Pyroclastic density currents, PDCs, are ground-hugging mixtures of hot gases and pyroclastic material that propagate at high velocities down the flanks of volcanoes. While PDCs are some the most hazardous volcanic phenomenon, they are challenging to study because the voluminous amounts of ash produced during an eruption hinders visual observation of the interior flow dynamics. Consequently much of the understanding of PDC dynamics comes from analysis of the deposits they produce. The May 18, 1980 eruption at Mount St. Helens has had a significant impact on the world's awareness and understanding of explosive eruptions, and study of the eruptive products produced during this eruption has aided our interpretation of deposits from many other eruptions. Earlier studies of the PDCs produced during this eruption were correlated with changes in eruptive intensity and behavior. Since then, however, deeply incised drainages have provided new, extensive exposures that contain important information about the currents that produced them, allowing for a more complete study of these deposits to take place. This project represents a multidisciplinary approach to better understand PDC dynamics at one of the best-documented eruptions. It is planned to combine the estimates of mass flux, detailed measurements of recently exposed strata, including ground penetrating radar studies, and multiphase numerical modeling techniques to better constrain the dynamics of the PDCs produced on May 18th.
In particular, this proposal focuses on three questions regarding the transport of PDCs. 1) How is flow intensity and concentration related to the transport capacity of lithic clasts, and how does this lead to current segregation and density stratification with distance from source? One of the striking observations of many PDC deposits is the very large (> 1 m) pumice and lithic clasts that are embedded within the finer-grained matrix. The advent of new multiphase numerical techniques allows for a better accounting of particle trajectories from the vent to their eventual deposition. This information will be combined with a careful analysis of the grain size distribution and architecture of recently exposed deposits to better explain the conditions necessary to transport these large lithics. 2) How much clast comminution occurs during PDC transport, and how does this influence subsequent dynamics? When volcanic particles collide or slide past each other they tend to break up, or comminute. The cumulative effect of this process is rounding of large particles and the production of a higher percentage of fine ash in the current. Early models suggest that this higher fine ash proportion can generate emergent dynamics in these flows, resulting in greater runout distances. Detailed correlation between the units and source conditions make Mount St. Helens an excellent candidate for detailed roundness studies, which in turn will yield information about the small-scale momentum transfer in these flows. 3) How does the self-channelization of PDCs influence transport distance? It is hypothesized that self-channelization, aided by the ease of the erosion of recently deposited unconsolidated pyroclasts, played a role in some of the largest Mount St. Helens PDCs. Improved maps of newly exposed gully cross-sections, ground-penetrating radar, and multiphase simulations will provide further information on St. Helens PDC self-channelization and flow feedback. In summary, the combined modeling-field study of Mount St. Helens will improve the general understanding of transport and deposition in PDCs. The results of this work will advance our knowledge and ability to assess the hazards related to PDCs in similarly explosive eruptions elsewhere, including relative runout distance and quantification of fine ash produced during lateral transport, which has significant relevance aviation hazard assessment and potential climactic effects.
