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Igneous Rocks and Types of Igneous Rocks

What are Igneous Rocks:

Igneous rocks are formed when molten magma is cools. Igneous have got their origin from word Agni which means made from heat or fire.When the rock is liquid and inside the earth,it is called magma. When the magma gets hard inside the earth crust, it turns into granite.It cools very slowly and is very hard.

When the magma gets up to the surface and flows out,like what happens when a volcano erupts,then the liquid is called lava. Lava flows down the sides of volcano.When it cools and tunns hard it is called obsidian lava rock or pumice-depending on what it looks like.About 46.7% of the earth crust by volume consist of igneous rocks , making it the most plentiful category.Of these 66% are basalts and gabbros, while 16% are granite. The oceanic crust is 99% basalt,which is an igneous rock of mafic composition.

01. Acid Igneous Rocks:

It has an excess of acid forming radical,silicon which is about 80% . The rest has magnesium,sodium,potassium,etc.  It cools fast and does not flow or spread away. High mountains are formed of acid igneous rocks.It is light in colour.  The main example of this type of rock is granite. Its erosion is very slowly. They are mostly compact and massive. They belong to the portion of the earth known as Sial.

02. Basic Igneous Rocks:

The silicion content in these rocks is below 40%. The rocks contain 40 magnesium and the rest 20 contains iron ,aluminium and potassium. It cools slowly due to low silicon content. The molten lava spread to far off distance and plateaus are formed from these rocks.Basalt,and Dolesite are the main examples. These rocks are not very hard and are eroded comparatively faster. They belong to the portion of the earth know as Sima.

03. Plutonic Igneous Rocks:

These rocks are formed by the solidification of magma at moderate depths beneath the earth surface. Since the solidification of magma occurs at great depths below the earth surface, the rate of cooling is much slower.As a result of this slow cooling very large sized crystals are formed in such rocks. These deep seated instructions are known as Plutonic rocks. Highly coarse grained granite is a typical example of plutonic rock.

04. Volcanic Igneous Rocks:

These rocks are formed as a result of solidification of lava ashes on the surface of earth. This lava hot gases and water vapours erupt from the weaker portion of the earth crust and spread on the surface of earth. These rocks are also known as volcanic rocks.As lava cools rapidly on coming out earth hot interior,the mineral crystals of extrusive igneous rocks change their structure and are very small,giving them very fine-grained look. Basalt is very typical example of extrusive igneous rocks.

05. Hypabyssal Igneous Rocks:

Hypabyssal igneous rocks are formed at depth in between the plutonic and volcanic rocks. These are formed due to cooling and resultant solidification of rising magma just beneath the earth surface.Hypabyssal rocks are less common than plutonic or volcanic rocks and often form dikes,sills, laccoliths or phacoliths.

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