Special Online Presentation–Thermally Induced Fracturing: New Insights from Active Exfoliation at Arabia Mountain, Georgia

This presentation provides geologic background prior to a field trip to the Arabia Mountain on August 4, 2024

The speaker will be Dr. Aislin N. Reynolds, President’s Postdoctoral Fellow at Georgia Tech

Abstract: The formation of surface parallel exfoliation fractures in rock domes produces some of the most celebrated and curious landforms on Earth. In 1904, G.K. Gilbert outlined three mechanisms to produce exfoliation fractures: (1) original cooling of the rock, (2) decompression during rock exhumation, or (3) post-exhumation surface processes. More than a century of observations, including direct measurement of active exfoliation events, highlight the key roles of solar heating and subcritical fracture propagation in rock exfoliation. Here we present new insights from Arabia Mountain, a biotite orthogneiss dome near Atlanta, Georgia (USA) that experienced a spontaneous exfoliation event of a formerly quarried surface in July 2023. The event uplifted a ~250 m2 area over 30 cm, as revealed by digital elevation model differencing and field measurements. Following the event, instrumentation was installed to monitor continued fracturing during summer 2024, including surface parallel stresses, local seismic waves, and surface air and rock temperatures. Multiple additional exfoliation events were witnessed in-person during equipment installation, including direct observation of progressive fracture propagation and explosion. Observations and stress and seismic measurements all suggest a strong correlation with diurnal thermal cycles as captured by thermal sensors. These observations are compared with predicted stress distribution models informed by bulk-property measurements and regional geologic conditions to investigate the broader question: is the formation of exfoliation joints more sensitive to surface processes (e.g. solar heating and weathering), confining stresses related to topography and erosion, or interactions of regional (i.e. tectonic) stresses with topography? Understanding the mechanisms driving exfoliation fracturing in rock domes will better constrain the sensitivities of these domes to future climate evolution, with implications for forecasting rockfall hazards.