HOW PEBBLES ROUND: AN EXPERIMENTAL STUDY OF THE BREAKDOWN OF RIVER ROCKS BY TRANSPORT

Rocks primarily break through two breakdown mechanisms, chipping and fragmentation. Chipping occurs in the semi-brittle failure regime and is characterized by low-energy impacts that cause small particles to break off the rock, eventually resulting in the rock taking a round shape. Fragmentation occurs in the brittle failure regime and is characterized by high-energy impacts. The impacts cause the rock to break into large daughter particles where the particles take a jagged shape. This study sought to empirically delineate a boundary between chipping and fragmentation. In order to experimentally determine this boundary, strong and weak rock-like particles were created out of concrete mix and sand. The particles were then rotated in a metal drum. While they were being rotated, the different particles were regularly removed from the metal drum, massed, and imaged. The images were then processed in ImageJ to determine circularity, aspect ratio, and other parameters. The mass and image data was then compared against a universal mass-loss curve. The weaker particles that broke into large daughter particles did not follow the universal mass-loss curve, suggesting that deviation from the universal mass loss curve indicates fragmentation.