2.6: Metamorphic Rocks
- Page ID
- 20335
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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 than those under which it formed. Metamorphic rocks typically have different mineral assemblages and different textures from their parent rocks, but they may have the same overall composition.
Controls on Metamorphism
The main factors that control metamorphic processes are:
- 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
The parent rock is the rock that exists before metamorphism starts. Sedimentary or igneous rocks can be considered the parent rocks for metamorphic rocks. Although an existing metamorphic rock can be further metamorphosed or re-metamorphosed, metamorphic rock doesn’t normally qualify as a “parent rock”. For example, if a mudstone is metamorphosed to slate and then buried deeper where it is metamorphosed to schist, the parent rock of the schist is mudstone, not slate. The critical feature of the parent rock is its mineral composition because it is the stability of minerals that counts when metamorphism takes place. In other words, when a rock is subjected to increased temperatures, certain minerals may become unstable and start to recrystallize into new minerals.
Temperature
The temperature that the rock is subjected to is a key variable in controlling the type of metamorphism that takes place. As we learned in the context of igneous rocks, mineral stability is a function of temperature, pressure, and the presence of fluids (especially water). All minerals are stable over a specific range of temperatures. For example, quartz is stable from surface temperatures all the way up to about 1800°C. If the pressure is higher, that upper limit will be even higher. If there is water present, it will be lower. On the other hand, most clay minerals are only stable up to about 150° or 200°C; above that, they transform into micas. Most feldspars are stable up to between 1000°C and 1200°C. Most other common minerals have upper limits between 150°C and 1000°C.
Some minerals will crystallize into different polymorphs (same composition, but different crystalline structure) depending on the temperature and pressure. The minerals kyanite, andalusite, and sillimanite are polymorphs with the composition Al2SiO5. They are stable at different pressures and temperatures, and, as we will see later, they are important indicators of the pressures and temperatures that existed during the formation of metamorphic rocks (Figure 7.1.1 ).
| Depth (kilometers) | Kyanite | Andalusite | Sillimanite |
|---|---|---|---|
| 5 | Less than 300°C | 300 to 650°C | Greater than 670°C |
| 10 | Less than 400°C | 410 to 580°C | Greater than 590°C |
| 15 | Less than 500°C | Not stable | Greater than 500°C |
| 20 | Less than 570°C | Not stable | Greater than 590°C |
| 25 | Less than 640°C | Not stable | Greater than 620°C |
| 30 | Less than 700°C | Not stable | Greater than 700°C |
Pressure
Pressure is important in metamorphic processes for two main reasons. First, it has implications for mineral stability (Figure 7.1.1). Second, it has implications for the texture of metamorphic rocks. Rocks that are subjected to very high confining pressures are typically denser than others because the mineral grains are squeezed together (Figure 7.1.2 a), and also because they may contain minerals that have greater density because the atoms are more closely packed.
Because of plate tectonics, pressures within the crust are typically not applied equally in all directions. In areas of plate convergence, for example, the pressure in one direction (perpendicular to the direction of convergence) is typically greater than in the other directions (Figure 7.1.2 b). In situations where different blocks of the crust are being pushed in different directions, the rocks will likely be subjected to sheer stress (Figure 7.1.2 c).
One of the results of directed pressure and shear stress is that rocks become foliated—meaning that they’ll have a directional fabric. Foliation a very important aspect of metamorphic rocks, and is described in more detail in the next section.
Fluids
Water is the main fluid present within rocks of the crust, and the only one that we’ll consider here. The presence of water is important for two main reasons. First, water facilitates the transfer of ions between minerals and within minerals, and therefore increases the rates at which metamorphic reactions take place. So, while the water doesn’t necessarily change the outcome of a metamorphic process, it speeds the process up so metamorphism might take place over a shorter time period, or metamorphic processes that might not otherwise have had time to be completed are completed.
Secondly, water, especially hot water, can have elevated concentrations of dissolved elements (ions), and therefore it is an important medium for moving certain elements around within the crust. So not only does water facilitate metamorphic reactions on a grain-to-grain basis, it also allows for the transportation of elements from one place to another. This is very important in hydrothermal processes, which are discussed toward the end of this chapter, and in the formation of mineral deposits.
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 millimeter 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. For example, one important metamorphic setting is many kilometers deep within the roots of mountain ranges. A mountain range takes tens of millions of years to form, and tens of millions of years more to be eroded to the extent that we can see the rocks that were metamorphosed deep beneath it.
Classification of Metamorphic Rocks
There are two main types of metamorphic rocks: those that are foliated because they have formed in an environment with either directed pressure or shear stress, and those that are not foliated because they have formed in an environment without directed pressure or relatively near the surface with very little pressure at all. Some types of metamorphic rocks, such as quartzite and marble, which can form whether there is directed-pressure or not, do not typically exhibit foliation because their minerals (quartz and calcite respectively) do not tend to show alignment.
When a rock is squeezed under directed pressure during metamorphism it is likely to be deformed, and this can result in a textural change such that the minerals appear elongated in the direction perpendicular to the main stress (Figure 7.2.1). This contributes to the formation of foliation.
When a rock is both heated and squeezed during metamorphism, and the temperature change is enough for new minerals to form from existing ones, there is a strong tendency for new minerals to grow with their long axes perpendicular to the direction of squeezing.
Table provides a nice summary of the types of metamorphic rocks the form for various protoliths at different temperatures.
|
Very Low Grade |
Low grade |
Medium Grade |
High Grade |
|
|---|---|---|---|---|
| Shale, mudstone | Slate | Phyllite | Schist | Gneiss |
| Ultramafic igneous rocks | Serpentinite | Serpentinite | Serpentinite | Serpentinite |
| Granite | No change | No change | Almost no change | Granite gneiss |
| Quartz sandstone | No change | Little change | Quartzite | Quartzite |
| Limestone | Little change | Marble | Marble | Marble |
Foliated Metamorphic Rocks
The various types of foliated metamorphic rocks, listed in order of the grade or intensity of metamorphism and the type of foliation are: slate, phyllite, schist, and gneiss (Figures \(\PageIndex{2-5}\)). Slate is formed from the low-grade metamorphism of shale, and has microscopic clay and mica crystals that have grown perpendicular to the stress. Slate tends to break into flat sheets. Phyllite is similar to slate, but has typically been heated to a higher temperature; the micas have grown larger and are visible as a sheen on the surface. Where slate is typically planar, phyllite can form in wavy layers. In the formation of schist, the temperature has been hot enough so that individual mica crystals are big enough to be visible, and other mineral crystals, such as quartz, feldspar, or garnet may also be visible. In gneiss, the minerals may have separated into bands of different colors. In the example shown in Figure \(\PageIndex{5}\), the dark bands are largely amphibole while the light-colored bands are feldspar and quartz. Most gneiss has little or no mica because it forms at temperatures higher than those under which micas are stable. Unlike slate and phyllite, which typically only form from mudstone, schist, and especially gneiss, can form from a variety of parent rocks, including mudstone, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.
Schist and gneiss can be named on the basis of important minerals that are present. For example a schist derived from basalt is typically rich in the mineral chlorite, so we call it chlorite schist. One derived from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets present it might be mica-garnet schist. Similarly, a gneiss that originated as basalt and is dominated by amphibole, is an amphibole gneiss or, more accurately, an amphibolite.
Nonfoliated Metamorphic Rocks
Metamorphic rocks that form under either low-pressure conditions or just confining pressure do not become foliated. In most cases, this is because they are not buried deeply, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust. This is contact metamorphism. Some examples of non-foliated metamorphic rocks are marble, quartzite, and hornfels.
Marble
Marble is metamorphosed limestone. When it forms, the calcite crystals tend to grow larger, and any sedimentary textures and fossils that might have been present are destroyed. If the original limestone was pure calcite, then the marble will likely be white (Figure \(\PageIndex{6}\)), but if it had impurities such as clay, silica, or magnesium, the marble could be “marbled” in appearance. Marble that forms during regional metamorphism (most marble) may or may not develop a foliated texture, but foliation is typically not easy to see in marble.
Quartzite
Quartzite is metamorphosed sandstone (Figure \(\PageIndex{7}\)). It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Most sandstone contains some clay minerals and may also include other minerals such as feldspar or fragments of rock, so most quartzite has some impurities with the quartz.
Hornfels
Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rock (Figure \(\PageIndex{8}\)). In some cases, hornfels has visible crystals of minerals like biotite or andalusite. If the hornfels formed in a situation without directed pressure, then these minerals would be randomly orientated, not aligned with one-another, as they would be if formed with directed pressure.
Serpentinite: California's State Rock
All 50 states have an official state rock, and California’s state rock is serpentinite, a metamorphic rock common in the Coast Ranges, Klamath Mountains, and the Sierra Nevada foothills of California. Serpentinite is a rock composed of the minerals of the serpentine group. The serpentine minerals are named for their smooth, but scaly appearance. The name is derived from the Latin serpentinus meaning "serpent rock". The serpentine group has over 20 varieties of silicate minerals containing abundant magnesium and iron.
Chrysotile is one polymorph of serpentine mineral. It has a fibrous crystal habit, and importantly, is one of the asbestos minerals. "Asbestos" is a generic commercial term for a group of naturally occurring silicate minerals that form in a particular fibrous crystal habit (this group also includes actinolite, amosite, anthophyllite, crocidolite, and tremolite - all amphibole minerals). Asbestos fibers make it an excellent fire retardant and thus it was used widely in construction (insulation in buildings and ships), the automotive industry (for brake shoes and clutch fittings), and in fabric treatment (clothing, kitchen linens). Over the first half of the 20th century, growing evidence indicated a link between inhalation of asbestos mineral fibers and lung disease (asbestosis) and what is now known as mesothelioma cancer. In the 1970s, The United States Environmental Protection Agency (EPA) enacted the Clean Air Act in 1970. The act classified asbestos as a hazardous air pollutant.
Not all serpentine minerals exhibit asbestos crystal habit. Other polymorphs have a platy (non-asbestos forming) crystal habit. Antigorite (Figure \(\PageIndex{9}\) center) and lizardite (Figure \(\PageIndex{9}\) right) are examples of non asbestos serpentine.
Serpentine minerals form from the metamorphism of mafic and ultramafic igneous rocks such as those derived from the ocean crust (basalt) and the upper mantle (peridotite). Serpentine minerals tend to form where hydrothermal alteration of ultramafic rocks is possible such as the hydration of olivine rich rocks at low temperatures 0°-60° Celsius.
Serpentinite is most common in the Coast Ranges, the Klamath Mountains, and the Sierra Nevada foothills (Figure \(\PageIndex{10}\). Interestingly, the soils derived from serpentine bedrock are toxic to many plants. The flora that grow from such soil is thus very distinctive. These areas are called serpentine barrens because they often consist of grassland or savannas where the climate would normally lead to the growth of forests. Grass Valley, California is one of these serpentine barrens. Just as many plants fail to grow in serpentine soils, some are uniquely evolved to thrive there. Calamagrostis ophitidis is an endemic species of grass more commonly known as Serpentine Reedgrass (Figure \(\PageIndex{12}\)). Similarly, the Tiburon Mariposa Lily (Calochortus tiburonensis) grows only in challenging serpentine soils, and is known in only one location just north of San Francisco, Ring Mountain (Figure \(\PageIndex{13}\)).
Officially, the state rock of California is listed as "serpentine." Technically, this is incorrect as serpentine is the mineral that makes up the rock serpentinite.
The following are some key characteristics of the mineral serpentine:
- Composition: Mg6Si4O10(OH)8
- Crystal system: Monoclinic (antigorite has a hexagonal polymorph, chrysotile two orthorhombic polymorphs).
- Habit: Crystals unknown: the serpentine minerals usually occur in structureless masses, except when asbestiform.
- Cleavage: None observable
- Hardness: 4-6
- Density: 2.5-2.6
- Color: Usually green, also yellow, brown, reddish brown and gray.
- Streak: White
- Luster: Waxy or greasy in massive varieties, silky in fibrous material.
- Occurrence: Serpentine is formed by the alteration of olivine and enstatite under conditions of low-and medium-grade metamorphism. It sometimes occurs as large rock masses. Massive serpentinite is sometimes cut and polished as an ornamental stone.
Plate Tectonics and Metamorphism
References
- Earle, S. (2019). Physical Geology – 2nd Edition. Victoria, B.C.: BCcampus. Retrieved from https://opentextbc.ca/ March 2024