RELATIONSHIPS AMONG MINERALOGY, GRAIN SIZE, MICROCRACKING, AND EXPOSURE AGE OF GRANITE AND CARBONATE ROCKS IN EASTERN CALIFORNIA, USA

Mineralogy plays a crucial role in rock identification and understanding geological history, but it may also be a key characteristic contributing to cracking habits and erosional processes when rocks are exposed at the Earth’s surface. How a rock chemically and mechanically responds to external forces like climate or tectonics is related to its primary mineralogy and to secondary alteration of in-situ mineral grains. We hypothesize that rock mineralogy correlates with microcracking morphology and density. To explore these relationships, we conducted petrological analysis on 31 thin sections of granitic rocks and 8 thin sections of carbonate rocks. Thin sections were cut from loose clasts collected from stable depositional surfaces, including alluvial fans, glacial moraines, glacial outwash, alluvial terraces, and modern alluvial deposits at three sites in Eastern California. These localities were chosen because the depositional ages of the surfaces are known. Thus, the cracking observed in the boulders is assumed to have occurred since their deposition. Oriented thin section billets cut at 1.5 cm depth and parallel to the natural upward-facing rock surface were processed from the rough samples.

Modal mineralogy was determined from an average of 500-point counts per thin section. Granitic samples were classified using the IUGS classification diagram for plutonic rocks, and carbonate rocks were classified using the Folk classification system. In addition to mineralogy, the length and width of cracks and non-linear void space were collected. Preliminary results show carbonate rocks have more linear cracks and overall void space than granites, possibly due to the weaker and more chemically soluble mineralogy, consisting of >80% calcite and 20% groundmass. Cracking occurred along grain boundaries in both granites and carbonates, with alteration occurring within feldspar and amphibole grains more frequently, and comparisons based on manual workflows designed to structure and verify A.I. software were used to process results of crack features from the thin sections. Understanding the mineralogical controls of cracking can help predict and understand the geomorphology of a geological region.