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Distal Enhanced Sedimentation From Volcanic Plumes: Insights From the Secondary Mass Maxima in the 1992 Mount Spurr Fallout Deposits

Abstract : 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.
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https://hal.uca.fr/hal-01710767
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Submitted on : Friday, February 16, 2018 - 12:20:03 PM
Last modification on : Wednesday, March 4, 2020 - 12:12:02 PM

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Julia Eychenne, Alison Rust, Katharine Cashman, Wolfram Wobrock. Distal Enhanced Sedimentation From Volcanic Plumes: Insights From the Secondary Mass Maxima in the 1992 Mount Spurr Fallout Deposits. Journal of Geophysical Research : Solid Earth, American Geophysical Union, 2017, 122 (10), pp.7679 - 7697. ⟨10.1002/2017JB014412⟩. ⟨hal-01710767⟩

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