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Summary Earth: Portait of a Planet - Chapter 8 - Metamorphism: A Process of Change
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September 2022CHAPTER 8
Metamorphism: A Process of Change
8.1 Introduction
A metamorphic rock forms when a pre-existing rock, protolith undergoes a solid-state
change in response to the modi cation of it environment. The overall process of change is
called metamorphism.
8.2 Consequences and Causes of Metamorphism
Metamorphic rocks can possess metamorphic minerals, new minerals that grow in place
within solid rock during metamorphism. A whole new group of minerals produced is called
metamorphic mineral assemblage.
- Metamorphic textures are de ned by the arrangement of mineral grains.
• These can manifested by metamorphic foliation; this is characterised by the parallel
alignment of inequant grains.
Recrystallization: changes the shape and size of grains without changing the identity of the
mineral making up the grains.
Phase change: transforms one mineral into another mineral with the same composition but
a di erent crystal structure.
Metamorphic reaction or neocrystallization: results in the growth of new mineral crystals that
di er fro those of the protolith.
- Chemical reactions digest minerals of the protolith and grow new minerals of the
metamorphic rock. For this to take place, atoms must slowly di use through solid
crystals, or dissolve and reprecipitate at grain boundaries.
Pressure solution: when a wet rock is squeezed more strongly in one direction than in the
other. Mineral grains dissolve where their surfaces press against other grains, producing
ions that migrate through the water to predicate elsewhere. Liquid-water lms must coat
grain surfaces during compression for this process to take place.
Plastic deformation: when a rock undergoes squeezing or sheering at elevated
temperatures and pressures, conditions during which minerals behave like soft plastic.
Heat causes the atoms in a mineral grain to vibrate which causes chemical bonds to lock
atoms to their neighbours to stretch and bend. If bonds stretch and bend too far, these
bonds can break.
- The depth in the Earth at which metamorphic temperatures exist depends on the
geothermal gradient, which re ects the geologic setting.
Pressure can cause a material to collapse inward.
- Mineral subjected to extreme pressure, the atoms pack more closely together to yield
denser minerals.
- High-pressure metamorphic rocks transform ma c protoliths, into eclogite; a rock
composed of brilliant maroon garnets embedded in greenish pyroxene.
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, The stability of certain minerals and of mineral assemblages depends on both pressure and
temperature. Each mineral can survive in a stability eld.
If a metaal is squeezed unequally from di erent sides, it has been subjected to di erential
stress.
- Normal stress: Pushes or pulls perpendicular to a surface. We call a push compression
and a pull tension. Compression attens a material whereas tension stretches a material.
- Shear stress: Shear stress, or shear, moves one part of a material sideways relative to
another part.
Compression and shear at metamorphic temperatures and pressures may cause a body of
rock to change shape without cracking or breaking. A preferred orientation develops
during this process.
- Plate and elongate grains are examples of inequant grains.
- Equant grains have roughly the same dimensions in all directions.
- As a result of pressure solution grains become shorter in the direction of compression.
- At higher temperatures, weaker grains atten by means of plastic deformation due to
di erential stress; the grains become narrower in one direction and become longer in the
other.
Metamorphic reactions take place in the presence of hydrothermal uids. A supercritical
uid is a substance that forms under high temperatures and pressures and has
characteristics of both liquid and gas.
- These react chemical with rock by dissolving, transporting, and providing ions.
- Metasomatism; The process by which a rock’s overall chemical composition changes
during metamorphism because of reactions with hot water that bring in or remove
elements.
- Some hydrothermal uids, precipitate in spaces that open up where rock cracks,
producing a mineral- lled crack known as a vein.
8.3 Types of Metamorphic Rocks
Foliation can give metamorphic rocks a striped or streaked appearances in an out crop.
- It either foliates because of a preferred orientation or because it has alternating dark-
colours and light-colours layers.
Slate; is the nest-grained foliated metamorphic rock, it forms by metamorphism of shale
or mudstone.
- It contains a type of foliation called slaty cleavage, this allows it to split into thin sheets
that make excellent roo ng shingles.
- The cleavage develops when compression causes clay akes to reorient and grow in an
orientation roughly perpendicular to the direction of compression.
- The preferred orientation of clay in slate develops in response to pressure solution and
recrystallisation.
Phyllite; a ne-grained metamorphic rock with a type of foliation caused by the preferred
orientation of very ne-grained white mica.
- The alignment of translucent ne-grained mica gives phyllite a silky sheen known as
phyllitic lustre.
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