A Chronosequence of Cracking in Mill Creek, California
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Abstract
Mechanical weathering –aka physical weathering; the process by which rock is 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. Cracking is one form of weathering, but before we can completely understand any surficial process that is predicated on rock cracking, we must characterize and understand cracking itself. How cracking changes and affects rocks through time is a feature of physical weathering that is relatively unknown compared to chemical weathering. I hypothesize that mechanical weathering rates will initially be fast, with a sharp increase in the size and numbers of cracks, followed by a period in which crack size and numbers plateau. This hypothesis was tested through the collection of crack characteristic data – length, width, density, and a variety of other rock and crack metrics that provide insight into both the style and rate of cracking. Data were collected from boulders naturally deposited on the surface of glacial moraines and stream floodplains and terraces on the eastern flank of Sierra Nevada Mountains, in the vicinity of Mono Lake, in California. Previously these surfaces had been dated by Rood (2011) using measurements of 10Be on the surface of large (>1 m) boulders found on the surfaces. The surfaces studied here included a streambed, referred to as the "Modern Wash" at 0 years, and a Holocene age terrace (with an estimated age of 5000 years), and five more surfaces that had been previously dated. The average exposure ages of these surfaces previously calculated by Rood (2011) were as follows; Lundy Canyon Outwash Terrace: 40 ka, Lundy Canyon Moraine: 19 ka, Mono Basin Moraine: 40 ka, Buckeye Creek Outwash Terrace: 227 ka, and Sherwin Moraine: 77 ka. In this study, I collected data both on "dated boulders" – the large boulders that Rood (2011) had sampled, as well as "transect boulders" – a set of 100 clasts that were identified on each of 5 surfaces. Boulders were selected on a size criteria, the length had to be greater than 15 cm, but no more than 50 cm. For each of the "dated boulders" crack data was collected from 20x20 cm representative squares drawn on the boulder. For each transect boulder, crack data was collected for all cracks found in the 100 boulder transect. Results suggest that more recently exposed rock deposits from the Holocene Outwash Terrace display higher crack densities and crack lengths than the rocks on the older surfaces. Using dimensional data for both the boulders themselves and the cracks on them, a trend was able to be seen. For most of the variables data was collected on, crack length, width, density, etc. a spike in these variables was seen around 5000 years, followed by an immediate decline and gradual plateau as the surfaces got older. For the other, non-crack specific data, a similar, if slightly less defined time trend can also be seen. This result does not disprove the initial hypothesis, rather it highlights the idea that cracks can disappear through time as pieces fall off boulders. So in sum, rates of rock cracking appear to decrease through time as the rock is exhumed. This results have important implications for understanding the effect of mechanical weathering on rocks both out in nature and ones used for building within our cities.