Metamorphic textures

Metamorphic textures are important criteria in the description, classification, and understanding of the conditions under which metamorphic rocks form. Textures vary widely and develop as a result of the interaction between deformation, recrystallization, new mineral growth, and time.

Metamorphism Types

In the classification and description of sedimentary, igneous, or metamorphic rocks, texture is a very important factor. Texture has a relatively straightforward definition—it is the size, shape, and arrangement of particles or minerals in a rock. How texture is related to the rock fabric and rock structure may be confusing. Rock structure may refer only to features produced by movement that occurs after the rock is formed. Rock fabric sometimes refers to both the structure and the texture of crystalline (most igneous and metamorphic) rocks. There is so much disagreement that the three terms are used interchangeably. Although there is no agreement on the categories of metamorphism, some are more generally accepted than others. Regional, contact, and cataclastic are three principal types. Shock, burial, sub-seafloor, and hydrothermal metamorphism are other varieties of metamorphism.

Metamorphic textures develop in response to several factors, the most influential being elevated temperatures and pressures and chemically active fluids. In each type of metamorphism, the relative influence of each of these three factors varies greatly. Generally, the effect of temperature on metamorphism is easy to understand. Converting dough to bread by baking provides a common analogy. The roles of pressure and fluids, however, are more difficult to explain. Two types of pressure, confining and directed, play a role in metamorphism. Confining pressure is the force exerted on a rock in all directions by the overlying material. Directed pressure, as the name implies, is a force exerted more strongly in some directions than others by some external process, usually tectonic in origin. Of these two types, directed pressure produces more apparent textural changes in metamorphic rocks. The primary role of chemically active fluids is to facilitate the metamorphic reactions and textural changes that occur.

Of all the types of metamorphism, the most pervasive is regional metamorphism. Regional metamorphism occurs in the roots of mountain belts as they are forming. Thousands of square kilometers can be involved. Temperature and directed pressure are the critical agents in producing the new mineral assemblages and textures distinctive of this type of metamorphism. The most diagnostic feature in regionally metamorphosed rocks is a planar fabric, which forms as platy and elongate minerals develop a preferred orientation. This planar fabric, or foliation, generally forms only in the presence of the directed pressure, and in a perpendicular orientation to it. Directed pressure may also impart a linear texture to some rocks. This lineation may develop because elongate minerals or groups of minerals are recrystallized, deformed into a preferred direction, or both. Lineations may also develop where two planar fabrics intersect. Of all the different rock types, shales and other fine-grained rocks are the most sensitive to increasing conditions of metamorphism, and they show marked textural changes as metamorphic conditions increase. Not all regionally metamorphosed rocks develop foliations or lineations; many rock types show very little textural change from the lowest to highest conditions of metamorphism.

As metamorphism begins, shale is converted to slate, which diagnostically splits along smooth surfaces called cleavage. Most cleavage is pervasive or penetrative fabric; directed pressure influences every portion of the rock, with most of the micaceous minerals undergoing recrystallization. A variety of penetrative fabrics characterize regionally metamorphosed rocks formed at different metamorphic conditions. In so-called fracture cleavage, the rock contains distinct planar fractures or cracks that are separated by discrete, relatively undeformed segments of rock. A large amount of fracture cleavage is probably the result of solution activity removing portions of the rock.

At the lowest grades of regional metamorphism, and as the penetrative fabric develops in slate, very fine-grained micaceous minerals start to grow in a preferred orientation, thus producing planes of weaknesses along which the slate readily splits. At slightly higher metamorphic conditions, the micaceous minerals continue to form. The rock typically takes on a pronounced sheen, although the individual mineral grains remain mostly too small to see with the unaided eye. This texture is referred to as a phyllitic texture, and the rock is called a phyllite. As metamorphic conditions continue to increase at ever greater depths, the micaceous minerals eventually grow large enough to be seen with the naked eye, and the preferred orientation of these platy minerals becomes obvious. This texture is referred to as schistosity, and the rock itself is called a schist. As the conditions of metamorphism approach very high levels, the micaceous minerals start to break down to form other minerals that are not platy, and the rock begins to lose its property to split along foliation surfaces. When rocks are metamorphosed, very large crystals of minerals such as andalusite, garnet, and cordierite may develop in an otherwise fine-grained rock. These large crystals constitute a porphyroblastic texture and are themselves called porphyroblasts. A new type of foliation develops, however, because light and dark minerals tend to separate into alternating bands. This banded texture is characteristic of rocks called gneisses, and the texture is referred to as a gneissosity. At even higher conditions, rocks begin to melt in a process called anatexis. Anatexis occurs at conditions that are considered to be at the interface between metamorphism and igneous processes.

Although not as widespread as regional metamorphism, contact metamorphism is of interest because many contact metamorphic mineral deposits are economically important. Contact metamorphism occurs in areas of relatively shallow depths where a hot, molten igneous body comes in contact with the cooler rock that it has invaded. The metamorphism takes place outside the margins of the igneous mass, with temperature as the key agent of metamorphism and recrystallization as the dominant process modifying the original rock. The area metamorphosed around the intrusion is called the aureole. The size of this aureole is a function of the size, composition, and temperature of the igneous body, as well as the composition of the host rock and depth of the intrusion. The intensity of the metamorphism is greatest at the contact between the two rock bodies and decreases away from the source of heat. Aureoles may be as thin as a meter or, on rare occasions, as wide as 2 to 3 kilometers; usually, however, they are tens of meters wide.

A number of rock types and textures are characteristic of contact metamorphism. Most rocks produced by contact metamorphism are fine-grained because the processes by which they were formed are relatively short-lived. Typically, the aureole is made of a hard, massive, and fine-grained rock called hornfels. Characteristically, hornfels is more fine-grained than metamorphosed rock. The uniform grain size gives the rock a granular appearance, particularly when magnified; this texture is termed granoblastic or hornfelsic. Because hornfels undergoes no deformation, textures in the original rock may be preserved, even when recrystallization is complete. If the aureole contains silicon-rich carbonate rocks (impure limestones), much coarser textures are likely to form at or near the contact of the igneous body. These coarser-grained rocks are termed tactites or skarns. In some cases the intrusion is emplaced forcefully and pushes the surrounding rock out of the way. Under these conditions, contact metamorphism may produce foliation or lineation.

Cataclastic Metamorphism

The third type of metamorphism is cataclastic or dynamic metamorphism, which occurs in fault or deep-shear zones. Directed pressure is the key agent, with temperature and confining pressure playing variable roles. The textures in cataclastic rocks are divided into four groups: incoherent, nonfoliated, mylonitic, and foliated-recrystallized. Incoherent texture develops at very shallow depths and low confining pressures. Based on the degree of cataclasis (from least to most), fault breccia and fault gouge are the rock types formed. Nonfoliated (or mortar) texture forms coherent rocks typified by the presence of porphyroclastic minerals (large crystals that have survived cataclasis) surrounded by finely ground material. Rocks with this texture lack obvious foliations. Also included in this group are cataclastic rocks with glassy textures. Unusually intense cataclasis produces enough frictional heating to melt portions of the rock partially. Mylonitic texture occurs in coherent rocks that show a distinct foliation. The foliation typically forms because alternating layers show differing intensities of grinding. Textural subdivisions and rock names are based on the degree of grinding, such as protomylonite (least amount of grinding), orthomylonite, and ultramylonite (most amount of grinding). The final group is the foliated-recrystallized textures. Cataclastic rocks that illustrate considerable recrystallization are also typically foliated. The degree of grinding determines the rock name. Mylonite gneiss shows the least cataclasis, whereas the blastomylonite is more crushed.

Shock Metamorphism

Shock metamorphism is the rarest type of metamorphism. It is characteristically associated with meteorite craters and astroblemes. Astroblemes are circular topographic features that are inferred to represent the impact sites of ancient meteorites or comets. Some geologists consider shock metamorphism to be a type of dynamic metamorphism because directed pressure plays the essential role in both, whereas temperature may vary from low to extremely high.

A number of textural features are associated with shock metamorphism. Brecciation (breaking into angular fragments), fracturing, and warping of crystals are common. On a microscopic scale, the presence of two or more sets of deformation lamellae in quartz crystals is considered conclusive evidence of a meteorite impact. (Deformation lamellae are closely spaced microscopic parallel layers that are partially or totally changed to glass or are sets of closely spaced dislocations within mineral grains.) Shock metamorphism is also detected when minerals are partially or completely turned to glass (presumably with and without melting) and also when shatter cones are present. Shatter cones are cone-shaped rock bodies or fractures typified by striations that radiate from the apex.

Although much of the work of describing metamorphic textures comes from the field of metamorphic petrology, the means of studying how these textures, fabrics, or structures are generated is more within the area of structural geology called structural analysis. Structural analysis relies heavily on the fields of metallurgy, mechanics, and rheology to describe and interpret the process by which rocks are deformed. Structural analysis considers metamorphic textures and larger features, such as faults and folds, from three different perspectives: descriptive analysis, kinematic analysis, and dynamic analysis. Synthesis of the information obtained from these three analyses typically leads to comprehensive models or hypotheses that explain problems concerning metamorphic textures.

Descriptive analysis is the foundation of all studies of metamorphic textures. In descriptive analysis, geologists consider the physical and geometric aspects of deformed rocks, which include the recognition, description, and measurement of the orientations of textural elements. Typically, descriptive analysis involves geologic mapping, in which geologists measure the orientations of metamorphic structures with an instrument that is a combination compass-clinometer. (A clinometer is an instrument or scale used to measure the angle of an inclined line or surface in the vertical plane.) These structural measurements are plotted in a variety of ways to determine if the data show any statistical significance. Field studies may also include analysis of aerial photographs or satellite imagery to determine if textures or structures that are evident on the microscopic scale, or those of a rock outcrop, are related to much larger patterns.

In the laboratory, descriptive analysis of metamorphic textures can involve several techniques. One of the most important involves the petrographic microscope, in which magnified images of textures result from the transmission of polarized light through thin sections of properly oriented rock specimens. The thin section is a paper-thin slice of rock that is produced by gluing a small cut and polished block of the rock specimen to a glass slide. This block, commonly a little less than 2.7 millimeters by 4.5 millimeters, is then cut and ground to the proper thickness.

Kinematic analysis is the study of the displacements or deformational movements that produce features such as metamorphic textures. Several types of movement are recognized, including distortion and dilation. The study of distortion (change of shape) and dilation (change of volume) in a rock is called strain analysis. Strain analysis involves the quantitative evaluation of how original sizes and shapes of geological features are changed. Although strain commonly expresses itself as movement along preexisting surfaces, distortion also creates new surfaces such as cleavages and foliations in metamorphic rocks. Often deformation reorients the crystal structures of deformed minerals. The alignment of mineral structures can be studied with the universal stage, a device that allows slides under the microscope to be tilted to any desired orientation.

Dynamic analysis attempts to express observed strains in terms of probable patterns of stress. Stress is the force per unit of area acting upon a body, typically measured in kilograms per square meter. One important area of study in dynamic analysis involves experimental deformation of rocks under various levels of temperature, confining pressure, and time. Such experiments are typically conducted on short cylindrical specimens on a triaxial testing machine, which attempts to simulate natural conditions with the variables controlled. By jacketing specimens in impermeable coverings, pressures can be created hydraulically to exceed 10 kilobars, which are comparable to those near the base of the earth’s crust, or outermost layer. Dynamic analysis also involves scale-model experiments, using clays and other soft substances to attempt to replicate naturally occurring structural features.

Applications in Building and Construction

Although a number of metamorphic minerals and rocks are used by society, in most cases the metamorphic textures of these materials play no role in their utility. One notable exception is slate. Because metamorphic processes impart a strong cleavage to this very fine-grained rock, it readily splits apart along these thin, smooth surfaces. The combination of grain size, cleavage, and strength makes slate useful in a number of products, including roofing shingles, flagstones, electrical panels, mantels, blackboards, grave vaults, and billiard tables. Most other metamorphic rocks, however, have foliations or other textural features that make the rocks structurally weak, and limit their usefulness.

In other cases, however, metamorphic rocks are useful because they lack pronounced foliations or do not readily break along planes of weakness. Many widely used marbles, particularly those quarried in Italy, Georgia, and Vermont, are products of regional metamorphism. They are prized in part because they are generally massive (lacking texturally induced planes of weakness) and can be cut into very large blocks. Several quartzites and gneisses also tend to be massive enough to be used as dimension stone or as aggregate because of high internal strength and relative chemical inertness. Some gneisses are prized as decorative stones because of their complex patterns.

Aside from direct usage, problems can develop in mountainous or hilly regions when construction occurs where weak, intensely foliated metamorphic rocks exist. When road or railway cuts are excavated through foliations inclined toward these cuts, rock slides are possible. In some cases, the roads can be relocated, or, if not, slide-prone slopes may be modified or removed. In other places, slide-prone exposures or mine walls and ceilings can be pinned and anchored.

Construction of buildings in metamorphic areas also requires careful evaluation. Where weak foliations run parallel to slopes, it must be determined whether the additional weight of structures and water for lawns and from runoff are likely to induce rock slides or other forms of slope instability. If so, land-use plans and zoning restrictions need to be adopted to indicate that these areas are potentially hazardous.

Principal Terms

coherent texture: an arrangement allowing the minerals or particles in a rock to stick together

foliation: a planar texture in metamorphic rocks

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

strain: deformation resulting from stress

stress: force per unit of area

tectonics: the study of the processes and products of large-scale movement and deformation within the earth

Bibliography

Andrei, Mihai. "Metamorphic Rocks: Formation, Types, Examples." ZME Science, 14 Feb. 2024, www.zmescience.com/feature-post/natural-sciences/geology-and-paleontology/rocks-and-minerals/metamorphic-rocks/. Accessed 29 July 2024.

Barker, A. J. Introduction to Metamorphic Textures and Microstructures. 2d ed. London: Routledge, 2004.

Best, Myron G. Igneous and Metamorphic Petrology. 2d ed. Malden, Mass.: Blackwell Science Ltd., 2003.

Blatt, Harvey, Robert J. Tracy, and Brent Owens. Petrology: Igneous, Sedimentary, and Metamorphic. 3d ed. New York: W. H. Freeman, 2005.

Chernicoff, Stanley. Geology: An Introduction to Physical Geology. 4th ed. Upper Saddle River, N.J.: Prentice Hall, 2006.

Davis, George H. Structural Geology of Rocks and Regions. 2d ed. New York: John Wiley & Sons, 1996.

Dietrich, Richard V., and B. J. Skinner. Rocks and Rock Minerals. New York: John Wiley & Sons, 1979.

Dolgoff, Anatole. Physical Geology. Boston: Houghton Mifflin, 1999.

Ernst, W. G. Earth Materials. Englewood Cliffs, N.J.: Prentice-Hall, 1969.

Fettes, Douglas, and Jacqueline Desmons, eds. Metamorphic Rocks: A Classification and Glossary of Terms. New York: Cambridge University Press, 2007.

Hyndman, Donald W. Petrology of Igneous and Metamorphic Rocks. 2d ed. New York: McGraw-Hill, 1985.

"Metamorphic Texture." Gelogia, 15 Nov. 2022, gelogia.com/metamorphic-texture/. Accessed 25 July 2024.

Newton, Robert C. “The Three Partners of Metamorphic Petrology.” American Mineralogist 96 (2011): 457-469.

Vernon, Ron H. A Practical Guide to Rock Microstructure. New York: Cambridge University Press, 2004.

Williams, Howel, F. J. Turner, and C. M. Gilbert. Petrology: An Introduction to the Study of Rocks in Thin Sections. 2d ed. San Francisco: W. H. Freeman, 1982.