Cracking Up: Lithological Controls on Non-Tectonic Rock Cracks, Mojave Desert, California
By Sarah Evans
Whitman College, Walla Walla WA
Physical weathering affects erosion rates, sediment production, and atmospheric concentrations of CO2, yet non-tectonic-related crack formation is poorly understood. Thermal stresses related to diurnal directional insolation may play the primary role in generating cracks initially, but it is unknown how specific rock properties affect this process. In this study we utilized field data from the 130 ka Providence Mountain (PM) alluvial fans and the ~140 ka Cima Volcanic Field basalt flows in the Mojave Desert to determine if crack population characteristics vary as a function of lithology. We measured crack density and orientation for more than 400 rocks along 19 transects.
Analysis suggests that rock type influences crack density, size, and orientation. Basalt clasts have a median crack density of 15 c/m2 (cracks per square meter), and metavolcanics have 35 c/m2. Generally about 70% of all rock types excluding basalt contained one or more cracks while only 38% of basalt clasts contained at least one crack. As the density of vesicles increases in basalts, the average number of cracks per clast decreases, possibly due to heat dissipation and reduction of thermal stresses. All rock types display preferred crack orientations with the majority of mean crack orientations toward the northeast. The lone exception to this trend is limestones, which show a southeast trend. The differences in orientations may be due to differences in mineralogy, heat capacity, and other thermodynamic properties of different rocks and minerals, making them susceptible to cracking at different times of the day or year.