6.1: Metamorphic Processes

Metamorphism occurs when solid rock changes in composition and/or texture without the mineral crystals melting. When melt occurs and rock forms from the melt, that forms igneous rock. Metamorphic source rocks, the rocks that experience the metamorphism, are called the parent rock or protolith, from proto– meaning first, and lithos- meaning rock. Most metamorphic processes take place deep underground, inside the earth’s crust. During metamorphism, protolith chemistry is mildly changed by increased temperature (heat), a type of pressure called confining pressure, and/or chemically reactive fluids. Rock texture is changed by heat, confining pressure, and a type of pressure called directed stress.

Temperature (Heat)

Temperature measures a substance’s energy—an increase in temperature represents an increase in energy [2]. Temperature changes affect the chemical equilibrium in minerals. At high temperatures, atoms may vibrate so vigorously they jump from one position to another within the crystal lattice, which remains intact. In other words, this atom swapping can happen while the rock is still solid.

The temperatures of metamorphic rock lie in between surficial processes (as in sedimentary rock) and magma in the rock cycle. So hot enough to cause a change, but not so hot that melt forms. Temperature increases with increasing depth in the Earth along a geothermal gradient (see Chapter 4) and metamorphic rock records these depth-related temperature changes.

Pressure

Pressure is the force exerted over a unit area on a material. 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.

Confining Pressure

Figure \(\PageIndex{1}\): Difference between pressure and stress and how they deform rocks. Pressure (or confining pressure) has equal stress (forces) in all directions and increases with depth under the Earth’s surface. Under directed stress, some stress directions (forces) are stronger than others, and this can deform rocks. Source: Peter Davis

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 pressure. Confining pressure has 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.

Confining pressure is measured in bars and ranges from 1 bar at sea level to around 10,000 bars at the base of the crust [6].

Figure \(\PageIndex{2}\): Pebbles (that used to be spherical or close to spherical) in quartzite deformed by directed stress

Directed Stress

Directed stress, also called tectonic stress, is an unequal balance of forces on a rock in one or more directions (see the previous figure). Directed stresses are generated by the movement of lithospheric plates at plate boundaries. Stress indicates a type of force acting on the rock. Strain describes the resultant processes caused by stress and includes metamorphic changes in the minerals. In contrast to confining pressure, directed stress occurs at much lower pressures and does not generate chemical reactions that change the mineral composition and atomic structure [3]. Instead, directed stress modifies the parent rock at a mechanical level, changing the arrangement, size, and/or shape of the mineral crystals. These crystalline changes create identifying textures, which is shown in the figure below comparing the phaneritic texture of igneous granite with the foliated texture of metamorphic gneiss (note the banding you see in the rock on the right).

Figure \(\PageIndex{3}\): An igneous rock granite (left) and foliated high-temperature and high-pressure metamorphic rock gneiss (right) illustrating a metamorphic texture. (Source: Peter Davis)

Fluids

A third metamorphic agent is chemically reactive fluids that are expelled by crystallizing magma and created by metamorphic reactions. These reactive fluids are made of mostly water (H₂O) and carbon dioxide (CO₂), and smaller amounts of potassium (K), sodium (Na), iron (Fe), magnesium (Mg), calcium (Ca), and aluminum (Al). These fluids react with minerals in the protolith, changing its chemical equilibrium and mineral composition, in a process similar to the reactions driven by heat and pressure. In addition to using elements found in the protolith, the chemical reaction may incorporate substances contributed by the fluids to create new minerals. In general, this style of metamorphism, in which fluids play an important role, is called hydrothermal metamorphism. Water actively participates in chemical reactions and allows extra mobility of the components.

Some hydrothermal alterations remove elements from the parent rock rather than deposit them. This happens when seawater circulates down through fractures in the fresh, still-hot basalt, reacting with and removing mineral ions from it. The dissolved minerals are usually ions that do not fit snugly in the silicate crystal structure, such as copper.

References

• 3. Winter JD (2009) Principles of igneous and metamorphic petrology, 2 edition. Pearson, New York
• 6. Petrini, Podladchikov (2000) Lithospheric pressure–depth relationship in compressive regions of thickened crust. Journal of Metamorphic Geology 18:67–77. https://doi.org/10.1046/j.1525-1314.2000.00240.x