High pressures and temperatures within the Earth's crust can change rocks from one type to another. Scientists call this process metamorphism, which comes from the Greek words for "after" and "form." At the junctions where the Earth's crustal plates collide, pushing up mountain ranges, or where one plate slides underneath another, the original rocks, or protoliths (from the Greek words meaning "first" and "rock") undergo reactions that change the chemical or crystal structure of the rocks with little or no actual melting. The grains of minerals within many metamorphic rocks are aligned in parallel due to the forces pushing on them. The pressure-temperature combination causes some sedimentary rocks to lose water molecules or become anhydrous.

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.