Processes in Hot Arid Environments (SL IB Geography)

• Overall, weathering rates are slow in hot arid and semi-arid environments 
• Rock breakdown is mainly through physical weathering

Physical weathering

• The main forms of mechanical or physical weathering in hot deserts are:
  • Thermal fracture
  • Pressure exfoliation

Thermal disintegration

• Also called thermal fracturing, thermal exfoliation or onion skin weathering
• Caused by extreme diurnal temperature
• During the day, rocks absorb insolation and expand
• At night heat is released and the rock contracts
• This process continues (cyclical process 1 in the diagram below) until eventually, fractures form along the surface
• These fractured pieces expose the rock beneath and the process continues (cyclical process 2)
• Thermal expansion and contraction occur at different rates on different parts of the rock

Image of cyclical thermal fracturing in hot deserts

Pressure exfoliation

• Also known as pressure-release exfoliation
• This is where overburden is removed through weathering and erosion and the rock beneath is gradually exposed
• The removal of the weight of the overburden releases the pressure on the rock beneath and the outer layer splits/fractures apart
• Once fractures develop, water enters and chemical weathering takes place, leading to the formation of new low-density minerals
• This enhances the fractures and encourages slabs of rock to detach from the rock surface

Image showing pressure release exfoliation over time

Chemical weathering

• Rates of chemical weathering are low due to a lack of water
• Chemical weathering depends on rock type and includes:
  • Crystal growth
  • Granular and block disintegration
  • Hydration

Crystal growth

• Crystal growth is the major cause of chemical weathering, especially in porous, sedimentary rock such as sandstone
• Coastal, fog-bound deserts are particularly prone to this type of weathering
• High temperatures draw salty groundwater to the surface, where it evaporates, leaving behind salt crystals
• Salt crystals grow between pores and joints, leading to granular and block disintegration

Salt crystallisation is where salty/saline water is drawn into the pores of a rock to evaporate and expand, exerting pressure on the rock

Granular disintegration 

• Found in rocks of different coloured minerals
• Darker minerals absorb more heat than lighter ones
• The rock will break down into grains to produce sand-sized material

Block disintegration

• Well-jointed and bedded limestone breaks into blocks along the natural weaknesses

Hydration

• The minerals in rocks expand when they absorb water, putting the surrounding rocks under more stress
• This build up of stress causes the rock to snap along its joints

• Wind and water are the main agents of erosion, transportation and deposition 

Wind action

• Winds in hot, arid environments are usually:
  • Strong (over 20 kilometres per hour)
  • Turbulent
  • Come from a mostly constant direction
  • Blow for a long time
• Friction slows down surface wind speed
  
  • The rougher the ground, the more the wind speed is reduced
• Transportation is greater where there is:
  • A lack of vegetation
  • Sediment is dry loose and small
• Deposition is mostly as sand dunes
• The shape and size depend on:
  • The supply of sediment
  • The direction of wind
  • Type of ground surface
  • Presence of vegetation
• Wind erodes in three ways:
  • Deflation
  • Abrasion
  • Attrition

Deflation

• The wind removes fine sediments and lowers the desert floor
• This leaves behind coarser gravels, forming a slag-type deposit of duricrust from the exposed bedrock
• This hardened desert floor, known as desert pavement, limits any future deflation and forms the features of a 'reg' desert
• Strong wind eddies can further hollow out the desert surface to form deflation hollows (blowouts)
• These deflation hollows are usually small but can extend several square kilometres 

Deflation of the desert floor

Image showing the formation stages of a desert pavement through deflation

Formation of deflation hollows

Image showing how deflation hollows form 

Abrasion

• Wind laden with sand carves and sculpts rock (sandblasting), usually within a metre of the desert floor
• The strength, duration and direction of the wind will dictate the rate and intensity of the abrasion
• The geology of the rock also factors into the rate of abrasion, with sandstone and limestone being the easiest to erode

Abrasion in deserts

Image showing how wind abrades rocks in the desert

Attrition

• Attrition happens during transportation
• The wind picks up small pebbles.
• These pebbles are too heavy to be carried far so attrition happens within 0.5 m of the desert floor
• As they move, the pebbles collide with each other, and bits of rock are broken 
• The wind picks up these smaller fragments and carries them along the desert floor, crashing into one another and break into even smaller pieces.
• Eventually, these particles become part of the abrasion process 

Attrition in deserts

Image showing how desert attrition occurs

Wind transportation in hot, arid environments

• Wind transports sand particles away from an area in 3 ways:
  
  • Creep occurs when sand grains (>0.25 mm) slide and roll across the surface
    • Small changes in air pressure result in a lift of no more than 2 mm
  • Saltation occurs when the wind is strong enough to bounce particles close to the ground
    • Material moved by saltation may move around and push bigger particles that are too heavy to lift.
    • The steady bombardment moves small rocks and pebbles across the desert's surface 
    • The wind can not carry the particles very far because they are too heavy, and they only rise 10 cm above the desert floor
  • Suspension occurs when high-speed winds carry small dust particles (fine silt and clay) of less than 0.15 mm high into the atmosphere
    • This process is the main cause of dust storms
    • These dust particles can travel for thousands of kilometres
      • Saharan dust has been found in the Amazon basin, the Himalayas, the UK, Japan, and other regions

Transportation of sediment in deserts

Image showing how material is transported along the desert surface

Wind deposition

• Deposition will only happen when the wind speed falls below a certain level
• This depends on how big the particle is and how strong the wind is
• The wind can then shape the object that has been dropped
• Only 30% of all deserts are covered in wind-blown sands and are called 'erg' deserts
• Some of the most unique landscapes on Earth are sand seas, also known as great erg deserts
  • The Great Sand Dunes National Park in Colorado, USA, has over 600 km² of different types of sand dunes

Water action in hot, arid environments

• The impact of water is significant in shaping desert landscapes
• Rainfall is sporadic; however, rare, substantial, and intense flash floods generate considerable runoff
• Due to convectional mountain storms, these flash floods flow as water sheets or are held within the channel 
• Although short-lived, huge amounts of sediment are washed down from the mountains and deposited on alluvial lowland plains 
• There is not much vegetation to soak up water, so soil levels are low
• This limits plant roots disturbing the soil, making it dense and hard
• Rain is able to hit the surface hard, moving small particles around and covering soil pores, which slows the rate of infiltration even further
• Because of this low rate of infiltration, slopes of less than 2° have a lot of horizontal flow

Erosion

• Hydraulic action
• Abrasion
• Corrosion
• Attrition

Transportation

• Traction
• Saltation
• Suspension
• Solution

Deposition

• This takes place when velocity drops 
• Usually occurs:
  • On the inside of meander bends
  • Where streams and channels flow out of the mountain edges onto flat desert plains

Sheetwash processes

• Studies show that surface flow is most likely to happen within 10 minutes of the rain starting
• First, there is sheet flooding, in which water flows widely over land and is not limited to channels.
• Second, water from all sources gathers in deep, steep-sided wadis or arroyos
• It happens about once a year on average along the semi-arid edges of the Sahara and once every ten years in the very dry interiors
• The fact that floods do not happen very often compared to the number of wadis indicates that they were built when storms were more frequent; they are a relict feature

Other water processes

• Splash erosion
• Rill erosion
• Gully erosion
• Bank erosion