Seminar by Martin Chaigne

Abstract

Rock erosion is responsible for a wide variety of geological landscapes and structures on the Earth’s surface. For soluble rocks such as gypsum or limestone, the main erosion mechanism is dissolution by a water flow. Couplings between rock topography, flow and dissolution rate can then lead to the appearance of regular patterns at different scales. The thesis begins with a field characterisation of two of these patterns, karren – parallel grooves found on limestone slabs – and scallops – concave cavities adorning cave walls. It then explains certain aspects of the formation and morphology of these patterns, by comparing simplified theoretical and numerical models with experiments using fast dissolving materials. In particular, a simple interface propagation model is used to explain the generic presence of crests by describing the emergence of surface singularities in finite time via a geometric mechanism. However, in order to understand the dimensions or asymmetry of the patterns, the links between flows and morphogenesis need to be studied more specifically, which is what we do next. Two experimental set-ups are implemented, in which soluble blocks dissolve with or without an imposed flow – in the latter case, the flow results from solutal convection generated by the concentration gradients due to dissolution. We then describe how parameters such as the velocity of the imposed flow or the inclination of the blocks modify the formation of patterns on their surface, before discussing the potential application of the results to the natural setting. Finally, we briefly present a propulsion mechanism based on dissolution, experimentally demonstrated during this thesis in addition to the main subject.