Geological evidence: New insights into rock avalanche dynamics

Abstract Rock avalanches (RAs) are among the most enigmatic geological processes on Earth and other planets, characterized 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 a lack of geological evidence20-21. Microscope observations and mineral analysis of the basal and body facies of the Nyixoi Chongco RA on the Tibetan Plateau, China, revealed that grains in the RA deform by brittle fracturing with the frictional temperatures ranging from 270 ~ 300 ℃. The basal facies features a dilated regime with a high fragmentation degree, but the body facies displays a dense regime with relatively low fragmentation, although their temperatures are similar. A jigsaw structure characterized by intense collision is observed in the basal facies but is lacking in the body facies. With these geological evidences, we propose that the frictional temperature of most RAs characterized by brittle fracturing should be low that cannot cause thermal decomposition or melt. A mechanism similar to the Leidenfrost effect should occur in the basal facies, which is related to landquakes and dominates the high mobility of RAs by the generation of a relatively dilated regime with the main mass propagating as a cataclastic flow.