Relief in Africa and the World
Senior 3 · Geography
Relief in Africa and the World
Lesson 2 of 4
25 min
Let's begin
Figure 4 is a true side view of Africa, drawn from satellite height data along latitude 2°S, a line that passes through Rwanda. Before you read on, study it. Where is the land lowest? Where does it suddenly drop and rise again? Where is Rwanda? The shapes you see (a wide low basin in the west, high plateau in the east, deep narrow troughs holding lakes) are the main relief forms of Africa, and this lesson names and explains them.
Figure 4. A cross-section of Africa along latitude 2°S. Drawn by Ganzaa from SRTM data.
Definition
Plateau: a wide area of high land with a fairly level top, e.g. the East African Plateau.
Basin: a wide, low area of land surrounded by higher ground, e.g. the Congo Basin.
Rift valley: a long, steep-sided valley formed where a block of the Earth's crust has sunk between two parallel faults.
Escarpment: a long, steep slope or cliff at the edge of a plateau or rift valley.
Fold mountains: mountains formed when layers of rock are squeezed and bent into folds, e.g. the Atlas Mountains.
Volcanic mountain: a mountain built up by lava and ash from eruptions, e.g. Kilimanjaro. Block mountain (horst): a block of crust raised up between faults, e.g. the Rwenzori.
Africa is often called a plateau continent. Its average height is about 600 m, and much of it is a series of wide plateaus separated by basins. Geographers divide it in two:
Figure 5. The main relief features of Africa. Drawn by Ganzaa from Natural Earth data.
| Relief form | Examples | How it formed |
|---|---|---|
| Plateaus | East African Plateau; Southern African Plateau; Ethiopian Highlands | Very old, hard rock uplifted over millions of years; in Ethiopia, also built up by great floods of lava |
| Rift valley | East African Rift: Eastern Rift (Ethiopia, Kenya, Tanzania) and Western Rift (Lakes Albert, Edward, Kivu, Tanganyika, Malawi) | Tension pulled the crust apart; blocks sank between parallel faults (Figure 6) |
| Volcanic mountains | Kilimanjaro 5,895 m, Mount Kenya 5,199 m, Mount Cameroon, the Virunga volcanoes (Karisimbi 4,507 m) | Magma rose through cracks in the crust, often along the rift, and built cones of lava and ash |
| Block mountains | The Rwenzori (Mount Stanley, 5,109 m) | A block of crust pushed up between faults |
| Fold mountains | The Atlas Mountains in the north-west; the Cape Ranges in the south | Rock layers squeezed together and folded |
| Basins | Congo, Chad and Kalahari basins | Parts of the plateau sagged downwards; rivers and lakes drain into them |
| Escarpments | The Great Escarpment of southern Africa (highest in the Drakensberg); rift valley walls | The worn edges of plateaus, and fault scarps |
| Deserts and lowlands | Sahara; Namib; Lake Assal, 155 m below sea level, the lowest point in Africa | Dry plateaus and hollows; Lake Assal lies in the Afar part of the rift |
The East African Rift is the most striking relief feature of the continent. Its two branches run from the Afar region near the Red Sea southwards for thousands of kilometres. Their floors hold long, deep lakes: Lake Tanganyika is the longest freshwater lake in the world. Volcanoes such as Kilimanjaro, Mount Kenya and the Virunga rose along the lines of weakness beside the rifts.
Figure 6. How a rift valley forms. Drawn by Ganzaa.
Earth Science Classroom: how the East African Rift System formed. Watch for the rising heat below the crust, the stretching and faulting, and the volcanoes that grew along the rift.
Rwanda sits on the high plateau beside the Western Rift. Figure 4 shows it well: Lake Kivu lies in the rift floor on the western border, the land rises steeply east of the lake to the Congo–Nile ridge, and then the plateau falls gently eastwards towards Lake Victoria. The ridge is a watershed: rain falling west of it drains to the Congo, and rain falling east of it drains to the Nile. In the north-west, the Virunga volcanoes rise to Karisimbi, at 4,507 m Rwanda's highest point; the lowest point is the Rusizi River in the south-west, at 950 m (Rwanda Development Board). Much of the country lies between 1,500 m and 2,500 m.
Worked example
Worked example 1: reading Figure 4. Along 2°S, Lake Kivu's surface is at 1,461 m (at 29.1°E) and the Congo–Nile ridge reaches about 2,100 m (at 29.5°E). One degree of longitude at this latitude is about 111 km. Find the rise and the average gradient from the lake to the ridge.
Answer: about 640 m of rise, an average gradient of about 1 in 70. The real slopes are much steeper in places: the section is an average over hills and valleys.
Worked example
Worked example 2: classifying by origin. Sort these into the process that made them: Kilimanjaro, the Atlas Mountains, the Rwenzori, Lake Tanganyika's trough, the Congo Basin.
| Process | Landform |
|---|---|
| Volcanic activity | Kilimanjaro |
| Folding (compression) | Atlas Mountains |
| Faulting: block pushed up | Rwenzori |
| Faulting: block sunk (tension) | Lake Tanganyika's trough |
| Downwarping (sagging) | Congo Basin |
Warning
Not every high mountain is a fold mountain. Africa's highest peaks, Kilimanjaro and Mount Kenya, are volcanoes, and the Rwenzori is a block mountain. Africa's only large young fold mountains are the Atlas, in the far north-west. Candidates often write "the Himalayas and Kilimanjaro are fold mountains": only the first is.
Tip
Memory hook: "Squeeze, stretch, sag, spill". Squeeze makes fold mountains, stretch makes rift valleys (and block mountains beside them), sag makes basins, and spill (of lava) makes volcanic mountains and lava plateaus.
Practice
Sources and credits
Video: East African Rift Valley System - Part 2 - Formation, Earth Science Classroom, embedded from YouTube.
Figures 4 to 6, the worked examples and the practice exercises are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
