Hydrometeor‐enhanced tephra sedimentation: Constraints from the 18 May 1980 eruption of Mount St. Helens

Abstract

[1] Uncertainty remains on the origin of distal mass deposition maxima observed in many recent tephra fall deposits. In this study the link between ash aggregation and the formation of distal mass deposition maxima is investigated through reanalysis of tephra fallout from the Mount St. Helens 18 May 1980 (MSH80) eruption. In addition, we collate all the data needed to model distal ash sedimentation from the MSH80 eruption cloud. Four particle size subpopulations were present in distal fallout with modes at 2.2 F, 4.2 F, 5.9 F, and 8.3 F. Settling rates of the coarsest subpopulation closely matched predicted single-particle terminal fall velocities. Sedimentation of particles <100 mm was greatly enhanced, predominantly through aggregation of a particle subpopulation with modal diameter 5.9 ± 0.2 F (19 ± 3 mm). Mammatus on the MSH80 cloud provided a mechanism to transport very fine ash particles, with predicted atmospheric lifetimes of days to weeks, from the upper troposphere to the surface in a matter of hours. In this mechanism, ash particles initiate ice hydrometeor formation high in the troposphere. Subsequently, the volcanic cloud rapidly subsides as mammatus develop from increased particle loading and cloud base sublimation. Rapid fallout occurs as the cloud passes through the melting level in a process analogous to snowflake aggregation. Aggregates sediment en masse and form the distal mass deposition maxima observed in many recent volcanic ash fall deposits. This work provides a data resource that will facilitate tephra sedimentation modeling and allow model intercomparisons.

CHIMERE-2017: from urban to hemispheric chemistry-transport modeling

“…The granulometry of the ashes are taken as 80 % in a coarse mode, with a lognormal distribution centered at 30 µm and 20 % in a finer mode with a lognormal distribution centered at 4 µm, consistent with the results of Durant et al (2009).…”

Brief Communication &quot;Rain effect on the load of tephra deposits&quot;

“…(2), we obtain ρ d = 800 kg m −3 , in agreement with the bulk density of other pumice deposits of past eruptions in the Neapolitan area (Cioni et al, 2003;Pfeiffer and Costa, 2004;Macedonio et al, 2008;Costa et al, 2009). In order to estimate a typical range of values for φ tot , we can consider that bulk densities of proximal and medial deposits of pyroclastic material range from 600 to 1500 kg m −3 (Durant et al, 2009;Pfeiffer and Costa, 2004). For example, tephra deposit densities are of about 900 kg m −3 for the 472 AD (Pollena) Vesuvius eruption (Cioni et al, 2003;Macedonio et al, 2008), and of about 700 kg m −3 for the Agnano-Monte Spina (AMS) eruption in the Campi Flegrei (Pfeiffer and Costa, 2004;Costa et al, 2009).…”

Atmospheric processes affecting the separation of volcanic ash and SO&lt;sub&gt;2&lt;/sub&gt; in volcanic eruptions: inferences from the May 2011 Grímsvötn eruption

“…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.…”