Metamorphism

Metamorphism is the process by which the structure and mineral content of rocks transform in response to changes in temperature, pressure, fluid content (gas or water), or a combination of these. Because the minerals that make up rocks are stable only within certain ranges of temperature and pressure, large changes in these conditions cause minerals to change chemically or to change shape, or both. Minerals that form during metamorphism include spectacular varieties of garnet, mica, amphibole, and serpentine, to name a few. Metamorphism produces characteristic textures in metamorphic rocks, the type of rocks that have undergone metamorphism, such as alignment of elongate crystals or differentiation of different minerals into layers. Distinctive minerals and textures are keys to distinguishing rocks that have experienced metamorphism from unmetamorphosed sedimentary and igneous rocks.

Metamorphism has captured the interest of geologists for many years. James Dana, a noted American geologist, wrote about the alteration of rocks by metamorphism in his Manual of Geology, first published in 1862. By the 1920, the Finnish geologist Pentii Eskola began to note differences in the degree of metamorphism of rocks in different areas on the basis of the different groups of minerals that typically occur together in metamorphic rocks.

The minerals that typically occur in distinctive groups in metamorphic rocks are known as metamorphic facies. Metamorphic facies reflect different conditions during metamorphism. For example, ongoing metamorphism of shale at continuously increasing temperature and pressure initially produces slate, then phyllite, schist, and gneiss. As the pressure and temperature change, the structures and minerals in rocks change to forms that are stable in those conditions. Thus, by studying the minerals present in an area, scientists can estimate the pressure and temperature at which metamorphism occurred.

Matching metamorphic rocks to their unmetamorphosed precursors is not always easy. However, some metamorphic rocks typically form from certain precursor rocks. For example, marble is a metamorphic rock that forms during metamorphism of limestone, a sedimentary rock. Metamorphism of other sedimentary rock, such as shale, can produce slate. Sandstone metamorphoses into quartzite. Granite, an igneous rock, can become gneiss during metamorphism.

Products of metamorphism occur worldwide. Familiar examples include slate roofs and marble floors, garnet jewelry, and asbestos insulation made from serpentine and amphibole minerals. Because of their unusual minerals and structures, as well as their association with majestic mountain ranges, metamorphic rocks are among the most beautiful.