Deserts are landforms of sand and sediments. They are the cradles of centrifugal eolian forces. Among all the terrestrial ecosystems, deserts are typical landforms due to their unique ecological conditions. Desert and near-desert areas cover nearly one-third of the land surface of the globe . Deserts alone covers about one-seventh of the land surface.

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ECOLOGY OF DESERTS
By
Prof.A.Balasubramanian
Centre for Advanced Studies in Earth Science,
University of Mysore,
Mysore

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Introduction:
Deserts are landforms of sand and sediments.
They are the cradles of centrifugal eolian forces.
Among all the terrestrial ecosystems, deserts are
typical landforms due to their unique ecological
conditions.
Desert and near-desert areas cover nearly one-
third of the land surface of the globe .
Deserts alone covers about one-seventh of the
land surface.

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We normally call deserts as “sand seas or
oceans of sands”.
Most of the major desert areas like as
the Sahara, the Arabian, the Kalahari, and
the Deserts of Australia all lie between 10 and 30
degrees north or south of the equator.
Deserts are dry ecosystems comprising a
substantial part of the globe.

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The following are the modules covered in this
unit:
1. Geomorphology of Deserts
2. Types of Deserts
3. Flora of deserts
4. Fauna of deserts
5. Natural Resources of deserts.

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1. Geomorphology of Deserts
Deserts are created by changes in climate and
accumulation of sands and other rocky wastes. A
high proportion of the desert floor is an erosion
surface of a bedrock.
Sand covers about 10 to 20 per cent of the
deserts. The rest of the land consists mostly of
gravels, boulders, mountains, and various types of
soils.

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A desert landscape includes various landforms
created by wind erosion. In the heart of the desert
the wind has a free play. Wind erosion creates
mounds of sand dunes and flat-topped hills known
as mesas and buttes.
Sand drifts, Crescentic Dunes or Barchans, Loess
and Longitudinal dunes and sand sheets are the
notable wind-borne geomorphic features.

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One of the most remarkable features of desert
dunes is their power of collecting all the sand
from their neighborhood.
Dunes are large piles of wind-borne sands
reaching a maximum height of 250 metres above
the surface.
Dunes show many shapes and patterns that change
continually due to the highly active winds.
Deserts are considered to be highly dynamic
geomorphic features.

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The sand bodies of deserts are always on rolling
motion due to the prevailing action of wind.
Dunes are characterized by two-sided slopes one
along the windward direction and the other along
the leeward side.
Deserts are characterised by the following
ecological factors:
a. Sandy soils and a rocky substratum
b. Scanty rainfall and high evaporation

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c. Hot and Cold weather
d. Prevailing Wind action
e. Poor Soil Moisture and water resources
Soils in desert regions are generally fertile but
lacks soil moisture to encourage plant growth.
Rainfall is a determining factor of deserts.
Rainfall is very scanty in all desert regions. It
spite of these deserts are not barren wastelands.

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Most of the deserts receive less than 200 mm
rainfall per year.
However, the amount of rainfall may vary greatly
from year to year.
A desert may not receive any rain for several
years and in some cases about 250mm of rain
might fall within a few hours.
Desert plants cannot use so much water at once,
and the desert soil cannot absorb all of it. Most of
the water runs off, carrying away the soil
particles.

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Climatologically, deserts are the hottest places in
the world because they absorb more heat from the
Sun than any other land in humid climates.
Deserts are mostly under arid to semi-arid
climates. The temperature varies during day and
nights.
In summer, desert temperatures often reach upto
38 °C during the day. They drop upto 25 degrees
C or more at nights.

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During the winter, temperatures in the desert
range from 10 to 21 °C. Clouds would reflect
much of the sun's intense radiation during the day,
slowing the rate of heating of the air near the
surface.
At night, clouds and water vapour would absorb
much of the earth's radiation--most of which
consists of infrared rays--slowing the rate of
cooling. Winters are much colder in middle
latitude semiarid areas and deserts.

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Winds that descend the leeward slopes of these
ranges are warm and dry.
Another controlling factor of desert is the
presence of water. Due to very scanty rainfall and
sandy soil, deserts have no or very little
possibility of holding water. Only in some
depressions very little water may exists. Soil
moisture is a rare feature in deserts. Even if little
soil moisture is present, due to hot climate and
prevailing winds, it will be evaporated quickly.

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The drainage system is made up of dried streams
called arroyos. After a rainfall, water fills the
arroyos.
They run down the mountains and cut away the
land, carrying deposits of gravel, rock, and sand to
the bottom.
Oasis is a wet and fertile zone in a desert with
vegetation.
Underground water comes nearer to the ground
surface.

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Open wells and springs do exist in such spots.
Water that occurs within an oasis has been drawn
through groundwater base flow from distant
catchments like mountains or hills.
Because oases have some water, farming and
settlement are expected to be more.
Some oases may be small and can support only a
few people, but others are large enough to support
millions of people.

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The typical desert topography includes playas,
alluvial plains, Pediments, inselbergs, mesas,
buttes and badlands.
Playa are dry lake beds formed by evaporation
from temporary (few hours to several months),
shallow accumulations of excess water (playa
lake) following infrequent and intense
rainstorms.

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Playas are characterized by mudcracks and
precipitated salt crystals, forming salt pans.
Alluvial fans are sediments deposited downslope
of the land, typically as aprons at the mouth of
canyons or as a piedmont plain.
Alluvial fans coalesce to form a bajada, a broad
alluvial apron with an undulating surface.
Pediments are the sloping low-relief surfaces
adjacent to mountains resulting from erosion and
retreat of the mountain front.

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Most covered by thin veneer of debris, alluvial
fans, or bajadas.
Inselbergs are isolated, steep-sided erosional
remnants of bedrock (characterized by greater
resistance to weathering than surrounding
mountains) that rise above flat desert plains.
Inselberg is a German word meaning "island
mountain".
Mesas are broad, flat-topped erosional remnants
bounded on all sides by steep slopes.

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Mesas consist of relatively easily weathered
sedimentary rocks capped by nearly horizontal
and more resistant rock layer.
Buttes are isolated pillar-like structures resulting
from continued eathering and erosion of mesas
Badlands are areas of closely spaced ravines with
little or no vegetation.

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Types of Deserts
The distribution of arid regions or deserts is
determined by the climate and topography, such
that arid terrains may be subdivided into
a) tropical deserts and
b) topographic deserts.
Characteristics common to all deserts include
irregular rainfall of less than 250 mm per year,
very high evaporation rates often 20 times the
annual precipitation, and low relative humidity
and cloud cover.

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Tropical Deserts:
The Tropical deserts have a big diurnal
temperature range and very high daytime
temperatures, whereas mid-latitude deserts have a
wide annual range and much lower winter
temperatures (in the Mongolian desert the mean
temperature is below freezing point for half the
year).

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Heat enhances evaporation over deserts.
Deserts and adjacent semi-arid regions
(steppes), which may have rainfall between 10
and 20 inches per year, are characterized by a
non-continuous vegetation cover. The tropical
desert lie within latitudes from 5º to 30º and are
mainly caused by the descent of air. This is
responsible for the warmness of land and loss of
moisture.

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Topographic Deserts:
Topographic controls on desert formation also
reflect a deficiency in rainfall. Topographic
deserts are located near the center of continents,
where climates are hot in summer and cold and
dry in winter. These regions are typically far
from ocean moisture sources, and most often are
cut off from rain-bearing winds by high
mountains.

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Based on their surface forms and soil
composition, deserts are classified into the
following four types:
a) Rocky Deserts
b) Stony Deserts
c) Sandy Deserts
d) Clayey Deserts.
Rocky deserts have uneven topography between
mountain ridges and groups of small hills.
Mountain ridges have rugged summits and crests,
steep slopes with outcrops of rocks.

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Talus and scree debris are numerous on hill sides.
They are not completely devoid of vegetation.
Isolated small and large shrubs can be located on
the lower parts of the slopes., in valleys and
depressions. Some water springs appear on valley
floors.
Stony deserts are completely flat or gently
undulated areas with stones of rocky wastes
comprising sharp edged rock fragments or
pebbles.

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There may not be any vegetation and water.
Sahara and Arabian Deserts are stony deserts.
Sandy deserts are areas of loose sand which forms
hillocks in the shape of barchans and dunes. These
are characterised by uneven surfaces of
undulating chains of dunes with isolated short
valleys or hollows with little vegetation. They
resemble like sea-waves. Barchans reach a height
of 200m. It is difficult to walk straight across on
these zones.

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One has to clim the crets and descend into the
hollows. It is easier to walk aided by wind. Sand
storms are common creating an atmospheric haze.
Clayey Deserts are not extensive. The clayey
floors are crackd into polygonal sections. Soil is
of fine silt in composition with sparse vegetation.
Soils of Clayey deserts are mostly saturated with
salts.

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The geographical classification of deserts also
include:
a) Continental deserts- eg. Gobi desert
b) Rain-shadow deserts- due to tall mountain
ranges- eg. Judean desert in Israel
c) Coastal deserts- Eg. Atacama Desert in S.
America, Namib Desert of Africa.
d) Trade wind deserts- Eg. Sahara desert-
temperature goes upto 57deg.C
e) Monsoon deserts- eg. Thar Desert in India,
Cholistan Desert of Pakistan

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f) Polar deserts- Eg. The Dry Valleys of
Antarctica – are ice free for thousands of years.
g) Extra-terrestrial deserts- deserts of other
planets, Eg. Mars have shown eolian features.
h) Montane deserts.
2. Flora of deserts
Most deserts plants are drought or salt tolerant,
such as Xerophytes. Some store water in their
leaves, roots and stems.

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Other desert plants have long tap roots that
penetrate the water table, anchor the soil, and
control erosion. The stemps and leaves of some
plants can lower the velocity of sand-carrying
winds and protect the ground from erosion.
The following three life forms of plants that are
adapted to deserts:
a) The annuals, which avoid drought by growing
only when there is adequate moisture

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b) The succulents- such as the cacti, which store
water and survive
c) The desert shrubs- which have numerous
branches with small but thick leaves to store
water.
Adaptation to desert environment and arid climate
involves the ability to avoid wilting and remain
dormant for long periods rather than an increase
in transpiration efficiency. Desert soils are
infertile, lacking in humus and generally grey or
red in colour.

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Deserts cannot support the abundant plant and
animal life as found in humid climates. But many
kinds of plants and animals thrive in deserts. The
few plants capable of surviving such conditions
are widely spaced, scrubby and often thorny.
Long-rooted plants (phreatophytes) such as the
date palm and musquite commonly grow along
dry stream channels.

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Salt-loving plants ( halophytes) such as saltbushes
grow in areas of highly saline soils and near the
edges of playas (dry saline lakes).
Xerophytes are drought-resistant and survive by
remaining leafless during the dry season or by
reducing water losses with small waxy leaves.
They frequently have shallow and widely
branching root systems and store water during the
wet season (for example, succulents and cacti
with pulpy stems).

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Desert plants survive compete with small amount
of water available.
Some desert plants obtain water from deep
beneath the surface of the earth. For example, the
American mesquite tree has roots that extend as
deep as 12 metres.
Other plants store large amounts of water in their
leaves, roots, or stems.

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The stem of the barrel cactus bulges with water
after a rainfall and shrinks as the plant uses the
water.
From an ecological point of view, desert can be
classified into a) Hot deserts and b) cold deserts.
All desert vegetation has a highly characteristic
spatial distribution plants.
Individual plants are scattered.
This spacing reduces competition.

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Some of the typical flora of deserts are:
a) Creosote bush- (Larrea) – North America, hot
deserts
b) Sagebrush –( Artemisia) Great basin- Cool
Deserts
c) Bur sage( Franseria)- high altitude plants
d) Giant cactus- ( Sahuaro)-
In addition to these little grass also grows on
deserts but are grazed by the animals.
Greesewood( sarcobatus) is a typical vegetation in
cool deserts.

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The extensive bar grounds of deserts is not
necessarily free from plants.
Mosses, algae, and lichens may be present.
Blue green algae are the nitrogen fixers in these
regions also.
After every rainfall, colourful flowers and lush
vegetation cover parts of deserts.
This dramatic change occurs because many desert
plants do not grow during a drought.

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Due to rain, plants sprout, flower, scatter their
seeds, and die.
The entire process usually takes only a few weeks.
The seeds of annuals can survive even the longest
dry periods.
The seeds lie buried until the rains return. They
then sprout, and the plants complete their life
cycle within a few weeks.

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In the deserts of Australia, the dominant plants are
tussock grasses, such as porcupine grass or
spinifex, and succulent herbs, such as parakeelya.
After the rains, the deserts come alive with
beautiful flowers, such as the purple mulla-
mullas.
In the deserts of North and Central America,
cactuses are prominent.

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Cactuses are protected from grazing animals by
their sharp spines.
Many cactuses have beautiful flowers.
In Africa, the most abundant desert plants are the
euphorbias, which have a poisonous milky sap.
The Sonoran Desert of the American Southwest
has the most complex desert vegetation on earth.
The giant “saguato cacti” provide nests for desert
birds and serve as “trees”of the desert.

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Saguaro grow slowly but may live upto 200 years.
Its branches develop after 75 years. When fully
grown it is 15 m tall and weigh 10 tons.
In addition to these, other types of plants
belonging to pea family and sunflower family are
seen in deserts.

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3. Fauna of deserts
Animals of the deserts have developed special
body structure and ways of life that enable them
to survive under the extreme heat.
Centipedes, gerbils, kangaroo rats, snakes, and
scorpions spend the day in burrows.
They come out to search for food only when
temperatures drop at night.

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Many insects, lizards, and tortoises can tolerate
high desert temperatures and are active in the
daytime.
But even they must retreat underground or find
the shade of a tree during the hottest part of the
day.
Some snails, insects, frogs, lizards, mice, and
ground squirrels aestivate in deserts, i.e they sleep
through the summer.

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Elf owls, roadrunners, snakes, spiders, bees and
butterflies are the other fauna of deserts.
Many desert dwellers have light-coloured skin,
which helps keep them cool by reflecting sunlight.
Desert animals include many kinds of insects,
spiders, reptiles, birds, and mammals.

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Deer, foxes, wolves, and other animals may visit a
desert after a rainfall.
Most desert animals avoid the extreme midday
heat by feeding at night after the temperature has
dropped.
Many small animals dig burrows underground and
stay there during the day.
Some of these animals are dormant (inactive)
throughout the summer.

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Desert foxes and hares have long ears.
When overheated, these animals move to a cool
cave or burrow where they can get rid of excess
body heat through their ears.
The Cape ground squirrel makes its own shade by
using its fluffy tail like a parasol.
Fairy shrimp and spadefoot toads may spend
months or years underground waiting for rain to
create ponds.

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Then they quickly feed and reproduce before the
ponds dry again.
Larger desert animals try to remain in shady areas
during the day.
The evaporation of water from their bodies lowers
their body temperature, but this water must be
replaced.

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Such animals obtain water from the food they eat
and from the few water holes that exist in a desert.
Desert animals also use water that is produced in
their bodies during digestion.
This source of water is particularly important to
camels, which can go for long periods without
food and water.
Large amounts of fat are stored in the humps of
camels.

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A camel can live for months on the water
produced when its body breaks down this fat for
use as energy.
Most deserts lie near the edges of the tropics.
Food and water are often scarce in deserts, and
temperatures in the summer can be scorching.
Despite these conditions, many kinds of animals
live there.

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Whenever deserts are discussed no one can forget
anout the “Ship of the Deserts” ie. The Camels.
These are unique creatures which can not only
survive, but also help the human beings across
hot, dry deserts with little food or water. They
walk easily on soft sand where even trucks would
get stuck, and carry people and heavy loads to
places that have no roads.
Camels also serve the people of the desert in
many other ways.

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A Camel carries its own built-in food supply on
its back in the form of a hump.
The hump is a large lump of fat that provides
energy if food is hard to find.
There are two chief kinds of camels:
(1) the Arabian camel, also called dromedary,
which has one hump, and
(2) the Bactrian camel, which has two humps.

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Hybrids (crossbreeds) of the two species were
used widely in Asia.
These hybrid camels had one extra-long hump and
were larger and stronger than either of their
parents.
Camels have been the domestic animals to support
humans for thousands of years. There are several
million Arabian camels, and most of them live
with the desert people of Africa and Asia.

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This desert ship can go for long distances without
water for days or even months.
The amount of water a camel drinks varies with
the time of year and with the weather. The
camels that graze in the Sahara can go all winter
without water and may refuse to drink if water is
offered to them.
But a large, thirsty camel can drink as much as
200 litres a day. This water is not stored in the

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camel's body but replaces water previously used
up.
Most animals sweat when hot, and the
evaporation of the water from their skin keeps
them cool.
But camels do not sweat much. Instead, their
body temperature rises by as much as 6 degree
Celsius during the hot summer day and then cools
down during night.

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On extremely hot days, a camel keeps as cool as
possible by resting rather than feeding.
Adaptation to the topography, landforms and
climate is a unique feature of desert animals. They
live with very scanty food resources.
Human population in deserts are also equally
facing the same issues. Deserts do not support a
large number of people as in humid regions.

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People living in desert regions must adjust to the
local and prevailing hot or dry climate.
Air-conditioning and irrigation projects have
made life more comfortable for other desert
dwellers.
4. Natural Resources of deserts.

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Deserts have enormous sands and soil resources.
The soil of deserts are mineral soils often called as
aridisols with low organic content.
Most desert soil is too dry to support widespread
vegetation, but much of it is rich in salt, uranium,
and other minerals. Playas are sources of mineral
deposits formed by evaporation.
Gypsum, sodium nitrate, sodium chloride salts
and Borates are the major precipitates.

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Minerals formed by evaporation are often referred
to as evaporites.
Many of them are used in the manufacture of
glass, ceramics, enamel, agricultural chemicals,
water softeners and pharmaceuticals.
Sodium nitrites are used for explosives and
fertilizers. Many metallic and non-metallic
mineral resources also are obtained from the
desert zones.

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Walter in 1954 has measured the net productivity
of a series of deserts and semi-arid communities.
The annual production of dry matter was a linear
function of rainfall upto 600cms.
This has also shown that the overall limiting
factor of deserts in productivity is the moisture.
It has been found that he annual net productivity
of true deserts is less than 2000 kg per hectare.

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When this is overcome due to rain, then the soil
becomes a limiting factor.
The primary production in deserts depend on the
little amount of available water and also the
consumption strategy adopted by the plants.
It is very low as 30-200 gm/ sq.m in arid zones,
above the ground.
The herbivores have a pronounced impact on
primary producers. The detrital food chain seems

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to be less important in the desert than any other
ecosystems.
Because of the limited production and
decomposition, nutrients are limited in the deserts.
Date palm tree produces date fruits. They thrive
in hot, dry climates.
They grow along oases throughout northern
Africa and the Middle East.

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The date palm is one of the oldest crop plants
known to have been used for at least 5,000 years.
Even today, dates form an important part of the
diet in many desert countries. Dates are also
supplied to other places from these lands.
In addition to these, large deposits of oil and
natural gas lie under many desert lands.
Some of the more productive petroleum areas on
Earth are found in arid and semi-arid regions of

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Africa and the Middle east although oil fields
were originally formed in shallow marine
environments.
Recent climate change, in geological time scale,
has placed these reservoirs in an arid
environment.
The hydrocarbon reservoirs of the North Sea is
associated with extensive evaporite deposits.

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Many of the major U.S. hydrocarbon resources
may come from aeolian sands.
The location of Oases has been of critical
importance for trade and transportation routes in
desert areas.
Caravans must travel through these places that
supplies water and food. The natural factors
creating deserts have not changed much for
thousands of years.

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It is observed over the world that various human
activities have caused expansion of desert regions.
This has happened due to continuous loss of
fertile land on the outskirts of such regions.
Planting shelter belts of trees will be one of the
solutions.
The role of human beings in reconstructing the
desert landscapes on their outskirts will certainly
stop their expansion over the other regions.