How are Igneous Rocks Formed

The earth is made up of three types of rocks, called igneous, sedimentary and metamorphic rocks. Igneous rocks are also called 'fire rock's that 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. So, what are igneous rocks? Igneous rocks are those rocks that are made from the cooling and solidification of magma or lava and therefore, are also called fire rocks. In this article, we shall learn how are igneous rocks formed.

How are Igneous Rocks Formed?

There are over 700 types of igneous rocks identified and most of them are formed below the surface of the earth. Based on their mode of occurrence, igneous rocks are either intrusive or plutonic rocks and extrusive or volcanic rocks. The most uncommon of the igneous rocks are hypabyssal igneous rocks. Let us understand how are igneous rocks formed based on their mode of occurrence.

Intrusive Igneous Rocks
Below the surface of the earth, about hundreds and thousand of kilometers underground, there is hot, molten magma that flows through the cracks and chambers of inner earth. This magma takes thousands and even millions of years to cool. This magma that slowly cools over the years, contains elements that form silicate minerals. These silicate minerals are the building blocks of igneous rocks. As per the amount of space available, these minerals grow very large. These rocks that form due to the gradual cooling of the magma are called intrusive igneous rocks or plutonic rocks. These rocks have minerals that are so large, that one can see them without 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
• Carbonatite
• Diorite
• Dunite
• Gabbro
• Granite
• Kimberlite
• Lamprophyres
• Monzonite
• Pegmatite
• Peridotite
• Pyroxenite
• Syenite

Extrusive Igneous Rocks
Magma flows deep inside the earth's belly. However, the magma finds its way onto the surface of the earth through various vents. When this molten magma erupts from volcanic mountains, it flows out in the form of lava. This lava comes from the upper mantle layer of the earth, which is about 50 km to 150 km below the surface of the earth. This erupted lava cools very quickly on the surface of the earth compared to the cooling of magma within the earth. 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 cool down within the volcanic glass and when it takes time to cool down like several days or weeks, the minerals form crystals leading to igneous rock formation. However, these crystals are not as large as the crystals of intrusive igneous rocks. The most common type of extrusive igneous rocks on earth is the basalt rock. The various types of extrusive igneous rocks are:

• Andesite
• Basalt
• Dacite
• Komatiite
• Obsidian
• Pumice
• Rhyolite
• Scoria

Hypabyssal Igneous Rocks
Hypabyssal igneous rock is the most rare type of igneous rock. It is formed at a depth between the plutonic and volcanic rocks. These rocks form dikes, sills and loccolith.

Basalt rocks make up the oceanic crust with ultramafic rocks beneath them. The subduction zones of earth are also made of basalt around the volcanic arcs and the continental edge. Granite rocks make up the continents and anywhere you dig in on the continents, you will reach granite rocks. The granite rocks are less dense as compared to basalt. This causes the continents to float higher than the oceanic rocks over the top of ultramafic rocks. Extrusive igneous rocks have fine grain texture compared to intrusive rocks. Intrusive rocks are thin or thick, huge irregular masses of rocks.

So, the answer to the question how are igneous rocks formed is, that they are formed by the cooling of hot magma. The magma is over 1200 degrees C in temperature and when it cools down it forms igneous rocks. Magma can directly cool and form crystals to form an igneous rock within the earth, or may erupt on the surface of the earth and cool down forming extrusive igneous rocks. I hope this article on how are igneous rocks formed helped clear your doubts regarding the existence of these fire rocks.