‘Ignis’ is a Latin word and its meaning is fire.
The cooling of intensely heated molten fluid liquid, known as magma, creates Igneous rocks.
It requires a higher amount of heat to melt the rocks under overlying pressure than on the surface. Magma forms at different depths not exceeding 40 km.
When magma comes at the surface of the earth it is called Lava.
Igneous rocks are formed below or on earth’s surface from solidified molten magma.
These rocks are parent rocks and also called as primary rocks.
In simple words, all rocks can be described as of igneous origin because, at one time or another, they erupted to the surface.
About 95% of the volume of outermost of the earth is composed of Igneous Rocks.
These types of rocks are largely hard and massive because of their magmatic origin and are crystalline in appearances.
Extrusive igneous rocks
Extrusive igneous rocks are formed on the earth’s surface by the cooling of lava.
As the lava cools very quickly when it comes out of the earth’s interior, the mineral crystals that make up these rocks are very fine.
These rocks are also called volcanic rocks.
Examples of such rocks are Gabbro and Basalt.
Intrusive igneous rocks
Intrusive igneous rocks are formed when magma solidifies below the earth’s surface.
The rate of cooling below the earth’s surface is very slow which gives rise to the formation of large crystals in the rocks.
Deep intrusive rocks are referred to as plutonic rocks.
Shallow intrusive rocks are referred to as hypabyssals.
Examples of intrusive rocks are Granite and dolerite.
In the Himalaya and the Deccan Plateau, Granitic rocks are found.
Common forms of intrusive igneous rocks are batholiths, sills, and dykes, etc.
Batholiths are huge masses of solidified magma.
These types of rocks vary in size; some are as much as several hundred kilometers across and thousands of kilometers thick.
They form the core of the big mountains.
Sill is the horizontal intrusion of solidified magma between the layers of preexisting rocks.
Dyke is similarly a more or less vertical formation from few meters to several kilometers in length and from few centimeters to hundreds of meters in thickness.
Acidic and Basic rocks
Acid igneous rocks contain more than 65 percent of silica.
Acid igneous rocks are light-colored very strong.
Granite is an example of an acidic rock.
Basic igneous rocks have more iron and magnesium.
These rocks contain less than 55% of silica.
These are dark-colored and weak rocks.
examples of basic rocks are Gabbro, basalt, and dolerite.