Impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 May 1980 Mount St. Helens fallout deposit

• Published in: Journal of geophysical research. Solid earth 2015-09, Vol.120 (9), p.6018-6038
• Main Authors: Eychenne, Julia, Cashman, Katharine, Rust, Alison, Durant, Adam
• Format: Article
• Language: English
• Subjects: Agglomeration; Aggregation; Area; Ashes; Asymmetry; Blast; Cloud formation; copyroclastic density current plumes; Crosswinds; Deconvolution; Deposition; Deposits; Destruction; Direction; Dispersion; Displacement; Distance; Earth Sciences; Enrichment; Entrainment; Fallout; Fields; fine ash; Geophysics; Grain size; Grain size distribution; Gravitation; Gravity; Height; Hydrometeors; Magma; Mass; Mount St. Helens; Particle size; Particle size distribution; Plumes; Satellite imagery; Satellites; Sciences of the Universe; Sedimentation; Sedimentation & deposition; Size distribution; Spatial distribution; Stratosphere; Strong winds; Transport; Velocity; Vents; Volcanoes; Volcanology; Wind; Wind fields; Winds
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2015JB012116

The 18 May 1980 eruption of Mount St. Helens started with a lateral blast that fed a pyroclastic surge, which then uplifted to form a co‐blast plume. Thirty minutes later, Plinian activity started at the vent and fluctuated in intensity for ~9 h. The resulting fallout deposit, documented to > 600 km from vent, presents some striking features: (1) displacement of the overall deposit to the north of the vent, (2) a secondary thickness and mass maximum at ~300 km from vent, (3) a total grain size distribution dominated by fine ash (62 wt % of the deposit