Widespread volcanic ash layers form important chrononstratigraphic horizons in the geologic record and have been critical in studies related to volcanology, paleoclimate, atmospheric circulation, and radiometric dating. They constitute event horizons that allow for precise temporal correlation across spatial domains of several tens of thousands of km2. Many aspects of their formation are well understood but critical questions remain about the dispersal and deposition of volcanic ash from the very largest eruptions that have occurred in the past. Their distribution patterns often extend significantly in an apparent upwind direction and their widths are wide relative to smaller historic examples of fallout. The factors that control the formation of widespread ash horizons will be investigated using a Lagrangian particle tracking model that was originally formulated from a pollution dispersal code. The model will be modified to 1) account for injection of volcanic ash from large-scale umbrella clouds, 2) treat particle settling variations as functions of the Reynolds number, and 3) account for stratospheric transport, including the typical variation of wind as a function of altitude and time. Simulations will be carried out on the 1991 eruption of Mt. Pinatubo in the Philippines in order to evaluate the predictive capability of the model. Model results will be compared with observational data on the movement of the eruption plume and the area of ash deposition. Additional simulations will be carried out on two eruptions of the Toba caldera in Indonesia (Youngest Toba Tuff and Oldest Toba Tuff events). Results of these simulations will be used to assess the role of tropospheric versus stratospheric transport in producing the exceptionally wide distribution of the tephra layers that are found in sediments both throughout the Indian Ocean and South China Sea.
