LiDAR derived morphology of the 1993 Lascar pyroclastic flow deposits, and implication for flow dynamics and rheology

Dynamics of pyroclastic density currents: Conditions that promote substrate erosion and self-channelization — Mount St Helens, Washington (USA)

“…The pumice-rich lobes and levees are characteristic surface features of pumice flows (e.g., Calder et al, 2000;Pittari et al, 2005;Jessop et al, 2012). We interpret that the pumice segregated and was transported to the outside of the current, where the larger pumice clasts became concentrated along the deposit margins or flow fronts (e.g., Sparks, 1976;Calder et al, 2000;Pittari et al, 2005).…”

High‐resolution bathymetry reveals contrasting landslide activity shaping the walls of the Mid‐Atlantic Ridge axial valley

“…The one on the left (deposit 1 in Figure f) displays an asymmetrical pattern of erosion gullies and a sinuous channel that could be a post‐deposition erosion feature. However, it also resembles the sinuous channels and complex morphologies observed for some natural debris and pyroclastic flows inland [ Calder et al ., ; Lube et al ., ; Mangold et al ., ; Jessop et al ., ]. These natural complexities deserve a more systematic study that would require in‐situ observations that are not presently available.…”

“…For small flux and/or slopes, intermittent granular flows are observed. At higher flux and/or slopes, the flow reaches a steady state and the deposit exhibits a levee/channel morphology as observed for aerial debris flows or pyroclastic flows [e.g., Lube et al ., ; Mangold et al ., ; Jessop et al ., ]. The variation of the thickness of the central channel (between the levees) as a function of the slope angle h c ( θ ) is shown to be very similar to the variation with slope angle of the maximum thickness of the deposit of steady uniform flows over inclined planes, usually called h stop ( θ ) [ Pouliquen , ; Figure in Félix and Thomas , ; Mangeney et al ., ].…”

“…As in Jessop et al . [], we choose here the following empirical relationship for h c ( θ ): (Equation 1)where d p is the particle diameter, a is the dimensionless constant and θ 1 the vertical asymptote of the h c ( θ ) curve, which is very close to the repose angle of the material involved [ Börzsönyi et al ., ; Mangeney et al ., , p 6]. In the experiments, the flow thickness h > h c strongly depends on the initial flux of material, while h c seems to be almost independent of the flow [ Félix and Thomas , ].…”

“…The ratio h c / h levee ( θ ) for the possible levee‐channel deposit ranges between 0.4 and 1 (Figure d), similar to the range measured by Jessop et al . [] for pyroclastic flows and by Félix and Thomas [] for laboratory experiments. The levee slope in the direction perpendicular to the flow is observed to be a few degrees smaller than θ 1 in the laboratory experiments of Félix and Thomas [].…”