THE INFLUENCE OF TEMPERATURE AND TEMPERATURE GRADIENTS ON ROCKS STRENGTH OVER TIME IN THE EASTERN SIERRA NEVADA FOOTHILLS, CA

Mechanical -or physical- weathering is a foundational process that a rock undergoes when it reaches Earth’s surface. Understanding mechanical weathering is crucial to fully comprehending numerous other surficial processes that involve rock. Currently, there is minimal documentation of how or why rock mechanical properties change through time as mechanical weathering progresses. Prior work (Eppes et al., 2016) demonstrates that thermal stresses play a major role in rock cracking. Here, we hypothesize that boulders experiencing greater temperature gradients across their surface -or higher surface temperatures overall- will initially experience more cracking, that in turn will lower their strength relative to rocks with lower temperatures or temperature gradients. We expect that these differences may reduce through time. We test these hypotheses by utilizing a Flir thermal imaging camera (to measure rock surface temperatures) and a Schmidt hammer (to measure rock surface compressive strength). Measurements were made on large (m+) boulders previously dated using 10Be exposure dating (Rood et al., 2011) in the sage shrubland of the eastern Sierra Nevada foothills, California. The ~30 boulders are similarly sized and of identical rock type, but range in exposure age from 13,400 to 167,000 years. Thermal images were acquired during a 2-hour time frame centered around solar noon, the time at which numerical modeling suggests that thermal stresses are highest (Eppes et al., 2016). Twelve Schmidt hammer measurements were made in a grid of 2 to 3 20 x 20 cm ‘boxes’ per boulder. So as not to introduce sampling bias, the boxes were located along predetermined transect locations down the center of the long axis of the boulder. In addition, we also acquired N, E, S, W, and Top thermal photos of 25 additional boulders. To find the average Schmidt hammer value for each box, the highest value and lowest value will be dropped and the remaining ten averaged, as is common practice in concrete testing. Preliminary data suggests a negative correlation between exposure age and compressive strength. This work has the potential to increase our understanding of how and why mechanical weathering progresses over time and space.