Particle size reduction in geophysical granular flows: the role of rock fragmentation

Abstract

Particle size reduction in geophysical granular flows is caused by abrasion and fragmentation. Controls on fragmentation are not well understood. In this study, I used laboratory experiments to measure fragmentation probability and resulting fragment sizes, to calibrate a numerical model that can predict how particle size distributions evolve with travel distance in laboratory drums and in the field. Using free-fall single-particle experiments with granodiorite, basalt and serpentinite samples, I found that fragmentation probability is a power function of impact energy, with an exponent that varies between 0.66 and 1.03 for different rock types. I also found that fragment size distributions can be represented with a single power relationship for each rock type, independent of impact energy. These results were used to calibrate a numerical code that simulates the production and size evolution of sediment particles by fragmentation and abrasion. I tested the code using particle size measurements from rotating drum experiments that physically model granular flows in nature. In a related project, I documented down-valley fining of debris flow deposits at Inyo Creek, California, which may result from particle fragmentation during high energy particle interactions.