Processes in Hot Arid Environments (DP IB Geography)

Physical & Chemical Weathering in Hot Arid Environments

• 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

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

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

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

Erosion, Transportation & Deposition in Hot Arid Environments

• 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

Formation of deflation hollows

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

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

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

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