Long-runout pyroclastic density currents: Analysis and implications

Pyroclastic density currents are hazardous, ground-hugging, hot mixtures of gas and solid particles produced by volcanoes. Currents generated by the largest explosive eruptions have traveled distances on the order of 100 km, and their devastating impact has repeatedly marked Earth’s geologic history. We show that pyroclastic density currents from super-eruptions during the Oligocene in the Great Basin of the United States had exceptional runout distances that may have exceeded 300 km. We present a quantitative analysis of the data from these currents and consider, in particular, the areal extent of their deposits (ignimbrites) as well as the relationship between their runout and the eruption mass discharge rate. The ignimbrites have elliptical distributions characterized by axis length ratios of ~2–6, in contrast to common subcircular ignimbrites, while the maximum runouts versus the discharge rates are clearly outside of the prediction intervals defined by other eruption data. We argue that the long runouts resulted from the channeling of concentrated currents in regional paleovalleys whose gentle slope worked to lengthen the travel distance. Our study demonstrates that these additional factors should be considered in assessing hazards posed by future super-eruptions.