99 Geomorphology (Posters)

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

INFLUENCE OF CLAST EXPOSURE AGE on CRACK CHARACTERISTICS OF DESERT PAVEMENT ROCKS, MOJAVE DESERT, CALIFORNIA

Paper number: 99-3
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

World-wide, rates of erosion are known to vary by many orders of magnitude. This variability may be driven by variations in physical or chemical weathering, however little data exists to document spatial variability in weathering processes. McFadden et al. (2005) suggest that directional insolation may play a key role in mechanical weathering of exposed rock, and provides a plausible mechanism whereby thermal stresses induced by diurnal insolation result in the development of rock cracks with preferred orientations. Both McFadden et al. (2005) and Eppes et al. (2010) show that desert pavement clasts across the world exhibit cracking consistent with this model, however it is unknown if insolation cracking varies temporally or spatially as a function of environmental factors. The purpose of this study was to document rock crack characteristics as a function of clast exposure length. 3,141 cracks longer than 2cm were characterized on 1,027 desert pavement clasts. These clasts were located on five Quaternary alluvial fan surfaces, aged 1, 4, 12, 50, and 130 ka (McDonald et al., 2003), of the western flank of the Mojave Desert National Preserve's Provenance mountains.

Overall numbers of cracks per rock decrease with surface age for large rocks, and stay the same when all rocks are examined. Crack length and width increase with surface age when all rocks are examined. Crack orientation does not vary in a consistent manner over time. Cracks exhibited preferred orientations that varied with fan age within a 0 to 90 degree vector mean azimuth for all but the 130 ka surface, which showed the most random crack orientation pattern and a vector mean of 170 degrees. However, the 130 ka surface was greatly reworked, as indicated by the number of clasts showing signs of recent disturbance.

In part, differences in preferred orientation may be attributable to the dominant rock type of each surface. Cracks for different rock types may have different preferred orientations due to the thermo-dynamic properties of individual minerals. Alternatively, cracks may form during discrete intervals when environmental conditions are favorable. Such conditions may have occurred at different times of the day and/or year under past Holocene and Pleistocene climate conditions resulting in different crack orientations.