On the basis of their formation rocks are of three chief types as shown in the diagram, namely— Igneous, Sedimentary and Metamorphic.

1. Igneous: These rocks are formed from cooling and solidification of molten material rising from interior of the earth called magma. When cooling and solidification happens just near the ground or above the ground they are called Extrusive Igneous Rocks. If cooling and solidification happens inside the earth, these rocks are called Instrusive Igneous Rocks. Basalt and Rhyolite are examples of extrusive igneous rocks and granite and gabro are examples of intrusive igneous rocks.

Igneous Rocks

Characteristics:

• When igneous rocks are exposed on the surface of the earth they are weathered and provide materials for other types of rocks. As a result the igneous rocks are called Primary Rocks.
• These rocks contain crystals whose size depends on rate of cooling.

2. Sedimentary Rocks: The material for sedimentary rocks may be derived from weathering and erosion of all types of rocks- igneous, metamorphic or old sedimentary rocks. They may contain fossils formed from remains of dead plants and animals.

There are two types of sedimentary rocks: inorganic and organic,

(a) Organic Sedimentary rocks are formed from organic matter derived from plant and animal remains, shell and skeletons of dead marine animals, decayed and decomposed plant and animal life as well as direct precipitation of minerals in solution of water.

(b) Inorganic Sedimentary rocks are formed due to weathering and erosion of all types of old rocks, clay, sand and mud deposited in oceans and lakes.

Characteristics:

• They have a layering arrangement.
• They may contain fossils.
• They may have ripple marks left by water.
• They are softer than any other rock.
• They are pervious or water can easily enter.
• There are no crystals in sedimentary rocks.
• Secondary ores of bauxite, manganese and other minerals are found in secondary rocks. Only sedimentary rocks are sources of fossil fuels. They also provide rich soils and materials for building and construction industry.

3. Metamorphic Rocks: Igneous and sedimentary when subjected to pressure and heat form sedimentary rocks. Examples- sandstone, granite, gnesis. Metamorphic rocks are made up by two ways and are found deep inside the surface of the earth.

The two ways are:

1. Transformation by heat. It happens because of hot magma, which solidifies the rocks that comes into its contact.

2. Transformation by pressure. Sometimes the pressure of upper rocks transforms the structure of lower rocks.