THERMALLY-INDUCED ROCK EXFOLIATION AS A MODULATOR FOR GRANITE DOME WEATHERING

Rock exfoliation domes form spectacular landscapes in many settings and are particularly well-expressed in the Sierra Nevada mountains of California. Exfoliation, whereby so-called “sheets” of rock detach along surface-(sub)parallel exfoliation fractures (or joints), is the primary macroscopic mode by which rock domes weather and subsequently erode, thereby resulting in their domal shape. Although exfoliation processes have long been recognized as integral to shaping domes, the exact mechanism(s) by which exfoliation occurs remains elusive, in part due to the lack of direct observation of the natural events that lead to their fracture. During the summer of 2014, an exfoliation dome near the town of Twain Harte, California, located in the Sierra Nevada foothills, began to seemingly spontaneously fracture, opening existing exfoliation joints and creating new fractures, which included the thrust of a 10 m² sheet of rock over 40 cm upward. A total of five energetic (i.e., with rapid fracture opening) exfoliation events were recorded in 2014; some of these were witnessed first-hand and captured on video. In 2015, additional non-energetic (i.e., slow) fracturing occurred. This was followed by two additional energetic fracturing events in 2016. All events occurred on particularly hot summer days.

To determine the cause of these exfoliation events, and to better understand the role of exfoliation in shaping landscapes, we undertook a comprehensive study of Twain Harte Dome utilizing a suite of mapping and instrumentation methods. The instrumentation captured several of the exfoliation events, resulting in the first-ever measurements of natural exfoliation sheet deformation and stress conditions. Through a comprehensive review of plausible explanations that might have triggered exfoliation, we show that thermally-derived stresses, coincident with hot summertime periods, was the most likely cause for fracturing. The new exfoliation from the 2014–2016 events caused fracturing over an area totaling 3,030 m² (i.e., approximately 30% of the planar dome summit surface) with a resultant 2,340 m³ of displaced rock. Thus, we show that thermally-induced weathering is an important contributor to macroscopic rock dome evolution in this environment, and potentially many others.