Dissolution patterns in limestone caves

Abstract:

Erosion by dissolution is a type of erosion occurring in nature when water flows over soluble rocks. The coupling between the shape of the rocks and the flow can lead to the formation of various patterns. A precise understanding of the mechanisms leading to their formation is valuable, as it could help “decode the shapes of nature”, and thus provide information on past physical and hydrodynamic conditions.
With this in mind, we turn our attention to a rather common pattern called “scallops”, which consists of a cellular array of cups-like concavities surrounded by crests. They can be found typically on the walls of limestone caves carved by underground rivers. We therefore reconstruct in 3D the scallops found on the walls of the Saint-Marcel cave (Ardèche) and characterize their morphology.
One striking feature is the presence of very sharp crests, which can be seen as singular structures. In the first and main part of the talk, I will explain the emergence of these singularities. To do so, we compare the field measurements to numerical models of interface propagation and to a solutal convection experiment. It allows us to show that the global shape of scallops results from a generic, geometric effect.
Another feature of the scallops of the Saint-Marcel cave is their asymmetry, in both the horizontal and vertical directions. In the final part of the talk, I will briefly present experiments in which patterns are formed on soluble materials placed in a water flume. I will discuss how these experiments can help us interpret the asymmetry of natural scallops.