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Formation of Igneous Rocks
Igneous Rocks, also known as "rocks of fire," are formed by the hardening of magma or lava.
What is the difference between magma and lava?
Therefore, rocks formed by magma hardening inside of Earth are called intrusive rocks, whereas rocks formed outside of Earth are called extrusive rocks.
What's the difference?
Extrusive igneous rocks cool quicker that intrusive rocks due to the outside interference. As the result, there is not enough time for large crystals to form on the rocks.
The main difference in the two types of igneous rocks are their crystal sizes. Intrusive rocks have a longer time to form, and therefore will form larger crystals. They are described as coarse grained, whereas extrusive rocks as fine grained.