Rocks at Work: Why Rocks Matter – Rocks (Senior 1 Geography) | Ganzaa

Rocks

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Senior 1 · Geography

Rocks

3 lessons

Lesson 3 of 3

Rocks at Work: Why Rocks Matter

22 min

Count the rocks in your classroom

Look around your classroom. The walls are bricks (fired clay) or blocks (cement, sand and gravel). The floor is concrete (more cement, sand and gravel). The window glass is melted quartz sand. The pencil "lead" is graphite, a mineral mixed with clay, and your phone contains tin solder and tantalum. Almost everything around you started as rock. This lesson asks what Rwanda gains from its rocks, and what it costs to use them.

Rocks for building

Figure 1. Almost every building in Rwanda is made from local rocks and sediments.

Definition

Quarry: a place where rock, sand, gravel or clay is dug from an open pit in the ground for use in building or industry. Example: quarries on the edge of Kigali supply crushed stone for concrete and roads.

Definition

Mining: digging valuable minerals (especially ores) from the ground, either from open pits or from underground tunnels, and then separating the useful mineral from the waste rock. Example: tungsten ore is mined underground in Rulindo District.

The importance of rocks

Use	How rocks help	Rwandan example
Building materials	Stone, gravel, sand, clay and cement	Volcanic stone houses in Musanze; bricks and tiles in every district; cement made at Bugarama
Minerals and export income	Ores are mined, sold and exported, bringing in foreign currency and jobs	Cassiterite, coltan and wolframite; mining is one of Rwanda's biggest earners of export income
Soils and farming	Weathered rock becomes the mineral part of soil	Rich volcanic soils around Musanze grow potatoes, beans and pyrethrum
Water	Cracked and weathered rocks store groundwater that feeds springs and wells	Springs at the foot of hills supply many villages
Tourism	Striking rock landscapes attract visitors	The Virunga volcanoes, the Musanze caves, the rock scenery around Lake Kivu
Energy	Peat, the first stage in the making of coal, forms in some wetlands and can be burned in power stations	Peat-fired power stations in Rusizi and Gisagara districts
Art and culture	Clay for pottery; stone for grinding and carving	Traditional pottery; grinding stones (urusyo) for sorghum and millet

Worked example 1: how much stone is in a wall?

A farmer at Kinigi builds a stone wall 20 m long, 1.5 m high and 0.5 m thick. Basalt has a density of about 3 tonnes per cubic metre, but a dry-stone wall is only about two-thirds solid rock (the rest is gaps). Estimate the mass of stone.

Step 1. Volume of the wall.

V = 20 \times 1.5 \times 0.5 = 15\ \text{m}^3

Step 2. Volume of rock. $\frac{2}{3} \times 15 = 10\ \text{m}^3$ .

Step 3. Mass.

m = \text{density} \times \text{volume} = 3 \times 10 = 30\ \text{tonnes}

So what? Every stone had to be cleared from the fields by hand, which also made the land easier to farm.

Rocks and mining in Rwanda

Figure 2. Rwanda's three main ores by mass, 2010 (data: Wikipedia, "Mining industry of Rwanda"). Percentages are rounded.

Worked example 2: reading the pie chart

In 2010 Rwanda produced 3,874 t of cassiterite out of 5,466 t of the three ores. What percentage was cassiterite, and how many degrees of the pie does it take?

\text{percentage} = \frac{3\,874}{5\,466} \times 100 \approx 70.9\%

\text{angle} = \frac{3\,874}{5\,466} \times 360^\circ \approx 255^\circ

Interpretation. By mass, most of Rwanda's ore was tin ore. By value the picture changes, because a tonne of coltan sells for much more than a tonne of cassiterite. A chart must always say what it measures.

Important

Case study: the costs of quarrying and mining

Rocks bring jobs and income, but digging them out has costs:

Land and soil: pits and waste heaps remove farmland and leave bare slopes that erode in the rains.
Water: mud washed from mines and quarries makes rivers such as the Nyabarongo brown and harms fish and water supplies.
Health and safety: dust, falling rock and collapsing pits injure workers, especially in small, unregulated sites.

Responses: mines need a licence from the Rwanda Mines, Petroleum and Gas Board; companies must plan how to refill and replant old pits, build ponds to trap mud, and give workers helmets and training. A quarry that is closed properly can become farmland, a fish pond or a forest again.

From Earth to Industry: the Story of Rwanda's Thriving Mining Sector, by the Rwanda Mines, Petroleum and Gas Board (38 min). Watch the first 10 minutes and list the minerals shown and the jobs people do at a mine.

Discuss

A company wants to open a stone quarry on a hill above your village. It will bring 40 jobs, but trucks, dust and noise will pass the school. Would you support it? Give two arguments for, two against, and one condition you would ask the company to accept.

Tip

For "importance of rocks" questions, think B-M-S-W-T: Building, Minerals, Soils, Water, Tourism, and always add a Rwandan example to each point.

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Practice

Give four ways in which rocks are important to people in Rwanda, with an example of each.
What is the difference between a quarry and a mine?
Name the rock material used to make (a) bricks; (b) cement; (c) glass.
A wall is 12 m long, 2 m high and 0.5 m thick. Find its volume. If it is solid basalt (3 t per m³), what is its mass?
Using Figure 2, calculate the percentage of the three ores that was coltan in 2010.
Give two environmental problems caused by quarrying and one way to reduce each.
Explain why the volcanic area around Musanze is both good for farming and a source of building stone.

Show answersHide answers

Any four: building (volcanic stone, bricks, gravel, cement); minerals and export income (cassiterite, coltan, wolframite); soil formation (volcanic soils near Musanze); groundwater and springs; tourism (Virunga volcanoes, Musanze caves); energy (peat); pottery and grinding stones.
A quarry is an open pit for building materials such as stone, sand, gravel or clay; a mine extracts valuable minerals such as ores, from open pits or underground tunnels.
(a) Clay; (b) limestone (heated with clay); (c) quartz sand.
$V = 12 \times 2 \times 0.5 = 12$ m³; mass $= 3 \times 12 = 36$ tonnes.
$\frac{749}{5\,466} \times 100 \approx 13.7\%$ .
E.g. soil erosion from bare pits (replant and terrace old pits); muddy rivers (build settling ponds); dust (spray water, plant trees as a screen).
The volcanic rocks weather into deep, fertile soils rich in nutrients, and the same hard, dense basalt lying in the fields can be split into strong building stones.

Remember

Rocks give building materials, ores and export income, soils, groundwater, tourist attractions and some energy.
Rwanda's main ores are cassiterite (tin), coltan (tantalum) and wolframite (tungsten).
Quarrying and mining must be managed to limit erosion, muddy rivers and danger to workers.

1Count the rocks in your classroom

2Rocks for building

3The importance of rocks

4Rocks and mining in Rwanda

5Discuss

Count the rocks in your classroom

Rocks for building

The importance of rocks

Rocks and mining in Rwanda

Discuss