Presentation
Erosional Self-Channelization of Pyroclastic Density Currents: Evidence from Ground-Penetrating Radar Imaging at Mt. St. Helens, Washington (USA)
2016 American Geophysical Union Fall Meeting
(2016)
Abstract
The causes and consequences of substrate erosion are among the least understood attributes of pyroclastic density current (PDC) dynamics. Field evidence of substrate erosion is often limited by the location and quality of exposed PDC deposits. Here we present evidence for one of the most spatially extensive cases of PDC erosion to date, found within the 18 May 1980 deposits of Mt. St. Helens, Washington (USA). An 8 m deep and 300 m wide PDC scour and fill feature, which extends into PDC deposits from earlier in the eruption, is exposed along a distal outcrop of the shallow-dipping (<15º) pumice plain. Near surface geophysical techniques allow us to investigate the nature, extent, and cause of this large scour.
We used 50 MHz ground-penetrating radar to track the distal scour from outcrop toward its source. Beginning ~700 m up-flow from the large scour and fill exposure, the feature progressively widens from 205 m to 280 m and deepens from 10 m to 13 m, suggesting the PDCs became more erosive along the length of the scour. We extended our transects across the pumice plain to locate additional scours and to establish the topography at the time of erosion. We found a second ~420 m wide and ~11 m deep scour that extends at least 500 m from its inception point. Apparent dips of the sides of both channels are asymmetrical, due to pronounced erosion on the up-slope side of the flow in cross-section. Our data show no evidence of subsurface topographic irregularities or high slope angles up-flow from either erosional feature.
These features imply large PDCs from semi-sustained or fluctuating eruptions can self-channelize by erosional mechanisms. Our findings suggest that (1) concentrated PDCs are capable of producing large scours on shallow slopes with negligible surface roughness, analogous to the erosional channels of submarine turbidity currents, (2) substrate properties, including partial fluidization of fresh PDC deposits, may facilitate substrate erosion during semi-sustained eruptions, and (3) large PDCs can undergo self-channelization, whereby axial zones of high flow energy erode channels that confine subsequent flows. Erosion and self-channelization of this nature is not accounted for in models of concentrated PDCs, which may result in underestimates of run-out distance.
Disciplines
Publication Date
December 12, 2016
Location
San Francisco, CA
Citation Information
Andrew Gase, Brittany D. Brand and John Bradford. "Erosional Self-Channelization of Pyroclastic Density Currents: Evidence from Ground-Penetrating Radar Imaging at Mt. St. Helens, Washington (USA)" 2016 American Geophysical Union Fall Meeting (2016) Available at: http://works.bepress.com/brittany_brand/56/