adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Definition of Metamorphism
The word "Metamorphism" comes from the Greek: Meta = change, Morph = form,
so metamorphism means to change form.In geology this refers to the changes
in mineral assemblage and texture that result from subjecting a rock to
pressures and temperatures different from those under which the rock
Note that Diagenesis is also a change in form that occurs in sedimentary
rocks. In geology, however, we restrict diagenetic processes to
those which occur at temperatures below 200oC and pressures below about
300 MPa (MPa stands for Mega Pascals), this is equivalent to about 3000
atmospheres of pressure.
Metamorphism, therefore occurs at temperatures and pressures higher than
200oC and 300 MPa. Rocks can be subjected to these higher
temperatures and pressures as rocks become buried deeper in the
Earth. Such burial usually takes place as a result of tectonic
processes such as continental collisions or subduction.
The upper limit of metamorphism occurs at the pressure and temperature of
wet partial melting of the rock in question. Once melting begins,
the process changes to an igneous process rather than a metamorphic
Grade of Metamorphism
As the temperature and/or pressure increases on a body of rock we
say the rock undergoes prograde metamorphism or that the grade of
metamorphism increases. Metamorphic grade is a general term
for describing the relative temperature and pressure conditions under
which metamorphic rocks form.
Low-grade metamorphism takes place at temperatures between about 200 to
320oC, and relatively low pressure. Low grade metamorphic rocks are
characterized by an abundance of hydrous minerals, minerals that contain
water, H2O, in their crystal structure.
Examples of hydrous minerals that occur in low grade metamorphic rocks:
High-grade metamorphism takes place at temperatures greater than 320oC and
relatively high pressure. As grade of metamorphism increases,
hydrous minerals become less hydrous, by losing H2O and non-hydrous
minerals become more common.
Examples of less hydrous minerals and non-hydrous minerals that
characterize high grade metamorphic rocks:
Muscovite - hydrous mineral that eventually disappears at the
highest grade of metamorphism
Biotite - a hydrous mineral that is stable to very high grades of
Pyroxene - a non hydrous mineral.
Garnet - a non hydrous mineral
As temperature and pressure fall due to erosion of overlying rock or due
to tectonic uplift, one might expect metamorphism to a follow a reverse
path and eventually return the rocks to their original unmetamorphosed
state. Such a process is referred to as retrograde
metamorphism. If retrograde metamorphism were common, we would not
commonly see metamorphic rocks at the surface of the Earth. Since we
do see metamorphic rocks exposed at the Earth's surface retrograde
metamorphism does not appear to be common.
The reasons for this include:
chemical reactions take place more slowly as temperature is
during prograde metamorphism, fluids such as H2O and CO2 are driven
off, and these fluids are necessary to form the hydrous minerals that
are stable at the Earth's surface.
Chemical reactions take place more rapidly in the presence of fluids, but
if the fluids are driven off during prograde metamorphism, they will not
be available to speed up reactions during retrograde metamorphism.
Factors that Control Metamorphism
Metamorphism occurs because some minerals are stable only under certain
conditions of pressure and temperature. When pressure and
temperature change, chemical reactions occur to cause the minerals in the
rock to change to an assemblage that is stable at the new pressure and
temperature conditions. But, the process is complicated by such
things as how the pressure is applied, the time over which the rock is
subjected to the higher pressure and temperature, and whether or not there
is a fluid phase present during metamorphism.
Temperature increases with depth in the Earth along the Geothermal
Gradient. Thus higher temperature can occur by burial of rock.
Temperature can also increase due to igneous intrusion.
Pressure increases with depth of burial, thus, both pressure and
temperature will vary with depth in the Earth. Pressure is
defined as a force acting equally from all directions. It is a
type of stress, called hydrostatic stress, or uniform stress. If
the stress is not equal from all directions, then the stress is called
a differential stress
If differential stress is present during metamorphism, it can have a
profound effect on the texture of the rock.
rounded grains can become flattened in the direction of maximum
minerals that crystallize or grow in the differential stress field
can have a preferred orientation. This is especially true of the
sheet silicate minerals (the micas: biotite and muscovite, chlorite,
talc, and serpentine).
These sheet silicates will grow with their sheets orientated perpendicular
to the direction of maximum stress. Preferred orientation of sheet
silicates causes rocks to beeasily broken along approximately parallel
sheets. Such a structure is called a foliation.
Fluid Phase - Any existing open space between mineral grains in a
rocks can potentially contain a fluid. This fluid is mostly H2O,
but contains dissolved mineral matter. The fluid phase is
important because chemical reactions that involve one solid mineral
changing into another solid mineral can be greatly speeded up by
having dissolved ions transported by the fluid. Within
increasing pressure of metamorphism, the pore spaces in which the
fluid resides is reduced, and thus the fluid is driven off.
Thus, no fluid will be present when pressure and temperature decrease
and, as discussed earlier retrograde metamorphism will be inhibited.
Time - The chemical reactions involved in metamorphism, along with
recrystallization, and growth of new minerals are extremely slow
processes. Laboratory experiments suggest that the longer the
time available for metamorphism, the larger are the sizes of the
mineral grains produced. Thus coarse grained metamorphic rocks involve
long times of metamorphism. Experiments suggest that the time involved
is millions of years
Responses of Rock to Increasing Metamorphic Grade
Slate - Slates form at low metamorphic grade by the growth of fine grained
chlorite and clay minerals. The preferred orientation of these sheet
silicates causes the rock to easily break along the planes parallel to the
sheet silicates, causing a slatey cleavage. Note that in the case
shown here, the maximum stress is applied at an angle to the original
bedding planes, so that the slatey cleavage has developed at an angle to
the original bedding.
Schist - The size of the mineral grains tends to enlarge with increasing
grade of metamorphism. Eventually the rock develops a near planar
foliation caused by the preferred orientation of sheet silicates (mainly
biotite and muscovite). Quartz and Feldspar grains, however show no
preferred orientation. The irregular planar foliation
at this stage is called schistosity.
Gneiss - As metamorphic grade increases, the sheet silicates become
unstable and dark colored minerals like hornblende and pyroxene start to
grow.These dark colored minerals tend to become segregated in distinct
bands through the rock, giving the rock a gneissic banding. Because
the dark colored minerals tend to form elongated crystals, rather
than sheet- like crystals, they still have a preferred orientation with
their long directions perpendicular to the maximum differential stress.
Granulite - At the highest grades of metamorphism all of the hydrous
minerals and sheet silicates become unstable and thus there are few
minerals present that would show a preferred orientation. The
resulting rock will have a granulitic texture that is similar to a
phaneritic texture in igneous rocks.
Metamorphism of Basalts and Gabbros
Greenschist - Olivine, pyroxene, and plagioclase in an original
basalt change to amphiboles and chlorite (both commonly green) as
water in the pore spaces reacts with the original minerals at
temperatures and pressures of low grade metamorphism.
Amphibolite - As pressure and temperature increase to intermediate
grades of metamorphism, only dark colored amphiboles and plagioclase
survive and the resulting rock is called an amphibolite.
Granulite - At the highest grade of metamorphism the amphiboles are
replaced by pyroxenes and garnets, the foliation is lost and a
granulite that has a granulitic texture is produced.
Metamorphism of Limestone and Sandstone
Marble - Since limestones are made up of essentially one
mineral, Calcite, and calcite is stable over a wide range of
temperature and pressure, metamorphism of limestone only causes the
original calcite crystals to grow larger. Since no sheet
silicates are present the resulting rock, a marble, does not show
Quartzite - Metamorphism of sandstone originally containing only
quartz, results in recrystallization and growth of the quartz,
producing a non foliated rock called a quartzite
Types of Metamorphism
Cataclastic Metamorphism - This type of metamorphism is due to
mechanical deformation, like when two bodies of rock slide past one
another along a fault zone. Heat is generated by the friction of
sliding along the zone, and the rocks tend to crushed and pulverized
due to the sliding. Cataclastic metamorphism is not very common
and is restricted to a narrow zone along which the sliding occurred.
Burial Metamorphism - When sedimentary rocks are buried to depths of
several hundred meters, temperatures greater than 300oC may develop in
the absence of differential stress. New minerals grow, but the
rock does not appear to be metamorphosed. The main minerals
produced are the Zeolites. Burial metamorphism overlaps, to some
extent, with diagenesis, and grades into regional metamorphism as
temperature and pressure increase.
- Occurs adjacent to igneous intrusions and results from high temperatures
associated with the igneous intrusion. Since only a small area
surrounding the intrusion is heated by the magma, metamorphism is
restricted to zone surrounding the intrusion, called a
metamorphicaureole. Outside of the contact aureole, the rocks are
unmetamorphosed. The grade of metamorphism increases in all directions
toward the intrusion. Because temperature differences between the
surrounding rock and the intruded magma are larger at shallow levels in
the crust, contact metamorphism is usually referred to as high
temperature, low pressure metamorphism. The rock produced is often a
fine-grained rock that shows no foliation, called a hornfels
- This type of metamorphism occurs over large areas that were subjected to
high degrees of deformation under differential stress.
Thus, it usually results in forming metamorphic rocks that are strongly
foliated, such as slates, schists, and gniesses.
The differential stress usually results from tectonic forces that produce
a compression of the rocks, such as when two continental masses collide
with one another.
Thus, regionally metamorphosed rocks occur in the cores of mountain ranges
or in eroded mountain ranges.
Compressive stresses result in folding of the rock, as shown below, and
result in thickening of the crust which tends to push rocks
down to deeper levels where they are subjected to higher temperatures and
A map of a hypothetical regionally metamorphosed area is shown in the
figure below. Most regionally metamorphosed areas can be divided
into zones where a particular mineral, called and index mineral, is
characteristic of the zone . The zones are separated by lines
(surfaces in three dimensions) that mark the first appearance of the
index mineral. These lines are called isograds (meaning equal grade)
and represent lines (really surfaces) where the grade of metamorphism is
In general, metamorphic rocks do not change chemical composition much
during metamorphism. The changes in mineral assemblages are due to
changes in the temperature and pressure conditions of metamorphism.
Thus the mineral assemblages that are observed must be an indication of
the temperature and pressure environment that the rock was subjected
to. This pressure and temperature environment is referred to as
metamorphic Facies. (This is similar to the concept of
sedimentary facies, in that a sedimentary facies is also a set of
environmental conditions present during deposition).
The sequence of metamorphic facies observed in any metamorphic terrain,
depends on the geothermal gradient that was present during
metamorphism.&nbrp; A high geothermal gradient such as the one labeled
"A" in the figure shown here, might be present around an igneous
intrusion, and would result in metamorphic rocks belonging to the hornfels
facies. Under a normal geothermal gradient, such as "B" in the
figure, rocks would progress from zeolite facies to greenschist,
amphibolite, and eclogite facies as the grade of metamorphism (or depth of
If a low geothermal gradient was present, such the one labeled "C" in the
diagram, then rocks would progress from zeolite facies to blueschist
facies to eclogite facies. Thus, if we know the facies of
metamorphic rocks in the region, we can determine what the geothermal
gradient must have been like at the time the metamorphism occurred.
Metamorphism and Plate Tectonics
At present, the geothermal gradients observed are strongly affected by
Along zones where subduction is occurring, magmas are generated near
the subduction zone and intrude into shallow levels of the
crust. Because high temperature is brought near the surface, the
geothermal gradient in these regions becomes high (geothermal gradient
"A" in the figure above), and contact metamorphism (hornfels facies)
Because compression occurs along a subduction margin (the oceanic
crust moves toward the volcanic arc) rocks may be pushed down to
depths along either a normal or slightly higher than normal geothermal
gradient ("B" in the figure above). Actually the geothermal
gradient is likely to be slightly higher than B, because the passage
of magma through the crust will tend to heat the crust somewhat.
In these regions we expect to see greenschist, amphibolite, and
granulite facies metamorphic rocks.
Along a subduction zone, relatively cool oceanic lithosphere is pushed
down to great depths. This results in producing a low geothermal
gradient (temperature increases slowly with depth). This low
geothermal gradient ("C") in the diagram above, results in metamorphism
into the blueschist and eclogite facies.