1. Metamorphism and Metamorphic Rocks
Chapter 8
Metamorphism and Metamorphic Rocks

2. Introduction
Metamorphism - The transformation of rocks without melting, usually beneath Earth's surface, as the result of heat, pressure, and/or fluid activity, produces metamorphic rocks.
During metamorphism, rocks are subjected to sufficient heat, pressure and/or fluid activity to change their mineral composition or texture, or both.
All this occurs below the melting point in the solid state.

3. Introduction
Metamorphism is responsible for producing a number of economically valuable materials, like marble, a favorite of sculptors throughout history.
Metamorphism is an important process that is closely related to plate tectonics, the growth of continents, and even climate change.

4. Introduction Distribution of Metamorphic Rocks
1. Shields – oldest part of the continental crust
2. Cores of large mountain ranges

5. The Agents of Metamorphism
The three principal agents of metamorphism are heat, pressure and fluid activity.
Intrusive magmas or deep burial provide heat which causes metamorphism
Pressure is produced by overlying rocks (lithostatic) or differential pressure is produced by various stresses, especially from plate collisions.
Fluid activity increases the rate of metamorphism.

6. The Agents of Metamorphism
Heat
Heat is an important agent of metamorphism
Heat increases the rate of reactions
Sources of heat include:
Lava
Magma
Deep burial
Temperature increases with depth. The geothermal gradient averages about 25oC/km.

7. The Agents of Metamorphism
Pressure
What are lithostatic and differential pressures, and why are they important?
Lithostatic pressure is a uniform field of pressure experienced by most rocks beneath Earth’s surface. Like the hydrostatic pressure experienced by divers underwater, the pressure acting on a rock embedded in the crust “feels” the same from all directions.
Fig. 8.2, p. 184

8. The Agents of Metamorphism
Pressure
What are lithostatic and differential pressures, and why are they important?
Differential pressure is a nonuniform field of pressure; the pressure acting on a rock in some directions is stronger than it is in others. Many metamorphic rocks form under conditions of differential pressure, which influences the development of metamorphic structures and textures in significant ways.
Fig. 8.2, p. 184

9. The Agents of Metamorphism
Fluid Activity
Fluids within sedimentary rocks or issuing from magmas can accelerate chemical changes, which occur during metamorphism and can cause new minerals to form.
Common metamorphic fluids include water and carbon dioxide.

10. The Three Types of Metamorphism
Contact
Dynamic
Regional

11. The Three Types of Metamorphism
Contact Metamorphism
Contact metamorphic rocks form under conditions of high temperature and low pressure. Metamorphic "baked zones" or aureoles occur around magmas.
Fig. 8.5, p. 186
Fig. 8.6, p. 187

12. Fig. 8.5, p. 186

13. Fig. 8.6, p. 187

14. The Three Types of Metamorphism
Contact Metamorphism
The size of an aureole depends on:
Size of the intrusion
Temperature of the intrusion, mafic magmas tend to be hotter than felsic ones
Amount of fluids, water and other fluids generally increase the size of the metamorphic aureole

15. The Three Types of Metamorphism
Dynamic metamorphism
Dynamic metamorphism occurs when rocks are ground and crushed in faults, such as at convergent or transform plate boundaries. High pressure builds up, but the temperature is low.
Fig. 8.7, p. 188

16. The Three Types of Metamorphism
Dynamic metamorphism
Mylonites are fine-grained rocks in faults that result from grinding and crushing during fault movements (earthquakes).
Fig. 8.7, p. 188

17. The Three Types of Metamorphism
Regional metamorphism
The most common type of metamorphism
Fig. 8.4, p. 186

18. The Three Types of Metamorphism
Regional metamorphism
As the name implies, regional metamorphism has a broad range. Temperature and pressure both act as driving forces for metamorphic reactions in regional metamorphism.
Fig. 8.4, p. 186

19. The Three Types of Metamorphism
The type of metamorphism that results largely depends on which of the three agents was dominant.
Fig. 8.4, p. 186

20. Fig. 8.4, p. 186

21. Environments of metamorphism (T = temperature, P = pressure)
Deeply buried
sediments
(low T, moderate P)
Water-saturated
seafloor lava flows near
mid-ocean ridges
(moderate T, low P)
Meteorite impacts
(ultrahigh T and P)
Active accretionary
wedges (low T, high P)
Country rock
in active mountain
belts (moderate to
high T, moderate
to high P)
Country rock around active Plutons
(high T, low P)
Boundary
between upper
and lower mantle
high T, high P)
Subducting
oceanic lithosphere
(high T, high P)
Stepped Art
Fig. 8-4, p. 186

22. The Three Types of Metamorphism
Index Minerals and Metamorphic Grade
Metamorphic grade – the degree of metamorphic change a rock has undergone, usually listed as low, intermediate, or high
Fig. 8.8 p. 188

23. The Three Types of Metamorphism
Index Minerals and Metamorphic Grade
Index minerals – certain minerals are only known to form under specific ranges of metamorphic temperatures and pressures. The chemistries of the parent rock and metamorphic fluids also affect the formation of index minerals.
Fig. 8.8 p. 188

24. The Three Types of Metamorphism
Index Minerals and Metamorphic Grade
Quartz, feldspars, and some other minerals occur in a wide variety of metamorphic rocks at different grades. They are not index minerals.
Fig. 8.8 p. 188

25. Index and other Minerals during the
Metamorphism of an Aluminum-rich Shale
Fig. 8.8 p. 188

26. How are Metamorphic Rocks Classified?
Metamorphic rocks are classified principally according to texture.
Foliated textures are produced by the preferred orientation of platy minerals because of pressure.
Nonfoliated textures do not exhibit preferred orientation of minerals. The minerals are randomly oriented.
Fig. 8.9a, p. 189

27. How are Metamorphic Rocks Classified?
Classification of Common Metamorphic Rocks
Table 8.1, p. 190

28. Table 8.1, p. 190

29. How are Metamorphic Rocks Classified?
Foliated Metamorphic Rocks
Form a graded series of grain size and the development of foliation, from fine grained slate, to phyllite and coarser grained schist, to gneiss, with segregated bands of minerals
Fig. 8.9b, p. 189
Fig. 8.10, p. 190

30. How are Metamorphic Rocks Classified?
Foliated Metamorphic Rocks
Schist and Gneiss
Fig , p. 191

31. How are Metamorphic Rocks Classified?
Foliated Metamorphic Rocks
Migmatite: Actually a mixed igneous and metamorphic rock
Contains streaks of granite from partial melting
Fig. 8.14, p. 192

32. How are Metamorphic Rocks Classified?
Foliated Metamorphic Rocks
Amphibolite is another fairly common coarse grained foliated metamorphic rock.
Table 8.1, p. 190

33. How are Metamorphic Rocks Classified?
Nonfoliated Metamorphic Rocks
Nonfoliated textures do not exhibit preferred orientation of minerals.
Common nonfoliated metamorphic rocks are marble, quartzite, anthracite coal, greenstone, and hornfels.
Fig. 8.16, p. 193
Fig. 8.17, p. 193

34. How are Metamorphic Rocks Classified?
Nonfoliated Metamorphic Rocks
Marble - metamorphosed limestones and dolostones
Quartzite - metamorphosed quartz sandstones
Greenstones - low grade metamorphic basalts
Hornfels - fine-grained contact metamorphic rocks
Anthracite - metamorphic coal
Fig. 8.15, p. 192

35. Metamorphic Zones and Facies
Metamorphic zone – a belt of rocks showing roughly the same degree of metamorphism
Fig. 8.18, p. 194

36. Metamorphic Zones and Facies
Metamorphic facies – refers to a group of rocks containing a distinctive mineral assemblage formed under similar conditions of temperature and pressure
Fig. 8.19, p. 194

37. Fig. 8.19, p. 194

38. Metamorphic Zones and Facies
How do metamorphic zones and metamorphic facies differ?
Metamorphic zones show the gradational metamorphic change within a single rock composition.
Metamorphic facies are groups of many different rock compositions whose mineral contents all indicate common temperature and pressure conditions during metamorphism.

39. Plate Tectonics and Metamorphism
Metamorphism can occur along all types of plate boundaries, but is most common and extensive along convergent boundaries.
Fig. 8.20, p. 195

40. Plate Tectonics and Metamorphism
Subducting plates at oceanic-continental boundaries reach great depths (high pressures), but the plate may heat slowly (low temperatures), resulting in blueschist facies metamorphism.
Fig. 8.19, p. 194

41. Plate Tectonics and Metamorphism
Higher temperatures and pressures in subduction zones produce higher grades of metamorphism in a subducting oceanic plate.
Fig. 8.20, p. 195

42. Fig. 8.20, p. 195

43. Metamorphism and Natural Resources
Metamorphic rocks, including marble and slate, are resources.
Slate
Fig. 8.21, p. 195
Fig. 8.10, p. 190

44. Metamorphism and Natural Resources
Marble

45. Metamorphism and Natural Resources
Metamorphic mineral resources include graphite, talc, asbestos, and garnet.
Geo-Focus Fig. 1, p. 183
Fig a, p. 191

46. Metamorphism and Natural Resources
Metamorphic mineral resources
Graphite: Used in pencils and lubricants
Talc: Once used in talcum powder
Asbestos: Once used in insolation and fire-proofing Crocidolite variety caused lung cancer in miners
Garnets: Used in "sand" paper
Andulasite, sillimanite, kyanite: High-temperature porcelians
Metallic ores, such as: hematite, copper and zinc sulfides, tungsten ores in contact metamorphic zones

47. Metamorphism and Natural Resources
Metamorphic metallic mineral resources
Table 8.2, p. 195

48. End of Chapter 8