5.6: Metamorphism and Metamorphic Rocks

Metamorphism is the change that takes place within a body of rock as a result of it being subjected to conditions that are different from those in which it formed. In most cases—but not all—this involves the rock being deeply buried beneath other rocks, where it is subjected to higher temperatures and pressures and no longer in equilibrium as it was in its original environment of formation. Metamorphism typically includes the recrystallization of minerals plus the formation of new minerals and different textures though often retaining the same overall chemical composition of the parent, or source rock. This parent rock is referred to as the protolith, from the Greek proto- meaning first, and lithos- meaning rock.

Most metamorphism is the result of tectonic forces during mountain building (orogenic) episodes. Plate collisions, especially continent to continent collisions, introduce tremendous stress and heat, take place over vast areas and last millions of years. These are the primary factors, or conditions needed to allow the dramatic changes to occur from protolith to metamorphic rock product.

Figure \(\PageIndex{1}\): The rock cycle, showing the processes related to metamorphic rocks at the bottom. (CC BY; Steven Earle from https://opentextbc.ca/physicalgeology2ed/part/chapter-7-metamorphism-and-metamorphic-rocks/)

Types of Metamorphism

There are two primary environments that produce metamorphism that are important to Historical Geology:

Regional metamorphism. This occurs over a wide expanse of the crust, hundreds to more than a thousand kilometers across, resulting from the collision and compression of tectonic plates. Regional metamorphism can affect the pre-existing rock of the continental crust, any oceanic crust caught in the collision, plus the sedimentary rock cover that exists on each. Regional metamorphic processes can also affect the entire thickness of the crust and upper mantle and produce tremendous change in the pre-existing rock occurring tens of kilometers to several hundred kilometers deep.

Contact metamorphism is much simpler and affects pre-existing rock by baking them through contact with molten rock material either by intrusion of magma or extrusion of lava on the surface. Intrusion can occur very deep in the crust and involve large areas affected by heat associated with vast magma chambers. Intrusion can also be much smaller and take place closer to the surface or involve the baking of the surface rock through volcanic eruption of lava.

Metamorphic Processes

The main factors that control metamorphic change of pre-existing rock include:

• the mineral composition of the parent rock,
• the temperature at which metamorphism takes place,
• the amount and type of pressure during metamorphism,
• the types of fluids (mostly water) that are present during metamorphism, and
• the amount of time available for metamorphism.

Parent Rock (Protolith)

The parent rock is the rock that exists before metamorphism starts. As we see in the rock cycle, any type of rock can be a protolith including pre-existing metamorphic rock. The critical component of the parent rock is its mineral composition because it is the stability of each mineral that matters when metamorphism takes place. In other words, when a rock is subjected to increased temperatures, certain minerals may become unstable and begin to recrystallize in size, orientation or into completely new minerals.

Temperature

As we learned in the previous discussions of Bowen’s Reaction Series, minerals gain stability as temperature cools. Minerals remain stable over a specific range of temperature. For example, quartz is stable under all surface environmental temperatures will remain stable all the way up to about 1800 \(^{\circ}\)C. On the other hand, most clay minerals are only stable up to about 150\(^{\circ}\) or 200 \(^{\circ}\)C; above that, they transform into micas. Feldspar, the most common mineral of the crust, is stable up to approximately 1200 \(^{\circ}\)C.

Pressure

Pressure is the force exerted on the protolith by burial and/or tectonic stresses. Like heat, pressure can affect the chemical equilibrium of minerals in a rock. The pressure that affects metamorphic rocks can be grouped into confining pressure and directed stress. Stress is a scientific term indicating a force. Strain is the result of this stress, including metamorphic changes within minerals.

Figure \(\PageIndex{2}\): An illustration of different types of pressure on rocks. Confining pressure, where the pressure is essentially equal in all directions, and differential stress, where the pressure from the sides is greater than that from the top and bottom. In both diagrams, there is also pressure in and out of the page. (CC BY; Steven Earle.)

Confining Pressure

Pressure exerted on rocks under the surface is due to the simple fact that rocks lie on top of one another. When pressure is exerted from rocks above, it is balanced from below and sides, and is called confining or lithostatic pressure. Confining pressure results in equal pressure on all sides (see figure) and is responsible for causing chemical reactions to occur just like heat. These chemical reactions will cause new minerals to form.

Differential Stress

Differential stress is an unequal balance of forces on the protolith in one or more directions (see previous figure). Differential stress is most commonly associated with the tectonic movement of plates during mountain building (orogeny). Differential stress modifies the parent rock at a mechanical level, changing the arrangement, size, and/or shape of the mineral crystals. This creates an identifying texture, known as foliation, which is shown in the figure below.

Figure \(\PageIndex{3}\): A comparison of granite and gneiss. The texture of the protolith granite is the same in every direction (equigranular) but the gneiss has a pronounced foliation, which we infer to have developed from squeezing (compressional stress) in a direction that was perpendicular to the resulting foliation. (Peter Davis. Modified by Callan Bentley.)

Differential stress produces rock textures with a visible parallel alignment. In response to the differential stress, mineral crystals can rotate, changing their orientation in space. This is a process of recrystallization in a solid state. The atomic structure of the mineral will respond to the stress and atoms will migrate within the structure in response to the stress. A good example would be the reorientation of the mineral biotite in the samples above. Biotite is a black, sheet silicate that can be easily peeled apart in thin layers. The biotite crystals are randomly oriented in the igneous granite sample on the left. When subjected to the differential stress of tectonic forces, the biotite crystals will realign perpendicular to the directions of stress providing the visually foliated metamorphic rock texture.

Recrystallization can also lead to minerals increasing in grain size as atoms migrate from an area of rock experiencing high stress and precipitate or regrow in a location having lower stress. You can visualize this similar to the phenomenon that occurs with small, adjacent soap bubbles coalesce to form larger ones. A good example of this would be the recrystallization of calcite that occurs in the metamorphism of limestone to marble (below). Note the crystalline fabric of the marble on the right versus the very fine mud-like texture of the sedimentary limestone, micrite, on the left.

Figure \(\PageIndex{4}\): Laminated micrite (left) compared to a crystalline banded marble (right). (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Fluids

Water is the main fluid present within rocks of the crust. Water facilitates the movement of ions within the protolith being subjected to metamorphic conditions. Water assists in metamorphic mineral growth and also acts as a catalyst that can increase the rate at which metamorphic reactions take place.

Time

Most metamorphic reactions take place at very slow rates. For example, the growth of new minerals within a rock during metamorphism has been estimated to be about 1 millimetre per million years. For this reason, it is very difficult to study metamorphic processes in a lab.

While the rate of metamorphism is slow, the tectonic processes that lead to metamorphism are also very slow, so in most cases, the chance for metamorphic reactions to be completed is high. A mountain range takes tens of millions of years to form, and tens to hundreds of millions of years to be eroded to the extent that we can see the rocks that were metamorphosed within the deep interior.

Classification of Metamorphic Rocks

Metamorphic rocks are classified based on two characteristics: Texture and Composition. Texture is first characteristic observed in the identification process. There are two main types of metamorphic rocks:

Foliated: formed in an environment subjected to differential stress resulting in a distinct alignment of minerals. Foliated metamorphic rocks are named based on the style of their foliations. Each rock name has a specific texture that defines and distinguishes it.

Non-foliated: either not subjected to directed pressure or, the dominant mineral does not tend to display any alignment.

Table \(\PageIndex{1}\): Metamorphic Rock Identification. (Created by Belinda C. Madsen for Salt Lake Community College)

1. Identify Rock's Foliation	1. Identify Rock's Foliation	2. Textural Features	3. Mineral Composition	4. Rock Name	5. Parent Rock
FOLIATED layered texture	Fine-grained or no visible grains	Flat, slaty cleavage is well developed. Dense, microscopic grains, may exhibit slight sheen (or dull luster). Clanky sound when struck. Breaks into hard, flat sheets.	Fine, microscopic clay or mica.	SLATE	Shale
		Finely crystalline; micas hardly discernible, but impart a sheen or luster. Breaks along wavy surfaces.	Dark silicates and micas.	PHYLLITE	Siltstone or shale
	Medium- to coarse-grained	Schistose texture. Foliation formed by alignment of visible crystals. Rock breaks along scaly foliation surfaces. Medium to fine-grained. Sparkling appearance.	Common minerals include chlorite, biotite, muscovite, garnet and hornblende. Recognizable minerals used as part of rock name. Porphyroblasts common.	MICA SCHIST GARNET SCHIST	Siltstone or shale
		Gneissic banding. Coarse-grained. Foliation present as minerals arranged into alternating light and dark layers giving the rock a banded texture in side view. Crystalline texture. No cleavage.	Light-colored quartz and feldspar; dark ferromagnesian minerals.	GNEISS	Shale or granitic rocks
NON-FOLIATED no layered texture	Fine-grained or no visible grains	Medium- to coarse-grained crystalline texture.	Crystals of amphibole (hornblende) in blade-like crystals.	AMPHIBOLITE	Basalt, gabbro, or ultramafic igneous rocks
		Microcrystalline texture. Glassy black sheen. Conchoidal fracture. Low density.	Fine, tar-like, organic makeup.	ANTHRACITE COAL	Coal
		Dense and dark-colored. Fine or microcrystalline texture. Very hard. Color can range from gray, gray-green to black.	Microscopic dark silicates.	HORNFELS	Many rock types
		Microcrystalline or no visible grains with smooth, wavy surfaces. May be dull or glossy. Usually shades of green.	Serpentine. May have fibrous asbestos visible.	SERPENTINITE	Ultramafic igneous rocks or peridotite
	Fine- to coarse-grained	Microcrystalline or no visible grains. Can be scratched with a fingernail. Shades of green, gray, brown or white. Soapy feel.	Talc.	SOAPSTONE OR TALC SCHIST	Ultramafic igneous rocks
		Crystalline. Hard (scratches glass). Breaks across grains. Sandy or sugary texture. Color variable; can be white, pink, buff, brown, red, purple.	Quartz grains fused together. Grains will not rub off like sandstone.	QUARTZITE	Quartz sandstone
		Finely crystalline (resembling a sugar cube) to medium or coarse texture. Color variable; white, pink, gray, among others. Fossils in some varieties.	Calcite or dolomite crystals tightly fused together. Calcite effervesces with HCl; dolomite effervesces only when powdered.	MARBLE	Limestone or dolostone
		Texture of conglomerate, but breaks across clasts as easily as around them. Pebbles may be stretched (lineated) or cut by rock cleavage	Granules or pebbles are commonly granitic or jasper, chert, quartz or quartzite.	META-CONGLOMERATE	Conglomerate

Foliated Metamorphic Rocks

Figure \(\PageIndex{5}\): Slate in many different colors, depending on the composition of the protolith. (By C. E. Jones, 2003, from: Geologic Image Archive at the University of Pittsburgh with permission for educational purposes.)

Figure \(\PageIndex{6}\): A slate boulder on the side of Mt. Wapta in the Rockies near Field, BC. Bedding is visible as light and dark bands sloping steeply to the right (yellow arrows). Slaty cleavage is evident from the way the rock has broken (along the flat surface that the person is sitting on) and also from lines of weakness that are parallel to that same trend (red arrows). (CC BY; Steven Earle.)

Slate is a fine-grained metamorphic rock that exhibits a foliation called slaty cleavage that is the flat orientation of the small platy crystals of mica and chlorite recrystallized perpendicular to the direction of stress. The minerals in slate are too small to see with the unaided eye. The thin layers in slate may resemble sedimentary bedding, but they are a result of differential stress and may lie at angles to the original strata (see photo below). In fact, original sedimentary layering may be partially or completely obscured by the foliation. The protolith for slate was a shale-type sedimentary rock composed of clay. Slate represents low-grade metamorphism.

Phyllite is also a fine-grained foliated metamorphic rock in which the platy chlorite and mica minerals have grown larger and the surface of the foliation shows a sheen from light reflecting from the grains. The texture is “phylittic” where we can detect an increase in size of the minerals over slate however the individual minerals are still too small to be differentiated with the naked eye.

Figure \(\PageIndex{7}\): Phyllite displaying “phylittic” texture. Note displaying cross-bedding of the sedimentary protolith. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Schist. The metamorphic rock schist displays a “shistose” foliation in which the minerals are now visible as individual crystals. Common minerals are muscovite, biotite, and porphyroblasts of garnets. A porphyroblast is a large crystal of a particular mineral surrounded by small grains. Varieties of schist are named for their dominant minerals such as mica schist (mostly micas) or garnet mica schist (mica schist with garnets).

Figure \(\PageIndex{8}\): Left: Biotite schist displaying coarse-grained “schistose” texture. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.) Right: Garnet Mica Schist (both biotite and muscovite). Also displaying the coarse-grained “schistose” texture with large porphyroblasts of garnet. (CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Gneiss and Migmatite. Mineral banding in a metamorphic rock produces a “gneissic” foliation in which visible silicate minerals separate into dark and light bands. The mineral grains tend to be coarse and the bands are often folded due to the extreme pressure and temperature conditions at which this type of metamorphic rock forms. A rock with this texture is called gneiss. Some gneisses may be subjected to temperatures at which partial melting can occur. This partially melted rock is a transition between metamorphic and igneous rocks called a migmatite. The melting will affect the minerals in the gneiss formed at the lowest temperatures on Bowen’s Reaction Series (quartz and potassium feldspar) and will collect in small lenses within the migmatite.

Figure \(\PageIndex{9}\): (Top) Muscovite, biotite, garnet gneiss displaying “gneissic” foliation. (Bottom) Migmatite. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Non-foliated Metamorphic Rocks

Non-foliated textures do not display alignment of mineral grains. Non-foliated metamorphic rocks are typically composed of just one mineral, and therefore, usually show the effects of metamorphism with recrystallization in which crystals grow together, but with no preferred direction. The two most common examples of non-foliated rocks are quartzite and marble.

Amphibolite is the result of metamorphism of a mafic igneous protolith. This may have originally been basalt or gabbro with olivine, pyroxene (augite) and calcium-rich plagioclase. The dominant mineral now is amphibole (hornblende). Zoom in on this sample and you will also see the green clay mineral chlorite.

Greenstone is a fine grained metamorphic rock that is largely composed of the green chlorite mineral plus other typically green metamorphic minerals, epidote and serpentine. The protolith of a greenstone is most commonly the mafic igneous rock, basalt. The original fine grained, iron and magnesium rich, minerals of olivine and pyroxene have recrystallized through metamorphic processes to form the greenstone rock. Greenstone is very important to Historical Geology, preserving some of the oldest slivers of early ocean crust. This is discussed in much more detail elsewhere in this text. The photo on the left, below is a hand sample of greenstone taken from the outcrop location on the right. Zoom into the sample on the left to see the fine-grained green minerals. Zoom into the photo on the left to get a feel for the metamorphic fabric and overall green color of the rock.

Figure \(\PageIndex{10}\): Left: Greenstone. Right: Outcrop location. (CC BY Attribution 3.0; Left photo: Robin Rohrback; right photo: Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Quartzite and Marble

Figure \(\PageIndex{11}\): Left: Quartzite is a metamorphic rock with the protolith being quartz sandstone. In quartzite, the quartz grains and silica cement of the original sandstone become fused and interlocking by the recrystallization process. Zoom in and note the cohesive fabric produced by the interlocking crystals of quartz. (CC BY Attribution 3.0; Callan Bentely, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.) Right: Marble is metamorphosed limestone (or dolostone) composed of calcite (or dolomite). Recrystallization typically generates visible interlocking crystals of calcite or dolomite. (CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan.)

Marble and quartzite often look similar however there are simple tests that can distinguish the two. First, the quartz minerals that compose the quartzite are considerably harder than the calcite that composes the marble. A simple hardness test on a piece of glass will show that quartz will scratch the glass, however calcite will not. Another way to easily distinguish quartzite from marble is with a drop of dilute hydrochloric acid. Marble will effervesce (fizz) because it is made of calcite. Remember to scratch the sample to make a little powder and then use a drop of hydrochloric acid to test. It will slowly effervesce if it is a dolomite marble.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Metamorphism caused by tectonic plate movement that takes place over a great expanse of the crust and upper mantle is called

a. confining metamorphism

b. contact metamorphism

c. differential metamorphism

d. regional metamorphism

Answer

d. regional metamorphism

Exercise \(\PageIndex{2}\)

The "protolith" is

a. the resulting metamorphic rock.

b. the parent rock that exists before metamorphism starts.

c.the final rock at the end of the rock cycle.

Answer

b. the parent rock that exists before metamorphism starts.

Exercise \(\PageIndex{3}\)

Differential stress will create a metamorphic rock texture called

a. crystallization

b. non-foliation

c. foliation

d. mineral organization

Answer

c. foliation

Exercise \(\PageIndex{4}\)

The rate of mineral growth during the recrystallization process of metamorphism is estimated to be

a. 1 millimeter per million years

b. 10 millimeters per 10 thousand years

c. 1 millimeter per billion years

d. 1 meter per million years

Answer

a. 1 millimeter per million years

Exercise \(\PageIndex{5}\)

The mineral calcite is the dominant composition of limestone, both chemical and biochemical limestone. When limestone is buried deeply and subjected changes in pressure and temperature the calcite will recrystallize to form the metamorphic rock, marble. Which of these would be considered the protolith?

a. marble

b. limestone

c. calcite

Answer

b. limestone

Exercise \(\PageIndex{6}\)

Metamorphism that occurs adjacent to a magma chamber would be of which type?

a. regional

b. temporary

c. contact

Answer

c. contact