Metamorphic Rocks Study Notes

Metamorphic rocks form when other rocks undergo changes due to various factors.
- "Meta" (Greek for change) and "morpho" (Greek for shape).
- These rocks are considered some of the oldest on Earth.
- Metamorphic rocks are significant components of continental shields.

Metamorphism is defined as the process where heat, pressure, or fluids alter the composition or texture of minerals in the rock.
- This process occurs deep within the Earth, specifically at temperatures below the melting point of the rocks.

The three primary agents of metamorphism are:
- Heat: Increases the reaction rates within the rock. Sources of heat include:
- Magma
- Lava
- Deep burial in the Earth.
- The temperature generally increases by about 25°C for every kilometer of depth into the Earth.
- Pressure:
- Lithostatic Pressure: The uniform pressure applied to rocks beneath the surface, exerted equally in all directions.
- Differential Pressure: Non-uniform pressure, where pressure is greater in one direction, usually happening at convergent plate boundaries.
Time: A crucial factor that allows minerals to stabilize and reach equilibrium with their new environmental conditions.

The most common metamorphic fluids are water and carbon dioxide.
- Sources of these fluids:
- Water trapped in sedimentary rock pore spaces.
- Volatile fluids present in magma.
- Dehydration of water-bearing minerals, such as gypsum.

Contact (Thermal) Metamorphism:
- Occurs when magma or lava alters the surrounding country rock.
- Typically forms under conditions of high temperature and low pressure at shallow depths.
- Aureole: The zone of metamorphism surrounding an igneous intrusion, with its size dependent on the following factors:
- Size of the igneous intrusion.
- Temperature of the intrusion.
- Amount of fluids present; more fluids contribute to more significant metamorphism.
Dynamic Metamorphism:
- Characterized by high pressure and low temperature, typically occurring at convergent or transform boundaries.
- Most metamorphic rocks are formed through this process, as rocks are crushed and ground along faults.
Regional Metamorphism:
- A broader type of dynamic metamorphism affecting larger areas.

Index Minerals: Minerals that form only under specific temperature and pressure conditions, including:
- Chlorite
- Muscovite
- Biotite
- Garnet
- Staurolite
- Kyanite
- Sillimanite
Metamorphic Grade: Refers to the degree of change that a rock undergoes, typically categorized as low, intermediate, or high.
- Example of mineral grades at different temperatures (°C):
- Clay (Unaltered)
- Chlorite (Low grade)
- Muscovite (Intermediate grade)
- Garnet, Staurolite, Sillimanite (High grade).

Foliated vs. Non-foliated:
- Foliated Rocks: Minerals are aligned in parallel bands due to applied pressure.
- Examples of foliated rocks, listed from low to high grade metamorphism:
  - Slate: Fine-grained, with slaty cleavage.
  - Phyllite: Coarser grained, with more than 50% platy elongated minerals.
  - Schist: Notable for its wavy foliation.
  - Gneiss: Coarsest grained, consists of segregated bands of light and dark minerals, typically formed in regional metamorphism.
- Non-foliated Rocks: Do not show parallel arrangement of minerals. Common examples include:
- Marble: Metamorphosed from limestones and dolostones.
- Quartzite: Metamorphosed from quartz sandstones.
- Greenstones: Low grade metamorphic from basalts.
- Hornfels: Fine-grained and result of contact metamorphism.
- Anthracite: The highest grade of metamorphic coal.

Metamorphic Zone: A belt of rocks that exhibits approximately the same degree of metamorphism.
Metamorphic Facies: Comprising a group of rocks containing a distinctive assemblage of minerals formed under similar conditions of temperature and pressure.
As intensity of metamorphism increases, the following mineral zones can be represented in a diagram related to different conditions (increasing metamorphic intensity involved with certain minerals):
- Chlorite, Biotite
- Garnet, Staurolite
- Sillimanite.

Metamorphism can occur at all types of plate boundaries, but it is most prevalent and extensive along convergent boundaries.
Blueschist Facies Metamorphism: Defined by low temperature and high pressure. It occurs in subduction zones which lead to higher grades of metamorphism in the subducting oceanic plates.

Many ore deposits form through contact metamorphism. Some notable ores found in metamorphic zones include:
- Bornite
- Chalcopyrite
- Galena
- Pyrite (iron)
- Sphalerite (zinc).
Metamorphic Mineral Resources:
- Graphite: Utilized in lubricants.
- Talc: Traditionally used in talcum powder.
- Garnets: Frequently employed in high-temperature porcelains.
- High-grade metallic ores such as hematite, copper, zinc sulfides, and tungsten ores can also be associated with metamorphic activity.