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Largest deserts
A
desert is a
landscape
or
region that receives almost no
precipitation. Deserts are
defined as areas with an average annual precipitation of less than
per year, or as areas where more water is lost by
evapotranspiration than falls as
precipitation. In the
Köppen climate
classification system, deserts are classed as
BWh (hot
desert) or
BWk (temperate desert). In the Thornthwaite
climate classification system, deserts would be classified as arid
megathermal climates.
Terminology
Deserts are part of a wider classification of regions that, on an
average annual basis, have a moisture deficit (i.e. they can
potentially lose more than is received). Deserts are located where
vegetation cover is sparse to almost nonexistent.
Geography
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The snow surface at Dome C Station in
Antarctica is representative of the majority of the continent's
surface.
Deserts take up about one third (33 percent) of the Earth's land
surface. Hot deserts usually have a large
diurnal and seasonal
temperature range, with high daytime temperatures, and low
nighttime temperatures (due to extremely low
humidity). In hot deserts the temperature in the
daytime can reach 45 °C/113 °F or higher in the summer, and dip to
0 °C/32°F or lower in the winter. Water acts to trap
infrared radiation from both the sun and the
ground, and dry desert air is incapable of blocking
sunlight during the day or
trapping heat during the night. Thus,
during daylight most of the
sun's heat reaches
the ground, and as soon as the sun sets the desert cools quickly by
radiating its heat into space. Urban areas in deserts lack large
(more than 14 °C/25 °F) daily temperature variations, partially due
to the
urban heat island
effect.
Many deserts are formed by
rain shadows;
mountains blocking the path of precipitation to the desert (on the
lee side of the mountain). Deserts are often composed of
sand and
rock surfaces.
Sand dunes called
ergs and stony surfaces called
hamada surfaces compose a minority of desert
surfaces. Exposures of
rocky terrain
are typical, and reflect minimal soil development and sparseness of
vegetation. The soil is rocky because of
the low chemical weathering.
Bottomlands may be
salt-covered flats.
Eolian processes are major factors in
shaping desert landscapes.
Cold deserts (also
known as
polar deserts) have similar
features, except the main form of precipitation is
snow rather than
rain.
Antarctica
is the world's largest cold desert (composed of
about 98 percent thick continental
ice sheet and 2 percent barren
rock). Some of the barren rock is to be found in the
so-called Dry
Valleys
of Antarctica that almost never get snow, which can
have ice-encrusted saline lakes that
suggest evaporation far greater than the rare snowfall due to the
strong katabatic winds that
evaporate even ice.
The largest hot desert is the Sahara in northern Africa, covering 9
million square kilometres and 12 countries.
Deserts sometimes contain valuable mineral deposits that were
formed in the arid environment or that were exposed by erosion. Due
to extreme and consistent dryness, some deserts are ideal places
for natural preservation of artifacts and fossils.
The 10 largest deserts
Rank |
Desert |
Area (km²) |
Area (mi²) |
1 |
Antarctic
Desert (Antarctica) |
|
5,339,573 |
2 |
Arctic |
|
5,300,000+ |
3 |
Sahara (Africa) |
|
3,320,000+ |
4 |
Arabian Desert (Middle East) |
|
900,000 |
5 |
Gobi Desert (Asia) |
|
500,000 |
6 |
Kalahari Desert (Africa) |
|
360,000 |
7 |
Patagonian Desert (South
America) |
|
260,000 |
8 |
Great Victoria Desert (Australia) |
|
250,000 |
9 |
Syrian Desert (Middle East) |
|
200,000 |
10 |
Great Basin Desert (North
America) |
|
190,000 |
Etymology
English
desert and its
Romance cognates (including
Italian and
Portuguese deserto,
French désert and
Spanish desierto) all come from
the
ecclesiastical Latin
dēsertum (originally "an abandoned place"), a participle
of
dēserere, "to abandon." (See
desertion.) The correlation between aridity and
sparse population is complex and dynamic, varying by culture, era,
and technologies; thus the use of the word
desert can
cause confusion. In English prior to the 20th century,
desert was often used in the sense of "unpopulated area",
without specific reference to aridity; but today the word is most
often used in its climate-science sense (an area of low
precipitation)—and a desert may be quite heavily populated, with
millions of inhabitants. Phrases such as "
desert island" and "
Great American Desert" in previous
centuries did not necessarily imply sand or aridity; their focus
was the sparse population. However, the connotation of a hot,
parched, and sandy place often influences today's popular
interpretation of those phrases.
Classification
In 1961,
Peveril Meigs divided desert
regions on Earth into three categories according to the amount of
precipitation they received. In this now widely accepted system,
extremely arid lands have at least 12 consecutive months without
rainfall, arid lands have less than 250 mm (10 in) of annual
rainfall, and semiarid lands have a mean annual precipitation of
between 250 and 500 mm (10-20 in). Arid and extremely arid lands
are deserts, and semiarid grasslands are generally referred to as
steppes.
Definition
Measurement of rainfall alone cannot provide an accurate definition
of what a desert is because being arid also depends on evaporation,
which depends in part on
temperature.
For
example, Phoenix,
Arizona
receives less than 250 millimeters (10 in) of
precipitation per year, and is immediately recognized as being
located in a desert due to its arid adapted plants. However,
the
North Slope of Alaska's
Brooks Range also receives less than
250 millimeters (10 in) of precipitation per year, and is not
generally recognized as a desert region.
Potential
evapotranspiration
supplements the measurement of rainfall in providing a scientific
measurement-based definition of a desert. The water budget of an
area can be calculated using the formula
P −
PE ±
S, wherein
P is precipitation,
PE is
potential evapotranspiration rates and
S is amount of
surface storage of water. Evapotranspiration is the combination of
water loss through atmospheric
evaporation and through the life processes of
plants. Potential evapotranspiration, then, is the amount of water
that
could evaporate in any given region.
As an example,
Tucson,
Arizona
receives about 300 millimeters, (12 in), of rain
per year, however about 2500 millimeters, (100 in), of water could
evaporate over the course of a year. In other words, about 8
times more water could evaporate from the region than actually
falls. Rates of evapotranspiration in cold regions such as Alaska
are much lower because of the lack of heat to aid in the
evaporation process.
There are different forms of deserts. Cold deserts can be covered
in
snow or
ice; frozen water
unavailable to plant life. These are more commonly referred to as
tundra if a short season of above-freezing
temperatures is experienced, or as an
ice
cap if the temperature remains below freezing year-round,
rendering the land almost completely lifeless.
Most non-polar deserts are hot in the day and chilly at night (for
the latitude) because of the lack of the moderating effect of
water. In some parts of the world, deserts are created by a
rain shadow effect in which air masses
lose much of their moisture as they move over a
mountain range; other areas are arid by
virtue of being very far from the nearest available sources of
moisture.
Deserts are also classified by their geographical location and
dominant weather pattern as trade wind, mid-latitude, rain shadow,
coastal, monsoon, or
polar deserts.
Former desert areas presently in non-arid environments are
paleodeserts.
Montane deserts are arid places with a very high
altitude; the most prominent example is
found north of the Himalaya
especially
in Ladakh region of Jammu and Kashmir, in parts of the Kunlun
Mountains
and the
Tibetan
Plateau
. Many locations within this category have
elevations exceeding 3,000 meters (10,000 ft) and the thermal
regime can be
hemiboreal. These places
owe their profound aridity (the average annual precipitation is
often less than 40 mm/1.5in) to being very far from the
nearest available sources of moisture. Montane deserts are normally
cold.
Rain shadow deserts form when tall
mountain ranges block clouds from reaching areas in the direction
the wind is going. As the air moves over the mountains, it cools
and moisture condenses, causing
precipitation on the windward
side. When that air reaches the leeward side, it is dry because it
has lost the majority of its moisture, resulting in a desert. The
air then warms, expands, and blows across the desert. The warm,
desiccated air takes with it any remaining moisture in the
desert.
Desert features
Sand covers only about 20 percent of Earth's
deserts. Most of the sand is in sand sheets and sand seas—vast
regions of undulating dunes resembling ocean waves "frozen" in an
instant of time. In general, there are six forms of deserts:
- Mountain and basin deserts
- Hamada deserts, which consist of plateau
landforms
- Regs, which consist of rock pavements
- Ergs, which are formed by sand seas
- Intermontane Basins
- Badlands, which are located at the margins of arid lands
comprising clay-rich soil
Nearly all desert surfaces are plains where eolian
deflation—removal of fine-grained material by the wind—has exposed
loose gravels consisting predominantly of
pebbles but with occasional
cobbles.
The remaining surfaces of arid lands are composed of exposed
bedrock outcrops,
desert soils, and
fluvial deposits including
alluvial fans,
playas,
desert
lakes, and oases. Bedrock outcrops
commonly occur as small mountains surrounded by extensive erosional
plains.
Several different types of dunes exist. Barchan dunes are produced
by strong winds blowing across a level surface and are
crescent-shaped. Longitudinal or seif dunes are dunes that are
parallel to a strong wind that blows in one general direction.
Transverse dunes run at a right angle to the constant wind
direction. Star dunes are star-shaped and have several ridges that
spread out around a point.
Oases are vegetated areas moistened by
spring,
well, or by
irrigation.
Many are artificial. Oases are often the only places in deserts
that support crops and permanent habitation.
Flora and fauna
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Organ Pipe cactus in Arizona
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Flora of Baja California Desert,
Cataviña region, Mexico
Deserts have a reputation for supporting very little life, but in
reality deserts often have high
biodiversity, including
animals that remain hidden during daylight hours to
control body temperature or to limit moisture needs. Some fauna
includes the
kangaroo rat,
coyote,
jack rabbit, and
many
lizards. These animals adapted to live
in deserts are called
xerocoles. Many
desert animals (and plants) show especially clear evolutionary
adaptations for water conservation or heat tolerance, and so are
often studied in
comparative
physiology,
ecophysiology, and
evolutionary physiology. One
well-studied example is the specializations of mammalian kidneys
shown by desert-inhabiting species. Many examples of
convergent evolution have been
identified in desert organisms, including between
cacti and
Euphorbia,
kangaroo rats and
jerboas,
Phrynosoma and
Moloch lizards.
Some flora includes shrubs,
Prickly Pears,
Desert Holly, and the
Brittlebush. Most desert
plants are drought- or salt-tolerant, such as
xerophytes. Some store water in their leaves,
roots, and stems. Other desert plants have long
taproots that penetrate to the water table if
present, or have adapted to the weather by having wide-spreading
roots to absorb water from a greater
area of the ground. Another adaptation is the
development of small, spiny
leaves which shed
less moisture than
deciduous leaves with
greater surface areas. The stems and leaves of some plants lower
the surface velocity of sand-carrying winds and protect the ground
from erosion. Even small fungi and microscopic plant organisms
found on the soil surface (so-called
cryptobiotic soil) can be a vital link in
preventing erosion and providing support for other living
organisms.
Deserts typically have a plant cover that is sparse but enormously
diverse.
The giant saguaro
cacti of the Sonoran
Desert
provide nests for desert birds and serve as "trees"
of the desert. Saguaro grow slowly but may live up to 200
years. When 9 years old, they are about 15 centimeters (6 in) high.
After about 75 years, the cacti develop their first branches. When
fully grown, saguaro cacti are 15 meters tall and weigh as much as
10 tons. They dot the Sonoran and reinforce the general impression
of deserts as cactus-rich land.
Although
cacti are often thought of as
characteristic desert plants, other types of plants have adapted
well to the arid environment. They include the
pea and
sunflower families.
Cold deserts have grasses and shrubs as dominant vegetation.
Water
Atacama
is the driest place on Earth
and is virtually sterile because it is blocked from moisture on
both sides by the Andes mountains and by the Chilean Coast Range. The cold
Humboldt Current and the
anticyclone of the Pacific are essential
to keep the dry climate of the Atacama.
The average rainfall
in the Chilean region of Antofagasta
is just 1 mm per year. Some weather
stations in the Atacama have never received rain. Evidence suggests
that the Atacama may not have had any significant rainfall from
1570 to 1971. It is so arid that mountains that reach as high as
6,885 metres (22,590 feet) are completely free of
glaciers and, in the southern part from 25°S to
27°S, may have been glacier-free throughout the
Quaternary, though
permafrost extends down to an altitude of 4,400
metres and is continuous above 5,600 metres.
Rain
does fall occasionally in deserts, and desert storms
are often violent. A record 44 millimeters (1.7 in) of rain once
fell within 3 hours in the Sahara. Large Saharan storms may deliver
up to 1 millimeter per minute. Normally dry stream channels, called
arroyos or
wadis,
can quickly fill after heavy rains, and
flash floods make these channels
dangerous.Though little rain falls in deserts, deserts receive
runoff from ephemeral, or short-lived, streams fed considerable
quantities of sediment for a day or two. Although most deserts are
in basins with closed or interior drainage, a few deserts are
crossed by 'exotic' rivers that derive their water from outside the
desert. Such rivers infiltrate soils and evaporate large amounts of
water on their journeys through the deserts, but their volumes are
such that they maintain their continuity.
The Nile River, the Colorado River
, and the Yellow River
are exotic rivers that flow through deserts to
deliver their sediments to the sea. Deserts may also have
underground springs, rivers, or reservoirs that lie close to the
surface, or deep underground. Plants that have not completely
adapted to sporadic rainfalls in a desert environment may tap into
underground water sources that do not exceed the reach of their
root systems.
Lakes form where rainfall or meltwater in interior drainage basins
is sufficient. Desert lakes are generally shallow, temporary, and
salty. Because these lakes are shallow and have a low bottom
gradient, wind stress may cause the lake waters to move over many
square kilometers. When small lakes dry up, they leave a salt crust
or
hardpan. The flat area of clay, silt, or
sand encrusted with salt that forms is known as a playa. There are
more than a hundred playas in North American deserts. Most are
relics of large lakes that existed during the last
ice age about 12,000 years ago.
Lake
Bonneville
was a 52,000
kilometers² (20,000 mi²) lake almost 300 meters (1000 ft) deep
in Utah, Nevada, and Idaho during the Ice Age. Today the remnants of
Lake Bonneville include Utah's Great Salt Lake
, Utah
Lake
, and Sevier
Lake
. Because playas are arid landforms from a
wetter past, they contain useful clues to climatic change.
When the occasional precipitation does occur, it erodes the desert
rocks quickly and powerfully.
The flat terrains of hardpans and playas make them excellent
racetracks and natural runways for airplanes and spacecraft.
Ground-vehicle speed records are commonly
established on Bonneville Speedway
, a racetrack on the Great Salt Lake hardpan.
Space
shuttles land on Rogers Lake Playa at Edwards Air
Force Base
in California.
Formation of hot deserts
There are four main, interlinked causes of hot deserts:
- The formation of the subtropical high-pressure cell.
- The rain shadow effect in the belt of easterly trade
winds.
- The effect of the cold currents off the west coast of the
continents at these latitudes.
- The depositing sands of a desert along its border into the
fertile land
It must be noted that also hot deserts (like
cold deserts) may result in average
temperature cooling
[702158] because they reflect more of the
incoming light (their
albedo is higher than
that of water or forests).
Mineral resources
Deserts may contain great amount of mineral resources over their
entire surface. This occurrence in minerals also determines the
color. For example, the red color of many sanddeserts is a result
of the occurrence of
laterite.
Some
mineral deposits too are formed,
improved, or preserved by geologic processes that occur in arid
lands as a consequence of climate.
Ground
water leaches ore minerals and redeposits them in zones near the
water table. This leaching process
concentrates these minerals as ore that can be mined.
Evaporation in arid lands enriches mineral accumulation in their
lakes. Lake beds known as
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
borate. The minerals formed in these
evaporite deposits depend on the composition and
temperature of the saline waters at the time of deposition.
Significant evaporite resources occur in the
Great Basin Desert of the United
States, mineral deposits made famous by the "20-mule teams" that
once hauled borax-laden wagons from Death Valley
to the railroad.
Boron, from
borax and
borate evaporites, is an essential ingredient in the manufacture of
glass, enamel, agricultural chemicals, water softeners, and
pharmaceuticals.
Borates are mined from evaporite deposits at
Searles
Lake
, California, and other desert locations. The
total value of chemicals that have been produced from Searles Lake
substantially exceeds
US$1
billion.
The
Atacama
Desert
of Chile
is unique
among the deserts of the world in its great abundance of saline
minerals. Sodium nitrate has been mined for
explosives and
fertilizer in the Atacama since the middle of the
19th century. Nearly 3 million
tonnes were
mined during
World War I.
Valuable
minerals located in arid lands include copper
in the United States, Chile, Peru
, and
Iran
; iron and lead-zinc ore in Australia; and gold, silver, and uranium deposits
in Australia and the United States. Nonmetallic mineral
resources and rocks such as
beryllium,
mica,
lithium,
clays,
pumice, and
scoria also occur in arid regions.
Sodium carbonate,
sulfate, borate,
nitrate,
lithium,
bromine,
iodine,
calcium, and
strontium compounds come from sediments and
near-surface brines formed by evaporation of inland bodies of
water, often during geologically recent times.
The
Green River Formation of
Colorado
, Wyoming
, and Utah
contains
alluvial fan deposits and playa evaporites
created in a huge lake whose level fluctuated for millions of
years. Economically significant deposits of
trona, a major source of
sodium
compounds, and thick layers of
oil shale
were created in the arid environment.
Some of the more productive
petroleum
areas on Earth are found in arid and semiarid regions of Africa and
the Mideast, although the
oil fields were
originally formed in shallow marine environments. Recent climate
change has placed these reservoirs in an arid environment.
It's
noteworthy that Ghawar
, the world's
largest and most productive oilfield is mostly under the Empty Quarter
and Al-Dahna
deserts.
Other oil reservoirs, however, are presumed to be
eolian in origin and are presently found in humid
environments.
The Rotliegendes, a hydrocarbon reservoir in the North Sea
, is associated with extensive evaporite
deposits. Many of the major U.S. hydrocarbon resources may
come from eolian sands. Ancient
alluvial
fan sequences may also be hydrocarbon reservoirs.
Solar energy resources
Deserts are increasingly seen as sources for
solar energy.
The Negev Desert
and the surrounding area, including the Arava Valley, are the sunniest parts of Israel
and little of this land is arable, which is
why it has become the center of the Israeli solar industry.
David Faiman, a world expert on solar energy,
feels the energy needs of a country like Israel
could be met
by building solar energy plants in the Negev. Faiman also
feels the technology now exists to supply all of the world's
electricity needs with 10 per cent of the
Sahara.
Solel has nine fields of
solar collectors in the
Mojave Desert of
California. It recently signed a contract to build the
Mojave Solar Park, which will be the
world's largest solar generating plant.
Human life in deserts
A desert is a hostile, potentially deadly environment for
unprepared humans. In hot deserts, high temperatures cause rapid
loss of water due to
sweating, and the absence
of water sources with which to replenish it can result in
dehydration and death within a few days. In
addition, unprotected humans are also at risk from
heatstroke.
Humans may also have to adapt to
sandstorms in some deserts, not just in their
adverse effects on
respiratory
systems and eyes, but also in their potentially harmful effects
on equipment such as
filters, vehicles
and communication equipment. Sandstorms can last for hours,
sometimes even days. This makes surviving in the desert quite
difficult for humans.
Despite this, some cultures have made hot deserts their home for
thousands of years, including the
Bedouin,
Tuareg and
Pueblo
people. Modern technology, including advanced
irrigation systems,
desalinization and
air conditioning have made deserts much
more hospitable.
In the United States
and Israel
for example,
desert farming has found extensive
use.
In cold deserts,
hypothermia and
frostbite are the chief hazards, as well
as
dehydration in the absence of a
source of heat to melt ice for drinking. Falling through pack-ice
or surface ice layers into freezing water is a particular danger
requiring emergency action to prevent rapid hypothermia. Starvation
is also a hazard; in low temperatures the body requires much more
food energy to maintain body heat and to
move. As with hot deserts, some people such as the
Inuit have adapted to the harsh conditions of cold
deserts.
Most traditional human life in deserts is
nomadic. It depends in hot deserts on finding water,
and on following infrequent rains to obtain grazing for livestock.
In cold deserts, it depends on finding good hunting and fishing
grounds, on sheltering from blizzards and winter extremes, and on
storing enough food for winter. Permanent settlement in both kinds
of deserts requires permanent water and food sources and adequate
shelter, or the technology and energy sources to provide it.
Many deserts are flat and featureless, lacking landmarks, or
composed of repeating landforms such as sand dunes or the jumbled
ice-fields of glaciers. Advanced skills or devices are required to
navigate through such landscapes and inexperienced travellers may
perish when supplies run out after becoming lost. In addition
sandstorms or
blizzards may cause
disorientation in severely reduced visibility.
The danger represented by wild animals in deserts has been featured
in explorers' accounts but does not cause higher rates of death
than in other environments such as rain forests or savanna
woodland, and generally does not by itself affect human
distribution. Defence against
polar bears
may be advisable in some areas of the Arctic, as may precautions
against
venomous snakes and
scorpions in choosing sites at which to
camp in some hot deserts.
See also
References
- What is a desert?
- According to What is a desert?, the 250 mm threshold
definition is attributed to Peveril Meigs.
- Glossary of Meteorology. Megathermal Climate. Retrieved on 2008-05-21.
- 1911 Encyclopedia Britannica
- http://ngm.nationalgeographic.com/ngm/0308/feature3/
- http://www.extremescience.com/DriestPlace.htm
-
http://quest.nasa.gov/challenges/marsanalog/egypt/AtacamaAdAstra.pdf
- Tsjaad by Dorrit van Dalen
- Head of Kibbutz Movement: We will not be
discriminated against by the government, Ehud Zion Waldoks,
Jerusalem
Post, March 10, 2008.
- Giant solar plants in Negev could power Israel's
future, John Lettice, The Register, January 25, 2008
- [1]
- Looking to the sun, Tom Parry, Canadian Broadcasting
Corporation, August 15, 2007.
- [2]
External links