Africa straddles the equator, having an almost equal south and north extent. This division of Africa into almost two equal parts (lengthwise) across the equator makes the climatic and physical conditions in the north repeat themselves in the south. For example, the Kalahari Desert is the south's answer to the Sahara; the Karoo matches the Maghreb, while the conditions in the Cape area are almost identical to those of the Mediterranean region.
Africa is the most tropical of all continents. Climate and vegetation range from equatorial rainforests, tropical deserts and savanna grassland to Mediterranean. The Sahara Desert, the largest of its kind anywhere in the world, is over 10.4 million km2 n North to south is approx. 1800 kms and east-west is 5600km.
GEOLOGY, RELIEF AND DRAINAGE Fig 2.1a page 26
In terms of structure and relief features, Africa appears to be different from the other continents. Its surface is almost invariably consisting of a geologically stable land mass made up of the pre-Cambrian basement rock overlain in part by sedimentary cover of a later period. The continent is made up of very old crystalline, metamorphic, and sedimentary rocks of great hardness (collectively known as 'basement complex'). Africa's land mass is made up of a simple tectonic plate (some geographers attempt to separate the Rift Valley system from the rest of the continent). The continent has some extensive areas of faulted plateau. It has however suffered little in terms of folding hence young folded mountain similar to the Rockies and Andes are missing. Only the Atlas Mountains in the North and the Drakensburg mountains in the South are exceptions.
Most of the highlands and mountains are the result of recent volcanic activities (e.g east African mountains like the Kilmanjaro -19,340 ft (5895 m); Kenya 17,058 ft (5200 m); Meru -4569 m and Elgon 4053 m and in the Ethiopian highlands Ras Dashen is about 15,000ft (4,573 m).
In West/Central Africa, mountains such as the Cameroon (4070m), Jos Plateau in Nigeria 5,840 ft (1,780 m), Fouta Djalon highlands in Guinea, the Ahaggar and Tibesti Massifs fall within the same category of highlands. Lower average altitudes characterize most of the areas in the North. The Southern and Eastern areas have most of the highest areas of the continent (900-1200m). The African plateau is underlain primarily by Precambrian rock that dates back to more than 600 years.
The Rift Valley System (Fig 2.3 page 28)
A unique feature of Africa's physiography is its Y-shaped integrated Rift Valley system that is believed to have been caused by the movement of the continental plates. The Rift Valley begins from the Red Sea and extends through the Ethiopian highlands to the Lake Victorian region where it subdivides into an east and west segments and continues southward through Lake Malawi to Mozambique. Its total length is estimated to be 6,000 miles (9,600 m). Average width ranges between 20 miles (32 km) and 50 miles (80 km).
Rift Valley Lakes: The valley system has been occupied by elongated lakes lying within very deep trenches (Lakes Tangayika, 2133 feet (650 m) below sea level, Malawi, Turkana, Albert, Nyansa). Volcanic highlands and block moountains (e.g. Ruwenzori massif 16, 404 feet, (5000 m) rises within the valley floor in places.
Africa’s coastline is generally straight and relatively short compared with that of other continents thereby resulting in more artificial harbors than natural ones. The shape of Africa is relatively simple with a remarkably smooth outline. The rivers plunge off the edges of the plateau into the sea in a series of falls and rapids. The coast is fringed with coral reefs, sand bars, mangrove swamps and lagoons that block passage to the continents interior. The coastline of Africa is remarkably straight, free from the indentations that make for good natural harbors. The narrow continental shelf is related to the steep face that the continent generally presents to the sea and the faulting that has produced its general shape.
The only place with an extended continental shelf is the Agulhas Bank (extends 320 kms or 200 miles off shore). The absence of any major continental shelf as in Europe and the northeastern part of North America limits the development of fishing grounds and the opportunities of exploring for major petroleum sources of the coast. No major coastal island (except Madagascar - the 4th largest island in the world). The coastline is generally smooth.
DRAINAGE Fig 2.1b page 27
Some of the world's largest and longest rivers found in Africa, e.g the Nile, Zambezi, Congo and the Niger. The rivers are not effective as transportation routes due to the existence of a large amount of rapids and cataracts. Almost all the rapids are found at points where the rivers descend the interior plateau some few kilometers from the coast. For example, the Zaire River is interrupted by 32 rapids and cataracts (which constitute the Livingstone Falls) in a descent of approximately 200 meters over a distance of 214 kilometers. In other continents the rivers offer the best entry into the interior from the coast (e.g the St. Lawrence and Mississippi and also served as a cheaper means of transportation for industrial goods but this is not the case in Africa.
Despite being impediments to transportation, most of the rivers provide a great potential for the production of hydro-electricity. Indeed, Africa has the greatest potential than any of the other continent. For instance, out of the world's total potential of 555,000 megawatts, Africa has 26% (145,000 mw). Less than 10% of this potential is being tapped now. Highest potentials of HEP: Zaire-78,000; Madagascar-11,500; Zambia- 17,000.
Although Africa consists of one landmass, it has a number of islands, which are structurally not different from the main land. Major Islands are Madagascar, Zanzibar and Pemba; the Comoros; Mauritius; Reunion, Seychelles (all in the Indian Ocean); Cape Verde, Fernando Po, Principe, Sao Tome and Annobon (all in the Atlantic). Africa linked to Asia by an isthmus that was cut into a canal in 1869 (Africa's location between the Atlantic and Indian Oceans and its great latitudinal extent constituted a barrier to trade between Europe and Asia).
Africa is the most tropical of all continents. Africa is the only continent that straddles the equator and therefore incorporates both the Tropic of Cancer and Capricorn. The climate south of the equator mirrors that of the north of the equator but the shape of the northern half of Africa reduces any maritime influence. This shape difference results in a large desert, Sahara in the north and a much smaller one, Kalahari, in the south. The shape difference again makes the southern portion of Africa cooler than the northern section of the continent (thereby making East Africa more favorable for European settlement than West Africa: Zambia and Niger are both located on Latitude 15 degrees from the equator but Niger is warmer). Archaeological studies and Satellite images show that the climate of Africa has not always remained the same. Conditions that are more humid had once existed in the Sahara desert and other drier parts of the continent.
Rainfall is the most significant climatic factor in Africa. Temperature is high through the continent because of the continent’s location relative to Equator. Range of temperature is quite small and Wind is much less of a feature than in temperate climates
Located on about Latitude 30o North and South of the Equator are Sub-tropical High Pressure Belts that dictate surface wind patterns and also influence rainfall and temperature regimes on the continent. The Subtropical High Pressure Systems on both sides of the Equator generate two wind systems that converge on the equator in a zone termed Inter-Tropical Converge Zone (ITCZ). From the north, Subtropical High Pressure Belt zone blows the Northeast Trade Winds (locally called Harmattan). The Harmattan is dry and cool and blows over Sub-Saharan Africa from about November to April. From the south Sub-tropical High Pressure belt zone blows the Southwest Trade Winds (locally called Monsoon). The Monsoon winds are moist and bring rainfall to the coasts of West Africa. The African continent does not extend much beyond 35o of latitude from the equator. The implication is that the range of climatic conditions is limited and that the general direction of wind movement is towards the equator (or in more accurate terms towards the inter-tropical convergence zone -ITCZ). The ITCZ shifts with the seasonal movement of the sun across the tropics:
· In June: the northern Summer season, the ITCZ is located at about 13 degrees of latitude in North Africa at the southern boundary of the Sahara;
· In December: the Northern winter season, the ITCZ moves southward along the West African Coast and to the northern and eastern margins of the Zaire basin and continues to Madagascar
Movements in the ITCZ are closely related to distribution of rainfall and climatic zones. The climatic zones assume symmetry around the equator, although the high altitudes in some parts of the continent and the adjacent disturb the symmetry. Rainfall averages from 5000mm (200 inches) in the coastal strips of Sierra Leone, Liberia, Nigeria, Cameroun and eastern Madagascar to 500m (20 inches) in the Sahel and 200mm (8 inches) or less in the arid regions (Sahara and Kalahari). Temperatures range from 12o C (54oF) to over 32oC (90oF). The highest recorded 57.7oC (136.4o F) in Azizia, Libya. Mean annual temperature is 3oC to 5.5oC (5.5-10oF) range for a greater part of the continent with <3oC (5.5oF) in the forest belt around the equator. Diurnal mean range of temperature: 10 to over 15oC (18-27o F+)
Major Ocean Currents (Refer to any Atlas map of Africa):
Winds that tend to blow persistently over the ocean tend to drag a thin surface layer of the water in their direction of flow. This layer of ocean water called Ocean Current dictates the temperature and moisture characteristics of the wind and the coastal regions over which the wind blows. When ocean currents blow from low latitudes (near to the equator) towards higher latitudes (towards the pole) the currents carry WARM ocean water into relatively cool regions. Such an ocean current is called Warm Ocean Current. Examples are the Gulf Stream in the Atlantic coast of the Eastern USA and Mexico.
Warm Ocean Currents supply moisture to winds blowing over them to develop rainfall on the adjacent Coasts. In Africa Warm Ocean Currents include, Warm Guinea Currents -–West Africa, Warm Mozambique current – Southeast Africa. The cool surface of Cool Ocean Currents causes moisture in winds blowing over them to condense and form fogs, etc. The winds are deprived of their moisture so they tend to absorb rather than deposit moisture at the adjacent coasts. Cool currents therefore cause dry conditions and in Africa the Cool Canary Currents - the western coast of the Sahara Desert, the Cool Benguela Currents – the Western coast of Kalahari Desert. The cool ocean currents tend to create rich fishing grounds. Rich fishing grounds exist along the Morocco and Spanish Saharan Coasts washed by the Cool Canary Currents. The Namibian coast that is washed by the Cool Benguelan current also has rich fishing grounds.
Maritime versus Continental Climates
Large water bodies such as the Ocean and huge lakes modify climates in adjacent lands. In the continental interior, where there are no large bodies of water, temperatures get VERY WARM in summer or during the day. Land is solid so it heats up more rapidly during the day. The compact nature of the land means that only a thin surface gets heated. As a result, heat absorbed into the thin layer of surface rocks is released very rapidly. The land surface is therefore VERY COOL at nights and in winter. The interior location does not also allow rainfall to reach such places thereby creating Warm, humid, hot and dry climates. These extremities in climate affect all states located in the interior of continents. Such climates are called Continental Climates as against Maritime climates experienced on lands located along coasts. In places such as Timbuktu, Mail, the diurnal and annual ranges in Temperature are very high because of continentality.
Since energy from the sun is transformed into heat on the surface of the earth, air temperature decreases with altitude at an average rate of 3.5 o F per 1,000 feet (6.4 o C per 1000 meters) called the LAPSE RATE. Because of this decrease of temperature with height, mountainous regions such as the Ethiopian highlands have very cool temperatures. Very high peaks such as Mountain Kilimanjaro located along the equator even have permanent snow cover. The combination effect of the above factors create variations in temperature, rainfall and other climatic factors resulting in the following climatic types.
1. Humid Equatorial Climates (Af)
2. Semi-arid cimates (Bsh)
3. Desert Climate (BWh)
4. Mountain climates (H)
5. Humid Temperate climates (C)
a. Cool summers
b. Dry summers and wet winters (Mediterranean)- Eastern South Africa and North Africa
c. Cool summers- Cape province in South Africa
Plant and animal life distribution and the different climatic/soil belts are closely related. Although some parts of the continent retain their natural/virgin flora and fauna, increasing population and human activities seem to be endangering these places. The Following types of vegetation types identified:
1. Tropical Rain forest:
This vegetation develops in low land areas with year-round precipitation. It extends throughout the Congo (Zaire) basin and along the West coast of Africa with its widest in Sierra Leone. It is also found in a narrow belt along the Southeastern coast south from the equator. Madagascar has a unique rain forest with special species of fauna and flora not available on the main land. Generally, a heavy canopy of foliage with lianes and epiphytes characterizes the rain forest. It occurs in regions where high temperatures are combined with heavy precipitation.
2. Temperate Forest:
This vegetation is poorly represented in Africa. It is only found on the lower slopes of the Ethiopian highlands and elsewhere on the highlands of considerable elevation and rainfall
3. Mediterranean Woodland
These are evergreen trees and shrubs adapted to the hot dry season and mild wet season. In North West Africa, the hilly belts support forests of evergreen oak, including cork oak and cedar, although they are now much reduced by clearing and grazing. Evergreen shrubs with hard and usually small leaves are common as growth is slow but continuous throughout the year. The vegetation in the Southwest corner is similar
4. Mangrove Swamps
This vegetation is found at the inter-tidal flats of the coastlines within the tropics.
The margin of the Savannah coincides with the rain forest belt where dry conditions begin. In such areas, there is often a mosaic of forest savanna. Further away from the forest, as the dry season becomes more pronounced and more prolonged, the Savannah woodland thins out, and trees that are more drought resistant are found. Generally, the prevailing vegetation cover is grass interspersed with trees (except in areas around streams and moist hollows). In the zone of lower rainfall the huge baobab tree illustrates storage of water while 'umbrella' trees show adaptation to dry wind. Acacias, some producing gum, are a feature of the drier Sudan savanna. The Savanna extends with similar differences in character over the Lake plateau across the Zambezi to the Drakensberg and dries towards the Kalahari in the thorny scrubs of the Bushveld. Major crops produced in the Savanna: millet, sorghum, peanuts, and cotton.
6. Temperate Grassland or Veld
This is found in the veld areas of South Africa. Characterized by short grass and plants adapted to a brief growing season but there are considerable differences due to altitude. It is by no means adapted everywhere to cultivation, but maize, tobacco and temperate cereals are grown
7. Semi-desert and desert
Areas further north, the Savanna degenerate into semi-arid and then desert. This is the environment of the Sahel ranging from thorn, wooded grassland to tussocky grasses with large bare patches of bare earth between. Human and animals overpopulate the semi arid areas and both take their toll on the environment. As pasture is destroyed through overgrazing and cultivation, the desert advances southward further restricting populations and increasing densities in a vicious circle of desertification.
AFRICAN SOILS. Fig 2.12 page 40
The best soils are alluvial deposits found in the major river valleys. With a few exceptions, most of the soils are difficult to cultivate although improvements can be made to increase natural fertility. Soils in the humid tropics can be quite rich due to the forest cover and the rapid decomposition of organic matter.
However, intense rainfall leaches out most of the plant nutrients. This results in a hard pan formed of iron or aluminum oxides - latosols/ferrosols. Between the wet tropics are well-formed soils rich in plant nutrients. But the high iron-oxide content turn and the alternate wet and dry climates turn it into a hard pan - luvisols. Towards the deserts, the soils are sandy and deep but low in humus and quite infertile (arenosols). These give way to xerosols that are quite low in humus.
1. Examine the advantages and impediments that Africa's location and physiography places on the development of its people.
Discussion - Advantages:
1. Centrally located among continents hence the opportunity to develop early trade and embark upon the conquest of surrounding countries. The central location also allows external powers to converge on the continent and to scramble for its people and resources. It seems however that Ancient Africans were content with what they had and had no incentive to leave the continent.
2. Most tropical of all continents means warm weather occur throughout the continent
3. Very few natural disasters such as devastating earthquakes, floods, volcanoes etc occur on mainland Africa
4. Several long and wide rivers cut across the continent in both north-south and east-west directions that could provide transportation and hydroelectric power.
5. Relatively flat land with very few mountains - ideal for transportation development
6. Very old crystalline rocks rich in valuable minerals such as gold, diamond and bauxite.