Air-layer lubrication, that is, nearly frictionless support on a layer of trapped and compressed air, has been suggested for the Blackhawk-type landslides and for some nuées ardentes, snow avalanches, and cratering fallback. To ensure a sufficiently small rate of air loss by leakage through a typical Blackhawk-type landslide, the harmonic mean permeability must be less than about 1 darcy, a value that is reasonable for the extremely poorly sorted debris involved. A further necessary requirement for air-layer lubrication to occur, rather than simultaneous deposition and fluidization, is that the product of the permeability and the bulk density of the basal debris be less than 0.7 times the product of the harmonic mean permeability and the arithmetic mean density for the debris as a whole, as is probably in fact the case in these landslides.

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