Classification of rocks

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Learn : Classification of rocks,Geological classification of rocks, Physical classification of rocks, Chemical classification of rocks


Stones are obtained from rocks. A rock forms portion of earth’s crust having no definite shape or chemical composition. It is usually a mixture of two or more minerals and is not homogeneous.

            Rocks from which stones can be found for building purposes are classified in the following three different ways:

(i)        Geographically (depending upon how the rock was originally formed).

(ii)       Physically (depending upon what its structure is).

(iii)     Chemically (depending upon its chemical composition).

Geological classification of rocks :

Geologists classify rocks into three main categories:

(i)        Igneous, primary, unstratified or eruptive rocks.

Inner layers of earth are at such high temperatures that masses of silicates melt. This molten mass, known as magma, is forced up. Magma solidifies into rocks when it reaches the surface of earth and forms Basalts and Traps. If, however, the magma solidifies before reaching the surface of earth it forms solid crystalline rock known as Granite.

(ii)       Sedimentarhoty, aqueous or stratified rocks.

Surface of earth is subjected to the destructive action of rain, frost, winds and chemical actions. These destructive agents break up the surface of earth which gets further broken up when carried down by rains and rivers. In the journey of river from mountains to sea the velocity goes decreas­ing as the river moves further. As the velocity of river decreases so it goes on depositing the debris carried by it. The …… ones being the first to be deposited followed by the deposit of smaller and finer ones. Sand and silt get deposited in the voids of deposited debris-the latter having the binding properties (Fig. 1.1).

Classification of rocks
Classification of rocks

Due to seasonal variations the materials are deposited in layers. The deposit in layers continues for millions of years in which period the deposited layers subjected to enormous pressure of overlying layers and of flowing water get consolidated forming stratified, sedimentary or aqueous rocks. Sand stones and lime stones belong to this class.

Sedimentary rocks are often well stratified and show well-defined bedding planes. These rocks can be split and cleared easily both in the bedding direction and normal to the bedding planes giving building blocks of fairly regular shape. Properties of sedimentary rocks vary considerably depending upon the nature of sediment and the type of bonding that exists between the adjacent sediment grains. Sand stone formed by consolidation of sand deposits consist primarily of small rounded grains of silica. The sand stone will be rela­tively soft or hard depending upon the nature of binding material.

(iii)     Metamorphic rocks.

Due to structural changes in earth igneous or sedimentary rocks find their way deep in earth where they are subject to high temperature and heavy pressure which cause changes in texture or in mineral composition or in both resulting in the formation of new types of rocks known as metamorphic rocks; Following are some of the changes to “metamorphic rocks”: (i) Granite (igneous) changes to Gneiss, (ii) Sand stone (sedimentary) changes to Quartzite, (iii) Lime stone (sedimentary) changes to Marble, and (iv) Shale (sedimentary) changes to Slate.

  Physical classification of rocks,

Physically rocks are classi­fied as

 Statrified, rocks

Statrified, rocks showing distinct layers along which it can be easily split into this labs e.g., Slate, Sand scone and Lime stone,

  Unstratified rocks

Unstratified rocks which show no sign of stratification and can not be easily split into thin layers e.g., Granite, Basalt and Trap.

All sedimentary rocks are essentially stratified whereas all igneous rocks are unstratified. Metamorphic rocks may be either stratified or unstratified depending upon the type of rock that has undergone transformation.

Chemical classification of rocks.

Chemically rocks are classified on the basis of Chief Constituent Mineral as:


Agrillaceous, where the principal  constituent is clay (alumina Al2O3) as in Slate and Laterite;


Silicious, where the chief constituent is Sand (Silica SiO2) as in Quartzite and Granite; and


Calcareous, where the chief constituent is Lime as in lime stone and marble stone.

Classification of rocks
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