Revisiting dirt cracking as a physical weathering process in warm deserts

Abstract

A half century ago C.D. Ollier proposed that insolation-driven temperature changes expand and contract fill in fissures enough to widen cracks, a process that would permit progressively deeper penetration of fissure fills, that would in turn generate a positive feedback of greater and greater strain until desert boulders and bedrock shatters. Although desert physical weathering by “dirt cracking” has occasionally been cited, this hypothesized process remains without support from subsequent research. Here, field observations, electron microscopy, X-ray powder diffraction, particle-size analysis, and laboratory experiments shed new light on dirt cracking. Little clear evidence supports the original notion of expansive pressures from thermal fluctuations. However, mineralogical, high resolution transmission electron microscopy, back-scattered electron microscopy, and experimental evidence support two alternative processes of widening fractures: wetting and drying of fills inside fissures; and the precipitation and remobilization of calcium carbonate. A re-envisioned dirt-cracking wedging process starts with calcium carbonate precipitating in fissures less than 5 μm wide. First precipitation, and then ongoing dissolution of this laminar calcrete, opens enough space for dust to penetrate into these narrow fractures. Wetting of expansive clays in the fissure fill exerts enough pressure to widen and deepen the fissure, allowing the carbonate precipitation process to penetrate even deeper and allowing even more dust to move into a fracture. As the dust infiltrates, its texture changes from a chaotic mix of particles to an alignment of clays parallel to fissure sides. This parallel alignment could increase the efficiency of fill wedging. Ollier's concept of a positive feedback remains supported; each increment of fracture deepening and widening permits more, even deeper infiltration of laminar calcrete and dust. Field and electron microscope observations of rock spalling in the winter of 2010 are consistent with Ollier's hypothesis that dirt cracking is a common physical weathering process in deserts that splits rocks of all different sizes.