Grotzinger • Jordan
Understanding Earth
Sixth Edition
Chapter 6:
METAMORPHISM
Modification of Rocks by
Temperature and Pressure
© 2011 by W. H. Freeman and Company
Chapter 6:
Metamorphism:
Alteration of
Rocks by
Temperature and
Pressure
About Metamorphism
• Changes in heat, pressure, and the
chemical environment of rocks can alter
mineral compositions and crystalline
textures, making them metamorphic.
• Metamorphic changes occur in the solid
state, so there is no melting.
Lecture Outline
1. Causes of metamorphism
2. Types of metamorphism
3. Metamorphic textures
4. Regional metamorphism and
metamorphic grade
5. Plate tectonics and metamorphism
1. Causes of Metamorphism
● internal heat of Earth
● internal pressure of Earth
● fluid composition inside Earth
1. Causes of Metamorphism
● temperature increases with depth
● rate = 20º to 60ºC per km
● at 15 km depth: 450ºC
1. Causes of Metamorphism
● pressure and temperature increase
with depth in all regions
1. Causes of Metamorphism
1. Causes of Metamorphism
● the role of temperature
● geothermal gradient
● shallow (20ºC / km)
● steep (50ºC / km)
1. Causes of Metamorphism
● the role of pressure (stress)
● confining pressure
● directed pressure
1. Causes of Metamorphism
● the role of pressure (stress)
● rate of increase =
0.3 to 0.4 kbar / km
● minerals are geobarometers
Thought questions for this chapter
At what depths in the Earth do metamorphic rocks form?
What happens if temperatures get too high?
Why are there no metamorphic rocks formed under
natural conditions of very low pressure and temperature,
as shown in Figure 6.1?
2. Types of Metamorphism
● the role of fluids
● metasomatism
● accelerated chemical
reactions
2. Types of Metamorphism
Depth,
km
0
Oceanic
crust
35
75
Oceanic
lithosphere
Shock
metamorphism
Depth,
km
0
Oceanic
crust
35
75
Oceanic
lithosphere
Shock
metamorphism
Depth,
km
0
Regional
metamorphism
Oceanic
crust
35
75
Oceanic
lithosphere
Shock
metamorphism
Depth,
km
0
Regional
metamorphism
High-pressure
metamorphism
Oceanic
crust
35
75
Oceanic
lithosphere
Shock
metamorphism
Depth,
km
0
Regional
metamorphism
High-pressure
metamorphism
Contact
metamorphism
Oceanic
crust
35
75
Oceanic
lithosphere
Shock
metamorphism
Regional
metamorphism
Depth,
km
0
High-pressure
metamorphism
Contact
metamorphism
Oceanic
crust
35
Oceanic
lithosphere
75
Burial
metamorphism
Shock
metamorphism
Regional
metamorphism
Depth,
km
0
High-pressure
metamorphism
Contact
metamorphism
Oceanic
crust
35
Oceanic
lithosphere
75
Water
Seafloor
metamorphism
Burial
metamorphism
Thought questions for this chapter
Draw a sketch showing how seafloor metamorphism might
take place.
3. Metamorphic Textures
3. Metamorphic Textures
Staurolite
crystal
Mica
Foliated rocks contain platy
minerals that are aligned along
a preferred orientation.
Staurolite
crystal
Mica
3. Metamorphic Textures
Feldspar
Quartz
Mica
Pyrite
Staurolite
Foliation is the result
of compressive forces.
Feldspar
Quartz
Mica
Pyrite
Staurolite
Foliation is the result
of compressive forces.
Mineral crystals become
elongated perpendicular
to the compressive force.
Feldspar
Quartz
Mica
Pyrite
Staurolite
3. Metamorphic Textures
● preferred orientation of crystals
● slaty cleavage
● foliation
3. Metamorphic Textures
shale
sandstone
layers
5 cm
The original bedding
can be seen in the
thin sandy layers.
shale
sandstone
layers
5 cm
The original bedding
can be seen in the
thin sandy layers.
foliation
plane
shale
sandstone
layers
original
bedding
5 cm
The original bedding
can be seen in the
thin sandy layers.
Regional metamorphism
causes cleavage planes
to develop.
foliation
plane
shale
sandstone
layers
original
bedding
5 cm
3. Metamorphic Textures
Low grade
Intermediate grade
3. Metamorphic Textures
Diagenesis
Slate
Slaty cleavage
Low grade Intermediate
grade
Phyllite
High grade
Schist
(abundant
micaceous
minerals)
Gneiss
(fewer
micaceous
minerals)
Migmatite
Schistosity
Banding
Banding
Foliated rocks are classified by the degree
of cleavage, schistosity, and banding.
Diagenesis
Slate
Slaty cleavage
Low grade Intermediate
grade
Phyllite
High grade
Schist
(abundant
micaceous
minerals)
Gneiss
(fewer
micaceous
minerals)
Migmatite
Schistosity
Banding
Banding
3. Metamorphic Textures
● classification of foliated rocks
● metamorphic grade
● crystal size
● type of foliation
● banding
3. Metamorphic Textures
● nomenclature of foliated rocks
● slate
● phyllite
● schist
● gneiss
● migmatite
Foliated
texture:
schist
with garnet
porphroblasts
3. Metamorphic Textures
● nomenclature of granoblastic
(non-foliated) metamorphic rocks
● hornfels
● quartzites
● marbles
● greenstones
● amphibolites
● granulites (granofels)
Granoblastic Texture
Thought questions for this chapter
How is slaty cleavage related to tectonic forces? What
forces cause minerals to align with one another?
Would you choose to rely on chemical composition or
type of foliation to determine metamorphic grade? Why?
4. Regional Metamorphism
and Metamorphic Grade
● grades of metamorphism
● low
● intermediate
● high
4. Regional Metamorphism
and Metamorphic Grade
● mineral isograds (zones of change)
● index minerals reflect pressure
and temperature conditions
● groups of 2 to 3 index minerals
form an isograd
4. Regional Metamorphism
Canada
ME
NY
Isograds
VT
NH
Key:
MA
CT
RI
Low
grade
Medium
grade
High grade
Not
metamorphosed
Chlorite zone
Biotite zone
Garnet zone
Staurolite zone
Sillimanite zone
Index minerals define
metamorphic zones.
Canada
ME
NY
Isograds
VT
NH
Key:
MA
CT
RI
Low
grade
Medium
grade
High grade
Not
metamorphosed
Chlorite zone
Biotite zone
Garnet zone
Staurolite zone
Sillimanite zone
Index minerals define
metamorphic zones.
Isograds can be
used to plot the
level or degree of
metamorphism.
Canada
ME
NY
Isograds
VT
NH
Key:
MA
CT
RI
Low
grade
Medium
grade
High grade
Not
metamorphosed
Chlorite zone
Biotite zone
Garnet zone
Staurolite zone
Sillimanite zone
Low
Grade
Intermediate
Grade
Phyllite
High
Grade
Schist
Blueschist
Gneiss
Migmatite
Temperature (°C)
Depth (km)
Pressure (kilobars)
Slate
0
0
Hornfels
0
20
25
13.5
15
15
Depth (km)
10
Granulite
10
20
Amphibolite
5
Greenschist
Pressure (kilobars)
5
30
200
400
Eclogite
35
600
40
1000
800
Temperature (°C)
Metamorphic facies correspond
to particular combinations of
pressure and temperature...
… and can be used
to indicate specific
tectonic environments.
Thought questions for this chapter
You have mapped an area of metamorphic rocks, such as
the region in Figure 6.9a, and have observed a series of
metamorphic zones, marked by north-south isograds,
running from sillimanite in the east to chlorite in the west.
Were metamorphic temperatures higher in the east or west?
Which kind of pluton would produce the highest grade of
metamorphism, a granite intrusion 20 km deep or a gabbro
intrusion at a depth of 5 km?
5. Plate Tectonics and Metamorphism
● metamorphism occurs in or near
● plate interiors
● divergent plate margins
● convergent plate margins
● transform plate margins
Tectonic transport moves
rocks through different
pressure-temperature
zones, …
Low P,
Low T
High P,
High T
Tectonic transport moves
rocks through different
pressure-temperature
zones, …
Low P,
Low T
High P,
High T
…and then transports them
back to the shallow crust or
the surface.
5. Plate Tectonics and Metamorphism
● metamorphic pressure-temperature
paths
● history of burial and exhumation
● prograde and retrograde paths
5. Plate Tectonics and Metamorphism
The garnet crystal initially grows in a schist
but ends up growing in a gneiss.
Intermediate
Grade
Depth (km)
Pressure (kilobars)
Low
Grade
High
Grade
Temperature (°C)
High
Grade
Peak metamorphism
Temperature (°C)
Low temperature–
high pressure metamorphism
within a subduction zone
Continental
crust
Deep-ocean
sediment
Shelf
sediment
Oceanic
crust
Trench
Mélange
ophiolites
Continental
crust
Prograde
path
Peak
metamorphism
Retrograde
path
Depth (km)
Pressure (kilobars)
Low
Grade
High
Grade
Depth (km)
Pressure (kilobars)
Low
Grade
Peak metamorphism
Temperature (°C)
Low temperature–
high pressure metamorphism
within a subduction zone
Continental
crust
Deep-ocean
sediment
Shelf
sediment
Oceanic
crust
Trench
Mélange
ophiolites
High temperature–
high pressure metamorphism
within a mountain belt
Suture
Continental
crust
Multiple
thrusts
Deformed and
metamorphosed
shallow- and deepocean sediments
Continental
crust
Continental
crust
Prograde
path
Peak
metamorphism
Prograde
path
Retrograde
path
Peak
metamorphism
Retrograde
path
5. Plate Tectonics and Metamorphism
● rapid erosion (exhumation) rates
of mountain ranges show a
relationship between
● tectonics (orogeny)
● climate
● controls the flow of metamorphic
rocks to the surface
Thought questions for this chapter
Draw a sketch showing how seafloor metamorphism might
take place.
Subduction zones are generally characterized by highpressurelow temperature metamorphism. In contrast,
continent-continent collision zones are marked by moderate
pressurehigh temperature metamorphism. Which type of
plate boundary has a higher geothermal gradient? Explain.
Key terms and concepts
Amphibolite
Blueschist
Burial metamorphism
Contact metamorphism
Eclogite
Exhumation
Foliated rock
Foliation
Gneiss
Granoblastic rock
Granulite
Greenschist
Greenstone
High-pressure metamorphism
Hornfels
Key terms and concepts
Marble
Mélange
Metamorphic facies
Metasomatism
Migmatite
Phyllite
Porphroblast
Quartzite
Regional metamorphism
Schist
Seafloor metamorphism
Shock metamorphism
Slate
Stress
Suture
Key terms and concepts
Ultra-high pressure metamorphism
Zeolite