What Are They?
Tafoni are ellipsoidal, pan- to bowl-shaped, natural rock cavities. 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 exquisite and fascinating cavernous weathering landforms are present on the surfaces of many different kinds of rocks located in a multitude of geographic regions around the world.
Tafoni are reported on most often on medium to coarse-grained, granular, silicate rocks like granite, granodiorite, gneiss, and sandstone.
The shape of cavity openings tends to be ellipsoidal, elongated, and parallel to the ground in both desert and coastal settings.
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
a) 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.
b) Chemical Weathering. Chemical weathering processes associated with tafoni development tend to include hydration ,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.
c) 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.
A perplexing property of the tafoni pattern is that uniform, seemingly homogenous rocks develop tafoni. Blackwelder (1929) discovered that tafoni form in rocks with no apparent structural or compositional irregularities like lithologic variations, fractures, or bedding planes. He suggested tafoni develop due to differential moistening of the rock surface and the creation of microclimatic conditions inside cavities, which cause hydration of feldspar minerals and subsequent exfoliation and removal by wind and rain.
a)Positive feedback mechanisms. Tafoni interiors and the microclimates they afford promote positive feedback cycles and differential weathering of the rock. When air enters a shaded tafone and cools, it condenses moisture onto cavity walls, thereby promoting chemical and physical weathering and creating a larger surface area on which more moisture can condense, further enhancing differential weathering feedback processes.
b)Upwards and backwards. Backwalls and roofs weather more readily because such sheltered areas are cooler and the rock surface remains in contact with the moisture for longer periods, thereby perpetuating positive feedback mechanisms. Consequently, tafoni grow “upwards and backwards” and create visors or broad overhanging margins on larger tafoni.
c)Negative feedback mechanisms. An example of a negative feedback mechanism slowing or preventing weathering is the interior of cavity walls that are reinforced with organisms or secondary mineral precipitates.
d)Climate. 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.
How can you log the cache?
To claim this cache as found you should send to this cache owner a message answering correctly the following questions:
- How many holes can you see at the top of the rock ? And behind?
- Which are the weathering processes that gave rise to this tafoni?
If you believe you have successfully completed this Earth Cache goals and has already sent to me all the requirements as requested, Please, feel free to log it as found. Later i will verify the requirements sent and, if necessary, contact you in order to make the necessary corrections to your log.