would have fallen out of the cloud very rapidly.…”

“…Indeed, the physical characteristics (e.g. shape, size, porosity) and chemical composition of volcanic ash particles are likely to greatly alter the aggregation efficiency (James et al, 2002;Durant et al, 2009;Brown et al, 2012;Telling et al, 2013a) and these properties vary substantially for different eruptions. Furthermore, in describing the evolution of aggregating particles, knowledge of the initial particle size distribution is required.…”

“…However, the lower part of the plume typically has higher velocities, leading to greater kinetic energy of particle collisions, which reduces the efficiency of aggregation . The presence of liquid water substantially increases the aggregation efficiency (Telling et al, 2013a) and, because condensation and freezing typically occur at high altitudes in the plume, the lower velocity of the plume reduces the kinetic energy of collisions. We therefore expect that aggregation proceeds rapidly in wet conditions, resulting in a pronounced increase in the size of particle clusters, while electrostatically dominated aggregation in dry regions results in more gradual growth of clusters.…”

Analyzing Glacier Surface Motion Using LiDAR Data

“…Applying PIV, an image processing technique [15][16][17][18], to LiDAR data provides an opportunity to measure velocity across entire glaciers rapidly and the ability to measure high resolution nuances in the flow field that may be missed using other techniques. When compared to other methods such as feature extraction, and other image correlation techniques, PIV offers an approach that is sensitive to small scale, locally variable changes.…”