Sheeting joints, widespread manifestations of the interaction of the Earth's internal stresses and topography, are opening-mode fractures formed subparallel to the Earth's surface beneath convex topography. They bound slabs of rock whose curvature decreases with depth. They occur in all major rock types, attain dimensions of hundreds of meters, and are important in landscape development, slope stability, and groundwater flow. The joints are widely regarded as forming in response to removal of overburden, but large fractures do not open in rocks merely by relieving compression: tensile stresses or high fluid pressures are required. This project tests the hypothesis that sheeting joints form due to topographically induced tensile stresses, and that sheeting joints develop perpendicular to the local most tensile stress. High surface-parallel compression is widely documented where sheeting joints occur, and where the compression is sufficiently strong and the topography is convex, tensile stresses perpendicular to the ground surface must develop to maintain equilibrium. The research combines detailed field characterization, existing stress measurements, calculations of curvature and slope, and stress analyses. Field investigations occur in Yosemite National Park, where the near-surface stresses have been measured and sheeting joints are superbly exposed on accessible bedrock surfaces of different size and shape. The joints and topography are mapped using high-resolution digital aerial photographs and digital airborne laser altimetry (LIDAR) data. The orientations of joints and the relative displacements across them are measured in the field. The topographic curvature is determined from the LIDAR data using methods of differential geometry. The stress field in the bedrock is investigated with exact analytical solutions and with two- and three-dimensional boundary element procedures that account for topography, gravity, horizontal regional stresses, and the presence of fractures to test how well the distributions of the joints can be predicted from curvature measurements and existing stress measurements in the park. Sheeting joints, also known as exfoliation joints, have intrigued geologists for more than two centuries. Thousands of students each year are introduced to sheeting joints through textbooks that attribute sheeting joints to unloading or pressure release. This research offers a new explanation for these joints, should solve a classic problem in structural geology, and should rewrite the textbooks. New insights into near-surface stresses, which critically affect many geologic processes (e.g., fracturing, groundwater flow, mass wasting (including rockfall in Yosemite Valley), volcanic processes, physical weathering, etc.) will be gained and shared with through public outreach at Yosemite National Park. Graduate students and undergraduate students are involved in all aspects of the project.
