1. Modification of Rocks by Temperature and Pressure
Grotzinger • Jordan
Understanding Earth
Sixth Edition
Chapter 6:
METAMORPHISM
Modification of Rocks by Temperature and Pressure
© 2011 by W. H. Freeman and Company

2. Alteration of Rocks by Temperature and Pressure
Chapter 6:
Metamorphism:
Alteration of Rocks by Temperature and Pressure

3. 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.

4. Lecture Outline Causes of metamorphism 2. Types of metamorphism
3. Metamorphic textures
4. Regional metamorphism and metamorphic grade
5. Plate tectonics and metamorphism

5. ● internal heat of Earth ● internal pressure of Earth
1. Causes of Metamorphism
● internal heat of Earth
● internal pressure of Earth
● fluid composition inside Earth

6. ● temperature increases with depth ● rate = 20º to 60ºC per km
1. Causes of Metamorphism
● temperature increases with depth
● rate = 20º to 60ºC per km
● at 15 km depth: 450ºC

7. 1. Causes of Metamorphism
● pressure and temperature increase with depth in all regions

9. 1. Causes of Metamorphism

10. ● the role of temperature ● geothermal gradient ● shallow (20ºC / km)
1. Causes of Metamorphism
● the role of temperature
● geothermal gradient
● shallow (20ºC / km)
● steep (50ºC / km)

11. ● the role of pressure (stress) ● confining pressure
1. Causes of Metamorphism
● the role of pressure (stress)
● confining pressure
● directed pressure

12. ● the role of pressure (stress) ● rate of increase =
1. Causes of Metamorphism
● the role of pressure (stress)
● rate of increase =
0.3 to 0.4 kbar / km
● minerals are geobarometers

13. 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?

14. ● accelerated chemical reactions
2. Types of Metamorphism
● the role of fluids
● metasomatism
● accelerated chemical
reactions

15. 2. Types of Metamorphism Depth, km Oceanic crust 35 Continental crust35
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere

16. Shock metamorphism Depth, km Oceanic crust 35 Continental crust35
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere

17. Shock metamorphism Regional metamorphism Depth, km Oceanic crust 3535
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere

18. Shock metamorphism Regional metamorphism High-pressure metamorphism
Depth, km
Oceanic
crust 35
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere

19. Shock metamorphism Regional metamorphism High-pressure metamorphism
Contact
metamorphism
Depth, km
Oceanic
crust 35
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere

20. Shock metamorphism Regional metamorphism High-pressure metamorphism
Contact
metamorphism
Depth, km
Oceanic
crust 35
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere
Burial
metamorphism

21. Shock metamorphism Regional metamorphism High-pressure metamorphism
Contact
metamorphism
Depth, km
Oceanic
crust 35
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere
Water
Seafloor
metamorphism
Burial
metamorphism

22. Thought questions for this chapter
Draw a sketch showing how seafloor metamorphism might take place.

23. 3. Metamorphic Textures

25. 3. Metamorphic Textures
Staurolite
crystal
Mica

26. Foliated rocks contain platy minerals that are aligned along
a preferred orientation.
Staurolite
crystal
Mica

27. 3. Metamorphic Textures
Feldspar
Quartz
Mica
Pyrite
Staurolite

28. Foliation is the result of compressive forces.
Feldspar
Quartz
Mica
Pyrite
Staurolite

29. Foliation is the result of compressive forces. Mineral crystals become
elongated perpendicular
to the compressive force.
Feldspar
Quartz
Mica
Pyrite
Staurolite

30. ● preferred orientation of crystals ● slaty cleavage ● foliation
3. Metamorphic Textures
● preferred orientation of crystals
● slaty cleavage
● foliation

31. 3. Metamorphic Textures
shale
sandstone
layers
5 cm

32. The original bedding can be seen in the thin sandy layers. shale
sandstone
layers
5 cm

33. The original bedding can be seen in the thin sandy layers. foliation
plane
shale
sandstone
layers
original
bedding
5 cm

34. Regional metamorphism causes cleavage planes to develop.
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

35. 3. Metamorphic Textures
Low grade
Intermediate grade

36. 3. Metamorphic Textures Diagenesis Low grade Intermediate grade
High grade
Slate
Phyllite
Schist
(abundant
micaceous
minerals)
Gneiss
(fewer
micaceous
minerals)
Migmatite
Slaty cleavage
Schistosity
Banding
Banding

37. Foliated rocks are classified by the degree
of cleavage, schistosity, and banding.
Diagenesis
Low grade
Intermediate
grade
High grade
Slate
Phyllite
Schist
(abundant
micaceous
minerals)
Gneiss
(fewer
micaceous
minerals)
Migmatite
Slaty cleavage
Schistosity
Banding
Banding

38. ● classification of foliated rocks ● metamorphic grade ● crystal size
3. Metamorphic Textures
● classification of foliated rocks
● metamorphic grade
● crystal size
● type of foliation
● banding

39. ● nomenclature of foliated rocks ● slate ● phyllite ● schist ● gneiss
3. Metamorphic Textures
● nomenclature of foliated rocks
● slate
● phyllite
● schist
● gneiss
● migmatite

40. Foliated
texture:
schist
with garnet
porphroblasts

41. ● nomenclature of granoblastic (non-foliated) metamorphic rocks
3. Metamorphic Textures
● nomenclature of granoblastic
(non-foliated) metamorphic rocks
● hornfels
● quartzites
● marbles
● greenstones
● amphibolites
● granulites (granofels)

42. Granoblastic Texture

44. 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?

45. 4. Regional Metamorphism
and Metamorphic Grade
● grades of metamorphism
● low
● intermediate
● high

46. 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

47. 4. Regional Metamorphism
Canada ME NY
Isograds VT NH MA
Key:
Not
metamorphosed CT
Chlorite zone
Low
grade RI
Biotite zone
Garnet zone
Medium
grade
Staurolite zone
High grade
Sillimanite zone

48. Index minerals define metamorphic zones. Canada ME NY Isograds VT NHMA
Key:
Not
metamorphosed CT
Chlorite zone
Low
grade RI
Biotite zone
Garnet zone
Medium
grade
Staurolite zone
High grade
Sillimanite zone

49. Index minerals define metamorphic zones. Isograds can be
used to plot the
level or degree of
metamorphism.
Canada ME NY
Isograds VT NH MA
Key:
Not
metamorphosed CT
Chlorite zone
Low
grade RI
Biotite zone
Garnet zone
Medium
grade
Staurolite zone
High grade
Sillimanite zone

50. Low
Grade
Increasing metamorphic grade
Slate
Intermediate
Grade
Phyllite
High
Grade
Pressure (kilobars)
Schist
Depth (km)
Gneiss
Blueschist
Migmatite
Temperature (°C)

54. Metamorphic facies correspond to particular combinations of
Hornfels
Contact metamorphism
Zeolite 5
Conditions beneath
mountain belts 5 10
Blueschist 15
Partial melting begins
Amphibolite
Granulite
Greenschist
Pressure (kilobars)
Depth (km) 10 20 25
13.5 15
Subduction zone 30
Eclogite 35 20 40
200
400
600
800
1000
Temperature (°C)
Metamorphic facies correspond
to particular combinations of
pressure and temperature...
… and can be used
to indicate specific
tectonic environments.

55. 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?

56. ● metamorphism occurs in or near ● plate interiors
5. Plate Tectonics and Metamorphism
● metamorphism occurs in or near
● plate interiors
● divergent plate margins
● convergent plate margins
● transform plate margins

57. Tectonic transport moves
rocks through different
pressure-temperature
zones, …
Low P,
Low T
High P,
High T

58. 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.

59. ● metamorphic pressure-temperature paths
5. Plate Tectonics and Metamorphism
● metamorphic pressure-temperature paths
● history of burial and exhumation
● prograde and retrograde paths

60. 5. Plate Tectonics and Metamorphism

61. The garnet crystal initially grows in a schist
but ends up growing in a gneiss.
Low
Grade
Slate
RETROGRADE PATH
Intermediate
Grade
Phyllite
PROGRADE PATH
Depth (km)
Pressure (kilobars)
Schist
Gneiss
High
Grade
Temperature (°C)

62. Pressure (kilobars) Depth (km) Temperature (°C) Low Grade
RETROGRADE PATH
RETROGRADE PATH
Pressure (kilobars)
Depth (km)
PROGRADE PATH
High
Grade
PROGRADE PATH
Peak metamorphism
Temperature (°C)
Low temperature–
high pressure metamorphism within a subduction zone
Deep-ocean
sediment
Trench
Continental
crust
Shelf
sediment
Oceanic
crust
Mélange
ophiolites
Continental
crust
Prograde
path
Peak
metamorphism
Retrograde
path

63. Pressure (kilobars) Depth (km) Temperature (°C) Low Grade
RETROGRADE PATH
RETROGRADE PATH
Pressure (kilobars)
Depth (km)
PROGRADE PATH
High
Grade
PROGRADE PATH
Peak metamorphism
Temperature (°C)
Low temperature–
high pressure metamorphism
within a subduction zone
High temperature–
high pressure metamorphism
within a mountain belt
Suture
Deep-ocean
sediment
Trench
Continental
crust
Deformed and
metamorphosed
shallow- and deep-
ocean sediments
Shelf
sediment
Continental
crust
Oceanic
crust
Multiple
thrusts
Mélange
ophiolites
Continental
crust
Continental
crust
Prograde
path
Prograde
path
Peak
metamorphism
Retrograde
path
Peak
metamorphism
Retrograde
path

64. ● rapid erosion (exhumation) rates of mountain ranges show a
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

65. Thought questions for this chapter
Draw a sketch showing how seafloor metamorphism might take place.
Subduction zones are generally characterized by high-pressure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.

66. Key terms and concepts Blueschist Burial metamorphism
Amphibolite
Blueschist
Burial metamorphism
Contact metamorphism
Eclogite
Exhumation
Foliated rock
Foliation
Gneiss
Granoblastic rock
Granulite
Greenschist
Greenstone
High-pressure metamorphism
Hornfels

67. Key terms and concepts Mélange Metamorphic facies Metasomatism
Marble
Mélange
Metamorphic facies
Metasomatism
Migmatite
Phyllite
Porphroblast
Quartzite
Regional metamorphism
Schist
Seafloor metamorphism
Shock metamorphism
Slate
Stress
Suture

68. Key terms and concepts
Ultra-high pressure metamorphism
Zeolite