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

Journal of Geophysical Research F: Earth Surface

By:
, , , ,
DOI: 10.1029/2008JB005756

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Abstract

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 ??, 4.2 ??, 5.9 ??, and 8.3 ??. Settling rates of the coarsest subpopulation closely matched predicted single-particle terminal fall velocities. Sedimentation of particles <100 ??m was greatly enhanced, predominantly through aggregation of a particle subpopulation with modal diameter 5.9 ?? 0.2 ?? (19 ?? 3 ??m). 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. Copyright 2009 by the American Geophysical Union.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Hydrometeor-enhanced tephra sedimentation: Constraints from the 18 May 1980 eruption of Mount St. Helens
Series title:
Journal of Geophysical Research F: Earth Surface
DOI:
10.1029/2008JB005756
Volume
114
Issue:
3
Year Published:
2009
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Journal of Geophysical Research F: Earth Surface