Molten rock, or magma, does not always reach Earth's surface. It may flow upward through cracks that end below the surface, where it gets trapped and cools slowly. Some intrusions, called plutons, are several kilometers or miles wide. During the slow cooling process, the magma freezes into crystals. Magma containing higher iron, magnesium, and calcium levels is the first to turn solid and forms dark, coarse-grained rocks such as gabbro. The lighter-colored granite comes from magma with high levels of silica (silicon dioxide) and relatively little iron and magnesium. Humans cannot witness the formation of intrusive igneous rock in the same way we see volcano eruptions. However, over millions of years, the crustal rocks above some intrusions wear away, leaving the solidified magma exposed to the environment in places such as the Sierra Nevada Mountains and Yosemite National Park in California.

Igneous rocks may be classified by their overall color, which is a clue to their chemical and mineral composition. Geologists call most dark-colored igneous rocks, such as gabbro and basalt, mafic (pronounced "MAY-fic") rocks. In the word "mafic," the first syllable represents the element "magnesium" and the F stands for "ferric," an adjective describing the element iron. Mafic rocks have a relatively high content of iron, magnesium, and calcium and less silicon dioxide than other types of igneous rocks. They also begin to solidify at higher temperatures, usually above 1,000 degrees C (1,800 degrees F). By contrast, felsic rocks, such as granite and rhyolite, are lighter in color overall, though they may contain dark grains. "Felsic" is a word coined from the minerals "feldspar" and "silica" (quartz); felsic rocks contain 70 to 75 percent silica and do not start to crystallize until magma, or molten rock, cools to around 700 degrees C (1,300 degrees F). In other words, in a body of magma that is cooling down, the mafic minerals will tend to form before most of the felsic minerals.