Chapter 8: Metamorphism: A process of change
What is metamorphism?
Metamorphic rock is one that forms when a pre-existing rock, called protolith, undergoes a solid state
change in response to the modification of its environment at depth of the Earth. This process of change
is called metamorphism.
By change, we mean that metamorphism produces new minerals that did not occur in the prolith,
and /or produces a new texture (arrangement of mineral grains).
By modification of environment we mean that metamorphism takes place when a protolith endures a
rise or fall in temperature/ pressure compression and shear/ very hot water.
Metamorphism can take place when heat from an intrusion cooks the rock into which it intrudes. And
can also take place when rock becomes deeply buried.
8.2 Consequences and causes of metamorphism
What is a metamorphic rock?
1)A metamorphic rock can posses metamorphic minerals, new minerals
that grow in place within the solid rock only under metamorphic
conditions
2) It can have metamorphic texture defined by the arrangement of mineral
grains. The texture can be manifested1 by metamorphic foliation,
characterized by the parallel alignment of platy minerals and or the
presence of alternating light-colored and dark-colored layers.
Common processes
Re-crystallization = changing the shape and size of grains without changing the identity of the
mineral making up the grains.
Phase change = which transforms one mineral into another mineral with the same composition
but a different crystal structure. Same formula but different crystal structures!
Metamorphic reaction / neocrystallization = growth of a new mineral. Chemical reactions
digest minerals of the protolith and yield new minerals of the metamorphic rock.
1 Manifested= zichtbaar duidelijk worden
, Pressure solution = wet rock is squeezed more strongly
in one direction. Mineral
grains dissolve where their
surfaces are pressed against
other grains, producing ions
that migrate through the
water to precipitate
elsewhere. Precipitation
takes place at less strongly
squeezed parts. So, one side
shortens and the other side
grows. (LOW temperature
metamorphism because
liquid water must be able to exist.)
Plastic deformation
Rock is squeezed or sheared at high temperatures and pressure. Minerals behave soft plastic,
and change shape without breaking.
Figure right: pressure solution
Metamorphism due to heating
Heat causes the atoms to vibrate rapidly so the chemical bonds stretch and bend. If bonds stretch and
bend too far they break. These atoms will form new bonds with other atoms.
Processes; Neocrystallization (new minerals form), recrysallization (size and shape of grains change)
Metamorphism due to pressure
Pressure can cause a material to collapse 2 inward. Open ( much space between the atoms) crystal
structures can be stable near the Earth’s surface, but pressure can cause them to pack more closely
together and denser minerals form.
Processes: Neocrystallization (new minerals form) and phase changes (crystal structure changes)
Changing both pressure and temperature
Pressure and temperature in the Earth change together. The stability of minerals (the ability of a
mineral to form and survive) depends on both pressure and temperature. So, rock formed at 8 km
depth does not contain the same mineral as one formed at 20 km. The specific
range of temperature and pressure under which the mineral exists is called the
stability field.
Compression, shear, and development of preferred orientation
If a material is squeezed or stretched unequally from different sides, we say that it
is subjected to differential stress. Not that differential stress differs from pressure-
in the latter3 the push is the same in all directions. We distinguish two types of
differential stress:
2 Collapse= ineenstorten
3 Latter=laatstgenoemde
What is metamorphism?
Metamorphic rock is one that forms when a pre-existing rock, called protolith, undergoes a solid state
change in response to the modification of its environment at depth of the Earth. This process of change
is called metamorphism.
By change, we mean that metamorphism produces new minerals that did not occur in the prolith,
and /or produces a new texture (arrangement of mineral grains).
By modification of environment we mean that metamorphism takes place when a protolith endures a
rise or fall in temperature/ pressure compression and shear/ very hot water.
Metamorphism can take place when heat from an intrusion cooks the rock into which it intrudes. And
can also take place when rock becomes deeply buried.
8.2 Consequences and causes of metamorphism
What is a metamorphic rock?
1)A metamorphic rock can posses metamorphic minerals, new minerals
that grow in place within the solid rock only under metamorphic
conditions
2) It can have metamorphic texture defined by the arrangement of mineral
grains. The texture can be manifested1 by metamorphic foliation,
characterized by the parallel alignment of platy minerals and or the
presence of alternating light-colored and dark-colored layers.
Common processes
Re-crystallization = changing the shape and size of grains without changing the identity of the
mineral making up the grains.
Phase change = which transforms one mineral into another mineral with the same composition
but a different crystal structure. Same formula but different crystal structures!
Metamorphic reaction / neocrystallization = growth of a new mineral. Chemical reactions
digest minerals of the protolith and yield new minerals of the metamorphic rock.
1 Manifested= zichtbaar duidelijk worden
, Pressure solution = wet rock is squeezed more strongly
in one direction. Mineral
grains dissolve where their
surfaces are pressed against
other grains, producing ions
that migrate through the
water to precipitate
elsewhere. Precipitation
takes place at less strongly
squeezed parts. So, one side
shortens and the other side
grows. (LOW temperature
metamorphism because
liquid water must be able to exist.)
Plastic deformation
Rock is squeezed or sheared at high temperatures and pressure. Minerals behave soft plastic,
and change shape without breaking.
Figure right: pressure solution
Metamorphism due to heating
Heat causes the atoms to vibrate rapidly so the chemical bonds stretch and bend. If bonds stretch and
bend too far they break. These atoms will form new bonds with other atoms.
Processes; Neocrystallization (new minerals form), recrysallization (size and shape of grains change)
Metamorphism due to pressure
Pressure can cause a material to collapse 2 inward. Open ( much space between the atoms) crystal
structures can be stable near the Earth’s surface, but pressure can cause them to pack more closely
together and denser minerals form.
Processes: Neocrystallization (new minerals form) and phase changes (crystal structure changes)
Changing both pressure and temperature
Pressure and temperature in the Earth change together. The stability of minerals (the ability of a
mineral to form and survive) depends on both pressure and temperature. So, rock formed at 8 km
depth does not contain the same mineral as one formed at 20 km. The specific
range of temperature and pressure under which the mineral exists is called the
stability field.
Compression, shear, and development of preferred orientation
If a material is squeezed or stretched unequally from different sides, we say that it
is subjected to differential stress. Not that differential stress differs from pressure-
in the latter3 the push is the same in all directions. We distinguish two types of
differential stress:
2 Collapse= ineenstorten
3 Latter=laatstgenoemde
