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1) How many holes can be counted at the rock?
2) What are we looking here? It is a taffoni or a pit? Explain why
3) What do you think, will It continue to grow?
4) Add a photo to your local, another one that you can see an object, or one nick in a piece of paper
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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
Rock basins are depressions formed in solid bedrock (Twidale and Corbin, 1963). Morpho-logically they vary in detail, but most are oval, elliptical or circular in plan. Some, strongly influenced by jointing, are angular in form, and others, resulting from the coalescence of two or more individuals, are irregularly lobate in plan. All are temporary water storages. Water accumulated from rainfall and runoff is lost either through evaporation, or use by animals (including humans), or by underground Rock basins are depressions formed in solid bedrock (Twidale and Corbin, 1963). Morpho-logically they vary in detail, but most are oval, elliptical or circular in plan. Some, strongly influenced by jointing, are angular in form, and others, resulting from the coalescence of two or more individuals, are irregularly lobate in plan. All are temporary water storages. Water accumulated from rainfall and runoff is lost either through evaporation, or use by animals (including humans), or by underground seepage. Alternatively, if supply exceeds storage and losses, the water in the basin overflows, cutting an outlet so that the depression eventually becomes part of an integrated drainage system.
1- Minor depressions and bowls develop from surface irregularity and/or lithologic weaknesses (i.e., jointing,xenoliths, textural change).
2-Depressions enlarge and deepen, commonly seasonally ﬁlling with water.
3-Bowls develop vertical walls resulting in pan-forms. Spillways are incipient. Seasonal precipitation removes accumulating silt, sand, and some pebbles.
4-Lateral growth rate increases and deepening decreases.Bowls enlarge laterally and bottoms ﬂatten to produce broader pans. Surface skins and coatings may develop to facilitate lipped and ﬂask-like edges.Weathering byproducts accumulate.
5-Lateral growth rate decreases. Adjacent pits and pans may coalesce into larger gnammas. Spillways enlarge with deﬁned channels. Rock coatings may develop into case-hardening that promotes the development of lips, visors, and ﬂask-shaped proﬁles. Sediment inﬁlling begins with weathering by-products (i.e., grus) or through aeolian contributions of silt and sand.
6-Channel and spillways link adjacent gnammas. Sediment accumulation supports plant growth (grass, moss, sedge, small plants) to colonize the depression. Once sediment and plant-ﬁlled, they act as sumps and/or as reservoirs for adjacent downslope gnammas.
-Rock basins have developed on scree blocks in some areas suggesting that the forms evolve quite rapidly in geological terms - in a matter of a few thousands of years.
Several morphological types have been recognised :
- pits are hemispherical and developed on gentle slopes;
- pans are comparatively shallow, are flat floored, and are also developed on gentle slopes;
- cylindrical hollows vary in plan shape, though they are generally circular and are rectangular in vertical section so that they are appropriately referred to as being of cylindrical form;
- armchair-shaped hollows are asymmetrical in section normal to the contours, having high backwalls on the upslope side, but being open downslope. They are typical of the moderately steep (20 -30 ) slopes that lead down from the flattish crestal areas to the steep bounding slopes and the plains.