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Metamorphism and Metamorphic Rocks
A. Metamorphic rocks: Form at conditions between
igneous and sedimentary rocks
Metamorphism and Metamorphic Rocks
B. Metamorphism
Heat, pressure, and chemical
reactions deep within the
Earth alter the mineral content
and/or structure of preexisting
rock without melting it
Any rock (igneous, sedimentary,
metamorphic) can be
metamorphosed
Some atomic bonds are broken
so atoms and ions can
rearrange themselves into new
minerals.
If all the atomic bonds break, the
rock would melt.
Metamorphism and Metamorphic Rocks
B. Metamorphism
Derived from older rocks through
solid-state crystallization.
Does not involve a significant melt
phase.
The lack of melting distinguishes
igneous from metamorphic
processes.
Metamorphism and Metamorphic Rocks
B. Metamorphism
1. Less stable minerals will become
more stable minerals.
Clay transforms into mica
2. Relatively stable minerals (e.g.,
quartz) will recrystallize or
change structure.
Can’t witness metamorphism on
the Earth’s surface.
Occurs under all the rocks we see
at the surface, we see
metamorphic rocks when the
area is uplifted and eroded.
Metamorphism and Metamorphic Rocks I. Factors controlling
metamorphism
A. Heat: most important factor this
drives chemical reactions
Subject rock to a change in
temperature
1. Bury Rocks
Geothermal gradient = 75F/mile
(average 25°C/km)
So bury rocks deep enough (either by
tectonic movement or deep burial
under sediments) and they will
metamorphose
Deepest mine: 2.5 miles deep—rock
is hot enough to burn human skin
2. Burial and geothermal gradient
Deepest well: 7.7 miles, temperature
is 473F (212F is boiling)
Metamorphism and Metamorphic Rocks I. Factors controlling
metamorphism
A. Heat: most important factor this
drives chemical reactions
Geothermal gradient = 75F/mile
(average 25°C/km)
What would be the temperature at
5 km depth within the interior of
North America?
Every point within the Earth we could
measure or calculate a T value. If
nothing perturbed our simple system,
then we could easily predict how T
changed with depth.
However, it not that simple.
Metamorphism and Metamorphic Rocks
I. Factors controlling
metamorphism
A. Heat: most important factor this
drives chemical reactions
Geothermal gradient = 75F/mile
(average 25°C/km)
Temperature gradient in volcanic
arcs and mid-ocean ridges is
significantly greater than in the
interiors of continental areas.
Volcanic regions - the geothermal
gradient can reach 50°C/km or
more.
The geothermal gradient in regions
oceanward of arcs is only about
10°C/km.
Metamorphism and Metamorphic Rocks
I. Factors controlling
metamorphism
B. Pressure
1. Confining pressure (lithostatic
pressure)
Deep in the Earth
Equal pressure from all directions
Rock will be compressed into a
smaller volume (no change in
rock shape)
Ions migrate within minerals from
high pressure to low pressure
regions
Metamorphism and Metamorphic Rocks
I. Factors controlling
metamorphism
B. Pressure
1.Confining pressure (lithostatic
pressure)
Pressures are given in kilobars
(kbars) or MPascals (MPa)
For every ~3 km increase in depth
pressures increases by ~1 kbar.
The continental interiors, the
geobarometric (or geopressure)
gradient is ~0.33 kbar/km.
What would be the pressure at 5
km depth?
Metamorphism and Metamorphic Rocks
Metamorphism and Metamorphic Rocks
I. Factors controlling
metamorphism
B. Pressure
1. Confining pressure (lithostatic
pressure)
Deep in the Earth
Equal pressure from all directions
Rock will be compressed into a
smaller volume (no change in rock
shape)
Ions migrate within minerals from
high pressure to low pressure
Metamorphism and Metamorphic Rocks
I. Factors controlling
metamorphism
B. Pressure
2. Directed pressure
 Pressure is greater in one direction
that in others
 Occurs at plate boundaries
 Changes the shape of a rock
 Flattens a rock in the direction of
greatest pressure
 Minerals growing align
perpendicular to the pressure 
foliation
 May also stretch, bend, or fold
existing features
Metamorphism and Metamorphic Rocks
I. Factors controlling
metamorphism
C. Circulating Fluids
• Predominantly water with dissolved ions
• Increases potential for metamorphic
reactions
• Aids in migration of unbonded atoms
and ions; ions move easily in fluids
• Water sources: percolated from Earth’s
surface trapped in subducting slab
between sediment grains or cracks
• Release from water rich minerals
(amphiboles, clays)
• Carry in new ions to the rock and carry
away ions so they can change a rocks
overall chemistry
Metamorphism and Metamorphic Rocks
I. Types of Metamorphism:
heat, pressure, and fluids interact
differently in different geological
settings to produce different
metamorphic rocks
A. Contact Metamorphism
Solid rock near magma may be
baked by heat
High temp, low pressure
(no foliation)
Rocks not good conductors of heat,
so area of metamorphism is
localized
Extent depends on size of intrusion:
Small dike or sill  centimeters to
meters or
Large batholith  several
kilometers
Metamorphism and Metamorphic Rocks
II. Types of Metamorphism:
Heat, pressure, and fluids interact
differently in different geological
settings to produce different
metamorphic rocks
A. Contact Metamorphism
Solid rock near magma may be
baked by heat
High temperature, but low pressures.
Near cooling magma (igneous
intrusions)
Metamorphism and Metamorphic Rocks
II. Types of Metamorphism:
A. Contact Metamorphism
Metamorphism and Metamorphic Rocks
II. Types of Metamorphism:
A. Contact Metamorphism
Metamorphism and Metamorphic Rocks
II. Types of Metamorphism:
B. Regional Metamorphism
Alters rock for thousands of square
kilometers
1. Continental Collision Zones
(rocks once at surface get
buried)
Directed pressure  foliation
2. Burial--Sediment gets buried in
deep sedimentary basins
Confining pressure  no foliation
Louisiana, Mississippi River delta,
bottom deposits are 12 km down
This is high temperature and high
pressure
Metamorphism and Metamorphic Rocks
II. Types of Metamorphism:
B. Regional Metamorphism
Metamorphism and Metamorphic Rocks
II. Types of
Metamorphism:
C. Subduction Zone
Blueschist
metamorphism
Low temperature, high
pressure
In a subduction zone
environment
What is the
geothermal gradient
Low or High?
~5°-10°C/km
Metamorphism and Metamorphic Rocks
II. Types of Metamorphism:
D. Hydrothermal
Metamorphism: chemical
alteration of preexisting
rocks by hot water
Usually happens at divergent
plate boundaries—beneath
ocean floors, where
seawater percolates down,
warms up by magma and
alters rock
High temperature, low
pressure
Similar tectonic setting to
contact metamorphism
Metamorphism and Metamorphic Rocks
Metamorphism and Metamorphic Rocks
High pressure and medium
pressure metamorphic rocks
in the Caledonides of western
Norway.
These rocks all have broadly
basaltic compositions, and
were metamorphosed in the
Paleozoic.
The high pressure rocks were
brought rapidly enough to the
surface so that these high
pressure rocks scarcely have
any evidence of retrograde
metamorphism.
Metamorphism and Metamorphic Rocks
Metamorphism and Metamorphic Rocks