1. Regional metamorphism The Rock Cycle
Temperature, fluids, pressure, deviatoric stress
Metamorphic grade
Metamorphic facies
Contact metamorphism
Regional metamorphism
The Rock Cycle

2. Rocks are classified by their mode of formation
Rocks are classified by their mode of formation. There are three major rock forming processes on Earth, producing three kinds of rocks.
Igneous Rocks
Formed when magma (molten rocks) solidifies
Environment: Hot enough to melt rock, pressure varies
Sedimentary Rocks
All non-igneous rocks formed by processes acting on the surface of the Earth
Environment: normal for Earth’s surface
Metamorphic Rocks
Formed by chemically and physically altering rocks under heat and pressure deep within the Earth’s crust.
Environment: High pressure, not hot enough to melt rock

3. Metamorphic Rocks
Metamorphic rocks form deep within Earth’s crust under high pressure and temperature. The buried rock (protolith) undergoes solid state transformation, altering the mineral composition and texture of the rock. The rock can not melt, or it would become an igneous rock after cooling.
Metamorphism that occurs around igneous intrusions is called contact metmorphism.
Metamorphism occurring due to stresses caused by mountain building is called regional metamorphism. These complex metamorphic environments may include zones of contact metamorphism if igneous intrusions occur.
The type of metamorphic rock produced is controlled by the composition of the protolith and the temperature, pressure and duration of the metamorphism. Metamorphic rocks come in two flavors – foliated and non-foliated.

4. Factors in Metamorphism
Metamorphism is effected by four main factors:
Fluids
Temperature
Pressure
Deviatoric stress

5. Factors in Metamorphism
Temperature
Controlled by:
Geothermal gradient (increasing temperature with greater depth in Earth)
Tectonic setting (higher in areas of igneous activity)
Most rocks melt at less than 2000oC, the upper limit of metamorphism (rocks formed from melt are igneous!)

6. Factors in Metamorphism
Fluids
Moving fluids within the rock body can effect:
Pressure (increases with temperature)
Heat (moves it around)
Dissolved ions and gases (moves them around)

7. Factors in Metamorphism
Pressure
A uniform compressive stress created by
Load (weight of overlying rocks)
Fluid pressure
Increases greatly deeper in the crust.
With temperature, controls what minerals are stable.

8. Factors in Metamorphism
Deviatoric
Stress
Stress imbalance causes compression in some directions and expansion in others. Controls:
Foliation (orientation of minerals into layers)
Lineation (linear orientation of minerals)

9. Hydrostatic Pressure vs Deviatoric Stress

10. Foliated Metamorphic Rocks
Metamorphic rock with aligned flat minerals
Foliation occurs when a protolith with flat minerals like mica is subjected to deviatoric stress. The flat minerals align perpendicular to the direction of maximum stress.
Increasing metamorphism causes more severe reordering and alteration of the rock’s metamorphic texture.

11. Marble Quartzite Anthracite Amphibolite Marble Quartzite Anthracite
Metamorphic rock without minerals that can be foliated
Non- Foliated Metamorphic Rocks
Marble
Quartzite
Anthracite
Amphibolite
quartz
sandstone
Protolith
limestone
coal
mafic rocks
Mineralogy
carbonates
quartz
none
amphiboles
Marble
Quartzite
Anthracite
Amphibolite

12. Deviatoric Stress
Maximum crystal growth occurs perpendicular to direction of maximum stress

13. Deviatoric Stress Foliation - alignment of flat minerals
Deviatoric stress is the primary control on foliation and lineation
Foliation - alignment of flat minerals
Slaty Cleavage – Alignment of small mica flakes
Schistosity – Alignment of large mica flakes
Gneissic Banding – segregation of different minerals into light and dark bands
Lineation - alignment of pointy minerals

14. Foliation and lineation
Deviatoric Stress
Foliation and lineation

15. Foliation

16. Foliation - Slaty Cleavage
Orientation of existing minerals

17. Foliation - Slaty Cleavage
Slate
slaty cleavage.

18. Foliation - Slaty Cleavage
Slate
slaty cleavage.
Protolith– fine grained rock like shale, mudstone, or siltstone

19. Foliation - Schistosity
Orientation of existing minerals and crystallization of new minerals

20. Foliation - Schistosity
exhibits schistosity
Protolith – fine grained rock like shale, mudstone, or siltstone

21. Foliation - Schistosity
New minerals crystallize - in this case garnets

22. Foliation - Gneissic Banding
Orientation and crystallization continue, minerals become segregated into bands

23. Foliation - Gneissic Banding
shows gneissic banding
Arizona State University

24. Low Grade High Grade Foliated Metamorphic Rocks
Metamorphic rock with aligned flat minerals
Slate
Schist
Gneiss
Low Grade
High Grade
Foliation perpendicular to direction of maximum differential stress
Foliation and crystallization perpendicular to direction of maximum differential stress
Foliation, crystallization and differentiation perpendicular to direction of maximum differential stress

25. Metamorphic Grade
Metamorphic grade increases with both increasing temperature and pressure, both of which increase with depth in the Earth

26. Metamorphic Grade
Materials with the same chemical formula can have different crystalline structures, and thus different physical properties, depending on the conditions of formation
Graphite
Temperature (K)
1000
3000
Pressure
Diamond
Rocks containing diamonds formed under different P/T conditions than rocks containing graphite

27. Metamorphic Grade
Polymorphs - minerals with the same chemical formula but different crystallize structures (=different chemical and physical properties)
Finding one polymorph in a rock narrows down the P/T conditions at metamorphism

28. Metamorphic Grade kyanite sillimanite andalusite

29. Metamorphic Grade

30. Metamorphic Grade
Index Mineral – a mineral in a metamorphic rock which indicates a certain metamorphic grade
Isograd – line marking the first appearance of an index mineral on a map or cross-section

31. Contact Metamorphism

32. Metamorphism adjacent to an igneous intrusion
Contact Metamorphism
Metamorphism adjacent to an igneous intrusion
Shale Rock
Igneous Intrusion

33. Contact Metamorphism Map
Finds first specimens of “green” metamorphic index mineral
Finds first specimens of “yellow” metamorphic index mineral
Finds igneous intrusion
Map
igneous
low grade
high grade
high grade
low grade

34. Metamorphic Grade Map Contact Metamorphism
Index Mineral – a mineral in a metamorphic terrain which indicates a certain metamorphic grade
Isograd – line marking the first appearance of an index mineral on a map. A B C A B C
Map
Contact Metamorphism

35. Metamorphic Grade
igneous
high grade
unmetamorphosed
low grade

36. Metamorphic Grade 300oC 700oC

37. Isograds of index minerals
Contact Metamorphism
Isograds of index minerals

38. Regional Metamorphism
Large-scale metamorphism is usually related to tectonic events (rifting, mountain building, etc.)
A single regional event can produce many different kinds of metamorphism, including smaller zones of contact metamorphism.

39. Metamorphic Facies
Metamorphic Facies – Rocks that have been exposed to the same degree of metamorphism (P/T regime), and thus are the same metamorphic grade

40. Metamorphic Facies
All rocks that have experienced this much metamorphism belong to Greenschist facies.

41. Minerals in Metamorphic Rock
Metamorphic Facies
Protoliths
Mudstone
Limestone
Granite
Gabbro
Quartz
K-feldspar
Plagioclase
Muscovite
Biotite
Chlorite
Epidote
Calcite
Magnetite
Actinolite
Minerals in Metamorphic Rock
Schist
Marble
Schist
Schist
Metamorphic Rocks
However, all the rocks have undergone the same amount of metamorphism, and all belong to the Greenschist facies.

42. Carolina Slate Belt Greenschist facies
Metamorphic Facies
Carolina Slate Belt Greenschist facies

43. The Rock Cycle

44. The Rock Cycle
Sedimentary Rocks
Igneous Rocks
Metamorphic Rocks

45. Igneous Process 1. Melt (form magma) 2. Cool (crystallize)
making igneous rocks
1. Melt (form magma)
2. Cool (crystallize)
geothermal gradient, Bowen’s Reaction Series, felsic, intermediate, mafic, magma, lava, texture, basalt, gabbro, andesite, diorite, rhyolite, granite

46. Sedimentary Processes
making sedimentary rocks
1. Chemical and physical weathering (breaks down rock)
(air/water)
(ice)
2. Transport clasts and/or ions

47. Sedimentary Processes
making sedimentary rocks
3. Deposition/ precipitation in basin
4. Lithification
weathering, transportation, deposition, lithification, clasts, clastic rocks, chemical rocks, depositional environment

48. Metamorphic Processes
making metamorphic rocks
Add heat and pressure to pre-existing rock (protolith)
metamorphism, temperature, fluids, pressure, deviatoric stress, metamorphic grade, metamorphic facies, index minerals, isograds, contact metamorphism, regional metamorphism

49. igneous rock metamorphic rock sedimentary rock Igneous Process
1. Heat (melt)
2. Cool (crystallize)
original rock mass
Metamorphic Process
Add heat and pressure
metamorphic rock
Sedimentary Process
The Rock Cycle
1. Breakdown (weather)
2. Transport
3. Deposit/precipitate
4. Lithify
sedimentary rock