Rocks differ in their properties, size of particles and mode of formation. On the basis of mode of formation rocks are grouped into three types.
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. Molten rocks produce an increase in volume which is responsible for causing fractures or cracks in the crust. The overlying pressure gets weakened along these openings, thus forcing out the magma through them.
When magma is ejected to the surface, it is called lava. Igneous rocks are formed from solidified molten magma below or on the earth’s surface. As they comprise the earth’s first crust and all other rocks are derived from them, these are called the parent of all rocks or the primary rocks.
On the basis of their mode of occurrence, igneous rocks can be classified as extrusive or volcanic rocks and intrusive rocks. Extrusive igneous rocks are formed by cooling of lava on the earth’s surface. Gabbro and basalt are very common examples of such rocks. These rocks are found in volcanic areas. Deccan plateau’s regur soil in India
is derived from lava.
Intrusive igneous rocks are formed when magma solidifies below the earth’s surface. Deep seated intrusive rocks are termed as plutonic rocks and shallow depth intrusive rocks are termed as hypabyssal. Granite and dolerite are common examples of intrusive rocks.
These rocks are formed by successive deposition of sediments. These sediments may be the debris eroded from any previously existing rock which may be igneous rock, metamorphic or old sedimentary rock. Sedimentary rocks have layered or stratified structure. So these rocks are also called stratified rocks.
The individual rock particles are first broken from rocks and then transported by running water, ocean currents, glaciers or even by wind from one place to another. The process by which rock forming material is laid down is called sedimentation or deposition. The sediments are often loose, unconsolidated, soft rock material, in the beginning like sand and clay, but in course of time they get hardened to a compact material by excessive pressure and cementation to form sedimentary rocks.
Sandstone, shale, limestone and dolomite are examples of sedimentary rocks. Huge folded mountains of the world like Himalayas, and Andes are made up of sedimentary rocks. All the alluvial deposits of the world are also due to sedimentary accumulations. All river basins, particularly their plains and deltas, e.g. IndoGangetic plain and Ganga-Brahmaputra delta are good examples of sedimentary accumulations.
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 temperature change the colour, hardness, structure and composition of all types of pre-existing rocks. The process which bring about the change is known as Metamorphism. Metamorphic rocks are hard and tough in comparison to the parent rocks from which they are formed.
Different types of metamorphic rocks are found all over the world. In India, marble is found in Rajasthan, Bihar and Madhya Pradesh, whereas slates are available in plenty in Orissa, Andhra Pradesh and Haryana. In Kangra and Kumaun regions of Himalaya, slates of different colours are found.