99 Geomorphology (Posters)

Monday, 1 November 2010
8:00 AM - 6:00 PM
Hall D

THE CONTROL OF ROCK TYPE IN NON-TECTONIC ROCK CRACK CHARACTERISTICS

Paper number: 99-1
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

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 initially generating cracks, but it is unknown how specific rock properties affect this process. In this study we utilized field data from the Providence Mountain alluvial fans and the Cima Volcanic Field basalt flows in the Mojave Desert to determine if crack population characteristics vary as a function of rock type. Over 400 rocks were measured along 19 transects from a 130 ka alluvial fan surface and a ~140 ka basalt surface. ‘All Rock’ and ‘Boulder’ transects were completed by measuring characteristics of every crack with a length greater than 2 cm on each rock with a length between 5-50cm for ‘All Rock’ and 15-50 cm for ‘Boulder’ under every half-meter tick mark of tape. Rock types present included coarse and fine plutonic rocks, meta-volcanic rocks, basalt rocks, and carbonate rocks.

Initial data analysis suggests that rock type influences rock cracking. For the ‘All Rock’ transects, meta-volcanic rocks have an average of 3.4 cracks, fine plutonics 2.9 cracks, carbonates 1.9 cracks, and basalt rocks only average 0.8 cracks. Basalts have the widest cracks with an average width of 1.52 mm, compared to 1.07 mm for all other rock types. Average crack lengths vary by rock type from 38 mm for basalts to 48 mm for fine plutonics. As the density of vesicles increases in basalts, the average number of cracks per rock decreases, possibly due to heat dissipation and reduction of thermal stresses. For all rock types, as more desert varnish accumulates (76-100% cover) darkening the clast surface, the average number of cracks increases.

All rock types display preferred crack orientations, but crack vector mean orientation varies by rock type (NE 75° for all plutonic rocks to SE 126° for carbonates). Plutonic rocks exhibit a strongly unimodal preferred crack orientation, while other rock types have preferred multimodal crack orientations. These differences in orientations may be due to differences in mineralogy, clast energy threshold, and/or other thermo-dynamic properties of different rock types and minerals, making them susceptible to cracking at different times of the day or year.