Pyroclastic flows are ground-hugging dense mixtures of gas and particles generated during volcanic eruptions.
Field evidence shows that they can entrain blocks from underlying substrates formed by earlier geological events, yet, counter-intuitively, they are less likely to erode unconsolidated layers of fine particles.
O. Roche and colleagues report laboratory experiments that reproduce these seemingly contradictory observations and also offer a means to infer pyroclastic flow velocity. Experiments demonstrate that the sliding head of a granular flow generates a dynamic upward pore pressure gradient at the flow-substrate interface. Associated upward air flux is enough to fluidize a smooth substrate of fines, so that particles are not entrained individually but the substrate instead results in small shear instabilities.
In contrast, coarse particles forming a non-fluidized rough substrate are lifted at a critical upward force due to the pore pressure gradient, according to their individual masses, which provides a basis for a model to calculate the flow velocity.
Application to the 18 May 1980 pyroclastic flow deposits at Mount St. Helens gives velocities of about 9 to 13 meters per second at about 6 to 7 km from the vent on gentle slopes (less than 4 to 6 degrees), in agreement with field observations at this volcano and others.
O. Roche et al., Laboratoire Magmas et Volcans, Université Blaise Pascal, CNRS DOI:10.1130/G34668.1.