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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.
Because such slow cooling allows time for large crystals to form, intrusive igneous rock has relatively large mineral crystals that are easy to see. Granite is the most common intrusive igneous rock (Figure 4.12). Figure 4.12: A close-up of a granite sample.he holes make pumice so light that it actually floats in water. The most common extrusive igneous rock is basalt, a rock that is especially common below the oceans (Figure 4.15). Figure 4.13: Extrusive igneous rocks form after lava cools above the surface.