Stone pavement

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Desert pavement. The cube has an edge length of 1 cm.
Desert pavement in the Rub al-Kali . The cube has an edge length of 1 cm.

Stone pavements are flat surfaces in deserts , consisting of mineral debris . Their maximum grain sizes rarely exceed 100 millimeters. They are formed by pedogenic enrichment processes and can cover large areas.

Defining description

Stone pavement, desert pavement or desert mosaic , in English called desert pavement , are typical surface forms of arid areas . They consist of a thin, single-ply, gravel fraction surface layer, the thickness of which is only one or two particle diameters (3.3 to 10 centimeters). Smaller grain sizes are shifted aeolian and larger stones no longer serve as suitable sediment traps.

The total area of ​​stone paving can vary from a few square meters to a few hundred hectares. Their degree of coverage is 50–90%. The rock particles making up the pavement can be rounded or angular. Their origin is either allochthonous or autochthonous , a distinction being made between primary (source rock in its original state) and secondary (source rock crushed by weathering) provision. The exposed surface of the stones is usually covered with a dark brown to black desert varnish, which consists of iron and manganese oxides.

The pavement covers a much thicker and finer-grained mantle of predominantly Aeolian sediments, which does not contain any coarser pebbles or rock fragments. This underlying mantle is usually characterized by a characteristic, stone-free to stone- poor topsoil, the so-called vesicular horizon or Av horizon . An interaction of this bubble-rich horizon with the desert pavement on top is often suspected in the literature.

Designations

Stone pavement has different regional names depending on the type of appearance. If the pavement is built up mainly from rock fragments and significantly fewer, loose rubble, the term rubble hammada is appropriate. The word hammada comes from Arabic .همادة hammāda is derived from hāmid هامدand means dead, lifeless, frozen, extinguished, sterile . Fels-Hamadas are extensive areas that are largely littered with large, angular rock fragments (with grain sizes> 100 millimeters). They occur, for example, in the Sahara of Libya . If there is a predominance of smaller, rounded fragments and a closed surface, the Arabic term Reg is appropriate, which means to become smaller and denotes gravel deserts. Examples of regs can be found e.g. B. at the Dead Sea and in Sinai . Reg surfaces are called Serir in the Central Sahara , but unlike Regs they are composed of angular material. In Australia , stone pavements are called gibber , gibber plains or stone mantle and in central Asia gobi or saï .

Emergence

In contrast to the visual impression created by stone pavement, it is not the visible, eroded surface of a more or less densely packed, loose rubble breccia, but an often only 20-30 millimeter flat layer of stone, which has a very loose rubble shell with a high proportion of silt covers. The foot can therefore sink several centimeters when stepping on stone pavement.

Rather, stone paving is created through an accumulation of the gravel fraction on the surface. As a rule, this sorting process lasts for several decades, with processes such as deflation or turbation blowing out, transporting and relocating the fine material originally present. Stone paving therefore always marks recent or fossil erosion areas.

The following geomorphological processes can bring about the enrichment:

  • Sediment blown out by the wind (deflation).
  • Surface rinsing (lessivation).
  • Upward migration of the gravel fraction (turbation).
  • Slow soil formation (accumulation).
  • Differential weathering (substrate erosion and surface erosion).

deflation

The deflation is usually cited as a first-rate development process for paving. Early attempts to explain this were based on the expulsion of the aeolian relocatable fine material. Accumulation is therefore carried out by the wind, which blows out small grain sizes and leaves the gravel fraction as a residue ( lag deposit ).

The Aeolian deflation is therefore selective. This means that the fine material is attacked and blown out first, while the coarse components remain and passively accumulate on the surface. A soil horizon created in this way will eventually reach a stable, inactive state. Once fully developed, there is hardly any further redesign. Even at higher wind speeds, the smoothness of the resulting surface no longer offers any surface to attack the ground under the stone pavement, which is therefore protected from further deflation. In contrast to the angular, coarse-grained Hammada, in which there is strong macro-turbulence between the rock fragments, which further intensifies the blowing out of the silt, the fine-grained components remain under and between the components of the stone pavement.

The deflation thesis is based on field tests that prove deflation of exposed fine material. 10 kg of sieved material from the Av horizon were artificially applied to one square meter. The fine material was completely blown out within three days.

The very low proportion of large rock fragments in the subsurface is contrary to the stone pavement on top. This is explained by the superficial blowing out of smaller stones during pavement genesis. The substrate is an allochthonous material that was provided by erosion of the surrounding highlands and was possibly transported and deposited during wetter climates in the form of mass movements (e.g. mudslides).

Examples of proven deflation can be found in Israel and the Peruvian coastal desert. In Peru , the pavement is underlain by sand and gravel and not, as is usual with stone paving, of silt and clay.

However, some researchers question the efficiency of wind erosion because of:

  • The observed binding of the fine material in crusts.
  • The increasing shielding of the fine material by the slowly emerging plaster that gains in relief.
  • The very thick layer of fine material in the coat under the pavement.

Deflation is therefore only seen as a partial process in the genesis of stone paving. Rather, swelling and shrinking processes (turbation) are considered to be the causes for the growth of the coarse soil portion. Furthermore, reference is made to the influence of surface runoff (washout) as the main cause of extensive erosion, the interrelationship with the underlying vesicular horizon and other mechanical and chemical processes.

Irrigation

The flushing (Lessivierung), engl. wash , is seen as another important enrichment process, whereby inundations of the surface remove fine material. Several researchers have been able to provide quantitative evidence that significant amounts of fine material are released in stone pavements that have been affected by lavage. For the area around the Aguila Mountains in Arizona , McHargue even sees surface washing as a necessary condition for stone paving. So that the first patches can form, 1 to 3 centimeters of fine material must be washed out; The plasters only become stable at 3 to 15 centimeters.

In this context, however, the widespread surface crusts, which call the efficiency of the flushing process into question, are contradictory.

Turbation

The third and undoubtedly most frequently added solid enrichment process is an upward migration of gravel fraction by Turbation . This theory represents a continuation of the deflation theory. This is understood to mean the upward dynamics of coarser rock material in the swellable fine material of the underlying mantle. Depending on climatic conditions, substrate and other factors, it can be salt dynamics , peloturbation or cryoturbation . In the literature, the influence of bioturbation is mostly limited to the lateral movement of stones within the discussion about the regeneration of stone pavers and is not seen as a cause of the stones growing over a large area.

Cryoturbation is mainly based on the increase in volume of water when it changes into the solid state. Due to the climatic conditions - it is particularly important in the periglacial, glacial and alpine milieu - it does not play a current role for some of the study areas, but it may have had an impact in times of a wetter and colder climate. Salt dynamics, on the other hand, are widespread within arid climates. When sodium chloride or gypsum precipitates , a crystallization pressure arises. The mode of action is comparable to the principle of cryoturbation.

The active principle of growth through peloturbation is based on an increase in the volume of the sediment body. The upward movement is set in motion by alternating wetting and drying, which causes the fine material to swell and shrink again. The entire substrate experiences an upward movement. The swelling of fine-grained material pushes larger fragments upwards. At the onset of desiccation, the larger fragments cannot sink back into their starting position, since fine material has now penetrated the space that has been created. By repeating the process of wetting / drying out, the coarse particles are slowly shifted to the surface.

The swelling and shrinking processes during the moistening and drying of clay minerals are seen as the main cause of the upward movement. During the drying process, superficial vertical cracks appear in a hexagonal arrangement, which are usually too narrow for the stone pavement to move downwards and are therefore filled with silt. After a precipitation event, the swellable clay minerals increase in volume, which leads to the material being pressed out. This thesis is supported by a laboratory experiment in which 12 stones were buried in a glass beaker with a fine bottom and sprinkled and dried with 22 repetitions. Some soil material was sprinkled over the cups prior to each cycle. At the end of the experiment, the height of the stones was measured and compared with the initial value. The highest upward displacement was 1.02 centimeters. On the other hand, stones from a control experiment which were not irrigated showed no upward movement.

The enrichment process of the turbation is most effective in mantles, the bottoms of which show a strong structural contrast between the A and B horizons. However, a layer of gypsum below the stone pavement can hinder the enrichment process.

The turbation theory remains difficult as a whole, since it can hardly be observed in nature - so no evidence has yet been found for rock fragments in transit . Furthermore, in extremely arid regions, a sufficient depth of penetration of moisture appears to be quite questionable.

Slowly accumulating soil formation

The enrichment process of slow, accumulating soil formation (English cumulic pedogenesis ) is possibly applicable for the center of Australia. According to Mabbutt (1977 and 1979), dust is trapped by the rough surface of the stone pavement. As the accumulation of dust increases, the stone pavement moves further and further upwards. Any size sorting takes place only in the uppermost area of ​​the fine-grained casing.

A comparable educational mechanism is also used by McFadden et al. a. (1987) cited. In their opinion, the formation of stone paving in the Cima Volcanic Field in California goes back to the in-situ weathering of the basalt and the removal of the weathering products on the surface of the earth. After the accumulation of Aeolian silt and clay beneath the gravel fraction, the stone pavement rises. It thus arises directly on the earth's surface and changes little after its formation. However, the bottoming out below is continuing. Wells et al. a. (1991) were able to largely confirm this model through their dating work using cosmogenic nuclides ( 3 He and 21 Ne). They found that the age of the lava and the age of the clasts could not be statistically separated.

A comparable, more up-to-date model is the so-called floating pebble hypothesis . As a result, desert pavements are not deflation areas, but deposition areas whose stone paving was never covered.

Two processes are dominant for the genesis of the surface. On the one hand, there is the alluvial displacement of basaltic rock fragments from topographically higher lying areas into depressions already filled with fine material. On the other hand, rock fragments of the original material grow under the accumulating fine material. The rough surface of the stone pavement causes a decrease in the wind speed and thus the transport force of the wind, which results in a deposition of the silt fraction. A vertical movement of the stones, which prevents the pavement from being buried, takes place via the volume change of the fine material through swelling and shrinking processes. Fine material is deposited between the stones and part of it is shifted into the dry cracks. In the event of a precipitation event, the volume increases and the previously blown material is pushed out. In addition to the growth of the stone pavement due to the swelling and shrinking dynamics of the fine clay-containing material, a close connection with the underlying vesicular horizon is assumed.

Differential weathering

The fifth enrichment process in 1977 by Mabbutt was differential weathering of the substrate (Engl. Substrates weathering ) proposed. Due to the distribution of moisture, mechanical weathering is more effective below the surface than on the dry surface. There is therefore an increased rate of weathering in the depths, with the result that large clasts disintegrate more quickly and a layer of fine-grained material without clasts results. According to Mabbutt, this process should be particularly effective in the formation of stone paving with a granitic composition . He also considers the weathering of coarse clasts below the surface in the presence of salts to be very efficient.

The weathering in the soil is intensified by the higher moisture supply, which can lead to small grain sizes in the substrate with a coarser stone layer at the same time. Insolation , frost and salt weathering are primarily mentioned .

Another approach traces the genesis of pavements back to intensive chemical or physical weathering of the stony surface. The weathered fine material falls into the gaps and is deposited there.

In summary, it can be said that stone paving finds an explanation in a whole series of enrichment processes. Some of these processes can run independently, while others work together. They can be designed differently depending on the location, depending on the prevailing climate, the geomorphological conditions, the available raw material and the locally realizable soil formations.

Vesicular horizon

Vesicular horizons are chemically and physically complex topsoils, which are usually covered by a stone layer. The vesicular structure is characteristic, from which the nomenclature A v for vesicular is derived. The designation is debatable, however, because within an Av horizon the lessivation as well as lime and salt accumulations play a role, which represent typical processes of a B horizon. Furthermore, the constant supply of raw material is to be seen as a characteristic of the C-horizon. The genesis of the predominantly stone-free to stone-poor topsoil is explained by the accumulation of fine-grain Aeolian material, which is deposited on the rough surface of the stone pavement. The formation of the vesicles lead Evenari et al. (1974), based on their laboratory experiments, traced back to the thermal expansion of the air trapped by the superficial moistening of the soil. The surface compaction can also be caused by silting up or by the rock material lying on top. In the experiment Evenari et al. (1974) only observed vesicle formation under the Petri dish and not under uncovered soil. However, a decrease in the thickness of the Av horizon was found both under larger stones and with increasing slope inclination. In contrast, vesicle formation correlates positively with a higher clay and silt content. The capillary pressure and the water pressure lead to a compression of the air and to the formation of vesicular pores within the damp and therefore very unstable soil structure.

Influencing factors

The formation of desert pavements with an underlying vesicular horizon is a complex process that is influenced by several factors. In the following, the interactions of the influencing factors with the stone pavement and the foam floor are shown, as well as connectivities with each other.

climate

The basis for the formation of a vesicular horizon is the accumulation of fine material. Erosion, transport and deposition of loess can only happen under arid conditions and the associated low vegetation density and sufficiently high wind energy. Since the distribution of stone pavers is strongly linked to the distribution of precipitation, the decisive influence of the climate will be discussed in more detail below.

Influence of temperature

In addition to various processes that can lead to the growth of the coarse material, a stone pavement can also form due to frost elevation. The temperature has a fundamental influence on pavements indirectly through the interaction with other factors. The temperature-related high evaporation reduces the water available to plants and thus affects the distribution of vegetation. A high daily temperature change favors the insulation weathering, which can lead to a higher degree of coverage of the pavement.

Influence of precipitation

Although the amounts and frequencies of precipitation in arid areas are very low, they have a major influence on the morphogenesis of deserts. Splash effects and a superficial runoff are responsible for the encrustation of the topsoil. The mechanical energy of the raindrops destroys the ground aggregates on impact, which in turn close the macropores and the clay minerals are regulated. Clay minerals and salts also act as binders between coarser fine material. As a result of the compaction, on the one hand, the aeolian discharge of fine material is reduced and, on the other hand, the infiltration rate decreases, which leads to increased surface runoff. Furthermore, the surface runoff is seen as the cause of the lateral movement of stones, which is also possible in areas with very low relief energy. Precipitation is therefore seen as a necessary part of the formation of stone paving.

Influence of wind

The formation of a predominantly stone-free topsoil under stone paving is attributed to the accumulation of fine material transported aeolian. This makes wind a major cause of accretionary pedogenesis. The influence on stone pavement is controversial. The existence of wind canters in the Atacama Desert shows a high level of wind energy, which makes desert pavement appear possible in this area as a result of deflation processes. Experiments also show the eroding effect of wind on unpaved fine material. After the formation of a stone pavement and the encrustation of the vesicular horizon, on the other hand, only a very slight deflation can occur, since hardly any material is available that can be moved. It is assumed that a maximum of stones up to 2 cm in diameter can be relocated aeolian, with wind speeds of up to 60 km / h only leading to the transport of fine material up to 1 cm. The subordinate role of the wind is justified by the presence of the desert varnish, which is a sign of low wind erosion. Wind as a possible cause of the lateral movement does not only play a role in stone paving. The phenomenon of moving stones in Racetrack Playa is mainly attributed to wind energy. In doing so, stones weighing up to 320 kg are moved regardless of the minimum gradient. In order to reduce the static friction, it is necessary to moisten the very clayey substrate.

water

Stone paving has a decisive influence on the infiltration rate and thus on the water available to plants. The reduction in the amount of infiltrating water is caused by the interception memory of the stones and the sealing of the surface. Surface runoff, which is able to move coarse material as well as sediment, is made responsible for the low relief energy of most pavements.

relief

Within gently sloping levels, the water saturation of the top soil layer can cause the fine material to flow. A decrease in the density of the stone pavement with increasing height of about 3% per 100 meters of altitude can be determined. Furthermore, the formation of the vesicular horizon decreases with increasing slope, which is possibly due to the lower infiltration rate.

Creature

Influence of flora

Vegetation is the main cause of desert pavement destruction and is indirectly proportional to the density of the stone pavement. In areas with shrub communities, there is an increased aeolian accumulation of fine material under shrubs, with the stone paving occurring only in the spaces between the diffuse vegetation. Furthermore, vegetation leads to a destruction of the vesicular horizon or prevents its genesis. The immigration of the stones may be prevented by the lack of the Av horizon. Disturbances of the pavement, such as upturned stones, correlate spatially strongly with areas of annual plants and can be caused directly by the growth of the plants or indirectly by the influence of animals.

Influence of the fauna

In addition, the activity of animals is believed to be the cause of both rock turning and lateral movement of rocks. This can be done by the locomotion of living beings on the ground, or by flocks of birds that land to search for food and search the ground for seeds. A disturbance of the pavement is therefore directly linked to the presence of recent vegetation. An influence of the microfauna on the nature of the surfaces is also discussed. Furthermore, filaments of bacteria on the soil surface are considered relevant for the movement of the stones in the Racetrack Playa.

Substrate

In addition to an arid climate, the vesicular horizon also seems to be bound to a suitable substrate. Vesicular genesis occurs under most types of soil, with sand being an exception. Accordingly, a minimum proportion of silt and clay is necessary for the genesis. A positive correlation of the vesicular horizon with the proportion of silt and clay could also be demonstrated. Precipitated calcium carbonate as well as clay membranes on the inner walls of the vesicles can increase their stability.

The arid climate leads to an accumulation of salt in the soil. The increase in volume as the salt crystallizes out can induce the stone paving to grow, similar to the processes of frost uplift. The crystallization pressure is also responsible for the weathering of the salt, which reduces the grain size of the stone layer and thus increases the degree of coverage. A higher degree of coverage in turn leads to less infiltration. An increased salt content in the topsoil reduces the root depth of most plants (with the exception of halophytes) and thus plant growth in general. The destructive effects of vegetation have been discussed in the section above.

time

Stone paving is one of the oldest surface forms in the world. Due to extremely low erosion, their shape can remain unchanged for more than 2 million years. A climax stage can be reached in the development of desert pavements. In this condition, the stone layer would have reached a minimal diameter due to weathering and thus the greatest protection against erosion would exist. A smaller diameter of the rock fragments on top is consequently a sign of prolonged weathering and therefore of older origin. Furthermore, the intensity of the desert varnish is seen as an indicator of the age of the pavement.

On the one hand, desert pavements need tens of thousands of years to fully develop, on the other hand, a regeneration process takes a few decades to centuries. Studies show a significant regeneration of a cleared stone pavement within a few years. In 80 years such an area in the square decimeter range could regenerate completely.

Recent desert pavement

Meteorite find on desert pavement (chondrite, 408.50 g)

Desert pavements often represent very old surfaces. All larger objects made of resistant materials that got into or on the (loess) soil before or during the deflation process end up on the recent surface. These include prehistoric tools as well as modern artifacts, but also meteorites. Meteorite finds on desert pavement often represent very old cases that were initially sedimented in the ground and thus escaped chemical and mechanical weathering. Uncovered by wind erosion, they come to rest on the recent surface when the desert pavement is formed.

distribution

The name desert pavement implicitly includes the main distribution area. However, in this case deserts are not limited to arid and semi-arid areas within the tropics. Similar manifestations are also known for periglacial areas and mountain regions, but no vesicular horizon has been detected here. For example, sand deposits more than 50 centimeters thick are found under a stony surface in Iceland . In the area of ​​the Sør Rondane mountains in Antarctica , a 30-40 centimeter thick, salty, silty layer was detected, which is covered by a stone paving made of easily weatherable gneiss . It is questionable whether these phenomena in the periglacial and glacial areas are merely forms of convergence that arose from frost uplift and intensive weathering, or whether comparable morphogenetic processes took place. There is agreement on the thesis that the stone pavement also functions as a sedimentation trap in Iceland.

Fossil stone paving is often found at the base of loess deposits .

Their distribution in detail:

literature

  • Detlef Busche: The central Sahara. Surface shapes changing. Perthes, Gotha 1998.
  • S. Buhl: The Hammadah al-Hamra Meteorite Field after 20 Years of Prospecting. In: Meteorite Magazine , Nov 2004, pp. 37-48.
  • AJ Parsons, AD Abrahams: Geomorphology of Desert Environments. Springer, Dordrecht (Netherlands) 2009.

Web links

Individual evidence

  1. ↑ LD McFadden, SG Wells, JC Dohrenwend: Influences of quaternary climatic changes on processes of soil development on desert loess deposits of the cima volcanic field, California . In: Catena . tape 13 , no. 4 , 1986, pp. 361-389 .
  2. ^ A b c R. Cooke, A. Warren, A. Goudie: Desert Geomorphology . 1993.
  3. ^ RU Cooke, A. Warren: Geomorphology in Deserts . University of California Press, Berkeley 1973.
  4. ^ A b c d e K. Anderson, S. Wells, R. Graham: Pedogenesis of vesicular horizons, Cima Volcanic Field, Mojave Desert, California . In: Soil Science Society of America Journal . tape 66 , no. 3 , 2002, p. 878-887 .
  5. ^ R. Amit, R. Gerson: The evolution of Holocene reg (gravelly) soils in deserts: an example from the Dead Sea region . In: Catena . tape 13 , 1986, pp. 59-79 .
  6. ↑ a b c J. A. Mabbutt: Desert landforms . MIT Press, Cambridge, Mass. 1977.
  7. ↑ a b c d e f g R. Cooke: Stone pavements in deserts . In: Annals of the Association of American Geographers . tape 60 , no. 3 , 1970, p. 560-577 .
  8. ↑ J. Dan et al. a .: Evolution of reg soils in southern Israel and Sinai . In: Geoderma . tape 28 , 1982, pp. 173-202 .
  9. ^ A b c P. M. Symmons, CF Hemming: A Note on Wind-Stable Stone-Mantles in the Southern Sahara . In: The Geographical Journal . tape 134 , no. 1 , 1968, p. 60-64 .
  10. ^ CS Denny: Alluvial fans in the Death Valley region, California and Nevada . In: US Geological Survey, Professional Paper . tape 466 , 1965.
  11. ↑ D. Sharon: On the nature of hamadas in Israel . In: Journal of Geomorphology . tape 6 , 1962, pp. 129-147 .
  12. ↑ LE McHargue: Late Quaternary deposition and pedogenesis on the Aguila Mountains piedmont, south-eastern Arizona (PhD) . University of Arizona, Tucson 1981.
  13. ^ ME Springer: Desert pavement and vesicular layer in some soils of the Lahontan Basin, Nevada . In: Soil Science Society of America Journal . tape 22 , 1958, pp. 63-66 .
  14. ↑ RW Jessup: The stony tableland soils of the southeastern portion of the australian arid zone and their evolutionary history . In: European Journal of Soil Science . tape 11 , no. 2 , 1960, p. 188-196 .
  15. ↑ JA Mabbutt: Pavements and patterned ground in the Australian stony deserts . In: Stuttgart Geographical Studies . tape 93 , 1979, pp. 107-123 .
  16. ^ SG Wells, LD McFadden, CT Olinger: Use of cosmogenic 3He and 21Ne to understand desert pavement formation . In: Geological Society of America Abstracts with Programs . tape 23 , no. 5 , 1991, pp. 206 .
  17. ↑ LD McFadden, SG Wells, JC Dohrenwend: Influences of quaternary climatic changes on processes of soil development on desert loess deposits of the cima volcanic field, California . In: Catena . tape 13 , no. 4 , 1986, pp. 361-389 .
  18. ↑ a b N. Matsuoka, CE Thomachot, CE Oguchi, T. Hatta, M. Abe, H. Matsuzaki: Quaternary bedrock erosion and landscape evolution in the Sør Rondane Mountains, East Antarctica: Reevaluating rates and processes . In: Geomorphology . tape 81 , no. 3–4 , 2006, pp. 408-420 .
  19. ^ WB Bull: Geomorphic responses to climate change . Oxford University Press, New York 1991.
  20. ^ A b M. Evenari, D. Yaalon, Y. Gutterman: Note on soils with vesicular structure in deserts . In: Z. Geomorph. tape 18 , no. 2 , 1974, p. 162-172 .
  21. ^ PK Haff, BT Werner: Dynamical processes on desert pavements and the healing of surficial disturbances . In: Quaternary Research . tape 45 , no. 1 , 1996, p. 38-46 .
  22. ↑ O. Arnalds, FO Gisladottir, H. Sigurjonsson: Sandy deserts of Iceland: an overview . In: Journal of Arid Environments . tape 47 , 2006, p. 359-371 .
  23. ^ Hammada de Tindouf at Geonames
  24. ↑ Hammada Tounassine at Geonames