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Classification of rocks

Er. Parbhakar Dwivedi


The building unites ate chanted Irons the rocks which are classified in the following three ways:

(1) Geological classification

(2) Physical classification

(3) Chemical classification

(1) Geological classification: According to this classification, the rocks are of the following three types:

(i) Igneous rocks (ii) Sedimentary rocks (iii) Metamorphic rocks

(i) Igneous rocks: The inside portion of the earth‘s surface has high temperature In a to cause fusion by heat at even ordinary pressures. The molten or pasty tot" material in known as the lug-u and this magma occasionally tries to come out to the earth surface through cracks or weak portions. The rocks which are formed in the cooling of magma are known as the igneous rocks. The igneous rush are recognized in the following dim classes:

(a) Plutonic rocks: Such rocks are formed due to cooling of magma at a considerable depth from earth's surface. The cooling is slow and the rocks process coarsely grained crystalline structure. The igneous rocks commonly used in building industry are of plutonic type. The granite II the leading example of this type of rock.

(b) Hypabyssal rocks: Such rocks are formed due to cooling of magma at a relatively shallow depth from the earth’s surface. The cooling is quick and hence these rocks possess finely grained crystalline structure. The dolerite is an example of this type of rock.

(c) Volcanic rocks: Such rocks are formed due to pouring of magma at earth’s surface. The cooling is VCI’Y rapid as compared to the previous two cases Hence these rocks are extremely fine grained in structure They frequently contain some quantity of glass which is a nun-crystalline material. The basalt is an example of this type of rock.

(ii)Sedimentary nets: These rocks are formed by the deposition of products of weathering on the pre-existing rocks. All the products of weathering are ultimately carried away from their place of origin by the agents of transport. Such agents are frost. Rain, wind. Flowing water Etc. Following four types of deposits occur:

(a) Residual deposits: some portion of the products of weathering remain at the site of origin such a deposit is known as a residual deposit.

(b) Sedimentary deposits: The insoluble products of weathering are carried way in suspension and when such products are deposited. They give rise to the Sedimentary deposits.

(c) Chemical deposits: Some material that is carried away in solution may be deposited by some physic-chemical processes such as evaporation, Precipitation, etc. It gives rise to the chemical deposits.

(d) Organic deposits: Some portion of the product of weathering gets deposited through the agency of organisms. Such deposits are known as the organic deposits

The examples of sedimentary rocks are gravel, sandstone, limestone Gypsum, lignite, etc.

(iii) Metamorphic rocks: These rocks are formed by the change in character of the pre-existing rocks. The igneous as well as sedimentary rocks are changed in character when they are subject to great heat and Pressure. The process of change is known as the metamorphism.

Following four types of metamorphism occur with various combinations of heat, uniform pressure and directed pressure:

(a) Thermal metamorphism: The heat is the predominant factor in this type of metamorphism.

(b) Catalectic metamorphism: At the surface of earth, the temperatures are low and metamorphism is brought about by directed pressure only. Such metamorphism is known as the catalectic metamorphism.

(c) Dynamo-thermal metamorphism: There is a rise in temperature with increase in depth. Hence, the heat in combination with stress, brings about the changes in rock. Such metamorphism is known as the Dynamo-thermal metamorphism.

(d) Plutonic metamorphism: The stress is effective only upto a certain depth. This is due to the fact that rocks become plastic in nature at certain depths. At great depths a stage is reached when stress cannot exist as it is converted into uniform pressure because of the Plasticity of rocks. The metamorphic changes at great depths are therefore brought about by uniform pressure and heat. Such is known as the plutonic metamorphism.


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