Physical context

Geology

Central Africa is formed mainly of an enormous bloc of Precambrian rocks that crop out in an arc around a central sedimentary basin, separating it from the sedimentary Chad Basin to the north and the coastal sedimentary basin to the west.

Geological formations of Central Africa

Cenozoic formations

The sedimentary formations comprise:

  • Neogene (7) formations, between 26 and 1.8 millions years old, that cover the cretaceous formations in the centre of the basin. They are also found in the coastal sedimentary basin along the Gulf of Guinea.
  • Quaternary and Holocene formations (8), less than 1.8 millions years old, that cover the two preceding formations around the confluence where the Ounagui and the Sangha meet the River Congo. They are most extensive in the Congo, and are also found in the coastal sedimentary basin, especially in the Ogooué delta.
  • Volcanic formations dating back to the late Neogene, the Pleistocene and the Holocene (9). They are limited to western Cameroon to the west and the Albertine rift region, which is the western branch of the African-Arabian rift, to the east.

The Archean formations (1)

These formations are between 3.2 and 2.6 billion years old and cover the Monts du Chaillu on the Gabon-Congo border, the north of Gabon (Monts de Cristal and the Minkébé region), a large part of South Cameroon, the southern part of CAR and the northeast of DRC. Outcrops are also found in the south of DRC, around Kasaï. The rocks are mainly gneiss with more ancient inclusions dating back to 3.9 - 3.8 billion years ago.

Mesozoic formations

These are mainly sedimentary formations:

  • Permian-Carboniferous formations, 350 to250 million years old, limited mainly to the eastern part of the Congo basin. They include some glacial sediments. There are also small formations of this age in northwestern Gabon, on the fringe of the coastal sedimentary basin
  • Triassic formations that are 250 to 200 million years old, also located to the southeast of the central basin but covering only small areas.
  • Cretaceous formations dating back 140 to 65 million years and covering a large part of the central basin and the coastal plain along the Gulf of Guinea

Proterozoic formations (2)

These are between 2.6 billion and 550 million years old. They include sedimentary formations and Archean rock that is generally strongly metamorphic and also, in some areas, intrusive rocks often in the form of inselbergs. The Proterozoic formations subdivide into several series that mainly cover the Paleoproterozoic (2.6-1.6 billion years ago) and the Neoproterozoic (1 billion to 550 million years ago). Together, these formations form an almost continuous arc around the edge of the Congo basin, with the Archean curving around it. The most extensive formations mainly cover the north, east and southeast of DRC. They are discontinuous in the Bas-Congo in DRC, form an east-west band through the centre of Gabon, and are also found in southeast Cameroon. The Proterozoic formations are rich in minerals, especially in areas where they are in contact with the Archean formations.

Soil Atlas of Africa

The Soil Atlas of Africa was published on 29 April 2013. The Atlas explains the origin and functions of the soils, describes the different types of soil and the usefulness for addressing local and global problems. It describes the main threats to the soils and the measure taken to protect Africa's soil resources.

The Soil Atlas of Africa is the result of collaboration between the European Union, the African Union, and the United Nations FAO, to support and encourage sustainable use of soil resources in Africa, and the Global Soil Partnership for Food Security.

To find the Atlas and download the data:
http://esdac.jrc.ec.europa.eu/content/soil-map-soil-atlas-africa

Important information about the Atlas

  • 98% of the calories consumed in Africa are from crops and livestock produced on African soils.
  • Soil organic matter can store more than ten times its weight in water, thereby reducing the risk of flooding and protecting groundwater resources.
  • African soils contain close to 200 gigatons of organic carbon, which is 2.5 times more than is contained in the continent's plants
  • The soils of tropical rainforests are not naturally fertile; they require constant supply of organic matter from the natural plant cover. Deforestation breaks this cycle
  • More than half of Africa's land surface is composed of sandy soils (22%), shallow stony soils (17%) and young, weakly developed soils (11%).
  • Many of Africa's soils are severely degraded by erosion and excessive nutrient depletion. This explains their low productivity, mainly due to lack of plant nutrients, which are not adequately replenished by artificial fertilizers. Because of rural poverty, African farmers on average can only apply 10% of the fertilizer applied by farmers in the rest of the world

Hydrography

Central Africa is part of five major river basins or groups of river basins.

River Length (km) Basin area(km2) Flow rate (m3/sec)
1 Congo 4374 3600000 41000
2 Nile 6670 3349000 5100
3 Chad (Chari) 949
4 Niger (Bénoué) 1400 870
The coastal rivers of the Gulf of Guinea
5 Cross 75000 569
6 Wouri 11700 308
7 Sanaga 1043 133000 2060
8 Nyong 27800 443
9 Ntem 31000 348
10 Wele
11 Muni
12 Komo
13 Ogooué 1200 205000 4400
14 Nyanga 25000 450
15 Kouilou-Niari 60000 700

Source: WRI-OFAC

The Chad Basin

This is an endohreic basin (with no outlet to the sea) centred on Lake Chad, a vast, shallow freshwater lake fed mainly by the Chari and its affluent the Logone. The area covered by the lake is prone to enormous variation. 50,000 years ago it covered 2 million km², but during the last glaciation it dried up completely. The lake filled up again in recent times but has prone to considerable variation. In 1908 it was practical dry; by 1963 it covered 22,000 km².

The Niger basin

The centre of Cameroon, which is drained by the Bénoué River, is part of the Niger basin.

The Nile basin

The northeast of DRC (around lakes Albert and Edward), the east of Rwanda and the northeast of Burundi are drained towards the Nile.

The Congo basin

The Congo drainage basin drains much of the DRC and the Republic of Congo, as well as the regions around Lake Tanganyika, including the Malagarazi and the Moyowozi basins in Tanzania, and the west of Rwanda and Burundi through Lake Kivu and the Rusizi river. To the south the basin covers the north of Angola and part of Zambia; to the north it drains part of the Central African Republic via the Ubangi River. Lake Kivu and the western part of Rwanda were part of the Nile basin but their outflow was blocked by the formation of Virungas at the end of the Miocene. The Akagera River then reversed its course, but was finally captured by Lake Victoria which is also part of the Nile basin. Lake Kivu, on the other hand, was captured by Lake Tanganyika and thus is part of the Congo basin

The coastal rivers of the Gulf of Guinea

The south of Cameroon, Equatorial Guinea, Gabon and part of the Republic of Congo are drained by numerous coastal rivers, the main ones being the Sanaga in Cameroon, the Ogooué and the Nyanga in Gabon and the Kouilou-Niari in Republic of Congo.

The Congo River Basin, where annual renewable water resources amount to about 1.3 billion m3 is the biggest of these basins and the biggest in Africa. It accounts for close to 30% of Africa's water resources and covers 4 million km2. The Congo River has an average annual flow rate of 41,000 m3/sec (Nkounkou and Probst, 2017). The basin covers parts of ten countries but 85.3% of the Congo River Basin is located in largely forested areas of four countries : Cameroon, Central African Republic, Democratic Republic of Congo and Republic of Congo. The Congo basin's stream network is very dense and consists of a complex network of rivers, vast freshwater swamp forests, and lakes.

For more information on the stream network, particularly its relationship to the forest and to ecosystem goods and services, see Chapter 9 of the State of the Forests 2008.

Rainfall

Annual average rainfall in Central Africa is highest in the area closest to the Gulf of Guinea: 3000 mm in Libreville, 3200 mm in Bata, 3400 in Douala, 6000 mm in the southern part of Bioko Island and close to 12,000 mm in Debunscha in the southwestern foothills of Mount Cameroon. Towards the south, precipitation declines rapidly: at the mouth of the Congo River, it is less than 1000 mm.

Further from the coast, the Congo basin's rainforests receive between 1700 and 2000 mm of rain. The areas with the highest rainfall are located close to the equator on the Chaillu mountains and on the centre and eastern edge of the central basin. Further from the equator, rainfall drops to under 1000 mm in the north of Cameroon near Lake Chad, the northeast of the Central African Republic and in certain parts of southeast DRC. The Albertine Rift valley also has some very arid areas near lakes Albert and Edward and in Tanganyika.

Topography

The forests of Central Africa mainly cover the vast alluvial plain of the Congo, lying at an altitude of between 300 and 600 m and surrounded by generally tabular plateaux with altitudes between 600 and 1000 m. To the north and south, these plateaux (which separate the Congo basin from the Chad and Zambesi basins) cover areas that are almost completely outside the rainforest zone.

To the east, forests cover the plateaux that form the foothills of the mountain range along the Albertine rift. This range is close to 1000 km long from north to south, peaking at 5100 m in the Ruwenzori range and 4500 m in the Virunga Mountains.

To the west, the Congolese plain is separated from the alluvial coastal plain which extends over 150 km where it reaches Gabon, by a series of plateaux and mountain ranges barely over 1000 m high that separate the waters of the Congo from those of the numerous coastal rivers.

In Cameroon, these plateaux spread over a large part of the country, punctuated to the northwest and the north by high volcanoes forming an arc from Bioko Island and Mount Cameroon to the Bamenda and Adamawa plateaux and even the Yade plateau in Central African Republic.

About 65% of Central Africa's tropical rainforests are located at altitudes of 300-600 m, 30% at altitudes between 600 and 1650 m and 5% between 0 and 300 m. Only 1% of these forests are composed of montane forests at altitudes over 1650 m.