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Expansion of tafoni seems to be based on weathering (seems reasonable enough.) But what creates them in the first place? There are a number of explanations online (Wikipedia lists eight plausible causes!) Many websites trying to give a simplified explanation simply say "salt weathering", but from a brief search it's clear that salt weathering is not always the cause of tafoni [0]. They develop on a wide range of rock types, from granite to sandstone to tuffs and conglomerates, and in many situations where they'd be expected to develop they don't.

From what I can tell, they seem somewhat mysterious, and there are explanations that make sense in some cases but not every case of tafoni is well explained. Is there a widely accepted explanation for the formation of tafoni?

Picture of tafoni (Martin Sharman, Wikipedia):

enter image description here

[0] for example: page 75, Arid and Semi-Arid Geomorphology, Andrew S. Goudie

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I had a good geomorphology teacher. He was so good that I bought his book, published on Prentince Hall at 2008. He is an aged an experimented geomorphologist, so I totaly trust there is not an agreement about the physical origin of tafonies (or at least there wasn't an agreement at 2008 when his book was published).

"Its origin [tafonis] is controversial (Twidale and Vidal Romaní, 2005). Some authors differentiate between wall tafonis and basal tafonis, linked to the processes of basal undermining (Smith, 1978, Bradley et al., 1978). It is agreed that the dominant processes are those related to the movement of water with salts close to the surface, which correspond to haloclastism and wetting and drying. Thermoclastism, cryoclasty, dissolution of carbonated cements and wind erosion are also mentioned (Evans, 1969-70, Mustoe, 1983, Young, 1987, Robinson and Williams, 1992). In addition to these subaerial processes, some taphonis can be generated by weathering under the soil surface and subsequent exhumation by erosion (Dragovich, 1969)."

Source: Geomorfología, Prentince Hall (2008), page 152. Translated from spanish by Google.

-Gutiérrez Elorza, M. (2008) "Geomorfología". Ed Prentince Hall.

-Twidale, C. R.; Bourne, J.A., y Vidal Romaní, J.R. (2005): "Beach etching and shore platforms" Geomorphology 67, 47-61.

-Smith (1978) "The origin and geomorphic implications of cliff foot recesses and tafoni on limestone hamadas in the northwest Sahara" Zeitscrift für Geomorphologie, 22, 21-43.

-Bradley, W.C.; hutton, J.T., y Twidale, C.R.(1978). "Role of salts in development of granitic tafoni, South Australia". Journal of Geology 86, 647-654.

-Evans, I.S. (1969-70) "Salt crystallization and rock weathering: a review" Revue de Géomorphologié Dynamique, 19,153-177.

-Mustoe, G.E.(1983)"Cavernous weathering in the Capitol Reef Desert, Utah" Earth Surface Processes and Landforms 8, 517-526

-Young, R.M. (1987) "Salt as an agent in the development of cavernous weathering" Geology 15, 962-966.

-Robinson, D.A. Williams, R.B.G.(1992) "Sandstone weathering in the High Atlas, morocco" Zeitscrift für Geomorphologie 4,413-429.

-Dragovich, D. (1969): "The origin of cavernous surfaces (tafoni) in granitic rocks of Southern Australia". Zeitscrift für Geomorphologie 13, 163-181.

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  • $\begingroup$ It is importan to note processes may differ within the different types of rocks where tafonies are developed. $\endgroup$ – user12525 Jun 25 '19 at 13:14
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    $\begingroup$ Ah, beautiful! This is exactly what I was looking for - multiple credible recent sources saying there was not (or was) a consensus! I'm pretty convinced. It'd be nice to have some sort of literature review here, I'm quite surprised none exists. Still, without one, I suppose this is the next best thing. Thank you! $\endgroup$ – Owen Versteeg Jun 25 '19 at 14:43
  • $\begingroup$ @Owen Versteeg. You are welcome and thanks for accepting my answer. Glad to help. They are not very recent quotes but trustable sources. I think multiple proccess are involved at different climates and rocks where this forms develops so the proccesses involved in its formation vary from regions/rocks. The book mention tafonis take a lot of time (some million years) to form. $\endgroup$ – user12525 Jun 25 '19 at 15:59
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It is called preferential weathering, but there is another element at play. Before the rock eroded into circular features, there were softer materials present. Those softer materials weathered away first leaving the more resistant host rock (or matrix) behind. The matrix is harder than the softer "clastic" material. When pebbles get washed into the cliff face, the pebbles will slowly break down the softer material first and eventually the wave-action of pebbles will continue to grind as they spin and get rolled around in these little crevasses, which can grow in size as is seen in many Tafoni rock faces.

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  • $\begingroup$ Thanks for the answer! However, I'm not convinced. I've seen this explanation all over the internet and it hand waves over the whole interesting part here, aka the weird pattern. Why, exactly, does the weathering happen in this specific way? And if the explanation is so straightforward, then why is the formation of tafoni so unpredictable? I'm leaning towards the other answer to this question, which cites a number of sources (some quite recent) saying there are many competing hypotheses but no agreement in the field as to their origin. $\endgroup$ – Owen Versteeg Jun 25 '19 at 14:22
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Here are some pictures of a supposed tafoni in Finland (a natural 23 m deep hole in bedrock in the municipality of Jalasjärvi).

Preferential weathering is the preferred geological explanation.

If you wish to see the place, come within seven years from now since the rental of the place elapses after that date.

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  • $\begingroup$ Interesting input but this does not answer OP's question: 'What create those in the first place' $\endgroup$ – Etienne Godin Dec 29 '19 at 14:41
  • $\begingroup$ Looks like an interesting place! However, that does not look anything like tafoni to me. Perhaps you're confusing it with another place that does have tafoni, also called the Devil's Nest and located in Finland: atlasobscura.com/places/pirunpesa-the-devils-nest $\endgroup$ – Owen Versteeg Dec 30 '19 at 20:21

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