Rock size characteristics in rock avalanches and blocky lava flows are different and statistical techniques can be used both to distinguish between and to constrain the different emplacement mechanisms of these two processes. At the Chaos Jumbles rock avalanche deposit in California, rock sizes decrease as a function of distance. The rock avalanche mechanism is an example of a secondary process that can only break existing blocks. In contrast, lava flows at Sabancaya, Peru, were emplaced by primary mechanisms capable of creating as well as breaking blocks. These deposits exhibit uniform block sizes as functions of distance and significantly larger blocks overall than Chaos Jumbles. Rock sizes at Inyo domes, also emplaced by a primary process, are significantly smaller in areas with compressional ridges as compared to vent or jumbled areas.