Igneous rocks are classified differently than the other two types of rocks. When identifying igneous rocks, you first have to decide whether it is intrusive or extrusive. Intrusive means that the rock was formed by magma cooling slowly inside the Earth's crust. These can usually be identified by their large crystals. Extrusive means it cools rapidly on the outer side of Earth's crust. These usually have small crystals. Phaneritic means the rock has large, visible crystals; aphanitic means the crystals are too small to be seen with the naked eye. Felsic means that the rock is light in color, and mafic means it is dark in color.
Granite and basalt are two examples of igneous rocks.
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