Tafoni (singular tafone) are cave-like structures that take the form of hollows or cavities and generally occur in granular rock (i.e., granite, sandstone). They can be as little as 0.1 m to several metres in height, width, and depth, with arched-shaped entrances, concave walls, sometimes with overhanging hoods or visors, and smooth and gently sloping, debris-strewn floors.
The origins of tafoni are complex and still subject of controversy among geologists. Salt action is the process commonly invoked, but researchers cannot agree whether the salts promote selective chemical attack or physical weathering.
In the first case, a solution rich in salts dissolves "mineral matter from the sandstone cement”, while in the second, successive layers of salt moisture cover the rock surface and evaporate, leaving growing crystals that forcing grains of rock apart.
Both chemical and physical processes may operate, but not all tafoni contain a significant quantity of salts. Once formed, tafoni are protected from rain-wash and may become the foci for salt accumulation and further salt weathering. Parts of the rock that are less effectively case-hardened are more vulnerable to such chemical attack.
Honeycomb weathering (or alveolar weathering) is a term used to describe numerous small pits, no more than a few centimetres wide and deep, separated by an intricate network of narrow walls and resembling a honeycomb. They are often thought of as a small-scale version of multiple tafoni. Honeycomb weathering is particularly evident in semi-arid and coastal environments where salts are in ready supply and wetting and drying cycles are common.
References:
[1] Paradise T.R. (2013) Tafoni and Other Rock Basins. In: John F. Shroder (ed.) Treatise on Geomorphology, Volume 4, pp. 111-126. San Diego: Academic Press. [online]
[2] Huggett, R. (2013). Fundamentals of Geomorphology. Taylor & Francis. [online]
all sources last accessed on 21.02.2015