Igneous Rocks

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Igneous Meaning

The word ‘igneous’ has been derived from the Latin word ‘Ignance’ that means hot or fiery. Igneous rock is a Latin word that means hot or molten rock that crystallizes and solidifies in the following shape:

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The Earth’s crust is made up of different types of rocks, the rock that makes up the maximum volume of Earth’s crust is called the Igneous rocks.

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Igneous rocks are found beneath the ocean crust and these are formed entirely of basaltic rocks with peridotite below the mantle. These basalts also erupt above the Earth’s subcontinent zone either in volcanic islands or along the sides of the continents.


What are Igneous Rocks?

In this article, you will learn about the types of igneous rock and their formation.

The rocks are associated with volcanic eruptions. The liquid matter coming out of these volcanoes is called ‘magma’, it is a hot liquid that is made up of molten minerals.

Now, when this magma reaches the Earth’s surface, it is known as ‘lava’. The cooling and solidification of lava at the Earth’s surface results in the formation of Extrusive Igneous Rocks; these rocks are also known as ‘Volcanic Rocks’. Due to rapid cooling, their crystals are fine-grained.

The image below shows the volcanic or extrusive igneous rock:

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However, some of the magma cannot come out and settles inside the surface; this magma cools slowly under the Earth’s surface, as it is surrounded by the soil and rocks present all around it.

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So, the cooling process and solidifying of magma at a depth within the Earth’s crust result in the formation of rocks having large crystals known as Intrusive Igneous Rocks. 

The below image shows the intrusive igneous rock:

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So, we studied the two types of igneous rocks viz: Intrusive igneous rocks and Extrusive igneous rocks with their formation. 


Fun Fact

It is fascinating to note that igneous rocks were the first to be formed and are also known as ‘primary rocks’. 


Igneous Rock Example

  • Pumice and Obsidian are the categories of extrusive igneous rocks. We use Pumice in making a lightweight concrete and Obsidian is used in ornaments 

  • Granite and Diorite are intrusive igneous rocks. Both of these are used as building materials.

Since there are many igneous rocks and it’s really hard to distinguish between these. Now, let’s identify the types of igneous rocks.


How to Identify Igneous Rocks?

So far we understood that igneous rocks are those formed by the cooling and solidification of magma or molten rock. As the melted rock cools, new minerals and textures are formed on the basis of things like the original magma composition and the addition of any new chemical components surrounding rocks. 

The time it takes for cooling and the temperature and the pressure at which it cools depends on various factors. Since there are so many variations in this process of formation of igneous rocks because of varying textures and compositions, so it becomes paramount to study their types to classify each one of these.

When we identify or classify igneous rocks, we look for textural and compositional clues in the rock that describes the story of magma formation and its cooling process. It’s because the magma formation and cooling process and the link between these two help us determine the formation environment of igneous rocks with its history. Now. let’s get started. 


Igneous Rocks: Textures and Composition

The below table describes each igneous rock according to its respective texture and composition


Phaneritic

100% visible crystals

Aphanitic

Most crystals are tiny to be seen

Glassy

100% glass

Frothy

Greater than 50% vesicles

Pyroclastic

Small pieces of ash, crystals, pumice rocks

Ultramatic

Peridotite

-

-

-

-

Mafic

Gabbro

Basalt

-

Scoria

Volcanic Tuff

Intermediate

Diorite

Andesite

-

Pumice

Volcanic Tuff

Felsic

Granite

Rhyolite

Obsidian

Pumice

Volcanic Tuff


If we look at the above table, the five columns represent the five basic textures and the four rows represent the four basic compositions. Through this table, we can classify something about igneous rock formation and their history.

Magnum that solidifies and cool so slowly underneath often take hundreds to thousands of years or quickly above the ground following an eruption.


Igneous Rocks and its Types

Magna never reaches underground instead they reach the magnum chamber or insert themselves into cracks or between layers in the rock over a long period of time. They cool and the atoms in the molten state bind with each other to form minerals that are most stable based on their existing temperature, pressure, and chemistry. 

The slow cooling process allows for large crystals formation; the texture of these rocks is described as Phanretic and these crystals are large enough to be visible through the naked eye. The subcategory of Phanretic rocks are usually large crystals and are called Peg Mafic rocks; the rock displaying this texture is called Pegmatite.

In any case, large crystals are formed when magma cools slowly and the magma viscosity is very low then the water is added to it. Due to the low viscosity, the atoms flow quickly that small inches of crystals are formed because when magma erupts as lava, atoms don’t get enough time to form large crystals. Such is the case of extrusive igneous rocks.

FAQ (Frequently Asked Questions)

1. Name the oldest type of rock on the Earth

Zircon is the oldest type of rock on Earth; however; it is not considered as a rock. It is believed to be the oldest geographical material persisting on Earth for 6 million years

2. Is Earth a rock?

Earth is a rocky, terrestrial planet because it has a solid and active surface with mountains, valleys, canyons, plains, and much more. 

The speciality of our mother Earth is, being rocky, it is also called an oceanic planet because its surface is covered by 70% water.

3. How old are the youngest rocks and where are they found?

The youngest rocks are found either in volcanic eruptions or mid-sea ridges. On the other hand, the youngest of the oldest rock is around 4 billion years old. Most of these rocks are fossils and these fossils are important clues to ascertaining the history of the Earth.

4. Name the Indian places where igneous rocks are found.

Below is the list of main regions of India where igneous rocks are found:

  • Karnataka

  • Tamil Nadu

  • Andhra Pradesh

  • Bundelkhand (the oldest gneiss)

  • Madhya Pradesh

  • Odisha

  • Chotanagpur plateau in Jharkhand, and

  • South-Eastern parts of Rajasthan