Metamorphic Rocks
 These are rocks that have changed (meta)
their form (morphic).
 Under the influence of heat, pressure and fluids, preexisting rocks are modified in form and even in
internal atomic structure to produce new rocks stable
at the new conditions.
Formation of Metamorphic Rocks
 In every case, a metamorphic rock is produced from an
another rock (parent rock). The parent rock may be
sedimentary, igneous or even another metamorphic
rock through heat, pressure and the movement of hot
(hydrothermal) fluids.
Shale
Schist
Formation of Metamorphic Rocks
 WHAT CHANGES OCCUR:
1) New minerals form - even though the minerals
change, most elements are provided by parent rock
except water and some dissolved ions. (Same
chemistry)
2) A new texture is provided – pressure often results in
foliation (layering) and bent (or deformed) layers
are common.
3) Crystalline texture – mineral grains grow larger
THE RESULTANT ROCKS TEND TO BE LAYERED,
HARDER AND MORE COLOURFUL.
Formation of Metamorphic Rocks
Formation of Metamorphic Rocks
 Thus, metamorphic rocks are pre-existing rocks whose
mineralogy and/or texture has been changed by
processes within the Earth.
 Metamorphic rocks form because of changes in
temperature and pressure due to the depth of
burial within the Earth without actual melting of the
rocks taking place.
 The changes that affect these rocks occur in the solid
state.
Metamorphism
 The important factors that produce metamorphic
changes are:
1) Pressure deep within the crust
2) Temperature
3) Strain (shape and volume changes as a result of stress
during deformation)
4) Fluid activity (pressure due to fluids in pore spaces
within the original rocks)
Where do Metamorphic Rocks form?
The cratonic regions of all continents are made up almost
exclusively of metamorphic rocks and most of the oldest
rocks on Earth are therefore metamorphic.
2) Fold mountain belts such as the Alps, Himalayas and
Andes contain large amounts of metamorphic rocks
which were deformed by folding, faulting and thrusting
3) Subduction Zones
4) Near the intrusion of granitic magmas.
1)
The fact that most metamorphic rocks are deformed
indicates that metamorphism and tectonism (e.g.
deformation) occur together.
New Minerals Form
1) Metamorphism is an internal process of the Earth
and occurs as a result of changes in temperature
and/or pressure. Most minerals are only stable at
particular temperatures and pressures.
2) Changes in temperature and pressure result in the
formation of new minerals (those stable at the
new temperatures and pressures).
Aluminosilicates (Al2SiO5) are a
group of minerals found
commonly in metamorphic rocks.
A different mineral grows
depending on the pressure and
temperature of surrounding rocks.
What Happens during Metamorphism?
The geothermal gradient:
 Another principle source of
heat is the natural increase
in temperature as the depth
increases. This is known as
the geothermal gradient.
The typical geothermal
gradient is about 30
degrees per kilometer
 Although high
temperatures can bring
about a significant change,
pressure causes the most
obvious changes.
What Happens during Metamorphism?
3) Other than water, few other components (these
include fluorine, boron) enter or leave during
metamorphism. No overall chemical changes occur,
the original chemical constituents are just readjusted during recrystallisation and the texture of
the rock changes, usually becoming coarser-grained.
4) Changes that occur at temperatures below 200°C
and pressures below about 300 MPa (MPa stands
for Mega Pascals) are considered to be Diagenesis.
Diagenesis is considered to be a sedimentary process.
What Happens during Metamorphism?
 Pressure affects sedimentary rocks
by squeezing the mineral grains
together. This eliminates the pore
spaces, and expels the fluids that
are present.
 Under increasing pressure, the
mineral grains will form a tightly
interlocking mosaic. If the
pressure continues, the crystals
may reform into fewer but larger
grains.
 This process is known as
recrystallization. The
recrystallized mineral may remain
the same as it was, or it may be a
new, more dense mineral that is
more stable at high pressures.
An example of metamorphic change:
Sedimentary to Metamorphic
The following is a very general progression from sedimentary
rock to metamorphic rock based primarily on pressure.
SURFACE
MUD
5 km DEEP
SHALE (sedimentary)
10 km DEEP
SLATE (low grade metamorphic)
Foliation appears, minerals recrystallize
15 km DEEP
SCHIST (medium grade metamorphism)
Micas form, garnet grows
20 km DEEP
GNEISS (high grade metamorphic)
25 km DEEP
HORNFELS (Cordierite appears)
Types of Metamorphism
The three main types of metamorphism are:
 1) Contact (Thermal) Metamorphism
 2) Regional Metamorphism
 3) Dynamic Metamorphism
Contact (Thermal) Metamorphism
Occurs when hot magma forms an intrusion into rock - the
surrounding rock faaces high temperatures, but no increase in
pressure.
2) Thus Contact Metamorphic rocks are produced in high
temperature, low pressure environments with low strain
(pressure) and variable fluid pressures.
3) They are usually formed at shallow depths within the crust
(usually less than 6 km). The heat sources responsible for
contact metamorphism are bodies of hot magma (e.g.
igneous intrusions) which raise the temperature of the
surrounding rocks.
4) The metamorphosed region is called the contact aureole.
These zones contain high temperatures but low pressures
and are responsible for the growth of new minerals that are
stable under these conditions.
1)
Contact Metamorphism
Hornfels – note the lack of foliation
and the very fine-grained texture
The Contact Aureole
Well-defined contact between
intrusive granite and contact
metamorphic slate.
Contact Metamorphism
These thermal effects are usually restricted to the contact
zones of the intrusions, hence the term contact
metamorphism. However, sometimes hot fluids are released
from the intrusions and penetrate the enclosing rocks along
fractures and produce contact metamorphic zones.
5) Unlike other metamorphic rocks, distinct layering and the
growth of coarser grained minerals is NOT found. Since
the rock is often fine-grained and not foliated, it is hard to
identify contact metamorphic rocks. Usually only after
observing large areas of rock surrounding an igneous intrusion
can we see a ‘contact aureole’. The common rock name for
contact metamorphic rock is called a hornfels.
6) These rocks show high temperature, low pressure minerals.
4)
Regional metamorphism
 This is the most common type of metamorphic
rock with zones of metamorphism covering large
areas of continental crust
 We will focus on these rocks in Rock
Identification!
 Regional metamorphism is produces a wide variety of
temperatures, pressures, strains and fluid flow and is
normally created by tectonic processes within the
Earth at a variety of depths. More simply put, this is
metamorphism caused by the burial of rocks at great
depth.
Regional metamorphism
 Most metamorphic rocks occur in fold mountain
belts or cratonic areas. Such rocks cover large areas of
the Earth's crust and are therefore termed regional
metamorphic rocks.
 They arise by the combined action of heat, burial
pressure, differential stress, strain and fluids on
pre-existing rocks.
 The resulting rocks are always deformed (as a result
of the differential stress) and commonly exhibit folds,
fractures and cleavages.
 Typically, grain size increases with increasing
temperature and pressure.
Regional Metamorphism
 Granitic intrusions are
also associated with
regional metamorphic
rocks. The most common
regional metamorphic
rocks are slates, schists
and gneisses. Regional
metamorphism covers a
wide range of
temperature and pressure
conditions from 200° C 750° C and 2 kbar - 10
kbar (or 5 km - 35 km
depth).
Metamorphic Grades
Rock Types, Crystal Size, Foliation
Classifying Regional Metamorphic Rocks
Classifying Regional Metamorphic Rocks
Common Regional Metamorphism of Clastic Sedimentary Rocks:
rock name
metamorphic grade
new minerals
texture
Sedimentary
no metamorphism yet
none
layered, soft
Slate and Phyllite
Low Grade
chlorite
platy cleavage
Schist
Intermediate Grade
micas
layered, shiny
Gneiss
High Grade
Eclogite
Partial melting
(almost igneous)
amphibole, quartz
and feldspar
garnet, pyroxene
Shale (sedimentary)
phyllite-slate
garnet muscovite schist
banded, partially
melted
gneiss
eclogite
New Mineral Growth in
Regional Metamorphic Rocks
Aluminosilicates
The polymorphic aluminosilicate
minerals, Andalusite, Sillimanite and
Kyanite (all Al2SiO5), are common in
metamorphic rocks of sedimentary origin
(pelitic). Since each forms at a different
pressure and temperature (see the phase
diagram above) they are a great measure
of the metamorphic grade. Finding these
minerals is very important to geologists.
andalusite
kyanite
sillimanite
Dynamic Metamorphism
 Dynamic metamorphism is the type of metamorphism that
normally occurs in active fault zones and areas where the
ocean crust is being subducted under continental crust.
Rocks found in these zones are highly deformed and they
tend to be exposed to very high pressures but only low
or medium crustal temperatures.
 Found where ocean crust is being subducted
into the Mantle!
 Since the original rocks being metamorphosed are formed
from ocean crust, they are rich in basalt and andesite
(rocks high in mafic minerals).
Dynamic Metamorphism
 THESE MINERALS ARE NOT INCLUDED IN OUR




COLLECTION!
Low Grade – Zeolite Basalt (holes in the Basalt are filled with
light coloured, soft zeolite minerals.
Medium grade – Greenschists – dark schist rocks rich in
chlorite (green mica) and actinolite*(green amphibole). (*
not always present)
Medium – High Grade – Blueschist – dark schist rocks rich
in blue amphiboles.
High Grade – Eclogite – hard banded rock containing garnet,
pyroxene, quartz and kyanite but no plagioclase feldspar.
(very close to the melting point)
Classifying Dynamic Metamorphic Rocks
Common Regional Metamorphism of Basalt
rock name
Basalt
Zeolite
metamorphic grade
no metamorphism yet
Very Low Grade
new minerals
none
zeolites
Greenschist
Low Grade
Amphibolite or
Blueschist
Granulite
Intermediate Grade
chlorite and
amphiboles
dark amphiboles and
plagioclase
garnet, pyroxene
Zeolite (in Basalt)
High Grade
Greenschist
Amphibolite
texture
no layering, dark
Soft, white or
colourful minerals
found in pore spaces
of basalt
layered, shiny, green
to grey
Banded, green to
black
Foliation is lost!
Granulite
Comparing Metamorphic Types
Comparing Metamorphic Types
Final Important Facts
1) Size of Metamorphic zones
 Contact and dynamic metamorphism are usually restricted
to localized areas whereas Regional metamorphism affects
large areas of the crust, sometimes over tens of thousands
of square kilometres. Contact metamorphism occurs as
zones a few hundred metres wide around large igneous
intrusions while dynamic metamorphism is restricted to
fault and thrust zones only a few tens of metres thick. All
three types of metamorphism can overlap.
2) Metamorphic rocks and ore deposits:
 Metamorphism is also strongly associated with many ore
deposits. This is because metallic elements (such as lead,
zinc, copper) are particularly mobile during
metamorphism, especially when fluid is involved.
Final Important Facts
3) Retrograde metamorphism
 Normal Increasing metamorphism is called Prograde
Metamorphism
 Many metamorphic rocks contain evidence of retrograde
mineral changes, that is, alteration of higher grade minerals
into lower grade ones. Retrograde metamorphism is normally
produced by repeated regional metamorphism where a lower
grade episode is superimposed on a higher grade one.
 Most retrogressive events are probably just a consequence of the
metamorphic system cooling down after peak metamorphism
has been reached.
 i.e. the system has to cool down with time and as the region
undergoes uplift with time, both pressure and temperature are
dramatically reduced.