Rocks – SS1 Geography Lesson Note (First Term)

A rock is any mineral material of the earth’s crust. Rocks differ in texture, structure, colour, permeability, mode of occurrence and resistance to denudation. They are classified into igneous, sedimentary and metamorphic rocks. Igneous rocks form from cooled magma, while sedimentary rocks form from sediments deposited in layers and hardened over time.

Behavioural objectives

By the end of the lesson, students should be able to:

• define a rock;
• name the three classes of rocks;
• state the characteristics of igneous rocks and sedimentary rocks;
• explain how igneous rocks and sedimentary rocks are formed;
• give examples of the types of igneous rocks and sedimentary rocks.

What is a rock?

The earth’s crust consists of rocks. A rock is any mineral material of the earth. It may be a combination of different mineral elements, such as silica, which contains silicon and oxygen. All rocks differ from one another in texture, structure, colour, permeability, mode of occurrence and degree of resistance to denudation.

What are the types of rocks?

Rocks are classified into three types:

1. Igneous rocks
2. Sedimentary rocks
3. Metamorphic rocks

What are igneous rocks?

Characteristics of igneous rocks

• They are crystalline in structure, i.e. they contain crystals.
• They do not occur in layers.
• They do not contain fossils.
• They are usually hard and impervious.
• They are resistant to erosion and the elements of weather.

How are igneous rocks formed?

Igneous rocks are formed by the cooling and solidification of molten magma ejected from beneath the earth’s crust. As the magma moves towards the surface, it comes into contact with lower temperatures. It then cools and solidifies to form igneous rock.

Types of igneous rocks

There are two major types of igneous rocks:

• Plutonic (intrusive) igneous rocks: These are formed inside the earth’s crust, when the molten magma cools and solidifies before it gets to the surface. This results in rocks with large crystals. Examples include granite, gabbro and diorite.
• Volcanic (extrusive) igneous rocks: These are formed when molten magma cools and solidifies on the surface of the earth. This results in rocks with small crystals. An example is basalt.

What are sedimentary rocks?

Characteristics of sedimentary rocks

• They occur in layers or strata.
• They may be coarse, fine, soft or hard.
• They do not exist in crystals, i.e. they are not crystalline.
• They may contain fossils.
• They are not resistant to erosion.

How are sedimentary rocks formed?

Sedimentary rocks are formed from sediments deposited by water, wind or ice. The sediments are laid down in layers or strata, one on top of another. After a long period, they become hardened by compression to form sedimentary rocks. Sedimentary rocks are therefore said to be stratified or layered.

Types of sedimentary rocks

There are three main types of sedimentary rocks. The classification is based on their mode of formation.

• Mechanically formed sedimentary rocks: These are formed from sediments of other rocks that have accumulated over some time. Examples include sandstone, breccia, shale, clay and conglomerates.
• Organically formed sedimentary rocks: These are formed from the remains of living organisms, such as plants and animals. Those formed from plant remains are called carbonaceous rocks, e.g. coal, peat, lignite and petroleum. Those formed from animal remains are called calcareous rocks, e.g. limestone and chalk.
• Chemically formed sedimentary rocks: These are precipitated chemically from solutions. Examples include potash, sodium chloride (common salt), nitrate, gypsum and dolomite.

Evaluation

1. Define a rock.
2. State four characteristics of igneous rocks.
3. Differentiate between plutonic and volcanic igneous rocks.
4. Name the three main types of sedimentary rocks and give one example of each.
5. What is the difference between carbonaceous and calcareous rocks?

Assignment

1. Explain how sedimentary rocks are formed.
2. Give two examples each of mechanically formed and chemically formed sedimentary rocks.