Dynamics of pyroclastic density currents: Conditions that promote substrate erosion and self-channelization — Mount St Helens, Washington (USA)

Abstract

The May 18th, 1980 eruption of Mount St. Helens (MSH) produced multiple pyroclastic density currents (PDCs), burying the area north of the volcano under 10s of meters of deposits. Detailed measurements of recently exposed strata from these PDCs provide substantial insight into the dynamics of concentrated currents including inferences on particle–particle interactions, current mobility due to sedimentation fluidization and internal pore pressure, particle support mechanisms, the influence of surface roughness and the conditions that promote substrate erosion and self-channelization. Four primary flow units are identified along the extensive drainage system north of the volcano. Each flow unit has intricate vertical and lateral facies changes and complex cross-cutting relationships away from source. Each flow unit is an accumulation from an unsteady but locally sustained PDC or an amalgamation of several PDC pulses. The PDCs associated with Units I and II likely occurred during the pre-climactic, waxing phase of the eruption. These currents flowed around and filled in the hummocky topography, leaving the massive to diffusely-stratified deposits of Units I and II. The deposits of both Units I and II are generally more massive in low lying areas and more stratified in areas of high surface roughness, suggesting that surface roughness enhanced basal shear stress within the flow boundary. Units III and IV are associated with the climactic phase of the eruption, which produced the most voluminous and wide-spread PDCs. Both flow units are characteristically massive and enriched in vent-derived lithic blocks. These currents flowed over and around the debris avalanche deposits, as evidenced by the erosion of blocks from the hummocks. Unit III is massive, poorly sorted, and shows little to no evidence of elutriation or segregation of lithics and pumice, suggesting a highly concentrated current where size-density segregation was suppressed. Unit IV shows similar depositional features but typically has a basal lithic-rich region, is variably fines-depleted and contains pumice lobes, suggesting density segregation in a less concentrated current relative to Unit III. Deep, erosive channels cut by the Unit III current and thick lithic levee deposits within Unit IV occur in an area where debris avalanche relief is limited, suggesting self-channelization developed as a function of internal flow dynamics. An increase in the proportion and size of lithic blocks is found (1) downstream of debris avalanche hummocks, suggesting the PDCs were energetic enough to locally entrain accidental lithics from the hummocks and transport them tens of meters downstream, and (2) within large channels cut by later PDCs into earlier PDC deposits, suggesting self-channelization of the flows increased the carrying capacity of the subsequent channelized currents. Finally, the combination of thick, massive deposits with a high percentage of fine ash within Unit III and in the medial–distal depositional regions of Units II–IV suggests that the PDCs developed and maintained a high internal pore pressure during transport and deposition. The most important results include our ability to understand the role of internal pore pressure on current mobility, the influence of self-channelization on carrying capacity of the currents and the influence of surface roughness on substrate erosion. These observations have critical consequences for understanding the flow dynamics and hazard potential of PDCs.