Rocks in Rwanda

Lessons

Senior 4 · Geography

Rocks in Rwanda

3 lessons

Lesson 1 of 3

Rock Groups, the Rock Cycle and Identifying Rocks

22 min

Three samples on a desk

A teacher in Muhanga puts three stones on the desk: a speckled grey stone made of interlocking crystals, a dark stone full of small holes from a road in Musanze, and a hard, pale, sugary-looking stone from a ridge-top quarry. Each formed in a different way, and each tells part of the story of how Rwanda was built. This lesson gives you the tests to tell them apart. (S1 introduced the three rock groups; here you will apply them to Rwanda's own rocks.)

Definition

Rock: a natural, solid aggregate of one or more minerals. A mineral is a naturally occurring solid with a definite chemical composition and crystal structure, such as quartz or feldspar.

Igneous rock: formed when magma or lava cools and crystallises. Sedimentary rock: formed from sediments that are deposited, compacted and cemented. Metamorphic rock: formed when existing rock is changed by heat and pressure without melting.

The rock cycle

Figure 1: the rock cycle, with the rock types that BGS maps in Rwanda. Weathering and erosion act on every rock type that reaches the surface, not only on igneous rock.

Characteristics of the three groups

Group	How it forms	What you see	Examples in Rwanda
Igneous, intrusive (plutonic)	Magma cools slowly deep underground	Large, interlocking crystals; no layers; hard	Granite and the "older granites", gneisses and migmatites of parts of eastern and southern Rwanda (BGS)
Igneous, extrusive (volcanic)	Lava cools quickly at the surface	Tiny crystals or glass; often gas holes (vesicles); dark	The northern lavas and south-western basalts, mostly lava flows (BGS)
Sedimentary	Sediments are deposited in layers, compacted and cemented	Layers (strata); grains cemented together; often porous; may contain fossils	Alluvium and lake sediments (sands, silts, gravels, clays), largest along the Akagera floodplain (BGS)
Metamorphic	Heat and pressure recrystallise an older rock	Interlocking crystals; often banded or foliated (flaky layers); hard	Quartzites and metamorphosed sandstones and shales over much of the country (BGS)

So the three samples on the desk are probably granite (crystals, no layers), vesicular basalt (dark, with holes) and quartzite (hard, sugary, from a ridge).

Quartzite at Ord on the Isle of Skye, Scotland: cream-coloured, hard, bedded rock tilted by folding, of the same kind as the quartzites that form many ridges in Rwanda. Photo: Jonathan Wilkins, 16 July 2008, CC BY-SA 2.0, via Wikimedia Commons.

Tip

Field tests for a rock sample: (1) Look: crystals or grains? (2) Layers or bands? (3) Holes (vesicles)? (4) Hardness: does a steel nail scratch it? Quartzite and granite resist; shale and chalk do not. (5) Acid: a drop of dilute acid fizzes on limestone and other carbonates. (6) Weigh it and measure its volume to find its density. Wear eye protection for the acid test and wash your hands afterwards.

Worked example 1: identifying a rock by its density

Problem. A dry sample has a mass of 405 g. Lowered into a measuring cylinder, it raises the water from 250 cm³ to 400 cm³. Typical densities are about 2.6–2.7 g cm⁻³ for granite and quartzite and 2.8–3.0 g cm⁻³ for basalt. Which is it likely to be?

Solution. Volume $V = 400 - 250 = 150\ \text{cm}^3$ .

\rho = \frac{m}{V} = \frac{405\ \text{g}}{150\ \text{cm}^3} = 2.7\ \mathrm{g\,cm^{-3}}

This fits granite or quartzite, not basalt. Crystals with no layers would point to granite; a sugary, very hard rock with no visible minerals other than quartz would point to quartzite.

Worked example 2: porosity, and why it matters

Problem. A dry block of sandstone of volume 100 cm³ gains 18 g when soaked in water; a granite block of the same volume gains 1 g. Find each rock's porosity (1 g of water fills 1 cm³).

Solution.

\text{porosity} = \frac{\text{volume of pores}}{\text{total volume}} \times 100\%

\text{sandstone: } \frac{18}{100} \times 100\% = 18\%, \qquad \text{granite: } \frac{1}{100} \times 100\% = 1\%

Porous rocks store ground water and let it flow. That is why BGS describes Rwanda's coarse alluvium as a highly productive aquifer, while unweathered granite stores water only in its cracks.

Warning

Colour is a weak test. Red or brown stones in Rwanda are often just stained by iron oxide from weathering, whatever the rock. Use texture (crystals, grains, layers, holes), hardness and density instead.

Earth.Parts: "Igneous, Metamorphic & Sedimentary rock types and the rock cycle". How each rock group forms and how one becomes another. Pause at each group and name a Rwandan example from the table.

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Practice

Define a rock and a mineral, and give an example of each.
Explain why granite has large crystals but basalt has tiny ones.
Draw and label the rock cycle, adding one Rwandan example of each rock group.
A sample of mass 594 g displaces 200 cm³ of water. Find its density and suggest what rock it might be.
A 250 cm³ block absorbs 30 cm³ of water. Find its porosity.
How would you tell quartzite from sandstone in the field?
Explain how a granite hill in eastern Rwanda could, over millions of years, become part of a metamorphic rock.

Show answersHide answers

Rock: a natural solid aggregate of minerals, e.g. granite. Mineral: a natural solid with a definite composition and crystal structure, e.g. quartz.
Granite cools slowly deep underground, giving crystals time to grow; basalt is lava that cools quickly at the surface, so crystals stay tiny.
See Figure 1: magma → igneous (granite, basalt) → weathering and erosion → sediment → deposition and cementation → sedimentary (alluvium, sandstone) → heat and pressure → metamorphic (quartzite, schist) → melting → magma.
$\rho = \frac{594}{200} = 2.97\ \mathrm{g\,cm^{-3}}$ : within the range for basalt.
$\frac{30}{250} \times 100\% = 12\%$ .
Sandstone breaks around its grains and feels gritty; quartzite breaks through the grains with a smooth, glassy surface and is much harder (a nail barely scratches it).
The granite is weathered and eroded; its sand and clay are carried away and deposited, then buried and cemented into sedimentary rock; deep burial under heat and pressure changes that rock into a metamorphic rock such as quartzite or schist.

Quick check

Match each rock to its group.

Tap one on the left, then its partner on the right.

Key points

Igneous rocks form from cooling magma (granite, basalt); sedimentary rocks from cemented sediments (alluvium, sandstone, clay); metamorphic rocks from changed rocks (quartzite, schist, gneiss).
The rock cycle links them through cooling, weathering, erosion, deposition, heat, pressure and melting.
Identify rocks by texture, layering, vesicles, hardness, the acid test and density $\rho = \frac{m}{V}$ .
Porosity controls how much water a rock stores.

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 (the rocks of Rwanda and where they occur).

Photo: Quartzite outcrop (geograph 901061), Jonathan Wilkins, CC BY-SA 2.0, via Wikimedia Commons.

Figure 1, the worked examples and the exercises were drawn and written by Ganzaa.

Video: "Earth.Parts #27: Igneous, Metamorphic & Sedimentary rock types and the rock cycle", Earth.Parts, embedded from YouTube.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Three samples on a desk

2The rock cycle

3Characteristics of the three groups

Three samples on a desk

The rock cycle

Characteristics of the three groups