Rock weathering, or the mechanical and chemical breakdown of rock over time, creates the landscape on which all terrestrial life is built. Here, I quantify the rates and controls over mechanical weathering [rock cracking/fracturing] f surficial boulder deposits in Eastern California using by collecting rock and crack field measurements, clast size distribution data from the field, and rock elastic properties using laboratory testing. I used a chronosequence or space-for-time approach, whereby data are collected from rocks or sediments that have been exposed to natural weathering conditions for a range of times, using the properties of the stable deposits to represent the amount of weathering that occurs over the time span of exposure. I studied rocks at three sites, with rocks being exposed to Earth surface conditions from 0 to 148,00 years [148 ka].
I manually measured 8763 crack lengths, widths, and orientations from 2221 in situ boulders on Earth’s surface and found that that rock cracking is initially fastest when rocks are exposed to Earth’s surface conditions, with rocks accumulating cracks at a rate of 9-1502 mm of cracks per m^2 rock surface over a thousand years, or 0.1-36 individual cracks per m^2 rock surface over a thousand years. After this point, rocks continue to crack, but the rate of crack growth slows down. After about 30 ka, the growth rate is <36 mm of cracks/m^2 of rock surface per ka, or <1 individual cracks/m^2 of rock surface per ka. Using all rock and crack data I determined that age itself has the most consistent, positive, statistically significant correlation with the number of fractures per rock surface area [fracture number density] and the total length of fractures measured per rock surface area [fracture intensity].
From two sites, I collected a granitic boulder from each dated deposit for rock mechanics testing. These data show that rock compliance increases over time while mechanical weathering leads to an increase in microscale cracks, which alter the rock’s strength and elastic strain response under stress. Using the laboratory analyses and local weather station data, I implemented a simple daily stress model applying Paris’ law of subcritical crack growth to predict single crack growth after each day of weather conditions, for a period of 5000 years. Cracking occurred over only a limited number of unusually intense weather days when the daily range of air temperatures was the largest. In the two semi-arid sites, these cracking days were hot, dry summer days; in the arid site, the day when the most crack growth was predicted coincided with summer monsoonal rains. The model is highly sensitive to rock elastic properties, which supports the theory that a gradual increase in bulk compliance allows rocks to withstand stress without cracking over thousands of years.
Finally, I present clast size data to show that for volcanic and carbonate rocks, there is a correlation between the geometry of cracking observed on the rocks and the shape of sediments on older deposits: when many cracks are parallel to the rock surface, older deposits tend to have more flattened rocks on them. This shows that cracking rates and crack geometries can play a strong role in clast size and shape evolution over geologic time, and mechanical weathering should be considered when interpreting sediments in the geologic record.
These findings are directly applicable to geoscientists attempting to understand weathering, landscape evolution, and geological hazards. More broadly, the decreasing rock cracking rates that accompany slow mechanical property changes represent a real-world example of material fatigue vs. material failure.