Rocks in Rwanda
Senior 4 · Geography
Rocks in Rwanda
Lesson 2 of 3
22 min
The British Geological Survey (BGS), working with the Rwanda Water and Forestry Authority, mapped Rwanda's rocks in three broad groups to show where ground water can be found. The map is also the simplest geological map of the country.
| Rock group (youngest first) | Age | Rocks and characteristics | Where |
|---|---|---|---|
| Unconsolidated sediments | Quaternary | Alluvium and lake sediments: loose sands, silts, gravels and clays filling valleys and forming floodplains | Largest along the Akagera floodplain on the eastern border; small strips in river valleys everywhere |
| Volcanic rocks | Cenozoic (some possibly Cretaceous) to Quaternary | Mostly lava flows: the northern lavas (Quaternary) and the south-western basalts. Dark, fine-grained, often with gas holes | The north (Virunga) and the far west (south-west) |
| Metasedimentary rocks (Burundian Supergroup) | Middle Proterozoic | Quartzites, metamorphosed sandstones and shales (schists, phyllites), locally intruded by granite. Hard quartzite bands; softer, flaky schists | Across much of the country |
| "Older granites" and other basement rocks | Lower Proterozoic | Granites, granitic gneisses and migmatites: crystalline, hard, weathering to sandy clay | Parts of eastern and southern Rwanda |
Source: BGS, Africa Groundwater Atlas: Hydrogeology of Rwanda (2018). Together the metasedimentary rocks and granites are called the basement: very old, hard, crystalline rocks that underlie everything else.
Figure 2: the ages of Rwanda's rock groups as described by the British Geological Survey, on the international geological time scale. The lower bar enlarges the last 150 million years, which is too short to see on the upper one.
Definition
Basement rocks: the old, usually crystalline igneous and metamorphic rocks that lie beneath younger rocks and sediments. Unconsolidated sediment: loose sediment that has not yet been cemented into rock, such as river alluvium.
Pegmatite: a very coarse-grained igneous rock, formed from the last, water-rich part of a granite magma, often rich in rare minerals. Vein: a sheet of minerals deposited from hot fluids in a crack in rock.
The Burundian rocks belong to a belt of very old folded rocks that geologists call the Karagwe–Ankole belt, which runs through Rwanda, Burundi, Uganda, Tanzania and the eastern DR Congo. Where granites intruded the metasediments, pegmatites and quartz veins formed, and these carry Rwanda's main metallic minerals: cassiterite (tin ore), columbite–tantalite or coltan (tantalum ore) and wolframite (tungsten ore). Gold also occurs. Mining areas such as Rutongo and Gatumba, named by REMA, work these deposits (REMA, 2009); geologists have described, for example, the vein-type tungsten deposit at Nyakabingo (Dewaele and others, 2016). The minerals themselves are studied in the S4 unit on mining.
Worked example 1: how much older?
Problem. The Burundian metasediments are Middle Proterozoic, at least 1,000 million years old. The northern lavas are Quaternary, less than 2.6 million years old. At least how many times older are the metasediments?
Solution.
The rocks under most of Rwanda are at least 385 times older than the volcanoes of the north. If the whole history were squeezed into one day, the northern lavas would have appeared only in the last 4 minutes or less: minutes.
Worked example 2: what the rocks are worth
Data (RMB, 15 February 2024). In October, November and December 2023 Rwanda exported 431,035 kg, 416,231 kg and 446,342 kg of cassiterite, worth US$6,487,192, US$6,274,000 and US$6,923,495; and 159,297 kg, 128,887 kg and 180,393 kg of coltan, worth US$6,907,161, US$5,364,535 and US$6,630,391.
Problem. Find the average export value per kilogram of each ore in that quarter.
Solution.
A kilogram of coltan was worth more than two and a half times a kilogram of cassiterite. RMB reports that all mineral exports together rose to over US$1.1 billion in 2023, from US$772 million in 2022.
Warning
"Basement" does not mean "deep and useless". In Rwanda the basement rocks are at or near the surface almost everywhere, covered only by their own weathered regolith. Most of the country's soils, ground water and minerals come from them.
Practice
Key points
Sources and credits
Sources: Africa Groundwater Atlas: Hydrogeology of Rwanda, F. Tetero, K. Upton, B. Ó Dochartaigh and I. Bellwood-Howard, British Geological Survey, 2018, CC BY-SA 3.0, adapted by Ganzaa; Rwanda's Mineral Export Earnings Surge in 2023, Rwanda Mines, Petroleum and Gas Board (RMB), 15 February 2024; Rwanda State of Environment and Outlook Report, REMA, 2009 (mining areas).
Also: Genesis of the vein-type tungsten mineralization at Nyakabingo (Rwanda) in the Karagwe–Ankole belt, Central Africa, S. Dewaele and others, Mineralium Deposita, 2016; Occurrence and lithogeochemical composition of iron ore in Ngororero District, Rwanda, C. Nshimiyimana, E. Chisakulo and C. Nsengiyumva, Heliyon, 2024 (Rwanda in the Karagwe–Ankole belt). Facts only; no text copied.
Map: Rwanda Hydrogeology2, British Geological Survey, CC BY-SA 3.0, via Wikimedia Commons.
Figure 2, the worked examples and the exercises were drawn and written by Ganzaa; the time-scale boundaries are those of the international geological time scale.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
Quick check
According to the BGS, which rocks cover most of Rwanda?
