Metamorphic rock classification

Metamorphic rocks bear witness to the instability of the earth’s surface. They reveal the long history of interaction among the plates that constitute the surface and the deep-seated motions within the plates. Among the metamorphic rocks are found many ores and stones of value to human civilization.

Metamorphism

A significant part of the earth’s surface is made up of rocks quite different from sedimentary or igneous rocks. Many of them have distinctive textures and structures, such as the wavy, colored bands of gneiss or the layered mica flakes of schist. They often contain certain minerals not found or not common in igneous or sedimentary rocks, such as garnet and staurolite. Studies of their overall chemical composition and their relationships to other rocks in the field show that they were once igneous or sedimentary rocks, but, after being subjected to high pressure and temperature, have been altered or recrystallized through a process called metamorphism. Metamorphism involves both mechanical distortion and recrystallization of minerals present in the original rock, the protolith. It can cause changes in the size, orientation, and distribution of grains already present, or it can cause the growth of new and distinctive minerals built mostly from materials provided by the destruction of minerals that have become unstable under the changed conditions. The chemical components in the rock are simply reorganized into minerals that are more stable under higher pressure and temperature.

Descriptive Classification of Metamorphic Rocks

Metamorphic rocks can be classified in a purely descriptive fashion according to their textures and dominant minerals. Because the growing understanding of metamorphic processes can be applied to interpret the origin and history of the rocks, they are also classified according to features related to these processes. The most common classification schemes, in addition to the purely descriptive schemes, categorize the rocks by general metamorphic processes (metamorphic environments), the original rocks (protoliths), metamorphic intensities (grades), the general pressure and temperature conditions (facies), and the ratios of pressure to temperature (pressure-temperature regimes).

The oldest classification is purely descriptive, based on rock texture (especially foliation) and mineral content. Foliation is an arrangement of mineral grains in parallel planes. The most common foliated rocks are slate, schist, and gneiss. In slate, microscopic flakes of mica or chlorite are aligned so that the rock breaks into thin slabs following the easy cleavage of the flakes. Schist contains abundant, easily visible flakes of mica, chlorite, or talc arranged in parallel; it breaks easily along the flakes and has a highly reflective surface. Gneiss contains little mica, but its minerals (commonly quartz, feldspar, and amphibole) are separated into different-colored, parallel bands, which are often contorted or wavy. The foliated rocks can be described further by naming any significant minerals present, such as “garnet schist.” Nonfoliated metamorphic rocks lack parallel structure and are usually named after their dominant minerals. Common types are quartzite (mostly quartz), marble (mostly calcite), amphibolite (with dominant amphibole), serpentinite (mostly serpentine), and hornfels (a mixture of quartz, feldspar, garnet, mica, and other minerals). Quartzite breaks through its quartz grains, whose fracture surfaces give the break a glassy sheen. Marble breaks mostly along the cleavage of its calcite crystals, so that each flat cleavage surface has its own glint. Hornfels often exhibits a smooth fracture with a satiny luster reminiscent of horn.

Classification by Processes or Environments

The second classification of metamorphic rocks is based on the general processes that formed them or the corresponding environments in which they are found. The recognized categories are usually named regional, contact, cataclastic, burial, and hydrothermal metamorphism.

Regional metamorphism is characterized by compression along one direction that is stronger than the pressure resulting from burial. The compression causes foliation, typified by the foliated rocks slate, schist, and gneiss. These rocks are found in extensive regions, often in long, relatively narrow belts parallel to folded mountain ranges. According to the theory of plate tectonics, folded mountain ranges like the Appalachians and the Alps began as thick beds of sediments deposited in deep troughs offshore from continents. The sediments were later caught up between colliding continents, strongly compressed, and finally buckled up into long, parallel folds. The more deeply the original sediment is buried, the more intense is the metamorphism. Clay in the sediment recrystallizes to mica, oriented with the flat cleavage facing the direction of compression. Thus, pelitic (clay-bearing) sediments become slates and schists with foliation parallel to the folds of the mountains. At higher temperatures, the mica recrystallizes into feldspar, and the feldspar and quartz migrate into light-colored bands between bands of darker minerals so that the rock becomes gneiss. At yet higher temperatures, some of the minerals melt (a process called anatexis), and the rock, called a migmatite, becomes more like the igneous rock granite.

Contact metamorphic rocks are commonly found near igneous intrusions. Heat from the intrusive magma causes the surrounding rock, called country rock, to recrystallize. Though the rock is under pressure because of burial, there is usually no tendency toward foliation because the pressure is equal from all directions. Some water may be driven into the rock through fine cracks, or conversely water may be driven from the rock by the heat. Mobile atoms such as potassium especially can migrate into the rock and combine with its minerals to form new crystals (a process called metasomatism). Usually, however, the chemical content is not greatly altered, and recrystallization chiefly involves atoms from smaller crystals or the cement migrating into larger crystals or forming new minerals. In quartz sandstone, for example, the quartz crystals grow to fill all the pore space in a tight, polygonal network called crystalloblastic texture, and the rock becomes quartzite. Similarly, the tiny crystals in limestone or dolomite grow into space-filling calcite crystals, forming marble or, if other minerals are present, the mixed rock called skarn. Pelitic rocks recrystallize to hornfels, containing a variety of minerals such as quartz, feldspar, garnet, and mica.

Cataclastic (or dynamic) metamorphism occurs along fault zones, where both the rock and individual grains are intensely sheared and smeared out by stress. In deep parts of the fault, the sheared grains recrystallize to a fine-grained, finely foliated rock called mylonite.

Burial metamorphism occurs in very deep sedimentary basins, where the pressure and temperature, along with high water content, are sufficient to form fine grains of zeolite minerals among the sedimentary grains. The process is intermediate between diagenesis, which makes a sediment into a solid rock, and regional metamorphism, in which the texture of the rock is modified.

Hydrothermal metamorphism (which many prefer to call alteration) is caused by hot water infiltrating the rock through cracks and pores. It is most common near volcanic and intrusive activity. The water itself, or substances dissolved in the water, may be incorporated into the crystals of certain minerals. One important product is serpentine, formed by the addition of water to olivine and pyroxene, which is significant in sub-seafloor metamorphism.

Classification by Original Rocks

A third classification is based on the original rocks, or protoliths. This classification is possible because relatively little material is added to or lost from the rock during metamorphism, except for water and carbon dioxide, so the assemblage of minerals present depends on the overall chemical composition of the original rock. The most abundant protoliths are pelitic rocks (from clay-rich sediments, usually with other sedimentary minerals), basaltic igneous rocks, and limestone or other carbonate rocks. Each kind of protolith recrystallizes into a different characteristic assemblage of minerals. The categories can be named, for example, metapelites, metabasalts, and metacarbonates.

Classification by Grade

Metamorphic intensity, or grade, is the basis for a fourth classification scheme. As pressure and temperature increase, certain minerals become unstable, and their chemical components reorganize into new, more stable minerals in the surrounding conditions. The presence of certain minerals, called index minerals, indicates the intensity of pressure and temperature. The grades most commonly used are named for index minerals in pelitic rocks; in the late nineteenth century, they were described by George Barrow in zones of metamorphic rocks in central Scotland. These Barrovian grades, in order of increasing intensity, are marked by the first appearances of chlorite, biotite, garnet, staurolite, kyanite, and sillimanite.

Metamorphism is a slow process, however (especially at low temperatures), and conditions sometimes change too rapidly for the mineral assemblage to come to equilibrium. It is not uncommon to find crystals only partially converted into new minerals or to find lower-grade minerals coexisting with those of higher grade. At low grade, some structures of the original rock, such as bedding, may be preserved. Even the outlines of earlier crystals may be seen, filled in with one or more new minerals. High-grade metamorphism usually destroys earlier structures.

All metamorphic rocks available for study are at surface conditions, so the pressures and temperatures that caused them to recrystallize have been relieved. Usually the loss of water and carbon dioxide prevents metamorphic reactions from reversing, but if conditions were relieved slowly enough, and particularly if water was available, the rock may have undergone retrograde metamorphism, reverting to a lower grade and thus adjusting to the less intense pressure and temperature. Retrograde metamorphism is usually not very complete, and some evidence of the most intense conditions almost always remains. For example, the distinct outline of a staurolite crystal might be filled with crystals of quartz, biotite mica, and iron oxides.

Classification by Facies

A fifth classification scheme categorizes the rocks according to the intensity of pressure and temperature, or facies, without reference to protoliths. The concept of facies was developed by Pentti Eskola, working in Finland around 1915, who enlarged on the work of Barrow in Scotland and V. M. Goldschmidt in Norway. Eskola realized that each protolith has a characteristic mineral assemblage within a given facies, or range of pressure and temperature. The facies are named for one of the assemblages within a specified range of conditions; for example, the greenschist facies, named for low-grade metamorphosed basalt, refers to equivalent low-grade assemblages from other protoliths as well. Other examples are the amphibolite facies, the range of pressures and temperatures that would give the staurolite and kyanite grades in pelitic rocks; and the granulite facies, which corresponds to extreme conditions bordering on anatexis. Facies is a more precise version of the concept of grade, and is defined by combinations of minerals that define specific chemical reactions rather than single minerals that may form via many different reactions.

Classification by Pressure-Temperature Regimes

Finally, the facies themselves, or the metamorphic assemblages in them, can be classified according to ratios of pressure to temperature, called pressure-temperature regimes. The greenschist, amphibolite, and granulite facies include mineral assemblages of increasing metamorphic intensity whose pressure and temperature rise together approximately as they would with increasing depth under most areas of the earth’s surface. This sequence is sometimes referred to as the Barrovian pressure-temperature regime because Barrow’s metamorphic grades in pelitic rocks fall in these facies. In another regime, called the Abukuma series after an area in Japan, the temperature is much cooler for any given pressure. The blueschist facies, characterized by blue and green sodium-rich amphiboles and pyroxenes, is typical of this series. The converse situation, in which temperature rises much faster than pressure, corresponds to contact metamorphism and is called the hornfels facies.

Investigating Rock Features and Histories

Initial studies of metamorphic rocks are almost always done in the field. The tectonic or structural nature of the region suggests the processes to which the rock has been subjected. For example, an area of folded mountains can be expected to exhibit regional metamorphism; a volcanic area, some contact metamorphism; and a fault zone, some cataclastic metamorphism. Some features are obvious at the scale of an outcrop, such as banding and foliation, or the halo of recrystallized country rock abutting an igneous intrusion. Some textural features—such as foliation, large crystals, or the luster of a fracture surface, as in quartzite or marble—are easily seen in a hand specimen. Similarly, a preliminary estimate of mineral content can be made from a hand specimen.

Many features, however, are best seen in thin section under a petrographic microscope. Usually all but the finest grains can be identified. From the relative abundance of the various minerals and their known chemical compositions the overall chemical composition of the rock can be calculated. The protolith can then be identified by comparing the calculated composition to the known compositional ranges of igneous and sedimentary rocks.

Textures seen under the microscope reveal much about the history of the rock. Foliation, for example, usually indicates regional metamorphism. A space-filling, polygonal texture can show contact or hydrothermal recrystallization, and a crumbled, smeared-out cataclastic texture indicates faulting.

More recent methods of investigation sometimes applied to metamorphic rocks are X-ray diffraction and electron microprobe analysis. The pattern of X rays scattered from crystals depends on the exact arrangement and spacing of atoms in the crystal structure, which is useful for identifying minerals. X-ray diffraction can be used to identify crystals that are too small or too poorly formed to be identified with a microscope. The microprobe can analyze the chemical composition of crystals even of microscopic size. Determination of exact composition or of variation in composition within growth zones of a single crystal can be especially useful for identifying variations in conditions during crystal formation.

Recognizing Metamorphic Rocks

Most people encounter metamorphic rocks while traveling through mountains and other scenic regions. Recognizing these rocks is easier if one has a general idea of where the various types occur and how they appear in outcrops.

Regional metamorphic rocks are best exposed in two kinds of localities: the continental shields and the eroded cores of mountain ranges. Ancient basement rocks of the continental platform, composed of regionally metamorphosed and igneous rocks, are exposed in shields without a cover of sedimentary rock. The Canadian Shield, extending from northern Minnesota through Ontario and Quebec to New England, is the major shield of North America. Similar shields are exposed in western Australia and on every other continent. The old, eroded mountains of Scotland and Wales contain abundant outcrops in which some of the pioneering studies of metamorphic rocks were conducted. The Appalachians have even larger exposures, extending from Georgia into New England. Somewhat smaller outcrops occur in many parts of the Rocky Mountains and the Coast Ranges. The foliated rocks schist, slate, and gneiss make up the bulk of these exposures. Rock cleavage parallel to the foliation is an important clue to recognizing outcrops of these rocks; slopes parallel to foliation tend to be fairly smooth and straight, while slopes eroded across the foliation are ragged and steplike. Bare slopes of schist can reflect light strongly from the many parallel flakes of mica. Slate is usually dull and dark-colored but characteristically splits into ragged slabs. The colorful, contorted bands of gneiss are easily recognized.

Contact metamorphic rocks are much less widespread and are generally confined to areas of active or extinct volcanism. The Cascades and the Sierra Nevada show many examples, but some of the best exposures are found near ancient intrusive rocks in the Appalachians and New England. Contact metamorphic rocks are more of a challenge to recognize because they generally lack foliation. The best clue is physical contact with a body of igneous rock. They often form a shell or halo around an igneous body, most intensely recrystallized at the contact and extending outward a few centimeters to a few kilometers (depending mostly on the size of the igneous body), until they eventually merge into the surrounding unaltered country rock. The halo, called an aureole, is generally similar to the country rock, but because it is recrystallized it is usually harder, more compact, and more resistant to erosion. Broken surfaces can be distinctive. Depending on the nature of the country rock, one might look for the glassy sheen of fractured quartzite, the glinting cleavage planes of marble, or the smooth, hornlike fracture of hornfels.

Hydrothermally altered rocks from sub-seafloor metamorphism, when finally exposed on land, are found among regional metamorphic rocks and appear much like them except for the greenish-gray colors of chlorite, serpentine, and talc. Terrestrial hydrothermal alteration is most easily recognized in areas of recent volcanic activity. Good examples are exposed in the southwestern United States, such as the so-called porphyry copper deposits. Many such areas contain valuable deposits, and so may have been mined or prospected. The outcrops are often much fractured and veined near the intrusion. Where alteration is most intense, the rock may appear bleached; farther from the intrusion, it may have a greenish hue because of low-grade alteration. Quartz and sulfide minerals such as pyrite are common in the veins, but weathering often leaves a rusty-looking, resistant cap over the deposit called an iron hat or gossan.

Rocks formed by cataclastic and burial metamorphism require specialized equipment for their recognition. Zones many miles wide containing mylonite, the product of cataclastic metamorphism, are exposed along the Moine fault in northwestern Scotland (where mylonite was first studied) and along the Brevard fault, extending along the Appalachians from Georgia into North Carolina. Examples of burial metamorphic rocks are found under the Salton Sea area of California and the Rotorua area of New Zealand.

Principal Terms

contact metamorphism: metamorphism characterized by high temperature but relatively low pressure, usually affecting rock in the vicinity of igneous intrusions

facies: a part of a rock, or a group of rocks, that differs from the whole formation in one or more properties, such as composition, age, or fossil content

foliation: a texture or structure in which mineral grains are arranged in parallel planes

metamorphic facies: an assemblage of minerals characteristic of a given range of pressure and temperature; the members of the assemblage depend on the composition of the protolith

metamorphic grade: the degree of metamorphic intensity as indicated by characteristic minerals in a rock or zone

metamorphism: changes in the structure, texture, and mineral content of solid rock as it adjusts to altered conditions of pressure, temperature, and chemical environment

pelitic rock: a rock whose protolith contained abundant clay or similar minerals

pressure-temperature regime: a sequence of metamorphic facies distinguished by the ratio of pressure to temperature, generally characteristic of a given geologic environment

protolith: the original igneous or sedimentary rock later affected by metamorphism

regional metamorphism: metamorphism characterized by strong compression along one direction, usually affecting rocks over an extensive region or belt

texture: the size, shape, and relationship of grains in a rock

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