A2 Physical Geography - Hot arid and Semi Arid Environment
2. • Caused by the subtropical High pressure belt and
the cold Benguela current
• The Namib is a coastal desert and the Kalahari is
• High humidity in the coastal Namib results in the
formation of FOG
• Ocean currents lower temperatures in summer so
it has a small seasonal temperature range.
3. The deserts in North America are caused by:
• The subtropical high pressure belt
• The cold Californian current
• Rocky mountains which creates a rain shadow.
4. • The Atacama desert is caused by the cold Peruvian
• Humidity is high and fog is common in the Atacama
• The Patagonia desert is created because it is in the
rain shadow of the Andes mountains.
5. • These deserts are caused by continentality and rain shadow
of the Himalayan Mountains
• They also experience extreme high pressures in winter and
in this season, they are extremely cold. This is due to their
high altitude and their great distance from the sea.
6. • Caused by the subtropical High pressure belt and by the cold
ocean currents on the west coast.
• The Great dividing range also creates a rain shadow affect on the
western parts of Australia resulting in the formation of deserts.
7. TYPES OF WEATHERING THAT TAKES PLACE IN
AN ARID AREA
Weathering is the break up of rocks.
In desert regions because there is very little water, weathering tends to be superficial – confined to the surface
Weathering is greater in Semi Arid areas because semi arid regions have a slightly higher precipitation.
Weathering can be broken up into Mechanical (physical) weathering and chemical weathering
• Mechanical weathering – break up of rock without changing its chemical composition.
• Chemical weathering – break up of rock by changing its chemical composition.
8. PHYSICAL WEATHERING
A) THERMAL FRACTURE
Break up of rocks caused by heat.
During the day rocks expand due to heat and at night they contract
This expansion and contraction causes the rocks to break up.
Rocks are not very good conductors of heat so during the day only the surface of
the rocks are heated and expand and during the night only the surface
contracts. This causes the surface layers to peel away like an onion and this is
9. The break up of larger rocks into smaller rocks is called
The break up of rocks into smaller particles is
called GRANULAR DISINTEGRATION
10. B) FREEZE THAW
There are several deserts e.g. Gobi where
temperatures fall below freezing point. Water in the
pores and cracks of the rock expand when frozen and
this can shatter the rocks.
11. C) SALT WEATHERING (SALT CRYSTALIZATION)
Arid environments have lots of salt in them for the following
• Some rocks already have salt in them
• Rain water has salt, when it evaporates its leaves the salt behind
• Ground water has salt and when it is drawn to the surface be
capillary action, it leaves the salt behind.
• Salt left behind as the remains of former lakes or seas
• In coastal salt flats – sea water has salt.
• Endorheic drainage basins – rivers flow into these drainage
basins when they evaporate, they leave the salt behind.
The salt is carried by water into the cracks and pores of the rocks. Here salt crystals grow and start to break up the rock
12. CHEMICAL WEATHERING
Chemical weathering generally occurs when water is available in desert environments. Chemical weathering is limited in
deserts because there is very little water.
a) Oxidation – when iron in rocks take up additional oxygen and turn to rust
b) Hydration – occurs when rocks containing calcium sulphates take up water and expand.
13. With the aid of examples, assess the extent to which ocean currents are
the cause of aridity. 
14. EROSIONAL AND DEPOSITIONAL FEATURES
CREATED BY WIND IN A DESERT
Wind erosion is strong in a desert for the following reasons:
• The surface lacks moisture, soil and organic matter which binds the soil together
• Very little vegetation to slow the wind down
• Winds are often very strong
There are two types of wind erosion in a desert
1. ABRASION – impact of soil and rock particles hitting other rocks
2. DEFLATION – loose particles are picked up by the wind and carried away.
Sand particles are carried by the wind by TRACTION, SALTATION, and SUSPENSION
15. DESERT LANDFORMS CREATED BY ABRASION
A) ZEUGEN –MUSHROOM/PEDESTAL ROCKS
Zeugens are mushroom shaped rocks that
have been created by abrasion.
Wind picks up the sand and blasts it against
The sand blasting effect is concentrated in the
lower levels of the rock near the ground hence
the bottom part of the rock is eroded faster
than the top creating the mushroom shaped
16. B) YARDANGS
Horizontal ridges that range in size from very small to
They are formed by abrasion
The softer rock around the more resistant rock is
removed by abrasion.
The lie in the direction of the prevailing winds
Yardang’s are associated with only one type of rock
19. DESERT LANDFORMS CREATED BY
Sand covers about a quarter of the worlds deserts.
Sand dunes are landforms created by the deposition and movement of sand in a desert.
There are deferent types of sand dunes
• Akle dunes
• Seif dunes
• Star dunes
Dunes are also created by obstacles that trap the sand
• Wind-Shadow dune
20. BARCHAN DUNES
Small crescent formed were the supply of sand
Where there is more sand, these dunes join to
form a barchanoid ridge
24. EROSION AND DEPOSITION FEATURES
CREATED BY WATER IN A DESERT
Three types of rivers can be found in an arid environment
1. EXOGENOUS RIVERS – rivers that are formed in high rainfall areas outside the desert but
flow across it, e.g. The river Nile which is created by heavy rains in the ETHIOPIAN HIGHLANDS
2. ENDORPHEIC RIVERS – rivers that flow into inland lakes such as the river Jordan flowing into
the dead sea.
3. EPHEMERAL RIVERS – rivers that only flow after a rain storm. Sudden rain storms in a desert,
create fast flowing rivers which carry large amount of sediment with it.
27. SALT LAKES/PLAYAS
Landlocked body of water that has a high
concentration of salt and other dissolved
When a salt lake dries up, it becomes a PLAYA
• SOLUTION - underground limestone is
• TECTONIC COLLAPSE
• WIND EROSION –
The extensive playas in SW USA are fossil lake
basins of the PLEISTOCENE PLUVIALS.
Low laying salt flats are called SABKHAS
SABKHAS are formed from salt deposited by the
sea or from water drawn up from just below
the surface by capillary action.
29. ALLUVIAL FANS
As the river flows through the river valley
generally in the mountains and it comes to
flat land, it deposits its load in the shape
of a fan, called an ALLUVIAL FAN
The size of the fan depends on the volume
of the river and the rock type.
Easily eroded rock will form a larger fan.
E.g. Alluvial fan in Greenland
When the alluvial fans coalesce they
create BAJADAS – sometimes called the
Gently sloping rock surface
covered by gravel. Pediments
form in two ways
1.Retreat of a hill slope over
time – pedimentation
An isolated, steep sided hill is called
Inselbergs can form in two ways
1. Scarp recession
2. Previous climatic conditions – If
there was a period of warm and
humid weather in the arid areas,
chemical weathering occurred
on the joints in the rock. Further
erosion removed the weathered
rock, leaving behind the
32. Fig. 10.1 is a photograph which shows landforms in Death Valley, USA.
(a) With the use of a diagram, describe the landforms shown in Fig. 10.1. 
(b) Explain the role of water in the formation of two of the landforms described in (a) 
33. Lesson objects
• To understand what past climates are
• To understand how past climates shape the desert
• To know what a soil profile is
• To be able to identify a soil profile in a desert
• Pluvial – wet period in climate history
• Scree – lose material that accumulates at the bottom of the scarp slope.
• Soil profile – vertical section through the soil
• Soil horizon – horizontal layers within a soil profile
• Aridisols – dry soils in a desert environment
• Biomass – total volume of organic material in the soil
• Deflocculating – dispersal of salt particles
• O horizon – organic horizon
• A Horizon – zone through which organic material is transferred by water to the lower
layers of the soil. This process is called leaching (zone of illuviation)
• B Horizon – zone where the organic material accumulates (zone of eluviation)
• C horizon – partially decomposed parent material
• Bedrock – parent rock
35. PAST CLIMATES
Wet periods in history are called PLUVIALS
There were pluvial in the SAHARA DESERT 12000 to 5000 years ago
There is evidence of pluvial in the past
• Ancient shorelines which show that lakes used to be much larger than they are now e.g. Lake Chad
36. • River valleys that were cut by rivers that are no longer there e.g. valley’s found in the
south eastern Sahara desert
37. • Fossilized soils which can only be found in wet conditions e.g. LATERITES in large parts of Australia
38. • Animals and plant remains found in desert areas where these animals would not survive today.
39. • Evidence of previous human occupation e.g. Roman grain production in parts of Tunisia
which is now desert.
40. These is also evidence of stronger winds and drier conditions in the past.
• Sand dunes in deserts which have become stable suggests that they were formed in the
Pleistocene when wind speeds were stronger and conditions were drier.
41. There is also evidence of colder conditions in deserts
• In western USA there is evidence of colder conditions in the past in the existence of relict
frost screes – slopes of rock shattered by climates colder than today
45. CHARACTERISTICS OF SOILS IN ARID
AND SEMI ARID AREAS
• Little chemical weathering means that the soil is thin and lack soil horizons
• Little vegetation so there is little biomass or humus in the soil
• They have a low clay content because minerals are not broken down to a great extent
• The soils are salty
• - Salty soil in deserts causes the soil to become infertile and impermeable.
- Salts cause deflocculating of soil particles which causes the soil to become
impermeable and the air necessary for plant growth cannot enter
- salt prevents plants from absorbing water into their roots
- Salts are poisonous to many plants
• Surfaces of the soils are impermeable
• Soils are stony because the finer particles have been blown or washed away leaving
behind a stone pavement called regs or HAMMADA in North Africa.
• Hard crust called DURICRUST FORMS – Duricrust can be calcrete formed from Calcium
Occasionally when a lot of rainfall occurs in a short
space of time very little of the water is infiltrated into the
soil as it is very hard and dry. Therefore there is high
47. VEGETATION IN ARID AND SEMI-ARID ENVIRONMENTS
CHARACTERISTICS OF VEGETATION FOUND IN ARID AND SEMI-ARID ENVIRONMENTS
• Low biomass
• Adapted to low rainfall conditions
• Adapted to salty soils
• Low range of species
• Limited nutrient cycling – movement of nutrients from the soils to the roots to the plant
and back to to the soil
PLANTS NEED WATER FOR THE FOLLOWING REASONS:
• Support plant tissue growth
• Water is the medium through which nutrients enter the plant
• Water is needed for photosynthesis
49. ADAPTION OF PLANTS TO ARIDITY
• Spines and thorns act as leaves to
reduce water loss –
• Waxy stems in-order to retain
• Bark is thick to prevent plant from
collapsing when it dries out
• Roots are deep or spread out
laterally over a large surface to
absorb maximum ground water
• Ephemeral plants die in the dry
season but their seeds remain
dormant. E.g. Rose of
• E.g. Aloe Ferox in
• Have deep tap root systems that
can reach down to the water
• They commonly grow near lakes,
stream channels or springs.
e.g. Wild palm trees in Morroco
• Tolerant of salt
• Adapted to prevent the entry of
• They can secrete the toxic salt
onto their leaves
• Some avoid salt by growing
during the rainy season when
salt content is low
e.g. Date palms can grow in saline conditions that is why
you find palm trees near an oasis.
52. • During dry periods they do not perform many water-draining functions such as photosynthesis.
• These periods of dormancy allow the plant to survive, though not grow or reproduce, during the most
challenging months of the year. When plants do produce seeds, the new seeds remain in their
protective coat much longer than in more temperate climates.
• Wax coatings on leaves prevent water loss through evaporation, which in the hot desert can cause
loss of water from both the surface and the inside of leaves. Leaves are also smaller on desert plants,
further reducing the possibility for water loss.
• Plants such as aloes are equipped with fleshy leaves that contain much of their water supply. Because
of their moist inner bodies, these plants are called succulents.
• Water conservation is vital to surviving in the desert. Desert plants conserve water by minimizing
water loss through the surface of their leaves
• Many have the ability to close leaf pores, called stomata, through which gas and water are exchanged,
during drought conditions. Desert plants can even photosynthesize at night, so that stomata are not
open during the heat of the day.
53. 10.Fig. 10.1 is a photograph which shows vegetation in the Sonoran Desert, USA.
10. (a) Describe the characteristics of the vegetation shown in Fig. 10.1. 
11. (b) Explain how vegetation, such as that shown in Fig. 10.1, has adapted to aridity. 
54. ADAPTATION OF ANIMALS TO ARIDITY
A) BEHAVIOURAL ADAPTATIONS
• Birds and larger animals avoid the hotter seasons by migrating
• Rodents, snakes and invertebrates live in caves or burrows to
avoid the heat
• Many desert animals only come out at night – nocturnal
• Sand snake execute a winding motion meaning only a small
part of their body is in contact with the hot sand
55. B) STRUCTURAL ADAPTATION
• In mammals like camels, fur coats are short and well ventilated
to allow sweat to evaporate directly from the skin
• In Ostrich the bare head, throat, legs and abdomen allow for
• Sand dwellers have evolved to survive in dunes e.g. fleshy
footpads in camels and scaly fingers in Lizards
56. C) PHYSIOLOGICAL ADAPTATIONS
• Maintain water balance by maximizing water intake and minimizing
water loss e.g. Camels and wild Asses can drink large
quantities of water in a very short time
• In coastal deserts, animals obtain water by licking fog drenched rocks
• Herbivores obtain water from the shrubs they eat e.g. eland and oryx
• Many desert animals have adapted to survive in dry conditions eg.
Camels and Elands reach body temperatures of 440C with no harmful
• Some desert animals avoid the heat by entering into a dormant
phase e.g. grasshoppers survive dry spells in the form of eggs or
61. What is desertification?
•Desertification is land degradation process in arid and semi-arid areas.
•It is a process by which fertile land is transformed into desert.
•It becomes progressively drier
•Unable to support any plant growth for food production.
•It occurs due to various factors including climatic
variations and human activities.
•Major cause of desertification.
•Too many animals grazing on the land.
•Animals eat all the vegetation and this creates soil erosion. E.g.
Goats and camel can eat the woody portion of plants and this
kills the plant. They are called browsers. Sheep and cattle are
called grazers they mainly eat the grass.
•Overgrazing is caused by
•Increase in herd size due to population growth
•Decrease in available grazing land due to cultivation of crops
•Settling of Nomads in restricted areas by governments wishing
to tax and control them.
•It is cutting down or clearing of trees from a
•If trees are removed, the area can become
much warmer and drier, which may result in
desertification, which transforms fertile land
into desert land
•Cutting down trees for fuel results in increase
wind speed and increase runoff both of which
results in an increase in soil erosion.
65. Over cultivation
• Many crops being grown on the land year after year.
• No rotation of crops.
Crops take all the nutrients from the soil.
• The soil becomes infertile and then crop cannot grow.
• Increase in population caused by a high birth rate and a low
death rate results in over cultivation
• Cultivation extends into dry land previously used for grazing
of animals. It the soil is not given an opportunity to lie fallow
it will lose its nutrients and nothing will grow there resulting
in increase soil erosion and desertification
It can also cause issues with the soil due to chemicals and other
things that may harm the ground.
As areas become more urbanized, there are less places for
plants to grow, thus causing desertification.
67. Climate Change
• Climate change partially result of human
activities that play major role in
• As the days get warmer and periods of
drought become more frequent,
desertification becomes more and more
• The climate is always changing in response
to changes in the output of heat from the
sun, the movement of continents by
continental drift, volcanic eruptions and
• Areas of the globe that were once wet are
now dry and other areas that were dry are
A drought induced famine occurred in the Sahel
region in 2010. There are two climate theories that
were used to explain this drought.
1. ATLANTIC MULTIDECADAL OSCILLATION (AMO)
• Seas surface temperatures in the North Atlantic,
warm and cool in a 70 year cycle and affect
rainfall in Africa.
• AMO warm phase increases summer rainfall,
while a cool phase reduces it.
2. AIR POLLUTION THEORY
• Air pollution that was generated in Eurasia and
North America changed the properties of the
clouds over the Atlantic ocean, disturbing the
Monsoons and shifting the tropical rains
southwards away from north Africa.
68. Effects of Desertification
• Loss of biodiversity.
• More chance of Floods, drought and related losses.
• Soil erosion.
• Desertification and Drought.
• Changes in hydrological regime.
• Loss of nutrients
• Poor Water Quality
• Hunger problems
• Increase in sand storms
69. Soil becomes less usable
The soil can be blown away by wind or washed away rain. Nutrients in the soil can be removed by wind or
water. Salt can build up in the soil which makes it harder for plant growth.
Vegetation is Lacked or Damaged
Desertification reduces the ability of land to support plant life. Also, when overgrazing occurs, plant
species may be lost.
Drought and poor land management contribute to famine.
The soil is not suited for growing food; therefore the amount of food being made will decline.
People near Affected Areas
Desertification can cause flooding, poor water quality, dust storms, and pollution. All of these effects can
hurt people living near an affected region.
Water scarcity in dry lands limits the production of wood, crops, forage and other services that
ecosystems provide to our community.
70. Solutions for Desertification
•Rising awareness of the problem
•Planting indigenous trees and shrubs
•Developing sustainable agricultural practices
•Crop rotation (increase fertility & reduce soil erosion)
•Proper land management
•Soil and water management
•Introduce proper technologies
•Manage dry land resources
•Tree planting schemes to reduce soil erosion.
71. CASE STUDY - LIC
sustainable management of desertification in
Rajasthan, Northern India
Densely populated, semi-arid area in India partly occupied by the Thar desert.
Desertification is a problem in Rajasthan due to the following factors
• Overcultivation and over grazing
• Dunes have shifted into previously farmed areas.
• The Thar desert is characterized by high velocity winds, huge shifting dunes, high diurnal
temperature ranges, intense solar radiation and high rates of evaporation.
• The desert only receives between 10 to 500mm per year, 90% of which falls between July and
• The sandy soils have rapid infiltration, poor fertility, low humus content and high salinity.
72. ACTIONS TAKEN TO MANAGED DESERTIFICATION IN RAJASTHAN
• Planting trees as windbreaks (Prosopis and Acacia). These trees can be grown on sandy soil and provide food,
fodder, fuel and building timber.
• Fencing dunes to keep out livestock
• Fixing parallel strips of brushwood into dunes to stabilize them
• Planting cover crops to protect the soil in between the times when the main crop is grown
• Inter-cropping – planting crops after the main crop has been harvested in order to fully protect the soil
• Putting cut vegetation on the soil (mulch) in order to improve its capacity to retain moisture and improve soil
• Planting along contours in order to reduce the flow of water or loose soil downhill
• Planting new high yield grass varieties for desert livestock. These grass lands also stabilize the dunes
• Damming eroded gullies as soon as they start to appear.
• CENTRAL ARID ZONE RESEARCH INSTITUTE (CAZRI) have developed and improved dozens of traditional and non
traditional crops such as the BER TREE that produces larger fruit and can survive with minimal rainfall.
73. CASE STUDY – HIC
MOJAVE DESERT – sustainable management of desert
The Mojave desert is situated in south east California and southern Nevada in the USA
Rainfall is very low and temperatures can rise to 540C
The main land use in the Mojave desert is recreation, tourism and military
Las Vegas is a city based on gambling and entertainment
Three national parks were created in 1994 – DEATH VALLEY NATIONAL PARK,JOSHUA TREE NATIONAL PARK and MOJAVE
Las Vegas and these national parks attract a lot of tourists to the area.
Tourists also come to see ghost towns left behind by the 19th century miners digging for silver and gold.
There is also a large military presence in the area. The marine corps are based at Twentynine Palms and the Edwards Air force
base is used for testing fighter aircrafts
Inevitably there is conflict between the military, tourists, natural environment and the US governement
74. • Impact of millions of tourists on the natural environment
• Activities such as hiking, mountain biking, horse riding, camping, hunting, rock
climbing and off highway vehicles all have an adverse effect on the
• Research has shown that the off road vehicles compact the soil and reduce
infiltration which increases soil erosion and makes the soil infertile and
• The vehicles damage vegetation
• When it rains water is channeled into the ruts created by the wheels and this
leads to gully formation.
75. MEASURES TAKEN TO ADDRESS THESE PROBLEMS
• The center for Biological Diversity has taken action to force the bureau of land management to change its
policy on off road vehicles.
• In 2009 a court declared a proposed extension of the areas accessible to off-road vehicles illegal because
of the damage being done to the quality of the soil, plants and animals.
• The BLM is now required to place signs along the routes that can be used by off-road vehicles.
• In 1994 the desert protection act requires the secretary of the interior to ensure that the American
Indians can have access to land for cultural and religious purposes.
• The 2010 Californian desert protect act encourages the Military to used renewable sources of energy. The
desert regions are very good for wind and solar energy.
• The bill also establishes endangered species mitigation zones and requires developers to contribute
money to a fund that could be used to better manage habitat for desert tortoise and other endangered or
THE MILITARY, RENEWABLE ENERGY DEVELOPMENT, RECREATION AND THE NATURAL ENVIRONMENT CO-
EXIST IN RELATIONSHIP THAT IS MANAGED BY LAW.