Simulating the growth of tafoni

Abstract

Throughout the world, large caves in rocks (tafoni) are found, which originate from salt weathering. 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. The length of the drying period in a single cycle proved to be the key parameter. 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.

Using paper pulp poultices in the field and laboratory to analyse salt distribution in building limestones

“…This result further underpins that salt ions favourably crystallise near the surface in the course of the dehydration process (e.g. [29]; however, a certain minimum moisture content needs to be maintained for an effective salt transport towards the surface.…”

“…Inhomogenities in the stone fabric, which often lead to aveolar weathering are not visible at the blocks of the investigated masonry. Reasons for the development of cavernous patterns might also be a different behaviour and interaction of highly soluble and poorly soluble salts [29,35,36] or a concequence of mechanical breakdown of stone surfaces exposed after crust removal [37]. Furthermore, salt concentrations are highest in the area of the capillary fringe, and they are much higher on the garden side of the wall, which is surprising because the starting hypothesis was that road salts and air pollution should be rather concentrated on the road side.…”

Biogenic origin of coastal honeycomb weathering

“…Laboratory simulations by caused them to suggest that localized variations in wind velocity may play an important role. Huinink et al (2004) proposed on the basis of mathematical modeling that tafoni may originate from variations in rate of evaporation of moisture from porous rock. Another intriguing possibility is that the positioning of cavities on the rock surface represents a phenomenon akin to the natural processes that produce spots on a leopard or markings on a butterfl y wing.…”

Sculpting of Rocks by Reactive Fluids

“…4.16) has been studied extensively and it is generally believed that growth of salt crystals plays an important role (Bradley et al, 1978;McBride and Picard, 2004). Huinink et al (2004) presented an informative simulation of the growth of a single tafone cavity. They used a simple 2D model with a wet zone inside the rock, in which water flowed according to Darcy's equation for unsaturated flow (with a saturation dependent permeability and pressure) in a porous medium.…”