1. The document discusses desert landforms and ecosystems. It describes the key characteristics of deserts including scanty rainfall, high temperatures, and sandy or rocky soils. 2. Several types of deserts are defined based on their geography and climate, including tropical, coastal, and cold deserts. Desert plants and animals are adapted to the harsh, dry conditions through mechanisms like water storage, drought tolerance, and dormancy. 3. The dominant features of desert geomorphology created by wind include sand dunes, mesas, and yardangs. Wind erosion processes like deflation and abrasion shape the landscape. Deserts cover about one third of the Earth's land and are dynamic, sparsely vegetated environments

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

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Introduction:
While studying the physical features and
geomorphology of landforms, a very unique
nature of landmass comes into our mind. It is
very unique in various aspects. That is the dry
landmass called as the Desert.
Earth has numerous land areas covered by
deserts. Among all the terrestrial ecosystems,
deserts are very typical landforms due to their
unique ecological conditions.

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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.
We normally call deserts as “sand seas or
oceans of sands”.

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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.
Deserts are to be fully understood in earth
science studies.
This lesson in about the Desert landforms.

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Objectives:
The objectives of studying lesson are , to
understanding about :
1. The Geomorphology of Deserts
2. The Types of Deserts
3. The Flora of deserts
4. The Fauna of deserts and
5. The Natural Resources of deserts.

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Characteristics of Deserts
Deserts are characterised by the following
ecological factors:
a. Sandy soils and a rocky substratum
b. Scanty rainfall and high evaporation
c. Hot and Cold weather
d. Prevailing Wind action
e. Poor Soil Moisture and water resources

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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.

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The sand bodies of deserts are always on
rolling motion due to the prevailing action of
wind.
Sand drifts, Crescentic Dunes or Barchans,
Loess and Longitudinal dunes and sand sheets
are the notable wind-borne geomorphic
features.
One of the most remarkable features of desert
dunes is their power of collecting all the sand
from their neighborhood.

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Sand dunes
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.

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Deserts are considered to be highly dynamic
geomorphic features.
Dunes are characterized by two-sided slopes
one along the windward direction and the
other along the leeward side.
Soils in Deserts
Soils in desert regions are generally fertile but
lacks soil moisture to encourage plant growth.

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Temperature in deserts
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.

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They drop upto 25 degrees Celsius or more at
nights.
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.

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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.
Rainfall over Deserts
Rainfall is a determining factor of deserts.
Rainfall is very scanty in all desert regions.

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It spite of these deserts are not barren
wastelands.
Characteristics that are common to all deserts
include
a) irregular rainfall of less than 250 mm per
year,
b) very high evaporation rates often 20
times the annual precipitation, and
c) low relative humidity and cloud cover.

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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.

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Wind movement in deserts
Wind movements in deserts are capable of
eroding, transporting and depositing the sand
masses. The following are the processes
expected:
Deflation is a kind of Wind Erosion. It is a
process by which loose sediments are
removed by the wind.
The sediments must be dry. The sand and silt
sized material will be moved first.

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The larger sized material are generally left
behind. Deflation creates its own types of
landforms. The deflation landforms are:
a) Blowouts – these are shallow depressions
formed by removal of sediment. Their
diameters range from a few meters to 1000
meters.

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b) Desert Pavement – these are fine material
removed from the surface leaving a cover of
pebbles, cobbles
and boulders. The coarser material protect the
fine material beneath the surface from further
erosion.
Abrasion
Another kind of erosional mechanism in desert
by the wind is abrasion. Abrasion is a kind of
"sand blasting".

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In this process, the impact of windblown
particles on exposed surfaces will remove
material from that surface.
Abrasion will also reduce the size of the
particles that are being moved. It is generally
limited to heights of 1 meter (2 meters max.) .
Abrasion creates its own kind of landforms.
The notable ones are ventifacts and yardangs.

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Ventifacts are land surfaces containing
pebbles, cobbles or boulders that have been
eroded on one or more sides by wind abrasion.
Yardangs are ridges which run parallel to
dominant wind direction, believed to be
formed by abrasion.
They also create a pillar-like structure which
are undercut by abrasion.

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Dust Storms in deserts
Deserts experience severe dust storms
regularly. Dust storms are heavy concentration
of dust (silt-sized particles) in a turbulent air
mass.
The dust can be carried hundreds or thousands
of meters into the air.
The large dust storms can carry as much as 100
million metric tons of material for several
kilometers distance.

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Sand Storms
Sand storms are low moving blankets of wind-
driven sands.
In this, the sand reaches heights of 1-2 meters
above surface.
The sand storms often contain dust and sand.

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Water availability in Deserts
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.

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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.
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.

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Oasis is a wet and fertile zone in a desert with
vegetation.
Underground water comes nearer to the
ground surface.
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.

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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.
Desert Topography
The typical desert topography includes playas,
alluvial plains, Pediments, inselbergs, mesas,
buttes and badlands.

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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.
Playas are characterized by mudcracks and
precipitated salt crystals, forming salt pans.

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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
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".

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Mesas and buttes
Mesas are broad, flat-topped erosional
remnants bounded on all sides by steep slopes.
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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Major kinds of Deserts
The distribution of arid regions or deserts is
determined by the climate and topography.
Based these two factors, arid terrains are
subdivided into
a) tropical deserts and
b) topographic deserts.

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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).
Heat enhances evaporation over deserts.

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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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Steppes
Steppes are Deserts and adjacent semi-arid
regions. These regions may get rainfall
between 10 and 20 inches per year.
They are characterized by a non-continuous
vegetation cover.
The tropical desert lie within latitudes from 5º
to 30º.
The rainfall are mainly caused by the descent
of air.

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This is responsible for the warmness of land
and loss of moisture.
Types of Deserts
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.

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Rocky 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.
Talus and scree debris are numerous on hill
sides.

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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
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
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.

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They resemble like sea-waves. Barchans reach
a height of 200m.
It is difficult to walk straight across on these
zones.
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.

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Clayey Deserts
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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Geographical classification of deserts
Deserts are also classified based the
geographic conditions.
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.

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d) Trade wind deserts- Eg. Sahara desert-
temperature goes upto 57deg.C
e) Monsoon deserts- eg. Thar Desert in India,
Cholistan Desert of Pakistan
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.

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Hot and Cold Deserts
From an ecological point of view, deserts are
also classified into two major kinds as
a) Hot deserts and
b) cold deserts.
Flora of deserts
Desert plants cannot use so much water at
once, and the desert soil cannot absorb all of it.

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Most of the water runs off, carrying away the
soil particles.
Most deserts plants are drought or salt
tolerant, such as Xerophytes.
Some store water in their leaves, roots and
stems.
Other desert plants have long tap roots that
penetrate the water table, anchor the soil, and
control erosion.

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The stemps and leaves of some plants can
lower the velocity of sand-carrying winds and
protect the ground from erosion.
Life forms of Plants
The following three life forms of plants that are
adapted to deserts:

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a) The annuals, which avoid drought by
growing only when there is adequate
moisture
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.

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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.
Deserts cannot support the abundant plant and
animal life as found in humid climates. But
many kinds of plants and animals thrive in
deserts.

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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.
Salt-loving plants ( halophytes) such as
saltbushes grow in areas of highly saline soils
and near the edges of playas (dry saline lakes).

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Xerophytes
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.
The stem of the barrel cactus bulges with water
after a rainfall and shrinks as the plant uses the
water.

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All desert vegetation has a highly characteristic
spatial distribution plants.
Individual plants are scattered.
This spacing reduces competition.
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

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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.
The extensive sand 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.

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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.
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.

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The seeds lie buried until the rains return.
They then sprout, and the plants complete
their life cycle within a few weeks.
In the deserts of Australia, the dominant plants
are
tussock grasses, such as porcupine grass or
spinifex, and succulent herbs, such as
parakeelya.

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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.
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.

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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.
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.

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In addition to these, other types of plants
belonging to pea family and sunflower family
are seen in deserts.
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.

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They come out to search for food only when
temperatures drop at night.
Many insects,
lizards, and
tortoises can tolerate high desert temperatures
and are active in the daytime.
Many of them retreat underground or find the
shade of a tree, during the hottest part of the
day.

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Some snails, insects, frogs, lizards, mice, and
ground squirrels aestivate in deserts, i.e they
sleep through the summer.
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.

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Desert animals include many kinds of insects,
spiders, reptiles, birds, and mammals.
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. Then they quickly feed and
reproduce before the ponds dry again.

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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.
Such animals obtain water from the food they
eat and from the few water holes that exist in a
desert.

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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. A camel can live for months on the
water produced when its body breaks down
this fat for use as energy.

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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.
Whenever deserts are discussed no one can
forget about 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.

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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.
A Camel carries its own built-in food supply on
its back in the form of a hump.

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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.
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.

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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.
This desert ship can go for long distances
without water for days or even months.

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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
camel's body but replaces water previously
used up.

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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.
On extremely hot days, a camel keeps as cool as
possible by resting rather than feeding.

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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
Human population in deserts are also equally
facing the same issues. Deserts do not support
a large number of people as in humid regions.
People living in desert regions must adjust to
the local and prevailing hot or dry climate.

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Air-conditioning and irrigation projects have
made life more comfortable for other desert
dwellers.
Natural Resources of deserts
Deserts have enormous sands and soil
resources.
The soil of deserts are mineral soils often
called as aridisols with low organic content.

73. 73
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.
Minerals formed by evaporation are often
referred to as evaporites.

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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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Annual production of dry matter
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.

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The detrital food chain seems to be less
important in the desert than any other
ecosystems.
Because of the limited production and
decomposition, nutrients are limited in the
deserts.
Walter in 1954 has measured the net
productivity of a series of deserts and semi-
arid communities.

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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.
Other produces
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.

80. 80
Some of the more productive petroleum areas
on Earth are found in arid and semi-arid
regions of 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.
Location of Oases
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.

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The natural factors creating deserts have not
changed much for thousands of years.
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.

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The role of human beings in reconstructing the
desert landscapes on their outskirts will
certainly stop their expansion over the other
regions.
Desert Regions of the World
Desert Region is a geographic zone
experiencing a mean annual precipitation of 10
inches or less, and relatively devoid of
vegetation.

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There are Four types of desert regions which
can be classified as:
- Subtropical Deserts
- Rainshadow Deserts
- Continental Deserts
- Polar Deserts
The Subtropical Deserts exist between 25°-30°
Latitude.
These deserts form in response to dry
descending air mass in Hadley Cell circulation.

85. 85
Examples are, Sahara Desert, Arabian
Peninsula, Kalahari Desert, and the Great
Sandy Desert.
The Rainshadow Deserts exist over the
mountain ranges which block the passage of
moist air.
The leeward side of the ranges are normally
arid.

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Examples are, Atacama Desert, Patagonia,
Death Valley, eastern Washington and eastern
Oregon.
The Continental Deserts are centralized
locations of deserts with in the continental
interiors.
There may be no nearby, large sources of
water.
Examples are many including the Gobi Desert.

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The Polar Deserts are located around polar
regions.
Cold air is unable to hold much moisture.
This kind of Polar areas are arid having very
little precipitation.
The polar Deserts are not defined by
temperature.
Examples are- Greenland; Antarctica.

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Conclusion
Deserts are dry landforms comprising of sand
and sediments. They are the cradles of
centrifugal aeolian forces.
Deserts sometimes contain valuable mineral
deposits that were formed in the arid
environment or that were exposed by erosion.
Evaporation in arid regions enriches the
mineral accumulation in their lakes.

89. 89
Playas may be sources of mineral deposits
formed by evaporation.
Water evaporating in closed basins
precipitates minerals such as gypsum, salts
(including sodium nitrate and sodium
chloride), and borates.

90. 90
The minerals formed in these evaporite
deposits depend on the composition and
temperature of the saline waters at the time of
deposition.
The natural forces that create deserts have not
changed much for thousands of years.
However, various human activities have caused
desert regions to expand considerably.
The major causes of desert expansion include
mining, improper farming methods, and
destruction of trees.