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Chapter 8
Metamorphism and Metamorphic Rocks
Metamorphic Rocks
Metamorphism: to change from one form
to another
Metamorphic rock: any rock that has
undergone solid state changes in texture,
mineralogy, and/or chemical composition
How Do Rocks Metamorphose?
Rocks metamorphose by the partial or
complete recrystallization of minerals in the
rocks over long periods of time
These rocks remain essentially solid during
metamorphism, but can flow in a plastic-like
manner
Metamorphism
Three Principal Factors that Drive Metamorphism
Temperature
Pressure
Fluids
Temperature
Temperature (heat) is the most important of
the three factors in metamorphism
Temperature drives the chemical changes
that result in the recrystallization of existing
minerals or the creating of new minerals
Temperature
Earth’s internal heat comes from energy
being released by radioactive decay and
thermal energy left over from the formation
of the planet
Temperature
The rate at which the temperature increases as
you go deeper into the Earth’s crust is called the
geothermal gradient
Temperature
The geothermal gradient averages 30oC per
kilometer increase with depth (but it can vary
from 20oC to 50oC per kilometer of depth)
Temperature
Note that the geothermal gradient is lowered by
the subduction of the cooler oceanic plate
Temperature
In contrast, the rising magma increases the
geothermal gradient
Effects of Temperature
Heat, by itself, can greatly
affect a rock’s texture and
mineralogy
Heat breaks chemical bonds
and alters the crystal
structure
Atoms and ions re-crystallize
into new mineral
assemblages
Many new crystals will grow
larger than they were in the
parent rock
Effects of Temperature
Given the initial mineral composition of the rock,
the metamorphic changes that occur with a
change in temperature follows a predictable and
repeatable path
Minerals crystallize (and remain stable) at
different temperatures in a predictable manner
Therefore, given a specific set of minerals in a
metamorphic rock, you can infer the temperature
at which the metamorphic rock formed
Temperature
Metamorphic changes can occur with increasing
or decreasing temperature
Prograde refers to mineral changes that take
place during an increase in temperature
Retrograde refers to mineral changes that take
place during an decrease in temperature
Pressure
Pressure, like temperature, changes a rock’s
mineralogy and texture in a predictable manner
Pressure, like temperature, also increases with
depth
Increases 1 kbar per 4.4 kilometers
Pressure
There are two major types of pressure:
Confining pressure applies pressure from
all directions
Differential stress is pressure comes from
a particular direction (such as from the
collision of two tectonic plates)
Confining Pressure
Buried rocks are subject to confining pressure,
where the pressure is applied equally in all
directions
Confining Pressure
Confining Pressure
Confining pressure causes the spaces between
mineral grains to close, producing a more
compact rock with a greater density
Confining Pressure
Confining Pressure
Confining pressure does not fold or deform
rocks
Differential Stress
Differential stress is a pressure that is applied
from a direction (rather than all directions)
Differential Stress
Rocks subject to differential stress are
preferentially shortened in the direction that
pressure is applied ...
Differential Stress
... and lengthened in the direction perpendicular
to that pressure
Effects of Pressure
Directed pressure guides the shape and
orientation of the new metamorphic minerals
Metamorphic minerals can be compressed,
elongated and/or rotated by being forced into
preferred orientations
Can form
spectacular
and erratic
banding
Effects of Pressure
At low pressures, rocks are brittle and tend to
fracture when subjected to differential stress
At high pressures, rocks are ductile and flow like
plastic
Under ductile conditions, mineral grains tend to
flatten and elongate when subject to differential
stress
Fluids
Fluids composed of water and other volatile
components, such as carbon dioxide, play an
important roll in metamorphism
Metamorphism can add or remove chemical
components that dissolve in water
Water acts as a catalyst during metamorphism
Water aids in the exchange of ions between
growing crystals
Fluids
Clays minerals can contained up to 60% water
Water is part of the crystal structure in many
minerals, such as mica and amphibole
When subject to low to medium temperatures,
water molecules can be removed from minerals
Once expelled, the water moves along the
individual mineral grains and is available to
transport ions
At higher metamorphic temperatures, the water
and fluids are driven from the rock
Parent Rocks
The initial composition of the parents rocks is a
fourth major factor
Most metamorphic rocks have the same overall
composition as the parent rock from which they
formed
Except for the possible loss or accumulation of
volatiles such as water and carbon dioxide
Metamorphic Grade
Metamorphic rocks are classified by how much
metamorphic changes they have undergone
High-grade: Formed in deeper crustal regions,
perhaps as deep as the upper mantle, under high
temperature and/or high pressure
Low-grade: Formed in shallower crustal regions
under low temperature and/or low pressure
Metamorphic Grade
The grade is related to both temperature
and pressure, which is related to depth
Texture & Composition
Metamorphic rocks are classified by their texture
and composition, which is also related to their
grade
Low grade
Slate
Phyllite
Schist
Gneiss
High grade Migmatite
Metamorphic Texture
Metamorphism creates new textures on the rock
it alters
In general the grain size of crystals increase as
the grade of metamorphism increases
Four main criteria:
The size of their crystals
How the mineral grain shape is changed
The degree to which minerals are
segregated into light and dark bands
Metamorphic grade
Metamorphic Textures
There are three major types of metamorphic
rock textures:
Foliation
Granoblastic
Large-crystal
When we look at foliation, we will also discuss:
Schistosity
Gneissic texture
Foliation
A fundamental and prominent textural feature of
regional metamorphosed rock
A set of flat or wavy parallel planes produced by
directed stress deformation
Presence of platy or elongated minerals (chiefly
micas and chlorite) help create the foliation
Foliation
Stress deformation causes mineral grains in
preexisting rocks to develop parallel, or nearly
parallel, alignment
Foliation
(Left)
Existing
minerals
keep their
random
orientation if
force is
uniformly
applied by
confining
pressure
Foliation
(Right)
Differential
stress causes
rocks to
flatten, and
the mineral
grains to
rotate toward
the plane of
flattening
Foliation
A. Ductile deformation (flattening) of mineral
grains can occur in one of two ways
Foliation
B. The first mechanism is a solid-state plastic
flow by intracrystalline movement within each
grain
Foliation
C. The second mechanism involves the dissolving
of ions from areas of high stress and the moving
and deposition of the ions in low stress areas
Foliation
D. Both mechanisms change the shape of the
mineral grains (but the volume and overall
composition remains essentially unchanged)
Foliated Rocks
Slate
Phyllite
Schist
Gneiss
Migmatite
Foliated Rocks - Slate
Slates are of the lowest metamorphic grade
They are so fine-grained that you need a
microscope to see individual minerals
Parent rock is shale
Foliated Rocks - Slate
Slate can be spilt along cleavage into thin sheets,
which gives it economic value
Chalk blackboards
were made of thin
sheets of slate
Slate
Slate can be used for roofing, but weight is a
problem
Slate
The very best pool tables are made of slate
Foliated Rocks - Phyllites
Phyllites form during low-grade metamorphism
of mud- and clay-rich sedimentary rocks
They represent an intermediate step between
slate and schist
Phyllites are very fine
grained rocks with a
grain size barely
visible in a hand
specimen
They usually exhibit
cleavage
Foliated Rocks - Schist
The schists are a group of medium-grade
metamorphic rocks, which are characterized by
having medium- to coarse-grained minerals that
are platy or flakey in appearance
These platy
mineral grains
include micas,
chlorite, talc,
hornblende,
graphite, and
others
Foliated Rocks - Schist
The small, platy (flakey) grains of mica in this
schist can easily be seen with the unaided eye
Foliated Rocks - Schist
Schist is also used as a “catch-all” term to
describe the texture of a metamorphic rock
To indicate composition, mineral names are
used
For example, this
is a “mica garnet
schist”
(Note that the
garnets are of
gem quality)
Foliated Rocks - Schist
Most schists have in all probability been derived
from clay and mud sedimentary rocks which
have passed through a series of metamorphic
processes involving the production of shales,
slates and phyllites as intermediate steps
Foliated Rocks - Gneiss
Gneiss are a group of high-grade metamorphic
rocks, which are characterized by having
medium- to coarse-grained minerals that are
banded or laminated in appearance
Gneiss is a common and widely distributed
type of rock formed by high-grade regional
metamorphic processes
Foliated Rocks - Gneiss
Gneiss is metamorphosed shale, granite or
volcanic rocks
The most common minerals in gneiss are quartz,
potassium feldspar and sodium-rich plagioclase
feldspar (plus lesser amounts of mica and other
minerals)
(Red-colored lichen
growing on gneiss
in Canada)
Banding in Gneiss
The banding can
be very distinctive
Foliated Rocks - Migmatite
Migmatite is a very high-grade of metamorphic
rock
Migmatite is a rock at the frontier between
metamorphic and igneous rocks
Temperatures are just high enough to start
melting the rock
As a consequence, migmatite istypically very
badly deformed and contorted with veins, pods
and lenses of melted rock
Foliated Rocks - Migmatite
Granoblastic Rocks
Not all metamorphic rocks have foliated texture
Many metamorphic rocks have a massive or
coarse granular appearance and exhibit no
deformation
They are composed mainly of crystals that grow
in equidimensional shapes
Therefore they are catalogued by mineral
composition, and not texture
These are referred to as granoblastic rock
Granoblastic Rocks
Some of the more common granoblastic rocks
are:
Quartzite
Marble
(Hornfels)
(Greenstones)
(Amphibolite)
We will look at quartzite and especially marble
Quartzite
Quartzite is a very hard metamorphic rock formed
from quartz sandstone
Pure quartzite is white, but reddish/pinkish and
grayish colors caused by impurities are common
The recrystillization
is so complete that
when broken,
quartzite will split
through the quartz
grains rather than
along their
boundaries
Marble
Marble is a metamorphic rock resulting from the
metamorphism of limestone or dolostone
This metamorphic process causes a complete
recrystallization of the original rock into an
interlocking mosaic of calcite, aragonite and/or
dolomite crystals
The temperatures and
pressures necessary to
form marble usually
destroy any fossils and
sedimentary textures
present in the original
rock
Marble
White in its pure form, marble is available in a
beautiful variety of colors, which are caused by
mineral impurities
such as clay, silt, sand,
iron oxides, or chert
Marble
Quarries in the mountains of Carrara, Italy have
been yielding quality marble for thousands of years
You can even see the quarries
from outer space
Marble
Marble is commonly used
as a building material
For example, as façades,
flooring and countertops
Taj Mahal
Located in Agra, India, the Taj Majal is a huge
mausoleum built by Shah Jahan for his wife
Mumtaj Mahal, and both are interred in it in a
simple crypt
Taj Mahal
It was built between 1631 and 1648 in the Mughal
architectural style, which combines elements of
Islamic, Indian, Persian and Turkish design
The Taj Mahal is considered to be the greatest
masterpiece of Islamic architectural art in India
Taj Mahal
The picture on the left was taken a half century
ago and shows the natural, brilliant white of the
marble used in the construction
The picture on the right shows how acid rain,
generated from local foundries and an oil
refinery, is turning the white marble into a sickly
light tan color
Marble
Pure white marbles, from Italy and
China, have been prized for
sculpture since classical times
These white marbles are soft which
facilitates carving, have the ability
to take a fine polish and are
relative resistance to shattering
Most of the greatest statues in the
world, such as the Venus de Milo
and David, were carved by hand
out of marble by the old masters
Marble
The low index of refraction of calcite in white
marble allows light to penetrate several
millimeters into the stone, resulting in the
characteristic "waxy" look which gives "life"
to marble sculptures of the human body
Three Graces by Antonio Canova
Marble
Antonio Canova (1 November 1757 to 13 October
1822) was from the Republic of Venice and many
consider him to be the greatest Italian sculptor
Eros and Psyche in
the Louvre, Paris
Large-crystal Textures
Metamorphic rocks can exhibit a great variation in
crystal size
During the recrystallization process, certain
metamorphic minerals, including garnet, Staurolite
and andalusite, tend to develop a few very large
crystals
In contrast, minerals
such as muscovite,
biotite and quartz
typically form a large
number of small
crystals
Porphyroblasts
Garnet
Porphyroblasts are
metamorphic rocks
having a matrix of
fine-grained minerals
with large crystals
The garnets grew
much faster than the
matrix in this schist
Speaking of Garnets
Garnets are only found in
metamorphic rock and can
be used to judge the grade
of the metamorphism
Most garnet is not of gem
quality
In fact, the most common
use of garnet is as an
abrasive, such as in garnet
sandpaper
Speaking of Garnets
When of gem quality,
garnets are typically
red, but they occur in a
wide variety of other
colors
The rarest color is blue
Uvarovite is a calcium chromium
garnet which is green in color
It is found in crystalline marbles
and schists in Russia and Finland
Texture
Classification of Metamorphic Rocks on Texture
Texture
Texture
1. Metamorphism
causes sedimentary
rocks, like shale, to
form slaty cleavage
planes perpendicular
to their bedding
planes
2. Original bedding was thin
clay layers
3. Metamorphism changes
the shale to slate
Texture
4. Folliation is
the result of
directed
compression
5. Mineral
crystals become
elongated
perpendicular to
the compression
6. Platy minerals
develop a
preferred
orientation
Texture
7. As intensity of metamorphism increases, so
does crystal size and coarseness of folliation
Texture
8. Foliated rocks are classified by the degree of
cleavage, shistosity and banding, which corresponds
to the intensity of metamorphism
Metamorphic Environments
Contact (or thermal)
Hydrothermal
Burial
Regional
Shock (impact)
Fault Zone
Contact Metamorphism
Contact or thermal
metamorphism
occurs when an
intrusive magma
heats the
surrounding country
(or host) rock and
changes the
mineralogy and
texture
Contact Metamorphism
The zone where the
rocks are subject to
metamorphism is
called the
metamorphic aureole
Contact Metamorphism
The sedimentary rocks are turned into
metamorphic rock by contact metamorphism
Contact Metamorphism
Even small dykes can form aureole of
metamorphic rock a few centimeters thick
Contact Metamorphism
The metamorphic aureole is the darker rock
that once roofed over the igneous pluton
Hydrothermal Metamorphism
Hydrothermal fluids can carry dissolved calcium
dioxide, sodium, silica, copper and zinc
Ascending hydrothermal fluids can react with
overlying rock, creating new minerals (which
may have great economic value)
Hydrothermal Metamorphism
The most widespread
occurrence of
hydrothermal
metamorphism is
along the mid-oceanic
ridges
As seawater
percolates through the
newly created crust, it
is heated and
chemically reacts with
the mafic (Fe and Mg
rich) basalt
Hydrothermal Metamorphism
The ferromagnesian
igneous minerals, such
as olivine and
pyroxene, are changed
into metamorphic
minerals such as
serpentine, chlorite and
talc
Calcium-rich
plagioclase feldspars
become more sodiumrich as the sea salt
(NaCl) exchanges
calcium for sodium
Black Smokers
Large amounts of metals, such as iron, cobalt,
nickel, silver, gold and copper, are dissolved from
the newly formed crust
These hot (~350oC), metal-rich fluids rise along
fractures, generating particle-filled clouds called
black smokers
Black Smokers
Black smokers were
first discovered in
1977 around the
Galápagos Islands by
the small
submersible vehicle
called Alvin
Smokers have now
been found in all
oceans
Black Smokers
Although life is very sparse at these depths, black
smokers are the center of entire ecosystems
Sunlight is nonexistent, so many organisms must
convert the heat, methane, and sulfur compounds
provided by black smokers into energy through a
process called chemosynthesis
Black Smokers
Hydrothermal vents support a large population
of chemosynthetic bacteria
The bacteria then grow into a
thick mat which attracts other
organisms such as amphipods
and copepods which graze
upon the bacteria directly
Larger organisms such as
snails, shrimp, crabs, tube
worms, fish, and octopuses
form a food chain of predator
and prey
Burial Metamorphism
Burial metamorphism occurs when thick
accumulations of sedimentary strata on the
ocean floor are subducted beneath another plate
Burial Metamorphism
This is a low grade metamorphism that typically
begins when the subducted sediments reach a
depth of 6-10 kilometers (3-6 miles) or when the
temperature reaches about 200oC
Regional Metamorphism
Most metamorphic rocks are created during the
process of regional metamorphism associated
with mountain building
During these
dynamic events,
large segments
of the Earth’s
crust are
intensely
deformed along
convergent
plate
boundaries
Regional Metamorphism
The mountain building applies differential stress
literally over a wide regional area
Sediments and
crustal rock
lifted up from
the ocean
floor are
folded and
faulted
Metamorphism
of all grades,
from low to
high occurs
Regional Metamorphism
The Andes Mountains and the Himalaya
Mountains (below) are prime examples where
regional metamorphism has occurred along
thousands of miles of mountain range
Regional Metamorphism
The Swiss and Austrian
Alps in Europe are other
famous examples where
extensive regional
metamorphism has
occurred
Impact Metamorphism
Impact metamorphism
occurs when an
asteroid or comet
impacts the Earth’s
surface
These objects can be
moving as fast as
100,000 miles per hour
(~28 miles per second)
Impact Metamorphism
In a fraction of a second, the energy of the
rapidly moving object is transferred into heat
energy and shock waves as it smashes into the
Earth
Impact Metamorphism
The impacting asteroid or comet is vaporized
The impacted rock is shattered, pulverized and
sometimes even melted
Minerals in the rock are instantly subjected to
both high temperature and high pressure
Impact Metamorphism
Rare and unusual metamorphic minerals such as
coesite, which are normally never found on the
Earth’s surface, are nearly instantly formed
Staggering quantities of matter are blown into the
atmosphere
Fortunately for life on
Earth, this is a rare
event, but these
impacts have
repeatedly caused
mass extinctions
Impact Metamorphism
A crater one mile in diameter and 500 feet deep is
formed in only 30 seconds
A crater ten miles in diameter and a mile deep is
formed in 90 seconds
The largest known crater on the Earth in located in
South Africa and is 180 miles in diameter
(Above) Tenoumer Crater in Mauritania
Fault Zone Metamorphism
Near the surface, rock behaves like a brittle solid
So near the surface, movement along a fault zone
fractures and pulverizes the rock, creating what is
called fault breccia
Fault Zone Metamorphism
In contrast, at depth under higher heat and pressure,
rock is ductile and flows like plastic
At depth along a fault zone, the mineral structures
are deformed by the ductile flow, giving the
metamorphic rock a foliated or lineated appearance
Metamorphic Grade
Metamorphic grade tells us the maximum
temperature and pressure to which a rock was
subject
However, metamorphism is a dynamic process and
a metamorphosed rock may have a very complex
history
Most minerals are stable over a relatively narrow
range of pressure and temperature
The stability range of different minerals sometimes
overlap and provide insights into the metamorphic
history of rocks
Metamorphic Grade
1. During metamorphism, a garnet crystal grows
and its composition changes as the temperature
and pressure around it changes
Metamorphic Grade
2. We can plot the growth of the garnet on the P-T
chart from where it started growing at its center
[1] to its edge at [2]
Metamorphic Grade
3. The garnet started growing in a schist [1] and
continued growing in a gneiss [2] as the grade of
metamorphism increased along the prograde path
Metamorphic Grade
4. Note that the garnet survived the progression
along the retrograde path as it headed to the
surface
Metamorphic Facies
About a century ago, it was realized that there are
groups of associated metamorphic minerals that
were formed under similar temperatures and
pressures
Different metamorphic rocks containing the same
assemblage of minerals are said to belong to the
same metamorphic facies
Metamorphic Facies
Each facies is characteristic of particular tectonic
environments and will have certain index minerals
that are indicative of those conditions
Therefore the minerals in a rock can be clues to
the (pressure and temperature) history of the rock
Metamorphic Facies
Metamorphic Facies
This color plot
shows the change
in the grade of six
common “index”
metamorphic
minerals across
New England
Note the regional
progression from
low- to high- grade
Metamorphic Facies
Metamorphic Facies
With increasing
metamorphic
grade, mineral
compositions
change and these
minerals define
the metamorphic
facies for the
metamorphic
environment
Metamorphic Facies
Metamorphic facies are determined by the
temperature and pressure
In turn, these
temperatures
and pressures
define the
metamorphic
environment
Therefore we
can plot the
metamorphic
environments
Plate Tectonics & Metamorphism
The pressure-temperature history of the rock can
often be tied to the plate tectonic setting
Continent-continent collision
Continent-ocean convergence
Seafloor spreading
Transform-fault
Plate Tectonics & Metamorphism
Plate tectonics
moves rocks
through different
temperature and
pressure zones,
from shallow to
deep levels in the
crust
And the back to
the shallow crust
and even the
surface
Plate Tectonics & Metamorphism
We will look at
two examples
involving
continent-ocean
convergence
Plate Tectonics & Metamorphism
Plate Tectonics & Metamorphism
Plate Tectonics & Metamorphism
Plate Tectonics & Metamorphism
Plate Tectonics & Metamorphism
The association of metamorphic facies
with the various types of plate tectonic
metamorphic environments
Chapter 9
Geologic Time