Metamorphic Grade

Metamorphic grade is a scale denoting the level of pressure and temperature involved in forming a particular metamorphic rock. The scale is based on the first appearance of particular minerals, known as index minerals. Because each mineral crystallizes within a limited pressure and temperature range, the presence of particular index minerals indicates the relatively specific set of conditions that existed when the rock formed.

In the late 1800s, geologist George Barrow recognized that certain minerals were abundant in particular metamorphic rocks. He produced the first scale measuring metamorphic grade. For example, the low-grade metamorphic rock slate forms when relatively low pressure and temperature are applied to the sedimentary rock shale. If a greater intensity of pressure and temperature are applied, the slate is altered and becomes the rock phyllite—similar to slate, but somewhat coarsergrained; additional pressure and temperature yields a schist. An even greater increase in pressure and temperature transforms the schist into gneiss—a high-grade metamorphic rock. With each alteration, lower grade index minerals disappear and a new set of higher grade index minerals develops.

Keep in mind, however, that the types of metamorphic rocks formed under the application of pressure and temperature depend on the mineral composition and texture of the parent, or original, rocks, as well as the amount of pressure and the degree of temperature to which the rocks are subjected. In general, as the pressure and temperature increase so does the texture, or grain size, of the rocks formed.

Contact metamorphism

Contact metamorphism results mainly from an increase in temperature with little change in pressure. The increase in temperature is caused by injection of molten rock, or magma, into surrounding rock (referred to as country rock). The area of rock altered by the injection of magma is known as an aureole, whereas the body of rock formed from the molten magma is called an intrusion. In essence, the altered rock is "baked." The rock closest to the source of heat is the most altered; further from the source of increased temperature, less alteration occurs. Eventually, due to the distance from the intrusion, unaltered country rock is encountered.

Contact metamorphism produces hornfels facies from clay-rich parent rocks such as shale. If the parent rocks are impure limestones, skarn (low- to high-grade metamorphism) is produced. Skarn is a calcium-rich, silicate rock containing a variety of minerals, including garnet. Relatively pure sandstones and limestones do not typically form new minerals due to contact metamorphism.

Regional metamorphism

Regional metamorphism produces the bulk of the Earth's metamorphic rock. The volume of rock affected can be hundreds or even thousands of cubic miles. It is usually associated with mountain building processes.

Earth consists of rigid plates, composed of the crust and a portion of the upper mantle, known collectively as the lithosphere, and a flowing layer of plastic rock known as the asthenosphere. Plate tectonic processes—the way in which plates move and interact—are an integral part of metamorphic events. The plates consist of oceanic and continental lithosphere, and these plates are in continual motion, sliding past one another, pulling apart and colliding. Margins of plates that slide past one another are referred to as transform boundaries, those pulling apart are called divergent plate boundaries, and margins of colliding plates are known as convergent plate boundaries.

Each of these boundary types provides an impetus for the increased pressure and temperatures needed for metamorphism. This discussion of regional metamorphism focuses on the convergent plate boundary, and shows the different pressure and temperature environments produced by plate movements and the metamorphic facies that they form.

The collision of two plates is a source of great pressure, and intense deformation (folding and faulting), of rock occurs there. In addition, the denser plate may be subducted or forced under the less dense plate, pulling, or subducting, rock deep into zones of immense pressure and temperature. Within the collision and subduction zones, rocks are recrystallized.

Regional metamorphism produces greenschist facies (low-grade metamorphism), which contains slate, phyllite and greenschist; amphibolite facies (medium-grade metamorphism) containing schist and/or amphibolite; and granulite facies (high-grade metamorphism), which contains gneiss and/or granulite.

Two other metamorphic facies are formed on a regional scale and under unique circumstances. The blueschist facies forms in the low-temperature, high-pressure environment in the upper portion of a subduction zone. Land-derived sediments accumulated deep on the cold ocean floor are driven into an area of high pressure during subduction of an oceanic plate. These rocks often look blue when seen in outcrops.

The eclogite facies indicates high-grade metamorphism produced when oceanic crust containing magnesium and iron is subducted to extreme depths. The very high temperatures and pressures produce garnet and pyroxene.

Burial metamorphism

Burial metamorphism occurs when sediments or rocks are deeply buried and so subjected to increased pressure from the weight of the sediments above them. As the depth of burial increases, so does the temperature. Burial may occur separate from or as part of regional metamorphism. The weight of the overlying sediments forces fluids out of pore spaces between mineral grains. This fluid then reacts with the minerals, a chemical process called metasomatism. Metasomatism is responsible for many of the copper, gold, iron ores, tin and zinc deposits found in metamorphic rock. It may also occur during contact and regional metamorphism. Burial metamorphism produces rocks of the zeolite facies.

Knowledge of metamorphic grade and the facies produced allows geologists to map pressure and temperature zones within metamorphic rocks and to understand the intense forces required to form specific rocks, precious minerals and to build continents. By performing metamorphic facies interpretations, geologists can determine the geologic history of vast regions of the Earth.