Evidence of erosional self-channelization of pyroclastic density currents revealed by ground-penetrating radar imaging at Mount St. Helens, Washington (USA)

Andrew C. Gase, Brittany D. Brand, John H. Bradford
2017 Geophysical Research Letters  
The causes and effects of erosion are among the least understood aspects of pyroclastic density current (PDC) dynamics. Evidence is especially limited for erosional self-channelization, a process whereby PDCs erode a channel that confines the body of the eroding flow or subsequent flows. We use ground-penetrating radar imaging to trace a large PDC scour and fill from outcrop to its point of inception and discover a second, larger PDC scour and fill. The scours are among the largest PDC
more » ... features on record, at >200 m wide and at least 500 m long; estimated eroded volumes are on the order of 10 6 m 3 . The scours are morphologically similar to incipient channels carved by turbidity currents. Erosion may be promoted by a moderate slope (5-15 ∘ ), substrate pore pressure retention, and pulses of increased flow energy. These findings are the first direct evidence of erosional self-channelization by PDCs, a phenomenon that may increase flow velocity and runout distance through confinement and substrate erosion. Citation: Gase, A. C., B. D. Brand, and J. H. Bradford (2017), Evidence of erosional self-channelization of pyroclastic density currents revealed by ground-penetrating radar imaging at Mount St. Helens, Washington (USA), Geophys. Data were acquired in common offset mode with a Sensors and Software PulseEKKO Pro GPR. Two sleds with 50 MHz antennas at a fixed offset of 2 m were dragged over the deposit surface. A wheel odometer controlled the 0.5 m trace interval. Simultaneously, we recorded real-time kinematic GPS data for topographic correction. Generally, we could image within Units II-V (∼20 m deep). The vertical resolution of 50 MHz electromagnetic waves (i.e., 0.46 m at 0.09 m ns −1 ) is suitable for interpreting flow unit contacts and broad stratigraphic trends. Processing included time zero correction and band-pass frequency filtering (12-25 to 400-800 MHz). Automatic gain control (AGC) was applied to Line 5, and true amplitude recovery was performed on all other GASE ET AL. PYROCLASTIC DENSITY CURRENT EROSION
doi:10.1002/2016gl072178 fatcat:fgxmcdsccnaatpks2gvhfcp3aq