Ch. 6 - Metamorphic Rocks & Processes

Metamorphism

Occurs through changes in temperature, pressure, and the presence of fluids, causing new minerals to form and existing minerals to recrystallize without melting.

Role of Temperature in Metamorphism

Causes atoms to diffuse and minerals to grow or change.

Role of Pressure in Metamorphism

Compacts atoms, causing recrystallization and preferred alignment (foliation, lineation).

Confining Pressure

Equal in all directions.

Directed Stress

Unequal and causes deformation, foliation, and lineation.

Metamorphic Textures from Directed Stress

Foliation, lineation, schistosity (alignment of minerals), and mylonite textures from shearing.

Role of Fluids in Metamorphism

Hot fluids can cause chemical reactions, transport ions, form new minerals, and alter rock compositions (hydrothermal metamorphism).

Chemical Processes in Metamorphism

Recrystallization, remobilization, and new mineral growth.

Physical Processes in Metamorphism

Deformation, rotation, and shearing of minerals.

Difference between Igneous and Metamorphic Textures

Igneous textures are based on cooling rate and crystal size; metamorphic textures show alignment (foliation/lineation) or recrystallization from pressure/heat.

Foliation

Parallel alignment of minerals.

Lineation

Linear alignment of elongated minerals.

Non-foliated Texture

Granular texture with equidimensional grains (quartzite, marble).

Schistosity

Wavy layers of visible micas.

Metamorphic Grade

Refers to the intensity of temperature and pressure during metamorphism.

Low Grade Metamorphism

Fine-grained, low T/P (e.g., slate).

High Grade Metamorphism

Coarse-grained, high T/P (e.g., gneiss).

Crystal Size and Foliation Relation to Metamorphic Grade

Higher grade = larger crystals and more pronounced foliation.

Presence of Garnet

Indicates intermediate to high-grade metamorphism.

Protoliths for Common Metamorphic Rocks

Marble: Limestone/dolostone; Quartzite: Quartz sandstone; Greenschist: Mafic rocks (basalt); Slate → Phyllite → Schist → Gneiss: From shale/mudstone.

Types of Metamorphism

Contact, Regional, Subduction zone, Burial, Fault (dynamic).

Differences between Regional, Contact, and Subduction Metamorphism

Regional: Large-scale, high T and P, produces foliated rocks; Contact: Local, high T/low P near magma, produces non-foliated rocks; Subduction: High P/low T, produces blueschist, eclogite.

Contact Metamorphism

Produces only non-foliated rocks due to low and uniform pressure, with no directed stress to align minerals.

Regional and Fault Metamorphism

Produce foliated textures due to directed stress that aligns minerals.

Metamorphic Rocks Diagnostic of Subduction Metamorphism

Greenschist, blueschist, eclogite.

Metamorphism by Tectonic Setting

Divergent: Hydrothermal, contact, burial; Subduction zones: Subduction and regional metamorphism; Continental collision: Regional and fault metamorphism; Transform boundaries: Fault metamorphism.