This paper contains brief introduction on Metamorphism. Useful for geology students.

1. Asif Hasan
Geo “47”
Department Of Geology
University Of Dhaka

2. 1.Introduction
2.Metamorphism
3.Common Structures and Textures of Metamorphic
Rocks
4.Classification of Metamorphic Rocks
5.Descriptive Study of Common Metamorphic
Rocks (Quartzite, Marble, Slate, Gneiss, Schist).
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3. Metamorphic rocks are
geologically classified as
one of the major group of
rocks which have been
formed out of
metamorphism of
preexisting igneous and
sedimentary rocks.
Igneous Sedimentary
Metamorphic
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4. They are altered or changed beyond their recognition, i.e.
change in Chemical composition, texture and structure
When rocks are baked by heat or molten magma,
squeezed by the movements of huge tectonic plates or by
the pressure of overlying thick succession of rocks
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5.  The word metamorphism means change of form (meta=
change; morph= form). In petrology, it indicates the effect
of temperature, pressure and chemically active solutions
over the texture, minerals and composition of parent
rocks.
This means the constituent minerals, texture and
composition of parent rocks by metamorphism change
over to new minerals or new texture or new composition
which are more stable and suitable under new condition.
For example, As a result of metamorphism Granite
changes to gneiss, sandstone changes to quartzite,
limestone changes to marble.
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6. There are six major factors in metamorphism:
Chemical composition.
The change in temperature.
The change in pressure.
The presence or absence of fluids.
How long a rock is subjected to high pressure or high
temperature.
Whether the rock is simply compressed or is twisted
and broken during metamorphism.
Factors in Metamorphism
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7. Metamorphic Agents
1. High Temperature
2. High Pressure
3. Chemically Active Fluids
The process of metamorphism occur in rocks due to effect of
high temperature, pressure and chemically active fluids.
These three are known as metamorphic agents. Generally, all
these three act together and cause metamorphism. But
sometimes, any one or two of them dominate and play an
active role.
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8.  Temperature: The source of temperature which is
responsible for metamorphism is either due to depth or due
to the contact with magma. The metamorphic changes
mainly take place in the temperature range of 200 – 850 ° C.
The temperature rise also increases the chemical activity in
rocks and facilitates reaction during metamorphism.
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9. Pressure: The pressure which causes metamorphism is of two
different kind namely
1. Uniform pressure
2. Direct pressure
 Uniform pressure: increases with depth. It acts vertically
downwards and affects the volume of both liquids and solids.
Naturally, its effect is significant only at great depths. But not at or
near the surface. This also means that high temperature will also be
associate with high temperature will also be associated with high
uniform pressure. So, both of them act together and bring about
metamorphism.
 Direct pressure: which is also called stress, is due to tectonic
forces. Such pressure act in any direction, i.e. upward or downward
or sideward It acts only on solids and affect the shape of rocks or
minerals. It is effective in the upper layers of the crust and increases
with depth to some extent . The application of stress gives rise to
shearing movements in the rock and produces new minerals which
have platy habit and arranges approximately parallel to each other.
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10. AsifHasan

11. UNIFORM PRESSURE
Types of Pressure
increases with depth due to
increase in overburden.
acts vertically downwards and
affects the volume of both liquid &
solids.
increases with depth up to some
extent, effective in the upper part of the
crust.
DIRECT or Differential PRESSURE
acts in all direction and affects only on
solids resulting into deformation of
shape and change in mineral
composition
High temperature is also associated
due to depth factor high temperature is not always
associated. to (depth factor)
Lithostatic pressure- due to
overburden Stress- due to tectonic forces
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12.  Chemically Active Fluids:
play a key role in different ways in causing metamorphism.
 First, since metamorphism of any type cannot take place for the
solid minerals in a perfect dry state, the presence of a liquid
medium of some kind is indispensable. Thus the liquids act as a
carrier of chemical components that actually take part in
chemical reaction. In other words, liquids provide the necessary
medium and facilitates the reaction to take place. The most
common liquid which play such role is water.
 Second, the huge quantity of volatiles that are associated with
magmatic bodies ultimately permeate through the surrounding
rocks by means of diffusion and cause compositional changes
even in rocks. Far from magma.
 Third, the magma or the hot juvenile hydrothermal solutions may
react directly with those rocks with which they come in contact.
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13. Metamorphic change occurs slowly in the solid state.
Several processes are at work.
Metamorphic Processes
Recrystallization – Minerals change size and shape.
Phase change – New minerals form with
 Same chemical formula.
 Different crystal structure.
Example: Andalusite to kyanite.
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14. Neocrystallization – New minerals with changes in
temperature and pressure.
 Initial minerals become unstable and change to new
minerals.
Original protolith minerals are digested in reactions.
 Elements restructure to form new minerals.
 In this way, a shale can transform into a garnet mica
schist.
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15. Pressure solution – Mineral grains partially dissolve.
 Dissolution requires small amounts of water.
 Minerals dissolve where their surfaces press together.
 Ions from the dissolution migrate in the water film.
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16. Plastic deformation – Mineral grains soften and deform.
 Requires elevated temperatures.
 Rock is squeezed or sheared.
 Minerals act like plastic, changing shape without
breaking.
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17. Metamorphic Grade
 Prograde – Metamorphism via increasing T and P.
 Common in rocks that are buried in orogenic belts.
 Progressive changes.
 Recrystallization causes mineral growth.
 Neocrystallization results in new mineral
assemblages.
 Mineral changes release water
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18. Example: Prograde metamorphism of a pelitic rock.
 Low grade – Shale protolith.
 Clays recrystallize into larger, aligned clays to yield a slate.
 Clays neocrystallize into tiny, aligned micas in a phyllite.
 Intermediate grade –
 Micas recrystallize and grow large to form a schist.
 New minerals grow in the schist.
 High grade -
 Micas decompose; elements recombine into new minerals.
 Neocrystallization yields quartz and feldspars in a gneiss.
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19. AsifHasan

20.  Retrograde – Metamorphism via decreasing T and P.
 Common in rocks that are brought from depth by erosion.
 Accompanied by addition of H2O by hydrothermal fluids.
Many prograde rocks aren’t retrograded. Rocks at the surface
can preserve prograde conditions.
Metamorphic Grade
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21. Metamorphic Environments
 Metamorphism occurs in different settings.
 Different settings yield different effects via…
• Geothermal gradient.
• Differential stresses.
• Hydrothermal fluids.
 These characteristics are governed by tectonics.
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22. Metamorphic Environments
The types (and settings) of metamorphism are –
 Thermal – Heating by a plutonic intrusion.
 Burial – Increases in P and T by deep burial in a basin.
 Dynamic – Shearing in a fault zone.
 Regional – P and T alteration due to orogenesis.
 Hydrothermal – Alteration by hot-water leaching.
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23. Contact Metamorphism
 Due to heat from magma invading host rock.
 Creates zoned bands of alteration in host rock.
 Called a contact (or metamorphic) aureole.
 The aureole surrounds the plutonic intrusion.
 Zoned from high-grade (near pluton) to low-grade
(far from pluton).
 Grades of alteration form bands around the pluton.
• Bands range from highly altered to slightly altered.
• Analogous to changes in pottery with increased
heating.
 The width of each aureole zone is due to…
• The size of the plutonic intrusion.
• The degree of metasomatism.
 The dominant rock is hornfels.
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24. AsifHasan

25. Burial Metamorphism
 As sediments are buried in a sedimentary basin
• P increases because of the weight of the overburden.
• T increases because of the geothermal gradient.
 Requires burial below diagenetic effects
• This is ~ 8–15 km depending on the geothermal
gradient.
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26. Dynamic Metamorphism
 Breakage of rock by shearing at a
fault zone.
 Fault location determines type of
alteration.
 Shallow crust – Upper 10–15
km.
o Rocks behave in a brittle
fashion.
o Mineral grains crush-
forming fault breccia.
 Deeper crust – Below 10–15
km.
o Rocks behave in a ductile
manner.
o Minerals smear like taffy to
form mylonite.
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27. Regional Metamorphism
 Tectonic collisions deform huge “mobile belts.”
 Directed compression thickens mountains.
 Rocks caught up in mountain building are…
 Heated via the geothermal gradient and
plutonic intrusions.
 Squeezed and heated by deep burial.
 Smashed and sheared by differential stresses.
 Regional metamorphism creates foliated rocks.
 This type of metamorphism is, by far, the most
important in terms of the amount of rock altered.
 Collisional belts are often…
 Thousands of km long.
 Hundreds of km wide.
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29. Hydrothermal Metamorphism
 Alteration by hot, chemically aggressive water.
 A dominant process near mid-ocean ridge magma.
 Cold ocean water seeps into fractured crust.
 Heated by magma, this water then reacts with mafic
rock.
 The hot water rises and is ejected via black smokers.
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30. Exhumation
 How do metamorphic rocks return to the surface?
 Exhumation is due to uplift, collapse, and erosion.
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31. Finding Metamorphic Rocks
 Large regions of ancient high-grade rocks, called shields,
are exposed in continental interiors.
 Shields are eroded remnants of orogenic belts.
• Shield rocks form the basement under sedimentary
cover.
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