Rock
Major types of rocks
Rocks forming the crust of the earth differ in mineral composition, colour, texture and other physical properties. Rocks are classified into three major types on the basis of their origin. These three types are (i) Igneous rocks (ii) Sedimentary rocks and (iii) Metamorphic rocks. Each major type is further sub-divided on the basis of their physical and chemical features and forms of their occurrence.
Igneous Rocks
Igneous rocks are those rocks, which are formed by the process of solidification of molten rock materials called magma. Such molten magma originates from the interior layers of the earth. On the formation of rocks igneous rocks are divided into two types.
1.     Plutonic rocks
2.     Volcanic rocks
1. Plutonic rocks
Plutonic rocks are those which have solidified at some depth below the surface of the earth. This produces a coarse grained texture. Granite and dolerite are examples of plutonic rocks.
2. Volcanic rocks
At the time of eruption of the volcano the magma, which comes on the surface of the earth cools, down and formed the rocks. Such rocks are called volcanic rocks. Volcanic rocks consist of crystals of small size. These rocks are called fine- grained rocks. Basalt is an example is an example of fine-grained volcanic rock.
Characteristics of igneous Rocks
1.     Igneous rocks do not contain spherical grains.
2.     These types of rocks are found in the volcanic regions.
3.     Igneous rocks are not piled up. These are formed solid.
4.     Igneous rocks do not contain fossils.
5.     Igneous rocks are hard.
Sedimentary Rocks
Sedimentary rocks are those rocks, which are formed by the process of consolidation of sediments deposited commonly in water bodies like seas or lakes. The sediments are derived by the denudation of the rocks exposed on the land by rivers, glaciers, winds and waves. Sedimentary rocks are sub-divided on the basis of the nature of sediments. The sub-divisions are as follows: (i) Inorganic Sedimentary rocks and (ii) Organic sedimentary rocks.
(i)    Inorganic Sedimentary rocks
The rocks formed by the different types of minerals, sand soil, stone, etc. is called inorganic rocks. For examples: sandstone, mudstone etc.
(ii)  Organic Sedimentary rocks
Organic sedimentary rocks are those derived by the accumulation of remains of organisms, such as shells of marine organism. Limestone and chalk are examples of rocks composed of organic sediments. Coal is also derived from organic sediments consisting of remains of plants.
Characteristics of Sedimentary Rocks
1.     Minerals and pieces of other rocks form it.
2.     It is layered.
3.     It is found only in the surface of the earth.
4.     It consists fossils of animals and plants.
5.     It is soft rock.
Metamorphic rocks
The word metamorphic means changed. Metamorphic rocks are those which are formed by the process of metamorphism of alteration of pre-existing and sedimentary rocks. Metamorphism means change of form which may be physical or chemical or both. During the process of metamorphism, the original minerals get altered and new minerals are formed. Rocks are altered by two types. They are as follows: (i) Thermal Metamorphism and (ii) Regional Metamorphism.
(i) Thermal Metamorphism
Metamorphism caused by high temperature is called thermal metamorphism. These changes alter the old minerals and new minerals are formed. Marble is the example of this type.
(ii) Regional Metamorphism
Metamorphisms caused by large-scale earth movements affect rocks over a large are. This is known as regional metamorphism. Slate is the example of this type.
Characteristics of the Metamorphism rocks
1.     Metamorphism rocks are generally coarsely crystalline.
2.     Metamorphism rocks are distinguished easily.
3.     Kyanite, andaluride, sillimanite, zoisite, wollastine, staurolite, etc. minerals are formed by this process.