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Types of Rock – Igneous, Sedimentary and Metamorphic Rocks

Last Updated : 01 Apr, 2024
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Based on the origin of rocks, they are classified into three types – Igneous rocks, Sedimentary rocks, and Metamorphic rocks. There are also a few rock substances of organic origin which sometimes contain fossils, for example, coal, peat, etc.

Rock Formation takes place by naturally occurring accumulation of mineral particles. The solid part of Earth is made up of various types of rocks differing from one another in texture, structure, permeability, mode of occurrence, and degree of resistance to denudation.


Types of Rocks

What are Rocks?

Rocks refer to the accumulation of minerals with different combinations of properties of different mineral traces.

A unique combination of chemical compositions, mineralogy, grain size, texture, or any other characteristics can decide the type of rock.

Different systems of classification exist in each major type of rock and different types of rocks exist in nature. Rocks that are found naturally exhibit properties of different variations as the scale of measurement keeps changing.

Types of Rocks

The three important types of rocks include-

  1. Igneous Rocks
  2. Sedimentary Rocks
  3. Metamorphic Rocks

Let’s look at each type individually and see their formation, characteristics and examples.

1. Igneous Rock

Due to various reasons for example tectonic movements of plates, crustal loading, density differences, etc. the magma is forced upwards on the surface as lava through the crust along lines of weakness.

Thus after cooling of lava two types of igneous rocks formed, the first type of rock spill out on the surface and consolidate to form extrusive igneous rocks, and another one does not find an outlet, sometimes it solidifies in cavities and fissures within the crust to form intrusive igneous rocks.


Igneous Rocks

Types of Igneous Rock

There are two types of igneous rocks:

  1. Intrusive igneous rock- Intrusive indigenous rocks are those which are crystallized below the surface of the earth which results in the formation of large crystals as the process of cooling takes place very slowly some examples include diorite, granite, etc.
  2. Extrusive igneous rock- Extrusive igneous rocks refer to those rocks which erupt onto the surface which results in small crystals as cooling takes place at a faster rate and for some rocks, it is so quick, that it leads to the formation of amorphous glass. A few examples include basalt, tuff, etc.

Igneous Rock Properties

  1. These types of rocks are known as primary or basic rocks as this type was considered to be the first form of rock and the supply materials for other types of rocks.
  2. No fossils contain found in these types of Rock.
  3. These types of rock forms include more than one mineral deposit and these may be either glassy or coarse in nature.
  4. Based on the rate of cooling of lava, the granular size of the rocks was dependent, for very fast cooling the size of the granular becomes smaller, whereas for slow cooling the size of the granular becomes larger.
  5. Based on the chemical and mineral composition of Igneous rocks, some different types of rocks are formed.
    • Mafic or Basic Rocks: Rocks contain 44-52% of silica in nature e.g., Gabbro basalt, Dolerite, etc. This type of rock is denser and darker in color.
    • Ultrabasic or Ultramafic Rocks: Rocks contain less than 44% of silica in nature e.g., Peridotite, etc.
    • Silicic rocks: Rocks contain more than 66% of silica in nature e.g., Granite, Granodiorite, etc. This type of rock is less dense and lighter in color.
    • Intermediate Rocks: Rocks contain 52-66% of silica in nature e.g., Andesite, Diorite, etc. This type of rock is less dense and lighter in color.

Igneous Rock Examples:

  • Intrusive igneous rock – diorite, granite, etc.
  • Extrusive igneous rock – basalt, tuff, etc.

2. Sedimentary Rocks

These types of rocks are found over about 70% of the Earth’s upper surface and formed through a series of processes.

In the very early stage weathered materials are transported by exogenic agents, and in the later time phase, it is deposited at the lower gradient due to sedimentation.

In this way due to erosion and deposition of metamorphic, igneous rocks sedimentary rocks are formed.

After the deposit of materials debris or sediments in suitable conditions, it is converted into consolidated sedimentary rocks by the process known as Lithification. Then, by the further process of Cementation, the layer of many consolidated sedimentary rocks convert into a large bound of sedimentary rock structure.


Sedimentary Rocks

Sedimentary Rocks Properties

  • Fossils are generally found in such types of rocks.
  • Maximum sedimentary rocks are found in the form of Shale, Sandstone, and Limestone (More than 97%).

Types of Sedimentary Rocks

The following are 3 types of sedimentary rocks:

  1. Clastic sedimentary rocks – Formed by the accumulation and compaction of sediments from old rocks.
  2. Chemical sedimentary rocks – Formed due minerals precipitation from a solution due to changing chemical conditions.
  3. Organic sedimentary rocks – Formed due to accumulation of plant or animal debris, such as shells or plant material.

Sedimentary Rocks Examples

  • Mechanically formed sedimentary rocks :Shale, Clay rocks, etc.
  • Chemically formed sedimentary rocks: Gypsum, Salt rock
  • Organically formed sedimentary rocks: Coal, Peat, Limestones, etc.

3. Metamorphic Rocks

Metamorphic rocks form as a result of the transformation of existing rock types, a process known as metamorphism, which means “form change.”

Metamorphic rocks are generally a modified form of Igneous, Sedimentary, or previous Metamorphic rock by various external weather conditions like temperature, pressure, volume, etc. and for such reason, the appearance, texture, and composition of rocks were changed from the previous characteristics.


Metamorphic Rocks

Types of Metamorphic Rock

The types of metamorphic rock include the following-

  1. Foliated metamorphic rocks- These are produced due to exposure to heat and pressure because of which they appear to be layered. Examples include Genesis and Phyllite.
  2. Non-foliated metamorphic rocks- These rocks do not have layering and examples include marble and quartzite.

Metamorphic Rocks Properties

  • These types of rocks are the hardest among the three types of rocks.
  • These types of rocks do not contain fossils.
  • These types of rocks are formed from all three types of rocks
  • These rock types are generally found at the core of the various mountain ranges.

Metamorphic Rocks Examples

Since metamorphic rocks are formed from other rocks over large period of time, let’s look at each transformation example:

1. Sedimentary Rocks to Metamorphic Rocks:

  • Slate from shale
  • Marble from Limestone
  • Marble from chalk and dolomite

2. Igneous Rocks to Metamorphic Rocks:

  • Gneisses from granites
  • Amphibolite from basalt
  • Schist from Basalt

Types of Rocks in India

Some important types of rocks in India include:

  1. Rocks of the Archaean system- They are formed from hot molten earth and are the oldest as well are primary rocks an example is Gnesis.
  2. Rocks of the Dharwar system- They are formed due to erosion and sedimentation of the archaean system and are also the oldest sedimentary rocks.
  3. Rocks of the Cuddapah system- They are formed due to erosion and sedimentation of the Dharwar system and include limestone, sandstone, etc.
  4. Rocks of the Vindhyan system- They are formed from the silt of river valleys and shallow oceans. An example is red sandstone.
  5. Rocks of Gondwana system-  These types of rocks are formed due to depression in basins. Coal is an example.
  6. Rocks of Deccan trap-These are formed due to the eruptions of volcanoes and examples include dolerite and basalt.
  7. Rocks of Tertiary system- They are found mostly in Himalayan ranges.
  8. Rocks of the Quaternary system- They are found in the plains of the Indus and Ganga rivers.

Related Links

  1. Metamorphic rocks
  2. Classification of Minerals
  3. How are minerals formed in Sedimentary Rocks?

Most of you see rock lying on ground almost every day, but how many did people wonder about it’s origin? With the help of this article we have tried to answer this question. We have discussed about rocks, three type of rocks and their examples.

Rocks is a very basic concept taught in primary school, but most people forget about it. You can loose potential marks in competitive exams if you ignore this topic as some questions are asked about rocks.

Hope it will help you in your coming exams!

Also read:

Frequently Asked Questions

Q 1. What are rocks?

Rocks refer to mineral compounds with a combination of properties of all the mineral traces and have any unique combination of chemical compounds, mineralogy, grain size, texture, or distinguishing characteristics which can describe the type of rock.

Q 2. What are the different types of rocks?

The different types of rocks include-

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

Q 3. What is the most common type of igneous rock?

The most common type of igneous rock is basalt and the majority of the ocean floor is constituted with basalt.

Q 4. What type of rock is marble?

Marble is a metamorphic rock. The alternation process due to extreme pressure and heat, transforms limestone into marble, which is the first step in the marble formation process.

Q 5. Which is the hardest type of rock?

The hardest type of rock is metamorphic rock.

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