Formation of Relief Features in Rwanda
Senior 4 · Geography
Formation of Relief Features in Rwanda
Lesson 2 of 5
25 min
REMA describes Rwanda's relief as hilly and mountainous, with an average altitude of about 1,700 m. The highest point is the summit of Karisimbi, 4,507 m above sea level; the eastern part of the country is relatively flat, mostly well below 1,500 m; and the Bugarama plain in the south-west lies at about 900 m, part of the tectonic depression of the African Rift Valley (REMA, State of Environment and Outlook Report, 2009). Figure 2 groups this relief into five regions.
Figure 2: a simplified sketch of the relief regions; real boundaries are gradual. Heights from REMA (2009). The dashed line A–B is the section drawn in Figure 3.
Figure 3 is a real section across the country along the dashed line A–B. It shows the most important fact about Rwanda's relief: the land rises steeply from Lake Kivu to the crest of the Congo–Nile Ridge, then falls gently, over more than 100 km, towards the Akagera valley in the east.
Figure 3: a real cross-section of Rwanda, drawn by Ganzaa from NASA SRTM 90 m elevation data (accessed through OpenTopoData) at points every 0.01° of longitude.
| Region | Where | Height and landforms | How it formed |
|---|---|---|---|
| Volcanic region (Virunga) | North-west: Musanze, Burera, Nyabihu, Rubavu | Volcanic cones up to 4,507 m (Karisimbi); lava plains; lakes Burera and Ruhondo | Volcanic activity along the western rift |
| Lake Kivu shore and Bugarama plain | West, along the lake and the Rusizi valley | Lake surface about 1,460 m (SRTM); steep shores, bays and islands; Bugarama about 900 m | Faulting: part of the rift floor |
| Congo–Nile Ridge | A north–south belt east of Lake Kivu, through Gishwati and Nyungwe | The highest non-volcanic land; about 2,450 m on section A–B; steep, deep valleys | Uplift of the rift shoulder |
| Central plateau | Centre, around Kigali, Muhanga, Huye | Rounded hills and valleys, mostly between about 1,400 and 1,800 m on section A–B; swampy valley floors | Erosion of old folded rocks; gentle tilting |
| Eastern plateau and lowlands | East: Bugesera, Kayonza, Nyagatare, Kirehe | Gentle, low hills, mostly below 1,500 m (REMA); lakes and swamps along the Akagera | Long erosion; warping and drowned valleys |
Definition
Endogenic (internal) processes: forces from inside the earth that build relief: earth movements and volcanicity. Exogenic (external) processes: forces at the surface that wear relief down: weathering, mass wasting and erosion by rivers, waves, wind and ice.
Folding: the bending of rock layers by compression. Faulting: the fracturing of rocks with movement along the fracture (a fault). Warping: the gentle upward or downward bending of a large area of the crust, without sharp folds or breaks.
Most of Rwanda is built of very old sedimentary rocks (quartzites, sandstones and shales, mostly changed by heat and pressure) that were folded and intruded by granites during mountain building more than a billion years ago, in what geologists call the Kibaran belt. Those fold mountains have long since been worn away. What remains is their grain: hard quartzite layers resist erosion and stand up as long ridges, while softer shales and schists between them have been worn into valleys. This differential erosion of folded rocks is one reason why many of Rwanda's hills line up in parallel ridges.
Western Rwanda lies on the western branch of the East African Rift System, also called the Albertine Rift. Here the crust is being stretched. Long blocks have dropped down between parallel faults to form a rift valley, now partly filled by Lake Kivu and, further south, by the Rusizi valley and the Bugarama plain. REMA notes that Rwanda lies in a tectonic region whose earthquakes centre on Lake Kivu. The land east of the rift was raised as a rift shoulder: the Congo–Nile Ridge. Section A–B shows its two sides: a steep fault-guided escarpment facing the lake and a long, gentle back-slope to the east.
Worked example 1: the vertical exaggeration of Figure 3
Problem. On Figure 3, 200 km of ground spans about 450 px and 1,300 m of height spans 250 px. Find the vertical exaggeration.
Solution. Horizontal scale: . Vertical scale: .
Heights are stretched about 80 to 85 times, so slopes look far steeper than they are. Without exaggeration the whole section would be a line barely 3 px thick.
Worked example 2: two sides of the Congo–Nile Ridge
Data from section A–B. The lake shore (1,461 m) is at 29.325° E; the crest (2,453 m) at 29.41° E; the Akagera valley (1,286 m) at 30.73° E. At latitude 1.95° S one degree of longitude is km.
West face. VI m; HE km m.
East side. VI m; HE km.
The west face is about 13 times steeper on average than the east side: the shape of a block raised and tilted beside a rift.
As the rift shoulder rose, the land to the east was gently tilted. Warping of this kind reduces the gradient of rivers that flowed east, and in places tilts it backwards, so that water collects in the lower parts of valleys. The broad, flat, swampy valley floors of central and eastern Rwanda, and long, branching lakes such as Muhazi and Mugesera, are usually explained in this way: they are river valleys partly drowned after the land was warped. A lake formed like this has an irregular shoreline with many arms, each arm an old tributary valley.
Warning
Do not mix up the processes. Folding needs compression and produces bends; faulting produces breaks with blocks moving up or down; warping produces broad, gentle bending. Lake Kivu lies in a faulted depression; Lake Muhazi is usually explained as a drowned valley; lakes Burera and Ruhondo were dammed by lava (next lesson).
Earth Science Classroom: "Continental Rifting & Rift Valley Formation". How stretching of a continent, rising heat and normal faults produce a rift valley with raised shoulders. Relate each stage to Lake Kivu and the Congo–Nile Ridge.
Practice
Key points
Sources and credits
Sources: Rwanda State of Environment and Outlook Report, Rwanda Environment Management Authority (REMA), 2009 (relief, heights, rift). Geology of Rwanda, Wikipedia contributors, CC BY-SA 4.0, adapted by Ganzaa (rock types and the Kibaran belt).
Figure 2 was drawn by Ganzaa (boundaries: geoBoundaries, CC BY 4.0; Natural Earth). Figure 3 was drawn by Ganzaa from NASA SRTM elevation data (public domain) obtained through OpenTopoData.
Photos: Gishwati Natural Forest 01, Andr96, CC BY-SA 4.0; Akagera, Niyigenachristian, CC BY-SA 4.0; via Wikimedia Commons.
Video: "Continental Rifting & Rift Valley Formation", Earth Science Classroom, embedded from YouTube. The rest of the lesson was written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
Quick check
Match each relief feature to the process that mainly formed it.
Tap one on the left, then its partner on the right.
