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

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... 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