Coming up with a way to classify and name the great variety of metamorphic rocks on Earth hasn't been easy. After decades of debate, geologists have found it most convenient to divide metamorphic rocks into two fundamental classes: foliated rocks and non-foliated rocks. Each class contains several rock types. We distinguish foliated rocks from each other partly by their component minerals and partly by the nature of their foliation, whereas we distinguish non-foliated rocks from each other primarily by their component minerals.
Foliated Metamorphic Rocks
To understand this class of rocks, we first need to discuss the nature of foliation in more detail. The word comes from the Latin folium, for leaf. Geologists use foliation to refer to the parallel surfaces and/or layers that can occur in a metamorphic rock. Foliation can give metamorphic rocks a striped or streaked appearance in an outcrop, and/or can give them the ability to split into thin sheets. A foliated metamorphic rock has foliation either because it contains inequant mineral crystals that are aligned parallel to one another, defining preferred mineral orientation, and/or because the rock has alternating dark-coloured and light-coloured layers.
Foliated metamorphic rocks can be distinguished from one another according to their composition, their grain size, and the nature of their foliation. The most common types include
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| Slate is a foliated metamorphic rock that forms at relatively low temperature and pressure. |
- Slate: The finest-grained foliated metamorphic rock, slate, forms by metamorphism of shale or mudstone (rocks composed dominantly of clay) under relatively low pressures and temperatures. Slate contains a type of foliation called slaty cleavage, which allows it to split into thin sheets that make excellent roofing shingles (figure above a). Slaty cleavage develops when pressure solution removes portions of clay flakes that are not perpendicular to the compression direction, while clay flakes that are perpendicular to the compression direction grow. During the process, some flakes passively rotate into parallelism with the cleavage plane, pushed into the new orientation by compression. For example, end-on compression of a sequence of horizontal shale beds produces vertical slaty cleavage (figure above b). Commonly, such compression also causes the layers to bend into curves called folds.
- Phyllite: Phyllite is a fine-grained metamorphic rock with a foliation caused by the preferred orientation of very fine grained white mica. The word comes from the Greek word phyllon, meaning leaf, as does the word phyllo, the flaky dough in Greek pastry. The parallelism of translucent fine-grained mica gives phyllite a silky sheen known as phyllitic luster (figure above a). Phyllite forms by the metamorphism of slate at a temperature high enough to cause neocrystallization of white mica.
- Metaconglomerate: Under the metamorphic conditions that produce slate or phyllite, a protolith of conglomerate becomes metaconglomerate. Specifically, pressure solution and plastic deformation flatten pebbles and cobbles into pancake-like shapes. The alignment of inequant clasts defines a foliation (figure above b).
- Schist: Schist is a medium- to coarse-grained metamorphic rock that possesses a type of foliation, called schistosity, defined by the preferred orientation of large mica flakes (muscovite and/or biotite; figure above c). Schist forms at a higher temperature than does phyllite.
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| The formation of gneiss, which takes place at very high temperatures and pressures. |
- Gneiss: Gneiss is a compositionally layered metamorphic rock, typically composed of alternating dark-coloured and light-coloured layers that range in thickness from millimetres to meters. This compositional layering, or gneissic banding, gives gneiss a striped appearance (figure above a). How does the banding in gneiss form? Some evolved directly from the original bedding in a rock. For example, metamorphism of a protolith consisting of alternating beds of sandstone and shale produces a gneiss consisting of alternating beds of quartzite and mica. Gneissic banding can also form when the protolith undergoes an extreme amount of shearing under conditions in which the rock can flow like soft plastic (figure above b). Such flow stretches, folds, and smears out pre-existing compositional contrasts in the rock and transforms them into aligned sheets. Finally, banding in some gneisses can develop by an incompletely understood process called metamorphic differentiation. During differentiation, chemical reactions segregate different minerals into different layers (figure above c).
- Migmatite: Under certain conditions, gneiss may begin to melt, producing felsic magma and residual, still solid, mafic rock. If the melt freezes again before flowing out of the source area, a mixture of igneous rock and relict metamorphic rock forms. This mixture is called migmatite. In effect, a migmatite is part metamorphic and part igneous.
Nonfoliated Metamorphic Rocks
Nonfoliated metamorphic rocks contain minerals that recrystallized or grew during metamorphism, but have no foliation. The lack of foliation means either that metamorphism occurred in the absence of compression and shear, or that most of the new crystals can only grow in an equant form. We list below some of the rock types that can occur without foliation.
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| Examples of quartzite and marble typically, but not always, these are non-foliated. |
- Hornfels: Hornfels is a fine-grained nonfoliated rock that contains a variety of metamorphic minerals. The specific mineral assemblage in a hornfels depends on the composition of the protolith and on the temperature and pressure of metamorphism.
- Quartzite: Quartzite forms by the metamorphism of pure quartz sandstone. During metamorphism, pre-existing quartz grains recrystallize, creating new, larger grains. In the process, the distinction between cement and grains disappears, open pore space disappears, and the grains become interlocking. When quartzite cracks, the fracture cuts across grain boundaries in contrast, fractures in sandstone curve around grains. Quartzite looks glassier than sandstone and does not have the grainy, sandpaper-like surface characteristic of sandstone (figure above a). Depending on the impurities it contains, quartzite can vary in colour from white to gray, purple, or green.
- Marble: The metamorphism of limestone yields marble. During the formation of marble, calcite composing the protolith recrystallizes, so fossil shells, pore space, and the distinction between grains and cement disappear. Thus, marble typically consists of a fairly uniform mass of interlocking calcite crystals.
Sculptors love to work with marble because the rock is relatively soft and has a uniform texture that gives it the cohesiveness and homogeneity needed to fashion large, smooth, highly detailed sculptures. Marble comes in a variety of colours white, pink, green, and black depending on the impurities it contains. Michelangelo, one of the great Italian Renaissance artists, sought large, unbroken blocks of creamy white marble from quarries in the Italian Alps for his masterpieces (figure above b).
Not all marble is non-foliated. If the original protolith contained layers with different impurities, and shear caused the marble to flow plastically, the resulting marble has colour banding that makes it a prized decorative stone (figure above c).
Defining Metamorphic Intensity
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| Intensity of metamorphism is indicated by metamorphic grade |
Not all metamorphism takes place under the same physical conditions. For example, rocks carried to a great depth beneath a mountain range undergo more intense metamorphism than do rocks closer to the surface. Geologists use the term metamorphic grade in a somewhat informal way to indicate the intensity of metamorphism, meaning the amount or degree of metamorphic change. (To provide a more complete indication of the intensity of metamorphism, geologists use the concept of metamorphic facies; see Metamorphic Facies) Classification of metamorphic grade depends primarily on temperature, because temperature plays the dominant role in determining the extent of recrystallization and neocrystallization during metamorphism. Metamorphic rocks that form at relatively low temperatures (between about 250C and 400C) are lowgrade rocks, and metamorphic rocks that form at relatively high temperatures (over about 600C) are high-grade rocks. Intermediate-grade rocks form at temperatures between these two extremes (figure above a).
Different grades of metamorphism yield different metamorphic mineral assemblages. As grade increases, recrystallization and neocrystallization tend to produce coarser grains and new mineral assemblages that are stable at higher temperatures and pressures (figure above b).
Geologists discovered that the presence of certain minerals, known as index minerals, in a rock indicates the approximate metamorphic grade of the rock. The line on a map along which an index mineral first appears is called an isograd (from the Greek iso, meaning equal). All points along an isograd have approximately the same metamorphic grade. Metamorphic zones are regions between two isograds; zones are named after an index mineral that was not present in the previous, lower-grade zone. To compare rocks of different grades, you could take a hike from central New York State eastward into central Massachusetts in the eastern United States. Your path starts in a region where rocks were not metamorphosed, and it takes you into the internal part of the Appalachian Mountain belt, where rocks were intensely metamorphosed. As a consequence, you cross several metamorphic zones (figure above c).
Metamorphic Facies
In the early years of the 20th century, geologists working in Scandinavia, where erosion by glaciers has left beautiful, nearly unweathered exposures of rocks once buried very deeply in the crust, came to realize that metamorphic rocks, in general, do not consist of a hodgepodge of minerals formed at different times and in different places, but rather consist of a distinct set of minerals that grew in association with each other at a certain pressure and temperature. It seemed that such mineral assemblages more or less represent a condition of chemical equilibrium, meaning that the chemicals making up the rock had organized into a group of mineral grains that were to anthropomorphize a bit comfortable with each other and their surroundings, and thus did not feel the need to change further. The geologists also determined that the specific mineral assemblage in a rock depends on pressure and temperature conditions, and on the composition of the protolith.
This discovery led the geologists to propose the concept of metamorphic facies. A metamorphic facies is a set of metamorphic mineral assemblages indicative of a certain range of pressure and temperature. Each specific assemblage in a facies reflects the original protolith composition. According to this definition, a given metamorphic facies includes several different kinds of rocks that differ from each other in terms of chemical composition and, therefore, mineral content but all the rocks of a given facies formed under roughly the same temperature and pressure conditions. Geologists recognize several facies, of which the major ones are zeolite, hornfels, greenschist, amphibolite, blueschist, eclogite, and granulite. The names of the different facies are based on a distinctive feature or mineral found in some of the rocks of the facies.
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| The common metamorphic facies. The boundaries between the facies are depicted as wide bands because they are gradational and approximate. Note that some amphibolite-facies rocks and all granulite-facies rocks form only if the protolith is dry. The relatively rare P-P (”prehnite-pumpellyite”) facies, is named for two metamorphic minerals. |
We can represent the approximate conditions under which metamorphic facies formed by using a pressure temperature graph (figure above). Each area on the graph, labeled with a facies name, represents the approximate range of temperatures and pressures in which mineral assemblages characteristic of that particular facies form. For example, a rock subjected to the pressure and temperature at Point A (4.5 kbar and 400C) develops a mineral assemblage characteristic of the greenschist facies. As the graph implies, the pressure and temperature conditions defining boundaries between facies cannot be precisely determined, and the transitions between facies are gradual.
We can also portray the geothermal gradients of different crustal regions on the graph. Beneath mountain ranges, for example, the geothermal gradient passes through the zeolite, greenschist, amphibolite, and granulite facies. In contrast, in the accretionary prism that forms at a subduction zone, temperature increases slowly with increasing depth, so blueschist assemblages can form.
Credit: Stephen Marshak (Essentials of Geology)
