Geography

TOPIC 2 Sustainable Development: Wealth Creation

Key unit competence:

By the end of this unit, I should be able to assess the economic importance of rocks and minerals.

6.1. Rocks: Definition, types and characteristics
Learning Activity 6.1.
Observe the rock samples provided to you and identify their distinctive

characteristics.

6.1.1. Definition
A rock is a natural aggregate of minerals in the solid state; usually hard and consisting
of one, two, or more mineral varieties. Rocks form the solid part of the earth’s crust.
Rocks may also include substances like clay, sandstones, shells and
corals. Rocks which contain metallic compounds are called ores.
6.1.2. Types of rocks
There are three major groups of rocks namely igneous rocks, sedimentary rocks
and metamorphic rocks. Their classification is based on the mode of formation and
the nature of constituting minerals. Characteristics of each rock group are briefly
described below.
i. Igneous rocks
The word igneous comes from the Latin word ignis, which means fire. Igneous rocks
are rocks formed by cooling of molten material from a volcano or from deep inside
the earth. This molten material from inside the earth is known as magma. Igneous
rocks are also called magmatic rocks or volcanic rocks. Their formation is associated
with the cooling and hardening of molten material from the interior of the earth.
ii. Sedimentary rocks
Sedimentary rocks are the result of the accumulation of small pieces broken off from
pre-existing rocks (igneous rocks, metamorphic rocks and sedimentary rocks) or
precipitation of dissolved minerals. Sedimentary rocks form when sediments become
pressed or cemented together or when sediments precipitate out of solution.
iii. Metamorphic rocks
The metamorphic rocks get their name from “meta” (change) and “morph” (form).
Metamorphic rocks are formed from pre-existing rocks due to increases in heat
and pressure which alter rock structure and chemical composition. Therefore,
sedimentary and igneous rocks can become metamorphic rocks.
There are four factors that contribute to the formation of metamorphic rocks:
• Heat or high temperature: this speeds up the chemical reactions that result in
metamorphic rocks. The heat is from magma, steam from hot water and rocks
sinking deeper into the warmer layer of the crust
• High pressure which changes the mineral and feel of the original rock.
• Nature of the parent rock which determines how resistance it is to
change.
• Time which determines the period required for the chemical reactions
to take place.
6.1.3. Characteristics of rocks
a. Characteristics of igneous rocks
Below are the characteristics of igneous rocks:
• Igneous rocks are hard, and water does not pass through their joints easily,
that is why they are less affected by erosion;
• They have a lot of minerals;
• They do not have strata or layers;
• They do not contain fossils (fossils are remains of plants and animals fixed in
rocks);
• The number of joints increases upwards in any igneousrock;
• Igneous rocks are mostly associated with volcanic activities and are mainly
found in the volcanic zones. That is why they are also called volcanic rocks.
Igneous rocks can also be characterized basing on their classification. According
to chemical and mineralogical characteristics, texture of grains, forms and size of
grains, and the mode of origin, igneous rocks are classified as follows:
i. Classification based on the amount of silica
• Acidic igneous rocks (having more silica: more 65% of SiO2
)
• Basic igneous rocks (having low amount of silica: less than 45% of SiO2
)
ii. Classification based on the chemical and mineral composition
• Felsic igneous rocks (composed of the dominant minerals of the light
group, Silicon, Aluminum)
• Mafic igneous (composed of the dominant mineral of dark group:
magnesium and iron)
iii. Classification based on texture of grains
• Pegmatitic igneous rocks, (very coarse grained igneous rocks) for
example, granite
• Phaneritic igneous rocks (coarse grained igneous rocks)
• Aphanitic igneous rocks (fine grained igneous rocks)
• Glassy igneous rocks (without grains of any size)
• Porphyritic igneous (mixed grained igneous rocks)
iv. Classification based on the mode of occurrence
• Intrusive igneous rocks: They are formed when the rising magma, during
a volcanic activity, does not reach the earth’s surface but rather cools
and solidifies below the surface of the earth. Intrusive igneous rocks fall
into two categories:
- Plutonic igneous rocks: are formed due to the cooling of magma very deep
inside the earth.
- Hypabyssal igneous rocks: are formed due to the cooling and solidification of
rising magma during volcanic activity in cracks, pores, crevices and hollow
places just beneath the earth’s surface.
• Extrusive igneous rocks: They are formed due to the cooling 
and solidification of hot and molten lava on the earth’s surface (examples are basalt,
Gabbro). Extrusive igneous rocks are further divided into two major subcategories:
- Explosive type: The igneous rocks formed by a mixture of volcanic materials
ejected during explosive or violent volcanic eruptions.
- Quiet type: The appearance of lava through minor cracks and openings on
the earth’s surface is called ‘lava flow’. The lava forms basallic igneous rocks

after cooling and solidifying.

           Figure 6. 114 Common Igneous rocks

b. Characteristics of sedimentary rocks
Sedimentary rocks have the following characteristics:
• Sedimentary rocks are the product of other rocks that have already formed;
• They appear in the form of layers or strata;
• They are formed from materials from the older rocks, plant and animal
remains;
• Sedimentary rocks are found over the largest surface area of the earth;
• Sedimentary rocks have various minerals because they are a product of
different sources;
• Most of the sedimentary rocks allow liquids and gases to pass through them
(permeable and porous);
• Sedimentary rocks are characterized by different sizes of joints;
• Sedimentation units in the sedimentary rocks having a thickness of greater
than one centimeter are called beds;
• As highlighted in the figure below, the composition of sedimentary rocks
includes clay, sand, rounded pebbles, angular fragments, calcium deposits
and organic carbon.

c. Characteristics of Metamorphic Rocks
The following are the characteristics of metamorphic rocks:
• They are harder than the original rocks. Therefore, they are not easily eroded;
• They do not split easily;
• They contain minerals;
• Some are made up of just one mineral, for example, marble;
• They have a different texture or feel from the original rock.
Metamorphic rocks present two distinctive physical characteristics: Foliated
metamorphic rocks and Non-foliated metamorphic rocks. Foliated metamorphic rocks
such as gneiss, phyllite, schist and slate have a layered or banded appearance that
is produced by exposure to the heat and pressure. Non-foliated metamorphic rocks

such as hornfels, marble, quartzite do not have a layered or banded appearance. 

Application activity 6.1.
 1. In which area of Rwanda do we find igneous rocks? Explain their characteristics.
 2. Observe rocks found in your environment and classify them in the major

rock groups.

6.2. Composition and properties of rocks
Learning activity 6.2.
Rocks are composed of physical and chemicals elements. Make a research on
internet and in other geographical resources and describe the physical and

chemical properties of rocks.

6.2.1 Composition of rocks
All rocks are composed of minerals. Composition refers to both the types of minerals
within a rock and the overall chemical make-up of the rock. The mineral that compose

the three types of rocks are presented in the table below. 

6.2.2. Properties of rocks
i. Physical properties of rocks
Physical properties of a rock can be intensive (hardness and softness) and
extensive (volume, total mass and weight). Rocks, whether igneous, sedimentary or
metamorphic, are subject to powerful stress or pressure by tectonic forces and the 
weight of overlying rocks. The physical properties of rocks determine their behaviour
and respective deformations when a rock is subject to stress such as folding, faulting
or warping, and their resulting landscape deformation (see the figure below).
• Stress refers to forces that constantly push, pull, or twist the earth crust. There
are three types of stress: tension (stretching), compression (shortening), and
shear (twisting or tearing).
• Strain is how rocks respond to stress whether by stretching, shortening,
shearing.
• The surface expressions refer to the structure of landforms resulting from
the stress depending on whether the rock is brittle (hard) or ductile (pliable).
Surface expressions can be folding (bending) or faulting (breaking). Brittle
rock breaks (brittle deformation) while ductile rocks like clay bend or flow
(ductile deformation).
The figure below presents different types of stresses that are naturally applied on

rocks, their resulting strains and surface expressions.

ii. Chemical properties of rocks
a. Sedimentary rocks
All water falling onto the earth as rain and running over the earth surface carries
minerals in solution. These minerals may precipitate by direct evaporation of water,
chemical interaction or by the release of pressure where underground water reaches
the surface. Sedimentary rocks formed as chemical precipitates include halite,
gypsum, silcretes, ferricretes, limestone, and dolomite. The table below gives details

on their chemical composition. 

b. Metamorphic rocks
Metamorphism involves the alteration of existing rocks either by excessive heat
and pressure or through the chemical action of fluids. This alteration can cause
chemical changes or structural modification to the minerals making up the rock.
Metamorphism process results in the creation of new minerals by the substitution,
removal, or addition of chemical ions. Metamorphism may consist of three
minerals, kyanite, andalusite and sillimanite. These are all aluminum silicates
having the same chemical formula (Al2SiO₅) but different crystal structures and
physical properties.
Below is an example of a simplified representation of sediments products and

resulting metamorphic rocks from sea beaches to far shelf.

c. Igneous Rocks

The major indicator for the chemical classification of igneous rocks is the amount of
Silica (SiO₂). Igneous rocks with a high proportion of silica exceeding 65% are said to
be acidic or felsic, for example, the granite found on an extensive part of Muhanga
District of the Southern Province. Where the amount of silica is very low (less than
45%), the rocks are said to be ultramafic or ultrabasic. Rock having intermediate
silica content comprised between 65% and 45% are said to be mafic or basic rocks.

Igneous rocks are classified according to their forming minerals (see the table
below). Mineral groups include Felsic minerals (feldspars and silica), mafic minerals
(magnesium and iron), and ultramafic minerals (low silica content). Some of these
rocks form underneath the earth’s crust and are known as intrusive magmatic rocks,
whereas other form from the volcanic lave that reached the earth’s surface, forming

extrusive volcanic rocks.

Application activity 6.2.
1. Referring to the properties of rocks, explain how rocks react to the stress
and the resulting landscapes?
2. Identify a sedimentary rock in your local environment and describe the

process under which it might have been formed. 

6.3. Impact of rocks: advantages and disadvantages on the landscape
and human
Learning activity 6.3
Make research using books and internet to explain briefly the advantages of

rocks on landscape and society.

6.3.1. Advantages of rocks on the landscape and human being
• Some rocks are more resistant to weathering and others are less resistant. This
difference in rock resistances provides various landscapes such as alternation
of elevated topographies (hills, mountains or interfluves) and depressions

 (valleys and low-lying areas) which are sometimes drained; 

• Gravel and sand, being among products of rock weathering make beautiful
landscape at some location of the earth. Also, the weathering of rocks 
provides different types of soils including sand, silt and clay which are useful at
varying points for agriculture.
• Some rocks present beautiful landscapes which may attract tourists;
• Some rocks store, purify water and act as water sources to rivers.




6.3.2. Disadvantages of rocks on the landscape and human
i. Disadvantages of rocks on landscape
• Hard and resistant rocks hinder the penetration of plant roots hence, limiting
the weathering process or hindering the growth of vegetation;
• Rock forming minerals have different colours. The difference in colours make
minerals to absorb differently the heat. Dark-coloured minerals absorb
much heat during daytimes and therefore expand, causing the cracking and
fragmentation of rocks.
ii. Disadvantages of rocks on human
• The sand can blow, rocks can roll risking injury to people;
• Light-coloured rocks reflect sunlight and increase the temperature around the
plants during the daytime;
• Some environments such as sand rocks (dunes, reg, erg, etc.) are not suitable
for human settlement because of lack of water and soils;
• Some rocks may reflect landscape with steep slopes where human activities

such as agriculture or settlement cannot be possible.

Application activity 6.3.
1. Identify in your local environment the objects made from different rocks.
2. Observe your school buildings and describe different rocks used as
construction materials.
3. With relevant examples, discuss the disadvantages of rocks on landscape

and society

6.4. Minerals
Learning activity 6.4.
Use internet and books to search on the following:
1. Types and characteristics of minerals
2. The use of minerals to the society
6.4.1. Definition and characteristics of a mineral
A mineral is a solid inorganic substance that occurs naturally in the earth’s crust. A
mineral deposit is a concentration of naturally occurring solid material in or on the

earth’s crust. Mineral resources are non-renewable. 

There are five characteristics shared by all minerals.
i. All minerals are formed by natural processes. They can form when magma
cools, when liquids containing dissolved minerals evaporate, or when
particles precipitate from solution.
ii. Minerals are inorganic. They are not alive and are not made by life processes.
Coal, for instance, is made of carbon from living things. Although geologists
do not classify coal as a mineral, some people do. Miners, for example,
generally classify anything taken from the ground that has the commercial
value as a “mineral resource”.
iii. Minerals are solid and have a definite volume and shape. A gas such as air
and a liquid such as water aren’t minerals because they do not have definite
shape.
iv. Every mineral is an element or a compound with a chemical composition
unique to that mineral.
v. The atoms in a mineral are arranged in a pattern that is repeated over and
over again.
The table below shows two examples of mineral crystals (salt and quartz) with

defined shapes:

Note. Fuels like oil are not minerals because, as explained above, they do not meet the
following criteria of minerals:
• Inorganic (Oil is organic)
• Naturally occurring
• Solid (Oil is a liquid, and natural gas is not solid)
• Have a specific chemistry (Oil is mixture of various hydrocarbons with varying
chemistry)
• Internal crystalline structure
However, while Petroleum is not a mineral, it can contain mineral particles as sand
which is not a mineral often contains Quartz which is a mineral.
6.4.2. Types of minerals and ores
The wide varieties of minerals that have been explored by man for general and
commercial purposes to satisfy his needs are of two types: metallic minerals and
non-metallic minerals.
i. Metallic minerals
Metallic minerals include:
• Industrial metallic minerals: iron ore
• Ferroalloy metallic minerals: manganese, chromium, cobalt, molybdenum,
vanadium, nickel.
• Precious metallic minerals: gold, silver and platinum
ii. Non-metallic minerals
This category of non-metallic minerals includes salt, tin, potash, asbestos and
sulphur.
Rocks or minerals worked because they contain valuable (profitable) elements are
usually called ore-deposits. Minerals are extracted in a mineral ore. For instance,
Aluminum comes from the ore bauxite. The iron comes from the mineral ore
Hematite. A mineral can also be called an ore, for example Hematite is a mineral that
can also be called an ore. A mineral is an ore if it contains useful substance that can be
mined at a high profit and be processed and refined into more useful materials. For
instance, Aluminum can be refined from bauxite, and made into the useful products.
These products are worth more money than the cost of the mining, so bauxite is an

ore. The table below gives details on the main economic mineral ores.

6.4.3 Physical properties of minerals
The most common minerals in earth’s crust can often be identified
 in the field basing on their basic physical properties such as their
 form, hardness, fracture, cleavage, colour, streak, density,
luster, mass, taste, odour, feel, magnetism as described
below:
i. Form: Definite geometrical forms called crystals can be recognized in
minerals. These are for example: cubic, acicular (needle shaped), columnar,

fibrous, reniform (kidney shaped) and nodular forms.

Pyrite (left) has a cubic form; Tourmaline (middle) is prismatic; azurite and malachite
(right) are often amorphous.
ii. Hardness: The hardness of a mineral can be tested in several ways. Most
commonly, minerals are compared to an object of known hardness using
a scratch test developed by Friendrich Mohs. He assigned integer numbers
to each mineral, where 1 is the softest and 10 is the hardest. This scale is

shown below.

If the gem minerals are excluded, the scale has only 7 numbers. Substitutes may be
used when the scale minerals are not available:
• Easily scratched by nail;
• Not so easily scratched;
• Can be scratched by a piece (a copper coin);
• Scratched easily by knife;
• Can be scratched by knife with difficulty;
• Scratched by window-glass;
• Window-glass is scratched by the mineral.
iii. Fracture: Freshly broken surfaces of minerals present characteristic fracture
surfaces. The following important types are noted:
• Conchoidal (vitreous): the fracture surfaces are curved with a concave
or convex form; for example, quartz.
• Even: the fracture surfaces are nearly flat; for example, in chert.
• Uneven: the fracture surface is formed of minute elevations and depressions;
for example, most of minerals.
iv. Cleavage: This is how the mineral breaks. Certain minerals split easily along
certain planes called cleavage-planes. These planes are parallel to certain
faces of the mineral crystal, or to faces of a form in which the mineral may
crystallize.
v. Colour: When a body absorbs all the seven colours that make up white
light it appears black, and when it reflects all the colours it appears white.
When a body reflects the green vibrations of white light and absorb the
other vibrations it appears green. Thus, the colour of a body depends on
the selective reflection and absorption of the different vibrations of white
light.
vi. Streak: The colour of the powder of minerals sometimes differs from the
mineral in mass. Different specimens of the same mineral might show
variation in colour, yet the streak is fairly constant.
vii.Luster: The amount and the type of reflection from the surface of a mineral
determine its brightness. There are several types of luster, including the
following:
• Metallic: The luster of ordinary metals.
• Vitreous: The brightness of broken glass, for example, quartz.
• Resinous: The luster resembling that of resin.
• Pearly: The luster of peal. This is commonly seen in minerals that present more
or less platy surfaces.
• Silky: The luster similar to that of silk; generally shown by fibrous minerals like
some varieties of asbestos.
• Adamantine: The luster of diamond.
 viii. Mass: The mass of a mineral can be used to identify its type.
ix. Density: The density of a mineral can also be used to determine its type.
x. Taste: Some of the minerals which are soluble in water give distinctive taste
but the character is not very useful in identification of minerals because
there are only a few minerals which are soluble is water. For example, we
get a saline taste in case of common salt, and alkaline in case of soda or
potash.
xi. Odour: Only a few minerals give characteristic odour, e.g. the odour of
garlic from arsenic compounds.
xii. Feel: Minerals differ in the sensation they give by touch, e.g. minerals are
smooth, greasy or rough.
xiii. Magnetism: Generally, iron bearing minerals are magnetic, but not
necessarily all iron bearing minerals are magnetic. Some non-magnetic
minerals like monazite are also slightly magnetic. The electromagnetic
minerals depend on the varying magnetism of different minerals.
6.4.4 Chemical properties of minerals
Some minerals are affected by the variations in temperature and the pressure on
the earth’s surface. Others vary in the structure depending on the percentage of
water that they loose with the change of the temperature and the pressure. The
chemical composition influences the destruction of the rocks and development of
new minerals.

Chemical properties of minerals are identified from their chemical composition. We
refer to two elements that are Silicon and oxygen. These are the two most abundant
elements in the earth crust. They constitute approximately 90% of the crust of
the earth. Then we distinguish silicate minerals and non-silicate minerals. Silicate
minerals (silicates) are minerals containing Silicon and Oxygen atoms usually with

one or more other elements. Non-silicates are minerals other than silicate minerals.

6.4.5 Value of minerals and manufactured products
Minerals provide the material used to make most of the things of industrial-based
society; roads, cars, computers, fertilizers, etc. In more than 1600 minerals identi-
fied in earth crust, only 200 are extracted for commercial and industrial purposes
and less than 1/3 are the most economically significant.
i. Value of minerals
Some minerals have high economic value because of their uses or they are rare and
beautiful. For example, germs or Gemstones is a mineral with a distinctive colour
which makes it expensive. That is why it is used for jewellery. The value of minerals
depends on the following various factors:
• Chemical composition: Minerals containing (a) rare metals, (b) rare earths, (c)
several acid radicals have high value.
• Rarity: Rarity in minerals may be classified as due to the quality of the specimen
being greatly superior to the average, or to the scarcity of the species, variety
or form.
• Commercial value: If a mineral has only small commercial value this may be
disregarded in its appraisal.
• Form of a mineral: Large, perfect crystals, with brilliant faces and many of
them, or groups of such crystals, other things being equal, are the most valuable forms of minerals.
• Beauty of a mineral: Theoretically beauty is not a factor in scientific valuation,
as it is an element of art rather than science, but practically it is one of the most
important factors in determining the value of mineral specimens
• Size of a mineral: The mineral may be valued in proportion to their size,
though crystals of fine quality increase in value much more than proportionally
to their size.
• Hardness of a mineral: It has but slight mineralogical value, but it contributes
much to the commercial value of gems and it is the chief property of value in
the abrasives.
• Unusual characteristics: Freak specimens are always more valuable than
those which lack unusual characteristics. Thus a twisted tourmaline should be
worth two or three times as much as a simple crystal. A beryl broken, by nature
into several pieces with quartz filling the space between the pieces, will be
correctly appraised at several times as much as the same crystal in one piece.
• Associated minerals: While the associated minerals do not, as a rule, increase
the value of a specimen, there are many instances in which they do. This is
particularly true if the associates give a clue to the genesis of the mineral.

Manufactured products from minerals

Application activity 6.4.
1. What are the five characteristics shared by all minerals?
2. Differentiate a mineral from an ore.
3. Explain the factors influencing the value of minerals.
4. Identify minerals that are extracted in your district and describe their

advantages and disadvantages. 

End unit assessment
1. Describe the distinctive characteristics of igneous rocks, metamorphic
rocks and sedimentary rocks and the places where they are found.
2. Explain the formation process of each major groups of rocks described
above.
3. Discuss the economic importance of rocks and minerals.
4. What is the difference between the physical and the chemical properties

of the rocks?