The earth crust consists of rocks. A rock is any mineral material of the earth. It may be a combination of different mineral element 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.


Types of rocks: Rocks are classified into the following; Igneous rocks, Sedimentary rocks and Metamorphic rocks.

Characteristics of Igneous rocks

  1. They are crystalline in structure ie they contain crystals.
  2. They do not occur in layers.
  3. They do not contain fossils.
  4. They are usually hard and impervious.
  5. They are resistant to erosion and element of weather.


Mode of Formation

They are formed by the cooling and solidification of molten magma, ejected from beneath the earth crust.

As the magma moves towards the surface, it comes in contact with lower temperature, hence cooling and solidifying to form igneous rock.


There are two major types of Igneous rock. These are:

  • Plutonic (Intrusive) Igneous rocks: These are Igneous rocks formed inside the earth crust as a result of the cooling and solidification of the molten magma inside the crust i.e before it gets to the surface. This results to rocks with large crystals. Examples include granite, gabbro and diorite.
  • Volcanic (Extrusive) Igneous rocks: These rocks form outside the earth crust due to the cooling and solidification of molten magma on the surface of the earth. This results to rocks with small crystals. Examples include basalt.



  1. Give two examples of plutonic igneous rocks.
  2. Mention three characteristics of igneous rocks.

Characteristics of Sedimentary Rocks

  1. They occur in layers or strata.
  2. The rocks may be coarse, fine soft or hard.
  3. They do not exist in crystals i.e they are not crystalline.
  4. They do not contain fossils
  5. They are not resistant to erosion


Mode of Formation

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


There are three main types of Sedimentary rocks. This classification is based on their mode of formation.

  • Mechanically formed Sedimentary rocks: These are formed from sediments of other rocks that have accumulated over a period of time. Examples include sandstone, breccia, shale, clay and conglomerates.
  • Organically formed Sedimentary rocks: These are rocks formed from the remains of living organisms eg plants and animals. Those formed from plant remains are known as CARBONACEOUS Rocks e.g Coal, Peat, Lignite, Petroleum, etc while those formed from animal remains are known as CALCAREOUS Rocks eg Limestone and Chalk.
  • Chemically formed Sedimentary rocks: These are rocks precipitated chemically from solutions. Examples include Potash, Sodium Chloride(Common Salt), Nitrate, Gypsum and Dolomite.



  1. State two characteristics of sedimentary rocks.
  2. Explain the mode of formation of sedimentary rocks.



  1. State five importance of rocks to man.
  2. Explain the mode of formation of igneous rock.
  3. Describe organically formed sedimentary rock.
  4. State the characteristics of igneous rock.
  5. Give examples of chemically formed sedimentary rock.



  1. They are formed in layers and derived from old rocks and the remains of dead organic This description refers to (a) metamorphic rocks (b) sedimentary rocks (c) igneous rocks
  2. Granite and diorite are examples of (a) igneous rock (b) sedimentary rock (c) metamorphic rock
  3. All the following rocks are sedimentary except (a) gneiss (b) limestone (c) chalk
  4. Chalk is a good example of (a) plutonic igneous rock (b) organically formed sedimentary rock (c) mechanically formed sedimentary rock
  5. Which of the following is not a rock? (a) Grit (b) Petrol (c) Micro-organisms



  1. State two characteristics of sedimentary rock.
  2. Mention three importance of rock to man


See also






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