Reanalysis of the Pyroclastic Fall Deposit from the 18 May 1980 Eruption of Mount Saint Helens, USA
Abstract
A new analysis of the distal fall deposit from the 18 May 1980 Eruption of Mount Saint Helens (MSH80) is presented. The removal of fine distal volcanic ash from the atmosphere is complex: From the data presented here, it is clear that meteorological processes and atmospheric dynamics influence particle settling. Original samples were analysed by a Malvern Instruments Mastersizer 2000 to determine particle size through the range 0.1-2000 μm. MSH80 fallout consists of material from an initial directed blast and ~9 hour Plinian phase, which was later dominated by coignimbrite input. A striking feature of the distal ashfall deposit is a secondary maximum in mass deposited located ~300 km from the volcano, previously attributed to ash aggregation and enhanced particle settling. Median particle size averaged by transect decreases with distance from 2.91 Φ at ~150 km to 5.35 Φ at ~300 km, then increases again before decreasing to a minimum of 5.22 Φ at ~650 km. Investigation of size fractions averaged by transect show 0-3 Φ proportions decrease with distance and >4 Φ proportions increase with distance. Contribution of 5-7 Φ size fractions peak at the secondary maximum, which may suggest that ash aggregation selectively binds particles of this size. Proportions of particles >9 Φ (<1.95 μm diameter ~ PM2.5; the most pertinent phi size fraction for health hazard assessment) remain fairly constant over the deposit at ~4-5 wt%. Samples beyond ~300 km had remarkably similar grainsize characteristics. All samples are poorly sorted with transect average σ values ranging from 2.30-1.64 Φ; samples in the secondary maximum region have the highest measured sorting coefficients. By transect average, all samples are positively skewed (0.5-0.79 Φ) and strongly leptokurtic (2.75-3.21 Φ). Some bimodal samples are present at distances less than ~300 km. Enhancement of coarse fractions between 0-3 Φ was observed to the south of these transects and may represent the footprint of the Plinian phase, based on cloud trajectory analysis. In addition, a selection of the samples were analysed using a Malvern Instruments PharmaVision 830 to characterize morphological changes as a function of distance from the volcano. This work provides a unique opportunity to constrain some aspects of the atmospheric dispersion and sedimentation process through high resolution particle size analysis of the resulting ashfall deposit. In addition, an extensive database is now available for volcanic ash transport and dispersion model validation, calibration and development.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V33B0656D
- Keywords:
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- 0370 Volcanic effects (8409);
- 8404 Volcanoclastic deposits;
- 8409 Atmospheric effects (0370);
- 8428 Explosive volcanism;
- 8486 Field relationships (1090;
- 3690)