Most rocks 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 rocks formed due to operation of such processes are defined as the Metamorphic rocks.
Temperature, pressure, stress and access of chemically reactive substances are the main agents, which are responsible for metamorphism.
Heat causes the minerals to recrystallise in the rock.
The process of change by heat is called thermal or contact metamorphism.
When molten magma or lava comes in contact with surrounding rocks, it bakes them and changes them into metamorphic rocks.
Similarly, the formation of the rocks due to tremendous pressure is known as dynamic or regional metamorphism.
Slate, gneiss, schist, marble and diamond are good examples of these rocks.
These rocks are hard and tough in comparison to the parent rocks from which they are formed.
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.