Séminaire : 18 juin 2015 en salle jean Jung à 14h.
« Dynamics of pyroclastic density currents: Conditions that promote substrate erosion and self-channelization — Mount St Helens, Washington (USA) »
BRAND, Brittany, D., POLLOCK, Nicholas, GASE, Andrew
Boise State University 1910 University Drive, Boise, Idaho 83725-1535
Pyroclastic density currents (PDCs) are one of the most dangerous phenomenon associated with explosive volcanism. However, dominant processes that greatly influence run out distance, such as interaction with topography and the controls and consequences of substrate erosion, remain poorly understood. PDC deposits from the May 18th, 1980 eruption of Mount St Helens (MSH) are well-exposed along the steep flanks (10°-30°) and across the pumice plain (~5°) up to 8 km north of the volcano. Stratified and cross-stratified deposits along the flank suggest that confinement by topography along a steep slope results in high basal shear stress, traction transport and rapid aggradation in highly unsteady currents. Stratification grades downstream into primarily massive deposits along the eastern regions of the shallow-dipping pumice plain, suggesting that unconfined flow with negligible topography results in currents with a more pronounced density gradient and deposition with little to no basal shear stress. In contrast, PDCs that traveled (unconfined) along the western pumice plain interacted with decameter-scale debris avalanche hummocks. The deposits in this region are also primarily massive; however, an increase in the proportion and size of lithic blocks is found (1) downstream of debris avalanche hummocks, suggesting the PDCs flowed over and around debris avalanche hummocks and were energetic enough to locally entrain accidental lithics from the hummocks and transport them tens of meters downstream, and (2) within large channels cut by later PDCs into earlier PDC deposits, suggesting self-channelization of the flows increased the carrying capacity of the subsequent channelized currents, and also likely increased run out distance. The May 18, 1980 PDC deposits thus provide a unique opportunity to assess the influence of slope and topography-scale surface roughness on current dynamics. Our ability to interpret the deposits of PDCs is critical for understanding transport and depositional processes that control PDC dynamics, and provide hypotheses that can be tested experimentally.