PHYSICAL SETTING OF AFRICA

The physical setting of Africa is defined by its relief, drainage, climate and vegetation, among others. These factors define Africa’s physical environment and they are important influences on patterns of human activity on the continent. The physical environment affects such things as patterns and types of agricultural production, patterns of population distribution and of hydro electric power production, among others.

Africa’s physical environment is in turn affected in important ways by its location (within the tropics), and by its bisection by the equator. The location and bisection affect the continent’s climatic and vegetation patterns.

 

Relief and Drainage

Flat surfaces (plateaus) and extensive inland basins dominate the relief of Africa. The three most prominent basins are the Lake Chad basin, the Congo basin and the Bahr el Ghazal basin. Large mountain ranges are limited to the north east and south of the continent. In the north are the Atlas Mountains and the Hoggar, Tibesti and Darfur ranges, while in the east and south we have the Mts. Batu, Elgon, Kenya, Kilimanjaro, Ruwenzori, Karisimbi, Mitumba and the Cape Ranges, among others.

 

The Atlas Mountains and the Cape Ranges are fold mountains. Some of the mountains in East Africa are of volcanic origin, including Mts. Kilimanjaro, Kenya and Elgon. Witha few exceptions, altitudes in the east and south are much higher than elsewhere in the continent. Three elevated regions are particularly noteworthy. These are the Ethiopian Highlands, the East African Plateau and the High Veldt in South Africa. The Ethiopian Highlands and much of the East African Plateau are volcanic in origin, while the High Veldt and the Drakensberg Mountains are a result of uplifting. Overall, the average altitude of the continent is 675 metres and Mt. Kilimanjaro (5,895 metres), which is in Tanzania, is the highest point in Africa.

The plateau that dominates the relief of Africa is very extensive and it drops rather suddenly to the coast in many parts of the continent. As a result there is very little coastal plain in some parts of the continent. Of all the continents in the world, Africa has the smallest proportion of its total surface below 150m. The continent does not have extensive alluvial plains comparable to those of the Ganges in India and the Mississippi in the United States of America. The surface of the African plateau is uniform over extensive areas, though it is dotted in many places by steep-sided hills (or inselbergs).

However, some parts of Africa are not part of the plateau. These include the two fold mountain belts, namely, the Atlas Mountains in north Africa and the Cape Ranges in South Africa. Others are the volcanic mountains such as Mts. Kilimanjaro (5895 m), Kenya (5199 m) and Elgon (4321 m).

In addition to the plateau, the mountain ranges and the inland basins that characterize the landscapes of Africa, another important feature is the East African Rift Valley which has two branches, the eastern branch and the western branch. This remarkable landform runs roughly north – south and is more than 4000 km long. It is part of the Great Rift Valley System, a section of which is in the Middle East. It extends from the Middle Eastern country, Syria, in the north to Zimbabwe and Malawi (both of which are in Africa) in the south. The bulk of the Great Rift Valley is in Africa and the Red Sea is part of it. The East African Rift Valley contains a number of lakes, the most important of which are Lake Abaya, Lake Rudolf, Lake Tanganyika and Lake Malawi. These rift valley lakes are an important feature of the drainage of the continent, which also includes many rivers, both large and small.

An important influence on the drainage pattern of the continent is climate. As a general rule, there are more rivers and streams in the humid environments than in the arid or semi-arid environments. Rivers like the Nile which flows through arid regions have their sources in humid regions. Two other generalizations characterize the drainage pattern of Africa. First, only about 48% of Africa is drained directly to the oceans, and secondly, 12% of the continent is drained into interior basins which have no outlet to the sea. The remainder of the continent (40%) is without organized surface drainage. Much of the areas that drain directly to the sea consist of broad shallow basins (300 – 1000m above sea level) with single outlets. These outlets are the great rivers of Africa, namely, the Niger, upper Nile, Congo, Orange and Zambezi. These rivers dominate the drainage of Africa. However, there are numerous other rivers and streams of lesser significance. The areas without organized drainage are the hot deserts where present precipitation is not enough to support an integrated system of drainage. Finally, the areas that drain into interior basins are centred on Lake Chad, the Makarikari salt pan of Botswana and the eastern Rift Valley.

The River Nile is about 6,700 km long, and it flows from the well-watered basin of Lake Victoria in the south into the Mediterranean Sea in the north, through the eastern Sahara. The river derives no water from its immediate environmentfor the last 2,400 km of its flow. The only exceptions are the tributaries which have their headwaters in distant humid lands. River Atbara is one of such tributary. River Niger flows over a distance 4200 km from its source in the Fouta Djalon Hills to its mouth in the Atlantic Ocean, or more correctly, in the Gulf of Guinea. The Niger has an inland delta above Timbuktu in the margins of the Sahara Desert where it provides valuable irrigation water for agriculture. Its main delta is in its estuary. The River Niger has a number of tributaries, the largest of which is the River Benue. The Benue has its source in Cameroon but flows through Nigeria for much of its length before joining the Niger in Lokoja. Other important tributaries of the Niger are River Sokoto and River Kaduna, both of which are in Nigeria.

River Congo is one of the three great African rivers. It is 4800 km long and is second to the Nile in length. However, it has the greatest volume of flow of any river in the continent. The Nile, Congo and Niger are among the ten longest rivers in the world. The Nile is the longest (or the second longest, if the upper reaches of the Amazon are included), the Congo is the sixth longest, while the Niger is the tenth longest river in the world. Other important African rivers are the Gambia, Senegal and Volta in West Africa and the Orange, Limpopo and Zambezi. Along with the Niger and the Congo, the Orange flows into the Atlantic while the Limpopo and Zambezi flow into the Indian Ocean. The major rivers of Africa, therefore, drain into the Mediterranean Sea (the Nile), the Atlantic Ocean and the Indian Ocean.

 

Climate

Several factors affect the climate of Africa. These include its latitude, altitude and continentality. Also important are ocean currents and air masses. With regard to latitude, and as indicated earlier, Africa bestrides the equator and about three quarters of the continent lies between the Tropic of Cancer and the Tropic of Capricorn. Therefore, Africa is a hot continent where average annual temperatures are above 20° C between the two Tropics. The only exceptions are the extreme north and south of the continent which are in the temperate zone and have a Mediterranean climate. Concerning altitude; temperatures generally decrease with increasing altitude. The average rate of decrease is 6.4° C per 1,000 m of vertical ascent. This is known as the environmental lapse rate. For this reason, highland areas tend to be cooler than surrounding lowland areas. The Jos Plateau in Nigeria and the East African highlands are good examples of this phenomenon.

Continentality has to do with distance from the sea. This is an important factor because of the comparatively large size of Africa. The effect of continentality is very evident in temperature range, which is the difference between the highest and lowest temperatures in any given place. As a general rule, temperature range increases with increasing distance from the sea. For example, Lagos, Nigeria, which is a coastal city has a temperature range of 3° C, while Kano, some 611km (straight line distance) north of Lagos has a temperature range of 12°C. Coastal locations are subject to maritime influence which with among other things, reduces the differences between high and low temperatures in locations at or near the coast.

Ocean currents also affect climate, particularly in coastal areas. Cold currents and warm currents affect the climate ofadjoining areas differently. Coastal areas adjoining warm currents tend to have higher temperatures and heavier rainfall than coastal areas washed by cold currents. For example, the Guinea Current on the West African coast and the Agulhas Current on the southeastern coast of Africa are warm currents and the adjoining areas receive comparatively high rainfall.

On the other hand, the Benguela Current and the Canary Current are cold currents and the adjoining areas receive little rainfall. South Africa exemplifies the effect of ocean currents on temperature. Durban and Port Nolloth are on the east and west coasts of South Africa respectively and both are almost on the same latitude (30°S and 29.5°S respectively). Yet, the mean annual temperatures for Durban are in the range of 17°C to 19°C, while those for Port Nolloth range from 12°C to 16°C. The difference between Durban and Port Nolloth is due to the effect of the cold Benguela Current in the case of Port Nolloth and the warm Agulhas Current in the case of Durban.

Annual rainfall in Africa ranges from less than 50mm to over 3000mm. As a general rule, rainfall totals in the continent decrease from the coast inland, especially north of the equator.

 

This is largely because the rain _ bearing winds come from the sea, and so the farther away a location is from the sea, the less rainfall it receives. This is very evident from the distribution of rainfall in West Africa in which Lagos on the Nigerian coast has an annual rainfall of about 2000mm, while Kano, some 611km north of Lagos receives less than 900mm of rainfall per year. The rain-bearing winds in West Africa come from the Gulf of Guinea and are called the Southwest Monsoon. They bring rain to much of West and Central Africa from April/May to about October/November, depending on location. During this period, those parts of the continent south of the equator receive little or no rain.

In some areas, rainfall totals during the rainy season has two peaks, one around June/July and the other in September/October, with the socalled August break in-between. The August break is characterized by comparatively lower rainfall. For example, in Ibadan the mean monthly rainfall figures for May, June and July are 150.5mm, 181.0mm and 150.36mm respectively. The total for August is 108.6mm, while for September and October the totals are 180.7mm and 160.2mm respectively. South of the equator, much of the rain-bearing winds originate from the Indian Ocean and are known as the Southeast Trade Winds. They bring rain to much of the region from November to April, during which period most parts of Africa north of the equator receive little or no rain. Rainfall in Africa is very unevenly distributed and in virtually all parts of the continent there are distinct wet and dry seasons. Because of the seasonality in the distribution of rainfall, average annual rainfall figures can be misleading. Seasonal distribution is very important.

In all, there are six major climate types in Africa.

 

These are as follows:

1. Equatorial Climate.
2. Humid Tropical Climate.
3. Dry Tropical Climate.
4. Steppe (Sahelian) Climate.
5. Desert Climate.
6. Mediterranean Climate.

The epicentre of equatorial climate in Africa is the Congo Basin, on both sides of the equator. It is characterized by high temperatures (about 25°C), with a very small temperature range. In other words, there is very little variation in temperature throughout the year in the equatorial region. It is also the one region where the rain falls throughout the year, with rainfall totals exceeding 1,500mm per year. On the map, the equatorial region appears to be the one around which the other climatic regions are arranged. The humid tropical climate is roughly similar to equatorial climate from the standpoint of amount of rainfall, although it is often less than 1,500mm per year, except in high altitude areas. However, rainfall is seasonal in this region, with alternating wet and dry seasons.

The dry tropical climate is noted for long dry seasons. In regions with this climate, the duration of the dry season increases with distance from the equator. The length of the dry season ranges from three to six months on average, depending on the distance from the equator. An important feature of this climate is the Harmattan. The Harmattan is a hot, dry dusty wind that blows from the northeast across the Sahara Desert towards the Atlantic Ocean in West Africa. The Harmattan occurs during the dry season and given the origin of the wind, it is not rain-bearing. Rainfall totals vary in this climatic region. For example, in southern Mali, it is in excess of 1000mm per annum, while in northern Nigeria, it could be less than 800mm per year. The Sahelian climate is the one that borders the desert, both the Sahara Desert and the Kalahari Desert. In this region, desert conditions become predominant as the duration of the rainy season is only about three months. Rainfall totals diminish as one approaches the desert. Annual rainfall ranges from about 50mm at the desert margins to about 500mm farther away. In this region, temperatures are typically higher than in the humid regions, reaching some 40°C. Temperatures are in excess of this figure at the end of the dry season.

Desert climate is found in Africa’s two deserts, the Sahara Desert and the Kalahari Desert. The Sahara is by far the larger of the two. In fact, it is the largest desert in the world, covering more than 8 million km². In the desert regions, rainfall is mostly below 100mm per annum and drought conditions are common.

Temperature range (both daily and annually) is quite high. Temperature range is as high as 50°C between day and night and 25°C and 30°C per annum. Finally, the Mediterranean climate is found in the extreme north of the continent, along the Mediterranean coast and extreme south of the continent, in the Cape area. This climate is noted for hot dry summers and mild wet winters. In North Africa, average annual temperatures vary from 14°C in the mountains to 18°C in lowland areas. On the other hand, in Cape Town, South Africa, January temperature is about 23°C and July temperature is about 14°C.

 

Vegetation

Climate, especially rainfall, is an important influence on the vegetation of Africa. Therefore, there is some association between the pattern of climate and the pattern of vegetation. Other important influences on vegetation include relief, soil type and human activity. The natural vegetation of Africa has been considerably modified as a result of human activities, chief among which are crop cultivation and livestock grazing. Many forest areas have thus been lost to grasslands. For this reason, the vegetation in some parts of Africa, is strictly speaking not the natural vegetation.

Human impact hasbeen particularly substantial in Africa because agriculture remains the mainstay of the economy of many African countries, and therefore the majority of the population depends on the land for sustenance.

There are many different types of vegetation in Africa, but the dominant ones are as follows:

1. Mangrove Forest.
2. Rain Forest.
3. Tree Savannah and Degraded Forests.
4. Wooded Savannah.
5. Grass Savannah.
6. Desert Steppe.
7. Desert.
8. Mediterranean Vegetation.

 

Mangrove forests are found in some delta and coastal lagoon areas, and are tolerant of salt water.

The trees grow in muddy sediments and most of the species have stilt or aerial roots. The mangrove forests are exceptionally rich in species. The equatorial rain forest is the dominant vegetation in the equatorial zone. This zone has conditions that are ideal for plant growth: abundant and well distributed rainfall, abundant sunshine, good soils. For these reasons, the rain forests are evergreen and are noted for many tall trees, some as tall as 46m and a few are much taller. As a result of the heights and sizes of the trees, a number of them have buttress roots to support them.

These forests are also noted for several layers of vegetation, with the branches of the tall trees interlocking and forming a continuous canopy that shields the forest floor from the direct rays of the sun. Typically, there is very little undergrowth in well developed, undisturbed rain forests. But where the vegetation has been disturbed by human activity and abandoned, dense, almost impenetrable undergrowth often develops. The rain forest has a wide variety of tree species and is home to very valuable tropical hardwoods. These include the mahogany, ebony, obeche and iroko, among others. Extensive tracts of the rain forest have been depleted as a result of logging, land clearance by subsistence farmers and the establishment of plantations.

The savannahs are the most extensive category of vegetation in Africa and consist of a mixture of trees and grassland in different proportions. It is estimated that the savannahs cover about 40% of the total land area of the continent. The zone of tree savannah and degraded forests consists of open forests. Trees in this zone are about 25m tall and some of them are deciduous. Deciduous trees are those that shed their leaves in the dry season. These forests are described as open because individual trees stand separately and their crowns do not form a continuous canopy as in the equatorial rain forest. Therefore, lush grasses grow between the trees. As a general rule, there is an increase in the proportion of grassland as rainfall amount and the duration of the rainy season diminish. This zone has been substantially degraded as a result of human activity, and much of it has been transformed to grasslands.

The wooded savannah zone is dominated by grassland, just like most savannahs, but its defining characteristic is that it contains trees in clusters, especially gallery forests along river valleys. As rainfall diminishes, shrub and grass dominate the vegetation and trees become fewer and smaller. This is the grass savannah zone. The different savannahs have different combinations of grass and trees. As a general rule, tree density and size as well as the proportion of trees decrease as the length of the dry season increases and as rainfall totals diminish. The desert steppe vegetation is mostly found near the deserts, in areas where rainfall is less than 400 mm per annum and where the duration of the dry season is eight months or more. This vegetation is dominated by short grasses that grow in patches, thorn bushes, spiny plants, acacia trees and don palms. In this zone, crops can only be grown along rivers. Conditions are much harsher in the desert zones.

In these areas, total rainfall is often less than 250 mm per annum, barely enough to support vegetation. The plants found here are well adapted to drought conditions. The plants either have fleshy stems in which water is stored or they have thorns instead of leaves or thereby reduce transpiration considerably. Cactus is one example of desert plants.