Regional metamorphism

Regional metamorphism, which takes place in the roots of actively forming mountain belts, is a process by which increased temperatures and pressures cause a rock to undergo recrystallization, new mineral growth, and deformation. These changes diagnostically cause minerals to develop a preferred orientation, or foliation, in the rock.

Rock Texture and Composition

Metamorphism is a process whereby igneous or sedimentary rocks undergo change in response to some combination of increased temperature, pressure, and chemically active fluids. Several types of metamorphism are recognized, but regional metamorphism is the most widespread. Regionally metamorphosed rocks in association with igneous rocks make up about 85 percent of the continents. In order to describe regionally metamorphosed rocks and to understand the processes by which they form, one must understand two properties of these rocks: texture and composition. Texture refers to the size, shape, and arrangement of crystals or particles in a rock. Composition refers to the minerals present in a rock.

The effect of temperature and pressure on texture and composition is aided by fluids. Fluids transport chemical constituents and facilitate chemical reactions that take place in the rocks. As metamorphic conditions increase, metamorphic reactions readily occur if fluids are present; however, fluids are gradually driven off as recrystallization reduces open spaces. Most of the chemical reactions that occur during metamorphism result in the loss of water or carbon dioxide. Once fluids are eliminated and temperatures and pressures decrease, the reactions generally occur very slowly. Therefore, in most metamorphic rocks, the minerals preserved are those that represent the highest temperatures and pressures attained, assuming equilibrium when adequate fluids were still available.

Regionally metamorphosed rocks characteristically develop distinctive textures, largely because of the role of two types of pressure in concert with elevated temperatures: confining pressure, or the force caused by the weight of the material overlying a rock, and a stronger directed pressure, or a horizontal force created by mountain-building processes.

Foliation, Schistosity, and Gneissic Texture

As metamorphism commences, rocks become deformed, and original features are strongly distorted or obliterated. Original minerals either recrystallize or react to form new minerals and develop a pronounced preferred orientation, or foliation, in response to the directed pressure. Preferentially aligned platy minerals (such as the micas), formed during the early stages of metamorphism, enhance the development of foliation. Foliation assumes different forms depending on the degree of metamorphism.

Although the minerals in most rock types characteristically develop preferred orientations, shales and other fine-grained rock types display more distinct changes of texture. Shales undergo textural changes as they pass from conditions of low-grade metamorphism (low temperatures and pressures) at shallow depths to high grades (higher temperatures and pressures) deeper in the earth. At low grades of regional metamorphism, microscopic platy minerals grow and align themselves perpendicular to the orientation of the directed pressure. This realignment allows this fine-grained rock, called a slate, to split or cleave easily along this preferred direction. Under somewhat higher metamorphic grades deeper in the earth, the platy minerals continue to develop parallel arrangements in response to the directed pressures, but the resultant mineral grains, typically the micas, are now large enough to be identified with the unaided eye. This texture, a schistosity, forms in a rock called a schist. At higher grades, the platy minerals react to form new minerals, and a gneissic texture develops. This texture is distinguished by alternating layers of light and dark minerals that give the rock, called a gneiss, a banded appearance.

Index Minerals

Just as textural changes in metamorphic rocks are generally predictable as grade increases, minerals also appear or disappear in a systematic sequence and become even better gages of the conditions of metamorphism. A progression of key minerals in shales, called index minerals, forms as the grade of metamorphism increases. These index minerals, therefore, occur in rocks metamorphosed at a given range of temperature-pressure conditions. When mapped in the field, areas containing index minerals are called metamorphic zones. As the metamorphic grades increase, the typical sequence of the index minerals is chlorite, biotite, garnet, staurolite, kyanite, and sillimanite.

Because metamorphic zonation based on index minerals utilizes minerals most likely to form from shales, zonation can be limited if shales are not abundant. A more precise definition of metamorphic change is recognized, which is not based on single minerals in one rock type but on sequences of mineral assemblages in groups of associated rocks within ranges of temperature-pressure conditions. These ranges of temperatures and pressures, in which diagnostic mineral assemblages in different rock types may exist, are called metamorphic facies. The zeolite facies represents the lowest conditions of regional metamorphism. At the lowest extremes of this facies, conditions considered sedimentary merge with those that are metamorphic. This facies is named for the zeolite group of minerals that forms within this facies. As metamorphic conditions increase, the assemblages of minerals distinctive of the zeolite facies break down, and new assemblages form that are distinctive of either the blueschist or the greenschist facies. As the names of these facies imply, the textures formed under these conditions are commonly schistose, and a number of the minerals impart either blue or green tints to the rocks in their respective facies. In a given area of regional metamorphism, it is typical to find rocks that represent a sequence of metamorphic facies that follow one of two pathways. The greenschist path has temperature increasing more rapidly than pressure, relatively speaking, so the combination of facies that it crosses is referred to as a low pressure/high temperature sequence. The less common blueschist path, in which temperature increases more slowly relative to pressure, represents a high pressure, low temperature sequence.

Understanding Regional Metamorphism Processes

By understanding how mountains are formed, scientists can learn how regionally metamorphosed rocks are generated. Regional metamorphism occurs in the roots of actively forming mountain belts. Linear zones thousands of kilometers long and hundreds of kilometers wide can be involved. It will take millions of years before the products of regional metamorphism are exposed in the Andes or Cascades that are forming now. Among the youngest exposed metamorphic rocks in the world are those in the Olympic Mountains of Washington, some of which are only ten million years old. On all the continents, however, broad areas of exposed rocks, called shields, now reveal the products of numerous ancient mountain-building events.

In the 1960s, the concept of plate tectonics revolutionized geology and scientists’ understanding of the processes in regional metamorphism. For decades, geologists have known that the earth is divided into several concentric layers. The outermost zone, the crust, consists of relatively low-density rocks and averages 35 kilometers thick on the continents and 10 kilometers thick under the oceans. Beneath the crust is the mantle (nearly 3,000 kilometers thick), and then the core.

With the advent of plate tectonics, another important subdivision was recognized. The upper 60 to 100 kilometers, which includes the crust and a portion of the upper mantle, behaves in a relatively rigid fashion and is composed of about twenty pieces that move independently of one another. This rigid zone is called the lithosphere, and the pieces are called lithospheric plates. Because each plate is moving relative to its neighbors, three types of boundaries with other plates exist: those in collision, those pulling apart, and those sliding by one another. The rigid lithosphere apparently floats on a soft plastic layer called the asthenosphere, which extends from 60 to 100 kilometers to at least 250 kilometers into the mantle. Plate tectonics allows us to understand the processes that occur during mountain building.

Convergent Boundaries

Regional metamorphism is restricted to margins in collision, called convergent boundaries. The collision of two lithospheric plates produces several profound effects. One plate subducts, or sinks, under the other but manages to crumple the edge of the continent into a mountain belt. An oceanic trench forms where the descending plate sinks beneath a continent. As one plate plunges downward, melting occurs where temperatures sufficiently rise. The molten material rises through the lithosphere to produce volcanoes at the surface and masses of igneous rock at depth. It is in this realm—extending from the zone of collision to well beyond the igneous bodies—that regional metamorphism occurs. Within this area, two distinct sequences of regional metamorphism develop. The high pressure/low temperature sequence forms in the region nearest the trench. As sediments and volcanic rocks are carried rapidly (in geological terms) downward into the subduction zone, they attain relatively high pressures but remain relatively cold because they have not had the time to heat up. The minerals that form in these high-pressure conditions are those of the zeolite and blueschist facies. If the downward movement continued uninterrupted, the rocks would eventually heat up. Before that can happen, however, thin, cold slabs with high-pressure mineral assemblages within them are chaotically pushed or thrust back toward the surface. Why this thrusting occurs is not entirely clear, but it is not uncommon to find a region 100 kilometers wide, adjacent to a trench, composed entirely of highly deformed high pressure/low temperature facies rocks. This region is called an accretionary wedge because of its inferred shape in a vertical slice through the earth. It is this material that allows the continents to grow larger through time. The California Coast Ranges are a good example of rocks that formed in such a setting.

Study of Regionally Metamorphosed Rocks

Geologists study regionally metamorphosed rocks from several different perspectives. At the core of these studies are field observations. Descriptions of rock units and structures provide information on how these rocks have formed. Much of this fieldwork is synthesized into a geological map.

In the laboratory, samples from the field are studied by a variety of physical and chemical methods. Studies with the petrographic microscope yield information on mineral compositions and textural relationships that provide clues for classifying and determining modes of origin for these rocks. X-ray powder diffraction techniques are commonly used to identify metamorphic minerals not readily identified by visual inspection. Since metamorphic rocks are markedly changed texturally and mineralogically from their original state, chemical analysis provides data that can be compared to probable premetamorphic rock types. Standard classical chemical methods of analysis may be used, but a number of more sophisticated, faster, and simpler spectrographic methods are popular. Atomic absorption and X-ray fluorescencespectroscopy are two of the more widely applied techniques to determine elemental abundances, although emission spectrographic and neutron activation analyses are also used. Another important tool is the electron microprobe. Most published mineral analyses are generated by electron microprobe analyses. Mass spectroscopic analyses also provide information on the distribution of isotopes in metamorphic rocks and minerals. These data are useful in determining age and conditions of formation.

Rock deformation experiments provide information about the mechanical properties of metamorphic rocks. Rock samples subjected to stress and strain tests yield data on properties such as plasticity, strength, and viscosity. These data are particularly important in understanding how directed pressures influence textures. Another phase of the study of metamorphic rocks is experimental petrology. Here, metamorphic minerals are synthesized under controlled-equilibrium conditions. From these studies, geologists gain knowledge about the actual ranges of stability of these minerals in naturally occurring environments. The study of the crystal chemistry and thermodynamics provides information regarding actual conditions of formation and potential reactions. The roles of variables—such as entropy, volume change, and heat of reaction—provide clues to the behavior of mineral assemblages at varying temperature and pressure conditions. Theoretical petrology—the treatment of data on metamorphic rocks by mathematical models or the principles of theoretical physics—also provides important information in the study of regional metamorphism.

In addition to analyzing the samples collected, scientists analyze field data in the laboratory. Statistical analysis of orientations of foliations and other structural features is greatly facilitated by the computer. Laboratory analyses are recast to generate a variety of graphical treatments to check for chemical trends or metamorphic facies relationships.

Applications for Industry and

The products of regional metamorphism touch people’s lives in many ways. Marbles, gneisses, and slates are used as building and cut ornamental stones, and some quartzites, marbles, and gneisses are used as aggregate. The minerals graphite, talc, vermiculite, and asbestos are four of the more commonly used mineral products of regional metamorphism, whereas wollastonite, garnet, kyanite, emery, and pyrophyllite are mineral products of limited use. Graphite is most commonly used in the metallurgical industry, with lesser amounts used as lubricants, paints, batteries, and pencil “leads,” and in electrodes. Talc is also a product with diverse uses. The ceramics industry is the largest consumer, followed by paint and paper manufacturers. Vermiculite is a common product in thermal and acoustic insulation. Asbestos, despite concerns about its health risks, remains an important fire-retardant material for appropriate uses.

Regionally metamorphosed rocks provide certain advantages in construction but also present unique engineering problems. A scan of the skyline of the island of Manhattan in New York City reveals that the largest skyscrapers are restricted to several areas of the island, surrounded by expanses of much shorter buildings. The reason that buildings such as the Empire State Building occupy only certain areas is that they sit directly on structurally strong regionally metamorphosed rocks that occur at or near the surface. In other areas of the island, these rocks are too deeply buried, and the overlying sediments are too weak to support the larger structures. In other cases, inclined foliations paralleling slopes in hilly or mountainous regions represent potential planes of slippage that can give way and produce massive rock slides, particularly when road, mine, or dam construction modifies the landscape. In areas where foliations are not adequately taken into account, the cost can be millions of dollars in repairs and/or great loss of life. Special techniques and restrictive measures need to be applied when construction occurs in areas where foliations present potential problems.

Principal Terms

equilibrium: a situation in which a mineral is stable at a given set of temperature-pressure conditions

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

igneous rock: a rock formed from the cooling of molten material

mica: a platy silicate mineral (one silicon atom surrounded by four oxygen atoms) that readily splits into thin, flexible sheets

mineral: a naturally occurring chemical compound that has an orderly internal arrangement of atoms and a definite formula

sedimentary rock: a rock formed from the physical breakdown of preexisting rock material or from the precipitation, chemically or biologically, of minerals

shale: a sedimentary rock composed of fine-grained products derived from the physical breakdown of preexisting rock material

strain: change in volume or size in response to stress

stress: force per unit of area

texture: the size, shape, and arrangement of crystals or particles in a rock

zeolites: members of a mineral group with very complex compositions: aluminosilicates with variable amounts of calcium, sodium, and water and a very open atomic structure that permits atoms to move easily

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