A CHRONOSEQUENCE OF CRACKING: HOW FRACTURES EVOLVE OVER TIME IN THE EASTERN SIERRA NEVADA FOOTHILLS, CA

Mechanical weathering –aka physical weathering; the processes by which rock is cracked and mechanically broken down into smaller pieces– is one of the first modifications rock undergoes once it approaches Earth’s surface. Essentially, almost all processes that shape the landscape can’t occur without mechanical weathering. Thus, before we can fully understand landscape evolution, we must first characterize and understand rock cracking itself. There are few if any studies that document how mechanical weathering progresses over time. Numerous space-for-time studies (chronosequences) of chemical weathering demonstrate that as rock weathering progresses, the proportion of weatherable minerals decreases, and therefore so do rates of chemical weathering. Here, we hypothesize that similarly, the longer a rock sits exposed at or near the Earth’s surface, preexisting cracks or flaws that are most susceptible to naturally occurring stresses – like those related to tectonics, topography, or environmental exposure – will preferentially grow. As long as the rock does not erode to expose fresh mineral surfaces or experience a different stress regime we also hypothesize that through time, the rate of crack growth will decrease as growing cracks intercept regions of the rock that require greater stress magnitudes than the rock is regularly experiencing. We are testing these hypotheses in Lundy Canyon, California (7858 ft elevation, west of Mono Lake) by examining the length, width, density, strike, dip, and weathering profile of visible cracks in granite and metavolcanic boulders. These 1+m diameter boulders are located on the surface of two glacial moraines and five fluvial terraces, ranging in 10 Be exposure- and OSL- derived ages (Rood et al., 2011; this study) from <1 yr (modern gravel bars) to 167 ka. Preliminary results show that boulders found on active gravel bars exhibit fewer, smaller cracks than their older terrace counterparts. The results from this study may increase our understanding of the steps between initial exposure and subsequent erosion of exposed rock.