Geological evidences: New insights into rock avalanche dynamics

Abstract Rock avalanche (RA) is one of the most enigmatic geological processes on Earth and other planets featured by extremely high mobility1-7. Although great work has been conducted with many hypotheses being proposed8-19, the mechanism for its high mobility remains a challenging issue due to lacking geological evidence20-21. Microscope observations and mineral analysis of the basal and body facies of the Nyixoi Chongco RA on the Tibetan Plateau, China show that grains in the RA deform by brittle fracturing with a frictional temperature being 270 ~ 300 ℃. The basal facies is featured by a dilated regime with a high fragmentation degree, but the body facies displays a dense regime with a relatively low fragmentation although their temperatures are near. Jigsaw structure featured by intensive collision is observed in the basal facies but lacking in the body facies. With these geological evidences, it is reached that the frictional temperature of most RAs featured by brittle fracturing should be low that can not cause the occurrence of thermal decomposition or melt. A mechanism liking a leidenfrost effect should occur in the basal facies, which is related to landquake and dominates the high mobility of RAs by the generation of a relatively dilated regime with the main mass propagated as a cataclastic flow.