THE MYSTERY OF CAVE SCALLOPS AND FLUTES: WHY THEY SHOULDN'T EXIST

Scallops and flutes are erosional forms that develop because of coupling between turbulent flow structures and erosion rates. They are found in caves and surface channels that have developed in highly soluble rocks as well as in channels flowing through ice. The basic theory of scallop formation was developed over 50 years ago. Similarly, experiments on calcite and limestone dissolution have long established the controls on dissolution rates as a function of partial pressure of carbon dioxide, dissolved ion concentration, and temperature. The resulting kinetic rates laws are typically applied within numerical models of speleogenesis. While such models have focused on the initial stages of cave formation, where flow is dominantly laminar, there is increasing interest in developing numerical models for studying the later stages of cave formation and cave channel evolution. Beyond the incipient stage, flow within caves is typically turbulent. Within the current models, dissolution under turbulent flow conditions is calculated by coupling the kinetic rate laws to transport through a thin diffusion boundary layer (DBL) that forms near the wall. Within two end member regimes, dissolution rates can be controlled either by the surface reaction rate or the DBL thickness. A straightforward analysis using the theory shows that under turbulent flow conditions the surface reaction rates of calcite dissolution are sufficiently slow that dissolution rates at the surface should not be influenced by DBL thickness or, consequently, by any turbulent flow structures that create contrasts in DBL thickness. This presents a conundrum to karst scientists. The standard theory of dissolution under turbulent flow used within speleogenesis models suggests that scallops and flutes should not exist. On the other hand, such features are ubiquitous within caves. While I have no answer to the conundrum, I discuss potential resolutions and their implications for speleogenesis.