Rock avalanche-debris geometry and implications for rock-avalanche genesis

Abstract

Rock avalanches have an important impact on the geomorphological evolution of mountain landscapes. Rock avalanche-debris geometry holds important clues related to the processes of the disaster predisposition, occurrence and rapid runout of rock avalanche-debris flows. The morphological features of the debris of the Touzhai rock avalanche deposit in Southwest China are examined as a specific case with the help of three-dimensional laser scanning, polarization microscopy (for polished sections) and field-emission scanning electron microscopy (FESEM). The unexpectedly high roundness is the most impressive morphological characteristic of the Touzhai rock avalanche-debris, particularly that of sand-plus silt-sized particles. The shapes of 12 scanned metric-sized boulders are closer to cubes than to spheres. 90% of the 114 scanned subangular blocks with a mean particle diameter of 12.04 cm and an average specific surface area of 0.225 cm2/g fall into the spherical domain of Zingg's classification diagram according to their flatness ratio and elongation ratio, and the average sphericity of 0.798 further indicates that they are more akin to cubes compared with spheres. The average form factor of the 1201 sand-sized grains is 0.784 with a standard deviation of 0.044, indicating that they are nearly spherical, and the solidity ranging from 0.951 to 0.974 also strongly shows that these particles are close to smooth spheres. Most of the grains in the size range of <0.075 mm are sub-angular except slaty secondary clay minerals of <0.005 mm based on the visual estimation under FESEM. The detailed qualitative and quantitative contrasts between the morphometric characteristics of the Touzhai rock-avalanche debris and those of the fresh basalt fragments demonstrate that the geometry of the former is dominated by the pre-failure predesign in which the basalt, primary and secondary structures, unloading and weathering acted together. Weathering was the direct contributor to its high roundness. The dynamic fragmentation triggering newly generated ruptures during the post-failure motion was less important, which should be the case in some rock avalanches in other regions. The slope pattern of “large creeping main slide mass + lower intact locked segment” generalized according to this study should be applicable to some other rock avalanches worldwide, and can help the pre-event identification of the potential rock-avalanche sites. Our results support the hypothesis of “aerification of debris” to explain the high mobility of rock avalanches. Highly fragmented source rockmass with enough fine grains should be necessary for the occurrence of rock avalanches and long, rapid runout of the rock avalanche-debris flows.