8. Metamorphic rocks
In chapter 7, we talked about the transformation of soft sediments into hard sedimentary rocks bycompaction and cementation. These processes occur once sediments are buried under new layersof sediments. Thick accumulation of sediments and sedimentary rocks is made possible by aprocess called subsidence (see chapter 7). As depth increases, temperature and pressure
increase too. At a certain depth, temperature and pressure become high enough to affect themineralogy, texture or chemical composition of rocks (sedimentary, igneous or metamorphic). Thisprocess of transformation is called metamorphism. Most metamorphic rocks are produced in themiddle and lower crust at a depth of 10 to 30 km. Metamorphism can be caused by plate tectonics
where shallow rocks are forced down to great depths during the process of mountain building atconvergent plate boundaries (i.e. orogeny = process of mountain formation). But metamorphism
can also occur at or near Earth’s surface where rocks are in contact with hot magma (i.e. contact
metamorphism, see section 8.4). The metamorphic grade of a rock refers to the pressure andtemperature to which the rock has been subjected. Metamorphic rocks formed at relatively low
pressures and low temperatures experience a so-called low-grade metamorphism. Those
produced at high pressures and high temperatures result from a high-grade metamorphism.
It is important to understand that rocks remain solid during the process of metamorphism. If rocks
melt, the resulting magma will produce an igneous rock, which is another domain of the rock cycle.
Rock metamorphism is driven by three factors that are described below: (1) temperature, (2)pressure, and (3) heated fluids (= hydrothermal fluids).

8.1. Temperature
The average geothermal gradient of the Earth’s crust is 30°C/km. A rock buried 15 km below thesurface is subjected to a temperature of 450°C. At such temperature, a rock may re-crystallize.
Preexisting minerals grow larger and new ones may also form. Different minerals are stable atdifferent temperatures. For example, chlorite is a sheet-like silicate mineral commonly forming
during low-grade metamorphism. Garnet, another silicate mineral, is characteristic of ahigher-grade of metamorphism. The temperature at which one mineral assemblage changes intoanother has been determined experimentally. Therefore, by studying the mineral composition of ametamorphic rock, geologists can determine the temperature at which the rock has formed.
Metamorphic mineral assemblages are natural geothermometers.
The depth at which metamorphism occurs depends on the geothermal gradient. The geothermal
gradient can vary according to the geological setting. An old, stable continental crust has ageothermal gradient that is lower than average. On the other hand, regions where the crust is
stretched and thinned (e.g. Basin and Range in US, East African Rift) are characterized by ageothermal gradient higher than average, i.e., temperature rises faster with increasing depth.
18.2. Pressure
Due to the weight of overlying rocks, pressure increases with depth in Earth’s crust at a rate of300-400 bar/km. The pressure at 15 km is thus approximately 5000 bar. For comparison, theatmospheric pressure at sea level on Earth is 1 bar. In the ocean, the pressure in the deepest
trenches about 10,000 m below the surface is approximately 1000 bar. The pressure resulting fromthe weight of overlying rocks is called confining pressure. Crustal rocks are squeezed in all
directions by this confining pressure which increases with depth. There exists another kind ofpressure: the directed pressure, caused by tectonic forces. Tremendous compressional forces
affect crustal rocks at convergent boundaries. The shape and orientation of minerals formed duringmetamorphism are greatly influenced by tectonic forces. Elongated and platy crystals tend toorientate perpendicularly to the direction of compression. The parallel alignment of platy crystals ina metamorphic rock produces a distinct texture called foliation (see section 8.5). Crystal
orientation in a metamorphic rock is therefore useful to reconstruct the tectonic history of a region.
Moreover, like for temperature, different minerals are stable at different pressures. Geologists canuse the mineral assemblages they identify in metamorphic rocks to determine the pressure atwhich they formed. Metamorphic mineral assemblages are natural geobarometers.

8.3. Hydrothermal fluids
Another factor that can lead to changes in the mineralogical and chemical compositions of rocks is
the presence of heated fluids (hydrothermal fluids) in the crust. Even in dry rocks, water can bepresent in the crystal structure of some minerals (e.g. clay minerals). Increasing pressure
eventually squeezes out water molecules. Heated water can react with surrounding rocks anddissolve components like CO2, SiO2, Na, K, S, Cu, Mn, and Zn, changing the initial composition ofthese rocks. New minerals can also precipitate from a hydrothermal fluid as it penetrates different
regions of the crust. The transformation of rock mineralogical and chemical compositions due tointeractions with hydrothermal fluids is called metasomatism.
An example of such transformation is related to the formation of hydrothermal vents (alsosometimes referred to as black smokers) already mentioned in chapter 4. Seawater penetrating
the oceanic crust near mid-ocean ridges is heated and reacts with the surrounding basalt. The
heated water leaches elements such as sulfur and various metals from the rock. Meanwhile, themineralogical composition of the basalt changes and new minerals form (e.g., chlorite, which has agreen color). Due to the presence of these new minerals, the color of the rock changes from darkgrey to green (hence the name given to this type of rock: greenstone). When the heated water
returns to the ocean floor and cools suddenly in contact with the cold deep ocean water, metallic
sulfides precipitate and form large chimneys from which seeps a blackish mixture of hot water andmineral particles.
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8.4. Types of metamorphism
Regional metamorphism: metamorphism which affects large regions of the crust at convergent
boundaries, where rocks are subjected to high temperature and high pressure. Rocks that are
carried to great depths in subduction zones can undergo high to ultra-high pressure
metamorphism. Metamorphic rocks formed under very high pressure deep under subduction
zones are called eclogites. They are rarely brought back to the surface and therefore rarely seenat outcrops. They contain minerals indicative of extremely high pressures, such as coesite (ahigh-pressure polymorph of quartz) or microdiamonds which form under 40,000 bar at a depth of
120 km!
Contact metamorphism: metamorphism which affects rocks in contact of a magmatic intrusion
within the crust or in contact with lava at the surface. In this case rocks are subjected to hightemperature but not necessarily to high pressure.
Seafloor metamorphism: metamorphism which affects rocks of the oceanic crust near mid-ocean
ridges in contact with hydrothermal fluids (see also section 8.3. of this chapter).
Burial metamorphism: metamorphism which affects sedimentary rocks as they are buried under
an increasingly thick pile of sediments. This metamorphism typically starts at depths ranging
between 6 to 10 km where rocks undergo a pressure of about 3000 bar and temperatures arecomprised between 100 and 200°C.
Shock metamorphism: metamorphism which is caused by a meteorite impact. The kinetic energyof a meteorite is released in the form of heat and shock waves. Temperature can be high enough tomelt rocks at the impact site. The rock is pulverized and small molten pieces of rocks cool in the airand form small glassy bead-like objects called tektites. The presence of tektites in sedimentaryrock layers is used as one of the criteria to recognize a meteorite impact in the geological record.
The shock wave can modify the mineralogy of the rocks at the impact site. For examples, thecrystal structure of quartz can be modified to produce coesite. The presence of high-pressureminerals like coesite or microdiamonds can also be used as supporting evidence for a meteorite
impact in the distant past.

8.5. Metamorphic textures
Metamorphism guides the size, shape, and orientation of crystals, and therefore has a profoundinfluence on rock texture. A common texture observed in metamorphic rocks is foliation. Foliation
results from the preferential orientation of elongated and platy crystals which tend to align in thedirection perpendicular to compressional forces. Minerals forming platy crystals and common inmetamorphic rocks are micas. The foliation plane is a plane along which the rock splits more3

easily (metamorphic cleavage). For example, slate is a low-grade foliated metamorphic rockcomposed of small platy crystals whose preferential orientation enables the rock to be split easily
into flat sheets. Due to this property, slates are commonly used to make floor and roof tiles.
Based on their texture, two main families of metamorphic rocks can be distinguished: foliatedmetamorphic rocks and non-foliated (or granoblastic) metamorphic rocks.
8.5.1. Foliated metamorphic rocks
Foliation is produced when metamorphic rocks are subjected to directed pressure. As the grade ofregional metamorphism increases, foliation becomes more pronounced and the size of crystals
increases. Foliated metamorphic rocks include the following common rock types, classified here
from a lower (top of the list) to a higher (bottom of the list) grade of metamorphism (see slides forillustrations):-slate-phyllite-

schist-gneiss-migmatite

Foliation becomes more conspicuous as the grade of metamorphism increases from slate tophyllite to schist. In gneiss, minerals are segregated into distinct lighter and darker bands. In
migmatites, banding becomes more diffuse as temperature increases and the rock approaches its
melting point.
8.5.2. Granoblastic (non-foliated) metamorphic rocks
Granoblastic rocks are metamorphic rocks which have not been subjected to directed pressure
and consequently do not display any foliation. One example of such rock type is greenstone.
Greenstones form in the context of seafloor metamorphism as explained in section 8.3. Anotherexample is marble. Marbles form by recrystallization of sedimentary rocks composed primarily of
CaCO3 (limestones) subjected to high pressure and high temperature. Quartzite, anothergranoblastic metamorphic rock, is the name given to a sandstone rich in quartz grains that has
undergone high pressure and high temperature. Note that in metamorphosed sedimentary rocks
such as marble and quartzite, fossils are destroyed during the process of recrystallization.
8.5.3. Porphyroblastic texture
The term porphyroblastic is used to describe a rock composed of large crystals(porphyroblasts) “floating” in a much finer-grained matrix. The interpretation of this texture is thatthe large crystals have formed over a broad range of pressure and temperature. The small crystals
in the matrix on the other hand are not stable over such a broad range of pressure and4

temperature; they have re-crystallized repeatedly as pressure and temperature increased and they
did not have time to grow large.

8.6. Metamorphic rocks: a tool to study the history of crustal rocks
8.6.1. Index minerals
As explained in section 8.1 and 8.2, mineral assemblages in metamorphic rocks can be used as akind of geothermobarometer. Of particular interest are index minerals: minerals stable in a
relatively limited range of pressure and temperature. Mapping the distribution of index minerals in aregion where metamorphic rocks are exposed to the surface enables to delineate zones of thecrust which represent specific grades of metamorphism, each characterized by a certain range ofpressure and temperature. The lines delimiting these zones are called isograds.
Ancient mountain chains are often characterized by extensive belts of different metamorphic
grades (with different index minerals) parallel to their axis with isograds following the main
deformation features of the rocks (folds and faults). Therefore, mapping metamorphic zones isuseful to recognize ancient convergent plate boundaries. In addition, the mineralogy ofmetamorphic rocks gives access to the conditions of pressure and temperature existing when theyformed. The next step is to identify the precise geological setting to which these metamorphic
rocks are related.
8.6.2. Metamorphic facies
The mineralogy of metamorphic rocks does not only depend on pressure and temperature but also
on the composition of the parent rock. As an example, let’s consider shale (fine-grained
sedimentary rock mainly composed of clay minerals and quartz) and basalt (extrusive igneous rockmainly composed of plagioclase, pyroxene, and olivine). Let’s raise the pressure and temperatureso that both rocks experience the same intermediate-grade metamorphism. The mineralogy of themetamorphosed shale and that of the metamorphosed basalt will overlap (presence of garnet inboth rocks) but will not be exactly the same. Both metamorphic rocks however belong to the same
metamorphic facies (in this case the amphibolite facies) because they both contain a mineral
assemblage indicative of a specific domain of pressure and temperature typically found deep
under mountain chains. Another example is the greenschist facies which includes a variety ofrock compositions characterized by a high abundance of green minerals, such as chlorite, andwhich typically result from low-grade metamorphism under mountain chains. Rocks belonging tothe blueschist facies on the other hand are characterized by the presence of the blue mineralglaucophane and indicative of conditions of moderate temperature and high pressure typicallyexperienced by rocks of the oceanic crust caught in a subduction zone. There are several othermetamorphic facies indicative of other tectonic settings and useful for geologists.
By subjecting rocks of various compositions to various conditions of pressure and temperature in5the lab, geologists can match the compositions of metamorphic rocks from a surveyed area tospecific domains of pressure and temperature (i.e. define specific metamorphic facies) which
provides valuable information about the geological setting in which these rocks formed (forexample, if the rock formed in a subduction zone, in contact with a magma, or with hydrothermalfluids).
8.6.3. The pressure-temperature paths of metamorphic rocks
The history of pressure and temperature of a given metamorphic rock is called its
pressure-temperature path or P-T path. The P-T path consists of two segments: a prograde and a
retrograde segment. The prograde segment reflects the increase in pressure and temperaturesince the rock began to experience metamorphism. The retrograde segment reflects thedecrease in pressure and temperature when the rock is brought back to the surface through theprocess of exhumation (see section below). Rocks from different tectonic settings have different
P-T paths (see slides for illustration). It is therefore very useful to know the P-T path of ametamorphic rock to understand its history.
In order to reconstruct the P-T path of a metamorphic rock, one approach is to target large
porphyroblasts (see section 8.5.3). One example is garnet, a mineral which grows over a broad
range of temperature and pressure. The key point here is that the chemical composition of garnet
evolves as pressure and temperature increase. Experiments conducted in the lab have shown
exactly how the composition of garnet changes as a function of pressure and temperature. Hence,by analyzing changes in the abundance of specific elements along the transect connecting thecenter of a porphyroblast of garnet (oldest age) to the rim of the same crystal (youngest age), it ispossible to reconstruct the evolution of pressure and temperature when the crystal was growing(i.e. the prograde segment of the P-T path).

8.7. Exhumation process
Metamorphic rocks can sometimes be brought back to the surface (luckily for geologists who canthen study these rocks). This process is called exhumation.
Exhumation can result from the combined effect of tectonic uplift and erosion. At convergent
boundaries, rocks at the core of mountain chains are uplifted by tectonic forces. Meanwhile thesurface of the mountain chain is affected by erosion removing rocks from the top. The result is thatdeep-seated rocks become progressively closer to the surface as they are pushed upward bytectonic forces and the rocks above are removed by erosion. Since erosion is controlled by climate,this theory implies that climate and tectonic uplift control the rate of exhumation.

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