The controversy that raged through the 1950s--1970s over similarities and differences between meteorite impact and volcanic processes is revisited in this paper. We proposed that there are quantitative similarities in erosion caused by high-speed ejecta produced by either impacts or volcanic processes. Field and petrographic data from the Manicouagan impact crater, Canada, are used to demonstrate that during the emplacement of the impact melt sheet, erosion occurred at a rate of ~2562 kg m-2 s-1. Field data for the Mount St. Helens lateral blast of May 18, 1980, suggest an erosion rate of 21 kg m-2 s-1, and field data for a small pyroclastic flow on August 7, 1980, suggest an erosion rate of ~14 kg m-2 s-1. It is proposed that these three rates were determined dominantly by the momentum of the ejecta, and a quantitative formulation based on lofting theory is given. A new application of the Monte Carlo approach to analysis provides minimum, most likely, and maximum estimates for both the field and the theoretical analyses. The substantial erosion that occurs by fast moving flows results in mixing of stratigraphic components over large distances and to very fine scales. ¿ American Geophysical Union 1993