Classify the Rocks and Describe their Composition, Characteristics, and Usage.

Classification of Rocks

There are many types of rocks that can be classified into three main categories based on their fundamental nature and origin of formation:

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


(1) Igneous Rocks

These are the primary rocks of the Earth’s crust, which were formed when the Earth’s molten material cooled down. They were formed in the past when there were no animals and plants, so their remains are not visible in these rocks. They were the first to form on Earth, yet not all igneous rocks are the oldest. Due to the eruption of the volcano, the formation process of these rocks continues even today. But other rocks have certainly been formed from these. Therefore, it is better to call igneous rocks primary rocks. Characteristics of igneous rocks:

They are hard and dense and are formed when the liquid cools down. These rocks come in different sizes, shapes, and sequences. Water cannot penetrate them, which is why chemical changes do not take place in them, but physical disintegration is more common. They are easily weathered by the action of sunlight and water.

They have a place of deposition, but the joints are affected by erosion and weathering. They are compact and do not have layers. The illusion of layering occurs in igneous rocks due to the occurrence of a joint on the plane in the rocks and the re-deposition of another layer of lava on one layer of lava. Animal remains are also not found in them. These rocks are easily eroded. The classification of igneous rocks is done on various grounds. Among these, their position and structure are important.

Types of Igneous Rocks Based on Location

  1. Intrusive Rocks – Magma is a molten substance found in the inner part of the earth, with an average temperature of 5950 degrees Celsius. When the magma solidifies slowly, large deposits are formed, and it takes more time to form such rocks. These types of rocks are called intrusive igneous rocks. Examples of these are granite and gabbro rocks. These rocks are brought to the surface only after the upliftment or erosion of the earth’s surface. Rocks located at greater depths are called plutonic rocks, named after the word ‘Pluto’ which means ‘underworld’.
  2. Dyke rocks – When the internal liquid material is unable to reach the surface while coming out, it freezes in the form of a vertical wall, bridge, and dam between the sedimentary rocks of the path. These are called dykes, and they make right angles with the layers of the earth’s surface. These rocks are harder and stronger than the rocks that the lava entered. That is why on the surface of a country, lava walls are found in the form of walls or ranges. Lava rock is found in the Singhbhum district of Bihar, which is famous for the name of the latest dolerite. In North America, the stamped or ring wall is known as the Cleveland lava wall.
  3. Extrusive Rocks – When molten rocks solidify on the earth’s surface, they do not form rocks due to rapid cooling, and they are called extrusive igneous rocks. Generally, during the eruption of a volcano, molten material comes out on the surface, which is called lava. Slag rock is also called volcanic rock. Lava coming out of the joints has banked a wide part of the surface because due to fluidity, it flows like coal tar. Many such lava flows have formed the northwest cover in India and the Columbia Plateau of America. Obsidian is an example of this type of rock, which is like glass.

Types of Igneous Rocks Based on Composition

The chemical composition of igneous rocks varies, with a mixture of constituent elements in different amounts. However, all rocks contain a mixture of silica in small amounts. Based on composition, igneous rocks are divided into two classes:

  1. Silicic Rocks

In these rocks, the amount of silica is up to 80 percent. They are available in the upper layer of the earth and due to the deficiency of iron and magnesium, their color becomes yellow and faded. Generally, they have an abundance of crystals and feldspar. Due to their solid body form, silicic rocks are simple and are less affected by the process of erosion, which is why they are used in the construction of buildings.

  1. Micro-silicic Rocks

The amount of silica in the composition of these rocks is up to 52 percent. Iron oxide, lime, and aluminum predominate in these. There is absolutely no alkaline substance in them. The amount of sand and silica in these is less, due to which their color is deep and black. They are found in the marginal part of the earth. Although these rocks melt at higher temperatures than granite rocks, they are thinner than they are. These are more affected by seasonal action, and basalt is a simple example of this. These rocks are found more in oceanic volcanic regions and islands.

Apart from this, two additional categories can also be made:

(a) Secondary Igneous Rocks – In these rocks, the amount of silica ranges from 55% to 65%. Oxide is the main rock.

(b) Very Low Silica Igneous Rocks – These rocks have less than 45% silica content. Puedolites are the main rocks. The economic importance of igneous rocks lies in mineral deposits. The Larcenous Shield of North America is rich in valuable minerals like gold, iron, copper, etc. There are also gold and diamond mines in the Western Plateau of Australia and South Africa.

Sedimentary Rocks

Deposits of soil, pebbles, and stones transferred by water, wind, and ice are formed on the earth’s surface in the form of clay, which are called sedimentary rocks. These rocks contain small and large particles of different sizes and types, made of rock in which the remains of animals and plants that fell between the layers during the formation period are found. They are spread over three-fourths of the earth’s surface, but their sum in the composition of the earth’s surface is only 5 percent. Sedimentary rocks are most important for mankind. In the formation of sedimentary rocks, there is an abundance of crystal, clay, and lime. These rocks are classified based on their composition and place of origin.

Types of Sedimentary Rocks Based on Method of Formation:

  1. Rocks Made from Erosional Powder or Non-living Rocks – These rocks are deposited by transporting rock powder from one place to another by agents of erosion. The rocks formed in this way are called clastic rocks. Most of the rocks in the world fall into this category. There are two classes of them: (a) Sandy rocks and (b) Clayey rocks.

(a) Sandy Rocks – In these rocks, crystalline minerals predominate, which eventually break down into sand and gravel. Due to the fine particles of clay, lime, and silica, the hard and differently sized particles of sand harden and become rock.

(b) Clayey Rocks – These are the rocks formed by the deposition of minute particles of clay. The clay content in them is high, with white sugar, chloride, and aluminum as part of the composition. So these rocks are soft but impregnable.

  1. Rocks Made of Chemicals – Flowing water carries away some soluble elements with it. These chemical substances get deposited when the water is no longer flowing. The heavier particles settle in the lower layers and the lighter ones in the upper layers, and thus rocks are formed. These types of rocks are Shale, Gypsum, Rock Salt, Siltstone, and Anhydrite.
  2. Rocks Made of Organic Elements or Animal Rocks – Components of animals and plants gradually accumulate and turn into hard rocks. Some of them are high in lime, and some have carbon.

(a) Calcareous Rocks – Calcareous rocks have a high amount of lime and are formed by organisms such as the skeletons of animals and the remains of plants. These rocks are found more in the shallow seas of tropical and temperate zones. Limestone, dolomite, and chalk are examples of these types of rocks.

(b) Carbon-dominated Rocks – Carbon-dominated rocks are made from the remains of plants and trees. The carbon element is present in higher quantities in these rocks. As a result of heat and pressure, the modified form of the plant becomes a mineral. Coal and oil-bearing rocks are similar types of rocks.

Types of sedimentary rocks based on place of origin:

  1. Continental Rocks: Sedimentary rocks formed in desert and coastal regions are called continental rocks. Their layers are less organized and rigid, their particles are round and smooth, and their color is red. Sandstone and clay fall under this category.
  2. Marine Rocks: These rocks are formed in the shallow parts of the sea. The three categories are sandstone, shale, and limestone.

Economic Importance of Rocks:

Rocks are important substances for human life, as over 2000 things are obtained from them. Most of the coal is obtained from rocks. Earth’s mountain-building movements suppress large forests with soil through earthquakes and other outbreaks. After hundreds of years, they turn into coal. Building stones, industrial minerals, coal, iron, oil, and precious metals such as gold and silver are obtained from rocks. The upper crust of the entire earth, on which humans are settled, is made up of fine powder of rocks.

Economic Importance of Sedimentary Rocks:

Sedimentary rocks are valuable for human life. Coal and mineral oil, which provide power and are pillars of civilization, are products of these rocks. Lime and chalk are used to make cement, while sand is used to make glass. Sandstone is used in building construction, and metals like gold, tin, and copper are also found in these rocks. They provide homes for human residence, food grains for food, and power for industries. Smelting furnaces are made from fire clay metals, and salt, manure, and nitrogen are obtained from them.

Economic Importance of Metamorphic Rocks:

The form and properties of igneous and sedimentary rocks change due to heat, pressure, and earth movement, resulting in metamorphic rocks that are harder than their original forms. Mineral elements are also transformed and their properties become new. The original condition of the rocks remains, but after complete transformation, it becomes difficult to identify their original form, and they often become coarse. The universal form of metamorphic fields is the parallelepiped arrangement of crystals, and this type of texture is called scaling. This process of transformation is called metamorphosis. Altered shale is available in mountainous regions, and their expansion is less than that of sedimentary rocks.

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