Classification of Rocks

Rocks are classified in four different ways:

1. Geological classification
2. Chemical classification
3. Physical classification
4. Practical classification

Geological classification

According to the mode of origin, rocks are divided into three principal classes or groups, namely:

1. Igneous rocks
2. Sedimentary rocks
3. Metamorphic rocks

Igneous rocks

The molten material present in the inside portion of the earth’s surface is known as magma and this magma occasionally tries to come out to the earth’s surface through cracks or weak portions. The rocks which are formed by the cooling of magma are called igneous rocks. The portion of lava which comes outside the surface cools quickly and forms a rock of non-crystalline nature called as trap or basalt. The rest which remains inside the earth undergoes cooling at a slow rate and results in the formation of a rock of crystalline variety known as granite. The igneous rocks are classified into the following three types.

1. Plutonic rocks: They are formed by the cooling of magma at a considerable depth from the earth’s surface. The cooling is slow and the rocks possess coarsely grained crystalline structure. This rock is mostly used for construction purposes. Granite is the leading example of this type of rock.
2. Hypabyssal rocks: They are formed by the cooling of magma at a relatively shallow depth from the earth’s surface. The cooling is quick and, hence, the rocks possess a finely grained crystalline structure. Dolerite is an example of this type of rock.
3. Volcanic rocks: In the case of these rocks, solidification of magma takes place on or near the surface of the earth. The cooling is very rapid as compared to the previous two cases. Hence, the rocks are extremely fine grained in structure. Basalt is an example of this type of rock.

Sedimentary rocks

These rocks are formed by the weathering action of natural elements on the original rock and subsequent transportation by air, river, glacier and sea and deposition at a different locality. The following four types of deposits occur:

1. Residual deposits: Some portion of the products of weathering remains at the site of origin. Such deposit is known as a residual deposit.
2. Sedimentary deposits: The insoluble products of weathering are carried away in suspension, and when such products are deposited, they give rise to sedimentary rocks.
3. Chemical deposits: Some material that is carried away in solution may be deposited by some physio-chemical process such as evaporation and precipitation. It gives rise to chemical deposits.
4. Organic deposits: Some portion of the product of weathering gets deposited through the agency of organisms. Such deposits are known as organic deposits.

Examples of sedimentary rocks are sandstone, limestone, gypsum, lignite, etc.

Metamorphic rocks

These rocks are formed by the change in character of the pre-existing rocks. Igneous as well as sedimentary rocks change in character when they are subject to great heat and pressure. The process of change is known as metamorphism. Table 1.1 gives the names of the original and metamorphic rocks. Mineral composition and texture of a rock represent a system which is in equilibrium with its physio-chemical surroundings. Increase of temperature and pressure upsets this equilibrium and metamorphism results from an effort to re-establish a new equilibrium. In this process, original constituent minerals, which are unstable under the changed conditions, are converted into newer ones, which are more stable under the changed conditions. These minerals are arranged in a manner, that is more suitable to the new environment. It should, however, be noted that changes produced by weathering and sedimentation are not included in metamorphism.

There are three agents of metamorphism, namely heat, pressure and chemically acting fluids. Heat may be supplied by the general rise of temperature with depth or by igneous magma. Pressure may be developed due to the load of rocks or movement of the earth. Chemically acting fluids play a passive role only and they do not take active part in the process of metamorphism. Pressure may be uniform or directed. Uniform pressure may be applied to solids and liquids. Directed pressure or stress can exist only in solids and it is converted into uniform pressure if applied to liquids. Following are the four types of metamorphism that occur with various combinations of heat, uniform pressure and directed pressure.

1. Thermal metamorphism: Heat is the predominant factor in this type of metamorphism.
2. Cataclastic metamorphism: At the surface of the earth, temperature is low and metamorphism is brought about by directed pressure only. Such metamorphism is known as cataclastic metamorphism.
3. Dynamo-thermal metamorphism: There will always be a rise in temperature with an increase in depth. Hence, heat in combination with stress brings about the changes in the rock. Such metamorphism is known as dynamo-thermal metamorphism.
4. Plutonic metamorphism: Stress is effective only up to 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. Metamorphic changes at great depths are, therefore, brought about by uniform pressure and heat. Such metamorphism is known as plutonic metamorphism.

Various types of metamorphic rocks that originated from various types of rocks are given in Table 1.1.

 

Table 1.1 Various Types of Metamorphic Rock and Its Origin

Name of the original rock
Igneous	Sedimentary	Name of the metamorphic rock
		Gneiss
Granite	Limestone	Marble
	Sandstone	Quartzite
	Clay	Slate

Chemical classification

On the basis of dominant chemical composition, the building stone may fall into any of the following three groups:

1. Silicious rocks: In these rocks, silica predominates. These rocks are hard and durable. They are not easily affected by the weathering agencies. Silica, however, in combination with weaker minerals, may disintegrate easily. It is therefore necessary that these rocks should contain maximum amount of free silica for making them hard and durable. Granites, quartzite, etc. are examples of silicious rocks.
2. Argillaceous rocks: In these rocks, clay predominates. Such rocks may be dense and compact or they may be soft. Slates, laterites, etc. are examples of silicious rocks.
3. Calcarious rocks: In these rocks, calcium carbonate predominates. The durability of these rocks will depend upon the constituents present in the surrounding atmosphere. Limestone, marbles, etc. are examples of calcarious rocks.

 

Classification of Rocks According to Their Chemical Composition

Chemical classification	Composition	Name of the rock
1. Silicious rock	Predominance of silica	Granite, sandstone, basalt
2. Argillaceous rock	Predominance of clay	Slate, laterite, schist
3. Calcareous rock	Predominance of lime	Limestone, marbles, dolomite

Physical classification

This classification is based on the general structure of rocks. According to this classification, the rocks are divided into three types.

1. Stratified rocks: These rocks possess planes of stratification or cleavage and such rocks can easily be split up along these planes. Sedimentary rocks are distinctly stratified rocks.
2. Unstratified rocks: These rocks are unstratified. The structure may be crystalline granular or compact granular. Igneous rocks of volcanic agency and sedimentary rocks affected by movements of the earth are of this type of rocks.
3. Foliated rocks: These rocks have a tendency to be split up in a definite direction only. Foliated structure is very common in case of metamorphic rocks.

 

Classification of Rocks According to Their Structure

Physical classification	Characteristics	Typical name
1. Stratified rock	Has many strata	Slate
2. Unstratified rock	Does not have strata	Granite
3. Foliated rocks	Has foliated structure	Gneiss

1.2.4 Practical classification

Practical classification is based on the usage. Practically stones have been classified as granite, basalt, laterite, marble, limestone, sandstone and slate.