  Chemical erosion, one of the two major erosion processes along with
mechanical erosion, occurs when a soluble rock like salt, gypsum or limestone
is dissolved in contact with a water flow. The coupling between the geometry of
the rocks, the mass-transfer and the flow leads to the formation of remarkable
patterns, like scallop patterns in caves. We emphasize the common presence of
very sharp shapes and spikes, despite the diversity of hydrodynamic conditions
and the nature of the soluble materials. We explain the generic emergence of
such spikes in dissolution processes by a geometrical approach. Singularities
at the interface emerge as a consequence of the erosion directed in the normal
direction, when the surface displays curvature variations, like those
associated to a dissolution pattern. First, we demonstrate the presence of
singular structures in natural interfaces shaped by dissolution. Then, we
propose simple surface evolution models of increasing complexity demonstrating
the emergence of spikes and allowing us to explain at long term by coarsening
the formation of cellular structures. Finally, we perform a dissolution pattern
experiment driven by solutal convection and we report the emergence of a
cellular pattern following well the model predictions. Although the precise
prediction of dissolution shapes necessitates to perform a complete
hydrodynamic study, we show that the characteristic spikes which are reported
ultimately for dissolution shapes are explained generically by geometrical
arguments due to the surface evolution. These findings can be applied to other
ablation patterns, reported for example in melting ice.
