Geophysical Investigations of Pyroclastic Density Current Processes and Deposit Properties at Mount St. Helens, Washington (USA)

Abstract

Geophysical imaging has the potential to significantly improve investigations in pyroclastic deposits, either as a means of in situ property estimation or to provide geologic context where exposures do not exist. I perform two geophysical studies set in the deposits of the 1980 eruption at Mount St. Helens, Washington (USA); the aim is to investigate the physical properties and geology of pyroclastic deposits.Joint petrophysical modeling reveals the dependence of seismic and electromagnetic velocities in pyroclastic deposits on two-phase porosity (vesicularity and intergranular porosity) and water-saturation. Seismic first arrival travel-time tomography, multi-channel analysis of surface waves, and multi-offset GPR reflection tomography inversions from coincident seismic and GPR surveys demonstrate that seismic and electromagnetic velocities are consistent with partially saturated and moderate to poorly sorted siliciclastic sediments. The anomalously high critical porosity of pumice allows pyroclastic deposits to maintain higher rigidity despite total deposit porosities on the order of 0.80.A GPR survey of the pumice plain reveals two broad pyroclastic density current (PDC) scour-and fill features interpreted as erosional channels-the most spatially extensive case of scouring by PDCs found to date. Both channels are >200 m wide and >500 m long; estimated eroded volumes are on the order of 10 6 m 3 . Erosion appears to be promoted by moderate slope angle (5-15 • ), substrate pore-air retention, and pulses of increased flow energy. These findings are the first direct evidence of erosional self-channelization by sustained, waxing and waning PDCs, a phenomena v that may increase flow velocity and run-out distance.vi