The wild beauty of deserts has always been a source of fascination theworld over. Mankind and Deserts 2 the second of three volumesfocuses on water, its absence or indeed its extreme scarcity, as well ason the ways in which salts come to be formed in areas such as these. Aridity of the climate does not exclude rainfall, after which desertsflourish; wet mists, dew, exceptional events separated by years of totaldrought. Water flows into temporary and disorganized networks but,occasionally, large rivers cross the deserts, giving rise to vibrantcivilizations: the Nile, Tigris and Euphrates, Niger, to name a few.Temporary or permanent lakes collect water in basins without outlet tothe ocean, referred to as endorrheic basins, such as Lake Chad. Thisresults in salt accumulation and evaporitic formations. A large variety ofsalts crystallize, in addition to halite, among which is potash. Halitecommon salt is an essential resource and its trade leads to the creationof salt caravans, used to exchange it with gold, even on a 1-1 weightbasis, generating subsequent wealth. From ancient, almost mythical, exploration to modern scientific studies,deserts have come to be better known yet still hold great appeal. Thisbook traces the history of their knowledge while providing a basis forunderstanding their features and the tools needed for their protection, inan ever-changing world.
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Introduction: Water in Deserts
I.1. Hydrology – wild water
1 Water Falling onto Soil and the Effects It Produces
1.1. The arrival of water
1.4. Geodynamic and geomorphological effects of runoff in arid zones
2 Temporary Water Bodies and Lakes
2.1. Water bodies
2.3. The principal features of lakes in an arid zone
3 Hydrographic Networks
3.2. The disruption of hydrographic networks
3.3. Allogenous rivers and streams
4 Salts in Deserts
4.1. The nature of the salts
4.2. The origin of salts and evaporite sequences
4.3. Evaporation of seawater
4.4. Evaporation of continental freshwater
4.5. Systems mixing surface water and deep water
4.6. Atmospheric origin of nitrate deposits
4.7. Mankind and salts
List of Authors
End User License Agreement
A man in the desert: Fernand Joly Tademaït, Sahara, Algeria
Figure 1.1. Diagram summarizing weathering in arid conditions
Figure 4.1. The N’zirah palm grove in Algeria Downstream, the palm grove gives w...
Figure 4.2. Ion chromatography in landscapes, according to Tardy (1969), modifie...
Figure 4.3. Salt fields in the Sahara, Tegguiddan Tessoum, Niger. Salt is tradit...
Figure 4.4. Camels being loaded to transport salt, Tegguiddan Tessoum, Niger (Ph...
Table 4.1. The major neutral salts
Table 4.2. Major basic and acidic salts
Table of Contents
Introduction: Water in Deserts
List of Authors
End User License Agreement
Series EditorFrançoise Gaill
First published 2021 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.
Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:
ISTE Ltd27-37 St George’s RoadLondon SW19 4EUUK
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© ISTE Ltd 2021
The rights of Fernand Joly and Guilhem Bourrié to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.
Library of Congress Control Number: 2020949240
British Library Cataloguing-in-Publication Data
A CIP record for this book is available from the British Library
Fernand Joly1 departed from this world before he was able to complete this book, through which he had hoped to share his experiences of and passion for deserts.
“Yet another book on deserts!” some might think; another book to add to the numerous publications dedicated to these alien and fascinating worlds.
This book, however, is different from earlier books, as can be seen from its title “Mankind and Deserts”. It is based on the singular relationships that are formed between humans and the world of the desert – relationships that are unique because they can be traced back to the very origins of humanity. Indeed, it is from the arid Horn of Africa (East Africa) that large migrations began and it is along the deserts, if not within the deserts themselves, that we find the major cradles of burgeoning historical civilizations. This inhospitable world is also associated with great spiritual leaders such as Moses, Jesus, Mohammed and the Buddha, while serving as the backdrop for adventurers and empire builders from Alexander the Great to Genghis Khan, or from the Incans to the Conquistadors in the Andes and Mexico.
What is this universe that is so barren and yet so mesmerizing?
“All about a word” was how Fernand Joly introduced his book: “What is a desert?” The ambiguity in this word results from the fact that it has been used in different senses across literature and throughout history. For a geographer-writer such as Fernand Joly, the one fact that stood out was that there was no one desert; instead there were multiple deserts, diverse and varied, ranging from Death Valley to the Kalahari, from the Namib to the Atacama or the Gobi desert. Each of these is a unique landscape, whose uniqueness was born out of its position with respect to the general atmospheric circulation, its geographic location with respect to the sea and to its relief features. And yet, transcending all differences, there is one constraint that binds them all together: aridity, defined as a natural physical state characterized by persistent dryness with the corollary of extremely scarce water resources. Both these concepts, aridity and water, are at the heart of the following chapters. Aridity (Chapter 3) because it “transcends time and takes over space” and water (Volume 2, Chapters 1–3) because it is the essential resource for all life, especially in the desert. Aridity is distinct from “drought”, which is simply a “period with insufficient rainfall”. Water is seen through the lens of how it appears on land: “wild water”, which flows over slopes in an un-channeled manner (Volume 2, Chapter 1), under the impact of violent but brief downpours, and “concentrated waters”, i.e. waters “concentrated” into a channel, fed by distant precipitation upstream of the borders of the desert. As can be seen, there is in fact a true hydrography of the arid world. Satellite images, among other sources, offer us clear and accurate reproductions of these systems: fossil hydrographic networks, the legacy of ancient humid periods, a map of intermittent water bodies (Volume 2, Chapter 2): playas and sabkhas, permanent lakes with fluctuating shorelines, such as Lake Chad or Lake Eyre, or large allogenous rivers (Volume 2, Chapter 3) that are born outside the desert but travel through the desert, sustaining life, such as the Colorado, the Niger and the Nile, “the first and most remarkable of rivers in the arid world”.
The role played by salts in hot deserts is rarely discussed in a systematic manner. Guilhem Bourrié, geochemist and soil scientist at INRAE, has analyzed the origins and nature of these salts and demonstrated how important these salt deposits in the desert are for humans, whether they live off agriculture, livestock or, indeed, the salt trade (Volume 2, Chapter 4).
Chapter 1 of Volume 3, drafted by Joly, was edited after his demise by Yann Callot, a professor at the University Lyon 2 who is a specialist in ergs and dunes. This chapter examines the importance of wind in the desert. Wind, sometimes considered to be more emblematic of a desert than even dryness, counts among the earliest dynamics on Earth, an element that humans have not always been able to control. Indeed, this lack of understanding of wind has sometimes had disastrous consequences for certain projects (see the Green Dam in Algeria).
The final chapter in Volume 3, “Living in the Desert”, was taken up by Marc Côte, Professor Emeritus at the University of Provence, who worked as a professor for 20 years at the University of Constantine. He has drawn on his deep knowledge of the land and the people of the Saharan region to present what he calls “The Desert Civilization”.
Finally, it must be noted, with great regret, that, since 2010, “geopolitical turbulences have tended to change the fundamentals, to burn away knowledge and to prevent researchers from keeping in touch with this part of space and humanity.”
Most of the illustrations were refined by Éliane Leterrier.
Honorary Professor at the University Paris VII, Denis Diderot
A man in the desert: Fernand Joly Tademaït, Sahara, Algeria
Professor at the University Paris VII, Denis Diderot, who spent 15 years at the Moroccan Institute of Science in Rabat.
Because of the minuscule amounts of water and their meteoric variability, the discipline of arid zone hydrology is one of the highest forms of art and science.
V. Kotwicki, Floods of Lake Eyre
“Ce qui embellit le désert, dit le petit prince, c’est qu’il cache un puits quelque part…” (“What makes the desert beautiful,” said the Little Prince, “is that somewhere it hides a well…”)
Antoine de Saint-Exupéry
It is because of its extreme scarcity, or indeed its absence, that water is so important in a desert, even when it is invisible, when one is searching for it or it is altogether lacking. It is exceptional to find open water sources (there are long periods when these open water bodies simply do not exist) and they are most often found at some depth. Finding water, collecting it, transporting it, using it carefully and storing it are all key issues to be resolved in desert life. Sometimes there is a lack of water because the excessive aridity results in deficient hydrology and a lack of wild water (Joly 2006), while at other times there is a poorly developed hydraulic system, such that domesticated water resources (Prosper-Laget 2001) cannot fulfill the needs of the population they serve.
Sometimes the areas involved are vast, like the Sahara, Arabia or central Australia; and, sometimes, there are smaller spaces where water, though present, is unequally distributed, as is the case in central Asia, Mongolia, western North America and the Andes. The tragedy here is that across these regions there is a permanent imbalance between available water resources (highly limited) and the requirements of the ecosystem for ordinary consumption and development (Margat and Tiercelin 1998).
Hydrology is the science of water (Cosandey and Robinson 2000) and it is not a discipline that can be easily applied to deserts (Margat 1985). As with climatological data, this scientific field is hindered by a series of restrictions, arising, principally, from small human presence and the highly discontinuous nature of hydrological phenomena, which are scattered through time as well as space. There are, of course, a relatively large number of qualitative observations from nomads and travelers. However, these are often not very objective, difficult to access and unreliable. The quantitative observations that are archived are better monitored and valid, but they are far too scattered as they usually focus on inhabited sites: military or administrative stations, oases, mining or agricultural installations and construction sites.
Certain phenomena can also be studied using isotopic or magnetic labeling (Adar and Leibundgut 1995), through various mathematical or statistical models (Band 1985; Dassargues 2000) and, above all, through remote sensing (Mering 2008), especially phenomena such as floods, humidity in air and soil, and even the presence of subterranean water (Timmermans and Meijerink 1999). There have also been some systematic projects devoted to observation and measurement (Braquaval 1957; Dubreuil 1972; Roose 1977). However, apart from the large rivers, certain semi-permanent water courses and some large basins of economic interest (mainly related to oil), there has been rather a meager amount of data collected, and what does exist is fragmented, dispersed and patchy. This is partly because many desert zones lie in developing countries, which have economical constraints as well as a restricted qualified workforce. However, it is also because even among the large powers, such as the United States, Russia or China, equipment and regular tracking are allocated only to arid zones considered to be economically “useful” or profitable (Lacoste 2001).
Adar, E. M. and C. Leibundgut, eds. (1995). Vienna Symposium, 1994, Applications of tracers in arid zone hydrology. Red Books 232. Wallingford, UK: IAHS. 452 pp.
Band, L. (1985). “Simulation of slope development and the magnitude and frequency of overland flow erosion in an abandoned hydraulic gold mine.” In. Models in Geomorphology. Ed. by M. Woldenberg. Winchester, Mass.: Allen and Unwin, pp. 191–211.
Braquaval, R. (1957). Études d’écoulement en régime désertique - Massif de l’Ennedi et région nord de Mortchai. Paris: ORSTOM. 92 pp.
Cosandey, C. and M. Robinson (2000). Hydrologie continentale. Paris: A. Colin. 360 pp.
Dassargues, A., ed. (2000). Tracers and modelling in hydrogeology. Red Books 262. Wallingford, UK: IAHS. 571 pp.
Dubreuil, P. (1972). Recueil des données de base des bassins représentatifs et expérimentaux - Années 1951–1969. Paris: ORSTOM. 916 pp.
Joly, F. (2006). “Les eaux sauvages des régions arides - Notions de base sur l’hydrologie des déserts.” In Geomorphologie: relief, processus, environnement 4, pp. 285–298.
Lacoste, Y. (2001). “Géopolitique de l’eau.” In Hérodote 102, pp. 3–18.
Margat, J. (1985). “Hydrologie et ressources en eau des zones arides.” In Bulletin de la Société Géologique de France, pp. 1009–1020.
Margat, J. and J.-R. Tiercelin, eds. (1998). L’eau en questions: enjeux du XXIe siècle. Paris: Romillat. 301 pp.
Mering, C. (2008). “Analyse et cartographie des formations superficielles à partir d’images optiques et radar.” In. LES FORMATIONS SUPERFICIELLES, Genèse- Typologie — Classification — Paysages et environnement - Ressources et risques. Ed. by Y. Dewolf and G. Bourrié. Paris: Ellipses. Chap. 11.2, pp. 413–439.
Prosper-Laget, V., ed. (2001). Eaux sauvages, eaux domestiquées. Hommage à LucetteDavy. Aix-en-Provence: Presses Universitaires de Provence. 341 pp.
Roose, É. (1977). Érosion et ruissellement en Afrique de l’Ouest. Travaux et Documents 78. Paris: Éditions de l’ORSTOM. 108 pp.
Timmermans, W. and A. Meijerink (1999). “Remotely sensed actual evapotranspiration: implications for groundwater management in Botswana.” In International Journal of Applied Earth Observation and Geoinformation 01/1999 (3-4-1), pp. 222–233.
Introduction written by Fernand JOLY.
We have seen the drastic conditions in deserts that govern the presence of water: an atmosphere deficient in humidity given the lack of water vapor and, consequently, the inability for air to reach saturation; scattered and irregular rains; and intense potential evaporation that is exacerbated by high daytime temperatures, annual temperatures and the ever-present wind. The very rhythm of rainfall in deserts is unfavorable to hydrology. Thus, the study of rainfall in deserts must focus on how the rain falls, as well as the effects that it has on the soil.
In deserts, fine and persistent rain, lasting several hours and impregnating the soil with moisture, is rare, although not entirely unknown. This kind of rainfall chiefly occurs in regions fed by humid air: sea breezes, monsoons and disruptions along the polar front. However, the more common type of rainfall in deserts is intense precipitation (sometimes in excess of 1 mm/min), concentrated into isolated showers (between 0 and 5, or 10 showers per rainy season), lasting a short time (between a few minutes and 1 or 2 h), provoked by rising convection currents, air rising over relief features, or by storms.
In Tamanrasset, for example, on a single day in September 1950, 44 mm of rain fell over 3 h (of which, 36 mm fell in 40 min), while the annual average for 15 years was 40.7 mm! These spectacular showers are, however, less violent than is sometimes assumed, and are certainly less violent, in most cases, than Mediterranean showers. The most frequent showers over the Sahara bring in less than 4.5 mm over 24 h (Capot-Rey 1953), while at least 5–8 mm are required to effectively sustain vegetation, and an intensity of at least 0.5 mm/min is required to trigger off any streams following rainfall (Dubief 1959–1963). In any case, the maximum rainfall is, on average, less abundant in the middle of the deserts (between 13–25 mm/h) than along the semi-arid borders (between 35 and 50 mm/h) (Jansson 1982). Depending on the geographic location of these deserts, rainfall occurs either during one or two distinct seasons, or is distributed randomly over the year. Solid precipitation, in the form of snow, only plays a notable role in cold deserts or temperate deserts, or in high mountains in tropical regions. However, occult precipitation (mist, dew, frost) brings in a notable amount of water to the soil, occurring in almost all deserts at night, and through the day in fog deserts at high altitudes or along the coast.
Water arriving into the soil is an important step in the water cycle of a region or a specific site, from a hydrological and geodynamic point of view. In the desert, the effects produced by this water chiefly depend on the energy power of the rainfall, the angle of incidence of the rain striking the soil, the state of the green cover that comes in between the atmosphere and the soil, and the nature of the terrain. Green cover is largely scattered, producing a wide open space that is largely composed of bare rock. With very few exceptions, there is no grassy carpet and no trees. There are only bushes, low tufts or isolated individual plants. In these conditions, there is very little to absorb the energy of the rainfall or modify, or delay, its impact on the soil. Unless the wind blows in such a way that the rainfall falls very obliquely, the kinetic energy released by the shock of a raindrop falling onto the soil creates a splash, whose effects vary depending on the soil structure. On the hard surface of cohesive rocks, this splash only throws up water, producing a superficial and practically instantaneous flow with limited or no mechanical effects. On loose formations, however, the splash is softened but, through the impact, can bring about the destruction of aggregates, and the mobilization and saltation (see Volume 3, Chapter 1) of the constituent grains (at least those with a diameter smaller than 200 µm), which may be enough to cause rain erosion (Feodoroff 1965) in the form of the displacement of material over a very short distance. Even the gravel in regs may be destabilized and mobilized by the redistribution of the fine fraction that supports them. This results in flattening or residual compacting of the coarse elements and a spreading out of the fine particles; through glazing and the filling in of gaps this may lead to the surface becoming impermeable. At the same time, a part of the water that has fallen infiltrates through the grains and moistens the soil. The excess water runs off on the surface and/or flows into the subsurface (subsurface runoff, also called hypodermic flow (Tricart 1981)), into the interstices between rocks, following the topographic slopes. If the rain stops, the moisture front stabilizes over a few centimeters and then shrinks through evaporation. If the rain continues, surface runoff continues and the infiltrated water moves deeper.
A large part of this rain water returns to the atmosphere. Evaporation (see Volume 1, p. 113 sqq.) occurs immediately at the point where it fell and can even, sometimes, act upon the precipitation itself, preventing it from reaching the ground. The phenomenon is especially remarkable when the sun and wind follow a short but heavy shower. Further, the transpiration part of evapotranspiration is reduced to a minimum due to the relaxation in the green cover and, consequently, maximum water loss occurs through physical evaporation. Long intervals between brief showers, where the sky is clear or cloudless, give evaporation a clear field, because of the soil being directly heated by the sun’s rays. This heating of the soil, however, is very uneven and depends chiefly on the soil’s own properties, albedo, color, petrographic composition and the green cover it may have. A sharp difference in temperature is then produced between bare soil and the lowest layer of the atmosphere, which has a very low relative humidity (and, consequently, very high evaporating power). This difference is particularly striking in tropical deserts, being more variable in temperate deserts. However, even in temperate deserts the evaporating process is as effective, as can be seen in winter when there is noticeable sublimation of snow.
We cannot therefore deduce the hydrological consequences of water falling onto the soil by only studying the quantity of rainfall. Unfortunately, we lack measures that can quantify the combined effect of the complex factors that come into play from the time the water falls. However, there have been a few experiments with artificial rains over plots of land, which have provided some useful indicators on the subject (Yaïr and Lavee 1974; Roose 1977; Bryan and Luk 1981). Studying what happens to water after rainfall is only slightly easier. This analysis can help us establish the overall water balance and the circulation of water in the desert.
Weathering is the series of mechanisms by which the atmosphere (air, water, various aerosols) act upon the rocks, modifying them and, eventually, breaking them up (Carroll 1970; Goudie 1989; Smith 2009). Weathering involves physical, chemical and biological processes. These processes operate together on the whole, however the extent of their individual participation differs based on local or momentary climatic, lithological and biotic conditions. We must also specify here how non-running water, brought in through rain, snow, mist or dew, plays a discreet but significant role in producing debris or waste, which are finally fixed or displaced by their own weight, streams and/or the wind.
Given the absence of water, at least for long periods of time, it has long been believed that erosion in deserts occurs only due to wind. However, the destructive force of the wind by itself is actually limited (see Volume 3, Chapter 1). Thus, many explorers and researchers surmise that in an arid environment, the direct action of variations in temperature may be one of the leading agents behind the fragmentation of rocks and this is expressed by the term thermoclasty (Walther 1893; Coutard 2008). However, there is a distinction made between fissuration, where cracks open up without the material dissociating, and fragmentation, where there are open fissures, and flakes or chips break away.
It is true that temperature differences in deserts are quite extreme due to the dryness of the atmosphere and the bare soil, which facilitates radiation. These temperature differences include seasonal differences in the high and middle latitudes, but also the sharp daily differences, especially in summer and in hot deserts. Around the middle of the day, soil temperatures can easily exceed 50°C or even 60 to 70°C (Peel 1974) (depending on the rocks, their own albedo, the biotic environment and exposure to the sun), while late-night temperatures often fall below 20, 10 or even 0°C. It is not uncommon to have a temperature difference spanning 30–50°C in a single day. The differences become much smaller as we go deeper into the soil, but can sometimes be noticed up to a depth or 30 or even 50 cm, especially in porous or fissured rocks into which air can penetrate. These temperature differences are destructive due to the frequency and intensity of diurnal expansion followed by nocturnal contraction, especially with respect to the tensions resulting from the rapid cooling following sunset. Cold rain falling on overheated soil acts in the same manner. The earlier disintegration of rocks also comes into play, in ways or timeframes that may not always be measurable; many fissurations are fossil fissurations, resulting from processes that are no longer present, such as tectonic movement or the freezing process.
When rocks have existing fissures, thermoclasty acts upon them through the alternating expansion and contraction of air trapped in the cracks. This varies depending on the nature and color (albedo) of the rocks. In compact rocks, the expansion and contraction of the rock itself, along with that of its mineral constituents, plays an essential role. In grained rocks (granite, sandstone) and crystalline rocks, the differential expansion of the grains or crystals triggers the granular disintegration that results in arenization (Godard and Bourrié 2008). On rocks that are covered by desert varnish or a crust, the different physical reactions of the rock and of its outer layer produce a scaling or exfoliation of the superficial layer. The case of loose gravel found on regs (Volume 3, Figure 1.1, p. 31) is a little more complex (Bertouille et al. 1979): small pebbles (1–4 cm) are likely to be most affected by fissuration (Soleilhavoup 1977), while larger pebbles and more voluminous stones (larger than 8–15 cm) are more likely to be affected by fragmentation (Joly 1962).
It is difficult to gauge the actual effect of thermoclasty in the field and its effects are rightfully debated. No one can deny that there exists a temperature differential between day and night, in deserts, that is steep enough to cause a thermal shock that can weaken the structure of a rock. This shock must be repeated often enough to produce an actual breaking up of the material. Experiments carried out in laboratories using different protocols and on various kinds of rocks (Coutard and Journaux 1976) have demonstrated that in dry conditions, with a temperature difference of the order of 50°C, at least 8000–20,000 successive shocks are required to visibly start off fissuration in grained rocks such as granite or sandstone, or in compact rocks such as basalt. Indeed, thermoclasty weakens the rock’s resistance and exposes the rocky material to future fragmentation. But while its existence cannot be denied, its effects are limited to specific cases where there is a conducive convergence of microclimatic, lithological and biotic conditions, along with pre-existing fissuration. In any case, the fragmentation of rocks in deserts cannot only be attributed to this phenomenon.
The presence of water significantly increases the efficacy of fragmentation due to weathering. In this regard, the rain we consider is not so much the brutal showers, but the fine, “moistening” rains, poorly drained water and, above all, mists, dew and condensation of any kind. The factor that is most strongly implicated in the process is not variation in pluviometric volume, but rather the variation in moisture and the moistening or humectation of rocks, which can sometimes start off a chemical weathering (hydration) where some elements are dissolved. Due to this, the term hygroclasty was proposed for this particular process (Joly 1962). Studying this phenomenon is rather difficult, especially with respect to data collection and the fact that there are multiple factors, apart from the microclimate, that come into play. These include biological activity, the nature of the rock, its albedo, pre-existing fissuration in the rock, its porosity and the capillarity of the moistened zone (Smith 2009).
Fissures grow in size chiefly through mechanical means, which, like thermoclasty, consist of compressions and decompressions that affect the rock and the fluids within the voids. These mechanical means provoke a slow degradation through the disaggregation of grains and crystals in grained rocks; the scaling away of the walls of the cracks, joints and fractures in compact rocks (Freytet 2008b) results in an enlargement of the channels and greater circulation of air and water, which produces a clear fragmentation of the rocky material. In many cases, the role of past events, in more humid periods, has been preponderant. The phenomenon is simply continuing, albeit, more slowly, in present conditions.
As with thermoclasty, the effects of hygroclasty are difficult to evaluate (Peel et al. 1975), and laboratory experiments are still rare or too specific (Pissart and Lautridou 1984) to allow for generalizations. Field observations can, nonetheless, provide a few useful indicators. Certain limestone pebbles, especially marlstones, which are either isolated on a reg or contained within a loose formation that can retain moisture for a long time, present polygonal fissuration, somewhat similar to “crocodile skin”, or fragmentation resembling a “bread crust” (Joly 1962). The open cracks, which may be several centimeters deep, have sharp, clear-cut edges that are flared at their upper extremities; their walls are often flaking, marked by wavy lines (vermiculated), polished by the wind, or covered by a varnish. The intervention of water in these rocks can be clearly seen. They are morphologically analogous to the cracks and fissures caused by shrinking in the clayey or clayey-silty surfaces of playas and sebkhas as they undergo desiccation. The fragments that break away are themselves vermiculated, marked by raised, spalling domes, or glazed. Similar shapes and forms can also be seen on bare limestone slabs or crusts, which are highly exposed to humid winds. On surfaces that are cool or lie in the shade (which, consequently, retain moisture for a long time), scaling and granular disaggregation also cause residual cavities of varying depths, called tafoni (singular: tafone). The size of these tafoni can range from a few decimeters to a few meters, and they can lead to deep steep ridges, and even the formation of hollow rocks and arches of all sizes (e.g. in the deserts of the American West, or in the central Saharan region). Rocky walls and isolated rocks, which are chiefly attacked at their base, which are most exposed to excess moisture, often present basal grooves, and take the shape of sculpted rocks, or mushroom-shaped rocks. Despite being partly fossilized and incompletely formed, these shapes are still vivid and often truly spectacular.
Cryoclasty is the breaking or fragmentation of rocks caused by the freezing of water within gaps in the rock (Ozouf 2008). When discussing thermoclasty, we saw that the strain the rock undergoes during rapid cooling can lead to fissuration. In addition to this, when water present in the cracks and pores in the rocks freezes, it causes an increase in volume as the water changes from liquid state to solid state. This increase in volume can exert considerable pressure on the walls of these cracks during the freezing process, and the subsequent thawing results in a decompression that can cause the rock to shatter (Prick 1999). This “frost weathering” or “rock-splitting freezing” does not require very low temperatures and can occur even at temperatures of −2 or −3°C. The gelifraction process, however, which produces debris (gelifracts), requires multiple freezing/thawing cycles, oscillating around 0°C.
Cryoclasty chiefly affects rocks that are most sensitive to the effects of freezing, i.e. rocks in which freezing water is constricted by the narrowness of the voids. If the voids were not entirely filled, the freezing could occur easily without exerting pressure on the walls. In coherent rocks, cryoclasty operates through the discontinuities created by tectonic movements (cracks, faults), mineralogical assemblages, sedimentation (joints, stratification planes) or previous weathering, as well as the network of cavities (pores) and channels that open to the outside. In loose rocks, gelifraction primarily occurs at the interface between frozen soil (gelisol) and non-frozen or thawed soil (mollisol), and moves along the migration path of the frost front. This is complemented by vertical fissuration caused by shrinkage cracks that result from the drying that accompanies freezing.
In many hot deserts around the world, the current action of frost is quite limited. This is because the frost occurs for too short a period, or too rarely, to produce a significant number of the freeze/thaw cycles that are required for the mechanical disintegration of rocks. Further, the total volume of precipitation as well as the duration of the precipitation is generally too small to provide enough water for gelifraction or freezing. In cold, subpolar deserts, with long winters and permanently frozen soil (permafrost), there is no alternating cycle of freezing and thawing. This cycle is chiefly produced in zones where the gelisol and mollisol come into contact with each other, during autumn (freeze) and in spring (thaw). Paradoxically, this cycle is most active in temperate deserts, where winters are harsh, where the freeze/thaw cycles are more frequent, and where melting snow and atmospheric humidity contribute to the voids in rocks getting filled up. This can be seen notably in the deserts of Iran, in Central Asia, Mongolia and North America. The phenomenon can also episodically be seen in the septentrional Saharan region; in Negev; in the deserts of Syria, Iraq and northern Arabia; and in Pakistan, South Africa and southern Australia. In deserts that are continually hot, as in the central Saharan region, in Arabia and across most of Australia, cryoclasty is rarer, and is linked to cold climatic crises or associated with particular conditions, such as high altitude (Hoggar, Hedjaz, the northern Andes), lying in the shade and zones of maximum condensation. However, this activity is all relative. It is, naturally, more tangible along the subarid and semi-arid regions where the water supply is relatively more abundant. But humidification and gelifraction do not penetrate very deep in any region. Superficial cryoclastic formations found today are usually not very thick, measuring just a few decimeters at most. Indeed, we only largely find gelifracts left over from earlier ages, covering older and more abundant cryoclastic debris.
Haloclasty is the disintegration of rocks caused by the action of evaporite salts, chiefly gypsum (CaSO4 · 2 H2O) and “salt” or halite (NaCl) (Evans 1970; Cooke 1981). It is known that deserts are rich in saline formations due to their geological past (Paleozoic, Triassic, Cenozoic) and from present-day deposits (coastal shores, salt lakes, playas, sebkhas, salt-affected soils) that are easily formed due to the evaporating climate. Salts and saline particles originating from volcanic activity, the sea and lakes, salt outcrops and deposits, and salt-affected soils, are transported by the wind, sea spray and rain. They are deposited by fog, dew and streams and are scattered across rocks and within their interstices. Fortuitously, in hot deserts as well as in cold deserts, the overall lack of water protects these formations from disappearing through being dissolved.
The disintegrating action of salts chiefly works in two ways: one of these is the crystallization of salts following the evaporation of a saline solution that had entered fissures or cavities in rocks. As with ice, the passage from an ionic state to a crystalline state results in a considerable increase in volume and may occur in any kind of rock. The pre-fissured pebbles on regs are, thus, broken into pieces by the distension of their existing microfractures or microjoints. The second way is through the hydration of saline minerals. The classic example of this is the transformation of anhydrite (CaSO4) into gypsum (CaSO4 · 2 H2O), and this operation is also accompanied by a volumetric increase in the hydrated rock. In both cases (crystallization as well as hydration), the action that leads to the rock breaking up, like with freezing, is the dilation of an interstitial body that can exert pressure onto the walls of the gap. This pressure is comparable to that seen in cryoclasty (Prick 1999).
Various experimental research studies (Tricart 1960; Goudie et al. 1970) have shown that haloclasty is more effective than thermoclasty and is even more dependent on lithology than hygroclasty. However, it is a phenomenon that is widespread, due to the wind dispersion of salty dust.
In the desert, all living things are heavily dependent on meteorological conditions, both for their vital development as well as for their distribution over and in the soil. Reciprocally, their presence and dynamism create important counterparts for all kinds of weather formations that are “always carried out in association with biological activities” (Ozouf 2008).
Neither plants nor animals can, by themselves, fissure or fragment hard, coherent, granular or compact rocks. However, they are able to occupy existing cracks and cavities in the rocks and can, thus, participate in the transformation of the rocks. Bioclasty, such as the fragmentation of a rocky outcrop through the action of vascular plant roots filling up a crevice, for instance, may be a spectacular occurrence, but is simply the result of each individual species making use of a given biotope, in a favorable microclimate, to optimize the space in which it has implanted itself. We can also cite here the example of isolated pebbles on an erg breaking up, through a direct shock or through ricochet, when mobilized during the passage of a group of people or animals. And, of course, there is the loosening and destructive effect of burrowing animals on loose rocks or, on the other hand, the consolidating and stabilizing effect of mound-building animals such as termites.
In reality, the action of living beings on the desert is much more complex and goes beyond the mechanical breakup of rocks. Microorganisms in particular, such as bacteria, fungi, algae and lichens, are found everywhere (Freytet 2008a). Their metabolism produces a large part of the active chemical elements found in the environment, such as O2, CO2 and various corrosive organic acids, which modify the pH of the milieu and control the unfolding of various processes (bioalteration). In this way, these organisms participate in the liberation of certain elements, in the formation of new mineral combinations (neo-formations), and the fixation of part of the products of alteration such as silica, oxides and hydroxides of iron, aluminium and manganese. In the mineral world, despite the harsh climate, living beings act as one of the principal factors in the destruction of rocks.
Generally speaking, chemical alterations are governed by the presence of water and accelerated by high temperatures (Millot 1964; Muxart and Birot 1977; Duchaufour 2004; Bourrié, Trolard, and Freytet 2008). Some reactions are destructive through simple removal of matter, while others are more complex and involve the participation of living microorganisms, the formation of new minerals, the exportation of mobile elements or deposition of residual substances. In the desert, these chemical processes are more common than they would seem, given the aridity of the climate and the sparseness of vegetation.
The following principles underlie these reactions:
– We have already spoken of the hydration of rocks when discussing hygroclasty and the transformation of anhydrite to gypsum. This process involves the addition of water molecules to anhydrous or hygroscopic minerals, i.e. minerals capable of absorbing water, such as gypsum, marls or calcareous marls, or certain clayey formations that can “swell”. This addition of water leads to an increase in volume and the loosening of the affected rock. Dehydration, on the other hand, results in contraction of the rock, with the material hardening and shrinkage cracks widening. Water can be brought in by rain but, most notably, comes from dew and morning mists.
Hydration is more effective at lower temperatures, i.e. at night and in winter (condensation), and dehydration occurs more commonly at high temperatures, i.e. in the daytime and in summer (evaporation). Arid climates, whether temperate or warm, are most conducive to the alternating “hydration/dehydration” cycle. Hydration involves the processes of hygroclasty and haloclasty in the disintegration of rocky outcrops, and the creation of certain minor desert forms, such as spalling, exfoliation and arches along the base of rocky structures or, again, the network of polygonal cracks along surfaces covered in fine-grained, dry substances.
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