fotolia 2032309 XS

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 the rocks differ from one another in texture, structure, colour, permeability, mode of occurrence and degree of resistance to denudation.



All rocks are classified into three major types: igneous rocks, sedimentary rocks, metamorphic rocks based on their origin (mode of formation) and appearance.


  1. Igneous Rocks


  • Igneous rocks are crystalline in structure i.e. they contain crystals.
  • They do not occur in layers i.e. they are non-stratified rocks.
  • They do not contain fossils.
  • They are usually hard and impervious.
  • They are resistant to erosion and other elements of climate.

Mode of Formation

Igneous rocks are formed by the cooling and solidification 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

  • Plutonic (intrusive) 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. 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.
  • Volcanic (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.


  1. Sedimentary Rocks


  • They occur in layers or strata.
  • The rocks may be coarse, fine soft or hard.
  • They do not exist in crystals i.e. they are non-crystalline in structure.
  • They contain fossils of plants and animals.
  • They are not resistant to erosion.

Mode of Formation

Sedimentary rocks are rocks formed from sediments deposited either by water, by wind or by ice. These sediments are deposited 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 two strata is called bedding plane.

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

  • 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.
  • 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 shellfish, 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 forests, they are called carbonaceous rocks, e.g. coal, peat, lignite, petroleum, etc.
  • Chemically formed Sedimentary Rocks: These rocks are precipitated chemically from rock solutions. Examples include potash, sodium chloride (common salt), nitrate, gypsum and dolomite.

Importance of Sedimentary Rocks to Man

  • Sedimentary rocks are source of materials for construction.
  • They are source of minerals e.g. petroleum, coal and gold.
  • They determine the quality of soil nutrients.
  • They determine the availability and supply of underground water.
  • They provide tourists attractions. Example is Ogbunike cave in Anambra State.
  • They are also source of raw materials. For example limestones for cement.
  • They are also source of food e.g. rock salt.
  • They are used as medicine e.g. kaolin.
  • They are used for scientific research e.g. fossils.


  1. Metamorphic Rocks


  • Some may occur in layers or strata.
  • The rocks may be hard or soft.
  • They are not crystalline in structure.
  • They exist in different colours and texture.
  • They 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. Slate is formed from clay, marble is formed from limestone, quartzite is formed from sandstone, gneiss is formed from granite, and schist is formed from shale while graphite is formed from coal.


Differences between Sedimentary and Metamorphic Rocks

  • Metamorphic rocks are generally more resistant than most sedimentary rocks.
  • Metamorphic rocks are crystalline while sedimentary rocks are non-crystalline.
  • Sedimentary rocks are more permeable while metamorphic rocks are more impervious.
  • Metamorphic rocks contain non-ferrous minerals while sedimentary rocks contain hydrocarbons as minerals.
  • Sedimentary rocks generally sustain lower relief than metamorphic rocks.
  • The fossils in metamorphic rocks have been destroyed by heat and pressure while sedimentary rocks have beautiful fossils.
  • Sedimentary rocks are more jointed than metamorphic rocks.
  • Metamorphic rocks are formed out of existing rocks; whereas, sedimentary rocks are products of weathering. Metamorphic rocks are formed due to heat and intense pressure while sedimentary rocks are formed through depositing of weathered materials; remains of organic matter and evaporation of chemically-changed water.


Economic Importance of Rocks to Man

  1. Some rocks like granites are used for domestic purposes e.g. grinding stones.
  2. Rock salts such as sodium chloride (table salt) from sedimentary rocks provide minerals used in cooking.
  3. Huge rock masses on mountains serves as tourist centres e.g Olumo rock in Abeokuta and Zuma rock in Suleja.
  4. Some beautiful rocks such as marble can be polished as ornaments for decorating floors, walls of buildings, churches, tombstones, etc.
  5. Rocks are sources of metals which are derived from mines e.g. gold, silver, iron, aluminium, copper, etc.
  6. Soils are formed from the disintegration of rocks.
  7. Some rocks like granite and sand stone are quarried and used for road, bridge and building construction.
  8. Some sedimentary rocks are used as raw materials in industries e.g. limestone is used for making cement, dolomite and marble are used for manufacturing of glasses and house paints.
  9. Sedimentary rocks like petroleum and coal are sources of fuel for domestic and industrial uses.
  10. Some rocks are sources of minerals such as gold, diamond, limestone, petroleum, etc. which can be exported to provide foreign exchange to a country. Petroleum, coal, limestone are derived from sedimentary rocks while gold, diamond and tin are derived from igneous and metamorphic rocks.

You may also like...