Metamorphic Rocks Overview

Study Notes

Metamorphic rocks are rocks that have changed due to heat, pressure, or chemical reactions in fluids.
Any type of rock (igneous, sedimentary, or metamorphic) can be transformed into a metamorphic rock.
The changes occur without melting the rock.
Increased pressure, temperature, and chemical reactions in fluids cause new mineral and texture formations.
Metamorphic rocks are classified based on texture and composition into foliated and non-foliated types.

Metamorphism is the transformation of existing rock to a rock with a different mineral composition or texture.
Metamorphism takes place at temperatures exceeding 150 to 200 °C (300 to 400 °F), often with elevated pressure or chemically active fluids.
The rock remains mostly solid during transformation.

Various forms of metamorphism exist, including regional, contact, hydrothermal, shock, and dynamic metamorphism.
These differ in their characteristic temperatures, pressures, and the rate at which they occur, along with the involvement of reactive fluids.
Prograde metamorphism occurs when pressure and temperature increase.
Retrograde metamorphism occurs when pressure and temperature decrease.

As a rock undergoes metamorphism, some mineral grains grow larger while others shrink.
The shapes of the grains can also change.
Limestone is transformed into marble during metamorphism.
Marble has large interlocking grains with no fossils.

Key minerals found in metamorphic rocks include andalusite, garnet, sillimanite, kyanite, and staurolite.
These are examples of minerals characteristic of metamorphic rocks.

Parent rock composition is a key factor.
No new material is added during metamorphism.
The resulting metamorphic rock has a composition similar to the original rock.
If the parent material contains only one mineral, the metamorphic rock will have only that mineral.
Minerals will often be recrystallized leading to alterations in the texture.

Limestone has fragments of shells, while marble has interconnecting grains and no fossils.

If the parent material contains multiple minerals, the old minerals will combine to create new metamorphic minerals, like garnet or others.

Pressure in the Earth is equal in all directions throughout the depth of the Earth.
Pressure increases by approximately 1 kilobar (kbar) per 3.3 kilometers of depth
Increased pressure causes grains to pack together, decreasing the overall volume and producing more compact mineral grains.

Tectonic forces create forces that are unequal in all directions—differential stress
Compressive stress (pushing together) causes flattening that is perpendicular to stress.
Shearing, (one surface sliding past another) causes flattening that is parallel to the stress direction

Hot water (or water vapor) is the most significant fluid component involved in altering existing minerals.
Heat causes unstable minerals to release water.
Water reacts with surrounding rocks and transports dissolved minerals, altering the chemical composition over extensive periods of time.

Metamorphic rocks are categorized based on their texture.
Foliated rocks display layered textures (e.g., slate, shale, gneiss).
Non-foliated rocks have non-layered textures (e.g., marble, quartzite, hornfels).

Contact metamorphism occurs near magma bodies (intrusion) resulting in a zone of contact known as an aureole.
Fine-grained rocks (e.g., hornfels) or coarse-grained rocks (e.g., marble) are typical outcomes.

Regional metamorphism occurs over extensive areas, especially in mountain ranges.
Folded and deformed rocks yield foliated metamorphic types. Pressure and temperature conditions in these regions promote the creation of new and altered minerals.

Migmatites are a product of partial melting during metamorphism.
These high-grade metamorphic rocks exhibit both intrusive and metamorphic textures.

Hydrothermal metamorphism occurs when cold seawater interacts with hot basalt from intrusions.
This creates alterations to the chemical and mineral composition of the basalt in altered zones.

Regional metamorphism is associated with convergent plate boundaries.
This process involves increasing pressure and temperature with depth
These variations result in different metamorphic conditions that lead to different degrees or grades of metamorphism.