Rocks from where building and other stones for construction purpose is obtained are classified in the following three different ways:-
- Geological Classification.
- Chemical Classification.
- Physical Classification.
1. Geological Classification
This classification depends on the mode of formation of rocks i.e. evaluation of the rock strata. Geological rocks are further divided into:-
(i) Igneous Rocks (ii) Sedimentary Rocks (iii) Metamorphic Rocks.
Igneous Rocks.
The rock formed directly by solification of molten material, either inside or on the surface of the earth is called igneous rock e.g. granite, basalt, trap, etc. The name igneous related to the word ignite, suggests fire. The rocks are formed by cooling the molten lava outside or inside the surface of the earth during the volcanic eruption.
Sedimentary Rocks.
The rocks formed by the consolidation of particles of the pre-existing rocks under the effects of weathering agencies such as rain, wind, water, and glaciers are known as sedimentary rocks e.g. gravel, sandstone, limestone, gypsum, lignite, etc.
Metamorphic Rocks.
The rocks formed from the pre-existing rocks within the earth’s crust under the influencer of variation temperature or pressure are known as metamorphic rocks—e.g marble, slate, gneiss, etc.
These are the agents of metamorphism namely heat, pressure and chemically acting fluids. The heat may be supplied by the general rise of temperature with the depth of igneous magma. The pressure may be developed due to the load of rocks or the movement of the earth. The chemically acting fluids play a passive role only and they do not take an active part in the process of metamorphism. The metamorphic rocks so formed are having their original structure and composition totally changed.
2. Chemical Classification.
This classification is based on the chemical constitution of a rock. Mineral or chemical which is present in greater percentage is considered and the rock is named accordingly. Chemically rocks are classified into the following three types.
(i) Siliceous Rocks (ii) Argillaceous Rocks (iii) Calcareous Rocks
Siliceous Rocks
The rocks having silica (SiO2) or sand as their major constituent are called siliceous. Rocks having silica in free form are hard and those having silica in combined form are liable to disintegrate. Examples are granite, sandstone, etc.
Argillaceous Rocks
The rocks having alumina (Al2O3) or clay as their major constituent are known as argillaceous rocks. They may be hard like slate which is brittle or soft like mud stone which loses its hardness when it comes in contact with water. Examples are slate, shale, laterite, etc.
Calcareous Rocks
The rocks having calcium carbonate (CaCo3) or lime as their major constituent are called calcareous rocks. These are easily acted upon by even dilute HCI. So durability of these stones will depend upon the atmospheric conditions. Examples are limestone, marble, kankar, etc.
3. Physical Classification.
This classification is based on the physical properties of rocks and shows the arrangement of different particles forming the mass of rock. Physically rocks are classified into the following three types:-
(i) Stratified Rocks (ii) Unstratified Rocks (iii) Foliated Rocks
Stratified Rocks
The rocks which show distinct signs of layers and can be easily split up into layers are called stratified rocks. The plane along which these layers can be separated is known as the cleavage plane. From these rocks stone in the form of thin slabs can be obtained. Examples are sandstone, limestone, slate, etc.
Unstratified Rocks
These are the rocks that do not show distinct signs of layers, so these rocks can’t be separated into layers. These are formed by the cooling of magma or lava. Examples are granite, trap, basalt, etc.
Foliated Rocks.
These rocks have a tendency to be split up in a definite direction only. These rocks can be split up into thin sheets. In the foliated texture, the arrangement of minerals is in the form of layers. The foliated structure is very common in metamorphic rocks. Examples are slate, schist, etc.
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