Metamorphism in Geology

Study Notes

Occurs when rocks are altered by heat, pressure, or fluids.
Changes the texture, mineral composition, and chemical composition of rocks without melting them.
Derived from Earth's heat, buried under layers of rocks or adjacent to igneous intrusions.
The pressure required for metamorphism is from either deep burial or tectonic plate compression.

Minerals in rocks undergo solid-state alterations.
Change into new minerals that are more stable under the new temperature and pressure conditions.

Mineralogical Changes: Minerals change to reflect the new pressure and temperature equilibrium conditions.
- Recrystallization: Existing crystals change in size and shape to form layers or interlocking crystals.
- Recombination: Elements in pre-existing minerals recombine to form stable new minerals.
- Addition/Subtraction of ions: Common in the presence of fluids during metamorphism.
Metamorphic Textures:
- Foliated metamorphic rocks are characterized by layers and bands of minerals due to pressure during metamorphism.
- Foliation develops perpendicular to the direction of the pressure.
- Nonfoliated metamorphic rocks have blocky crystal shapes with no layering.
- Increasing grain size parallels changes in composition and development of foliation. This is not a factor for nonfoliated rocks.

Different combinations of temperature and pressure produce different grades: low, intermediate, and high.
Each grade is associated with a specific set of minerals and textures.
Metamorphism of shale produces the colorful minerals observed in slate.
Prograde metamorphism: A series of increasing changes in mineral composition and textures as the temperature and pressure increases. Example: slate → phyllite → schist → gneiss → migmatite

Regional Metamorphism: Occurs when high temperatures and pressures affect large regions of Earth's crust.
- Produces large belts of metamorphism that range from low to high grade.
- Results in mineral and rock type changes, folding, and deformation of rock layers.
Contact Metamorphism: Occurs when molten material, such as in igneous intrusions, comes in contact with solid rock.
- High temperatures and moderate-to-low pressure result in specific mineral assemblages.
Hydrothermal Metamorphism: Occurs when very hot water reacts with rock, altering its chemical and mineral composition.
- Commonly occurs near igneous intrusions and active volcanoes.
- Chemical changes are common.

Many economic mineral resources are produced by metamorphic processes.
Metallic Mineral Resources: Many metallic deposits are precipitated from hydrothermal solutions.
Nonmetallic Mineral Resources: Metamorphism produces resources such as talc, asbestos, and graphite.