Rock Formation: Meaning And Definition/Types/ And Economic Importance Of Rock In Agriculture

Agricultural Science

Topic: Rock Formation


  • Definition And Types Of Rocks
  • Modes Of Formation
  • Economic Importance Of Rocks In Agriculture
  • Other Economic Importance Of Rocks To Man


At the end of this article, readers should be able to:

1. Define rock.
2. Describe the types and characteristics of different rocks.
3. Describe the processes of rock formation.
4. State and explain the economic importance of rocks in agriculture.

Definition And Types Of Rocks

A rock is defined as any mineral material of the earth. The earth’s crust consists of rocks. A rock may be a combination of different mineral elements such as silica which contains silicon and oxygen.

Types Of Rocks

All rocks are classified into three major types: igneous, sedimentary and metamorphic rocks, based on their origin, mode of formation and appearance, all these rocks differ from one another in texture, structure, colour, permeability, mode of occurence and the degree of resistance to denudation.

1. Igneous Rocks


1. Igneous rocks are glassy in appearance.

2. They are crystalline in structure; that is, they contain crystals.

3. They do not occur in layers. That is, they are non-stratified rocks.

4. They do not contain fossils.

5. They are light and dark in colour.

Mode of Formation

Igneous rocks are formed by the cooling and solidation of molten rocks called magma ejected from beneath the earth’s crust. The magma, which results from high temperature and pressure beneath the earth, forces itself towards the earth’s surface through cracks.

As the magma moves towards the surface, it comes in contact with lower temperature hence it cools and solidifies to form igneous rocks.

Types Of Igneous Rocks

There are two types of igneous rocks which are:

a. Plutonic (or instrusive) igneous rocks: These are rocks formed when the molten magma cools and solidifies slowly before it gets to the surface of the earth to form large crystals.

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As a result of prolonged erosion, the plutonic igneous rocks will later be exposed to the surface. Examples of plutonic igneous rocks are granite, gabbro and diorite.

b. Volcanic (or extrusive) igneous rocks: These rocks are formed when the molten magma cools and solidifies rapidly on getting to the surface of the earth to form small crystals. An example of volcanic igneous rock is basalt.

2. Sedimentary Rocks

Characteristics of Sedimentary Rocks

1. They occur in layers, strata or sheets.

2. The rocks are coarse in texture.

3. They do not exist in crystals, i.e, they are non-crystalline in structure.

4. They contain fossils of plants and animals.

5. They are not resistant to erosion.

6. They are soft.

Mode Of Formation

Sedimentary rocks are formed from sediments deposited either by water, by wind or by ice. These sediments are accumulated in layers or strata, one on top of the other and after a long period of time, they become hardened by compression to form sedimentary rocks.

Sedimentary rocks are stratified rocks and the plane between strata is called bedding plane.

Types of Sedimentary Rocks

There are three types of sedimentary rocks, based on their process of formation. These are:

1. Mechanically formed Sedimentary Rocks: These are formed from sediments of other rocks that have accumulated and cemented together over a long period of time. Examples include: sandstone,breccia, shale, clay and conglomerate.

2. Organically formed Sedimentary Rocks: These are rocks formed from the remains of living organisms. When sedimentary rocks are formed from the remains of animals like corals or fish shells, such rocks are called calcareous rocks, e.g, limestone and chalk.

But when sedimentary rocks are formed from the remains of vegetable matter such as swamps and forest, they are called carbonaceous rocks. Examples include coal, peat, lignite, petroleum, etc.

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3. Chemically formed Sedimentary Rocks: These rocks are precipitated chemically from rock solutions. Examples include potash, sodium chloride (common salt), nitrate, gypsum and dolomite.

3. Metamorphic Rocks

Characteristics of Metamorphic Rocks

1. Some may occur in layers or strata.

2. The rocks may be hard and massive.

3. They are non-crystalline in structure.

4. They exist in different colours and texture.

5. Thay may contain fossils.

Mode of Formation

Metamorphic rocks are changed rocks. The rocks are formed from pre-existing igneous or sedimentary rocks which have been changed as a result of great heat and pressure.

The original character and appearance may be greatly altered or changed by such forces. Examples of metamorphic rocks include slate, marble, quartzite, gneiss, schist and graphite. For example, slate is formed from clay; marble is formed from limestone; quartzite is formed from sandstone; gneiss is formed from granite; schist is formed from shale and graphite is formed from coal.

Economic Importance Of Rocks In Agriculture

1. Formation of soil: Soil is formed from the disintegration of rocks.

2. Sources of plant nutrients: Plant nutrients like iron, calcium, potassium, etc are derived from rocks.

3. Construction purposes: Some rocks like granite and sand stones are quarried and used for road and farm building construction.

4. Erosion control: Rocks are used to make a pile up across sloppy lands to prevent soil erosion.

5. Obstacles to farm implements: Rocks are generally a source of hindrance to the free operations of farm implements.

6. Sharpening of farm tools: Some rocks are used for sharpening farm tools like knives and cutlasses.

7. Breaking hard objects: Some rocks are used for breaking hard objects like kernels and seeds.

8. Usefulness as Grits: Some rocks are used as grits in livestock feed formulation.

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9. Useful in fertilizer blending: Some rocks are useful in blending of fertilizers in agriculture.

10. Useful as liming materials: Rocks generally contain calcium and magnesium which are useful as liming materials.

11. Useful as feed additives: Some rocks are very useful as feed additives, e.g. Salt lick.

12. Sources of plant nutrients: Rocks are good sources of plant nutrients in the soil, e.g. Mica releases potassium in the soil.

Other Economic Importance of Rocks to Man

1. Domestic uses: Some rocks like granites are used for domestic purposes. An example is grinding stone.

2. Sources of metals: Rocks are sources of metals which are derived from mines, e.g. Gold, silver, copper and aluminium.

3. As ornamentals: Some beautiful rocks such as marble can be polished as ornamentals for decorating floors, walls of buildings, churches, etc.

4. Sources of fuel: Sedimentary rocks like petroleum and coal are sources of fuel for domestic, commercial and industrial purposes.

5. Sources of Food: Rock-salt, such as sodium chloride (table salt) from sedimentary rocks, provides minerals used in cooking food.

6. Sources of Minerals: Some rocks are sources of minerals such as gold, diamond, limestone and petroleum which can generate income to the nation.

7. Raw Materials for Industries: Some sedimentary rocks are used as raw materials for industries. For example lime-stone is used for making cement. Dolomite and marble are used to manufacture glasses and house paints.

8. Construction purposes: Rocks such as granite and limestone are used for construction e.g. Roads, buildings and bridges.

9. As centre of tourism: Rocks are also used as centre of tourism e.g. Olumo rock in Ogun State.

10. For Research Purposes: Some rocks are used by scientists for research purposes.

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