1. Alteration of Rocks by Temperature and Pressure
Lecture 7
Metamorphism:
Alteration of Rocks by Temperature and Pressure

2. 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.

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

4. ● 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

5. Temperature increases with depth =Geothermal gradient

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

8. ● 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)

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

10. ● 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

11. Pressure: Increases with depth
Different types of pressure result in different rearrangement of minerals (compaction or elongation)
Preferred mineral
orientation
Denser atomic structures

12. First type of metamorphism is called LOW GRADE metamorphism:
Takes place around 150 degree C
Which is typically at depths of 5km below the surface (and up to ~400C)
At this temperature and pressure new minerals are formed  change from one mineral to another that is stable within the new range of pressure-temperature

14. Second type of metamorphism is called HIGH GRADE metamorphism:
Takes place at temperature > 400C which is typically > 15 km below the surface
At this temperature melting of mineral occurs

15. 1. Causes of Metamorphism

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

18. Hot water dissolves mineral and deposits minerals that it carries (metals)
Metamorphism by hot fluids leads to concentration of precious metals (gold, silver, copper) ...

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

20. Shock
metamorphism
Depth, km
Oceanic
crust 35
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere
Heat and shock wave from impact transform rocks at impact site

21. Shock metamorphism Regional metamorphism
Depth, km
Oceanic
crust 35
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere
At convergent plate boundaries, occurs du to pressure and temperature increase

22. Regional Metamorphism:
The pre-existing rock is subject to intense stresses and strains (deformation) usually from forces of mountain building
Involves:
High heat/ high pressure
Wide spread geographically
Slow process

23. Shock metamorphism Regional metamorphism High-pressure metamorphism
Depth, km
Oceanic
crust 35
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere
At convergent plate boundaries, occurs due to very high pressure at depth

24. Shock metamorphism Regional metamorphism High-pressure metamorphism
Contact
metamorphism
Depth, km
Oceanic
crust 35
Continental crust
Oceanic
lithosphere 75
Continental mantle lithosphere
Asthenosphere
At any area in the vicinity of magma:
Affects a thin zone of country rock next to an intrusion due to temperature increase

25. Contact Metamorphism:
Magma “contacts” solid-rock
Heat is transferred to the overlying rocks through conduction, which chance the pre-existing rock mineral composition
Involves:
High heat/ low pressure
Localized geographically
Fast acting process

26. 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
Affect any sedimentary rocks at depth due to increase in pressure and temperature

27. Hydrothermal metamorphism
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
Seafloor
Metamorphism /
Hydrothermal metamorphism
At mid oceanic ridges due to circulation of seawater in the produced basalts (oceanic crust)

28. Hydrothermal Metamorphism:
Chemical alteration of pre-existing rocks from the action of hot water. Usually the hot water is due to the presence of magma and is iron rich
Involves:
Medium heat/ low pressure + Hot water
Localized geographically
Fast acting process

29. What do all metamorphic processes have in common?
Heat
Provides the energy to cause recrystallization of preexisting minerals to new minerals
Examples of heat sources: contact with hot magma (conduction); geothermal gradient (rocks are hotter when buried); contact with hot fluids

30. How do we know how much a rock has been metamorphosed?
The rock texture and composition reflects the degree of metamorphism
HP/HT – High grade (regional metamorphism): Rocks show a foliation (texture), a preferred mineral orientation, and signs of high pressure (deformation)
LP/HT – Low grade (contact metamorphism):
Rocks show a semi-parallel orientation of minerals and minimal signs of deformation

33. The metamorphic facies –mineral assemblages – reflects the temperature and pressure experienced by a rock

34. ● 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

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

36. 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.

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

38. 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)

39. 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

40. 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

41. ● 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

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