Desert pavement

Stone pavement, desert pavement or desert mosaic, plane, consisting of mineral weathering debris surfaces called in heat and cold deserts that exceed a maximum grain size of 100 mm rare. Such surfaces are called in Arabic Reg or Serir while Hammada referred stone deserts with grain sizes above 100 mm.

• 4.1 climate 4.1.1 Influence of temperature
• 4.1.2 Influence of precipitation
• 4.1.3 Influence of Wind

• 4.4.1 Effect of the flora
• 4.4.2 Effect of the fauna

Definition

Pavement surface shapes are typical of arid regions. The soils underlying are usually characterized by a characteristic stone-free stone to poor topsoil, the Vesikularhorizont. An interaction of this bubble- rich horizon with the overlying desert pavement is often assumed in the literature. The genesis of both phenomena can be explained by different theories.

Dissemination

Desert pavement imply its main distribution area in its name. However, deserts limit in this case solely on arid and semi-arid areas within the tropics. Similar phenomena are also known for periglacial areas and mountainous regions, but no Vesikularhorizont has been established here. In the field of Sør Rondane Mountains, East Antarctica, a 30-40 cm deep saline silty layer was detected, which is covered by a stone pavement of easy verwitterbarem gneiss. In Iceland you can find more than 50 cm thick sand deposits beneath a rocky surface. Whether it is in these phenomena merely by convergence forms which are caused by frost heave and intense weathering, or whether similar morphogenetic processes took place, is questionable. There is agreement with the thesis that the pavement acts as Sedimentationsfalle that even in Iceland.

Formation

Vesikularhorizont

Vesikularhorizonte are chemically and physically complex surface soils, which are usually covered by a stone mounting. Characteristic is the bubble- rich structure, resulting in the nomenclature Av derived for vesicular. The term is, however, debatable because within a Av - horizon and the Lessivierung lime and salt accumulations play a role, which represent typical processes of a B horizon. Furthermore, the steady supply of raw material as a characteristic of the C- horizon can be seen. The genesis of the predominantly free of stones to stone poor topsoil is explained by the accumulation of fine-grained eolian material deposited on the rough surface of the stone pavement. The formation of the vesicles lead Evenari et al. (1974), based on their laboratory experiments on the thermal expansion of the area enclosed by the superficial moistening of the soil air back. The surface compaction can also be caused by siltation or by the overlying rock material. In the experiment could Evenari et al. (1974 ) only observe a vesicle under the petri dish and not under bare bottom. However, there was a decrease in the thickness of the Av horizon are found both among larger stones as well as with increasing slope. Positive, however, correlates the vesicle with a higher clay and silt content. The capillary pressure and the water pressure result in compressing the air and within the wetted and therefore very unstable soil structure to form vesicular pores.

Desert pavement

Desert pavement caused by a several year tens of thousands of permanent usually sorting process, in which blown out and removed the originally existing fine material by deflation or Turbation. Desert pavement therefore always also feature recent or fossil erosion surfaces. Fossil stone paving are often found at the base of loess deposits.

If the original surface of various grain sizes, for example, silt, fine gravel and larger stones, the eolian deflation acts selectively. This means that the fine material is first attacked and blows, while the coarse components remain. A built in that way soil horizon eventually reaches a stable, inactive state. A further transformation takes place hardly after complete expression. The smoothness of the resulting surface provides even at higher wind speeds no more attack surface for the Auswehung located below the desert pavement soil. In contrast to the angular, coarse-grained Hammadah, when it comes to strong macro turbulence between the rock fragments that deflation, ie, blowing out the silt, exacerbate, the fine-grained constituents remain among and between the components of the block paving.

Unlike optical impression given by desert pavement, it is not the visible aberodierte surface of a more or less densely packed loose Geröllbrekzie, but an often only 20-30 mm flat stone layer that covers a very loose gravel layer with high Lößanteil. Therefore, the foot can when entering the desert pavement to sink several inches.

Desert pavement are typical surface forms in cold and dry deserts. They are defined by a generally single-layer thin film made ​​of rock fragments, which mainly have a diameter of 3.3-10 cm. Smaller particle sizes are shifted aeolian and larger stones no longer serve as suitable sediment trap. The total can range from a few square meters to several hundred hectares. The degree of coverage is 50-90 %. The origin of the stones can be allochthonous and autochthonous nature, distinguishing between primary ( parent rock ) and secondary ( crushed by the weathering of parent material ) provide a distinction. The exposed surface of the stones is usually coated with a dark brown to black desert varnish, which consists of iron and manganese oxides. Due to their worldwide distribution desert pavement have a variety of names. In Australia they are Gibber Plains or Stone Mantle called, in the USA, the term Desert Pavements has been established and in Central Asia, the Gobi name will be used. Hammada, Reg and Serir are Arabic terms, where one speaks of Hammada in the event of a covered with large rock rubble and gravel deserts Pavements ( round) or Serir be called ( edged) with small stones depending on their form as Reg. Different theories on the development are explained in detail below.

Deflation

The first attempts to explain the emergence of paving stones came from a Auswehung of aeolian movable fine material. A selective transport leads to the removal of the fine material. This leads to the exposure of relatively coarse ingredients which accumulate at the surface, until the system reaches a stable phase. The underlying material is protected from further deflation through the pavement. This thesis is based on field trials that demonstrate a deflation of exposed fine material. In this case, 10 kg of sieved material of the Av - horizon were applied artificially to a square meter. Within three days the fine material was completely blown out. The very low proportion of large rock fragments in the subsoil is contrary to the overlying pavement. This is explained with the superficial blow out smaller stones during Pavementgenese. The substrate is allochthonous material which was provided by erosion of the surrounding mountains and may have been transported and deposited during wetter climates in the form of debris flows. Other scientists, however, leads to deflation only as part of the process of genesis and see swelling and shrinking processes as the cause for the growth of the coarse soil fraction. Furthermore, reference is made to the influence of surface runoff on area as the main cause of erosion, the correlation with the unterlagerndem Vesikularhorizont and other mechanical and chemical processes.

Turbation

This theory represented a continuation of the deflationary theory dar. basic idea of this model is a virtuous circle of stones by Turbationsprozesse. Depending on climatic conditions, substrate, and other factors, it may be salt dynamics, Peloturbation or Kryoturbation. The influence of bioturbation is usually limited in the literature on the lateral movement of stones within the discussion about the regeneration of desert patches and not seen as the cause of a two-dimensional growth of the stones. The principle of growing up through Turbationsprozesse based on an increase in volume of the sediment body. The entire substrate experiences an upward movement. At the subsequent shrinking but the coarse material can not be moved back into the starting position, since the gap has been filled with fine material. Kryoturbation is mainly due to the increase in volume of water when switching to solid state. Due to the climatic conditions, it has no current role for some of the study areas, however, may have exerted influence in periods of wetter and colder climate. A salt dynamics, however, is common within arid climates. The precipitation of sodium chloride or gypsum results in a crystallization pressure. The operation is similar to the principle of Kryoturbation. The main cause of the swelling and shrinking processes during wetting and drying of clay minerals can be seen. It occurs during the drying of the formation of superficial vertical cracks in a hexagonal arrangement, which are mostly too narrow for a downward displacement of the block paving and are therefore filled with loess. After a rainfall event occurs for the increase in volume of the swelling clay minerals, which results in a pressing out of the material. This is supported by a laboratory experiment in which 12 stones were buried in a beaker with fine soil, respectively, and were irrigated with 22 repetitions and dried. Before each cycle some ground material was scattered over the cups. At the end of the experiment the amount of stones was measured and compared with the baseline. The highest upward displacement was 1.02 cm. Stones of a control experiment which were not watered showed no upward movement of

Weathering

Another approach traces the genesis of Pavements on intensive chemical or physical weathering of the rocky surface. The weathered fine material thereby falls into the interstices and deposits there. Is enhanced weathering in the soil due to the higher moisture supply, which can lead to small grain sizes in the substrate while coarser stone mounting. Priority may be mentioned Insolation, frost and salt weathering.

Accumulation

A current model is the so-called floating Pebble hypothesis. So it is with Desert Pavements not deflation areas, but deposition surfaces whose pavement were never covered. Two processes are dominant for the genesis of the surface. On the one hand this is the shift from alluvial basaltic rock fragments from topographically higher lying areas in already filled with fine material sinks. On the other hand grow rock fragments of the starting material to be accumulated under the fine material. The rough surface of the stone pavement causes a decrease in wind speed and thus the transport force of the wind, resulting in deposition of the loess result. A vertical movement of the stones, which prevent the buried Pavements, via the change in volume of the fine material through swelling and shrinking processes. Fine material is deposited between the stones and moved some to the dry cracks. With a precipitation event it comes to the increase in volume and thus to push out of the previously inserted blew material. In addition to the growth of the stone pavement by the swelling and shrinking dynamics of the fine clay material is a close connection with the subjacent Vesikularhorizont is suspected.

Factors

The emergence of Desert Pavements with an underlying Vesikularhorizont is a complex process which is influenced by several factors. The following interactions of the factors with the pavement and the foam floor to above, as well as connectivities with each other are shown.

Climate

The basis for the formation of a Vesikularhorizontes the accumulation of fine material. Erosion, transport and deposition of loess can only under arid conditions and associated low vegetation density and high enough wind energy to happen. Since the distribution of paving stones is strongly tied to the distribution of precipitation which will be discussed in more detail below on the decisive influence of the climate.

Influence of temperature

In addition to various processes that can lead to the growth of the coarse material, it can also result in the formation of a stone pavement by frost heave. Fundamental influence on Pavements exercises the temperature indirectly via interaction with other factors. The temperature-induced high evaporation reduces the plant-available water and thus characterizes the vegetation distribution. A high daily temperature change favors the Insolationsverwitterung, which can lead to a higher coverage of the Pavements.

Influence of precipitation

Although the precipitation amounts and frequencies are very low in arid areas, yet they have a great influence on the morphogenesis of deserts. So splash effects and a superficial runoff are responsible for the crusting of the topsoil. The mechanical energy of raindrops destroyed upon impact the soil aggregates, which in turn close the macropores and it comes to the regulation of the clay minerals. Furthermore, clay minerals and salts act as a binder between coarser fine material. As a result of the compression is reduced to a fine material discharge of the aeolian and on the other decreases the infiltration rate, resulting in an increased surface run-off. Furthermore, the surface runoff as a cause for the lateral movement of stones is seen, which is possible even in areas with very low relief energy. Precipitate is thus seen as a necessary part of the development of stone paving.

Influence of wind

The formation of a predominantly stone-free topsoil under paving stones is attributed to the accumulation of aeolian verfrachteten fine material. This wind is a major cause of the accretionary pedogenesis. The influence on pavement is controversial. So is the existence of ventifacts in the Atacama desert a high wind energy, which can be in this area desert pavement appear to be possible as a result of deflation processes .. Also, experiments show the erosive action of wind on unpaved fine material. After the formation of a stone pavement and the encrustation of Vesikularhorizontes it can, however, only come to a very low deflation, since hardly be displaced material is available. It is estimated that a maximum of stones up to 2 cm in diameter can be shifted aeolian lead with wind speeds up to 60 km / h only to a transport of fine material up to 1 cm. The subordinate role of the wind is justified by the presence of desert varnish, which is a sign of low wind erosion. Wind as a possible cause for the lateral movement plays a role not only in paving stones. Thus, the phenomenon of the moving rocks at the Racetrack Playa is mainly attributed to wind energy. It is independent of the minimum slope for movement of up to 320 kg stones. In order to reduce the stiction, a moistening of the very clayey substrate is necessary.

Water

Have stone paving a decisive influence on the infiltration rate and thus on the plant-available water. Reducing the amount of water is accomplished by infiltrating the Interzeptionsspeicher the stones and the sealing of the surface. Surface runoff, which is able to move alongside sediment and coarse material, is partly responsible for the low relief energy of most Pavements.

Relief

Within flat inclined planes may lead to a flow of movement of fine material by water saturation of the top soil layer. There may be a decrease in the bulk density of the stone pavement with increasing height by about 3% per 100 meters of altitude are found. Furthermore, takes the training of Vesikularhorizontes with increasing slope, which may be due to the lower infiltration rate.

Creature

Influence of Flora

Vegetation is the main cause of the destruction of desert patches and indirectly proportional to the density of the stone pavement. In areas with shrub companies increased aeolian accumulation of fine material is noted under bushes, where the pavement only occurs in the interstices of the diffuse vegetation. Furthermore, vegetation leads to a destruction of the Vesikularhorizontes and prevents its genesis. Maybe is prevented by the lack of Av horizon the migration of the stones. Disorders of Pavements, like inverted stones are spatially correlated strongly with areas annuals plants and can be directly caused by the growth of plants or indirectly through the influence of animals.

Influence of the fauna

In addition, the activity of the animals is thought to be due to both the instant of the stones as well as for the lateral movement of the stones. This can be done by the locomotion of creatures on the ground, or by flocks of birds that land in search of food, and search the ground for seeds. Thus, a disturbance of the Pavements is directly linked to the presence of extant vegetation. An influence of the microfauna on the nature of the surfaces is also discussed. In addition, filaments of bacteria on the soil surface are held to be relevant to the movement of the stones in the Racetrack Playa.

Substratum

Addition of an arid climate Vesikularhorizont also seems to be bound to a suitable substrate. A Vesikulargenese takes place under most types of soil, with sand, was an exception. Thus, a minimum percentage of silt and clay is necessary for the genesis. A positive correlation of Vesikularhorizontes with the proportion of silt and clay could also be detected. Precipitated calcium carbonate and Tonhäutchen to the inner walls of the vesicles, to increase their stability. Due to the arid climate, there is a salt accumulation in the soil. The increase in volume during the crystallization of salt can induce growth of the stone pavement, similar to the processes of frost heave. The crystallization pressure is also responsible for the salt decomposition, which reduces the particle size of the stone support and therefore increases the degree of coverage. A higher degree of coverage, in turn leads to a lower infiltration. An increased salinity in the topsoil reduces the rooting depth of most plants ( with the exception of halophytes ) and thus plant growth generally. The destructive effect of vegetation as outlined in the section above.

Time

Desert Pavements are among the oldest surface forms of the world. Due to an extremely low erosion, can their shape for more than two million years remain unchanged. In the development of Desert Pavements, a climax can be achieved. In this state, the stone pad would have reached by weathering a minimum diameter and thus stocks utmost protection against erosion. A smaller diameter of the overlying rock fragments is thus a sign of a prolonged weathering and thus older origin. Furthermore, the intensity of the desert varnish is seen as an indicator of the age of the Pavements.

On the one hand need Desert Pavements several ten thousand years in the future development, on the other hand runs a regeneration process over a few decades to centuries. So investigations show a significant regeneration of a beräumten stone pavement within a few years. In 80 years as an area in Quadratdezimeterbereich could completely regenerate.

Recent desert pavement

Desert pavement often represent very old surfaces. All larger objects made ​​of durable materials that came in or on the ( loess ) soil before or during the deflation process, eventually 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 first sedimented in the ground and so escaped the chemical and mechanical weathering. Exposed by wind erosion, they come to lie in the formation of desert pavement on the recent surface.