Geography and Environment

By the end of this unit, you must be able to differentiate among types of rocks and their
economic uses.

The unit objectives
By the end of this unit, you must be able to:
(a) define rocks
(b) state the types and characteristics of rocks
(c) outline the importance of rocks.

Definition of rocks

Do this in groups of four.
1. Go outside your classroom and collect different types of stones.
2. Observe the stones you have collected.
3. Describe their shape, colour and texture.
4. Write your findings in your notebooks.
5. Share your findings with other class members in a class discussion.

A rock is a naturally occurring solid made up of one or more minerals. Rocks form the solid part of the earth’s crust. A mineral is a natural inorganic substance possessing a definite chemical composition. Minerals are found below the earth’s surface. The scientific study of rocks is called petrology.

Types and characteristics of rocks

Individually, study the photograph below and answer the questions that follow.

                           Fig 6.1

1. Are the rocks shown on the photograph similar?
2. If your answer is no, identify the differences seen in the rocks.
3. Why do you think the rocks are different?

There are different types of rocks. Rocks are classified based on three main characteristics.
(a) chemical composition - mineral composition
(b) physical properties such as permeability, texture and the size of the mineral particles
(c) mode of formation.
Based on the above characteristics, there are three main types of rocks. They are igneous, sedimentary and metamorphic rocks.

Igneous rocks

Do this in pairs.
Study the photograph below and use it to answer the questions that follow.

                                 Fig 6.2

1. Tell your friend what you can see in the photograph.
2. Suppose you were staying in this area, which type of rock would you see?
3. What happens when the hot magma cools down?
4. Name the type of rocks found when the magma cools from within the earth’s crust.
5. Discuss your answers in class.

Igneous rocks are formed from the cooling and solidification of magma or lava. Rocks in the interior of the Earth are in a molten form. This is due to very high temperatures and pressure. Magma is forced out of the interior of the earth through cracks in the earth’s crust. The cracks through which
magma passes are also referred to as fissures or vents. The magma then cools and solidifies into hard rock. These rocks are called igneous rocks.

When magma that is on the earth’s crust cools, it forms rock crystals. Magma that cools slowly forms large crystals while magma that cools quickly forms small crystals.
The chemical composition of igneous rocks varies. This variation depends on the proportion of silica and basic oxides. There are four classes of igneous rocks based on their chemical composition. They are acidic, basic, intermediate and ultrabasic igneous rocks. Table 6.1 below shows the chemical composition of the different classes of igneous rocks.

Table 6.1 Chemical composition of igneous rocks

Type of rock	% silica	% basic oxides
Acidic	>65	35
Intermediate	55 – 65	35 - 45
Basic	455 – 55	45 - 65
Ultrabasic	<45	>55

There are two types of igneous rocks. They are intrusive and extrusive igneous rocks.

Intrusive igneous rocks
The word igneous comes from a Latin word that means fire. This is related to the melting process that forms these rocks. These are also known as plutonic rocks. They form from magma that cools and solidifies inside the earth’s crust. The magma cools and solidifies slowly leading to the formation of large crystals. Landforms of intrusive igneous rocks are sometimes seen on the
surface of the earth. This happens when the overlying rocks of the earth’s crust are removed by erosion. 

Examples of intrusive igneous rocks includes diorite, granite, gabbro, syenite and peridotite.

Fig 6.3 Igneous rocks.

Rocks that cool and solidify near the surface of the earth are known as hypabyssal rocks. Since they form at shallow depth, the rate of cooling is moderate. As a result, medium sized crystals are formed. Examples of hypabyssal rocks are porphyry, dolerite, porphyrite, diabase, lamprophyre and
granophyre.

Extrusive igneous rocks 

These rocks are also known as volcanic rocks. They form from lava that cools and solidifies on the earth’s surface. The lava cools and solidifies rapidly leading to the formation of small crystals.
There are two types of extrusive igneous rocks formed by lava flows and volcanic ejecta.

(a) Extrusive igneous rocks formed by lava flows

During volcanic eruptions, lava is ejected onto the earth’s surface. The lava flows for a long distance on the earth’s surface.
This happens before it cools and solidifies to form extrusive igneous rocks. Examples of extrusive igneous rocks include basalt and obsidian.

(b) Extrusive igneous rocks formed by volcanic ejecta

During volcanic eruptions, solid ash and semi-liquid materials are ejected out of the earth’s crust. This happens with great force.
The materials are pushed high up into the atmosphere. They then cool and solidify as they fall back onto the earth’s surface.
They form extrusive igneous rocks of various sizes and shapes. Volcanic ash and dust settle on the earth’s surface. They are compressed and hardened over a long period of time to form a rock called tuff.

Figure 6.4 shows examples of extrusive rocks.

Fig 6.4 Extrusive igneous rocks.

Sedimentary rocks

Do this in pairs.
Use the Internet and geographical documents.
1. Define sedimentary rocks.
2. Research on the formation of sedimentary rocks.
3. Identify three examples of sedimentary rocks.
4. Name some of the places where sedimentary rocks are found.
5. Discuss your finding in class.

Sedimentary rocks are formed by the deposition of materials on the earth’s surface and on the beds of water bodies.
They are formed through the process of sedimentation. Sedimentation is the process that causes mineral and other organic particles to settle and accumulate.
It also causes minerals to precipitate from a solution.

The sediments that form sedimentary rocks are derived from the pre-existing rocks through weathering and erosion. The sediments are transported and deposited in layers by wind, water or moving ice.
After a long period of time, the sediments are compacted to become hard rocks.
This is due to pressure from the overlying sediments and other materials.

Sedimentary rocks are classified according to their mode of formation and appearance.
The classifications are:
(a) mechanically formed sedimentary rocks.

(b) organically formed sedimentary rocks

(c) chemically formed sedimentary rocks.

(a) Mechanically formed sedimentary rocks

These rocks are formed when eroded rock materials are transported by agents of erosion. They are then deposited in layers either on land or in the sea. The rock materials are derived from pre existing rocks such as igneous or metamorphic rocks.

The rock particles are consolidated and hardened into hard rocks by pressure from overlying layers. Examples of mechanically formed sedimentary rocks include the following.

(i) Sandstone and gritstone – composed of rock particles, mainly sand.
(ii) Conglomerate, breccia and boulder clay – composed of rock particles larger than 2mm in             diameter.
(iii) Claystone, siltstone, shale, loess, mudstone – composed of very small rock particles.

                              Fig 6.5 Mechanically formed sedimentary rocks.

(b) Organically formed sedimentary rocks

These rocks are formed from the remains of organic materials. They are formed when the remains of plants and animals accumulate over time. They accumulate in layers on land or in the sea. Other materials accumulate over these organic remains. The weight of overlying materials exerts pressure on the organic materials. Eventually, they are consolidated and hardened to form rocks.

Organically formed sedimentary rocks are classified based on their composition and mode of formation. Table 6.2 below summarises their classification.

Table 6.2 Classification of organically formed sedimentary rocks.

Classification	Examples of rocks	Composition
Calcareous	Limestone, chalk, coral reef	Skeletons of tiny marine creatures e.g. coral polyps.
Ferruginous	Iron stone	Hydrated iron oxide.
Siliceous	Diatomite	Remains of diatoms rich in silica.
Carbonaceous	Coral (lignite coal, bituminous coal, brown coal, anthracite coal	Remains of plants.

   Fig 6.6 Organically formed sedimentary rocks.

Chemically formed sedimentary rocks
These rocks are formed when minerals are precipitated. They are also formed when solutions of salt evaporate and particles accumulate in layers. Soluble minerals such as salts, lime and other chemicals result from complicated chemical processes. They are then deposited on the lakes or ocean beds. High evaporation in areas with low rainfall also results in accumulation of salts.
The salts accumulate on the lake beds.

The mineral salts accumulate in layers over long periods of time. The overlaying layers exert pressure on the lower layers. The mineral particles are eventually consolidated and hardened into rocks.
Chemically formed sedimentary rocks are classified based on their mineral composition

Table 6.3 Classification of chemically formed sedimentary rocks.

Classification	Examples of rocks	Composition
Carbonates	Travertine, trona, dolomite	Calcium carbonate compounds
Sulphates	Gypsum	Calcium sulphate
Chlorides	Rock salt	Sodium chloride
Silicates	Flint	Silica
Iron stones	Limonite, hematite	Iron oxides

Fig 6.7 Chemically formed sedimentary rocks.

(c) Metamorphic rocks

Do this in pairs.
Make use of the Internet and geographical documents.
1. Define metamorphic rocks.
2. Research on their formation.
3. Get some clay and mix it with some water. Mould a small ball out of it. Put the clay ball in fire and observe what happens.
4. Discuss your finding and observations in class

These rocks are formed when pre-existing rocks are subjected to great heat or pressure.
Sometimes the rocks are subjected to both heat and pressure.
The pre-existing rocks are either igneous or sedimentary. The process of metamorphism leads to physical or chemical changes in the original rocks.

Table 6.4 Types of metamorphic rocks.

Work in groups of three. Use the Internet, geography textbooks and journals for your research.
1. Find out the characteristics of each of the types of rocks that you have studied in class.
2. Note down your findings in a notebook.

3. Discuss your findings within your groups.

4. Choose a leader to present your findings in a class discussion

5. Compare your findings with those of other groups

6. Write notes on the characteristics of the different rock types.

Work in groups of three.
1. Your teacher will take you to a field near your school.

2. Collect rock samples that fit the characteristics that you found out.

3. Take them to class.

4. Touch and feel them with your hands.

5. Classify the rocks according to their types by observing their characteristics.

6. Make a class presentation on your findings.

7. Ask your classmates questions on the characteristics of rocks they have collected.

Characteristics of rocks
(a) Rock colour
The colour of rocks gives a hint about its mineral composition. For example, gold is yellow and copper is reddish brown. The individual minerals that form these rocks have the same colour as the rocks.

(b) Rock texture and grain size This refers to the size of the individual grains that form the rocks. Rocks with very small grains have a very fine texture. Others are either medium or coarse grained.

(c) Rock structure
This refers to the arrangement of the minerals that form the rocks. Minerals have different shapes which determine the nature of rock formation. For example, quartzite crystals are hexagonal in shape and fluorite crystals are cube shaped.
Non-crystalline rocks like sedimentary rocks have a compact appearance. This is because they are made up of dense and solid minerals.
Granular rocks are made up of grain-like minerals as seen in sandstone.

(d) Mineral composition of rocks
This refers to the chemical composition of the minerals that form a rock. It determines the origin of the rock. The original materials of volcanic rocks are identified as basic or acidic. Those with high silica content are acidic while those that contain little silica are basic. Sedimentary rocks have a
chemical composition similar to the rocks from which they are derived. The chemical composition of a rock can be determined through laboratory tests.

(e) Rock hardness
This is the level of resistance of a rock to being broken or scratched. Rock hardness gives a clue to how a rock may have been formed. Generally, metamorphic rocks are more resistant than sedimentary rocks.
The hardness or resistance of a rock may be tested through different ways. They include scratching the rock with a fingernail, another rock, a knife or hitting using a hammer.

(f) Rock density
This refers to the weight of rocks. This can be determined by weighing the rock, tossing it or placing it in water. A heavy rock will sink in water and settle at the bottom of a container. A light rock of the same mass may float on water. For example, pumice rock will float on water while a granite rock will sink.
Rocks which form oceanic beds are heavier than those that form the continental crust.

(g) Mineral luster
Lustre refers to the brightness of rocks. The brightness is determined by the way light is reflected from the rock surface. There are two types of luster, metallic and nonmetallic.
Minerals with a metallic luster shine like metal. Examples of such minerals include gold, silver and lead.

Cleavage
This refers to the way a rock mineral breaks. Some rocks have a uniform cleavage. These ones break in one direction forming thin sheets. An example of this is mica. Other rocks break into irregular shapes as is the case with granites.

(i) Taste and odour
Rocks can either be porous or impervious. Porous rocks have pore spaces which allow water to pass through them. Hence such rocks are permeable.
Impermeable rocks do not allow water to pass through them. This is because they have very tiny pore spaces. Clay is impermeable despite the fact that it is porous. Its pore spaces allow water to
collect and remain within.

(i) Rock solubility
Some rocks such as limestone, chalk and trona easily dissolve in water.

Task 6.1
Discuss the distinguishing characteristics of the following.
(a) Igneous rocks.

(b) Sedimentary rocks.

(c) Metamorphic rocks.

The importance of rocks

Do this individually.
Read the passage below and answer the questions that follow.

Hirwa and his younger sister Muteteli were excited that they were closing school. They were to begin their long vacation. On their way home from school, they saw a road under construction. Beside the road was a quarry where stones were being dug out.

The stones were rectangular in shape. They were being loaded onto trucks that were on standby. Some rocks were being put into a large machine to crush them into smaller pieces.

When they reached home, their father was waiting to take them to town for shopping.
On their way, they saw a group of tourists taking photographs. They were standing around a rock that had a round top and narrow base.

After they passed the river, they saw some goats and cows licking some rocks. Their father told them that the rock contained salt.

As they neared town, they saw some buildings that were under construction.
Hirwa and his sister saw a blue lorry that they had seen at the quarry. The lorry was parked next to one of the buildings that was under construction. Some young men were offloading the rectangular blocks from the lorry. There was another lorry that was full of bags of cement. Muteteli asked his father where the cement was from. Their father told them that the cement was from a factory. He also told them that it was made by crushing limestone into powder. They then continued with their journey to the city of Kigali.
(a) From the above story, give some of the uses of the rocks that Hirwa and Muteteli saw.
(b) Apart from the uses identified in the story above, state other uses of rocks.

Rocks are of great significance to any country.

(a) Some rocks provide raw materials for industries leading to industrialisation. Coral limestone is used in the manufacture of cement. Cement is also exported and this earns the country foreign exchange. Rock chalk is used to produce chalk which is used on chalkboards. Diatomite is a
filtering agent in food industries.

(b) Some rocks such as trachyte and limestone are extracted to provide building and construction materials. Phonolite, sandstone and flint are also used for building. Gabbro and granite can be crashed to provide road chippings. Marble and gneiss are used to decorate buildings. Slate can be split into thin sheets for roofing. Loess can be cut into blocks used for building. Clay provides the
raw materials for brick making and pottery. Kaolin which is a type of clay is used in the manufacturing of ceramics and porcelain.

(c) Some rocks contain valuable minerals.
They are mined and exported to earn foreign exchange. Minerals include gold, diamonds, copper, silver and tin. Other rocks are sources of gemstones which are used to produce ornaments.

(d) Some rocks are extracted and used in art in the making of carvings. These carvings are sold to earn income. An example of such rocks is soap stones.

(e) Some rocks such as coal are a source of energy.

(f) Some rocks such as rock salt is used as a healthy substitute of common salt. This is because it is rich in minerals.

(g) Some rocks when weathered produce fertile soils which are suitable for agriculture. Limestone when ground produces lime which can be added to soils to improve them. Phosphates are used as fertilisers.

Some rock formations like granitic tors and coral reefs are tourist attraction sites. They earn foreign exchange.

(i) Some rocks store underground water. The water can be extracted and used for domestic, industrial or agricultural purposes.

Do this in groups of three.
Your teacher will take you for a field visit to a quarry.
1. Observe how the rocks are extracted.
2. Find out how they are used.
3. Find out their importance to the people who use them and to the people who extract them.
4 Observe and point out how some of the rocks have been used within your school compound.
5. Write your observations and findings in your notebooks.
6. Once you are back to school, present your findings in a class discussion.

Did you know?
• The earth’s crust is made up of rock.
• Ninety-five percent of the earth’s crust is made up of igneous rocks.
• Sedimentary rocks are extremely important resources that give us clues about the earth’s past.
• Metamorphic rocks are so called because they always begin as another type of rock.
• Rocks have been used by humans for millions of years.

1. Define a rock.

2. (a) Give the characteristics of igneous rocks.

(b) Give three examples of intrusive igneous rocks.

(c) Describe how extrusive igneous rocks are formed.

3. Explain how sedimentary rocks are formed.

4. (a) Classify sedimentary rocks based on their mode of formation and composition. Give examples of rocks in each class.

(b) Give the characteristics of sedimentary rocks.

5. (a) What are metamorphic rocks?

(b) Describe three ways in which metamorphic rocks are formed.

(c) Give four original rocks to match the metamorphic rocks that result after metamorphism.

6. Using specific examples, describe the importance of rocks to the economy of Rwanda.