Simulating the growth of tafoni

Throughout the world, large caves in rocks (tafoni) are found, which originate from salt weathering, see figure 1. The mechanisms that control their development are poorly understood. The growth of tafoni has been studied with a model that describes how a rock surface, containing a small pit, disintegrates by salt crystallization during wetting/drying cycles. In the model the rock is mapped on a grid. The migration and crystallization of salts are simulated explicitly in the drying phase of a cycle. At the end of each wetting/drying cycle the amount of salt deposited at the grid nodes is evaluated and the shape of the rock surface is adjusted by removing nodes

Figure 1.  A typical example of a 'tafone' (http://www.xtec.es/iesfortpius/img/tafoni.jpg)

The length of the drying period in a single cycle proved to be the key parameter, see figure 2. For short drying periods the amount of crystallized salt at the surface is proportional to the drying rate. Therefore, for short drying periods most salts are deposited at the more exterior parts of the rock surface. As a result, most damage will develop at these parts of the surface, which results in smooth surfaces.  Due to the characteristics of the drying process for long drying periods, most salts accumulate at regions with low evaporation rates, which are the sheltered parts of the rock surface.  These parts are not exposed to the wind or the sun.  As a result, the pit grows and a tafone develops.
 
 

Figure 3. The evolution of a pitted surface with increasing number of wetting/drying cycles (x10^3) for a) short and b) long drying periods. Green and medium grey refer to the dry and wet parts of the rock, respectively. White refers to the air.