Metamorphic Rocks and Processes

Metamorphic rocks

Formed when a pre-existing rock (protolith) undergoes solid-state change due to environmental modification. This involves new minerals and/or textures

Metamorphism

Solid-state changes in rocks due to heat, pressure, and/or chemical environment No melting occurs

Recrystallization (metamorphism)

The shape and size of mineral grains change, but the composition remains the same, such as limestone becoming marble

Phase change (metamorphism)

One mineral transforms into another with the same composition but a different crystal structure, like quartz to coesite

Neocrystallization (metamorphism)

The growth of new mineral crystals as protolith minerals break down and atoms rearrange

Pressure solution (metamorphism)

In wet rock under directed pressure, minerals dissolve at grain contacts, and ions migrate away

Plastic deformation (metamorphism)

At high temperatures, rocks can be squeezed or sheared, causing minerals to behave like soft plastic

Metamorphic grade

Reflects the degree or intensity of metamorphism, increasing with temperature and pressure and yielding different mineral groups

Index minerals

Minerals that are characteristic of specific metamorphic grades and can indicate the P/T conditions

Metamorphic facies

A set of metamorphic minerals that are representative of a particular pressure and temperature range

Foliation

An assemblage of parallel planar surfaces or layers in metamorphic rocks, often due to aligned inequant minerals or compositional banding under differential stress . Requires inequant minerals and differential stress

Differential stress

Stress that is different in different directions; pressure, in contrast, is the same in all directions . It includes normal stress (compression or tension) and shear stress

Slaty cleavage

A type of foliation in slate where fine-grained clay particles align perpendicular to the applied stress

Gneissic banding

Foliation in gneiss characterized by alternating dark and light compositional bands