Over the years my research has focused primarily on the processes of explosive volcanism and the interpretation of the deposits that are produced in both the terrestrial and marine environments. One of the main objectives of the work has been to utilize quantitative models of particle dispersal and deposition to produce methods for the interpretation of eruption dynamics based on grain size and other types of measurements collected from deposits in the geologic record. We have modified existing computer models for the atmospheric dispersal of volcanic ash in order to investigate the factors that control the formation of widespread volcanic ash layers that form important chronostratigraphic horizons in marine sediments and terrestrial sequences. The approach has been to simulate the dispersal of volcanic ash from several well-documented historic eruptions where various parameters such as the regional wind field, eruption column height, depositional area, and eruption duration are known. An important finding of the work is that there appears to be a strong control of the tropopause as a level where horizontal advection of particles is maximized, even for eruptions that inject particles to much higher levels. This finding has significant implications for short-term predictions of volcanic hazards to aviation associated with volcanic ash in the atmosphere.