Earth's crust, or outermost rocky layer, sits on top of a deeper layer called the mantle, which stores heat from two sources: the formation of the Earth 4.65 billion years ago and the radioactive decay of uranium, thorium, and potassium. When cracks between huge crustal plates open up, the gap causes the underlying mantle to rise up. The upwelling partially melts that region of the mantle; scientists call that decompression melting. The molten rock, or magma, is less dense than solid rock, so it moves upward, the way a cork bobs to the surface of water. As the magma reaches the upper layers of the crust or even Earth's surface, it cools and hardens into a solid known as igneous rock. Scientists categorize igneous rocks according to their chemical composition, the method of their formation, and their degree of crystallization.

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.