Ch. 7 Metamorphic Rocks

Metamorphic Rocks: Form from the alteration of preexisting rocks due to increased temperatures and pressures that exceed those at the Earth’s surface.
Grade: Refers to the level of temperature and pressure encountered during metamorphism.
Metamorphic rocks form at unnatural temperatures, and they do not melt

Rock types in order of metamorphism in increasing temperature

slate, phyllite, schist, and gniess

Environmental changes produce new minerals that did not occur in the protolith and/or produce a new texture

buried rocks are subject to confining pressure, where pressure is applied equally in all directions

It does not necessarily become more defined just by making it take up less space… makes it more dense

Protolith: The original rock from which a metamorphic rock forms; for example, granite is the protolith for gneiss.

Heat and Pressure: The solid-state changes occur without melting or sediment formation. Hot water can also contribute to changes.

Temperature Change: Increase in temperature leads to the breaking of chemical bonds, allowing atoms to move and form new minerals.
Pressure Change: Increased pressure can cause atoms to come closer together, forming new minerals. This can happen through burial or uplift mechanisms.
Compression and Shear: Involves forces that push or squeeze, leading to the distortion of rocks. Shear is sliding, Compression is pushing together
Hydrothermal Fluid Presence: Very hot water can facilitate the metamorphic process through chemical reactions and changes.
Combination of Factors: Often more than one factor acts simultaneously to produce metamorphic changes in rocks.

Definition: Applies pressure equally in all directions to buried rocks.
Effects:
- Closes the spaces between mineral grains, leading to increased compactness and density.
- Does not cause rocks to fold or deform.

Definition: Pressure applied unevenly from one direction.
Effects:
- Shortens the rock in the direction of applied pressure and lengthens it perpendicular to that direction.

Results of Compression and shear on mineral grains in rocks

before -

Equant grain - roughly the same dimensions in all directions '

after

inequant grains - dimensions are not the same in all directions

Certain minerals are stable at specific temperatures and pressures
mafic mineral form first, felsic forms last
Felsic melts first, mafic melts last
Metamorphic Rock Example:
- Initial texture as limestone with fossil fragments undergoes metamorphism to result in large calcite crystals, which display interlocking patterns.

Metasomatism: The process of changing a rock's chemical composition through reactions with hydrothermal fluids (hot water). It can change how hot minerals have to be to melt down
Recrystallization: Involves changing grain shape and size without altering the mineral chemistry, typically resulting in larger grains.
Metamorphic Reaction (Neocrystallization): Leads to the formation of new minerals differing chemically from the protolith.
Phase Change: Transforms one mineral into another of the same composition but with a different crystalline structure (e.g., andalusite to kyanite).
Pressure Solution: Involves the dissolution of grains under higher pressure and precipitation where pressure is lower. Water helps atoms move more easily/ quicker
Plastic Deformation: Changes in grain shape without fracturing due to compression at high temperatures. Ex a rubber band being stretched out, illustrating how materials can change shape and accommodate stress without breaking, similar to the behaviors exhibited by minerals during plastic deformation.

Metamorphic Texture: Arrangements of grains developed due to metamorphic processes.
Metamorphic Minerals: Specific minerals that develop under metamorphic conditions.
Metamorphic Foliation: Defined as the parallel alignment of minerals or the presence of alternating bands of light and dark minerals. They have layers

Foliated Metamorphic Rocks: Characterized by their mineral composition, grain size, and foliation nature. Examples include: The following rocks are oragnized by grade
- Slate: fine-grained, low-grade, fissile
- Schist: low to medium-grade, platy texture.
- Gneiss: high-grade, granular, wavy layers.
- Migmatite: a rock that is a partial melt of gneiss. It is considered the in-between stage
Nonfoliated Metamorphic Rocks: Classified primarily based on composition. Examples include:
- Hornfels: Contains various minerals based on protolith characteristics, formed due to contact metamorphism.
- Quartzite: Derived from pure quartz sandstone; quartz grains recrystallize to form textured rock.
- Marble: Formed from limestone; recrystallizes into a solid mass of calcite covering fossil structures.

Metamorphic Grade: Indicates the intensity of metamorphism or the level of metamorphic change, which is influenced by temperature and pressure.
Sequence of Metamorphic Rocks with Increasing Grade:
- Low Grade: Shale → Slate.
- Medium Grade: Slate → Schist. (color bands are still developing)
- High Grade: Schist → Gneiss. (color bands are clear)

Contact (Thermal) Metamorphism:
- Occurs when the temperature of the protolith rises due to nearby magma heat, where hydrothermal fluids also contribute to metamorphic processes. - this how we get hornfels
- The altered zone adjacent to magma is known as the Metamorphic/Contact Aureole.
- does not include pressure component
Burial Metamorphism:
- Results from deep burial (8-15 km) causing increases in temperature and pressure leading to low-grade metamorphism.
Dynamothermal (Regional) Metamorphism:
- Common during mountain-building processes at convergent plate boundaries where rocks are subjected to significant temperature, pressure, and shearing forces.
- Represents large scale metamorphism with increasing metamorphic grade correlated with depth.
- It’s considered all of nothing metamorphism
Shock Metamorphism:
- Involves high-energy events such as meteorite impacts that can transform minerals (e.g., quartz to coesite).