Insolation Weathering: An Instrumentation and Field Based Study (Invited)

Processes of mechanical weathering related to diurnal insolation are largely unexplored. Recent studies (McFadden et al., 2005, Eppes et al., 2010) demonstrated that rocks in a range of environments exhibit preferentially orientated (~N-S) cracks that are hypothesized to form as rocks are heated and cooled during the sun’s daily transit across the sky. In this study, we attempt to better understand the association between rock fracture and directional insolation. In Charlotte, NC we instrumented a ~30 cm diameter granite boulder sitting in full sun exposure with 8 thermocouples, 8 strain rosettes, 6 acoustic emission sensors and a moisture sensor, in order to spatially and temporally correlate rock cracking with rock surface conditions. Temperature and strain are recorded every minute along with a suite of meteorological data, and acoustic emissions are continuously monitored. As part of an NSF REU, in the Providence Mountains of the Mojave Desert of Southern California, we examined every crack greater than 2 cm in length on 1027 desert pavement rocks of varying types and on surfaces of varying age (~1 ka to ~150 ka) in order to examine crack characteristics as a function of rock shape, rock type and rock exposure age. Analysis of preliminary instrumentation data indicates that rock cracking as monitored by AE devices occurs in discrete intervals of events that initially appear to be related to rapid changes in temperature and/or temperature gradients on the rock surface. Using 3-D location software, we are also able to locate the foci of events within the rock to a reasonable degree of certainty. Our data will allow us to begin to quantify the stress and temperature conditions under which cracking occurs. Preliminary analysis of our field data indicates that cracks exhibit preferred strike orientations (~NE) and dip directions (~ESE). These data support the idea that cracking occurs in association with the extreme temperature gradients that arise as rocks are first heated in the morning sun. Rock shape appears to enhance this effect. For example, more cracks are observed parallel to large flat SE facing surfaces as well as to NE oriented long axes of elongated rocks. We also observe correlations with rock type and cracking. For example, the average number of cracks per rock range from 3.4 (Meta-volcanic) to 1.9 (carbonates) to 0.8 (basalts) on a 140 ka surface. There is not an obvious trend through time in crack orientations, and the mode(s) of crack orientations appears to vary with surface age. These differences in orientations may be due to differences in the thermo-dynamic properties of different rock types and minerals, making them susceptible to cracking at different times of the day or year. Alternatively, cracks may have formed during discrete intervals when environmental conditions were favorable. Such conditions may have occurred at different times of the day and/or year throughout the Quaternary.