On the basis of genesis or mode of origin, rocks are classified into following three major groups.
1. Igneous Rocks
These types of rocks are formed by the solidification of molten magma in the interior of the earth.
When a rock is formed by cooling and solidification of magma, below the surface of earth, it is called plutonic igneous rocks. Due to relatively slow rate of solidification, plutonic igneous rocks have coarse grained structure.
If a rock is formed from the solidification of lava on the surface of earth, the rate of cooling is faster and rapid solidification takes place. This type of rock is known as volcanic igneous rocks. Volcanic igneous rocks have fine grained structure.
Examples of igneous rocks are granite, dolerite, basalt, rhyolite etc.
2. Sedimentary Rocks
These types of rocks are formed due to weathering and decomposition of earth crust or from any rock type.
When a rock on earth crust is weathered or decomposed and transported and redeposited, and subsequently consolidated and cemented partly or fully, then the new product is known as sedimentary rocks.
Weathering agents act on the surface of a pre-existing rock which may be igneous, sedimentary or metamorphic. These weathered products later become constituents of the new sedimentary rocks. The weathering agents involved are rain, frost, wind, temperature, river, sea etc.
From an engineering point of view, the most important sedimentary rocks are sandstone, shale and limestone.
3. Metamorphic Rock
Due to high pressure, high temperature as well as high shearing stresses on existing igneous or sedimentary rock masses, under the earth crust, re-crystallization of rocks takes place and resulting mass is known as metamorphic rocks.
The principle agents of high temperature and pressure involved in the re-crystallization can be the following
Earth movement and pressure
Liquid and gas, chiefly water
Due to the effects of the above agents, new minerals are formed. Such as