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.

A collision between a celestial body and the Earth creates sudden, enormous pressures, far beyond those generated by volcanoes and plate tectonic processes. The violent blow from the impact changes the crystal structure of the quartz and feldspar found in normal, unshocked rocks. A microscope reveals tiny, distinctive parallel cracks in the silicate minerals; these cracks are called planar deformation features. Quartz may also transform into its high-pressure forms, such as suevite and coesite. The "shock metamorphism" may also create diamonds out of carbon. Huge impacts may throw off bits of melted rock debris, which quickly cool with a glassy texture and a round or elongated shape. Scientists call these glassy stones tektites and they provide evidence of long-ago impact events. Some evidence of shocked silicate rocks has also been found at atomic bomb testing sites.