1. Metamorphism and Metamorphic Rocks

2. What are metamorphic rocks?
Increases in heat and pressure and changes in chemical environment can alter the mineral composition and crystalline textures of sedimentary and igneous rocks
Metamorphism is the solid-state (without melting) transformation of pre-existing rock into texturally or mineralogically distinct new rock as the result of high temperature, high pressure, or both.
Limestone Marble
Sandstone Quartzite (meta-quartzite)
Shale Slate
Shale/sandstone Schist
Shale Hornfels
Shale/sandstone Gneiss

3. Metamorphic Rock Cycle Sedimentary Igneous Erosion + Deposition
Melting
Erosion + Deposition
Burial + Heating
Melting
Burial + Heating
Metamorphic

4. What are metamorphic rocks?
Causes of metamorphism
Heat & Pressure

5. Sedimentary Surface 15o C 5 km 200o C Metamorphic 30 km Igneous 600o C
Occurs at depths greater than that of lithification
(Diagenesis -> Sedimentary Process)
Sedimentary
Surface
15o C
5 km
200o C
Metamorphic
30 km
Igneous
600o C

6. The 3 “R”s of Metamorphism
Regrowth - crystals grow larger and more uniform in shape.
Recrystallization - new minerals form from existing minerals.
Reorientation - platy or elongated mineral crystals become layered and aligned.

7. Factors Controlling Metamorphism
Metamorphic changes bring a pre-existing rock into equilibrium with new surroundings
Temperature
Re-crystalisation – new mineral assemblages
Segregation of new minerals – banding
Pressure
Regrowth - Confining pressure – acts in all directions – alters mineralogy by squeezing atoms into a denser state
Reorientation - Directed pressure – acting in a particular direction – convergent plate boundaries – causes preferred orientation - foliation

8. Kinds of Metamorphism Regional Regional Contact Contact Burial
Cataclastic
Hydrothermal

9. Kinds of Metamorphism Regional Metamorphism
Occurs where both high temperature and high pressure are imposed over large belts of the crust
Destroys all original sedimentary or igneous textures through growth of new minerals
Occurs in deeper levels of the crust along convergent plate boundaries particularly continental-continental boundaries where active mountain building is taking place.

10. Regional Metamorphism
Most metamorphic rocks
Occurs over a large area as a result of intense heat and pressure
Most obvious at convergent plate boundaries where rocks are intensely deformed and compacted during subduction and compression
Metamorphic grades seen through index minerals

11. Taconic Mountain Building Event
(450 million years ago)

12. Taconic Mountain Building Event
(450 million years ago)

13. Taconic Mountain Building Event
Regional Metamorphism
Taconic Mountain Building Event
(450 million years ago)

14. Regional Metamorphism

15. Metamorphic Grade

16. Progression of metamorphism
Start with a shale and then hit
it with pressure and heat.
Slate
Phyllite
Schist

17. You end up with something that is really Gneiss!

18. Sedimentary Rock
200° C
3000 ATM
Low Grade Metamorphic Rock
High Grade Metamorphic Rock
600° C
Migmatite
Magma

19. Migmatite
gneiss that begins to partially melt into granite.

20. Kinds of Metamorphism Contact Metamorphism
Heat and pressure generated by igneous intrusions
Affects only a thin region of intruded rock.
At shallow depths the mineral transformations generally associated with heat – pressure becomes important for intrusion at great depth
A zone of alteration called an aureole forms in the rock surrounding the magma

21. Contact Metamorphism: The main metamorphic agent is heat.
Types of Metamorphism
Contact Metamorphism:
The main metamorphic agent is heat.

22. Contact Metamorphism

23. Contact metamorphism

24. Kinds of Metamorphism Dynamic / Cataclastic Metamorphism
Found along faults
Fault movement causes rocks on either side to fragment
The product is a rock with a broken and pulverised texture
Generally found in strongly deformed mountain belts – often found with regionally metamorphosed rocks

25. Dynamic Metamorphism Associated with Fault Zones
Low temp/high pressure
Mylonites – hard, dense, fine-grained rocks, with thin laminations (less than 1 cm thick)
San Andreas Fault, CA and Northern Scotland

26. Granite away from active fault

27. Granite next to active fault

28. Kinds of Metamorphism Hydrothermal Metamorphism
Often associated with mid-ocean ridges
Seawater heated by upwelling magma promotes chemical reactions with basalt causing alteration
Alteration is also common in other igneous rocks such as granite.
Alteration can result in the rock becoming weaker, more compressible and more permeable – the end product is very similar to that produced by chemical weathering

29. Kinds of Metamorphism Burial Metamorphism (precursor to Regional)
Diagenisis grades into burial metamorphism
Low grade metamorphisms resulting from heat and pressure exerted by overlying sediments and sedimentary rocks
Bedding and other sedimentary structures are preserved
Burial metamorphism can grade into regional metamorphism

30. Burial Metamorphism

31. Classification of Metamorphic Rocks
Foliated Rocks
A set of flat or wavy parallel planes produced by the preferred orientation of minerals, particularly platy minerals like mica

32. Foliated Rocks This slide is indicative of a schist.
A foliation is any planar fabric in a
metamorphic rock. In this case, the
foliation is defined by aligned sheets
of muscovite sandwiched between
quartz grains.
This slide is indicative
of a phyllite.
The foliation in this rock is a
crenulation cleavage, and is
developed after the horizontal
foliation.
This slide is indicative
of a schist.

33. Classification of Metamorphic Rocks
Criteria
Nature of foliation
Grain size
Degree of banding
Metamorphic grade
Foliated Rocks

34. Common metamorphic rocks
Foliated rocks
Slate
Very fine-grained
Excellent rock cleavage
Most often generated from low-grade metamorphism of shale, mudstone, or siltstone

35. Low Grade - Slate

38. Burial
Mountain Building

39. Common metamorphic rocks
Foliated rocks
Phyllite
Gradation in the degree of metamorphism between slate and schist
Platy minerals not large enough to be identified with the unaided eye
Glossy sheen and wavy surfaces
Exhibits rock cleavage
Composed mainly of fine crystals of muscovite and/or chlorite

40. Phyllite (left) and Slate (right) lack visible mineral grains

42. Intermediate Grade – Phyllite

43. Common metamorphic rocks
Foliated rocks
Schist
Medium- to coarse-grained
Platy minerals predominate
Commonly include the micas
The term schist describes the texture
To indicate composition, mineral names are used (such as mica schist)

44. A mica garnet schist

45. High Grade - Schist

47. Common metamorphic rocks
Foliated rocks
Gneiss
Medium- to coarse-grained
Banded appearance
High-grade metamorphism
Often composed of white or light-colored feldspar-rich layers with bands of dark ferromagnesian minerals

48. Gneiss typically displays a banded appearance

50. High Grade - Gneiss

51. Classification of Metamorphic Rocks
Foliated Rocks
Gneiss
Schist
Slate

52. Regional Revisited
Slate
Phyllite
Schist,
Gneiss

53. Metamorphic Grade

54. Non-foliated Metamorphic Rocks
granular texture
interlocking grains
composed primarily of one mineral
uniform grain size

56. Classification of Metamorphic Rocks
Non-foliated Rocks
Hornfels
Product of contact metamorphism
Uniform grain size- little or no deformation – plate/elongate crystals are randomly orientated
Often harder and more brittle than parent rock
Marble
Product of heat & pressure acting on limestone or dolomite
Often show irregular banding or mottling due to impurities
Granular texture
Coarse, crystalline
Parent rock was limestone or dolostone
Composed essentially of calcite or dolomite crystals
Used as a decorative and monument stone
Exhibits a variety of colors
Quartzite
Derived from quartz-rich sandstones (contact or regional metamorphism) often very hard and extremely strong

57. Marble – a nonfoliated metamorphic rock

59. Nonfoliated - Marble metamorphosed limestone (CaCO3)
bedding and fossils obliterated

60. Quartzite

61. Nonfoliated - Quartzite
metamorphosed quartz-rich sandstone
Pore spaces filled with SiO2

62. Metamorphic
Rocks

63. Metamorphic environments associated with plate tectonics

64. Now go to Metamorphic Revision PPT