The role of the sun in fracturing rocks has been debated by geomorphologists and soil scientists for over 100 years without resolution. Recent work indicates that diurnal heating and cooling are largely responsible for crack initiation in exposed rocks. The majority of cracks in exposed boulders in both deserts and more humid climates have preferential orientations centered on north-south, independent of rock fabric and shape. Such a ubiquitous preferential orientation is difficult to explain by any other mechanism other than directional solar heating. Cracks caused by freeze-thaw or salt shattering, for example, should exhibit random orientations. This project will examine and quantify the fundamental physics and environmental conditions required for cracks to form in rocks due to recurrent exposure to the sun. Methods include: 1) instrumenting a boulder to measure surface strain and surface temperature, and to monitor acoustic emission (cracking) events; 2) collecting field data of rock fractures in desert and temperate sites; and 3) using the field measurements and observations to model the process of crack growth with state-of-the-art numerical techniques. The project will involve several undergraduate students in the field data and analysis to provide a frim foundation for learning principles of scientific research.
Physical weathering is the mechanical breakdown of rocks and bedrock into smaller particles. It plays an elemental role in the rock cycle, through the breakdown of rock into sediment and through the subsequent increased exposure of rock surfaces to chemical weathering. Physical weathering also dictates the production of sediment on hillslopes and the formation of desert pavements. Physical weathering has also been linked by many workers to global phenomena such as atmospheric dust production, carbon cycling, and climate change. In addition, the physical degradation of natural and man-made materials has a direct and persistent impact on human civilization; phenomena ranging from rock fall to the deterioration of concrete bridges and archaeological treasures continuously cost humans lives, dollars and time. Despite its broad and significant role in surficial processes and societal infrastructure of our planet, however, there has been a critical gap in our understanding of physical weathering. The mechanical weathering of rock proceeds by a complicated suite of surface processes, all of which depend in some way on the creation of cracks and the exposure of fresh mineral surfaces. Surface crack initiation and propagation will determinethe pace for subsequent fracture and transport of rock debris in numerous environments. This research will build a foundation for significant gains in documenting the role of physical weathering in shaping Earth's landscapes.
