Objectives: By the end of this subtopic learners should be able to:
Describe factors influencing location of hot deserts.
Describe characteristics of hot deserts.
Describe and explain the main denudation processes operating in hot deserts and their resultant features.
Outline the influences of human beings on desert soils and water.
Describe causes, effects and methods of controlling desertification.
What is a desert?
A desert is a place that receives very little or no rainfall .The climate of a desert can either be extremely hot or cold.
There are two types of deserts namely cold and hot deserts.
Cold deserts have extremely low temperatures throughout the year.
Hot deserts have very low rainfall and high temperatures.
There are two types of hot deserts, the semi-arid and arid.
Arid deserts receive an average rainfall of less than 200mm per year.
Semi-arid deserts receive an average of 500mm per annum.
In this topic we are going to study in detail hot deserts.
Desert Locations
Major hot deserts are located between 25Ëš and 40Ëš north and south of the equator.
Most deserts lie on western sides of continents between latitude 15Ëš and 30Ëš North and South of the Equator.
The tropics of Cancer and Capricorn usually run through the centres of these deserts.
Trade winds blow offshore and are very dry having blown over continents resulting in loss of moisture.
Most deserts are located away from the convergence zone.
Africa has three major hot deserts: Sahara, Kalahari and Namib.
Sahara is the world largest desert covering countries like Morocco, Tunisia, Egypt, Libya and Chad.
Namib Desert is in Namibia and Kalahari Desert is in Botswana.
Development of hot deserts
They develop on the western sides of the continents, in the trade wind belts where the winds blow offshore.
The air loses its moisture along the way from the tropics, by the time it reaches the deserts it will be a dry air mass, for example the Harmattan winds of the Sahara desert.
Some deserts develop on mountain sides and are caused by the rain shadow effect.
As air moves up over a mountain range, it loses the ability to hold moisture so it loses it through rain or snow.
By the time the air moves down the leeward side of the mountain it will be dry and gives little or no rainfall thus a desert is formed for example Kalahari in Botswana.
The rain shadow effect
This explains the semi—arid desert like conditions experienced in the south-eastern Lowveld of Zimbabwe which is the rain shadow of the eastern highlands.
Deserts can also develop along coasts, e.g. when cool and dry air moves from the Antarctic and South American coasts.
The effect of cold current
A lot of fog is formed but no rainfall. This explains the location of Kalahari and Namib Deserts which are influenced by the cold Benguela current.
Characteristics of hot deserts
High pressure cells prohibit moisture laden air from entering, resulting in very little or no precipitation.
Temperature during the day is very high; it can go as high as 55ËšC on rock surfaces.
Humidity is near zero.
With little vegetation to check wind speed, wind travels at great speeds of up to 100km/hr.
A desert experiences very harsh sand storms.
These winds are very disruptive.
Very low temperatures at night which can go below zero.
High diurnal temperature range. (The difference between maximum and minimum temperatures).
Associated with cool dry sinking air coming from the tropics of Cancer in the north and Capricorn in the south.
There is lack of vertical air movement hence little or no condensation for example in Sahara desert.
Dry air masses are common.
Most deserts are rocky places that consist of gravel and large stones.
These are generated by high rate of physical weathering promoted by high diurnal temperature range and low rainfall.
Oases are the sources of water in hot deserts.
Deserts can spend years with no rainfall at all and when it falls it will be thunderstorms lasting for few hours.
Plants in deserts
Vegetation is sparse, drought resistant, mainly thorn bush and thick grass.
In deserts there are two main groups of plants Xerophytes and Ephemerals.
Xerophytes
They are found in deserts, common known type is the cactus.
They have more spikes than leaves to reduce water loss.
Fleshy stem to store water.
Extensive root system to retrieve as much water as possible.
White upper surface to reflect sun's heat.
Ephemerals
Some plants have altered their behaviour and bloom when conditions are correct, these are known as Ephemerals.
They spring into life following desert rain and have very short life spans.
They produce colourful flowers only a few days after rain.
The seeds can lie dormant for many years until growing conditions are conducive.
The plants are used to the harsh climatic conditions experienced in deserts.
They have long taproots that can reach 80metres below the surface for example the sage brush, they also have needle like leaves.
Others have waxy thick cuticle with fleshy stems that keep water for longer periods, for example yucca, aloe and cactus.
Animals in deserts
Many kinds of animals live in the deserts such as spiders, reptiles, birds and some mammals.
Most animals stay in shade during day time and look for food at night when temperatures are low.
They obtain water from the food they eat or in the oases found in deserts.
Rodents, invertebrates and snakes avoid heat by spending the day in caves and burrows.
Camels can go for two weeks without drinking water.
Birds in deserts migrate along desert plains or up into mountains.
Extent of Hot Deserts in Africa
a) Sahara Desert
Sahara covers almost a third of the African continent.
It is the world largest hot desert located in North Africa covering Algeria, Chad, Egypt, Libya, Mauritania, Niger, Sudan, Tunisia and Mali.
It covers approximately 9 million square kilometers.
Its boundaries are Atlantic Ocean to the west, Mediterranean to the north and the Red Sea to the East.
Nile River sustains life in the Sahara desert.
b)Namib Desert
It stretches for 806 000 square kilometres.
It has 3 500 plants species half of which are endemic.
Have sands dunes over 300m in height.
It extends southwards from the Atlantic Ocean to Carunjamba River in Angola and Olifants River in Western Cape, South Africa.
It receives rainfall ranging from 2mm to 200mm per year.
It is the oldest desert 55 to 88 million years old.
The cold Benguela current from the Atlantic Ocean is the main cause of the desert assisted by the Hardly cell that gives fog but no rainfall.
Very few people, only pastoral groupings like Ovahimba, Obatjimba Herero and the Topnaar, are the native to this desert.
c)Kalahari Desert
It covers mainly all parts of Botswana, eastern third of Namibia, and northern part of Northern Cape Province of South Africa.
It is roughly 930 000 square kilometers.
It is mostly covered with sand sheets, sand dunes, vleis or pans.
Receives less than 250mm of rainfall per year.
The Khoikhoi and San are the native inhabitants of the desert.
Denudation Processes in Hot Deserts
There are three main processes responsible for landform formation and these are weathering, wind and water action.
a)Weathering
Mechanical weathering is more dominant in deserts than chemical.
The very high and very low temperatures experienced in deserts promote exfoliation.
The temperature changes during the day and night are responsible for the sculpturing of desert landforms.
b)Wind Action
Wind is a very active agent of erosion, transportation and deposition in hot deserts.
It has the ability to erode, transport and deposit sediments.
c)Water Action
Deserts are very hot and arid but they do receive rainfall in the form of flash floods.
The storms are of short duration and high intensity. This results in flash and sheet floods.
The floods possess great power to erode, transport and deposit sediments resulting in landforms.
Wind Erosion Processes
There are two types of wind erosion processes in deserts and these are abrasion and deflation.
Deflation
Rock waste or loose material is blown and rolled away.
The size of particles picked up depends on the velocity of the wind.
Speeds of about 100km/hr lift up bigger particles and cause more undercutting resulting in lowered surfaces and low depressions (deflation hallows).
Attrition
Collision between rock and sand particles cause them to wear away.
This process produces round shaped soil particles.
Abrasion
It is a process whereby small sand and rock particles are blasted against rock surfaces to produce landforms.
The process involves scrapping and crushing rock surfaces and is very active one meter above ground surface.
It is responsible for undercutting rocks in that height; soft layers are eroded more than the hard layers.
The resultant features are pedestals, zeugens and yardangs.
Landforms formed by wind erosion
Rock pedestal
A rock pedestal resembles a mushroom.
It is formed by erosion of horizontal layers of soft and hard rocks.
The abrasive power of the wind is responsible for their formation.
The lower part of the rock is more susceptible to abrasion and is worn away faster than the top.
The soft layers are eroded faster leaving the hard layer protruding.
If the processes continue the soft rock will wear away and fall as what happens to the Finger of God, in Namib Desert that fell in 1989.
If the rock is of a homogeneous layer, a mushroom like landform will be formed with narrow footing and wide top.
Yardangs
They are created by wind erosion.
It is predominantly the action of wind abrasion that creates them.
They are formed when layers of hard and soft rock lie vertical and parallel to the prevailing wind.
The weaker layer is removed by abrasion, leaving ridges of hard rock.
Yardangs vary in size greatly.
They could be just a few centimeters high to tens of meters high and several kilometers long.
Large troughs and furrows are formed by wind abrasion and deflation.
Zeugens
They are formed when hard rocks lie on top of soft rocks.
The harsh turbulent winds of deserts erode the underlying soft rock.
Weathering and erosion turns the flat topped landscape into ridge and furrow landforms.
The ridges are formed by the hard rocks and the furrows by the soft rocks.
The diagram below shows zeugens.
Deflation hallows
These are enclosed depressions formed as a result of wind erosion in arid and semi-arid regions.
also known as hallows or blowouts.
They are formed when loose surface materials are scooped out by blowing wind creating hallows or depressions.
The continuation of deflation will deepen the hollow; it will stop when it reaches the water table.
After reaching water table, oases or pans are formed which are sources of water in deserts.
The wet pans are not easy to erode hence deflation ends with the emergence of water.
the diagram below shows the development of a deflation hallow.
Wind Transportation Processes
Wind moves material by three processes which are: suspension, saltation and traction or surface creep.
it is important to note that each of these three methods of transportation do not happen in isolation but, often all three take place simultaneously at any given time.
The diagram below summarises these processes.
Traction/creep
Transport materials that are above 0.25 mm in diameter.
Large particles of sand and stones are rolled by wind on the windward side of the particle.
The particles are too big to be lifted.
Saltation
Particles are lifted vertically by wind turbulence for a distance of about 2 meters before being dropped to the surface.
Happens when wind speeds exceed the speed required to move grain particles.
Particles movement is that of a leep - frogging bouncing manner.
Suspension
Finer sand and dust particles are carried in suspension for a long distance.
The material is very fine 0.15 mm in diameter; they are easily picked by wind.
At times its highly concentrated resulting in formation of dust storms.
Wind deposition
When the velocity of the wind drops, the load is deposited at any time and any part of the hot desert.
When the wind encounters a barrier, deposition takes place.
Landforms such as sand dunes, barchans, seif and transverse dunes are formed when sand is deposited.
Formation of dunes
Sand dunes are formed in regions of abundant sand and frequent winds.
These are erosional features resulting from obstacles encountered by wind along its way.
The obstacles force wind to lose speed and start depositing sand particles.
For dunes to be formed there must be abundant supply of sand and steady winds.
Strong winds slow down over some type of obstacle such as rock, vegetation and dead animals and drop their sand.
As the wind moves up and over the obstacle, it increases the speed.
It carries the sand up the gently sloping wide side of the dune by saltation.
There are live and fixed dunes.
Live dunes change their form and shape under effect of wind.
Fixed dunes are those whose shape and position are maintained over a long period of time.
Most dunes form on areas that are flat and sandy rather than those that are rocky and uneven.
Although they take many forms there are two common types of dunes, barchans and seif.
Barchans
These are crescent shaped sand mounds with horns facing downwind.
They are created by deposition.
The agent of deposition is wind.
They form around an obstruction for example vegetation or rock.
The obstruction causes the wind to lose velocity and deposit the sand it is carrying. This increases the size of the obstruction and encourages further sand deposition.
The windward slope is concave, leeward side steep and the tails point downwards.
Height of a few meters to 30 meters and can be 400 meters wide.
As the mound grows larger, its two edges are slowly carried forward.
The windward side is slightly concave caused by wind eddies set up by the prevailing wind.
Can occur singly or in groups
Barchan formation
It moves forward as grains of sand are carried up the windward side and slip down the lee wide side.
They range from 30 to 400 meters.
Most common barchans are found in Erg du Djourab in Algeria and Djabo plateau in northern Nigeria.
Seif dunes
There are two types of seif dunes namely transverse and longitudinal.
Transverse seif dunes are horizontal to the prevailing wind and form a wave like pattern.
Longitudinal seif dunes stretch parallel to the prevailing wind with rounded or pointed tops.
They can reach 300 meters in height and range up to 300 km in length.
They are parallel to each other and the crest is sharp.
Ridge shaped with steep sides and lie parallel to the prevailing wind.
separated by flat corridors which are between 25 meters to 400 meters wide
Corridors are kept clear of loose sand by the prevailing wind.
The Action of Water in Deserts
Semi — arid deserts receive considerable amount of rainfall.
This has formed outstanding landforms as shown in the diagram below.
Rainfall in deserts is of great intensity over short duration.
These unexpected heavy rainstorms can range from 100 mm to 250 mm.
They give rise to sheet and flash floods.
The floods have power to erode, transport and later deposit sediments.
Sheet floods operate on gentle slopes whilst flash floods operate on steep slopes.
It results in the formation of the following landforms: dry streams, inland drainage patterns, wadis, playas, alluvial fans, bajadas, mesas and buttes.
Wadi/ Canyons
It refers to a valley or steep sided flat floored dry channel that only experiences water when there is rainfall.
It is created by erosion, but the agent that creates it is water.
A wadi is formed after sudden but infrequent heavy rainfall known as flash flood.
Due to the infiltration excess and even saturation there is significant runoff in the form of sheet floods.
This flow occurs in the form of rills which in turn form gullies which in turn form into wadis.
Wadis are deepened by flood erosion.
The channel is littered with weathered materials which will turn into mud flows.
Deposition is very common because water quickly sinks and deposits the load of sediments that it has.
When the wadis open into plains, alluvial fans are formed.
Playas (salt lake)
It is a shallow lake in a desert.
also known as inland drainage basins.
The water dries fast.
Consists of fine grained sediments such as salt.
During the dry season, the surface of the lake is usually hard and rough and wet and soft in rain season.
Examples of these can be found in the Sahara desert.
Alluvial fan
Found in areas that experience periodical flash floods.
Flash floods carry large amounts of material including silt.
All this material is called alluvium, which forms a mud flow.
As water dissipates after the end of the storm the alluvium is deposited to form fan shaped features.
They have a funnel shaped basin at the top which then creates a very strong narrow stream which opens into an alluvial fan.
There is more plant life because the sediments are fertile and water is available.
Mesa
It is an elevated area of land that has steep sides and a flat top.
Formed by weathering and erosion of horizontal layers that were lifted by plate tectonic activity in the area.
Stronger rock on top and weaker rock on the sides and bottom.
Its top is wider than its height.
Variations in the ability of different types of rocks to resist weathering and erosion cause the weaker type of rock to be eroded away, leaving the mere resistant rock higher than their surroundings.
The picture below shows a mesa.
Buttes
These are small, isolated flat topped mountain with vertical steep sides.
The top is relatively small.
They are formed by erosion when hard cap-rock covers a layer of softer rock that is eventually worn away.
The hard rock thus resists erosion.
The top is further eroded by abrasion and weathering.
The excess material that falls away builds up the base.
The cap-rock provides protection for the less resistant rock below from wind abrasion which leaves it isolated.
Has height larger than its width.
The image below shows buttes.
Bajada
When alluvial fans come together at one place or deposition they create a bajada.
There is much more water and sediments.
Canyon
It is a landform feature created by the process of water erosion.
It is a deep sided valley carved from the landscape by a river.
They are formed when there is harder rock on either side of the water channel.
Water is able to cut vertically but not horizontally.
Human Activity in Deserts
The desert environments provide both limitations and possibilities to people who live in them.
Limitations are the prohibitive factors that make life in deserts difficult compared to other areas with favourable climates.
Possibilities are opportunities that people in deserts exploit and enjoy.
a) Agriculture
Both subsistence and commercial farming are practiced in hot deserts.
Agriculture in deserts mainly depends on irrigation
Subsistence farmers are nomadic pastoralists.
They keep animals like goats and camels which are drought tolerant animals.
Crop farming is done by irrigation using perennial rivers like the Nile as well as aquifers and oases in deserts.
Drip irrigation is used in Egypt to irrigate citrus fruits plantation.
Commercial farming is done by use of water from elsewhere to the deserts, crops like palm trees and cotton are also grown.
For example, in the Toshka project in Egypt, water comes from Lake Nasser into western desert through canals and pumps but it is very expensive.
b) Settlement.
Settlements are found around large oil fields and others evolving into permanent towns and cities known as Boom towns.
Most people settle along perennial rivers which gives lifeline for example the Nile River.
d) Mineral wealth
A good portion of the world's oil fields are found in desert regions and countries like Libya, Saudi Arabia, United Arab Emirates and Egypt.
Their economies are well supported by oil resources. Dubai is now a world class city because of oil.
Evaporation in deserts lands enriches mineral accumulation in the lakes.
Playas may be sources of mineral deposits formed by evaporation.
Some mineral deposits are formed, improved or preserved by geologic processes that occur in arid areas.
Ground water leaches ore minerals and re-deposits them in zones near the water table. The leaching process concentrates these minerals as ore that can be mined.
Water evaporation in closed basins precipitates minerals such as gypsum, salts and borates.
Atacama deserts have great abundance of saline minerals and it has been mined since the nineteenth century.
Valuable minerals found in hot deserts include limestone, phosphate to make fertilizer, feldspar used to make ceramics, gypsum for making plasters; diamonds are the pillars of the Botswana economy.
Examples include copper in USA, Chile and Iran deserts, iron and zinc in Australia, chrome in Turkey, gold, silver and uranium in Australia.
Deserts also have non-metallic mineral reserves and rocks such as beryllium, mica, lithium, clay, pumice and scoria.
c) Transport and communication
Various roads and railway lines have been constructed across deserts allowing for the fast transportation of goods across deserts without relying on camels.
A highway links Ethiopian regions with Egypt and is used to transport cotton from Ethiopian fields to Egyptian markets.
The development of satellite technology allows people in remote parts of deserts, for example oil fields, to use these devices to communicate with other parts of the world thus reducing the remoteness of the region and improving flow of information.
Large amounts of money from oil are used to develop desert infrastructure.
Using money from oil to develop recreational facilities, modern roads, railway lines and cities.
Dubai is now number one hub of business in the world but it is in the middle of a desert.
d) Mineral wealth
A good portion of the world's oil fields are found in desert regions and countries like Libya, Saudi Arabia, United Arab Emirates and Egypt.
Their economies are well supported by oil resources. Dubai is now a world class city because of oil.
Evaporation in deserts lands enriches mineral accumulation in the lakes.
Playas may be sources of mineral deposits formed by evaporation.
Some mineral deposits are formed, improved or preserved by geologic processes that occur in arid areas.
Ground water leaches ore minerals and re-deposits them in zones near the water table. The leaching process concentrates these minerals as ore that can be mined.
Water evaporation in closed basins precipitates minerals such as gypsum, salts and borates.
Atacama desert has great abundance of saline minerals and it has been mined since the nineteenth century.
Valuable minerals found in hot deserts include limestone, phosphate to make fertilizer, feldspar used to make ceramics, gypsum for making plasters; diamonds are the pillars of the Botswana economy.
Examples include copper in USA, Chile and Iran deserts, iron and zinc in Australia, chrome in Turkey, gold, silver and uranium in Australia.
Deserts also have non-metallic mineral reserves and rocks such as beryllium, mica, lithium, clay, pumice and scoria.
e) Tourism
Deserts offer clear skies that can be ideal for filming, military exercises and weapon testing, solar and wind power generation.
Motor sporting activities such as the Dakar rally attracts lots of tourist in Senegal.
Tourists are also attracted by the popular Valley of Kings.
Archaeological sites, like the Egyptian pyramids attract millions of global tourists annually.
Other tourists want the extreme conditions experienced in deserts.
Limitations
To most subsistence farmers, crop husbandry is limited because of the fragile soils and shortage of water for irrigation.
Oil and gas exploration disturbs sensitive habitats.
Dumping of nuclear wastes in deserts disturbs the ecosystems.
Running pipes carrying oil, the trenching associated with underground lines for gas, oil and water leaves soil and rock surfaces unstable, thus concentrating erosion and water run-off.
Military activities cause extensive damage to plants and soil in deserts.
Irrigation causes the building up of salts that kill plants.
Potassium cyanide used in gold mining may poison wildlife.
Population growth has increased pressure on the already fragile scarce resources.
Soil erosion.
Deforestation.
Desertification
Desertification is the spread of desert-like conditions into agricultural prime lands caused by human activities and climatic factors.
The land becomes drier and drier.
Productive lands are turned non-productive.
It is common in areas bordering deserts like the Sahel region.
This is a result of human activities and climate change.
Sign of desertification.
Ground water reserves decline.
Soil and surface water is affected by salinisation.
Water in ponds, lakes and streams is reduced.
The primary vegetation is destroyed.
Causes of desertification
Desertification is caused and accelerated by both human activities and natural phenomenon such as climate change.
Climate change
The change in global climate has caused more drought than usual.
As the climate becomes warmer and periods of drought become more frequent, huge areas of agriculturally productive land are turned to desert.
Human activities
Overgrazing by livestock removes the vegetation cover which is the stabilising factor in keeping soil intact.
In the past, it was not a problem as desert people practiced nomadic pastoralism but now they are controlled by national boundaries.
Deforestation due to the over reliance on firewood as source of energy.
The increase in population has led to the over-exploitation of resources leaving the soil bare and susceptible to desertification.
Agricultural activities in the vulnerable ecosystems of the arid and semi-arid environments strain them beyond their capacity leading to desertification.
Poor methods of irrigation lead to salinisation which is the accumulation of toxic salts that destroy plants.
Overpopulation has also put pressure on the land since the vulnerable dry lands of deserts cannot sustain high population numbers.
The Sahel region, south of Sahara is slowly turning into a desert because of climate change, overgrazing, population growth and the demand for wood fuel.
Effects of desertification
The loosened bare soil can be easily blown away by wind or washed away by rain.
Loss of soil fertility since sandy soils replace the agricultural prime soils.
Salt can build up in soil which makes it difficult for plant growth.
Loss of bio-diversity.
Loss of grazing and farming lands.
Increases the severity of famine and drought.
Food loss leading to loss of human and animal life.
Desertification can cause flooding, poor water quality, dust storm and pollution.
Case study: Sahel region.
People settled in the Sahel region around 1950s in areas that were close to sources of reliable water supply.
Population started to increase, the perennial shrubs were destroyed because of overgrazing and they were replaced by annuals.
Then the annuals were grazed out which left bare soils. A lot of the top soil was washed away and all that was left were rocks.
Silt turned hard when it was hit by rain. Therefore, the plants were not able to grow because the roots could not penetrate this hard layer.
Now the place has turned into a desert and is still expanding.
Records show that rainfalls in Sahel have decreased and sands have shifted about 96 kilometers south to the area.
People use the slashing and burning methods to clear land. This degrades the quality of soil just like overgrazing.
Solutions to desertification
Afforestation-The replanting of trees in areas bordering deserts. A green wall helps the soil regenerate and protect it from soil erosion.
Educate farmers on better farming methods so they use the land in a sustainable way.
Allow the soil time to regenerate using crop rotation methods.
Plant acacia trees to use as fodder so the animals don't strip the ground of vegetation.
Plant drought resistant crops.
Terracing the land to reduce surface run-off and this helps to reduce leaching of minerals as well.
