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Metamorphism: A Process of Change
8.1 Introduction
🪨 Metamorphic rock = rock that forms when a protolith (= the original rock from which a metamorphic rock formed), undergoes a solid-
state change in response to the modification of its environment.
→ Minerals: metamorphism produces new minerals that did not occur in the protolith.
→ Texture: metamorphism produces a new arrangement of mineral grains that differs from the protolith.
→ Modification of environment: metamorphism takes place when a protolith endures a rise in temperature or pressure, undergoes compression
shear or reacts with hydrothermal fluids (= solutions of very hot water).
8.2 Consequences and Causes of Metamorphism
How do you recognise a metamorphic rock?
1. Metamorphic minerals = a new mineral that grows in place within a solid rock by metamorphism.
→ Many metamorphic minerals make up a metamorphic mineral assemblage.
2. Metamorphic textures = a distinctive arrangement of mineral grains by metamorphism. →
Eg. metamorphic foliation = a fabric defined by parallel surfaces or layers that develop in a rock as a result of metamorphism.
The formation of metamorphic minerals and textures takes place very slowly, it may take thousands to millions of years.
🪨 Metamorphic processes occur when they are subjected to metamorphic conditions: heat, pressure, compression, shear and/or very
hot water. The most common processes include:
Recrystallisation: changes the shape and the size of the grains without changing the identity of the mineral.
Tightly fitting, large, irregularly shaped quartz grains replace a cluster of small, round, cemented together quartz grains.
Phase change: transforms one mineral into another mineral with the same composition but a different crystal structure.
The transformation of quartz (SiO₂) into coesite (SiO₂).
Metamorphic reaction or neocrystalisation: results in the growth of new mineral crystals that differ from those of the protolith.
Chemical reactions digest minerals of the protolith and grow new minerals, for this to take place atoms must slowly diffuse (migrate).
Pressure solution: happens when a wet rock in squeezed more strongly in one direction than in another.
Mineral grains dissolve where their surfaces press against other grains, producing ions that migrate through water to precipitate
elsewhere.
Chapter 8: Metamorphic Rocks 1
, Plastic deformation: happens when a rock undergoes squeezing or shearing at elevated temperatures and pressure.
Metamorphism due to heating Metamorphism due to pressure
Heat causes atoms to vibrate rapidly, so the chemical bonds stretch, Pressure = force per unit area, or the push acting on a material in
bend and eventually break. The atoms detach and form new bonds cases where the push is the same in all directions.
with other atoms which leads to a new arrangement of atoms.
When? Metamorphic pressure occurs at depths between 10 km and
When? Metamorphic temperatures lie between the diageneis 40 km where the pressure is between 3000 and 12 000 bar.
temperature (affects sedimentary rocks) and the melting temperature
High pressure metamorphic rocks: rocks formed under pressure
(affects igneous rocks). Usually between 250°C and 850°C, but it can
between 12 000 and 27 000 bar.
vary up to 1200°C.
→ Eg. basalt or gabbro turn into
The depth at which metamorphic temperatures exist, depends on the eclogite.
geothermal gradient (= the rate of change in temperature with respect
Ultrahigh pressure (UHP) metamorphic rocks: rocks formed
to increasing depth).
under pressure of more than 27 000 bar.
→ Eg. quartz turns into
coesite and graphite becomes diamond.
🧿 Mineral stability = the ability of a mineral to form and survive.
Compounds can exist as different polymorphs (= a mineral with the same chemical composition but a
different internal structure). In the phase diagram here, the three polymorphs of Al₂SiO₅ are indicated
by their stability field. It shows that both pressure and temperature have an impact on mineral
stability.
Differential stress = a condition causing a material to experience a push or pull in one direction of a
greater magnitude than the push or pull in another direction.
Normal stress: flattens or stretches a material by compression (push) or tension (pull).
Shear stress or shear: moves one part of the material sideways relative to another part.
Preferred orientation = the parallelism of inequant grains in a metamorphic rock.
→ Grains with different dimensions in different directions are inequant grains, whereas
grains with roughly the same dimensions in all directions are equant grains.
How does preferred orientation happen?
Pressure solution: under lower temperatures, grains become shorter in the direction of
compression.
Plastic deformation or recrystallisation: under higher temperatures, grains flatten due
to compression or grains align due to shear stress.
💧 Hydrothermal fluids = solutions of very hot water that may react with rock to form metamorphic rock.
→ Hydrothermal fluids react chemically with rock by: dissolving, transporting and providing ions. Hydrothermal fluid may carry ions up to tens
of kilometres from where they originated. When hydrothermal fluids pass through a rock and drop off ions of another rock, thus changing the
chemical composition of the original rock, it is called metsomatism.
Supercritical fluid = a substance that forms under high temperatures and pressure and has characteristics of both liquid and gas.
Where does the water in hydrothermal fluids come from? From groundwater, magma or product of metamorphic reactions.
Where does the hydrothermal fluid end up? Incorporated in rock or precipitate in cracks of rock creating a vein.
Chapter 8: Metamorphic Rocks 2
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