ARID ENVIRONMENT AND ASSOCIATED PROCESSES

ARID ENVIRONMENT AND ASSOCIATED PROCESSES

1. ARID ENVIRONMENT Content Overview Definition and arid environments Causes of aridity Sediment source and transport Significance of weathering in arid regions Insolation Moisture Salt weathering Zone of net erosion Slopes. Exposed lake sediment Prepared by; Rejina Karki

2. 1. Definition and arid environment Arid environments normally have very low rainfall, such as deserts. Many arid environments are hot, causing water to evaporate faster than it can be replaced.   A place that receives less than 25 centimeters (10 inches) of rain per year is considered a desert. Deserts are part of a wider class of regions called  drylands . Deserts cover more than one-fifth of Earth's land area, and they are found on every continent . 

3. Arid environments  encompass more than 35% of the world’s land area, nearly 60 million square kilometers, and represent the most common habitat on Earth after oceans (Mares, 1999) Figure 1: Aridity and humidity 1. Definition and arid environment

4. 1. Definition and arid environment Humidity index( Ih )= ×100% Aridity index( Ia )= ×100% Moisture index( Im )= Ih - Ia   Classification Aridity index Hyperarid AI < 0.03 Arid 0.03<AI < 0.20 Semi arid 0.20<AI < 0.50 Dry subhumid 0.50<AI < 0.65 Moisture Index % Climate Type Above 100 Pre humid(A) 100-80 Humid(B4) 80-60 Humid(B3) 60-40 Humid(B2) 40-20 Humid(B1) 0-20 Moist sub-humid (C2) to -33.3 Dry sub-humid (C1) -33.3 to -66.7 Semi-arid (D) Less than -66.7 Arid (E) Table 2: Classification of climates according to moisture index(%) climate type(Symbol) Table 1; UNESCO aridity classification;

5. Figure 2; World wide distribution of Deserts 1. Definition and arid environment

6. 2. Causes of aridity Aridity is basically a comparison between water supply and water need.  Aridity may be considered as an expression in a qualitative or quantitative manner of the dryness of an area . Causes of aridity Climate Tectonics Human

7. Climate Climate plays major role in aridity Arid environment are typified by variable rainfall Four main individual climate causes are; 2. Causes of aridity

8. 1. Global atmospheric circulation Areas dominated by high-pressure cells and hot (such as the tropics of Cancer and Capricorn) or cold dry subsiding air (the North and South Poles) will have limited rainfall capability. Figure 3; Global atmospheric circulation Climate 2. Causes of aridity

9. 2. Continentality Continentality refers to areas within the interiors of land masses away from the influence of the sea. Central area of continents are dry because air moving over landmasses doesn’t absorb large amount of water vapour . 3. Rain Shadow created by high mountain area Tall mountain ranges, such as the Andes, force air to rise on the windward slopes of the mountains, as pressure decreases and the air cools, condensation and cloud formation occurs and this produces rain. This results in windward slopes being much wetter than leeward slopes and can cause arid areas to form on the leeward side. Climate 2. Causes of aridity

10. 4. Cold upwelling ocean currents. Winds blowing across the ocean surface often push water away from an area. When this occurs, water rises up from beneath the surface to replace the diverging surface water Several deserts lie on the western coasts of continents where there is an upwelling of cold water due to the circulation of wind currents . Figure 4; showing cold upwelling ocean currents 2. Causes of aridity Climate

11. Tectonics Arid environments can be found in a range of tectonic settings . These include currently stable intraplate settings such as cratons (e.g . the Australian Desert) and passive margins (e.g. the Namib ). 1 Orogeny: 2 Continental drift: Anthropogenic agents The mismanagement of land through overgrazing and soil salinization has occurred since the Holocene increase in human population . Desertification , also called  Desertization , the process by which natural or human causes reduce the biological productivity of drylands(arid and semiarid lands) 2. Causes of aridity

12. 3. Sediment source and transport The dominant transport processes and in situ modification (weathering) that can occur after deposition . Water is the main agent of weathering, and lack of water slows weathering. Precipitation occurs in deserts, only less than in other climatic regions. Figure-5: Simplified flow diagram of sediment production and routing in arid environments Sediment in arid environment mainly transport through following processes; A. Suspension B. Saltation C. Creep

13. Physical weathering processes like thermal expansion and contraction caused by daily temperature variations, freeze-thaw cycles (in cold deserts), salt crystal growth, and wind abrasion are prevalent . Weathering in arid regions contributes to the development of unique landforms. Features such as inselbergs (isolated rock hills or mountains), mesas, buttes, etc. In arid regions, weathering can lead to changes in mineral composition. For instance, the breakdown of feldspars and other minerals can lead to the formation of clays and oxides, which are more stable under surface conditions. 4. Significance of weathering in arid region

14. Split stones are commonly occurring and widely reported features in arid regions Figure 7 -Small cobble-sized clasts that exhibit a regular form of in-sit parallel/subparallel splitting that cuts across geological structures such as bedding planes 5. Insolation Large temperature ranges (which can exceed 50°C, Goudie 1997) associated with the diurnal temperatures of deserts have been attributed with expansion and contraction of rocks insitu . Figure6 -showing the theoretical effects of differential heating of rock surface and subsurface layers within a rock and how this notionally varies in response to diurnal patterns of heating and cooling

15. Moisture Arid areas have limited water availability from rainfall, moisture from other sources may be significant. Such sources of moisture tend to be alkali as they are sourced from salt-rich waters such as the sea or salt lakes. This high pH greatly increases silica mobility during rock weathering. Where moisture is trapped within rocks the thermal expansivity of the rock (Goudie 1997) is increased . 6. Moisture

16. In arid environments salt can be sourced from a number of potential areas. These may include : 1. sea water or relict sea water which may be found in bodies of water (e.g. lakes ) formerly connected to the sea. 2 . the release of salts through rock weathering, especially where the bedrock is evaporitic in origin (e.g. halite ). Salt weathering can occur through number of processes ; Figure 8 -Death Valley saltpan showing well-developed evaporite deposits of salt with patterned ground phenomena evident in the form of salt polygons with clay pinnacles. 7. Salt W eathering

17. 1. Salt crystal growth: This may result from changes in solubility with temperature , evaporational concentration of solutions, and mixing of salt solutions with the same ion. Figure-9 Scanning electron microscopy image showing sodium chloride salt crystals between the cleavage planes of mica . 7. Salt weathering 2. Hydration Some of the largest hydration pressures occur during anhydrite (dehydrated calcium sulphate ) to gypsum (hydrated calcium sulphate ) transformation 3. Thermal expansion; When halite (sodium chloride) is heated from 0°C to 60°C it expands by 0.5%, whereas granite minerals only expand by up to 0.2%. This differential expansion can contribute to rock disintegration ( Goudie 1997).

18. Within arid environments sediment production will be dominated by source areas susceptible to erosion. These most commonly include areas that generate conditions of high erosivity (slopes) or areas of high erodibility reflecting lithologies less resistant to erosion, such as lake-bed sediments . 8. Zone of net erosion Slopes; These caprock may be a part of geologic sequence or it could be developed in insitu . Many arid areas are associated with landforms such as pediments, cuestas and mesas that are protected by a caprock . Rates of scarp retreat, and thus sediment production, vary according to the local balance between the geological characteristics and the climate

19. Exposed lake sediment; Lake-bed deposits in arid regions range in thickness from a thin veneer to hundreds of meters. Lake sediments contain a range of grain sizes, depending on the extent and depth of the lake system, and the characteristics of the surrounding catchments feeding the lake with water and sediment Lake sediments may have been sourced originally from terrigenous , allochthonous sediments external to the water body . The finest of these lake sediments will be prone to dust erosion. 8. Zone of net erosion

20. This is currently occurring in Lake Burdur in central Turkey, where tectonically elevated Pliocene lake sediments are being eroded by gullying, and redeposited in the smaller, and constantly shrinking, modern lake system. Figure -10 Satellite images of the Lake Burdur over time showing the water surface areal changes 8. Zone of net erosion Exposed lake sediment;

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