DESERT

DESERT• 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 land. • 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 surface (42 million square kilometers)

• No other land climate type covers such a large area

Semiarid, Arid and DesertsSemiarid 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

Within water deficient regions two climatic types are commonly recognized.

DESERT• Desert which is arid.• A desert is a dry, often

sandy region of little rainfall, extreme temperatures, and sparse vegetation.

STEPPE• 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.

Causes Of Natural Deserts

A variety of factors contribute to the formation of desert, so desert come in several different types , depending on their origins.

1. High surface temperature2. Topography

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 Circulation Pattern.

• Warm air holds more moisture than cold.• Global distribution of air pressure and wind is

cause of formation of deserts.

Global circulation pattern

• 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 pressure.

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 surface.• Rising air hits the tropopause move pole ward begins to

cool.• Falling air accumulates and create a high pressure system.• Some of this fallen air continues pole ward until it is

heated again and some of air returns.

• 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 increasing altitudes.

• 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 more water, so when it reaches earth’s surface. It cause rapid evaporation.

• Thus many of the World’s major deserts fall in belts close to these zones of sinking air at 30 degrees north and south of the equator, these are the tropical/ latitudes deserts.

2.Topography • Many 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 states.

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 America.

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

1. 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 blowouts)

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, Africa)

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 sand sheets)

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)

Desert Land Forms

Desert landforms• Plateau: A plateau is a

broad, level, elevated region commonly 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 that are usually steep cliffs.

• Butte: is an isolated hill with steep, often vertical sides and a small, relatively flat top; buttes are smaller than mesas, plateaus, and table landforms.

• Wadi: It refers to a dry (ephemeral) riverbed that contains water only during times of heavy rain or simply an intermittent stream.

• Alluvial fans: An alluvial fan is a fan or cone-shaped deposit of sediment crossed and built up by streams.

• Playa lakes: an temporary lake where water flows into the dessert depression. Eventually the water evaporates and salt pans or clay pans are formed.

• Inselberg: Inselberg is an isolated hill of harder rock.

• Bolson : A bolson is a semiarid, flat-floored desert valley or depression, usually centered on a playa or salt pan and entirely surrounded by hills or mountains.

• Arch: is a natural rock formation where a rock arch forms, with an opening underneath.

• Escarpment: An escarpment is a steep slope or long cliff that occurs from faulting and resulting erosion and separates two relatively level areas of differing elevations.

• Oasis: is an isolated area of vegetation in a desert typically surrounding a spring or similar water source. Oasis also provide habitat for animals and even humans if the area is big enough.

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 driving force • winds also help form the surface landscape • But without gravity and the sun this whole process would not

occur.

• Erosion 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 &

Deflation.

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 Ventifacts and Yardangs.

• Vent facts: are rocks that are polished and shaped by sand blasting (abrasion).

• Yardangs: 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.

• Deflation : is the wholesale removal of loose sediments, usually fine-grained sediment, by the wind.

• 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 finer particles.

• Blowouts: a 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.

Desert PavementIn 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 pavement

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: 1. 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 sand dunes.

• Suspended 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 DepositsAs with the case for running water, wind drops its load of sediment when its velocity falls and the energy available for transport diminishes

So windblown silt and sand can accumulate, forming distinctive geologic features.

Loess

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:

Deserts

Glacial outwash deposits

Desert Loess in ChinaHomes, called yaodongs, are created by digging into the loess

In the dry desert, these structures can last for many centuries

Windblown SandWhere sand is transported and later deposited by wind, the principle

depositional feature is called a dune (or sand dune)

Sand dunes form when there is1. a ready supply of sand 2. a steady wind 3. 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.

TypesBarchans Dunes:

• 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 wind direction.

Transverse Dunes• 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 wind direction.

Linear Dunes• Long straight dunes that form in areas with a limited sand supply

and converging wind directions.

Star Dunes -

• Are dunes with several arms and variable slip face directions that form in areas where there is abundant sand and variable wind directions.

Dune MigrationContinued sand accumulation, combined with periodic slides down the slip face, result in the slow migration of the dune in the direction of the wind.

The Highest DunesThe 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) high.

Desertification• 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 in deserts

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 interconnected.

• 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.

Sources:• http://pubs.usgs.gov/gip/deserts/types/• http://www.docstoc.com/docs/77146811/2---Three-Cell-Global-Circulatio

nppt---firstclass-Support• http://www.ask.com/question/why-is-the-desert-important• http://library.thinkquest.org/26634/desert/formation.htm• Environmental Geology (fifth edition) by Carla W. Montgomery• Essentials of Geology (tenth edition) by Frederick K.Lutgens & Edward

J.Tarbuck• http://en.wikipedia.org/wiki/Desert• http://courses.missouristate.edu/emantei/creative/glg110/deserts-

wind.html#page314• Physical Geology(seventh edition) by Charles C. Plummer• www.info.com/Desert +Dunes

Sources:

• www.kean.edu/.../Lecture%2016n%20Dry%20Climate.

• www.elmhurst.edu/~richs/EC/102/Lectures/Deserts • www.duneguide.com/worldwide_dunes.htm • www.worldofteaching.com • users.unimi.it/ paleomag/ppt/E-Environs .Deserts.