Arid and Semi Arid Environments: A Summary

Both these types of environments can be classified as having climates with repeatedly low and erratic rainfall, low humidity, high temperatures, high sunshine, high winds, sparse vegetation, low evapotranspiration and high diurnal ranges due to lack of cloud cover. Additionally, they occur at latitudes between 20-40 degrees North and South of the equator.

Arid environments receive under 250 mm of rainfall a year (e.g. Sahara)

Semi-arid environments receive between 250-500 mm of rainfall a year (e.g. Kalahari)

Factors Affecting Aridity

  • Low latitude – subtropical high pressure cells (Hadley Cell) causes air to sink and become warmer and drying resulting in a lack of rainfall. An animation of this is available here.
  • Continentality – e.g. Simpson Desert, Australia is far from rain-bearing winds
  • Offshore currents – remove moisture from the coast such as the case with the Namib Desert, advection fog is formed.
  • Highlands act as ‘rainshadows’ – such as with the Atacama desert in Chile with the Andes. The ‘shadow’ is an area of dryness caused as the mountains act as a ‘barrier’ to the rain causing it to fall before the desert as it condenses and falls at high altitudes. Animation available here.
Types of Weathering Occurring in Deserts: Mechanical, Biological, Chemical
Mechanical: Insolation and Crystal Growth
Insolation
  • High temperatures during the day lead to high surface temperatures and layers of rock heat up and expand. At night, the surface cools and contracts. Over repeated cooling and contracting, the surfaces crack creating what is called ‘onion skin’ weathering. In well-jointed rocks this process causes block disintegration and in varying rock types (e.g. rock with granite and sandstone) granular disintegration
Crystal Growth
  • Occurs in porous, sedimentary rocks (e.g. Sandstone)
  • High temperatures mean saline groundwater is drawn out and evaporated. Salt crystals form between pores and joints leading to weakening and open to insolation weathering.
Biological: Plant Growth
  • Limited – lichen and algae growth causes micro-morphological rock change
  • Respiration of plants leads to increased CO2 and carbonation which ultimately forms oxalic acid
Pressure release – not strictly considered weathering refers to the gradual removal of surface layer rocks and leads to horizontal lines of weakness that forms inselbergs subject to deep chemical weathering by hydrolysis.
Other forms of weathering include:
  • Wetting/drying – caused via flash flooding and seasonal rains causes clay minerals in rocks to swell in rocks and contract during drying causes rock disintegration (mechanical)
  • Frost Shattering – occurs in mountains, repeated cycle of freeze and thaw – in joints and pores of rocks causes shattering and ultimately scree (mechanical)
  • Hydration – minerals such as anhydrite absorb H2O and expand and cause stress on the rock as well as granular disintegration. Additionally, the addition of anhydrite to water forms gypsum that leads to increased carbonation (mechanical)
  • Solution – water dissolves minerals (such as rock salt) and removes it (chemical)
  • Oxidation – oxygen dissolved in water reacts with iron/manganese to form oxides and hydroxides that causes red staining on sandstone and basalt -> RUSTING (chemical)
  • Acid action – carbonic acid dissolves lime rich rocks (chemical)
Features of Weathering
  • Alveoles – small hollows in rocks between 5-50 cm, occur in clusters with thin partitions strengthened by case hardening (layer of salt-encrusted rock formed where salt is drawn to the surface by capillary action) (e.g. sandstone in Utah) formed by crystal growth, wetting and drying and hydration
  • Pedestal Rocks – originally thought to be created by wind erosion, however, now suggested that high evaporation rates draw out water with salts to the surface via capillary action. These salts form a case-hardened resistant layer and the rocks below are weathered creating an indented base (abrasion occurs at the base)
Types of Erosion Occurring in Deserts: Wind & Water
Why Wind is Effective
  • Extreme differences in pressure in deserts
  • Low vegetation cover
  • Level areas = lower friction and greater power
  • High levels of fine debris to transport
Wind: Deflation, Corrasion & Attrition:
  • Deflation: wind picks up and removes loose material forming deflation hollows (e.g. Qattara Depression, Sahara, Egypt), salt pans and desert pavements (whereby larger stones are left behind)
  • Corrasion: sand carried by wind abrades rock surfaces forming sculptured rocks and ventifacts
  • Attrition: sand grains collide and reduce in size
Types of Wind Transportation:
  • Suspension: fine sediment carried within air (between 0-2 metres height)
  • Saltation: sand grains hop along the surface
  • Surface creep: material rolls along the ground (sediment)
Features Created by Wind Erosion:
  • Yardangs: Linear ridge of clay, silt or rock formed via abrasion and deflation. They are orientated parallel to prevailing winds and occur in fleets. They develop in soft but cohesive sediments of silt and clay and also more resistant rocks such as sand and limestone. E.g. Kharga Yardang Field, Egypt
Wind Depositional Features:
  • Dunes: Three ways they form – around obstacles (rocks/plants etc), around rough surface causing frictional drag and deposition or when wind flows converge from different directions
  • The characteristics of the dune are dependent on the strength and direction of wind, the volume of grain size and sand supply, the shape of the land and the vegetation present
  • E.g. Namib Sand Sea, Namib Desert – 1600 km long
Types of Dunes:
  • Fixed – around an obstacle/plant (e.g. Nebkha, Parabolic, Climbing)
  • Free: no obstruction – splits in to two further categories:
    Transverse: sand moves in normal direction to the wind (e.g. Barchan, Namib Sand Sea)
    Linear: sand moves parallel to the wind (e.g. Seifs – Sinai)
Why Water is Effective in Deserts:
  • Heavy seasonal and convective rainfall
  • High surface runoff and low infiltration
  • Low vegetation so interception is low
  • High temperatures bake soil to a hard crust
  • Water forms sheets which form channels and creates features such as wadis, canyons and canyon-lands
Water Erosional Features:
Canyons
  • Gorge with a deep, narrow channel bounded by resistant rocks with an exogenous river
  • Aridity preserves the valley sides as limited erosion can occur
  • Deep canyons caused by tectonic uplift (e.g. The Grand Canyon, Arizona)
  • River discharge varies according to seasonal rains and flash flooding
  • When discharge is low braiding occurs (whereby river splits into multiple streams)
  • Tributary streams swell after flash flooding leading to transportation of large boulders and the formation of rapids
Mesas and Buttes:
  • Isolated and eroded by water
  • Horizontally bedded layers of rock capped by highly resistant bands to protect softer rock
  • Base of mesa pediment is often covered with scree from rock falls
  • Thought to be remains of plateau surfaces dissected by river erosion e.g. Arches National Park, Utah
Wadis:
  • Dry river valley with steep sides, wide floor with channel deposits
  • Can vary from being a dense network of deep channels for several kilometres or single streams
  • Forms when an ephemeral river forms during flash flooding
  • High discharge plus loose, dry sediment -> transport -> short distance
  • When discharge is low a shorter stream is formed. Additionally, storms are localised, water evaporates and water infiltrates
  • When discharge is low braiding occurs
Water Depositional Features:
Salt Pans (AKA Playas, Sabkhas)
  • Flat, low-lying ephemeral lake beds with fine-grained sediments with alkali salts. Vegetation is shallow, however, some grows on the edge of the salt beds (e.g. the mesquite bush – halophytic plant)
  • When filled with precipitation, groundwater seepage and surface run off the bed dries out and clay floor cracks and curls up leaving it open to salt weathering – material is loosened and weathered by wind which then is deposited as crescentic dunes
  • Pans can form where animals overgraze at watering holes
  • E.g. Death Valley, CA

Charlotte Tidman

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