DEFINITION OF SAHARA DESERT

The world’s largest desert, covering

5,400,000 square miles (8,600,000 km2) in northern Africa

including Mauritania, Morocco, Algeria, Tunisia, Libya,

Egypt, Sudan, Chad, Niger, and Mali. The desert is bordered

on the north and northwest by the Mediterranean Sea and

Atlas Mountains, on the west by the Atlantic Ocean, and on

the east by the Nile River. However, the Sahara is part of a

larger arid zone that continues eastward into the Eastern

Desert of Egypt and Nubian Desert of Sudan, the Rub‘ al-

Khali of Arabia, and the Lut, Tar, Dasht-e-Kavir, Takla

Makan, and Gobi Deserts of Asia. Some classifications

include the Eastern and Nubian Deserts as part of the Sahara

and call the region of the Sahara west of the Nile the Libyan

Desert, whereas other classifications consider them separate

entities. The southern border of the Sahara is less well

defined but is generally taken as about 16° latitude where the

desert grades into transitional climates of the Sahel steppe.

About 70 percent of the Sahara is covered by rocky and

stone or gravel-covered denuded plateaus known as hammada,

and about 15 percent is covered with sand dunes. The

remaining 15 percent is occupied by high mountains, rare

oases, and transitional regions. Major mountain ranges in the

eastern Sahara include the uplifted margins of the Red Sea

that form steep escarpments dropping more than 6,000 feet

(2,000 m) from the Arabian Desert into the Red Sea coastal

plain. Rocks in these mountains include predominantly Precambrian

granitic gneisses, metasediments, and mafic schists

of the Arabian Shield and are rich in mineral deposits, including

especially gold that has been exploited by the Egyptians

since Pharaonic times. The highest point in the Sahara is Emi

Koussi in Chad, which rises to 10,860 feet (3,415 m), and the

lowest point is the Qattara Depression.

High isolated mountain massifs rise from the plains in

the central Sahara, including the massive Ahaggar (Hoggar)

in southern Algeria, Tibesti in northern Chad, and Azbine

(Air Mountains) in northern Niger. Ahaggar rises to more

than 9,000 feet (2,740 m) and includes a variety of Precambrian

crystalline rocks of the Ouzzalian Archean craton and

surrounding Proterozoic shield. The Air Mountains, rising to

more than 6,000 feet (1,830 m), are geologically a southern

extension of the Ahaggar to the north, containing metamorphosed

Precambrian basement rocks. Tibesti rises to more

than 11,000 feet (3,350 m) and also includes a core of Precambrian

basement rocks, surrounded by Paleozoic and

younger cover. Northeast of Tibesti near the Egypt-Libya-

Sudan border, the lower Oweineat (Uwaynat) Mountains

form a similar dome, rising to 6,150 feet (1,934 m), and have

a core of Precambrian igneous rocks.

The climate in Sahara is among the harshest on the planet,

falling in the trade wind belt of dry descending air from

Hadley circulation, with strong constant winds blowing from

the northeast. These winds have formed elongate linear dunes

in specific corridors across parts of the Sahara, with individual

sand dunes continuous for hundreds of miles, and virtually

no interdune sands. These linear dunes reach heights of more

than 1,100 feet (350 m) and may migrate tens of feet or more

per year. When viewed on a continental scale (as from space,

or on a satellite image) these linear dunes display a curved

trace, formed by the Coriolis force deflecting the winds and

sand to the right of the movement direction (northeast to

southwest). Most parts of the Sahara receive an average of

less than five inches (12 cm) of rain a year, and this typically

comes in a single downpour every few years, with torrential

rains causing flash flooding. Rains of this type run off quickly,

and relatively little is captured and returned to the groundwater

system for future use. The air is extremely dry, with

typical relative humidities ranging from 4 percent to 30 percent.

Temperatures can be extremely hot, and the diurnal

variation is high. The world’s highest recorded temperature is

from the Libyan Desert, 136°F (58°C) in the shade, during

the fall of 1922. The temperature drop at night can be up to

90°F (30°C), even dropping below freezing after a scorching

hot day.

Most of the Sahara is sparsely vegetated, with shrub

brushes being common, along with grasses, and trees in the

mountains. However, some desert oases and sections along

the Nile River are extremely lush, and the Nile Valley has

extensive agricultural development. Animal life is diverse,

including gazelles, antelopes, jackals, badgers, hyenas, hares,

gerbils, sheep, foxes, wild asses, weasels, baboons, mongooses,

and hundreds of species of birds.

A variety of minerals are exploited from the Sahara,

including major deposits of iron ore from Algeria and Mauritania,

Egypt, Tunisia, Morocco, and Niger. Uranium deposits

are found throughout the Sahara, with large quantities in

Morocco. Manganese is mined in Algeria, and copper is

found in Mauritania. Oil is exported from Algeria, Libya,

and Egypt.

Much of the Sahara is underlain by vast groundwater

reservoirs, both in shallow alluvial aquifers and in fractured

bedrock aquifers. The water in these aquifers fell as rain

thousands of years ago and reflects a time when the climate

over North Africa was much different. In the Pleistocene,

much of the Sahara experienced a wet and warm climate, and

more than 20 large lakes covered parts of the region. The

region experienced several alternations between wet and dry

climates in the past couple of hundred thousand years, and

active research projects are aimed at correlating these climate

shifts with global events such as glacial and interglacial periods,

sea surface and current changes (such as the El

Niño–Southern Oscillation). The implications for understanding

these changes are enormous, with millions of people

affected by expansion of the Sahara, and undiscovered

groundwater resources that could be used to sustain agriculture

and save populations from being decimated. Many of the

present drainage and wadi networks in the Sahara follow a

drainage network established during the Pleistocene. In the

Pliocene the shoreline of the Mediterranean was about 60

miles (40 km) south of its present location, when sea levels

were about 300 feet (100 m) higher than today. Sand sheets

and dunes, which are currently moving southward, have only

been active for the past few thousand years. These are known

to form local barriers to wadi channels in the Sahara, Sinai,

and Negev Deserts and locally block wadis.

The sand of the Sahara and adjacent Northern Sinai

probably originated by fluvial erosion of rocks in the uplands

to the south and was transported from south to north by

paleo-rivers during wetter climate times, then redistributed by

wind. Dry climates such as the present, and low sea levels

during glacial maxima, exposed the sediment to wind action

that reshaped the fluvial deposits into dunes, whose form

depended on the amount of available sand and prevailing

wind directions. This hypothesis was developed to suggest the

presence of a drainage network to transport fluvial sediments.

Indeed, numerous channels incise into the limestone

plateau of the central and northern Sahara, and many lead to

elongate areas that have silt deposits. Several of these

deposits have freshwater fauna and are interpreted as paleolakes

and long-standing slack water deposits from floods.

Plio-Pleistocene lakebed sediments have also been identified

in many places in the mountains in the Sahara, where

erosionally resistant dikes that formed dams in steep-walled

bedrock canyons controlled the lakes. The paleo-lake sediments

consist of silts and clays interbedded with sands and

gravels, cut by channel deposits. These types of lakebeds

were formed in a more humid late Plio-Pleistocene climate,

based on fossil roots and their continuity with wadi terraces

of that age.

The fluvial history of the region reflects earlier periods of

greater effective moisture, as evident also from archaeological

sites associated with remnants of travertines and playa or lake

deposits. An early Holocene pluvial cycle is well documented

by archaeological investigations at Neolithic playa sites in

Egypt. Late Pleistocene lake deposits with associated early and

middle Paleolithic archaeological sites are best known from

work in the Bir Tarfawi area of southwest Egypt. Similar associations

occur in northwest Sudan and Libya.

An extensive network of sand-buried river and stream

channels in the eastern Sahara appears on shuttle imaging

radar images. Calcium carbonate associated with some of

these buried river channels is thought to have precipitated in

the upper zone of saturation during pluvial episodes, when

water tables were high. As documented by radiocarbon dating

and archaeological investigations, the eastern Sahara

experienced a period of greater effective moisture during

early and middle Holocene time, about 10,000–5,000 years

ago. Uranium-series dating of lacustrine carbonates from several

localities indicated that five paleo-lake forming episodes

occurred at about 320–250, 240–190, 155–120, 90–65 and

10–5 thousand years ago. These five pluvial episodes may be

correlated with major interglacial stages.

These results support the contention that past pluvial

episodes in North Africa correspond to the interglacial periods.

Isotopic dating results and field relationships suggest

that the oldest lake and groundwater-deposited carbonates

were more extensive than those of the younger period, and

the carbonates of the late wet periods were geographically

localized within depressions and buried channels.

This archaeological evidence of previous human habitation,

coupled with remains of fauna and flora, suggests the

presence of surface water in the past. Indeed, remains of lakes

and segments of dry river and stream channels occur

throughout the Sahara. Archaeological evidence of human

habitation during the early Holocene was recently uncovered

in the northeast Sinai Peninsula where an early Middle Paleolithic

site shows evidence for habitation at 33,800 years BP.

See also DESERT; FRACTURE ZONE AQUIFERS; NILE RIVER.

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