Introduction to Metamorphism

Metamorphism

A subsolidus process leading to changes in mineralogy, texture, and often in chemical composition in a rock, caused by physical and/or chemical conditions that differ from those at the surface.

Protolith

The original rock from which a metamorphic rock forms.

Equilibrium Metamorphism

Type of metamorphism where conditions remain stable over time, promoting the development of stable mineral assemblages.

Non-Equilibrium Metamorphism

Metamorphism where conditions may change rapidly, leading to the formation of metastable minerals.

Geothermal Gradient

The rate of temperature increase with depth in the Earth's crust.

Foliation

The alignment of mineral grains within a rock, resulting in a layered appearance.

Lithostatic Pressure

The pressure exerted on a rock due to the weight of overlying material, generally equal in all directions.

Differential Stress

Unequal pressure applied to rock in different directions, which can lead to deformation and foliation.

Prograde Metamorphism

Metamorphism that occurs as temperature and pressure increase, leading to changes in rock grade.

Retrograde Metamorphism

Metamorphism that occurs as temperature and pressure decrease, often leading to the reformation of minerals.

Contact Metamorphism

Metamorphism that occurs when rocks are heated by nearby molten magma.

Regional Metamorphism

Metamorphism that affects a large area of rock, typically associated with tectonic forces and heat from geologic processes.

Hydrothermal Metamorphism

Metamorphism that occurs when hot, chemically active fluids circulate through rocks, often at mid-ocean ridges.

Migmatite

A mixed rock formed from partial melting and metamorphic processes, containing both igneous and metamorphic components.

Granofels

A medium- to coarse-grained, non-foliated metamorphic rock, typically formed without distinct foliation.

Eclogite

A high-pressure metamorphic rock composed primarily of the minerals garnet and omphacite.

Slate

A fine-grained, foliated metamorphic rock that splits easily into thin slabs.

Phyllite

A fine-grained metamorphic rock with a sheen, characterized by a higher degree of metamorphism than slate.

Schist

A medium- to coarse-grained metamorphic rock characterized by schistosity and the presence of aligned minerals.

Gneiss

A banded or foliated metamorphic rock composed of granitic materials and characterized by its banding.

Hornfels

A fine-grained, non-foliated metamorphic rock formed from contact metamorphism.

Amphibolite

A metamorphic rock predominantly composed of amphibole and plagioclase, typically formed under high temperatures and pressures.

Granite

An igneous rock that often serves as a protolith for various metamorphic rocks.

Carbonates

Metamorphic rocks that originate from limestone or dolostone, featuring high concentrations of calcium and magnesium.

Younger Cratons

Stable continental crust areas that undergo little tectonic change, often characterized by low geothermal gradients.

Pelite

A metamorphic rock originating from fine-grained sedimentary rocks like mudstone or shale, often rich in alumina.