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.

Dikes, sills, and volcanic pipes are all types of intrusions, a word geologists use to describe places where magma has pushed its way upward through the Earth's crust and cooled close to the surface. Dikes are places where the molten rock has solidified in a sheet that cuts through layers of rock bodies. Sills, on the other hand, are intrusions that squeezed in between existing layers of rock. A volcanic pipe forms when a tube-like conduit of magma to a surface volcano solidifies after the volcano's eruption has ended. Because these large-scale intrusions are formed underground, humans must wait a long time, sometimes millions of years, until erosion, mining, or digging removes the surrounding rock, revealing the deep structures.