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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.
To give you an idea of the difference between intrusive igneous rocks and extrusive igneous rocks, take a look at this example of basalt, an extrusive igneous rock: You'll notice that you cannot see the different mineral crystals because this rock cooled from lava, much too quickly for mineral crystals to form.