4.7 Tafoni and Other Rock Basins

4.7 Tafoni and Other Rock Basins TR Paradise, University of Arkansas, Fayetteville, AR, USA r 2013 Elsevier Inc. All rights reserved.

4.7.1 4.7.1.1 4.7.1.2 4.7.1.3 4.7.2 4.7.2.1 4.7.3 4.7.3.1 4.7.4 4.7.5 4.7.5.1 4.7.5.2 4.7.5.3 4.7.5.4 4.7.5.5 4.7.6 References

Introduction Tafoni Gnamma Climatic and Geographic Influences Morphological Classification and Rate of Development Tafoni Stages of Tafone Development Gnammas Stages of Gnamma Progression Processes of Development Lithologic Influences Environmental Influences and Salinity Biotic Influences Climate and Insolation Feedback Cycles Summary

Glossary Alveolar weathering (singular alveolae, alveolus) Describes tafoni occurring typically on vertical surfaces and rarely larger than 3–5 cm in individual cavity diameter. ‘Alveolus’ come from the term ‘little cavity’ in Latin. Feedback cycle Commonly used to describe tafone and gnamma incipience and development, when the product from a process impedes its own process as negative feedback, or increases its own process as positive feedback. Gnamma From an Aboriginal word that derives from the Western Australia, gnammas are stone basins, cavities, or rock-holes generally found in on nearly horizontal surfaces (0–151), commonly in sandstone or granite. These basins are formed by weathering and are commonly narrow at the opening and wider at the bottom, water generally collects seasonally in these stone basins. Subcategories of gnammas can include pits, pans, bowls, canoe, armchair, flask-shaped, and paternoster gnammas. Also called ‘tinajas,’ ‘tinajitas,’ ‘cuencas’ (Spanish), ‘Opferkessel,’ ‘Baumverfallspingen,’ ‘Steingrube’ (German), ‘kociolki’ (Polish), ‘conche rocciose’ (Italian), ‘oric- angas’ (Portuguese), and ‘caldeiraos, poc- os’ (Brazilian Portuguese).

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Honeycomb weathering A subcategory of tafoni that represent small (o2 cm), regular and commonly patterned cavities found in granular rock (i.e., granite, sandstone). Stonelace A subcategory of tafoni that is commonly synonymous with honeycomb weathering. Tafoni (singular tafone) Small (o1 cm) to large (41 m) cave-like features generally occurring in granular rock (i.e., granite, sandstone) with smooth concave cavities, and often round rims and openings. Subcategories of tafoni can include honeycomb, stonelace, alveolar (o2 cm), sidewall, basal, nested, and relic. Also called ‘nido d’ape roccioso’ (Italian). Weathering Is the breakdown of rocks, minerals, and soils through contact with the climatic influences (i.e., insolation), technological influences (i.e., pollution) and/or humans (i.e., touch). Weathering occurs in situ and should not be confused with erosion, whereby the influences such as water, ice, wind, and gravity degrade through/by movement. Conventional classifications include: (1) mechanical or physical weathering (breakdown through direct contact with atmospheric conditions such as heat, ice); and (2) chemical weathering (effect of biologicallyproduced and/or atmospheric chemicals).

Paradise, T.R., 2013. Tafoni and other rock basins. In: Shroder, J. (Editor in Chief), Pope, G.A., (Ed.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 4, Weathering and Soils Geomorphology, pp. 111–126.

Treatise on Geomorphology, Volume 4

http://dx.doi.org/10.1016/B978-0-12-374739-6.00068-3

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Tafoni and Other Rock Basins

Abstract Tafoni and gnamma are cavernous weathering phenomena that have been extensively surveyed, discussed, and studied. From delicate, polygonal cells (2–4 mm) to huge circular pits (30 m þ ), they develop in a variety of rock substrates, commonly in sandstone (tafoni) and granite (gnamma). Early research on their initiation and development was divisive, but current studies indicate polygenetic, differential weathering from intrinsic factors including hydrolysis, hydration of lithologic constituents, to extrinsic influences like moisture availability, insolation, and salinity. Increasingly, it is believed that complex, feedback cycles are responsible for their sigmoidal rates of development.

4.7.1

Introduction

One of the most puzzling traits of honeycomb or panshaped weathering features is that they can develop in seemingly homogenous rocks, in one portion or face, and not another. So why does an intricate lacework pattern form on one cliff face or boulder, and not on the one adjacent? Are the processes responsible for their developed unknown, or are we slowly beginning to understand their complex yet beautiful nature? Studies have investigated the primary causes and influences for more than 100 years, but we are only now beginning to comprehend the integrated, coordinated, and feedback systems involved with their initiation, development, and progression. Contemporary research emphasizes the role of microclimatic conditions, salinity, and environmental factors in tandem with feedback mechanisms on their development (Hejl, 2005), but controversy still exists as to the polygenetic nature of these generally unique features called tafoni, gnammas, armchair hollows, alveoli, solution pits, or honeycomb weathering. Tafoni and gnammas are lace-like, honeycomb, bowl, or pan-shaped cavities occurring in a variety of rock types and locations that show a commonly unique assemblage and morphology. These oddly shaped, generally intricately created, naturally occurring weathering and erosional features occur in a wide range of lithologies including sandstone, limestone, granite, greywacke, rhyolite, quartzite, conglomerate, and tuff. They have been documented across diverse environments in deserts and along coasts, and from assorted climatic zones that include the mountainous, coastal, and arid regions of all continents, Antarctica, and even on Mars (Rodriguez-Navarro, 1998). These recessional and erosional features come in various forms and with assorted names. ‘Tafoni,’ or singular ‘tafone,’ commonly occur on vertical faces, whereas ‘gnammas’ are weathering and erosional depressions that develop on relatively horizontal solid rock substrates, with a continuum of weathering feature morphology from steep to flat settings. Tafoni and gnammas are natural rock cavities that typically take on an ellipsoidal, circular, oblate, or polygonal outer edges, generally bowl, honeycomb, or pan-like in form. They have been recorded as small as tiny pits, cells, or voids, to fistsize, bowl-size, or as large as an automobile or small house. Blackwelder (1929) described the largest as ‘shelter for horse and rider.’ They form from as delicate lacey features measured in millimeters, to the renown tafone cavities of the ‘Remarkable Rocks’ of Kangaroo Island, Flinders Chase National Park in Australia where these famous granite cavities measure up to 20 m across (Figure 1).

4.7.1.1

Tafoni

The etymology of the word ‘tafoni’ is obscure; however, its use across the Mediterranean implies that it may derive from the Italian dialectic verb ‘tafonare,’ ‘to perforate.’ It has become a colloquial term for ‘windows’ across the western Mediterranean Basin, especially on Corsica (Boxerman, 2006), Sicily, and Sardinia. However, the term ‘tafone’ is commonly synonymous in Italy with nido d’ape roccioso (rocky honeycomb), whereas in English it can be synonymous with a number of terms such as honeycomb, stone lace, alveoli, fretting, and stone lattice. Although the term ‘tafoni’ has not been specifically assigned to features occurring only on vertical faces, similarly appearing recessional forms on horizontal surfaces are most commonly called ‘gnamma,’ an Australian aboriginal word that derives from the Western Desert languages (i.e., Nyungar) which were/are spoken over a huge area of Western Australia. The Western Desert people use ‘gnamma’ to refer to rock-holes occurring in sandstone or granite, sometimes containing water. Described as basins formed by weathering that are commonly narrow at the opening and wider at the bottom, water generally collects seasonally in these stone basins (Moore, 1842). Honeycomb features were described by Strabo 2000 years ago (22AD), and then commonly in the reports of nineteenth century western explorers including Stephens (1837) and Burton (1879). However, the term ‘tafoni’ was first specifically used by De Prado (1864) to describe the unusual weathered

Figure 1 Photograph of extraordinary tafoni development on Kangaroo Island, Southern Australia. These ‘Remarkable Rocks’ are located in Flinders Chase National Park and have developed atop granite corestones resting atop a small, partially exposed granite dome. These boulders with tafoni have become a popular tourist destination on the western side of the Island. The largest tafone in this image measures 3  4 m, at an aspect of B310 1N.

Tafoni and Other Rock Basins

Figure 2 Sketch by De Prado (1864) of tafone in granite near Madrid. This is the first recorded reference to and illustration of tafone/tafoni in western research.

forms occurring in Spanish granites (Figure 2), and later by Reusch (1883) and Penck (1884) regarding Corsica’s unusual weathering features. Since the nineteenth century, more than 100 articles and reports alone have been published in the western literature on these often ubiquitous, yet interesting features (Hejl, 2005), and they continue to foment interest in curious tourists and researchers alike. In the western literature, the earliest reference to honeycomb weathering and their possible development is commonly attributed to Darwin (1839) with his discussion of weathering features during his voyages on the HMS Beagle. While sailing the coast of Western Australia, he described features not unlike tafoni, which appeared to be carbonate casts of tree roots. After the global instability of the First World War (WWI), exploration across the planet’s arid regions expanded, prompting elaborate travel journals and new research directions and studies. Tafoni research increased and so did the speculation as to their formation. Sir Charles Cotton’s notable work on the geomorphology of New Zealand is one of the first to document tafone development, in addition to postulations on their initiation and morphological evolution. He specifically attributed the formation of tafone and alveoli to lithologic variation. Cotton (1922) cited the example of limonite-rich seams that weathered at a slower rate than the surrounding sandstone, which created polygonal cavities that developed into tafoni. In his work on the Giza Plateau of Egypt, Hume (1935) speculated that sandstone deterioration and the formation of tafone were products of salt mobilization from external sources, and within the rock material itself. This was an innovative concept for the time and guides research to this day. Bartrum (1936) followed investigating the role of carbonate dissolution in less arid landscapes like New Zealand. Popoff and Kvelberg (1937) would then hypothesize about the importance of microclimatic effects and moisture mobility on the famous tafone of Corsica.

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However, many researchers consider the early works and related controversy of Bryan and Blackwelder as the fundamental watershed in tafoni research. Bryan’s examination of the Chaco Canyon sandstones (1928) led to his hypotheses on the crucial nature of moisture and rock permeability as the primary influence on the development of tafone. However, Blackwelder (1929) explained that the role of wind cannot be discounted in their development, arguing that Bryan’s work was site-specific and too narrow in his explanation of honeycomb weathering. Although Bryan stressed the occurrence of these cavities near ephemeral waterfalls, weeps and sapping indicated the role of moisture in ‘dissolving mineral matter from the sandstone cement,’ Blackwelder was quick to explain that moisture and mineral alteration was influential, whereas wind was the essential element needed to remove the by-products of weathering. He emphasized the ‘cooperative effect of wind’ not as an abrasive agent, but as an erosional force. This debate still haunts tafoni research today – is their development due to primary influences like wind or lithology, or is it a complex and polygenetic interaction of intrinsic and extrinsic controls on tafoni initiation and development?

4.7.1.2

Gnamma

In the comprehensive Descriptive Vocabulary of Aboriginal Words (Moore, 1842), the earliest reference is to ‘gnamar,’ or a ‘hole or pool of water in a rock.’ Later Austin (1856) described how his Aborigine guide led him to a water-hole in a parched area of Western Australia. He stated that the guide ‘‘depended upon the precarious supply of rainwater accumulated in the hollows of the rocks.’’ Calvert (1897) and Carnegie (1898) later coined the term ‘namma-hole’ while touring and reporting on their journeys across Australia’s outback. Gnammas are rock basins or cavities that generaly occur on nearly horizontal surfaces (0–151) in exposed granite landscapes where they can be common and well developed. Gnammas can occur atop other substrates like sandstone or greywacke, where the rock materials are relatively impermeable like granite. They can measure mere centimeters in width and depth, to as large as 5 18 m and to a depth of 2 m. Twidale and Corbin (1963) cited examples, however, of exceptionally deep gnammas at 6 m, whereas Netoff and Shroba (2001) later discovered huge basins (tanks) in the sandstones of the American Southwest that measured 16 m in depth and up to 70 m in diameter. Gnamma morphology is typically a round or oval depression atop a planar surface, generally wider than deeper. They also follow a rough trend from small and shallow to large and deep: circular and bowl-shaped at first termed ‘pits,’ to elliptical and flat-bottomed (generally sedimentfilled) as larger and older, called ‘pans.’ As gnammas develop on, or enlarge toward steeper slopes (415–201), they enlarge in dimension and often evolve into ‘armchair hollows,’ where the uphill depth is greatest, and the downslope depths decrease to essentially no rim, edge, or lip (Paradise and Yin, 1993). Gnammas also act as reservoirs for water from precipitation so when it is full, the water overflows at the edge with

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the lowest rim. This repeated drainage erodes a channel or spillway often characteristic of advanced morphology. In fact, spillways that connect gnamma to gnamma are distinctive of a constellation representing ‘mature’ gnammas, and gnamma landscapes in advanced stages. Gnamma enlargement has been attributed to the seasonal filling of water; however, a pit with water is not necessarily a gnamma. Their evolution and development morphologies indicate that there is indeed an ordered progression sequence, pointing toward forms and development that are not arbitrary. This has been repeatedly corroborated through gnamma morphometric analysis (GMA) where developmental relationships between width and depth indicate a coordinated morphological progression (Dominguez-Villar, 2006). Gnammas have been widely described in English and other languages and include tinajas, tinajitas or cuencas (Spanish: small tub, bowl, or basin), Opferkessel, Baumverfallspingen, or Steingrube (German: sacrificial cauldron, stone pit or hollow), kociolki (Polish), conche rocciose (Italian: rock basins), oric- angas (Portuguese), caldeiraos or poc- os (Brazilian Portuguese: cauldrons, rock basins), weathering pans, weathering pits, solution pits, cups, or armchair hollows. Unlike tafoni which develop on permeable substrates that enable moisture mobility (i.e., sandstone, limestone), gnammas are generally horizontal (or close) and develop on mostly impermeable substrates (i.e., granite). In his notable work in Yosemite National Park, California, Matthes (1930) coined the terms ‘weathering pit’ and ‘weather pit’ – terms that remain both in the scholastic studies and travel books today. In the western literature, research on gnamma morphology and development in granite has been dominated by Australian studies of Twidale and others (Twidale and Corbin, 1963; Vidal Romani and Twidale, 1998); however, in the past century other geomorphologists have continued to investigate these unusual rock pans, pits, and bowls in granite and other rock substrates in Australia and abroad. In the Americas, research intensified – Caldenius (1932) wrote extensively about gnamma development on Patagonian granites and Matthes (1930) wrote about their forms, distribution, and development on the exposed granite of the Sierra Nevada Mountains. Friese (1938) speculated as to the many influences on the progression of gnammas in Brazil. Investigations into their morphology and development leapt in number and depth after WWII with a number of important studies in the 1960s (i.e., Dahl, 1966). These studies have led to works today that link morphology, and morphology process. Gnammas have also been used as relative dating tools. By exploring pit widths, depths, and/or depths, research has utilized their morphology and distribution to date surface exposure after ice sheet retreat, or landscape change (i.e., Matthes, 1930; Dahl, 1966; Ives, 1978; Landvik, 1994). These same researchers have also noted the dangers associated with using gnammas in this manner; however, their use has led to interesting speculation and continued research on glacial retreat, weathering rates, and landscape denudation. This chapter will cover tafoni and gnammas, and their morphology, development, substrates, and rate of development, as well as the range of influences. The discussion will include salt-induced weathering, microclimatic effects, lithologic influences, positive and negative feedback cycles, and the

role of geographic controls in the development of some of nature’s most intriguing weathering phenomena. However, over the past century, an absence of consistent and standardized nomenclature has led to some confusion as what are tafoni, gnammas, honeycomb, or alveolar weathering features? In field studies, are gnammas being discussed, tafoni being studied or both, in the same study, or at the same site? In both Bryan’s (1928) and Blackwelder’s (1929) seminal works, although the terms ‘niches’ and ‘cavernous’ features were used, descriptions and images indicate that both tafone and gnamma morphologies were being addressed. In the work of Sancho and Benito (1990) on sandstones in central Spain, although the term tafoni was preferred, images show a range of morphologies and locations of tafone, cavernous weathering, armchair hollows, and gnammas, developing as a function of slope – vertical slopes exhibited tafoni, whereas on horizontal surfaces, gnammas were prevalent. This confusion in nomenclature and overlap in terms will continue to haunt this fascinating area in geomorphological and stone conservation research.

4.7.1.3

Climatic and Geographic Influences

It has been estimated that tafone development is so extensive along the Earth’s coasts that they account for more than 10% of all shoreline retreat (Gill et al., 1981; see Chapter 4.13). On a global dimension and within human timescales, weathering and erosional processes are responsible for the slow destruction of stone structures and monuments; however, most of these processes are often invisible within each generation of observation. Over time, nevertheless, tafone and gnamma development and subsequent surface recession affect landscapes differently – deserts at a slower rate than coasts. These phenomena, however, are undeniably responsible for extensive surface recession and subaerial denudation to an enormous degree. Although tafoni and gnammas have been recorded and studied across the planet (and Mars), distinctive relationships have been found between extrinsic factors including climate and environmental conditions, and intrinsic controls like lithologic structure, constituency, texture, and integrity (fresh vs. weathered). However, their geographic distributions vary widely and between themselves. They have been recorded at sea level and at above 2000 m in the Mountains of Corsica, and Patagonia, and above 2700 m in the Sierra Nevada of California. However diverse are the landscapes of tafone and gnamma occurrence, there is a broad consensus that their development is more prevalent in temperate coastal environments and more hot and/or cold deserts. Tafoni have been recorded and investigated in temperate regions that include the coasts of Africa, America, Asia, Australia, and Europe. The humid regions include Hong Kong, the U.S. Midwest, and the Mediterranean Basin, arid, warm landscapes in the American Southwest, North Africa and Australia, and the cold deserts of Antarctica, Finland, Iceland, China, and Mars. Similarly, gnammas have been recorded across the globe and at varying latitudes and climatic influences, from sea level to the summits of mountains and bornhardts. However influential is climate, the control of

Tafoni and Other Rock Basins

climate has been found to be minimal when compared to the substrate type, permeability, and surface conditions (i.e., varnish, glaze, crust, vegetative mats) (Figures 3 and 4). Geographically, tafoni most commonly occur in coastal environments and arid landscapes in permeable or quasipermeable rock substrates, whereas gnammas commonly occur in more diverse climatic zones, atop or flanking crystalline rock substrates like granite or granitoid schists, although the occurrence of gnammas has been recorded in

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igneous (extrusive, intrusive), sedimentary (clastic, chemical), and metamorphic substrates. In fact, some of the largest ‘weathering pits’ were documented in the Entrada sandstones of Utah (USA), measuring 3–70 m in diameter to 2–16 m in depth (Netoff and Shroba, 2001). Climate effects, like wind and environmental influences (i.e., salinity), are generally crucial in the development of tafoni. Although lithology and substrate composition commonly determine the location, distribution, and morphology

Figure 3 Map of recorded tafone locations from western research in English, French, German, and Italian languages. These sites have been compiled from research dating from the 1840s. Some neighboring sites have been identified with one marker only.

Figure 4 Map of recorded gnamma locations from western research in English, French, German, and Italian languages. These sites have been compiled from research dating from the 1840s. Some neighboring sites have been identified with one marker only.

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of gnamma as they are generally found in outcrops of hard, impermeable rock, particularly granite, and along the flanks and tops of monadnocks and inselbergs (Twidale and Corbin, 1963). Matthes (1930) noted the size and volume of weather pits increased toward the peaks of granite domes in Yosemite National Park, causing speculation as to whether this was a function of slope, exposure, temperature change, or another environmental factor. Paradise and Yin (1993) noted that their distribution was more a function of aspect, heating and cooling cycles, and moisture availability. Tafoni and gnammas both develop best in conditions where an ideal balance between wetting and drying exists. Too much water or too little moisture can arrest their progression, which controls their size, shape, and distribution. Research has indicated that it is the availability of moisture, and it is distribution that is paramount to the development of tafoni and gnammas (Rodriguez-Navarro et al., 1999; Huinink et al., 2004). Field observations and laboratory simulations point to the importance of the length of episodes between wetting and drying cycles in tafoni. In coastal environments where moisture is more evenly distributed, tafoni extents tend to be larger, whereas the individual cavities are smaller (Boxerman, 2006). Although in arid regions where the moisture is restricted to shaded alcoves and northern faces, tafoni may develop into larger cavities, however occupying a smaller extent. In near-shore areas where the rock is continually wetted or moist (diurnal cycles), the substrate never or rarely dehydrates completely; few salts or none can accumulate. Hence, salt is evenly distributed across the surface as a function of sea spray. This causes a relatively even development and distribution of relatively uniformly sized cavities and ribs in coastal settings where salt is more prevalent in comparison to arid environments. In arid regions like the U.S. Southwest or the Levant, periods of moisture availability occur from seasonal events more often than from diurnal variation. These cycles of seasonal precipitation are longer in duration, with lengthier episodes of drying between wetting and/or saturation. These dry periods are measured in weeks and months (arid) in contrast to hours and days (coastal). These longer dry periods in deserts allow for greater accumulations of salts inside the tafone cells and cavities. This enables positive feedback cycles to proliferate in arid landscapes where the sheltered cavities accumulate salts, deteriorate faster to create larger cavities, increasing the cavity volume. More salts accumulate in the larger cavities, accelerating in-cavity weathering to propagate the cycle. In the broadest sense, near-shore settings facilitate smaller and more uniform cells and ribs in tafoni, whereas hot and cold arid environments sustain the development of larger cells and ribs within a grouping of more irregularly size cavities. Similarly, hyper-arid landscapes do not facilitate the growth of gnammas, since moisture (and related heating and drying cycles) are needed in the initiation and development of bowls and pans. However, the occurrence of gnammas in humid and/or tropical climates indicates that their progression is more controlled by lithology than climate. Largely, it may be said that tafoni growth is primarily influenced by extrinsic factors, whereas gnamma development is affected by both intrinsic and extrinsic influences.

4.7.2

Morphological Classification and Rate of Development

Tafoni and gnammas evolve in distinct stages, from tiny cavities and depressions to large caves and pits over decades, centuries, and millennia. It is this relatively long period of development within human time scales that makes it difficult for geomorphologists, stone conservators, geologists, and geographers to effectively explain the rate of change, and the occurrence of similarly shaped niches, bowls, and pans on varied rock substrates, and in diverse environmental settings.

4.7.2.1

Tafoni

Tafoni, rock niches, or cavernous weathering can take on a number of forms and configurations that have been discussed and coined over the last 150 years. Some tafoni have been grouped by locality (near-shore), position (cliff base), size (huge, remarkable), shape (geometric, lacey), or having a unique feature (human face, eagle). Various terms have been collected from research conducted over the last 150 years: ‘Honeycomb’ is possibly the most common synonym or alternate term for small-cavity or cell tafoni and is often considered a subclass of tafoni. It has been interchanged with ‘alveolar weathering,’ implying a cell-like size, arrangement (o2 cm), and configuration of cavities. Smaller honeycomb features may be called lacework, stonelace, fretwork, or fretting. Some studies have identified honeycomb at centimeter scales or smaller, and tafoni at centimeter scales or larger (Viles, 2001). ‘Sidewall tafoni’ describe their occurrence on the steep faces of boulders, outcrops, and cliffs, whereas ‘basal tafoni’ specifically explain their development below steep faces as a function of moisture accumulation and wicking, environmental factors (i.e., sea spray, wind), and/or biotic instigation (i.e., lichens, insects). ‘Nested tafone’ is used to identify features that have developed within the cells, cavities, and walls of other tafone. This nesting indicates that the tafoni are redeveloping within already developed cavities and represent a later stage in tafone development. Nested tafoni may also be ‘relic tafoni’ where the existing cells, cavities, and walls are no longer actively enlarging, receding, or weathering. The presence of lichens, mosses, cyanolichens, or other coatings indicates either a stable surface on which they may propagate, or a surface with arrested weathering due to attachment and growth of biotic coatings. Lichen attachment can both indurate and weather the rock substrate. ‘Iconic tafoni’ describe tafoni that have developed into forms or shapes that resemble something else. These tafoni can bear a resemblance to an animal’s head, face, mushrooms, structures, and writing, Tafoni evolve in five distinctive stages that are categorized through changes in the cavity shapes, width–depth ratios, and height–depth ratios. First, small pits and cavities develop. Then the cavities and cells widen, deepen, and enlarge. The cells begin to develop orthogonal walls and bases. Then the cells enlarge, the walls thin, and the bases (backwalls) of the cavities flatten. Finally, as the cavities enlarge, some of the walls may collapse and/or breach, causing cavities to merge creating larger cavities. Pitting may initiate in existing cavities

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Stone 1

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Figure 5 Diagram of rock profiles illustrating the evolution of tafone morphology. Development is affected by complex intrinsic and extrinsic influences and negative and positive feedback cycles. (1, 2) Small depressions develop and cavities broaden and deepen into (3, 4) distinctive honeycomb or lace-like features consisting of a cell-and-wall structure. Development continues as the cell bases flatten, walls narrow, and the cavities deepen. (4, 5) Cavities merge as walls weather and/or break. Tafone can range in size from o1 cm to 410 m. Reproduced from Goudie, A.S., Migon, P., 1997. Weathering pits in the Spitzkoppe area, Cental Namib Desert. Zeitschrift fu¨r Geomorphologie 41, 417–444, and Boxerman, J. Z., 2006. The evolution of tafoni on coastal sandstones in northern California. Master’s Thesis, Department of Geosciences, San Francisco State University, unpublished.

to progress again. The tafoni evolve from cavity initiation and development, to enlargement, to coalescence, and sometimes back to cavity initiation (Figure 5).

4.7.3

Stages of Tafone Development

1. Small depressions and pits develop due to lithologic weakness or irregularity (i.e., permeability, porosity, bedding plane, crack, or joint). Dimensions smaller than 15–20 mm (cells o2 cm, cavities 42 cm). Rough walls and ridges develop. 2. Pits and depressions enlarge and deepen, and cells and cavities develop. Cavities round out in form (420–30 mm). Ribs and walls become defined and regular. Weathering byproducts may accumulate and/or be removed. 3. Cavity backwalls, rib and walls and wall intersections develop a more orthogonal and geometric configuration from rounded form. Walls begin to thin to similar dimensions. Coatings, skins, and surface rinds may develop on the original surface. Backwalls and cavity ‘roofs’ enlarge at a faster rate than cell and cavity floors.

4. Walls between cavities and cells begin to breach and/or collapse as walls near 2–5 mm in width, due to the enlargement of each cavity. Rock coatings may develop into case-hardening that promotes the growth of lips, flares, hoods, and visors. Weathering by-products accumulate. Overall deepening and enlargement continue until walls are fully breached. 5. Walls erode into recessed ribs within the cavity voids. Many cavities and cells coalesce to create relatively smooth, void surfaces. Pitting may re-initiate to renew the development progression. Deepening rates decrease, and breakdown rate of wall remnants and portions increase. New depression initiation may commence within existing cavities creating a nested arrangement and orientation for tafoni. Some researchers have speculated that tafone walls and ribs regularly thin to 4–8 mm since that is the thinnest width in which biotic overgrowth (i.e., lichen, algae, bacteria) is able to stabilize the wall by connecting through the permeable substrate of the walls (Boxerman, 2006). This may represent the thinnest width or depth at which the thalli or colonies on

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opposite sides of the cavity wall can connect into single colonies to stabilize the wall, and arrest recession. Tafoni develop at rates controlled by climate, lithology, environmental factors, and anthropogenic factors. Climate controls moisture availability, wetting–drying and freeze-thaw cycles, and the intervals of desiccation between periods of saturation. Lithology can affect variability in permeability and porosity that controls moisture mobility and salt accumulation. Environmental influences can include tree shading or removal, covering rockfalls or sand ramps, or groundwater fluctuations (either natural or induced) affecting variable wicking, and moisture transport. Prior conservation efforts, pollution, and human-induced damage can affect rates of development by modifying permeability or rock integrity. However, climate has a significant effect on rates of tafone development. It has been found that weathering increases when evaporation rates are slow and the period of desiccation is lengthy. In arid settings, since the drying episodes are longer between periods of wetting and saturation, cavity and cell deepening may be slower overall, although dry, hot climates foment deterioration at rates up to two to four orders of magnitude faster than in moister environments. This may be attributed to the reason why tafone in arid–hot settings can be so much larger than in coastal environments. However, along coasts, the constant wetting with seawater, cells and cavities develop at some of the fastest rates known; rate of deterioration in millimeters per year are not uncommon (Huinink et al., 2004; Boxerman, 2006). Rates of tafone development are rarely arithmetic in nature, but most commonly logarithmic, sigmoidal, and/or irregular. Under ideal tafone-producing conditions, once the rock substrate is exposed, tafoni do not develop immediately but lag in their pit initiation. Once the depression begins to enlarge, the cavity deepens at a faster rate than it widens. Wall thinning rates have been recorded as slower than back wall deepening. The rate of deepening then decreases as the rate of widening increases, until a uniform cell has developed (Sunamura, 1996). Cell and cavity wall breach and/or collapse commonly indicate a decrease in the rate of widening and deepening, representing a later stage in development. Also, not all rock substrates weather, enlarge, widen, and deepen at the same rates: tafoni tend to develop into the most voluminous cavities in arid, hot settings, and containing the most cells and cavities in near-shore environment. However, part of the wonder and complexity of tafoni is that large cavities occur along coasts, and intricate stonelace and multiple-cell tafoni are common in deserts. The fastest rates for tafone progression have been recorded in coastal settings with up to 4.9 mm yr1 for cavity widening, and 0.1–0.6 mm yr1 for cavity deepening, yet mean rates of development for a 4–5 cm cavity have been recorded to take 100–500 years (Mustoe, 1982; Sunamura, 1996; Pye and Mottershead, 1995). Due to the nature of insolation-induced drying and evaporation, lower walls in cells and cavities weather and enlarge at slower rates than cavity roofs and upper walls where evaporation rates are less. Consequently, for a tafone cell to progress from stage 1 (cell) to stage 2 (cavity), a century may pass for the initiation and development of a 10–14 mm cell with the greatest dimension occurring in the upper portions of each cell. Moreover, to extrapolate

Figure 6 Photograph of complex tafone development in the Moenkopi Formation at Wupatki National Monument, Arizona. The arrangement of ribs and cavities in the tafoni indicates the control of horizontal lithologic structure (bedding planes), in addition to the development stages of cavities-within-cavities-within-cavities representing stages 4–5. The tafoni in this image measure 1.5 m across, at an aspect of 255 1N.

weathering rates in near-shore marine settings, we can hypothesize about tafoni cycles as contributors to coastal retreat (assuming no uplift or major climate change) at 100–600 m of retreat over 100 000 years (Figure 6).

4.7.3.1

Gnammas

Solution pits, weathering pits, pans, and gnammas are terms that have been used to describe these pans and pits. Some gnammas have been grouped by climate (in alpine, desert), location (on bornhardts), topography (on summits, pediments), size (pans, pools), shape (oval, bowl), or having a unique outline (animal shape, geometric). These are various terms that have been collected from research conducted over the last 150 years. ‘Gnamma’ is the term now widely used to describe an oval or circular depression commonly containing water seasonally. ‘Pits, bowls’ are hemispherical in shape, whereas ‘pans’ possess flattened bottoms from increased lateral development over vertical recession. ‘Canoe’ is half of a pit developing against a wall or joint, an elongated modification to a pit, whereas ‘armchairs’ or ‘armchair hollows’ result from pan enlargement on or toward a steeper slope (4201). More unusual terms have been used for less common attributes of gnammas. ‘Flask-shaped hollows’ exhibit distinctive narrow openings to larger chambers. ‘Paternoster gnamma or pits’ is a rare but apt term that describes a series of pits and/or pans visibly connected by spillways and channels. This term is derived from their resemblance to rosary beads. ‘Flares,’ ‘lips,’ ‘hoods’, and ‘visors’ are somewhat synonymous terms that have been used to describe the small ridges or rims that overhang the pan or bowl. These generally occur in mature tafone (stages 4–6) and may be due to case-hardening, coresoftening, and/or differential weathering. ‘Pot-hole’ is an incorrect use that has conventionally applied to a circular depression as a result of stream channel erosion. Gnammas are affected by complex influences, and their development by complicated feedback cycles. They evolve through distinctive stages that are categorized through changes in the pan and bowl shapes, width–depth ratios, height–depth ratios, and sediment infilling. First minor depressions enlarge and deepen, becoming circular or oval in outline through progression. As bowls enlarge and bottoms flatten, spillways

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Stone

Figure 7 Diagram of profiles and photographs illustrating the evolution of gnamma morphometry. Development is affected by complex influences and feedback cycles but the cavities evolve from (1, 2) minor depressions and bowls, (3, 4) into broader and deeper hollows often seasonally filling with water, (5, 6) until enough weathering by-products (i.e., grus, sand) accumulate to act as a plant substrate facilitating grass, moss, sedge, and small plants to colonize the depression. Gnammas can range in size from o10 cm to 412 m. Reproduced from Twidale, C.R., Corbin, E.M., 1963. Gnammas. Revue de Ge´omorphologie dynamique 4, 1–20; Paradise, T.R., Yin, Z.Y., 1993. Weathering pit characteristics and topography, Stone Mountain, Georgia. Physical Geography 14, 68–81.

develop until channels linking adjacent gnammas develop. Lateral growth increases as deepening decreases and erosion of sediments decrease until they accumulate to support plant grown. Once completely sediment-filled overall length, width and depth weathering diminishes (Figure 7).

4.7.4

Stages of Gnamma Progression

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. Gnammas, like tafoni, develop at rates controlled by climate and environmental factors. Climate controls moisture availability both as atmospheric humidity and precipitation, wetting–drying and freeze-thaw cycles, and through the intervals of desiccation between episodes of bowl in-filling. Lithology can affect gnamma initiation due from differential weathering of the original surface (phenocrysts, xenoliths, variability in constituent minerals, density, integrity), and related disparities in permeability and porosity that control water containment, and sediment accumulation. Environmental influences can include surrounding and ingnamma plant growth, spillway obstruction, and/or adjacent or upslope tree growth or removal. Although anthropogenic influences can affect development through aerial and point-source pollution that may exacerbate weathering, and human-induced damage (intentional or accidental) through

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Irregular depressions: often multiple

Merging of depressions

with wth ng o r l g peni e era Lat ht de lig

Large, shallow irregular depressions

Flat-bottom, shallow pits: no sediment

Single pits: sediment-filled with connected channels

Ext dee ensiv pen e ing

tom Bot ing Sumps d l a ra gr Late th w old gro esh Thr nation i elim De epe Integrated nin g depressions

Deep isolated pits: scarce sediments

Figure 8 Diagram illustrating the pathways of gnamma development in granite in the Namib Desert. Reproduced from Goudie, A.S., Migon, P., 1997. Weathering pits in the Spitzkoppe area, Cental Namib Desert. Zeitschrift fu¨r Geomorphologie 41, 417–444.

spillway obstruction, trampling, and abrasion at popular American tourist sites such as Stone Mountain (Georgia), Yosemite National Park (California), and Enchanted Rock (Texas), where visitors are free to wander and walk atop mature and incipient gnammas (Figure 8). Gnammas are not believed to initiate, enlarge, widen, and deepen at the same rates. Unlike tafoni that have environments that facilitate their maximal development such as arid and coastal settings, gnammas occur across diverse landscapes from marine to alpine and from humid to arid. However, it is the substrate type and its permeability that appears to have the greatest influence on their occurrence and progression. The fastest rates for gnamma progression have been recorded on bare granite substrates such as the summits of bornhardts, inselbergs, and mountaintops, yet their occurrence on steep slopes as armchair gnammas has been recorded across the globe. However pervasive their occurrence on granite and crystalline rock, other substrates including sandstone (arkosic), quartzite, greywacke, schists (granitoid), porphyry (quartz), siltstone, conglomerate, and limestone have also been documented and studied. Gnammas are believed to develop at rates much slower than tafoni, and also at rates less able to be quantified due to their much slower progression. In central Australia, gnammas measuring 25 cm (1000 ) in diameter were believed to have developed in ‘a few thousands years at the most’ (Twidale and Corbin, 1963), whereas Victorian-constructed seawalls exhibit extensive tafone development over 150 years. In cold climates like Patagonia, juvenile gnammas were documented (stages 1 and 2) displaying no spillways, and measuring 6–45 cm in length, 6–30 cm in width, and 1.1–7.5 cm in depth. Their volumes ranged from 4 l to less than 0.1 l. Having initiated after glacial retreat between 600 and 1250 AD, it places their maximum recession for gnamma length at 0.6–0.3 mm yr1, for width at 0.4–0.2 mm yr1 for width, and 0.01–0.05 mm yr1 for depth (Dominguez-Villar, 2006). These represent rare measurements for gnamma development, owing to their slow progression. Like all studies of this nature, they cannot account for variations in rate over time or nonlinear dimensional changes, so as lateral growth increases over time, and deepening decreases, since these gnamma are incipient, their maximum growth rates represent only the earliest stages of morphological progression.

4.7.5

Processes of Development

Recent studies have determined the relative importance of diverse and interacting influences on the formation of tafone and gnamma. Increasingly research points toward the complex and commonly nebulous polygenetic nature for their development where initiation and development is controlled by many factors, both intrinsic (i.e., substrate composition, structure) and extrinsic (i.e., climate, lichen overgrowth). Here we will address the many influences that have been investigated and documented including the roles of lithology, climate, environment, and plants and animals in their occurrence and growth.

4.7.5.1

Lithologic Influences

Gnamma and tafone initiate in zones of differential weathering on the rock surface, including variations in lithology, structure, composition, and texture (Dragovich, 1969). Petrology and mineralogy of the substrate then play two primary roles in the development of tafone and gnamma. Variability in composition can influence differential weathering since some lithologic constituents act as relative indurating agents (i.e., iron, silica), or through the hydrolysis, dissolution, ionic exchange, or hydration of weathering-susceptible minerals like phyllosilicates (Mustoe, 1983), carbonates (Bartrum, 1936), and feldspars (Sancho and Benito, 1990) which can weather to fall out, enlarge to pry out neighboring constituents, and/or weaken to erode from the substrate. Other components can increase differential weatherability and subsequent cavity development through variations in integrity (i.e., jointing, cleavage, texture). These variations may facilitate recession in some portions, relative to little recession in others. The other important role of lithology, and possibly the more influential, lies in its structural and compositional effects on porosity (percentage of void space to material) and permeability (percentage of active porosity or connected pore space). In the ruined city of Petra, Jordan, Paradise (1995) investigated the occurrence of tafone and cavernous weathering in sandstone. He found that minor constituent variation of iron oxides from 2 to 4% in the rock matrix, significantly affected the rate of surface recession and tafone development. Using architectural surfaces that have been

Tafoni and Other Rock Basins

exposed since their construction (50 BC–150 AD), analyses showed that cavernous weathering features (tafoni, alveoli) developed to depths of 200 mm when the matrix contained only 2% iron oxides, whereas when the matrix contained 4% iron levels, the stone exhibited nearly complete arrested recession. These high iron sandstone surfaces displayed original Nabataean and Roman stonemason dressing marks. Although iron was found to decrease weatherability, some components (like carbonaceous matrices) showed accelerated recession and increased development of tafoni. Sunamura (1996) found that an increase in tafone development, distribution, and dimension was directly related to the presence of weak bedding planes, jointing, and fractures in the substrate rock. This was found to be especially important in sheer cliff faces and single expanses of rock where variations could be identified and related to tafone occurrence. In Weston-Super-Mare, UK, limestone and sandstone blocks were both used to build a seawall; the limestone remained relatively unweathered, whereas the sandstone exhibited obvious tafone development. Pye and Mottershead (1995) found that this was due to the low permeability of limestone and its decreased chemical susceptibility to saltinduced weathering, when compared to adjacent sandstone blocks in the seawall. Lithologic constituents can facilitate the initiation and development of tafoni and gnammas, and some can nearly halt surface recession. Where salt is present it can only facilitate tafone development when other factors (i.e., substrate type, permeability) are present. In their research in central Spain, Sancho and Benito (1990) confirmed that specific environmental factors influenced tafone (and gnamma) development in sandstone. They found a positive correlation (r ¼ 92) between feldspar constituency in the rock (410%) and the distribution density of tafoni – a previously speculated influence (Mustoe, 1983). It was explained that the relative susceptibility of feldspar to weathering through hydrolysis was the culprit. It was clear that the freed potassium cation (K þ ) mobilized to later recombine and form potassium salts (i.e., sylvite) which would further accelerate feldspar deterioration, releasing more potassium, thus propagating a positive feedback cycle. They also found a significant relationship between tafoni distribution and substrate conductivity (4150 mS cm1) in Spain – a previously unknown control. Increases in conductivity may be due to lithologic constituents that can increase electron mobility, or simply due to the increased presence of secondary salts known to foment tafone and gnamma development. Both in Mustoe’s (1983) seminal work on tafoni, and Twidale and Corbin’s (1963) research on gnammas, the decomposition of feldspars and phyllosilicates was cited as a crucial influence on tafone development through differential weathering. When all variables are normalized, then lithology has been found to dramatically affect relative, differential weathering. However, it may be the role of lithologic structure that has the greatest effect on cavernous weathering. The moment joint patterns found in sandstones across the Colorado Plateau have been shown to rapidly drain and dry these beds, allowing for little throughflow, although these sandstone are some of the more permeable rock formations on Earth. Yet when water mobility is diminished – such as in nonjointed areas, or at a

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contact with a less permeable contact – cavernous weathering develops. This indicates that there is a balance required for tafoni to develop between rock so permeable that water drains out freely and quickly, and rock with so little permeability that moisture is not mobilized within the strata. This may be the reason why arid environments (hot and cold) exhibit a broader diversity of cavernous features as a result of decreased moisture, its movement through the substrate, and within its ambient microclimate. Secondary deposition of rock coatings and skins may also influence permeability and subsequently the progression of tafone and gnamma morphology. For example, since iron and silica skins can diminish overall permeability, their initiation can also decrease cavernous development. Spatial variation in permeability due to lithologic changes or secondary skin development will cause variations in moisture mobility within and across the substrate, which in turn produces a variability in morphology. Coatings can also influence the rate at which cavernous weathering proceeds through the precipitation of secondary minerals or materials (i.e., iron oxide, calcite, kaolinite) into and among the gaps and pores of the substrate surface. It has been found that rock subsurfaces may weather faster than at the surface. In Baja California, it was found that intrusive rock constituents (i.e., biotite, feldspar) beneath the surface deteriorated to produce iron-rich solutions that mobilized outward to precipitated and indurate the surface. Accordingly, solutes produced from internal weathering were precipitated at the surface to produce a coating, which facilitated cavernous weathering through a core-softening and/or case-hardening (Conca and Rossman, 1985). Since the earliest observations of gnammas across the granite domes and outcrops of the Outback and Yosemite, iron staining on the rims, lips, pans, and bowls has been noted, measured, and studied. Prior work has detailed that iron salts (oxides, sulfides, sulfates), amorphous silica, silcretes, and/or calcretes can act as indurating and/or cementing agents that create coatings and skins. For instance, as rocks and minerals deteriorate through hydration, hydrolysis, and ionic exchange, weathering by-products (i.e., iron oxides and clays) can combine to produce coating that can indurate the surface (i.e., ferruginous cements). Measuring from 1 to 30 mm in thickness, although 1–5 mm skins are the most prevalent (Goudie and Viles, 1997), these brown and orange rinds have been recorded on tafoni and gnammas on each continent and are considered by many researchers as prerequisites in the development of tafone and gnamma. Research continues to underline and examine the role of this case-hardening from coatings in tafone and gnamma development, and may one day answer the question regarding their complex nature. With gnammas, lithology affects both their initiation and development. Incipient gnammas commonly display an irregularity in substrate granite or sandstone indicating that their initiation was indeed a function of a variation in petrology (i.e., phenocryst size, xenoliths, orientation, shape, texture). It was also found that on granite bornhardts like Stone Mountain, Georgia, the gnammas occurred larger and in greater frequency on spalled portions or ‘leaves’ relative to nonspalled substrates. It was hypothesized by Paradise and Yin (1993) that in its exfoliation and subsequent expansion,

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granites display less compact structures which permit greater intercrystal water penetration which facilitates mineral hydration and deterioration. This would weaken the substrate, accelerating granite breakdown, which aided in pit and pan incipiency.

4.7.5.2

Environmental Influences and Salinity

Salt and its role in tafone development have been discussed since the work of Bryan (1922) and Blackwelder (1929) in the U.S. Southwest. Simply put, the common occurrence of tafoni in saline environments like deserts and intertidal settings is a sound indication that salt plays a crucial role in their development. However, the larger question looms – why do tafoni develop on some surfaces and not others, but in the same saltrich environments? Is salt essential, but only in conjunction with other intrinsic and extrinsic influences? Sodium chloride (halite) is the most prevalent form of salt most commonly derived from seawater, however less common salts occur, including sodium sulfate, calcium sulfate (gypsum), magnesium sulfate (epsomite), and potassium chloride (sylvite) – all representing common contributors to cavernous weathering. These salts may be derived from seawater or formed through the dissolution of outcrops of rock and minerals, or water infiltration in and through strata containing chlorides, carbonates, and sulfates. Moreover, salt can foment both physical and/or chemical weathering. Dissolved salts may be transported atmospherically or as crystalline aerosols and deposited atop the rock substrate where they dissolve to precipitate, or simply accumulate as salt crystals, expanding to pry apart crystalline or grained rocks like granite or sandstone (Rodriguez-Navarro et al., 1999). As salt crystals develop, they can physically force apart crystal boundaries opening the rock surface, causing disaggregation and surface recession, and the subsequent deposition of its deterioration by-product, such as grus, sand, or silt. Salt also has an accelerating effect on the dissolution of silica. Increasing the concentrations of salt ions such as chloride, sulfides, and/or sulfates increases the weathering rates of silicate rocks (granite), minerals (quartz), and amorphous silicates (opal). In addition, the presence of salts can modify the pH of water that, in turn, can accelerate weathering and tafone and gnamma initiation and progression. The simple addition of seawater (and sea salt) has been found to greatly increase the solubility of quartz, whereas its presence can instigate the weathering of pyrite and other sulfites to produce acidic solutions (pH 3–5) that also increase weathering. Salts can speed up the deterioration of clasts and mineral components through mechanical and chemical means, which then expands the boundary interstices, permitting more salts or saline solutions to enter, thus weathering more and creating a positive feedback cycle of breakdown (Huinink et al., 2004). It has also been found that the length of the wetting and drying episodes and the rate of evaporation are crucial parameters in the role of salt in tafone and gnamma development. When drying periods are short, the evaporation boundary remains in contact with the substrate surface during desiccation, depositing salts in proportion to the period of drying. Consequently, salt deposition is greatest at the more

exposed portions of the cavities, pits, and pans. However, when the drying episode is long, the evaporation boundary is in little contact with the surface – the greatest salt deposition then occurs in the more sheltered parts of the cavities where the drying rates are lower: tafone backwalls, and beneath overhanging gnamma rims (Huinink et al., 2004). This disparity in salt deposition causes differential weathering which then triggers marginal and subsurface areas to enlarge and deepen. This may explain why tafone are commonly observed expanding upward with ‘floors’ weathering at a slower rate than ‘roofs,’ and gnammas expanding within the seasonal waterline thus producing lips, rims, and flask-forms, from the vertical pan walls. Development of tafoni and gnammas can indeed accelerate their own progression in saline environments. Many studies emphasized the primary influence of salt on tafone and gnamma development; however, it is increasingly clear that these weathering and erosional features are polygenetic in origin, attributed to complex tandem influences and feedback cycles.

4.7.5.3

Biotic Influences

The role of plants and animals in stone weathering has been discussed since Strabo’s observations across the Mediterranean 2000 years ago (22 AD). Like the effects of salinity, plants and animals have been found to act as mechanical and chemical weathering agents. The attachment of lichens on rock is commonly considered a destructive agent on rock integrity through the penetration of rhizines into the substrate and prying and separating the substrate constituents. Also, lichen attachment can ‘roughen’ the substrate beneath the cortex, abrading the surface, thus preparing it for differential weathering and the initiation of tafone or gnamma. In addition to rhizinal penetration, the production of oxalic acid beneath the lichen cortex can exacerbate the destructive capabilities of lichen overgrowth (Paradise, 1997). However, some lichen species and subspecies (i.e., Lecanora sp) have been found to indurate the rock beneath the cortex through the production of an oxalate skin, whereby the sandstone, granite, or limestone actually weathers at a slower rate than areas adjacent with no lichen overgrowth (Paradise, 2005). Additionally, organic acids from humic decay have also been identified as important weathering agents. Some organic compounds can act as coatings on reactive surfaces, thus decreasing weatherability; however, in general, organic acids consistently accelerate weathering rates (Young et al., 2009). Amorphous and crystalline silica solubility increases in the presence of organic acids. In the Hawkesbury Sandstone near Sydney, Australia, it was found that infiltrating acidic waters dissolved the iron oxide matrix cement to release the clasts to produce cavernous weathering (Young et al., 2009). Commonly mistaken as tafoni and gnammas are the cavities and bowls produced by pholads (Pholas dactylus). These small marine bivalve mollusks (2–6 cm) bore into wood, clay, concrete, or soft rock for protection leaving a cavity not unlike one or a group of tafone (Boxerman, 2006). Their appearance is much like incipient tafoni; however, they are bowl-shaped without thin ribbing and polygonal cell arrangements.

Tafoni and Other Rock Basins

Although these boreholes can be mistaken as small gnammas on horizontal surfaces, or tafoni on vertical faces, they were created through organic processes. It must be noted however that pholad boreholes can initiate the development of tafoni in near-shore environments.

4.7.5.4

Climate and Insolation

The degree of differential weathering across a surface can be further enhanced by differences in the environmental and climatic factors like wind, humidity, and insolation. The rate of salt weathering is known to accelerate as wetting and drying cycles increase; however, the duration of the wetting and drying episodes and the lapses between have been identified as instrumental in the progression of tafoni and gnamma. Although singular influences on tafone and gnamma development have been researched and discussed (and emphasized), most studies have also addressed an underlying effect(s) controlled by environmental factors. Temperature variations, insolation fluctuations, wind, and humidity variability have been shown to affect changes in the rate of cavernous weathering development. Since the late 1930s, insolation has been relatively abandoned as a primary factor in weathering due to the important laboratory simulations conducted by Blackwelder (1933). However rare, research since has examined the power of sunlight and heat in tafone and gnamma development. Popoff and Kvelberg (1937) found the highest frequency of gnamma (although they are called tafoni) on granite surfaces exposed to the greatest temperature fluctuations and refuted Blackwelder’s widely accepted findings. Using aspect as a surrogate for insolation and temperature, Paradise (2002) examined the frequency, arrangement, and dimensions of tafoni occurring on architectural surfaces in Petra, Jordan. By correlating tafone size and number to aspect, significant relationships were revealed. In studies as these, southern faces commonly exhibit the largest cavities; however, in Petra, the widest and deepest tafoni were found between 230–2701N and 140–1701N. This bimodal distribution indicates a different role of sunlight beyond simple heating since afternoon heating would have increased on western aspects,

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and direct insolation would have facilitated growth on southern exposures. Since the largest tafoni dominated on both western and eastern, insolation was found to have the greatest effect on the development of tafoni in arid settings, when in conjunction with increased wetting–drying and/or heating–cooling cycles. Temperature fluctuations and cycling have been found to loosen sandstone clasts or weaken matrix cements. (Figure 9) Not only are thermal expansion and/or wetting–drying cycles responsible for their growth, freezing and thawing fluctuations may be as important in frigid settings. In Antarctica, French and Guglielmin (2000) explained that the development of cavernous weathering on granitic and gneissic metasedimentary rocks was related to the microfracturing of quartz minerals in these cold environments. Quartz fractures under cryogenic conditions, especially when salts as by-products of weathering, lower the freezing temperature. Their findings explain why cavernous weathering like gnamma and tafoni may be so well developed in arctic and polar deserts. Wind has also been emphasized as an overlooked influence; however, it may present a quandary common in all geomorphological research. How important is wind in the development of cavernous phenomena when it cannot be separated from other effects like salt weathering, or hydrolysis of mineral constituents? In his early work, Futterer (1899) stressed the importance of wind in tafone and gnamma development in granite, whereas Blackwelder (1929) emphasized its role in deserts, explaining that it was an erosional factor, and not an abrasive agent. However, in many arid settings, ‘sand storms’ have been observed scouring cliff and boulder faces in a single event, removing large expanses of tafoni (Burton, 1879). So how does wind influence tafone and gnamma development; does it scour the rock to produce depressions, while somehow unaffecting its walls and rims? As Blackwelder implied, wind may not act an as abrasional element, but may be crucial in the removal of weathering by-products within the cavities and niches, like sand and grus. Rodriguez-Navarro et al. (1999) also corroborated the role of salt in cavernous weathering (in oolitic limestone), but they also found that wind was instrumental in accelerating surface evaporation to produce the greatest relative disparities between

Tafoni dimensions and aspect at Anjar quarry, Petra, Jordan 200 (n = 540)

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Aspect (degrees) Figure 9 Graph illustrating the relationship between aspect (0–360 1N) and tafoni dimension in Petra, Jordan. Reproduced from Paradise, T.R., 2002. Sandstone weathering and aspect in Petra, Jordan. Zeitscrift fu¨r Geomorphologie 46, 1–17.

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Dry rock substrate

Developing cavity

Enlarging cavity

Rock moisture Moisture boundary

Figure 10 Diagram of cavity development illustrating the importance of the moisture boundary and its relationship to tafoni or gnamma morphology. The cavity evolves fastest (through deepening and/or widening) at the moisture boundary where wetting and drying cycles may be diurnal or seasonal. Reproduced from Huinink, H.P., Pel, L., Kopinga, K., 2004. Simulating the growth of tafoni. Earth surface process. Landforms 29, 1225–1233.

dry and wet portions. This leads to ‘localized supersaturation’ that promoted salt weathering. Moreover, it was discovered that salt was not facilitating chemical weathering, but physical deterioration though constituent prying and heaving. Since Bryan (1922) and Matthes (1930), it has been emphasized that moisture plays an important role, if not primary, in the initiation and progression of rock cavities. Bryan mentioned visible moisture, dripping water, or sensed humidity in the niches and cavities of Chaco Canyon, New Mexico, whereas Matthes emphasized the need for channels from and between gnammas that acted to remove grus and debris from the deteriorating bowl walls in Yosemite. This action then leaves a fresh granite surface upon which weathering can progress. If water cannot drain from the bowl or pan, then erosion of the detritus is only possible through wind action – a rare occurrence within these receding ground level cavities. Finally, it was found that the role of moisture was explained in terms of the boundaries that shift as ambient moisture regimes fluctuate. Huinink et al. (2004) established that cavities like tafone grew as a function of mobilized moisture fronts within the substrate. In essence it was found that the cavity backside and interior develops within the dry rock, whereas the walls and edges break down within moist or saturated areas of the substrate. This brings new attention to the powerful influence of wetting and drying within and on the rock surface (Figure 10).

4.7.5.5

Feedback Cycles

In nature and society, feedback occurs when the product from a process impedes its own process (negative feedback), or increases its own process (positive feedback). Such mechanisms and processes have been identified in cavernous weathering and represent the sequence of causes and effects that accelerate the rate of development, or slow or stop the growth of tafone and gnammas. Generally, weathering processes exploit lithologic variability so once deterioration begins, rocks degrade at faster rates than neighboring unweathered rock, and tafoni and gnammas develop. The bowls, cavities, pits, and interiors then weather at rates faster than surface faces and walls and a self-reinforcing loop develops. Weathering accelerates and the rate of change is exponential. However, as cavities and bowls enlarge, they can instigate the growth of epilithic coatings (like rock varnish or cyanolichens), which decreases the rate of weathering, or altogether stops it. This

differential weathering can be attributed to intrinsic factors like variations in lithology, and/or to the extrinsic influences such as microclimatic variations between the substrate exterior and its interior (i.e., salinity, humidity). Feedback loops can be affected positively and/or negatively through these influences. For example, the induration of surfaces can result in core softening or case-hardening; crystalline rocks like granite and quartzites tend to core soften, whereas clast-matrix rocks like sandstone or arkose tend to develop case-hardening. As the cavity enlarges, it create an environment that can accelerate its own expansion or halt its development. Moisture fluctuations, wetting–drying, and/or freezing– thawing can also have exacerbating effects on cavernous development. During episodes of saturation or high humidity, water is retained in the sheltered cavities, and salt accumulation is diminished. However during dry periods, capillary water is drawn to the cavity wall to evaporate, precipitating interstitial salt crystallization. During both dry and moist episodes, the environment created in the cells, bowls, and cavities creates a positive feedback cycle that facilitates weathering. Tafone development, however, may pass through negative and positive feedback cycles from incipience to maturity since the sequence has been found to be sigmoidal in rate with a nonlinear progression: slow during incipiency to accelerate during its mature stages when cavities may deepen faster than widen, to a slow rate of growth as the stages pass (Goudie and Viles, 1997). In tafoni, the earliest stages of initiation and development may be limited by low salinity, followed by enlargement and deepening dominated by increasing salt concentrations within a positive feedback cycle. Cavity development then progresses until the cavity is too large to effectively wick moisture from the substrate and/or maintain a balance between salt mobilization, accumulation, and related weathering. Hence, the cavity outgrows its own capacity to enlarge and a negative cycle follows. Gnammas also initiate and enlarge due to feedback influences. When water collects in the irregularities on rock surfaces, then the water localizes physical (freeze-thaw) and chemical (hydrolysis, hydration) weathering and granular disintegration follows. Erosional forces like water and wind eliminate the weathering by-products (i.e., grus), and the depressions enlarge. This traps more water and the process accelerates as a positive feedback cycle.

Tafoni and Other Rock Basins

Negative feedback cycles in their development have also been identified through the growth and/or accumulation of secondary mineral precipitates, sediment accumulation, or plant overgrowth. Mustoe (1982) and Paradise (2002) both found that lichens decreased weathering by acting as protective biotic blankets. Although rhizinal penetration and oxalic acid production in lichens has been found to accelerate deterioration, adnate lichen overgrowth can diminish weathering and cavity enlargement in tafoni and gnammas. Secondary depositions like rock varnish may also develop, indurating surfaces by filling mineral and clast pores and boundaries. Such overgrowth and coatings act as negative feedback mechanisms whereby enlargement facilitates endolithic overgrowth and coating buildup until weathering is arrested and the tafoni and gnamma become relict, no longer actively weathering.

4.7.6

Summary

Why tafoni and gnammas develop still puzzles geographers, conservators, and geologists, although influences of salinity, mineral solubility, lithology, and microclimatic influences are still considered essential. Tafoni and gnammas develop over a wide range of scales and environments, and many weathering processes work in tandem, and in feedback cycles, to produce these often delicate and symmetrical, or large, deep, and cavernous features. However, little is still understood as to how these bowls, pans, honeycombs, alveoli, and armchairs initiate, develop, enlarge, and coalesce. Some develop within decades and others over millennia depending on the complicated controls that may be intrinsic like rock type, or extrinsic like microclimatic influences or broader shifts in climate. At smaller scales (centimeter to meter) it is now widely accepted that salt mobility and evaporation is critical in cavity development on vertical faces (tafoni, alveoli, stonelace, niches), whereas differential weathering through hydrolysis, hydration and/or ion exchange is crucial on horizontal to vertical surfaces (gnammas, tafoni, niches). However important single agents may have been identified at specific sites, to fully grasp the complicated and integrated processes responsible for tafoni and gnammas, we must begin to examine the hierarchal associations between the many processes known to affect their incipience and progression. Some sites may indeed indicate that one process is principal, whereas another may point to another influence as significant. Since Blackwelder (1929) speculated on the polygenetic origins of niches and cavernous weathering, we have identified a number of processes that operate in tandem and/or individually. With contemporary research advocating feedback sequences, the very nature of positive and negative feedback cycles supports the idea that tafoni and gnammas develop – from small pits and hollow to niches large enough for a ‘horse and rider’ or basins big enough ‘to bathe a family’ – through complicated and interconnected processes. Examples of such an integrated process chain may include the following: (1) Lithological disparity in the rock substrate initiates a depression. (2) Differential weathering deteriorates boundaries of clasts and mineral constituents that instigate disaggregation

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and cells and bowls develop. (3) As cavities enlarge, differential weathering (physical and chemical) continues. (4) Erosion through water and wind remove the by-products of weathering to support accelerated weathering and cavities enlarge, or by-products remain to decrease rate of weathering and cavity progression slows down or stops. Rodriguez-Navarro (1998) speculated on the causes of tafoni identified on Pathfinder’s photographs from Mars. He argued that salt weathering was the primary agent, in conjunction with chemical weathering and frost shattering. Such conjecture may be valuable regarding our terrestrial phenomena, as well as these fascinating cavities observed on Martian boulders. As research continues on Earth (and other planets), it is imperative that we continue to identify the influences on tafone and gnamma development and their interconnected hierarchies of associated factors, how they relate to each site, at varying scales (spatial and temporal), on a diversity of substrates, and within a varied range of environmental regimes.

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Biographical Sketch Dr. Tom Paradise is a geography and geosciences professor at the University of Arkansas and past director of the King Fahd Center for Middle East Studies. He comes from a diverse background in the geology, climatology, material sciences, cartography, architecture, and Mediterranean geography. Having researched the unique, decaying architecture of Petra, Jordan, since the 1980s, he has published more than 40 articles, chapters, and books on the subject and continues to advise foreign agencies on cultural heritage management, stone architectural deterioration, and Middle Eastern and North African architecture. In addition, Paradise has published four Atlases, including the recently released illustrated atlas of Arkansas. The Atlas of Hawai’i is one of the most popular books of its kind and won the Hawaii Book of the Year award (Ka Palapala Po’okela). Prof. Paradise has taught abroad at Universities in Rome, Venice, Amman, and Tunisia, as well as in the USA in Georgia, Hawai’i, Arizona, and California. He currently lives in Fayetteville, Arkansas, USA.