Abstract
AbstractThe 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 erosional features on record, at >200 m wide and at least 500 m long; estimated eroded volumes are on the order of 106 m3. 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.
Highlights
Pyroclastic density current (PDC) deposits from flows of moderate volume (∼106 m3) are often mantled by intertwined pumice lobes with lateral levees and depressed central channels [e.g., Wilson and Head, 1981; Calder et al, 2000]
Evidence is especially limited for erosional self-channelization, a process whereby pyroclastic density current (PDC) erode a channel that confines the body of the eroding flow or subsequent flows
The scour and fill feature is interpreted as evidence of PDC self-channelization, where PDCs scoured into fresh PDC deposits from earlier phases of the eruption and subsequently deposited within the scour
Summary
Pyroclastic density current (PDC) deposits from flows of moderate volume (∼106 m3) are often mantled by intertwined pumice lobes with lateral levees and depressed central channels [e.g., Wilson and Head, 1981; Calder et al, 2000]. The morphological similarity of pumice lobes to self-channelized granular flows implies that in certain conditions, PDCs from discrete or waning eruptions can self-channelize [Jessop et al, 2012; Kokelaar et al, 2014]. Self-channelization is commonly observed as levee formation and/or axial erosion (i.e., scouring) in sediment-laden flows, including but not limited to, granular flows [e.g., Pouliquen et al, 1997; Félix and Thomas, 2004] and turbidity currents [e.g., Clark and Pickering, 1996]. Constraining the conditions that promoted erosion and the nature of scouring is limited without complementary subsurface imaging
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