1. Define the term sedimentary rock and describe any two processes of its formation

Sedimentary rocks are rocks formed from sediments deposited either by water, wind or ice. These sediments are formed 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.

Two processes of sedimentary rock formation are:

  • Mechanical process: This involves the formation of sedimentary rocks from sediments accumulated from other rocks such as igneous and metamorphic rocks. These rocks settled in layers to form sedimentary rock e.g. Formation of sandstone, shale, etc.
  • Chemical process: Pre-existing igneous and metamorphic rocks are precipitated chemically from solution of one kind on the rock. Rock salts are derived from strata which once formed the beds of seas or lakes. Examples of rocks formed by chemical process include sodium chloride and gypsum, among others.


  1. Discuss any three ways in which sedimentary rocks are important to man
  • Some sedimentary rocks are used for road construction and for building houses.
  • Some sedimentary rocks are sources of some useful minerals such as coal and petroleum which serve as fuel energy to man.
  • Rock salt e.g. sodium chloride (table salt) is a source of body mineral needed for maintaining osmotic pressure of the blood.
  • Some beautiful rocks such as marble can be polished as ornaments for decorating floors, walls of buildings, churches, tombstone etc.


  1. Give an account of the economic importance of rocks


  • Sources of minerals: 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.
  • Sources of fuel: Sedimentary rocks like petroleum and coal are sources of fuel for domestic and industrial uses.
  • Construction purposes: Some rocks like granite and sandstones are quarried and used for roads, bridges and building constructions.
  • Tourist centres: Huge rock masses on mountains serve as tourist centres and attractions e.g. Olumo Rock in Abeokuta and Zuma Rock in Abuja.
  • As ornamentals: Some beautiful rocks such as marble can be polished as ornaments for decorating floors.
  • Sources of food nutrients: Rock salts such as sodium chloride (table salt) from sedimentary rocks provide minerals used in cooking our food.
  • Formation of soil: Soils are formed from the disintegration of rocks.
  • Sources of metals: Rocks are sources of metals which are derived from mines e.g. gold, silver, iron, aluminium, copper etc.
  • Domestic uses: Some rocks like granite are used for domestic purposes e.g. grinding stones.
  • Raw materials for industries: Some sedimentary rocks are used as raw materials for industries e.g. limestone is used for making cement. Dolomite and marble are used for manufacturing of glasses and house paints.


  1. Describe the characteristics and mode of formation of igneous rocks

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.


  1. Describe the characteristics of each of the following rocks types:
  • Granite: Granite rock is 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 granite rock is later exposed to the surface.
  • Limestone: Limestone rocks are formed from limestones deposited either by water, wind or ice. These limestones are deposited down in layers or strata one on top of the other and after a long period of time, they become hardened by compression to form limestone rocks. Limestone rocks are stratified rocks and the plane between two strata is called bedding plane. They are rocks formed from the remains of living organisms. When limestone rocks are formed from the remains of animals like corals or shells of fish, such rocks are called Calcareous.
  • Graphite: Graphite 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. Graphite is formed from coal.