Distal Enhanced Sedimentation From Volcanic Plumes: Insights From the Secondary Mass Maxima in the 1992 Mount Spurr Fallout Deposits
Résumé
Some tephra fallout deposits show an increase of mass and thickness at distances from the
source >100 km (areas of secondary mass maximum, ASMM) which demonstrates distal enhanced
sedimentation from volcanic plumes. We explore development of the ASMMs during the 1992 August and
September Mount Spurr eruptions, USA, by combining fi eld data on the spatial distribution of mass and grain
size with (1) simulations of individual particle settling through a homogeneous and horizontally stratified
atmosphere and (2) mesoscale models of the three-dimensional wind field that include the effect of the
underlying topography. The crosswind and downwind variations of deposit characteristics indicate that the
increase of sedimentation at the ASMMs is not formed solely because of preferential settling of small ash
particles (<125 μm), as commonly assumed in aggregation models. Instead, ASMM grain sizes correspond to
the fine modes of the bimodal total grain size distributions. There also appears to be a link between the
ASMM and the topography: the mass local minima occur across the windward flank of 2 km high mountain
ranges, while the ASMMs spread on the leeward flank. Mesoscale models of the three-dimensional wind field
show vertical oscillations in the wind over mountainous regions which may enhance mechanisms of en
masse sedimentation (aggregation, hydrometeor formation, and particle boundary layers), as well as strong
spatial variations of the horizontal wind field in the lower troposphere. Our study demonstrates the
importance of using grain size, as well as mass, data to constrain the complex processes responsible for
particle sedimentation from volcanic plumes.
Domaines
VolcanologieOrigine | Fichiers éditeurs autorisés sur une archive ouverte |
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