How are Igneous Rocks Formed

Batul Nafisa Baxamusa Jan 23, 2019
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The following piece of writing provides information about the formation process of igneous rocks.
The Earth is made up of three types of rocks called igneous, sedimentary, and metamorphic rocks. They are formed under the Earth's surface, and also above the ground. The term 'igneous' is derived from the Latin word, igneous that means fire. They are formed from the cooling and solidification of magma or lava, and therefore, are also called 'fire rocks'.

Formation Process

There are over 700 types of igneous rocks that have been identified, and most of them are formed below the surface of our planet.
Based on their mode of occurrence, they are either intrusive or plutonic rocks, and extrusive or volcanic rocks. The most uncommon of these rocks are of the hypabyssal variety.

Intrusive Igneous Rocks

Below the surface of the Earth, about hundreds and thousand of kilometers underground, there is hot and molten magma, which takes thousands and even millions of years to cool. It slowly consolidates over the years, contains elements that form silicate minerals, which are the building blocks of igneous rocks.
As per the amount of space available, the minerals grow very large. These rocks that form due to the gradual cooling of the magma are called intrusive igneous or plutonic rocks. They consist of minerals that are so large, that one can see them with the naked eye.
Of the different types of intrusive igneous rocks, granite is the most common rock type. The various types of intrusive igneous rocks are as follows.

Anorthosite

Diorite

Granite

Kimberlite

Pegmatite

Peridotite

Pyroxenite

Syenite

  • Carbonatite
  • Dunite
  • Gabbro
  • Lamprophyres
  • Monzonite

Extrusive Igneous Rocks

Magma flows deep inside the Earth's belly. However, the magma finds its way onto the surface of our planet through various vents. When this molten magma erupts from the volcanic mountains, it flows out in the form of lava, which further comes from the upper mantle layer of the Earth.
The erupted lava cools very quickly on the Earth's surface compared to the cooling of magma within our planet. This causes the elements to cool down quickly without having the time to form minerals. This happens when the lava cools in just 2 to 3 days.
The elements consolidate within the volcanic glass, and when it takes time to cool down like several days or weeks, the minerals form crystals leading to the formation of igneous rocks. However, these crystals are not as large as those of the intrusive ones. The most common type of extrusive rocks is basalt. The other types are as follows.

Andesite

Basalt

Obsidian

Pumice

Rhyolite

Scoria

  • Dacite
  • Komatiite

Hypabyssal Igneous Rocks

They are formed at a depth between the plutonic and volcanic rocks, and they form dikes, sills, and loccoliths.