Main types of rocks

Main types of rocks

Rocks differ in their properties, size and particles and mode of formation. On the basis of mode of formation rocks may be grouped into three types:-

1. Igneous rocks 

2. Sedimentary rocks

3. Metamorphic rocks

Igneous rocks

The word igneous is derived from the Latin word 'ignis' meaning fire. Igneous rocks are formed by the cooling of highly heated molten fluid material, known as Magma. The word Magma is derived from the Greek word which means 'dough'. It requires a greater quantity of heat to melt the rocks under the overlying pressure than at the surface. We do not know the exact depths at which magma forms but probably it is formed at different depths not exceeding 40km.

Sedimentary rocks

These rocks are formed by successive deposition of sediments. These sediments may be the debris eroded from any previously rock material  which may be igneous rock, metamorphic or old sedimentary rock. Sedimentary rocks have layered or stratified structure. The thickness of strata varies from few millimeters to several metres. So these rocks are also called stratified rocks. Generally, these rocks have some type of fossil between  their strata. 

Metamorphic rocks

Most rocks in mountainous regions show an evidence of change. All these in course of time become Metamorphic or changed forms of rocks. Metamorphic rocks are formed under the influence of heat or pressure on sedimentary or igneous rocks. Tremendous pressure and high temprature change the colour, hardness, structure and composition of all types of pre-existing rocks.