1. METAMORPHIC ROCKS

2. METAMORPHIC ROCKS What is Metamorphism? What Drives Metamorphism?
Metamorphic Textures
Common Metamorphic Rocks
Metamorphic Environments

3. Metamorphic Rocks
What Is Metamorphism?

4. WHAT IS METAMORPHISM? “meta” = change; “morph” = shape
GEOL 131: Metamorphic Rocks
WHAT IS METAMORPHISM?
“meta” = change; “morph” = shape
Change of one rock type into another
Heat and (usually) pressure
Parent rock
Rock that gets metamorphosed into something else
ANY rock type can be a parent rock, including a metamorphic rock

5. Metamorphic Grade Intensity levels of metamorphic conditions
GEOL 131: Metamorphic Rocks: What Is Metamorphism?
Metamorphic Grade
Intensity levels of metamorphic conditions
Higher grade = hotter and (usually) greater pressure

6. GEOL 131: Metamorphic Rocks: What Is Metamorphism?
Metamorphic Settings
Geologic environments where metamorphism can occur
Enough heat to drive chemical changes, but not enough to cause melting

7. What Drives Metamorphism?
Metamorphic Rocks
What Drives Metamorphism?

8. Four Metamorphic Agents
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Four Metamorphic Agents
Heat: most critical
Confining Pressure
Differential Stress
Chemically active fluids

9. Heat A form of energy Vibration of atoms and molecules
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Heat
A form of energy
Vibration of atoms and molecules
Hotter = faster vibration

10. Role of Heat in Metamorphism
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Role of Heat in Metamorphism
Provides energy for chemical reactions
Weakens atomic bonds in mineral crystals

11. Role of Heat in Metamorphism
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Role of Heat in Metamorphism
Compositional changes
Minerals become unstable at higher temps
Change into new, stable minerals
From: facweb.bhc.edu
Clay minerals in shale (above) change into micas when shale metamorphoses into schist (right).
From: itc.gsw.edu

12. Role of Heat in Metamorphism
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Role of Heat in Metamorphism
Atomic structure changes
Weakened atomic bonds allow atoms to be rearranged
From: pengellytrust.org
Calcite and aragonite have the same composition (calcium carbonate) but different atomic structures.

13. Sources of Heat Geothermal Magmas Remnant heat of formation
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Sources of Heat
Geothermal
Remnant heat of formation
Radioactive decay
Magmas

14. Confining Pressure Equal in all directions Deep burial
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Confining Pressure
Equal in all directions
Deep burial

15. Differential Stress Higher pressure from sides
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Differential Stress
Higher pressure from sides
Converging tectonic plates
Zone of intense differential stress

16. Chemically Active Fluids
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Chemically Active Fluids
Some minerals are hydrated
E.g. gypsum CaSO4 + 2H20
Water driven out by heat
Circulating water promotes transport of atoms

17. Metamorphic Rocks
Metamorphic Textures

18. Foliation Created by differential stress
GEOL 131: Metamorphic Rocks: Textures
Foliation
Created by differential stress
Preferred orientation of elongated minerals

19. Foliated Textures Slaty cleavage (low-grade) Gneissic texture
GEOL 131: Metamorphic Rocks: Textures
Foliated Textures
Slaty cleavage
(low-grade)
Gneissic texture
(high-grade)
Schistosity
(intermediate-grade)

20. How Foliation Develops: Three Ways
GEOL 131: Metamorphic Rocks: Textures
How Foliation Develops: Three Ways
Rotation
Recrystallization
Flattening

21. How Foliation Develops: Rotation
GEOL 131: Metamorphic Rocks: Textures
How Foliation Develops: Rotation
Elongated crystals rotate into alignment

22. How Foliation Develops: Recrystallization
GEOL 131: Metamorphic Rocks: Textures
How Foliation Develops: Recrystallization
Original grains replaced by more stable ones
New grains are elongated and aligned

23. How Foliation Develops: Elongation
GEOL 131: Metamorphic Rocks: Textures
How Foliation Develops: Elongation
Original equant grains are “squashed” into elongated ones

24. Nonfoliated Textures Confining pressure Absence of pressure
GEOL 131: Metamorphic Rocks: Textures
Nonfoliated Textures
Confining pressure
Absence of pressure
Equant grains, usually interlocking

25. Nonfoliated Textures GEOL 131: Metamorphic Rocks: Textures
Limestone metamorphoses into the nonfoliated rock marble.
Marble, made of interlocking equant calcite grains.

26. Nonfoliated Textures Porphyroblastic texture
GEOL 131: Metamorphic Rocks: Textures
Nonfoliated Textures
Porphyroblastic texture
Unusually large crystals (porphyroblasts)
Schist with garnet porphyroblasts

27. Common Metamorphic Rocks

28. Common Foliated Rocks Slate Phyllite Schist Gneiss
GEOL 131: Metamorphic Rocks: Common Types
Common Foliated Rocks
Slate
Phyllite
Schist
Gneiss
From: radford.edu

29. Common Nonfoliated Rocks
GEOL 131: Metamorphic Rocks: Common Types
Common Nonfoliated Rocks
Quartzite
Parent: sandstone
Marble
Parent: limestone
Anthracite
Parent: coal

30. Metamorphic Environments
Metamorphic Rocks
Metamorphic Environments

31. What is a Metamorphic Environment?
GEOL 131: Metamorphic Rocks: Environments
What is a Metamorphic Environment?
Defined by a given temperature and pressure
“P/T” conditions

32. Contact Low pressure, high temperature Contact with magma
GEOL 131: Metamorphic Rocks: Environments
Contact
Low pressure, high temperature
Contact with magma
Nonfoliated rocks

33. Hydrothermal Low pressure, low temperature Contact with hot water
GEOL 131: Metamorphic Rocks: Environments
Hydrothermal
Low pressure, low temperature
Contact with hot water
Common at undersea volcanoes
Nonfoliated rocks

34. Burial & Subduction Zone
GEOL 131: Metamorphic Rocks: Environments
Burial & Subduction Zone
Burial
Under sediment or volcanic deposits
High confining pressure, low temp
100s to 1000s of feet deep

35. Burial & Subduction Zone
GEOL 131: Metamorphic Rocks: Environments
Burial & Subduction Zone
Subduction zone
High differential stress, high temp
10s of miles deep

36. Regional Continental collision zones Very large-scale
GEOL 131: Metamorphic Rocks: Environments
Regional
Continental collision zones
Very large-scale
High confining pressure and differential stress
Low to high temp

37. Regional GEOL 131: Metamorphic Rocks: Environments
Collision of India with Asia created a large zone of regional metamorphism.

38. End of Metamorphic Rocks Chapter