Rocks in Rwanda: Importance and Problems

22 min

A country built of its own rocks

Look at a house going up in any Rwandan town: stone foundations, walls of fired or sun-dried clay bricks, concrete made of cement, sand and crushed stone, and a road outside built on layers of crushed rock. Almost all of it came out of the ground nearby.

Why rocks matter for Rwanda

Use	Rocks involved	Rwandan evidence
Building stone, aggregate and sand	Quartzite, granite, basalt; river sand	REMA reports that quarrying expanded to sand and stones for the construction boom and road building, and that quarry products made up 11% of internally generated revenues (REMA, 2009)
Bricks and tiles	Clay from weathered rock and valley floors	Clay from wetlands is used for bricks and tiles (REMA, 2009)
Cement	Limestone, with clay and other rock materials	CIMERWA's main factory is in Muganza Sector, Rusizi District, with a capacity of about 600,000 tonnes a year (Wikipedia, "Cimerwa Cement Limited")
Metallic minerals and exports	Pegmatites and quartz veins in the basement	Mineral exports rose to over US$1.1 billion in 2023 (RMB); mining and quarrying were estimated to employ about 50,000 people (REMA, 2009)
Ground water	Weathered and fractured basement; alluvium	Weathered basement is the most widespread aquifer; alluvium in valleys is highly productive (BGS, 2018)
Soils	All rocks, as parent material	Fertile volcanic soils near the Volcanoes National Park (REMA, 2009)
Fuel	Peat (an organic sediment)	Peat is used as fuel in prisons, industries and schools (REMA, 2009)
Tourism and landscape	Volcanic rocks; hard rock bands	Volcanoes, caves and waterfalls attract visitors

Figure 3: from rock to building, and the costs at each step.

Worked example 1: how much rock does a road need?

Problem. A new district road is 10 km long and 7 m wide and needs a base of crushed stone 0.25 m thick. Loose crushed stone has a density of about 1.6 tonnes per m³. How many tonnes are needed, and how many loads for 20-tonne lorries? (Illustrative figures.)

Solution.

V = 10\,000\ \text{m} \times 7\ \text{m} \times 0.25\ \text{m} = 17\,500\ \text{m}^3

m = 17\,500 \times 1.6 = 28\,000\ \text{t}, \qquad \frac{28\,000}{20} = 1\,400\ \text{loads}

One short road means a large quarry face, and 1,400 lorry trips of dust and noise.

Worked example 2: bricks for a wall

Problem. A wall is 10 m long and 3 m high. Each brick, with its mortar joint, covers 0.2 m by 0.1 m of the wall's face and is 0.1 m thick. How many bricks are needed, and what volume of clay? (Illustrative figures.)

Solution. Bricks per square metre: $\frac{1}{0.2 \times 0.1} = 50$ .

N = 10 \times 3 \times 50 = 1\,500\ \text{bricks}, \qquad V = 1\,500 \times (0.2 \times 0.1 \times 0.1) = 3\ \text{m}^3

About 3 m³ of clay, dug from a valley floor or a hillside pit, for one wall of one house.

Problems linked to rocks

Problem	Explanation and evidence	Possible solution
Damage from quarrying and sand harvesting	REMA lists resource depletion, energy use, waste, air pollutants, dust, noise, displacement of people and land-use change (REMA, 2009)	Licensing and environmental impact assessment; rehabilitating old quarries by back-filling and tree planting
Muddy, polluted rivers	Water used to wash ores at Rutongo and Gatumba carried clay and sand into the Nyabarongo and Nyabugogo (REMA, 2009)	Settling ponds; recycling washing water; buffer zones along rivers
Spoil heaps and open pits	Waste rock is eroded into valleys; open pits are dangerous to people and livestock	Back-filling; fencing; replanting spoil
Hard rock is costly to work	Quartzite and granite slow down road building, foundations and pit latrines	Proper equipment; route roads along softer valleys where safe
Weak, clay-rich regolith	Weathered schist and clay swell, crack walls and fail in landslides	Drainage, deep foundations, avoiding steep sites
Little ground water in much of the basement	BGS notes that most basement rocks have low productivity; only quartzite is moderate	Site boreholes on fractures, quartzite and alluvium; harvest rain water

Warning

"Rocks are only a resource" is half an answer. An exam question on the importance of rocks expects advantages and problems, each with an example. Quote a source and a year for any figure, such as RMB's US$1.1 billion of mineral exports in 2023.

Engineering World: "How Cement Is Made In Factories?". Quarrying and grinding limestone and clay, heating them in a rotary kiln to make clinker, and grinding clinker with gypsum. Compare the kiln with the photograph of the Muganza works.

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Practice

Give five uses of rocks in Rwanda, with the rock used in each case.
Explain why the construction boom increased quarrying in Rwanda.
A driveway 40 m long, 3 m wide and 0.2 m deep is filled with crushed stone of density 1.6 t per m³. Find the mass needed.
How many bricks covering 0.25 m by 0.1 m each are needed for a wall 8 m long and 2.5 m high?
Describe three environmental problems caused by quarrying and mining, using REMA's evidence.
Suggest how an abandoned quarry could be made useful again.
"Rwanda's rocks are more a blessing than a curse." Discuss.

Show answersHide answers

For example: building stone (quartzite, granite); aggregate for concrete and roads (crushed basalt or quartzite); bricks and tiles (clay); cement (limestone with clay); minerals for export (cassiterite, coltan from pegmatites); ground water (weathered basement, alluvium); fuel (peat).
New houses, offices and roads need stone, sand, aggregate, bricks and cement, so demand for quarry products rose; REMA (2009) reports that quarrying expanded to sand and stones for construction and roads.
$V = 40 \times 3 \times 0.2 = 24\ \text{m}^3$ ; $m = 24 \times 1.6 = 38.4$ t.
Per m²: $\frac{1}{0.25 \times 0.1} = 40$ ; area $8 \times 2.5 = 20$ m²; $20 \times 40 = 800$ bricks.
For example: dust and noise; land-use change and displacement of people; rivers made muddy by ore washing (Rutongo and Gatumba into the Nyabarongo and Nyabugogo); spoil eroded into valleys (REMA, 2009).
Back-fill and replant it; turn it into a water reservoir, a fish pond, a sports ground or a building site, after making the faces safe.
Blessing: building materials, cement, export minerals and jobs, ground water, fertile volcanic soils, tourism. Curse: environmental damage, polluted rivers, accidents, hard rock and weak clays raising building costs. A good answer concludes that rocks are a blessing when they are managed with licensing, rehabilitation and safety.

Quick check

Which rock material is the main raw material for making cement?

Key points

Rocks give Rwanda building stone, aggregate, sand, clay bricks, cement, export minerals (over US$1.1 billion in 2023, RMB), ground water, soils, fuel and tourism.
Quarrying and mining cause dust, noise, land degradation, displacement and river pollution (REMA, 2009).
Hard rocks raise building costs; weak clays and weathered schists fail in landslides; much of the basement holds little ground water.
Licensing, rehabilitation, settling ponds and safety rules reduce the problems.

1A country built of its own rocks

2Why rocks matter for Rwanda

3Problems linked to rocks

A country built of its own rocks

Why rocks matter for Rwanda

Problems linked to rocks