HR: 0800h
AN: V31E-0702    [Abstracts]
TI: Turbulent dynamics and pyroclastic flow generation during the Mount St. Helens May 18th, 1980 eruption
AU: * Andrews, B J
EM: andrewsb@mail.utexas.edu
AF: Dept. of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, 1 University Station C1100, Austin, TX 78712-0254, United States
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: Dept. of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, 1 University Station C1100, Austin, TX 78712-0254, United States
AB: Eruption behavior at Mount St. Helens changed greatly over the course of 18 May: a buoyant, Plinian column dominated the morning phases of eruption, whereas during the early afternoon, the column partially collapsed, such that a Plinian column and non-buoyant pyroclastic flows were simultaneously erupted. Changes in the plume's turbulent flow dynamics, pyroclastic fall and flow deposit grain size distributions (GSDs), and character of the plume reflect this evolution in eruption dynamics. Optical flow velocimetry of video of the plume immediately above the crater rim indicates the sizes of the largest structures in the plume decreased from a range of ~300 to >1000 m during the morning to 150-200 m during the afternoon. These measurements agree with visual inspection of photographs showing eddy size decreasing from a range of 200 to >500 m (average 300 m) in the morning to a range of 150-350 m (average 250) in the afternoon. During this same time interval, the rotation speed of eddies (as measured by the rms values of the 2D velocity field) increased by a factor of 1.6. Furthermore, the appearance of the column changed through the course of the eruption. In the morning, the column was characterized by discontinuous, large eddies frequently depositing "curtains" of pyroclasts, and an indentation was present on the column's southern margin. In contrast, the column margins were completely covered by smaller eddies and no curtains of sedimenting pyroclasts during the afternoon. Given that during the morning most mass erupted as buoyant plumes, we have estimated total eruptive GSDs from fall deposit GSDs using known mass fluxes and plume sedimentation models. Accounting for changes in buoyant mass flux and depositional axis, the afternoon Plinian fall deposits are 0.5 to 1 phi units coarser than models predict if the bulk, buoyant GSD remained the same. Although the majority of pyroclastic flows were emplaced to the north of the crater during the afternoon, smaller pyroclastic flows were emplaced over the rim of the crater. The GSDs of these "over-the-rim" flow deposits are significantly finer grained than those to the north, despite their shorter run out. Increased total mass flux caused the change from dominantly buoyant to dominantly non-buoyant behavior observed between the morning and afternoon of 18 May. This increase was likely the product of increases in at least two components of mass flux: column density and mean velocity. Shorter length scales of turbulence observed in the afternoon reflect shorter Kelvin-Helmholtz type instability wavelengths resulting from eruption of a denser jet into the atmosphere. Increases in jet velocity are indicated by the increase in turbulence intensity. Coarsening in bulk, buoyant plume GSDs and different groups of flow GSDs suggest either less efficient fragmentation during the afternoon, thus a comparatively coarse-skewed GSD erupted from the vent, or size segregation of pyroclasts between buoyant and non-buoyant parts of the column.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting