International audienceField evidence shows that pyroclastic flows can entrain blocks from underlying substrates formed by earlier geological events, yet, counterintuitively, they are less likely to erode unconsolidated layers of fine particles. Here we 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 substrate of fines, so that particles are not entrained individually but the substrate instead is subject to small shear instabilities. In contrast, coarse particles forming a non-fluidized 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 (Washington State, USA) gives velocities of ∼9-13 m s-1 at ∼6-7 km from the vent on gentle slopes (<4°-6°), in agreement with field observations at this volcano and at others

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This is an author-produced, peer-reviewed version of this article. The final, definitive version of this document can be found online at Icarus, published by Elsevier. Copyright restrictions may apply. DOI: 10.1016DOI: 10. /j.icarus.2013 Abstract Systematic temperature mapping and high resolution images reveal a previously unrecognized class of small, fresh lunar craters. These craters are distinguished by near-crater deposits with evidence for lateral, ground-hugging transport. More distal, highly insulating surfaces surround these craters and do not show evidence of either significant deposition of new material or erosion of the substrate. The nearcrater deposits can be explained by a laterally propagating granular flow created by impact in the lunar vacuum environment. Further from the source crater, at distances of ~10-100 crater radii, the upper few to 10's of centimeters of regolith appear to have been "fluffed-up" without the accumulation of significant ejecta material. These properties appear to be common to all impacts, but quickly degrade in the lunar space weathering environment. Cratering in the vacuum environment involves a previously unrecognized set of processes that leave prominent, but ephemeral, features on the lunar surface. HighlightsSmall lunar craters are surrounded by granular flow deposits Extensive insulating surfaces extend beyond the granular flowsThe insulating surfaces show no evidence for impact related deposition or erosion These properties appear common to all small lunar impacts and are ephemeralThe structure of the upper regolith is modified by relatively distant small impacts 2 This is an author-produced, peer-reviewed version of this article. The final, definitive version of this document can be found online at Icarus, published by Elsevier. Copyright restrictions may apply.

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