What is “metamorphism” ?
• meta (change) morphism (form)
• transformation of pre-existing
rock from one to another
through pressure and
temperature relationships
• all changes take place through
solid state transformation
Types of Metamorphism
• takes place around 150oC (temperature)
• typically at depths of 5 km below the
earth’s surface (Pressure)
At this temperature and pressure new minerals
are formed – changed from one mineral to
another
Low-grade Metamorphism
• between 5-15 km below the earth’s
surface
• temperatures up to 400oC
• common low-grade minerals
• quartz, chlorite, muscovite
Chlorite
Quartz
Muscovite
High-grade metamorphism
• 15 km below the surface to the point
melting (when melting occurs)
• Temperatures greater than 400oC
• Common high-grade minerals
•garnet, kyanite, sillimanite
Garnet
kyanite
sillimanite
How do fluids enhance metamorphism?
• minerals dissolve – re-precipitated in the rock
• fluids speed up chemical reactions – reduces
mineral melting points creating “migmatites”
Migmatite
Igneous rock material
Metamorphic rock
Melting occurs from lowered
melting points creating
small pockets of igneous
rocks surrounded by
metamorphic rocks
When does metamorphism take place?
• solid rock coming in contact with hot
magma
• solid rock contacts hot fluids (hot
water)
• during episodes of intense mountain
building
• along earthquake fault zones
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Solid-state transformations
1. Define metamorphism, and what is meant
by a “solid-state-transformation” ?
2. What conditions below the surface
characterizes the onset of metamorphism?
3. What conditions distinguishes the differences
between high-grade and low-grade metamorphism?
4. What is a migmatite?
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What are the various types of
metamorphism processes?
• contact metamorphism
• hydrothermal metamorphism
• regional metamorphism
• Metasomatism
What is contact metamorphism
• Contact metamorphism:
magma “contacts” solid rock –
heat is transferred into overlying
rocks through conduction – changing
the pre-existing rock it intruded
• high heat / low pressure
• localized metamorphism
• “fast acting” like touching a
hot frying pan
Country rock (surrounding) is baked by conduction
What is regional metamorphism?
Regional Metamorphism
pre-existing rock is subjected to intense
stresses and strains (deformation) usually
from the forces of mountain building
• high heat / high pressure
• very wide spread geographically
• like experiencing a slow sun burn
Mountain building
Wide spread belts
What is hydrothermal metamorphism
Hydrothermal metamorphism
the chemical alteration of pre-existing
rock from the action of hot water –
usually hot water from a magma source
that is ion rich
• low pressures / high heat
• hot water alteration
Black smokers on the ocean floor
hydrothermal alteration – “spewing” hot
water concentrated in sulfide metals
Metasomatism
• fluids pass in / out of the rock changing the
overall chemical composition
• fluids take away or add chemical compounds
to the rock
Chemical alteration from fluids --- moving and
out of the rock
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all types of metamorphism.
5. Briefly describe the conditions for the
following types of metamorphisms:
• contact metamorphism
• regional metamorphism
• hydrothermal metamorphism
• metasomatism
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What does each metamorphic process
have in common?
Heat:
provides the energy to cause
recrystallization of pre-existing
minerals into “new” type of minerals
Heat sources:
contact with hot magma – heat penetrates
overlying rocks (conduction)
geothermal gradient – heat increases with
depth – rocks are hotter when buried
Geothermal Gradient
Steep gradient
0
1000
2000
3000
4000
• Temperature increases
as depth increases
• Large thermal gradient
at the surface
• gradient decreases
after 1000 km depth
Pressure – increases with depth
various types of pressure rearrange
minerals – compacts or elongates
Confining pressure
Differential pressure
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heat and pressure.
6. Describe the characteristics of the geothermal
gradient and it influences metamorphism.
7. Distinguish the differences between both
confining and differential pressure.
8. How does confining and differential pressure
influence the orientation of mineral grains
found in a metamorphic rock?
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Chemically active fluids
• commonly hot water carries ions in
solution and ppt various solids (metals)
• hot water dissolves minerals and deposits
minerals to other sites – concentrated
gold, silver, other earth metals
Gold in Quartz vein
Native Copper (Cu)
How does one know how much a
rock has been metamorphosed?
• the degree of metamorphism is
reflected in the rocks texture
and composition
Regional Metamorphism (HP/HT) – high grade
• rock exhibits a foliation texture
• rocks show a “preferred” mineral
orientation
• parallel mineral alignments
How does one know how much a
rock has been metamorphosed?
Contact Metamorphism (HT / LP) low-grade
• rock textures decrease in
foliated textures – “sub” parallel type
textures
• rocks composed of one type of
mineral (same chemistry) are called
non-foliated
How does one know the amount of
temperature / pressure the rock has been
subjected?
The Metamorphic facies
• the set of metamorphic mineral assemblages
that form under various temperature/pressure
relationships
• Associated with different plate boundaries
• Blueschist / Eclogite facies occur in the
high pressure environment --- subduction
• Greenschist, amphibolite, granulite facies
occur along convergent boundaries
Metamorphic facies – mineral assemblages that
represent various P / T relationships
What plate tectonic settings do
metamorphic rocks form?
Divergent boundaries
• hydrothermal metamorphism
with black smokers
Convergent plate boundary
Contact
Regional
• metamorphism along plate
boundary
• regional and contact
metamorphism