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Sahara Territory



:: SAHARA GEOGRAPHY ::

The Sahara is the greatest warm desert in the world, occupying an area of around 9,7 millions km2. This area is somewhat changing, depending on the climate conditions of each year in its boundaries which consist of diverse steppes. The Sahara expands and contracts itself mainly in the southern area, called Sahel, which is an arid savannah that makes the transition between the desert and the fertile savannah. The rainfall in the Sahel is higher than in the inner desert, but it varies greatly each year, so the Sahara southern border is variable, expanding during drought seasons and contracting during rainfall seasons.

The Sahara presents a wide variety of terrain types:

- ergs or edeien are areas covered with sand dunes, reaching some of these dunes close to 300 meters high. The ergs cover approximately 25% of the total area of the Sahara and are the areas that have the highest temperatures and the lowest rainfall. Some of the most arid regions in the Sahara are the Erg of Tenere, the Libyan Desert and the Tanezrouft Desert.

- hammadas are plateaus covered with stones and gravels.

- regs or serirs are plains covered with sand and stones or gravels.

- chotts or sebkhas are depressions whose terrain is often under sea level and often contain salts deposits on their ground. The lowest depression on the Sahara is Qattara, reaching -133 meters under sea level.

Many mountainous massifs of volcanic origin are spread through the Sahara. These areas have their own climate conditions, having generally lower temperatures and higher rainfall than the lower desert areas. Snow is present in the highest areas of the Ahaggar and Tibesti massifs during some winter seasons, with an average frequency of each five years. Ahaggar and Tibesti massifs are the highest ones in the Sahara; in the Tibesti lies the highest mountain in the Sahara, a shield volcano called Emi Koussi, that reaches 3415 meters above sea level. In the Sahara maps, mounts can be referred in the arabic form jebel, also spelled gebel, yebel or djebel, in the berber form adrar or in the teda forms emi and tarso, depending the form used on the ethnical influence area where the mount is located.

Water is temporarily present in many places after rainfall seasons or permanently present in some especific places, called oases, where the underground water reaches the surface. Underground deposits containing fossile waters are present in many areas on the Sahara. This water, generally drinkable, is extracted with wells and exploited in any way possible, but as it happens with petrol deposits, these water deposits cannot regenerate and eventually would be depleted.

Vegetation is abundant in the oases and also present in the dried river basins, known as wadi or uadi in arabic language, as oued in berber language and as enneri in teda language. The oueds are always dry except just after one rare rain. In fact generally there is water present, but only at few meters under the surface of the ground. Some trees survive, if their roots are long enough. In the oases inhabited by humans, date palms are common, while in the wilderness are common the acacia trees and shrubs. In rainy seasons temporary grass fields can be seen.

Fauna species include the dromedary and goats, as domesticated animals, while in the wilderness live different species of gazelles and antelopes, jackals and hyenas, foxes, some ostriches and a reduced number of cheetah guepards and crocodiles. Lizards and venomous vipers are present mainly near water sources during summer or in stony and rocky areas, where scorpions are also frequent.

Erosion is specially strong in the desert. The most agressive form of erosion is generated by the seasonal rainfall; while rainfall is sparse in the desert, when it rains after a long drought, the rain is torrential and the desert soil, lacking any vegetation that could hold the terrain, is drawn by the water torrents and therefore eroded. Towns such as Tamanrasset, Djanet and Illizi have suffered strong inundations.

Another form of erosion, which acts more slowly, is the abrasive sandy wind. The sand dragged by the wind impacts against the rocks, eroding them. The rock formations are destined to be totally destroyed by these erosive processes when enough time has passed. The most vulnerable to these processes are the sandstone rocks, while the volcanic basaltic rocks are more resistent because of their hardness.

Temperatures vary greatly in the desert; while during the daytime the temperature can exceed the 50º C during summer season, during the nights it can fall below 0º C. These extreme temperature variations produce contractions and dilatations in the rocks that crack and slowly broke them into small stones.

Erosive processes produce bizarre geological formations in the deserts. The mountainous massifs show a variety of rock towers and needles, arches and inselbergs, and also yardangs. These are sandstone rock formations that have been eroded by a wind that always blows in the same direction, producing an elongated shape in the eroded rocks. Yardangs can have different sizes, from a meter in height and a few meters in lenght, to hundreds of meters in height and several kilometers in lenght. The Sahara posseses the greatest yardangs in the world, located to the west and south-east of the Tibesti massif. These mega-yardangs have been produced by the harmattan wind that blows from the north-east.

Left image: sand dunes and rocks in Tadrart Akakus, in south-western Libya.

Right image: a view of the landscape in the Ahaggar massif, in southern Algeria.

The landscape of Tadrart Akakus. The landscape of Ahaggar.

Left image: a guelta in the oasis of Iherir, in Tassili-n-Ajjer, southern Algeria.

Right image: the landscape of the Sahel, a strip of arid savannah that separates the desert from the fertile southern savannah.

A guelta in Iherir. The landscape of the Sahel.

Left image: a rock formation in southern Algeria.

Right image: a rock formation in Fezzan, in south-western Libya.

A rock formation in southern Algeria. A rock formation in Fezzan.

In the ergs, the sand dunes can adopt a variety of shapes depending on the way the wind blows. In the following images, sand dunes formations at Erg Chech in western Algeria (left image) and Erg Marzuq in south-western Libya (right image).

Satellite detail view of the sand dunes in the Erg Chech. Satellite detail view of the sand dunes in the Erg Marzuq.

In the following images, volcanic formations with basalt deposits in the Tassili-n-Ajjer plateau, in south-eastern Algeria. Tassili-n-Ajjer is probably the most interesting place in the entire Sahara, because of its wide diversity of features that makes this an unique place. Plateau canyons, volcanic areas, bizarre rock formations, red sand dunes, oueds and oases with permanent water, almost extincted flora species, archeological sites and prehistoric art collections are hidden in this place lost in the center of the Sahara.

Satellite detail view of volcanic formations in Tassili-n-Ajjer. Satellite detail view of volcanic formations in Tassili-n-Ajjer.

Two satellite detail views of the mega-yardangs located to the south-east of the Tibesti massif. In the left image it is visible an impact crater.

The mega-yardangs are a feature unique of the Sahara desert and they are said to be able to accelerate the wind that passes through them (in fact, the same wind that produced them). This wind should then be able to draw the dust accumulated in the bottom of the Bodélé depression (located south of Tibesti massif) and transport it as far as the South and Central America rainforests, where it deposits. The sand and dust present in the deserts contain mineral nutrients that serve as fertilizers for the forests.

An impact crater and the mega-yardangs next to Tibesti massif. Satellite detail view of the mega-yardangs next to Tibesti massif.



Saharan sandstorms or simoons are a cause of massive, though slow, erosion in the deserts and a way of transportation of mineral nutrients towards the far forests located in another continents. The areas of the Sahara that are most susceptible to produce sand or dust storms are the depression of Bodelé and the ergs of Chech and Iguidi. Saharan dust storms frequency has multiplied several times since the 1950s, causing topsoil loss in the Sahel region.

Sand and dust storms are a major problem in arid countries, since they seriously degrade the cultivable soils and even can damage the youngest crop plants, because of the abrasive effect, while prolonged and unprotected exposure is truly harmful for the respiratory system and the eyes. To get caught in the middle of a major sandstorm could be very dangerous; in the past times, many caravans and even some armies were supposedly buried by sandstorms.

Satellite view of a sandstorm in the Sahara.

The following satellite images show the huge size of these sandstorms that are so frequent in the Sahara. When the prevailing north-easterly saharan wind reaches the Mediterranean, it is known as sirocco and often reaches hurricane speeds.

Satellite view of a sandstorm in the Sahara. Satellite view of a sandstorm in the Sahara.

Satellite view of a sandstorm in the Sahara. Satellite view of a sandstorm in the Sahara.

Satellite view of a sandstorm in the Sahara. Satellite view of a sandstorm in the Sahara.

Satellite view of a sandstorm in the Sahara. Satellite view of a sandstorm in the Sahara.

The following images show the effect of wind on sand dunes and rocks. From left to right and from top to bottom:

Star dunes: these dunes have complex forms created by variable winds, that usually create several slopes that converge into a central high point, that can reach close to 300 meters in areas with abundant sand deposits. These dunes are present mainly in Tenere-Borkou, Marzuq, Awbari, Tin Merzouga, Admer, Grand Erg Oriental, Grand Erg Occidental and Iguidi.

Longitudinal dunes (seif): these dunes are formed in areas with a limited amount of sand, by a wind that always blows in a same general direction but having slight variations on its direction, not bigger that a compass quarter. In the Sahara these dunes can reach 100 meters high and more than 100 kilometers in lenght. These dunes are present mainly in the Tenere, the Libyan Desert, Chech, El Djouf and Azauad.

Crescent dunes (barchan): these are lonely dunes of a crescent shape with their ends pointing in the direction of the wind. These dunes can form in areas where sand supplies are limited and the surface is relatively flat, hard and devoid of vegetation. They migrate slowly in the wind at a speed of up to 15 meters per year. Its size is usually modest, reaching the largest, heights of 30 meters, while the maximum expansion between the points is close to 300 meters. When the wind is almost constant, the crescent of these dunes is almost symmetrical. However, when the wind is not perfectly fixed, one end is larger than the other. Good examples of these dunes can be found in western Mauritania or in the Djado and Erdi regions.

Transversal dunes: these dunes are formed in regions where the prevailing winds are uniform, there is plenty of sand and vegetation is sparse or non existent. These dunes are a series of long ridges separated by depressions and oriented in right angles to the prevailing wind. In the Sahara these dunes can reach heights of 200 meters and may extend over distances of more than 100 kilometers.

Barchanoid dunes: this relatively common form of dune is an intermediate type between the isolated barchans and the extensive transversal dunes. These dunes look somewhat like rows of barchans placed alongside each other.

Yardangs: The yardangs are formed over thousands of years by an abrasive wind that always blows in the same direction, creating this particular form of erosion. The wind impacts against the rock and then it is divided, flanking the rock and therefore eroding the flanks, giving the rock an aerodinamic shape that ressembles the hull of a boat when the wind is well fixed. The most impressive collection of yardangs lies in the Djado-Tibesti and Erdi regions.

Star dunes. Longitudinal seif dunes.

Crescent barchan dunes. Transversal dunes.

Transversal barchanoid dunes. Yardang formations.

Sahara Ethnography



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