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Very large rock avalanches, involving more than about 106 m3 of rock debris, exhibit anomalously low coefficients of friction. Consequently they travel much farther than conventional slope-stability criteria predict. Such long-runout landslides (sturz-strom) include the catastrophic Elm (1881), Frank (1903), and Sherman Glacier (1964) events. Attempts to explain this behavior have considered water or air lubrication, local steam generation, or even the formation of melt layers within the rock debris. Discovery of deposits of such landslides on Mars and the moon, however, appears to rule out the fundamental involvement of volatiles or atmospheric gases in the flow mechanism.

It appears that large, high-frequency pressure fluctuations due to irregularities in the flow of the debris may locally relieve overburden stresses in the rock mass and allow rapid pseudoviscous flow of even dry rock debris. If the avalanche volume is large enough, the rate of production of this vibrational (acoustic) energy exceeds its loss rate, and sustained motion is possible. Small-scale laboratory experiments have verified theoretical predictions of the rheology of such acoustically fluidized debris. This rheology is consistent with the rate and pattern of observed large rock avalanches. Although much work remains to be done, acoustic fluidization is the most plausible explanation of the fluidity of large, dry debris avalanches.

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