A comparison of pyroclastic flow and debris avalanche mobility

Many pyroclastic flows have runout distances longer than expected from basic frictional arguments. In these cases the ratio of the height descended H and the runout L are as small as ∼0.2, a feature which has been attributed to fluidization. However, significant fluidization by gases is now considered unlikely during emplacement of debris avalanches, where H/L and volume are inversely related. We determine whether pyroclastic flows are more mobile than debris avalanches of the same volume and if different emplacement mechanisms are needed to describe their runout. H/L and volume data were compiled for 34 nonvolcanic and 40 volcanic debris avalanche deposits, as well as 15 pyroclastic flow deposits. Systematic differences in the volume versus H/L plots were explored by statistical analysis. Results show an inverse relationship between H/L and volume for both nonvolcanic and volcanic debris avalanche deposits, as expected, and also for pyroclastic flow deposits. Regression lines for plots of pyroclastic flow and volcanic debris avalanche data are statistically indistinguishable and the non-volcanic debris avalanche line is parallel to the other two. We argue that the nonvolcanic and volcanic debris avalanche differences are due to material properties rather than emplacement mechanisms. Therefore since pyroclastic flows and volcanic debris avalanches have indistinguishable H/L versus volume, it is unlikely that pyroclastic flows have a significantly different primary emplacement mechanism. Based on the data in hand, we conclude that it is unnecessary to invoke gas fluidization to explain the mobility of the types of pyroclastic flows examined or the excess runout of large-volume debris avalanches.

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