In this unit, the will should be able to:
• Define the term rocks.
• State the types of rocks.
•State the characteristics of rocks.
• Outline the importance of rocks.
DEFINITION OF ROCKS
A rock is a naturally occurring solid
made up of two or more minerals.
Rocks form the solid part of the
A mineral: is a natural inorganic
substance possessing a definite
chemical composition. Minerals are
found below the earth’s surface.
Rocks are studied under the
scientific study called Petrology.
Quartz - Mineral
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
c) Their mode of formation
Therefore, there are three main types of rocks namely;
1. Igneous rocks,
2. Sedimentary rocks, and
3. Metamorphic rocks.
Igneous rocks are formed when magma cools and
solidifies inside or on the earth’s surface.
Igneous rocks have four classes according to the
chemical composition; acidic, intermediate, basic,
and ultrabasic rocks.
Type of rock Presence of
Presence of basic
Acidic Less than 65 35
intermediate 55-65 35-45
Basic 45-55 45-65
Ultrabasic Less than 45 Greater than 55
Table showing chemical composition of igneous rocks
CHARACTERISTICS OF IGNEOUS ROCKS
✓They are formed by the cooling of magma which solidifies
into or onto the earth’s surface
✓ They contain crystals after
✓ They do not contain strata
✓ They do not have fossils
(plant and animal remains).
TYPES IGNEOUS ROCKS
There are two types of igneous rocks. They are intrusive and
extrusive igneous rocks.
a) Intrusive igneous rocks: these are formed when magma cools
and solidifies inside the earth’s surface. They are also known
as plutonic rocks.
Examples: Diorite, dolerite, granite, gabbro, peridotite, diabase,
porphyry, and lamprophyre.
Notice: rocks that cool and solidify near the earth’s surface are
known as hypabyssal rocks. They include, porphyrite, diabase,
lamprophyre, and granophyre.
b) Extrusive igneous rocks: these are formed when magma cools and
solidifies on the earth’s surface. They are also known as Volcanic
There are two types of extrusive igneous rocks formed by lava flows
and volcanic ejecta.
❖Extrusive igneous rocks formed by lava flows: these are formed
lava flows a long distance on the earth’s surface before it cools
and solidifies to form extrusive igneous rocks. Example are basalt
❖Extrusive igneous rocks formed by volcanic ejecta: these are
formed when solid ash and semi-liquid materials ejected out of the
earth’s surface cool and solidify as they fall back onto the earth’s
surface. Examples are pumice and tuff rocks.
Sedimentary rocks are formed by erosion, deposition, and compaction of
weathered or eroded materials of former rock masses at the earth’s
surface and within water bodies. They are formed through the process of
Sedimentation: this is the
process that causes
mineral and other organic
particles to settle and
Sedimentation of Rocks Sedimentary rocks
CHARACTERISTICS OF SEDIMENTARY ROCKS
Characteristics of sedimentary rocks are the following:
❖ They contain both organic (plant and animal) and inorganic deposits
❖ They are laid down in strata (layers)
❖ They contain fossils (plant and animal remains)
❖ They are non crystalline because they do not form under heat.
They are laid down in strata
Skeleton of the bird-like specimen (Aurornis xui)
found in Yizhou Fossil & Geology Park, China
HOW SEDIMENTARY ROCKS FORM
There are three ways through which sedimentary rocks are
1) The pre-existing rocks are weathered and eroded and
broken into small crystals to form sediments (weathering
2) The weathered and eroded particles are transported and
deposited in layers by water, wind, and moving ice.
3) Thereafter, the sediments are compacted to become hard
rocks by the influence of pressure from the overlying
sediments and other materials.
TYPES OF SEDIMENTARY ROCKS
a) Mechanically (Physically) formed sedimentary rocks: these rocks are
formed when sediments are transported and deposited either on land
or sea by the agents of erosion, then after they are consolidated and
hardened to form hard rocks by the pressure from overlying layers.
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. clay stone: composed of very small rock particles.
b) Chemically formed sedimentary rocks: these are formed when salt
and other soluble minerals are precipitated, evaporate and
accumulate in layers by the influence of rain, temperature, and
Classification of chemically formed sedimentary rocks
Classification Examples of rocks Composition
Carbonates Travertine, trona,
Sulphates Gypsum Calcium sylphate
Chlorides Rock salt Sodium chloride
Silicates Flint Silica
Iron stones Limonite, hematite Iron oxides
c) Organically formed sedimentary rocks: these rocks are formed when
remains of plants and animals accumulate in layers over time. Then
after they are compacted by the pressure from overlying materials
hence to form hard rocks.
Classification of organically formed sedimentary rocks
Classification Examples of rocks Composition
Calcareous Limestone, chalk, coral
Skeletons of tiny marine
Ferruginous Iron stone Hydrated iron oxide
Siliceous Diatomite Remains of diatoms rich in
Carbonaceous Coral (lignite coal,
bituminous coal, brown
coal, anthracite coal)
Remains of plants
These rocks are formed when pre-existing rocks are subjected to
chemical or physical changes under the influence of heat and pressure.
Types of metamorphic rocks
Classification Type of rock
Characteristics of metamorphic rocks are the following:
❖ They are formed through a metamorphism process.
❖They are more resistant to erosion
❖ They are formed by chemical or physical changes from
the existing rocks
❖ They are crystalline and often have a squashed (foliated
or banded) texture.
FORMATION OF METAMORPHIC ROCKS
Metamorphic rocks are formed in two ways:
a) Contact metamorphism: this is when igneous rocks come
into contact with other rocks and change their chemical
and physical structure to form metamorphic rocks.
a) Dislocation (region metamorphism): this is when a large
region is subjected to different temperature conditions by
the advancing igneous rock. Instead dislocation occurs
along a line of weakness or fault follows a volcanic pipe.
CHARACTERISTICS OF ROCKS
Rocks are characterized by the following features:
✓Rock color: the color of rocks gives a hint about its mineral composition. For example; gold
is yellow and copper is reddish brown
✓Rock texture and grain size: this determines the size of the individual grain that form the
✓Rock structure: this is the arrangement of the minerals that form the rocks. It determines
the shape and the mode of formation of the rocks.
✓Rock composition: this is the composition of minerals that form the rock. It determines the
origin of the rocks
✓Rock hardness: this is the level of rock resistance to being broken or scratched. It
determines how rocks may have been formed.
✓Rock density: this refers to the weight of rocks. It determines how rocks are heavy.
✓Rock luster: this refers to the rock brightness. It determines how rocks reflect the light.
✓Rock cleavage: this refers to how rocks break. It determines the way by which rocks break
✓Rock taste and odor: this refers to rock porosity. It determines how rocks allow water to
pass or not pass through them
✓Rock solubility: this determines the level at which rocks dissolve in water. 1/11/2023
IMPORTANCE OF ROCKS
❖Some rocks provide raw materials for industries leading to industrialization
❖Some rocks are extracted to provide building and constructional materials for
example sandstone, limestone etc.
❖Some rocks contain valuable minerals such as gold, diamond, copper that are
exported to earn foreign exchange
❖Some rocks are extracted and used in art in the making of carvings that are sold to
❖Some rocks such as coal are source of energy for industrial and domestic uses
❖Some rocks such as rock salt are used as a healthy substitute of common salt
❖Some rocks when weathered provide fertile soils good for crop growing
❖Some rocks such as phosphates, calcium are used as fertilizers
❖Some rock formation like granite tors and coral reefs are tourist attraction sites
❖Some rocks store underground water which can be extracted and used for domestic
and industrial and agriculture
Therefore, rocks are used in promoting different economic sectors that boost the
country’s development. However they have some disadvantages.
DISADVANTAGES OF ROCKS
Some rocks have the following disadvantages on human activities:
❖Some rocks are obstacles and may hinder cultivation activities
❖They may also hinder road construction activities
❖Some rocks are very porous provide infertile soils that may reduce cultivation yield
❖Some rocks may hinder mining activities because they require much expenses for
❖Some rock formation results in rock mountainous may hinder transport and