WIND/AEOLIAN ACTION IN DESERT AND RESULTING LANDFORMS – ADVANCED PHYSICAL GEOGRAPHY
What is the wind?
Wind is the movement of air from high pressure centre to low pressure centre.
The wind actions are commonly occur at the Desert and Semi-desert areas.
BASIC FUNTION OF WIND
The wind like as river and glacier that performs the three (3) basic functions. These are: – 1. Wind erosion.
- Wind transportation.
- Wind deposition.
WIND EROSION
Wind erosion is the wearing a way of land surface by action of blowing winds.
The wind erosion is the most effective in the Desert.
PROCESSES OF WIND EROSION
There are basically three (3) processes of wind erosion. These are;
i. Deflation:
Is the removal of light loose materials from a desert surface by blowing wind. Such material can be lifted out or rolled away.
ii. Abrasion (corrasion / sand blast):
This is where wind hurls small particles of rocks against rock surface. This is done by grinding, scraping and polishing. The friction wears away the rock.
iii. Attrition:
The process by the rock particles rub or collide against each other as they are carried along with the wind. The particles get progressively reduced in size through this way until they become finer and finer.
WIND EROSIONAL FEATURES / LANDFORMS
There are different features produced by wind. These are: –
i. Yardang:
These are narrow steep sided ridge in the desert separated by trenches (furrow). Yardang form as wind erosion attacks rocks of alternate hard and soft lies vertically. The wind erosion erodes more the softer rocks bands to form trenches leaving the harder rock as ridges called Yardang. Example; Near Qunianga-Kebir in North East Chad, South West Libya, Saloh-Central Algeria, etc.
ii. Zeugen:
These are tabular rock masses of resistant rock standing up on softer underlying rock where the alternate layers of hard and soft laid horizontally to each other. The soft layers are eroded faster by abrasion than the hard layer, after erosion ridges and furrow landscapes develop. The ridges upstanding are called Zeugen.
iii. Rock pedestal / Mushroom block:
These are the tower like structure composed of alternate bands of soft and hard rocks produced due to wind abrasion in the desert. The wind erodes soft rock easily to give tower like structure. Example; Tibesti in China, Gara mountain in Saudi Arabia, Sahara Desert, Niger in W/Africa, etc.
iv. Mushroom blocks:
It is like a rock pedestal but it is made by homogenous rock of uniform resistance to erosion. This is a massive rock with a broad, smoothed and rounded top and a very narrow base formed when abrasion process attacks the lower parts as a result to a mushroom like feature called mushroom block. It is very common in Namibia.
v. Ventifacts / Dreikanter:
These are boulders stones or pebbles that have been broken from masses rocks through physical weathering. They are then smoothed and polished on their windward sides through the process of abrasion. Periodic changes in wind direction exposes other sides o of the rock pieces to the same process, that cause develop three wind faces surface.
vi. Deflation hollow / Blow out depression / Pans:
These are hollows or depressions produced by wind deflation. Small hollows are called Pans and are common in Kalahari. Sometimes the water table may be reached for water to seep through to form an Oasis. Example; Qattara depression in Egypt, Kufra and Siwa oasis in Sahara Desert, Etosha pan in Namibia, etc.
vii. Inselberg:
These are flat or rounded toped resistant hills made up by hard rock left upstanding on the arid and semi-arid areas. It is formed by wind action also by fluvial action in the Desert areas. It is formed by combine effects of abrasion and deflation processes. It occurs when Desert surface made up by soft and hard rocks, the wind abrasion worn away the weaker rock easily and left hard rock layer upstanding above the ground surface level. When inselbergs are smooth and round in shape are called bornardts, and when they are characterised by a lot of joints with rectangular rock blocks pilled together to produce a castellated form are called as Kopjes.
viii. Desert pavements:
These are the horizontal areas of bare, polished rock formed by the scouring action of grains of quartz.
ix. Millet seed / Rounded sand grain:
These are formed due to the collision of sand grain with or against one another. The result is to produce a characteristic of a rounded sand grain (millet seed). This forms the dominant end product of desert erosion.
WIND TRAPORTATION
Wind transportation is the movement of eroded materials form one place to another place by wind for deposition.
PROCESSES OF WIND TRANSPORTATION
There are three (3) basic process of wind transportation. These are;
i. Suspension:
This is the process where by wind transport very fine materials by swinging them in the air for greater distance and very faster.
ii. Saltation:
This is the process where by wind transport medium size particles by skipping and bouncing along the surface.
iii. Traction / Surface creep:
This is the process where by wind transport very heavy and large size particles like boulders by dragging or rolling or sliding over the surface under the influence of very strong wind.
FACTORS INFLUENCING WIND TRANSPORTATION
The transportation of materials by wind in the deserts is influenced by the following factors;
i. Speed and strength of the wind.
ii. Obstacles in the path of the wind.
iii. Nature of the land.
iv. Weather changes.
WIND DEPOSITION
Wind deposition is the process of laying down the materials that transported by wind due to decrease of its strength.
FACTORS THAT CAUSE WIND DEPOSITION
There are different factors responsible for wind deposition. These are: –
i. The nature of the ground surface eg. sandy, rocky, etc.
ii. Presence of obstacles eg. vegetation, large boulders of rocks, etc.
iii. Strength of the wind and their direction.
iv. Size and amount of materials carried by the wind.
v. Variation of weather conditions, etc.
WIND DEPOSITION LANDFORMS
These are the landforms which formed due to accumulation of materials was transported by wind like Loess, Draas, and Sand dunes (seif dunes, barchans, parabolic dunes, star dunes, ripples, etc). These are;
i. Loess:
This is an accumulation of very fine materials transported earlier by suspension process of wind and deposited by wind beyond the desert limits. The word loess come from German word which means “fine ground, light coloured porous and yellowish dust”. The deposition of materials is often very far from the original place. This dust was removed by wind from desert surface or from unconsolidated glacial and fluvioglacial materials during the dry inter-glacial and post glacial periods. Example; Gobi Desert, Yellow river (Hwang Ho) and Yellow Sea in China, etc. ii. Draas:
These are very large sand mounds of varying shapes. However most of the draas are similar in shape to seif and transverse dunes. Their height may reach 200m example; Sahara Desert in Western side.
ii. Ripples / Transverse dunes:
These are long waves like ridges separated by flat bottomed trough. These are smallest depositional feature, they are long and can be found when wind speed has been reduced. They tend to form a large field, but most are less than 50m height. They are built by light to moderate winds, and their length about 100km in distance. Example; Djanet in Algeria, Edeyin Uberi in Libya, etc.
iii. Seif dunes / Longitudinal dunes:
Are the long narrow ridges of sand which lie parallel to the direction of the prevailing wind. The word seif is derived from Arabic word meaning a sword. Therefore, it is similar to a sword. They are large in soze about 100m in lenght and 200m in height. They build up from small sand ridge which can be prolonged to enable it attain its linear characteristics features. The prevailing winds blows straight along the furrow between the dunes keeping them clear of sand, while eddy winds help to build the sides of the dunes. Example; Egyptian Desert, Namib Desert, etc.
iv. Barchans / Barkhans:
These are crescent shaped dunes with two horns pointing down wind, which occur individually or in groups. Their development is usually caused by the obstruction which may be a trees or large stone block. The wind ward side of the barchans is steep and the lee ward side which are concave. It may be up to 25m height and 400m wide. Depending on the direction of winds, barchans are sometimes mobile. Example; Sahara Desert, Northern Nigeria, Old Vai Gorge, etc.
v. Parabolic dunes:
These are also crescent shapes that has concave shape on wind ward side. The parabolic dunes are similar to barchans in terms of shape, but differ interm of size and direction of winds. The parabolic dunes are larger in size compare with barchans and the direction of winds are concave in wind ward side and steep with convex on lee ward side (downwind). Parabolic dunes are formed in association of deflation hollow, when deflation winds deposit their sediments opposite to the depression. Also, it common along the coast. Example; Near Pismo beach in Central California.
vi. Star dunes:
These are isolated hill of sand confined largely to parts of Sahara and Arabian Deserts. Their shape like stars with having multipole points. Usually they have four sharps created ridge diverge from a central high point that in some cases may approach a height of 90m. star dunes may form by accumulation of sand by winds whose direction are variable (from different direction).
vii. Attached dunes /Head dunes:
These are sand drift caused by an obstacle of vegetation or a rock in the path of the prevailing wind. Sand accumulates in the dead air space around the rock to form wind line shape on the windward side of the obstacle and formation of tail dunes on the leeward side.
THE DESERTS
What are the Deserts?
Deserts are the areas which are characterised by the scanty and unreliable precipitation as well as little or no vegetation.
What is the aridity?
Aridity is the state of the land being deficient in moisture leading to scanty vegetation. The areas that receives less than 250mm of rainfall per annually would generally be arid.
Therefore, the Desert is arid area with sparse vegetation.
In generally most of the hot desert occur in the western margin of the all continents (except for the Sahara Desert start from west to east) between latitude 150N/S and 300N/S of the equator.
Therefore, the desert called arid and semi-desert called semi-arid or steppe.
EXAMPLES OF DESERT FROM DIFFERENT CONTINENTS
i. Africa – Namib, Kalahari, and Sahara Deserts.
ii. North America – Colombia, Mahave, Mexican Desert, etc.
iii. South America – Atacama Desert, Patagonia Desert, etc.
iv. Asia – Jordan, Syria, Iran, Saudi Arabia, Gobi Deserts, etc.
v. Australia – Great Australia Desert.
WHY HOT DESERTS HAVE HIGH TEMPERATURE DURING THE DAY AND LOW TEMPERATURE DURING THE NIGHT
During the day time:
The temperature in hot deserts get hot because there are no clouds to block the rays of the sun (short waves solar radiation).
During the Night time:
The temperature gets cold because there are clear skies (no clouds cover) therefore, the radiation of the earth (long wave radiation) loss free without any hindering.
CLASSIFICATION OF DESERTS
Deserts can be classified according to the following criteria. These are;
i. Types of deserts according to surface appearance.
ii. Types of desert according to temperature.
I: TYPES OF DESERTS ACCORDING TO THE SURFACE APPEARANCE
There are three (3) basic of deserts. These are: –
Erg / Sandy desert:
These are deserts with surface composed of mounds of sands. The mounds of sand differ in size and shapes. Example; Edeyin Uberi Deser in Western Libya, Namib Desert, Egyptian Desert, etc.
Reg / Stony desert:
These are desert with a stony covered surface. Reg desert also called Serir in Libya. These compsed stones, boulders and pebbles. Example; Calansho serir in Libya, Tanezrouft west of the Hogger mountain in Algeria.
Hamada / Rocky desert:
These rae deserts with a bare rocky surface. Such deserts may be characterised by residual hills. Example; El Homra in Northern Libya, etc.
II: TYPES OF DESERTS ACCORDING TO THE TEMPERATURE
In this category, the temperature of the desert is observed, as a result of the following two (2) of deserts. These are: –
Tropical / Hot desert:
These are deserts which found within the tropics both north and south, between 150 to 350N/S of the equator. They are very hot due to high amount of insolation received. The temperature of these deserts exceeds 400C in day time and decrease to 150C or below in night time. Example; Sahara, Kalahari, Colorado, and Atacama deserts in Peru.
Cold / Midlatitude desert:
These are deserts located near to the north or south poles. These are called Polar deserts. These experience low temperature both day and night time due to low amount of insolation i.e below 150C. Example; South Russia, Tibesti desert in China, northern Alaska, Canada and Siberia desert, etc.
CHARACTERISTICS OF DESERTS
The following are the general characteristics of deserts. These are;
i. Little or unreliable rainfall i.e less than 250mm per annual.
ii. Low biodiversity. Both flora and fauna, they dominated by Xerophytes and halophytes plants.
iii. Low density of plants. iv. Poor soil productivity.
v. Irregular surface drainage, etc.
CAUSES OF DESERTS
There are different factors that cause occurrence of deserts. Both natural and manmade, these are;
A. Natural / Physical factors:
i. Latitudinal location eg. between 150 – 350N/S of equator.
ii. Presence of mountain (relief) barrier eg. lee ward of mountain. iii. Great distance from the ocean or large water bodies. Because the sea breeze decrease their moisture when the distance increases from water bodies.
iii. Off shore trade winds eg. harmattan winds which blows between Sahara Desert and Arabian Desert.
iv. Presence of cool ocean currents. On western sides of all continents eg. Benguela currents as a result of the formation of Namib Desert.
v. Drought/Insufficient rainfall: Desertification could occur in areas where rainfall is low and unreliable. Such areas receive less than 250 mm of rain annually.
B. Human / Anthropogenic factors:
Deforestation.
Poor irrigation practices. iii. Cultivation in marginal land. iv. Expansion and intensive use of agricultural lands (overcultivation). v. Overgrazing, etc.
DESERT LANDFORMS
The desert landforms as a result of the both effects of: –
• Wind action, and;
• Water (fluvial) actions.
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