Metamorphic Grade

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Metamorphic Grade

Metamorphic grade reflects the pressure and temperature involved in forming a particular metamorphic rock. It is based on the existence 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 coarser-grained; 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.

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.

Metamorphic rocks and facies

Metamorphic rocks form below the ground surface, beyond the reach of near-surface sedimentary processes of disintegration and consolidation. Pressure and temperature increase as depth below the ground surface increases. For each 1 mi (1.6 km) increase in depth below Earth’s surface, pressure typically increases by 0.56 kilobars, while temperature increases an average of 70^°F (40^°C). Minerals and rocks form under specific conditions, including unique pressure and temperature ranges. If the conditions are changed after the rock has formed, the rock becomes unstable and must undergo change to again reach stability. If these changes include the long-term application of increased temperature and pressure to any type of rock—whether igneous, sedimentary or metamorphic—are sufficient to cause solid-state recrystallization, the end result is a metamorphic rock. If melting and recrystallization from a liquid occurs, the newly formed rocks are igneous and not metamorphic. This is metamorphism.

In the order of increasing pressure and temperature, the metamorphic rocks formed from the sedimentary rocks shale or mudstones are slate, phyllite, schist and gneiss; from volcanic tuff (ash turned to

rock), various types of schist and amphibolite, a dark rock containing hornblende and feldspar; from sandy limestone or dolomite, marble, tremolite marble, and diopside marble; the latter two being coarse-grained, impure forms of marble.

Although metamorphism produces particular types of rocks, when interpreting metamorphic grade, geologists often focus on metamorphic facies, as opposed to a specific type of metamorphic rock. This is because the environment in which metamorphic rocks formed is not easily identified based on a single type of rock.

A metamorphic facies consists of metamorphic rocks that form within a similar environment with respect to pressure and temperature, and is identified by the presence of specific mineral groups.

Types of metamorphic facies

Common metamorphic facies include:

Hornfels facies: low- to high-grade metamorphism.
Zeolite facies: low-grade metamorphism.
Greenschist facies: low-grade metamorphism.
Amphibolite facies: medium-grade metamorphism.
Granulite facies: high-grade metamorphism.
Blueschist facies: low-temperature/high-pressure metamorphism.
Eclogite facies: high-grade metamorphism.

Contact, regional, and burial metamorphism produce the metamorphic facies listed above.

Types of metamorphism

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. 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

KEY TERMS

Asthenosphere— Flowing layer of plastic rock situated below the lithosphere.

Hornfels— A metamorphic rock containing micas, quartz and garnets and that is formed from clay-rich rocks.

Kilobar— A unit of measure used to express the high pressures found within Earth’s interior. It is referenced to air pressure, the force exerted by the weight of the atmosphere at Earth’s surface, which equals one bar. A kilobar is 1000 bars.

Lithosphere— The crust and a portion of the upper mantle, which is divided into rigid plates.

Metamorphic facies— A set of metamorphic rocks formed under the similar pressure and temperature conditions and identified by the presence of specific minerals.

Metasomatism— A group of chemical reactions that occur when water released during metamorphism of rocks is involved in the creation of new minerals.

Silicate minerals— A group of minerals, containing the elements silicon and oxygen plus various others, which compose most igneous rocks and many metamorphic and sedimentary rocks as well. The silicate minerals, including quartz, and the mineral groups feldspar, mica, pyroxene, amphibole, and garnet, make up over 90% of Earth’s crust.

Skarn— A metamorphic rock composed of silicate minerals, as well as the elements calcium, aluminum, iron and magnesium.

do not typically form new minerals as a consequence of contact metamorphism.

Regional metamorphism

Regional metamorphism produces the bulk of 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.

The outer shell of 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 that can give rise to intense deformation (folding and faulting). 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 ocean floor are driven into an area of high pressure during subduction of an oceanic plate. These rocks often have a blue cast or color.

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 in 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 Earth.