Geography

Topic area: Physical Geography

Sub-topic area: Rocks and minerals

Key unit competence By the end of this unit, you should be able to compare different types of rocks and minerals of Rwanda and evaluate their importance.

Unit objectives 

By the end of this unit, you should be able to: 

• Identify the different types of minerals and rocks in Rwanda.

• Locate the major minerals and rocks in Rwanda. 

• Recall the economic importance of rocks and minerals.

Types of minerals and rocks in Rwanda

 Activity 4.1

Work in groups. Use geographical knowledge that you have on rocks. 

1. Go outside your school compound. 

2. Collect different types of rocks. 

3. Examine the characteristics of the rocks you have collected and classify them. 

4. Write down your findings for discussion in a class presentation. In Senior One, you learnt about rocks.

You defined rocks and studied the types and characteristics of rocks as well as their importance. You defined a rock as a naturally occurring solid that is made up one or more minerals. In Senior Four, you are going to learn about the types of rocks and minerals that exist in Rwanda, their distribution in the country as well as their importance.

In Rwanda the largest parts are made up of Burundian rocks (1050-980 million years). These are associated with the folded sediments of Western and Eastern Provinces. These are rocks that changed their mineralogical and chemical compositions due to metamorphism. The specific examples include schist and quartzite that were exposed forming granite. These rocks occupy the greatest part of Rwanda. The components of precambrian rock basement (Burundian and Rusizian) spread over the Kibaran belt. This is associated with the Kibara Mountains of the Democratic Republic of Congo. There are young volcanic rocks (90-40 million years) that are found in some parts of North-west, West and Southwest of Rwanda. These rocks are known as Cenozoic rocks. Beneath the surface of Rwandan land, there are large deposits of lava which never came out to form visible volcanic features.

These rocks are believed to have formed in the mid-tertiary and quaternary era. In the north- western part of Rwanda, there is clear evidence of recent volcanism. These volcanoes are part of the Birunga range of southwestern Uganda. There are also young rocks of sedimentary nature whose date of formation is traced far back in tertiary and quaternary periods.

Classification of minerals and rocks in Rwanda

The land mass of Rwanda is made up of rugged land that is heterogenous in nature. Rwanda’s relief is composed of broken, rocky and hilly surfaces that formed as result of a wide range of geomorphologic processes. These processes led to the formation of steep mountain slopes and deep valleys characterised by different types of rocks. There are three major types of rocks that are classified according to their mode of formation. The rocks are as discussed below.

(a) Igneous rocks 

Activity 4.2 

Work in pairs. Use the knowledge previously gained in Geography. 

1. Define igneous rocks.

 2. Describe their formation. 

3. Find out where they are found in Rwanda. 

4. Record your findings and discuss them in a class presentation.

The word igneous is derived from a Latin word “ignis” which means fire. Therefore, the igneous rocks refer to “fire formed rocks, or rocks born of fire”. These are rocks that result from the cooling and solidification of molten material known as magma in the fissures, vents or pipes and lines of weakness at or near the surface of the Earth. There are two types of igneous rocks. 

• Extrusive igneous rocks 

• Intrusive igneous rocks

Extrusive igneous rocks form when magma solidifies on the surface of the Earth. Examples of such rocks include: basalt, obsidian and pumice rocks. Intrusive also known as plutonic rocks form when magma solidifies in the interior of the Earth’s crust. Examples of these rocks include; diorite, granite, gabbro and dolerite.

Task 4.1 

Study the table below showing examples of igneous rocks found in Rwanda. Fill in the missing information.

(b) Metamorphic rocks 

Activity 4.3

 Do this in groups of three. 

1. Collect clay and mould blocks of bricks from it. 

2. Dry the bricks and put them in intense fire where you will monitor the clay. 

3. After they turn red, remove them from the fire and let them cool. 

 4. Observe the clay and compare the final rock with the clay before moulding. 

5. Record your observations and discuss your findings in class.

The word metamorphic comes from two Greek words; “meta” which means change and “morph” which means form. The word metamorphism means to change form. Therefore, metamorphic rocks are those that changed their mineralogical and chemical composition due to intense heat and pressure.

Examples of metamorphic rocks found in Rwanda include the following. 

• Slate 

• Marble 

• Quartzite 

• Schist

 • Gneiss

About 80% of the land surface in Rwanda is made up of metamorphic rocks.

Task 4.2

 1. Define metamorphic rocks. 

2. Identify examples of metamorphic rocks found in Rwanda.

(c) Sedimentary rocks 

Activity 4.4

 Work in pairs. Use the knowledge previously gained in Geography: 

1. Define sedimentary rocks. 

2. Describe their formation. 

3. Find out where they are found in Rwanda. 

4. Record your findings and discuss them in a class presentation.

Sedimentary rocks are formed from the weathered and eroded materials of the already existing rocks. The weathered eroded materials are known as sediments. These materials are compacted in layers.

The sediments are transported by wind, water or glaciers. They are then deposited in layers known as strata. These rocks are formed from rocks that were originally igneous or metamorphic. There are mechanically formed, chemically formed and organically formed sedimentary rocks.

Task 4.3 

1. Describe the formation of sedimentary rocks. 

2. Name 3 examples of sedimentary rocks found in Rwanda. 

3. Name the places where sedimentary rocks are found in Rwanda.

Characteristics of rocks and minerals 

Activity 4.5 

Work in pairs. Use the Internet, Geography textbooks and journals. 

1. Find out the characteristics of the different types of rocks.

 2. Describe the physical properties of each type of rock with your friend.

3. Record your findings and present them in a class discussion.

The different types of rocks have different characteristics. They are discussed below. 

Igneous rocks are characterised by the following features. 

• They are formed after the cooling and solidification of molten material. 

• They have a crystal appearance after cooling and solidification. 

• They do not have strata or layers. 

• They have no fossils.

Metamorphic rocks are characterised by the following features.

 • They are harder and more compact than primary pre-existing rocks. 

• The minerals in these rocks are recrystallised.

Sedimentary rocks are characterised by the following features:

 • They have fossils. 

• They have both inorganic and organic materials. 

• They have no crystalline look because they do not form under the influence of heat. 

• They have rock layers known as strata as a result of the different and successive deposition of sediments at different intervals.

Task 4.4 

1. Describe the characteristics of the following types of rocks.

 (a) Igneous rocks 

(b) Sedimentary rocks 

(c) Metamorphic rocks.

The distribution of major rocks in Rwanda 

Activity 4.6 

1. Find out the areas in Rwanda that contain the following types of rocks: 

(a) Igneous rocks. 

(b) Metamorphic rocks.

 (c) Sedimentary rocks. 

2. List them down and present them in a class discussion.

(a) Igneous rocks Igneous rocks are common in the Northern and Western Provinces of Rwanda. The table below gives a summary of some of some of the igneous rocks and their locations in Rwanda.

(b) Metamorphic rocks

 About 80% of the land surface in Rwanda is made up of metamorphic rocks.

(c) Sedimentary rocks

 In Rwanda, these rocks are formed and found in valleys and lowland areas of the Eastern Province and along the foot hills of the Northern Province more especially in Gicumbi district. The table below gives a summary of some of the sedimentary rocks and where they are found.

The distribution of major minerals in Rwanda 

Activity 4.7 

Use the two photographs provided below to answer the questions that follow:

1. Name the activity taking place in the photographs. 

2. Explain what the person in Fig 4.4 photograph is looking for. 

3. Differentiate between rocks and minerals.

4. Name four minerals mined in Rwanda. 

5. Name the places where the minerals you have listed are mined in Rwanda. 

A mineral is a valuable rock extracted from the Earth’s crust. It is a solid inorganic substance that occurs in nature with a definite chemical arrangement. It is an element or chemical compound that is crystalline and has been formed as a result of geological processes. Minerals are classified as metallic and non metallic. There are various types of minerals found in Rwanda. Examples of metallic minerals found in Rwanda include; gold and micro-diamonds. Non-metallic minerals in Rwanda include peat coal and methane gas. The table 4.4 shows the major minerals found in Rwanda.

Activity 4.8

 Do this in pairs. 

1. Draw a sketch map of Rwanda. Include all the good elements of a map on it. 

2. Locate the areas where the major rocks and minerals are found. 

3. Mark the rocks and minerals in different colours. 

4. Put your map on the classroom notice board.

Importance of rocks and minerals in Rwanda 

Activity 4.9 

Work in groups. The following photograph shows a quarry in Gicumbi district. Study it and use it to answer the questions that follow. You can use the Internet, Geography textbooks and other materials for your research.

1. Define the term quarrying. 

2. Distinguish between a quarry and mine. 

3. Name at least one product got from each of the places mentioned in (2) above. 

4. Examine the value of the products obtained from the area represented in the photograph above. 

5. Suggest measures that you would put in place to avoid such an effect on the environment. 

6. Record your findings and present them in a class discussion.

Activity 4.10 

Work in groups. Study the photograph below and answer the questions that follow. You can use the Internet, Geography textbooks and other materials for your research. 

1. Describe the mineral got from the process below. 

2. State the value of the mineral mined to the economic development of Rwanda as a country. 

3. Where this is mineral obtained from in Rwanda. 

4. Apart from the mineral mentioned in 1. identify other minerals found in Rwanda and explain their importance.

5. Analyse the effects of mining to the environment in Rwanda. 

6. Suggest possible measures that can be undertaken to avoid the destruction of the environment when mining. 

7. Record your findings and present them in a class discussion

Rocks and minerals are important to Rwanda. They are important in the following ways.

 (a) Formation of soils 

Rocks are broken down into tiny particles through the process of weathering. This leads to the formation of soil that supports plant growth. For example, the igneous rocks around the volcanic mountains in the Northern and Western Provinces of Rwanda have been weathered leading to the formation of fertile volcanic soils. These soils have supported crop production in these areas. These mountains include Mts. Muhabura, Karisimbi, Bisoke and Sabyinyo.

(b) Minerals 

Rocks provide humans with valuable minerals that are used in various ways. For example, micro- diamonds in Gicumbi and tin in Muhanga are igneous rocks. These are exported earning the country foreign exchange.

(c) Construction materials 

Rocks are used in various ways in the building of infrastructure. For example, igneous and sedimentary rocks are obtained from quarries to provide stones that are needed in building. Some coloured stones are used to decorate houses and to construct fences. Some igneous rocks especially in Musanze area are used in the construction of fences around homesteads to provide security.

(d) Road construction 

Closely related to the above, all weather roads are constructed using granite chippings. The construction of road pavements and slope stabilisation systems, require strong stones that are resistant to weathering and erosion.

(e) Research and study purposes 

Rocks and minerals are used in various studies by learners at different levels of education. Geologists use rocks to understand their discipline and plan for the future. Learners in secondary schools use them to understand the topic on rocks.

(f) Source of water 

The nature and texture of rocks prevailing in a given area will determine the volume of the underground water present in the area. The underground water are sources of springs, wells and boreholes. The impermeable rocks do not allow infiltration and percolation of water hence limiting the level and volume of water table. This leads to the absence of underground water. Rocks that allow proper circulation of water make large volumes of underground water that is beneficial to humans through springs, wells and boreholes.

(g) Source of energy

 Peat coal is used as a source of energy in some homes in Rwanda. Hot rocks found beneath the Earth’s surface are responsible for the generation of geothermal energy. This project is still underway in Rwanda but has already picked up in places like Eburru in Kenya.

Tourism development 

There are many rocks in Rwanda that attract tourists from all over the world. For example, in the Southern Province, there are wonderful cliffs and rocks such as “Urutare-rwa-Kamegeri and Urutare-rwaNdaba”. These, together with many others contribute to the revenue earnings of the government through foreign exchange .

(i) Ornamental stones 

The precious stones got from highly valued rocks such as gold and diamond are used as ornaments. Some individuals keep their wealth by investing huge sums of money in purchasing them. Countries or governments also make gold reserves as a security measures for their economy.

(j) Millstones and grindstones

 Low income earners whose economic abilities cannot allow them to access better food processing depend on metamorphic and granite rocks to process dry products that need pounding or grinding.

The negative contribution of rocks

 Case study 

The day I will not forget 

It was a Sunday afternoon when the sun slowly moved towards the west. I was sight seeing, enjoying the scenery of my environment. The aroma from the flowering plants that decorated the rolling hills of my neighbourhood, made me admire nature as a geographer. The sunshine slowly faded away. Its rays were replaced by the cool breeze and mild winds that blew gently against my skin. I decided to climb the slope slowly since my home was on the other side of the slope. As I walked on, I kept appreciating the rocks and plants on the side of the slopes. In a place not far from where I was, I saw a steep cliff. As I kept looking, a big rock broke off and fell down the slope. I was a bit scared by that. I then heard someone screaming for help from below. I decided to run down slope so that I could see what happened. As I ran, the sharp rocks on the ground kept piercing my feet. It became hard for me to run. When I got there, a lady had been hit by the falling rock. She was hurt and everyone around the place tried to help her. The presence of rocks in the area made it impossible for roads to be constructed in the area. It was therefore very difficult to get the injured lady to hospital. The beauty I admired just turned into a disaster. 

(a) Point out the negative effects of rocks to humans and to the development of an area. 

(b) Apart from the problems mentioned in the story, what are the other negative effects of rocks?

There are various disadvantages of rocks. They include the following.

 1. Areas with many rocks such as outcrops make it difficult to develop transport and communication infrastructure, especially roads. This is clearly witnessed in some parts of Northern and Western Provinces of Rwanda where some areas are inaccessible due to their rocky nature.

 2. In steep areas, falling rocks lead to serious accidents where houses or homes are destroyed. Sometimes, people and animals lose their lives. 

3. Rocks provide habitat to dangerous reptiles such as snakes which put the lives of the people at great risk. 

4. It is difficult and expensive to construct houses in rocky areas. Sometimes this makes some people give up on such important developments. 

5. The formation of sedimentary rocks leads to creation of young soils that barely support agriculture. 

6. Impermeable and non-porous rocks such as those that have high measures of clay form water-logged areas thus creating swamps. This leads to the formation of acidic soils that do not support a variety of crops. Swampy areas are good breeding areas for mosquitoes that cause malaria which claims many lives.

7. The coral rocks which are a form of limestone rocks interfere with navigation and development of ports. This is because they make accessibility to the coastal areas difficult. 

8. The weathered sand stones and gravel lead to formation of soils that are unproductive hence being a problem to the agricultural sector. 

9. Rocks which are porous allow the surface run-off to infiltrate and percolate to the deep parts of the crust. This way, they lower the water table beyond man’s reach. In such areas, water becomes a problem since there are no wells or springs. This is due to lack of surface water. 

10. Some rocks such as limestone are soluble. When it rains, they dissolve in water thus polluting the surface and underground water. This makes the water hard and unsuitable for consumption.

Did you know? 

• Rocks are constantly changing in a rock cycle. They change from igneous to sedimentary then to metamorphic. It takes millions of years for rocks to change.

• There are rocks that come from space called meteorites that are made up of iron. 

• Ores are rocks that include minerals that have important elements such as metals like gold and silver. 

• Most of the ocean floor is made of basalt. This igneous rock continues to flow out of Earth from chains of underwater volcanoes known as midocean ridges. 

• Diamond is the hardest mineral.