Transcript of "Desert, types of desert, land forms, dunes and types of dunes"
A desert is a barren area of land where little
precipitation occurs and consequently living
conditions are hostile for plant and animal life.
A desert is a region with so little vegetation that no
significant population can be supported on that
It need not be hot or even
, technically dry. Ice sheets
are a kind of desert.
Deserts cover about 30% of the Earth’s land
Semiarid, Arid and Deserts
Semiarid lands, also called steppes,
typically receives 250 to 500 millimeters (10
to 20 inches) of rain per year
Arid lands receive less than 250 millimeters
(10 inches) of rain per year
Deserts are formally defined as regions that
rarely receive precipitation, typically less
than 100 millimeters (4 inches) per year
Desert which is arid.
A desert is a dry, often
sandy region of little
temperatures, and sparse
Steppe which is semiarid.
The steppe is a marginal and
more humid variant of the
desert and is a transition
zone that surrounds the
dessert and separates it from
bordering humid climates.
WITHIN WATER DEFICIENT REGIONS TWO CLIMATIC TYPES ARE COMMONLY
CAUSES OF NATURAL DESERTS
A variety of factors contribute to the
formation of desert, so desert come in
several different types , depending on their
High surface temperature
1. HIGH SURFACE TEMPERATURE
Most vegetation, under such conditions,
requires abundant rainfall and or slow
evaporation of what precipitation does fall.
The availability of precipitation is
governed, in part by the Global
Warm air holds more moisture than cold.
Global distribution of air pressure and wind
is cause of formation of deserts.
Some of the air at the horse latitudes is heated
and rises. When the air reaches the tropopause
it is pushed in opposite directions.
Creating 2 more low pressure areas.
Air then moves from high pressure to low
GLOBAL CIRCULATION PATTERN
Solar radiations falls most directly on the earth
near the equator and consequently, solar
heating of the atmosphere and surface more
intense there the sun rays are most dispersed
near the poles.
The air over the equatorial regions would be
warmer than the air over the poles.
Heated air rises creating low pressure at earth
Rising air hits the tropopause move pole ward
begins to cool.
Warm air holds more moisture than cold.
Similarly, when the pressure on a mass of air is
increased, the air can hold more moisture.
Air spreading outward from the equator at high
altitudes is chilled an at low pressure, since air
pressure and temperature decrease with
Thus the air holds little moisture when that air
circulates downward, at about 30 degrees north
and south latitudes, it is warmed as it
approaches the surface, and also subjected to
increasing pressure from the deepening column
of air above it. It can then hold considerably
desert in the middle latitudes are rains
shadow deserts. As moving air meats a
mountain barrier, it is forced to rise.
Clouds and precipitation windward side often
result. Air descending the leeward side is
much drier. The mountains effectively cut the
leeward side off from the sources of moisture,
producing a rain shadow desert.
The great basin desert is a rain shadow that
covers nearly all of Nevada and adjacent
On the other hand even coastal areas can
have desert under special circumstances.
If the land is hot and adjacent ocean cooled by
cold currents, the moist air coming off the
ocean will be cool and carry less moisture.
Then air over a warmer ocean as the cooler air
warms over the land and becomes capable of
holding still more moisture,
It causes rapid evaporation from the land
rather than precipitation.
This phenomenon is observed along portion of
the western coasts of Africa and south
DESERTS OCCURS IN 2 BROAD BELTS, AT 20-30O NORTH AND SOUTH OF THE
EQUATOR, ALONG THE TROPICS OF CANCER AND CAPRICORN.
TYPES OF DESERT
TRADE WIND DESERT An area of very little rainfall
and high temperature that occurs where the trade
winds or their equivalent blow over land. (linear dune)
The best examples are the Sahara and Kalahari
deserts. The trade winds blowing from higher latitudes
are very drying, and cloudiness is almost absent in
these desert regions. (57 degrees)
2. Midlatitude deserts : Midlatitude deserts occur
between 30° and 50° N. and S., pole ward of the
subtropical high pressure zones.These deserts are in
interior drainage basins far from oceans and have a
wide range of annual temperatures.
The Sonoran Desert of southwestern North America
is a typical midlatitude desert.(ripples, small
3.Rain Shadow desert: Rain shadow deserts
are formed because tall mountain ranges
prevent moisture-rich clouds from reaching
areas on the lee, or protected side, of the
range. As air rises over the mountain, water is
precipitated and the air loses its moisture
content. A desert is formed in the leeside
"shadow" of the range.(Tian Shan of China)
4. Coastal deserts: Coastal deserts generally are found
on the western edges of continents near the Tropics of
Cancer and Capricorn. They are affected by cold
ocean currents that parallel the coast. Because local
wind systems dominate the trade winds, these deserts
are less stable than other deserts. Winter fogs,
produced by upwelling cold currents, frequently blanket
coastal deserts and block solar radiation. A coastal
desert, the Atacama of South America, is the Earth's
driest desert.(Crescent-shaped dunes in Namib,
5. Monsoon desert : Monsoons develop in
response to temperature variations between
continents and oceans. The southeast trade
winds of the Indian Ocean, for example, provide
heavy summer rains in India as they move
onshore. As the monsoon crosses India, it loses
moisture on the eastern slopes of the Aravalli
Range.The Rajasthan Desert of India and the
Thar Desert of Pakistan are parts of a monsoon
desert region west of the range.(small patches of
6. Polar desert: Polar deserts can also be attributed to
the differences in moisture holding capacities between
warm and cold air. Air travelling from warmer latitudes to
colder near polar ones will tend to lose moisture by
precipitation ,so less remains to fall as snow near the
poles, and the limited evaporation from cold high latitudes
oceans contributes little additional moisture to enhance
local precipitation. Thick polar ice caps, then , reflect
effective preservation of what snow does fall, rather than
heavy precipitation. sand dunes are not prominent, but
snow dunes occur in those areas where precipitation is
more abundant. (mostly bedrock or gravel plains)
Plateau: A plateau is a
broad, level, elevated
bounded partly by cliffs.
Mesa: A mesa is the
American English term
for tableland, an
elevated area of land
with a flat top and sides
Butte: is an isolated hill
with steep, often
vertical sides and a
small, relatively flat top;
buttes are smaller than
mesas, plateaus, and
Wadi: It refers to a dry
that contains water only
alluvial fan is a fan or
cone-shaped deposit of
sediment crossed and
built up by streams.
temporary lake where
water flows into the
Inselberg is an
isolated hill of
: A bolson
is a semiarid, flatfloored desert
is a natural
rock formation where
a rock arch forms,
with an opening
escarpment is a
steep slope or long
isolated area of
vegetation in a
spring or similar
provide habitat for
animals and even
PROCESSES AFFECTING LANDFORM SHAPES
weathering, erosion, and deposition are a main
force behind landscape types
But they are not the only reason for a plain,
plateau, and mountain region to form.
Other factors such as glaciation and tectonic
activity lead to landscape building
the water cycle or hydrologic cycle is the
winds also help form the surface landscape
Erosion is the process by which soil
and rock are removed from the
Earth's surface by exogenetic
processes such as wind or water
flow, and then transported and
deposited in other locations.
Wind erosion: consists of Abrasion
Wind abrasion: is the wearing away of a solid
object by the impact of particles carried by
winds. It is a sort of natural sand-blasting.
Abrasion creates interestingly shaped stones
called Vent facts and Yardangs.
Vent facts: are rocks that are polished and
shaped by sand blasting (abrasion).
Yardangs are usually small,
wind sculpted landforms that are aligned
parallel with the wind. Individual
yardangs are generally small features
that stands less than 5 meters high and
no more than about 10 meters.
: is the wholesale removal of loose
sediments, usually fine-grained sediment, by
The resulting surface after deflation is called
Desert pavement and Blowouts.
Desert pavement: consists of a closely packed
veneer of pebbles and cobbles that is only one
or two stones thick. Beneath the pavement is
material containing a significant proportion of
blowout or deflation hollow is a
depression excavated by the wind in easily
eroded materials such as sand or silt
deposits with little or no moisture or
vegetation content--vegetation present in
materials at the surface impedes deflation.
In portions of many deserts, the surface is
a closely packed layer of coarse pebbles
and cobbles too large to be moved by wind
This stony veneer is called desert
TRANSPORTATION OF SEDIMENT BY WIND
Moving air, like moving water is turbulent and able to
pick up loose debris and transport it to other locations.
Wind erosion loads:
Bed loads: the bed loads carried by wind consists of
sand grains. The windblown sand moves by skipping
and bouncing along the surface (saltation)
the accumulation of this type of load forms the desert
load: suspended materials in
the wind consists primarily of silt or clay size
particles and if highly concentration can
result in dust storms--the deposition of the
suspended load can form large deposits
which lack layering called loess.
Wind running water, wind
As with the case for
drops its load of sediment when its velocity
falls and the energy available for transport
So windblown silt and sand can accumulate,
forming distinctive geologic features.
A deposit of windblown dust is called loess
The fine dust in loess is typically only 0.01 to 0.06
millimeter (0.0004 to 0.0024 inch) in diameter
There are two primary sources for loess deposits:
Glacial outwash deposits
Desert Loess in China
Homes, called yaodongs, are created by digging into the
In the dry desert, these structures can last for many
Where sand is transported and later
deposited by wind, the principle
depositional feature is called a dune
(or sand dune)
SAND DUNES FORM WHEN THERE IS
a ready supply of sand
a steady wind
an obstacle such as vegetation, rocks, or fences, to trap
some of the sand. Dunes migrate by erosion of sand by
the wind on the gentle upwind slope and deposition
and sliding on the slip face.
crescent-shaped dunes with the points of the crescents pointing
in the downwind direction, and a curved slip face on the
downwind side of the dune. They form in areas where there is a
hard ground surface, a moderate supply of sand, and a constant
Large fields of dunes that resemble sand ripples on a large scale. They
consist of ridges of sand with a steep face in the downwind side, and
form in areas where there is abundant supply of sand and a constant
Long straight dunes that form in areas with a limited sand supply
and converging wind directions.
Are dunes with several arms and variable slip face directions that form in
areas where there is abundant sand and variable wind directions.
Continued sand accumulation, combined with
periodic slides down the slip face, result in the
slow migration of the dune in the direction of
The Highest Dunes
The highest dunes in the world are located along
the southwest coast of Africa in the Namib Desert
in the country of Namibia.
For example, “Dune 7” is 383 meters (1245 feet)
Occurs as a result of climatic changes, such as changing positions of the
continents, or changes in ocean and air circulation patterns. Human
impacts, such as overgrazing, draining of land, and lowering of the
groundwater table, can also contribute to desertification. As vegetation
dies out, the soil is more easily eroded and may be lost so that other
vegetation becomes destabilized. Since soil can hold moisture, if the
soil erodes, the area may become arid, and the desert expands.
THE CHANGE BETWEEN DAY AND NIGHT TEMPERATURE
GLOBAL IMPORTANCE OF DESERT
The desert is important because it helps maintain biodiversity in
our planet. If were remove deserts, it will affect our
environment. Everything in our environment is linked and
A lot of oil is often found under many deserts. Two thirds of the
world’s crude oil is found by drilling in deserts and they hold
great potential for solar and wind energy too.
Environmental Geology (fifth edition) by Carla W.
Essentials of Geology (tenth edition) by Frederick K.Lutgens
& Edward J.Tarbuck