As conditions change after burial in basins or at convergent boundaries, the rocks and minerals begin to change in response to the new environment, to increased pressure, temperature, and chemically active fluid mobilization. The effect of these processes is gradual. As conditions become more extreme, the changes that occur in rocks themselves are more extreme. Gradually rocks lose their originals character and evolve into a new group of rocks called metamorphic rocks. 

Metamorphic rocks are formed as a result of environmental changes acting upon other rocks. While sedimentary rocks show the greatest change, and therefore are often discussed at greater length, all rocks are affected by metamorphism. Because the minerals in igneous and metamorphic rocks are already stable under high temperature and pressure conditions, they will undergo less obvious changes than those in sedimentary rocks. Nevertheless, metamorphism affects all rocks. Metamorphism can be caused by two sets of conditions. The first is associated with igneous activity, the second with broader regional trends. 

As magma stopes upward, or is injected into surrounding rocks, it is under tremendous pressure and at high temperature. It will affect the surrounding rocks and they will be metamorphosed. Such metamorphism is called contact metamorphism and, in continents, is restricted to areas in contact with igneous materials. 

Contact metamorphism is an integral process intimately associated with the formation of oceanic lithosphere. As new igneous materials rise from the asthenosphere into the rifts of oceanic ridges it interacts with sea water, which infiltrates along cracks and faults associated with these divergent boundaries to depths of several miles. The amount of interaction depends on the temperature of the surrounding rock and the amount of seawater that can infiltrate. In the upper zones of infiltration, the oceanic rocks become richer in Magnesium and Sodium and sea water becomes richer in Calcium. Deeper down, sea water leaches out metals such as copper, nickel, zinc and rises back to the ocean where it precipitates those metals back out again. This can be seen in black smokers. As the lithosphere spreads and cools, it becomes further enriched in Magnesium and Potassium and the iron becomes oxidized. Further away from the ridges, the igneous materials become sealed off from sea water by deep ocean sediments, and interaction is greatly decreased. This contact metamorphism is responsible for many basic chemical changes both in the oceanic crust as well as in the sea water itself. 

On continents, large areas are affected by metamorphism in deposit basins, and during episodes of convergence and mountain building. As sedimentary rocks are compressed by the weight of overlying materials or between moving sections of the lithosphere such as in accretionary wedges or colliding continents, large areas are heated and deformed. Such metamorphism, affecting large volumes of rocks is called regional metamorphism.