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Intrusive igneous rocks (also called plutonic rocks, named after Pluto, the Roman god of the underworld) are formed from magma that cools and hardens within the earth. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are course grained.
When magma cools either above or underground, igneous rock is formed. What is the difference between intrusive and extrusive igneous rocks? Since Intrusive igneous rocks were formed underground, they had more time to release air and the size of the crystals is bigger. Extrusive rocks cool quickly so they don't have much time to compress together with all the crystals.