Metamorphic Rocks
Metamorphism and Metamorphic Rocks
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What is metamorphism?
How do temperature and pressure change rocks?
Are fluids important in metamorphism?
How are metamorphic rocks brought to the surface?
What are the conditions of metamorphism?
How are metamorphic rocks classified?
What was the rock before it was metamorphosed?
Where does metamorphism occur in relation to plate
tectonics?
Metamorphism and Metamorphic Rocks
Metamorphic rocks:
• Form beneath Earth’s surface (metamorphic
processes cannot be directly observed).
• Are commonly found exposed in actively forming
mountains, e.g. the Himalayas today.
• Are always found in the centers of eroded ancient
mountain belts.
• Metamorphic minerals make up such economic
materials as talc, graphite, marble, garnet,
corundum, and coal deposits.
Metamorphism and Metamorphic Rocks
Preview: Three types of metamorphic rocks:
– Regional metamorphic rocks.
• Occur across vast regions such as convergent plate
boundaries (e.g. interior of Himalayas today)
• Increased temperature and directed pressure
– Contact metamorphic rocks.
• Thermally driven, common along boundaries of igneous
intrusions, sometimes under lava flows
– Hydrothermal metamorphic rocks.
• Produced by hot-fluid induced chemical changes
• May be associated with one or both of the previous types
Appalachian Mountains
- A belt of regional metamorphic
rocks >1500 km long and
200-300 km wide, produced
by continent-continent
collision.
From the Greek “meta” (change) and “morphe” (form)
Metamorphic rocks – Formed under conditions between those
of compaction and cementation (lithification – sedimentary
rocks) and melting (igneous rocks).
What is Metamorphism?
• Metamorphism describes the mineralogical,
chemical, and textural changes to preexisting
rocks due to increased temperature and
pressure.
• Metamorphic reactions occur in the solid
state – no melting is involved.
• Because these conditions occur at depth in
the Earth, they are not directly observable.
• Modern experimental petrology, however, can
reproduce these conditions in the laboratory,
including original rock, T, P and fluids.
Migmatite – formed by very high grade metamorphic
conditions where partial melting (anatexis) occurs.
What is Metamorphism?
• Rocks exhibit two types of change:
• Original minerals react to form new minerals.
• Rock texture is altered by changes in size, shape,
and orientation of the minerals (new and/or old).
• Original rock type is important:
• Chemistry controls what reactions will take place,
i.e. what new minerals may form.
• In general the whole rock starting composition is
the same, or similar to, the metamorphic rock
composition.
• In some cases the presence of chemically active fluids
may cause substantial changes.
How do temperature and pressure change rocks?
• We know: Temperature increases with depth in the Earth.
• This is referred to as the geothermal gradient, typically 20-30 oC/km
beneath continents and 60 oC/km beneath oceans.
How do temperature and pressure change rocks?
How do temperature and pressure change rocks?
• It takes ~10-12 km burial to reach low-grade metamorphic conditions.
• By contrast, lithification to form sedimentary rocks occurs at shallow depths.
How Are Higher Temperatures and Pressures Produced in
Mountain Building Events?
• Mountain building
events involve
directed pressure, e.g.
the collision between
India and Asia
producing the
Himalayas.
• When one block of
crust is forced over
another along a fault,
the lower one heats to
a temperature
associated with its
new depth.
•This is referred to as
tectonic burial and
produces regional
metamorphic rocks.
How Are Higher Temperatures Produced by Magma Intrusion?
• Intrusion of magma
raises the local
geothermal gradient.
This causes
metamorphism in
rocks adjacent to the
intrusion.
• This produces what
are called contact
metamorphic rocks.
How do temperature and pressure change rocks?
• Heat drives off fluids (volatiles) that may
have been in the rocks.
• High-temperature metamorphism causes
minerals containing volatiles to lose them.
Dehydration reactions = loss of water.
(HEAT)
KAl3Si3O10(OH)2 + SiO2
(muscovite)
(quartz)
Al2SiO5 + KAlSi3O8 + H2O
(sillimanite)
(K-feldspar) (water)
How do temperature and pressure change rocks?
• Minerals have ranges of temperature and pressure in
which they are stable.
– If the T and P change the minerals may transform into new
minerals which are stable in the new conditions.
A Contact Metamorphic Reaction:
A Common Example We All Know:
~40 km
How do temperature and pressure change rocks?
• Minerals have ranges of temperature and pressure in
which they are stable.
– If the T and P change the mineral may transform into a new
mineral which is stable in the new conditions.
We can use metamorphic
minerals as indicators of T and P,
since we know what conditions
they are stable at.
Phase diagrams like this have
been constructed via
experimental petrology.
Kyanite: high P
Sillimanite: high T and P
Andalusite: low T and P
What if we found a rock with all
three minerals in it?
How do temperature and pressure change rocks?
• Pressure can be simply thought of as
applying stress to a rock.
• Stress – The magnitude of the force divided
by the area the force is applied to.
• Normal stress – perpendicular to the surface.
Results in change in volume and often shape.
• Shear stress – force parallel to the surface.
Results in change of shape, but not volume.
• Strain – the deformation of a rock as a result
of applied stress.
How do temperature and pressure change rocks?
Directed pressure such as this is called Differential Stress.
How do temperature and pressure change rocks?
How do temperature and pressure change rocks?
•Directed pressure can cause platy minerals such as micas to
align perpendicular to the stress.
• This produces what is called foliation and is a characteristic
of regional metamorphic rocks.
How do temperature and pressure change rocks?
Foliation in thin section: Defined by alignment
of muscovite (brightly colored) intergrown with
quartz (greys and blacks)
How do temperature and pressure change rocks?
Metaconglomerate - formed from increase
in temperature and directed pressure.
How do temperature and pressure change rocks?
• Increased temperature and pressure can cause
recrystallization of minerals into new shapes and
sizes.
This process is clear
in these microscope
photos of a
sandstone (top) and
a metamorphosed
sandstone (bottom)
which is called
quartzite.
Fig 6.9
Are fluids important in metamorphism?
• Fluids can participate in two ways:
• React with old minerals to from new minerals that
contain the fluid (water or CO2)
• Help drive the changes of metamorphism by
delivering and removing dissolved ions. Fluid
makes reactions occur much faster and more easily.
Are fluids important in metamorphism?
• Fluid may be present during low-grade
metamorphism (and ultimately – taken away
during high grade metamorphism…)
• Most commonly during low-medium grade
metamorphism (similar to chemical weathering
reactions, but higher temperature and pressures)
2 Mg2SiO4 + 2 H2O + CO2
Mg-olivine
water carbon dioxide
Mg3Si2O5(OH)4 + MgCO3
serpentine
magnesite
Confining pressure may produce metamorphism,
but only differential stress produces foliation
Cataclastic (brittle) fault zone deformation at <10 km depth
Temperatures and pressures are too low for metamorphism
to occur, the rock is simply shattered and broken up.
Mylonite - ductile deformation at >10km depth.
Now T and P are high enough for metamorphism, the rock
may recrystallize to new minerals and plastic deformation
can occur.
How Are Metamorphic Rocks Brought To The Surface?
• Metamorphic rocks form many kilometers underground.
• They usually get exposed due to faulting, uplift of mountains
and subsequent erosion.
How Are Metamorphic Rocks Brought To The Surface?
• Metamorphism of rocks occurs very slowly, it may
take millions of years for a rock to convert to a
metamorphic rock.
• Uplift and exposure is usually relatively rapid and
rocks cool quickly during uplift, this “freezes” in the
metamorphic mineral assemblage.
• Loss of volatiles during high grade metamorphic
reactions is also a big factor.
– Reactions cannot reverse easily if volatiles are gone.
• Sometimes: High-grade metamorphic rock may rise to
low-grade conditions and stay there for extended
periods, re-metamorphosing them.
What Are The Conditions of Metamorphism?
Mineral stability is
experimentally
calibrated in the
laboratory with
high temperature
and pressure
instruments.
What Are The Conditions of Metamorphism?
Some minerals form only over a limited range of pressure and
temperature – these are good index minerals.
How are metamorphic rocks classified?
• Composition and texture:
– We know that metamorphic processes change the
minerals in a rock and often the rock texture as
well. Texture and composition are thus the primary
criteria for classifying metamorphic rocks.
• The first characteristic is either foliated or
non-foliated. Foliated rocks are characterized
by having aligned minerals, or layered
minerals, or otherwise linearly arranged
minerals.
How are metamorphic rocks classified?
This is a foliated texture: fine-grained with minerals aligned along
planes (producing rock cleavage). This rock is called slate and
may contain clays, muscovite, chlorite, quartz.
Parent rock is
shale or tuff
How are metamorphic rocks classified?
Compositional layering in outcrop - This rock is a gneiss.
How are metamorphic rocks classified?
Progressive changes in rock texture and mineral
content take place as the metamorphic grade increases.
For foliated metamorphic rocks - the type or degree of
foliation is the primary characteristic for classification.
Start with a sedimentary rock – a shale….
How are metamorphic rocks classified?
At low grade metamorphic conditions clay
minerals begin to convert to micas and chlorite,
but this process is incomplete. If differential
stress is present these newly grown minerals will
be aligned with the stress field (perpendicular).
How are metamorphic rocks classified?
At moderate metamorphic conditions all the clay is
recrystallized into mica and chlorite. Feldspars may
begin to form. The crystals are now visible with the
unaided eye, and the rock develops a “sheen” due to
the abundance of muscovite mica.
How are metamorphic rocks classified?
At medium grade metamorphic conditions the micas
grow much larger, and are all aligned perpendicular to
the stress. Chlorite is now gone. New higher grade
metamorphic minerals such as garnet may form.
How are metamorphic rocks classified?
At high grade metamorphic conditions minerals begin to
segregate into layers, with light colored high Si, low Fe
and Mg minerals in layers, and dark colored low Si, high
Fe and Mg minerals in other layers. This is called
gneissic banding, and is a type of foliation.
How are metamorphic rocks classified?
How are metamorphic rocks classified?
Some metamorphic
rocks do not have
foliation.
For these
metamorphic
rocks, composition
(mineral content)
is the primary
characteristic for
naming the rock.
What was the rock type before it was metamorphosed?
Here are some common metamorphic changes in rock
type for different starting (parent) sedimentary and
igneous rock types (these are called protoliths).
Where does metamorphism occur?
Where does metamorphism occur?
A contact metamorphic zone has increasing grade as you
get closer to the heat source.
Where do we find igneous intrusions? In convergent and
divergent plate tectonic boundaries.
Contact Metamorphism by Igneous Intrusions.
These are common over subduction zones.
Where does metamorphism occur?
• Hydrothermal metamorphism
– Involves migration and reaction of hot, geothermal
fluids
– Along mid-ocean ridges, divergent plate
boundaries.
– Above igneous intrusions, subduction zones.
Diagram showing hydrothermal ore deposits
at a mid-ocean ridge.
Where does metamorphism occur?
Tectonic setting for regional metamorphism at a convergent plate boundary,
in this case oceanic crust subducting beneath continental crust.
Where does metamorphism occur?
Regional Metamorphism by Continent-Continent Collision.
Where does metamorphism occur?
Progressively higher grade metamorphism of shale or
mudstone may produce this sequence.
Where does metamorphism occur?
There are three primary types of metamorphism—contact,
hydrothermal, and regional.
Contact metamorphism occurs along the margins of igneous
intrusions. It is very localized and driven by increased T.
Hydrothermal metamorphism is usually small scale and
localized also. It occurs mostly along mid-ocean ridges.
Involves hot fluids and the ions the fluids mobilize.
Regional metamorphism involves increasing temperature and
pressure over large volumes of crust. It is typical of converging
plate boundaries.