Earth’s crust consists of various types of rocks. The three main types of rocks are igneous rocks, sedimentary rocks and metamorphic rocks. Igneous rock covers the major portion of the Earth’s crust. Metamorphic rocks cover a lesser region of the crust than Igneous rock but more than the region of crust covered by sedimentary rocks.
What is a Rock?
A rock is a naturally formed, non-living earth material. Rocks are made of collections of mineral grains that are held together in a firm, solid mass.
Rocks are categorized mainly by the minerals they contain and their texture. All types of rocks have a unique specific set of minerals. A rock may be made of grains of a single mineral type, such as Quartzite. Much more commonly, rocks are made of a mixture of different minerals. The texture is a description of the size, shape, and arrangement of mineral grains.
These are known as Primary rocks. Because it forms the basis of the formation of other rocks. The word igneous comes from the word ‘Ignis’ that means fire. During volcanic eruptions, a huge amount of liquid rock matter comes out from inside the Earth’s crust. This is called Magma.
It is a hot and molten form of various minerals. When it flows through the surface of the volcano towards downward, it is called Lava. As the magma stops to come out from the volcano, lava begins to cool down and solidifies. The solid lavas or rocks are known as volcanic rocks or extrusive igneous rocks.
Some of the magma can’t come out from the surface of the Earth and solidifies inside the Earth’s crust. These rocks are known as Intrusive igneous rocks. These rocks have larger crystals than that of extrusive igneous rocks because the solidification of inner lava takes more time than outer lava. So, there are two types of rocks, one is extrusive rocks and the other is intrusive rocks.
Examples of extrusive igneous rocks are Basalt( Deccan plateau of India is made of Basalts), Pumice( use to make lightweight concrete), Obsidian( use to make ornaments), etc and intrusive rocks are Granite ( use to make buildings and roads), Diorite( use as construction materials) and so on.
As discussed above, we classified Igneous rocks into Intrusive, Extrusive, and Plutonic on the basis of the location where they were formed and their cooling time. Now we will categorize Igneous rocks into 4 types of rocks on the basis of their mineral composition and Physical properties. These are Felsic, Intermediate, Mafic, and Ultramafic Igneous rocks.
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Types of Igneous rocks
(ⅰ) Felsic Igneous rocks
The felsic igneous rocks contain a high amount of silica (SiO₂) around more than 63%, for example, granite and rhyolite. These are light in color. These are also rich in feldspar. Feldspar is a group of tectosilicates and forms compounds with sodium, calcium, and Potassium.
(ⅱ) Intermediate rocks
The Intermediate Igneous rocks contain 50% to 60% silica. Its examples are Andesite and Dacite and these have intermediate colour. These rocks are a mixture of felsic minerals (contain plagioclase) and mafic minerals (including hornblende, pyroxene or biotite). Continental margins are full of these types of rocks.
(ⅲ) Mafic rocks
The Mafic Igneous rocks have less silica content around 40% to 50% and more magnesium and iron contents. These form at high temperatures and are found deep inside the Earth’s surface and also around the tectonic plates. It is mostly composed of pyroxene, calcium-rich plagioclase(found in feldspar series) and a small amount of olivine. Examples are Gabbro and Basalt. These are dark in colour.
(ⅳ) Ultramafic rocks
The Ultramafic Igneous rocks have less silica around 40%. These have large quantities of Pyroxene and Olivine. Pyroxene and olivine are present in the upper mantle of the earth. Pyroxene is single-chain silicates or amphiboles. Olivines is iron and magnesium silicate.
It is a type of orthosilicate. It is olive and green in colour and also used as a gemstone. Examples of Ultramafic rocks are Picrite, Komatiite, Peridotite, etc. These have a lower viscosity than mafic rocks. These were the main types of rocks in the Igneous category.
Importance of igneous rocks.
Igneous rock is a very essential part of infrastructure like buildings, roads, bridges, etc. Granite is used as construction material, pumice is used as abrasive, diorite had been used in earlier times or in early civilization to make decorative artworks. It is also relevant in the case of study of Earth’s inside composition, temperature and environment.
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Sedimentary rocks form mainly by the cementing together of sediments, broken pieces of rock-like gravel, Clay, Silt, and Sand. Those sediments can be formed from the erosion and weathering of pre-existing rocks or by many processes mentioned below. Sedimentary rocks also include chemical precipitates, the solid materials left behind after a liquid evaporates.
These are formed from the deposition of sediments over an area for a long period of time. The formation of these types of rocks involves five processes that are Weathering, Erosion, Deposition, Compaction, and Lithification.
It is the first process of the formation of sedimentary rocks. In this process, the rocks are broken down into smaller pieces. There are three types of weathering which are physical weathering, chemical weathering, and biological weathering.
(a) Physical weathering
The process of Physical weathering includes temperature change, freeze-thaw, wind and waves. There is a creation of cracks in the rock due to abrupt changes in temperature, especially in hilly areas and the desert area where during the daytime the temperature is very hot and very cold at night.
When there is rainfall, water traps inside the cracks. As the water freezes, its volume increases and causes cracks to increase. This process continuously happens and finally breaks the rock into pieces.
In the desert, sandstorms or sand winds contain small sand particles. These small particles strike the big rocks. It causes wear away from rocks and forms small sediment. This process also happens in the rocky parts of coastal regions. The ocean waves, sea waves or tidal waves wear away the rocks through continuously striking.
(b) Chemical weathering
Chemical weathering occurs due to chemical reaction with the minerals of rocks. Rainwater combines with atmospheric gases like carbon dioxide, nitrogen oxides, sulfur oxides, etc, and falls as acid rain. These acidic chemicals react with rock’s minerals and degrade the rocks into smaller pieces.
(c) Biological weathering
Biological weathering is done by living organisms. When the plants or trees start to grow inside the crack of the rock, their roots become bigger and bigger. It creates large pressure inside the crack of the rock.
Over time, the rock gets apart. Burrowing animals like rabbits, mice, etc, make their holes inside these cracks and increase the size of cracks which can easily break down. Micro-organisms like Bacteria, Algae, and Mosses also stay on the surface of the rock. These organisms release chemicals on the surfaces that cause degrading of the upper layer of the rock.
This process involves movement or the transportation of the fragments of the rocks that are broken down during weathering. It is a mechanical process.
There are three main agents of erosion. These are Water, Wind, and Ice.
(a) Erosion by water:
It involves sediment’s transportation due to fluid movement with the help of water bodies like oceans, seas, rivers, lakes, etc. In the natural process, this is called the Fluvial process. The size of the segments are large and they are carried with the flow of water.
Larger sediments break into smaller pieces due to high speed and regular colliding with other hard objects. Due to this collision, they turn into tiny granules of sand.
(b) Erosion by the wind:
It is called the Aeolian process in the natural system. Aeolian means activities which are related to the wind in geology. Here, tiny fragments of weathered rocks are carried by winds and deposits where the speed of the wind slows down.
(c) Erosion by ice:
In the colder regions or the polar regions, sediments of the rocks are frozen inside the ice and carried by the ice and glaciers to the deposition areas.
It is the geological process in which the sediments are deposited on their deposition areas. It happens when the agents of the erosion aren’t able to carry the mass of load or aren’t able to overcome the force of gravity.
In the case of water and wind, when their speed slows down and their loading capacity increases then sediments are deposited inside the water bodies or near the banks and seashores (during the water erosion process) and in the sandy regions like sand dunes (during wind erosion).
After the deposition the next process is compaction. The deposition takes place several times which creates a large number of layers of fine granules of rocks or sand.
As the layers increase, the gap between the layers and fine particles of rocks decrease due to the heavy pressure of the upper layers. This squeezes the older sediments together and decreases their porosity which creates a compressed structure. This phenomenon is known as Compaction.
In this process, there is the conversion of loose sediments into rocks. This involves Cementation and Authigenesis. Authegenesis refers to the process in which reactions take place between various minerals and also between water in pores and minerals.These reactions help in the binding of sediments or cementation and form rocks.
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Types of Sedimentary Rocks.
There are three major types of rocks of sedimentary category on the basis of their origin. These are Chemical Sedimentary rocks, Clastic Sedimentary rocks and Organic Sedimentary rocks.
(a) Chemical Sedimentary rocks:
This type of sedimentary rocks forms when the water evaporates during compaction and only dissolved minerals in the water remain after that. Its examples are:- Chert, Dolomites, Flint, Rock Salt, Iron ore, etc.
(b) Clastic Sedimentary rocks:
These sedimentary rocks are formed from the older weathered and eroded sediments of rocks. These are buried deep in the earth and due to the accumulation of the sediments, compaction and cementation cause the formation of these rocks. Some examples are Breccia, Conglomerate, Sandstone, Siltstone, Shale, etc.
(c) Organic Sedimentary rocks:
These sedimentary rocks are the accumulation of organic debris like plant bodies and animal bodies. For example: Inside the water bodies when the animals died, then their debris accumulated on the water bed.
Over time there is a large accumulation of this debris and lithification takes place which causes the formation of organic sedimentary rocks. Some examples are Coal, Limestone, Chalk, etc.
Importance of the Sedimentary rocks:
These are used in construction as these are soft and easy to break. These are also helpful in research activities because most of the sedimentary rocks have fossils of old animals and plants which give us information about the past flora and fauna and climatic condition at that time.
Coal is an important source of fuel for power generation plants. Various fossil fuels that are used to run vehicles and build infrastructure also come from the sedimentary rocks. The world-famous buildings and monuments are made of these rocks. These are The Colosseum, Red Fort, Hawa Mahal of Jaipur, Jantar Mantar, Sphinx, etc. Sedimentary rocks have industrially, scientifically, and socially high importance.
These types of rocks are formed from the igneous rocks and sedimentary rocks under high pressure and high temperature. The word, Metamorphic, comes from “metamorphosis” which means there is a change in the existing form and finally becomes a different one from the existing one.
Igneous rocks, sedimentary rocks, and other metamorphic rocks that undergo metamorphosis are called Protolith as these are original rocks in the transformation. The places where metamorphic rocks are formed are deep in the Earth’s crust, line of the plate tectonic movement and igneous intrusion.
(Igneous intrusion is the dome-shaped formation which is created by excessive pressure of magma so that it comes out from the earth surface).
These places have excessive heat and pressure which change the protolith into metamorphic rocks. Metamorphism plays an important role in these rocks. Its mechanism involves a chemical reaction between minerals. During the chemical reaction, there is an exchange of the atoms, molecules and formation of new bonds that takes place that results in the formation of new compounds. These cause the formation of a new rock having new physical and chemical features.
Metamorphism is classified into two types. These are Contact metamorphism and Regional metamorphism.
a) Contact metamorphism:
It takes place at the igneous intrusion. Magma has extreme temperatures and comes out with pressure. When protolith comes in contact with the magma, there is contact metamorphism that occurs and forms the metamorphic rocks.
The area of its influence is usually small around 1 to 10 km and only affects the rock which is in contact with the magma. So, it is called low-grade metamorphism. These are low pressure and high-temperature metamorphism. In this phenomenon, there is recrystallization and recombination of minerals that take place in the contact area of rock with the magma.
Its examples are Marble (form from limestone and dolomite), Quartzite (form from sandstone), Hornfels (form from shale), etc.
b) Regional Metamorphism:
It takes place at the areas where two or more tectonic plates collide with each other generally at the base of mountains. During mountain formation, some parts of land go down the earth’s crust and some parts of it go up and it covers a large area and the region here is of extreme pressure and extreme temperature.
It modifies protolith rocks under extreme conditions and gives them different physical and chemical properties like various types of shades, texture, density, etc. These rocks have a foliated appearance which means due to high pressure and temperature, layers of rocks looking like thin sheets add over another layer to form new rocks.
Examples of the rocks which are formed from regional metamorphism are Slate (it is made from Shale), Gneiss (it is made from Granite), Schist (it is made from Slate), etc.
Types of metamorphic rocks:
There are two basic types of metamorphic rocks on the basis of textures. These are Foliated metamorphic rocks and Non-Foliated metamorphic types of rocks.
(a) Foliated metamorphic rocks:
These have layered structure and different types of color bands showing a mixture of various minerals. Their rocks grains are arranged in a regular manner in planes. Its examples are Slate, Schist, Gneiss, etc.
(b) Non-Foliated metamorphic rocks:
These rocks have rocks grains randomly arranged. These do not have a banded structure and texture. Some examples are Marble, Quartzite, etc.
Importance of the Metamorphic Rocks:
These rocks are commonly used for decorative elements, artwork, and building materials. Marble is a very important rock in the economic market. Tiles in the kitchen and bathrooms, statues, beautiful monuments like Taj Mahal which is known as one of the seven wonders of the world, etc, are all made of marble.
Metamorphic rocks are also a source of precious and economic minerals like diamond, gold, zinc, manganese, copper and lots more. Quartzite is a hard rock and is used in rail tracks.
Metamorphic rocks play an important role in the study of geology as scientists and researchers are able to find out the condition of temperature, stress, pressure and different minerals inside the earth’s crust and about the environmental condition of the mantle layer of the earth which is next layer down the crust. So, metamorphic rocks are a valuable resource of nature.
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