Tafoni - Amazing Geology of Sark

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Tafoni

The etymology of the word "tafoni" is unclear. Tafoni may come from the Greek word taphos, "tomb", or it may derive from a Corsican or Sicilian word for "windows", taffoni, or from tafonare meaning "to perforate".
Tafoni are ellipsoidal, pan- to bowl-shaped, natural rock cavities in granular rock such as sandstone, granite, and sandy-limestone with rounded entrances and smooth concave walls, often connected, adjacent, and/or networked.. These cavernous weathering features include tiny pits, softball-sized cavities, truck-sized caves, and nested and cellular honeycomb forms. Tafoni typically develop on inclined or vertical surfaces and occur in groups.

These refined and fascinating cavities are present on the surfaces of many different types of rocks located in a large number of geographic regions around the world, but are more common in maritime zones, in arid zones and in deserts. Its development and evolution is intriguing and continues to arouse curiosity.

Currently favored explanations controlling their formation include salt weathering, differential cementation, structural variation in permeability or wetting-drying.
 

Origin

There are many explanations about the origin of the tafoni: wind erosion, erosion due to salts, differences in the internal cohesion and in the permeability of the rocks, duration of the dry period between several wet periods, etc.
The current view is that they are the result of weathering initiated along the weakest lines in rocks, such as joints and fractures, or less resistant minerals, where water can reside. It is believed that the voids are enlarged by the progressive desquamation of the interior surfaces and their granular disintegration, probably as a result of the crystallization of dissolved material from the saline rock solution, carried to the rock by the wind, or splashed on the rock by sea water.

The smaller tafoni are also called "alveoli" in correspondence to honeycomb and the corresponding type of erosion is called honeycomb erosion.
The process of alveolar erosion in the sandstone is a result of the combined action of wind and the salt contained in the sea spray.
Tafoni can also be a result of saltwater-weathering. Saltwater-weathering is similar to frost-damage. The only difference between those two are salt crystals which are the reason for the damage at saltwater-weathering and at the frost-damage it is ice.
The salt water penetrates the rock. Then the sun's heat rapidly evaporate off the water and the salt contained in the rock crystallizes inside and on the surface, pushing up the grains of the sandstone which subsequently are blown away by the wind.
Due to this chemical and mechanical process the small cavities get bigger and merge together while the more resistant parts of the rock form the thin walls, forming an intricate and petrified network which covers the exposed surface of the rock.

Tafoni is limited by the speed of evaporation and the sun. Once the cavities are large enough so that the sun can not evaporate all the water inside the gap, its growth stops, since the salt can not dry.
These cavernous weathering features include tiny pits (even microscopic at times), softball-sized cavities, and even truck-sized caves. Sometimes smaller tafoni develop inside larger ones, and these smaller ones found inside other ones are called nested tafoni. Tafoni typically develop on inclined or vertical surfaces and occur in groups.

Erosion

is the general name for those processes that break down rocks (weathering) and the processes that carry away the breakdown products (transportation).
The physical processes of erosion are called corrasion or mechanical erosion; the chemical processes are called corrosion or chemical erosion. But most examples of erosion include both corrasion and corrosion.
The three main agents of erosion are water (eluvian erosion), wind (eolian erosion) and ice (glacial erosion).

Weathering Processes

Weathering defines the set of physical, chemical, and/or biological processes which decay and break rock down into smaller pieces. Weathering processes can act independently and in concert as well as at different scales of observation. Erosion is the transport of the weathered debris, often by wind and water, away from the cavity interior. Particularly in arid regions where wettings are infrequent, regolith, soil, and weathering products cover cavern floors

Biological Weathering.

By secreting organic acids in the shape of a halo, colonies of microorganisms chemically weather biotite. This biochemical process leaves behind an ellipsoidal area, depleted in ferric oxide cement, which is more readily weathered and eroded. While it might be possible that “scattered colonies of microorganisms cause the development of alveolar cavities, it is probably more likely that larger organisms such as pholads create borings that are the nucleation pits of some tafoni.

Physical Weathering.

Physical, or mechanical, cavernous weathering processes generally refer to the grain-by-grain destruction of a rock (also known as granular disintegration). Physical weathering is invoked by researchers more often than biological or chemical weathering when describing creative tafoni forces. Like granular disintegration, delamination of thin flakes of rock (also known as scaling and negative exfoliation) is augmented by salt crystallization pressures (salt weathering) as well as by hydration pressures caused by mineral swelling.

Chemical Weathering.

Chemical weathering processes associated with tafoni development tend to include hydration (e.g., anhydrite to gypsum), hydrolysis, and the exchange of cations between mineral solutions and the host rock. Sources of moisture promoting tafoni formation can be from saline-rich sea spray and splash, condensation from the air, melted snow, groundwater, and rainwater. The decomposition of feldspar to clay minerals, the chemical etching of quartz by salt, and the detachment of iron from biotite appear particularly important in the chemical weathering of tafoni.

Climate Aspects.

Large and small tafoni may differ because a large tafone is voluminous enough to create a specific microclimate with “sufficiently” humid conditions to differentially weather the interior. While the weathering processes may be similar such as the growth pressure of crystallizing salt, flaking, etc., the microclimate inside a large tafone (that is, the temperature and moisture content in the air in a given 24 hour period) likely differs from smaller tafoni, which might not be able support a more constant temperature and humidity profile. Honeycombs and small tafoni might be too small to support microclimates; instead, the brevity of drying period in wetter (coastal) setting may weather cavities rapidly, preventing them from developing into large, decimeter- to meter-scale tafoni.

There are many types of tafoni, for example:

Sidewall tafoni: Tafoni on the steep sides of cliff faces, boulders, or outcrops.


Basal tafoni: Tafoni that are form at the base of outcrops and boulders and on the undersides of outcrops and exfoliation sheets.

Nested tafoni: Cavities that occur inside one another in a fractal-like manner.
 

Honeycomb: In Livorno, Italy, locals know cell-like tafoni as sassoscritto, or stone writing.

Relic tafoni: A tafone no longer actively enlarging is a relic tafoni.

Sources:

https://bahiker.com/pictures/southbay/castlerock/121399/websize/64tafoni12.13.99.jpg
https://www4.ac-nancy-metz.fr/base-geol/fiche.php?dossier=183&p=5annexe1
https://en.wikipedia.org/wiki/Tafoni
http://www.tafoni.com/Welcome.html
www.thoughtco.com

To log this Eartcache, go to the given coordinates, answer the following questions and send them to my account:
 

Choose one of the rocks near the given coordinates and answer the following tasks.

1. What kind of Tafoni can you see? Describe the shape and justify your observation?
   If you look at the amount of Tafoni, would you say process has started long time ago?

2. Measure the average size of the small “caves” and how deep are they? (Measure or Estimate)

3. How do you think these particular tafoni were formed, given the rock they are in, the surrounding environment, etc?

4. Can you find any salt inside the Tafoni
   
   Additionally

5. Please make a picture with your GPS, Post-It with your name on it, personal Geocoin or Travel-Bug or even with your self, but please, do not spoil the answers and did not show the "special geological formation" of the other nearby earthcache.