This chapter on metamorphic rocks, the third major category of rocks in the rock cycle, completes our description of earth materials (rocks and minerals). The information on igneous and sedimentary processes in previous chapters should help you understand metamorphic rocks, which form from pre-existing rocks.
After reading the chapter on weathering, you know how rocks are altered when exposed at Earth's surface. Metamorphism (a word from Latin and Greek that means literally "changing of form") also involves alterations, but the changes are due to deep burial, tectonic forces, and/or high temperature rather than surface conditions.
As you study this chapter, try to keep clearly in mind how the chemical composition of a rock and the temperature, pressure, and water present each contribute to the metamorphic process and the resultant metamorphic rock.
We also discuss the hydrothermally deposited rocks and minerals, which are usually found in association with both igneous and metamorphic rocks. Hydrothermal ore deposits, while not volumetrically significant, are of great importance to the world's supply of metals.
Because nearly all metamorphic rocks form deep within the earth's crust, they provide geologists with many clues about conditions at depth. Therefore, understanding metamorphism will help you when we consider geologic processes involving Earth's internal forces. Metamorphic rocks are a feature of the oldest exposed rocks of the continents and of major mountain belts. They are especially important in providing evidence of what happens during subduction and plate convergence.
