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Explain the important types of Rocks with suitable examples.

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Rock refers to the hard and resistant materials of the earth’s crust. But scientifically rock includes even soft and loose materials like chalk, clay, etc. So, rock refers to any solid materials, hard or soft of which the crust of the earth is formed. All rocks do not have the same chemical composition. But minerals have their own chemical compost in and physical prosperities. The earth’s crust is made up of various types of rocks

Types of Rocks: Rocks can be classified into three major groups on the basis of their origin or mode of formation. They are:

A. Igneous rocks: The term Igneous is derived from the Latin word “Ignis”, means fire. Thus the igneous rocks are formed by the cooling and solidification of molten material which is called magma. Igneous rocks are also called primary rocks, because they were the firs to be formed. As they are the rocks from which all other types of rocks are derived, they are also called parent rocks.

Igneous rocks are commonly classified on the basis of mode of formation into two major types. 

1. Intrusive rocks 

2. Extrusive rocks.

1. Intrusive rocks: The magma cannot escape out to the earth’s surface, it cols slowly inside the earth’s crust and hardens into rock. This type of rock is known as Intrusive Igneous rock. E.g. Granite and dolerite. These rocks can be divided into two type’s 

a. Plutonic rocks and 

b. Hybabyssal rocks.

  • Plutonic rocks: The rocks which are formed due to cooling of magma at great depth inside the earth are called Plutonic igneous rocks. 
  • Hybabyssal rocks: These are intermediate rocks between the extruded volcanic rocks and the deep plutonic rocks. They are formed due to cooling and solidification of magma in cracks, pores, crevices etc.

II. Extrusive rocks: Rocks formed by cooling and solidification of lava on the surface of the Earth is called extrusive igneous rocks. E.g. Basalt.

B. Sedimentary Rocks: These rocks which are formed due to aggregation and compactness of sediments are known as sedimentary. In other words, sedimentary rocks are formed bye day the deposition of sediments derived form older rocks, planets and animals remains by river, winds, glaciers etc and these sediments are hardened into rocks by pressure. As they are formed by the consolidation of sediments. They are called sedimentary rocks. They are also called stratified rocks,

a. Mechanically-formed rocks: The rock which have been formed form the accumulation of rock materials, derived form other rocks and have been cemented together are known as “ Mechanically formed rocks.” The mechanically formed rocks consist of sediments which have been carried and deposited by rivers, glaciers, winds or waves and cemented together with clay or line. On the basis of rock materials. These rocks can be divided into three main categories. They are: Rudaceous rocks, Arernaceous rocks and Argilious rocks. 

b. Chemically formed rocks: The chemical often settle down and hardened to form rocks known as chemically formed rocks. For example: Gypsum and rock salt, running water dissolves and carries chemicals and where evaporation takes place, these chemicals are deposited at the mouth of springs, caves or caverns or in lakes. Rock salt and gypsum are formed form deposit of salt in strata on the beds of lakes.

c. Organically formed rocks: Organic rocks are formed form the remains of organisms, i.e. of animals and plants. Examples: coal, limestone etc. On the basis of lime and carbon content, organically formed rocks can be divided into two kinds, namely

  • Calcareous rocks: Calcareous rocks are formed mostly from the remains of living organisms. These rocks contain calcium carbonate or lime. They include limestone and chalk. They are porous and soluble.
  • Carbonaceous rocks: These are formed due to the transformation of vegetative matter. Under the impact of heat and pressure the remains of plants are turned into hard layers. E.g. coal.

C. Metamorphic rocks: Rock which has been changes either in form or in composition without disintegration is called metamorphic rocks. These rocks are metamorphosed form igneous sedimentary rocks. Igneous and sedimentary rocks may undergo chemical and physical changes because of pressure and heat and form metamorphic rocks. The intense heat and pressure in the earth’s curst alters the composition and appearance of rocks completely or partially to produce a new type of rocks.In this manner metamorphic rocks are formed. Marble, Diamond, Quartzite, Ruby, Emerald are the examples of metamorphic rock.

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