1. Metamorphic Rocks
PART 1

2. OBJECTIVES Define Metamorphism Classifying Metamorphic Rocks
Metamorphic Process
Metamorphic Environments

3. DEFINING METAMORPHISM

4. Metamorphism
… is the transformation of rock by temperature and pressure
Metamorphic rocks are produced by transformation of:
Sedimentary and Igneous rocks, and by the further alteration of other metamorphic rocks
Protolith: The source rock.

6. Origin of pressure in metamorphism
Confining pressure aka “lithostatic”
(due to burial)
(Convergent Margin)

7. CLASSIFYING METAMORPHIC ROCKS

8. CLASSIFICATION Texture Foliated: Parallel arrangement of minerals.
Nonfoliated (Granoblastic): Nonparallel arrangement of minerals.

9. What are metamorphic textures?
Texture refers to the size, shape, and arrangement of mineral grains within a rock
Foliation – planar arrangement of mineral grains within a rock

10. Metamorphic textures Foliation can form in various ways:
Rotation of platy or elongated minerals
Recrystallization of minerals in a preferred orientation
Changing the shape of equidimensional grains into elongated and aligned shapes

11. Texture Foliated Classification
Size of Crystals
Nature of Foliation
Degree of segreation of light & dark bands.
Metamorphic Grade
Nonfoliated (Granoblastic) Classification
Mineral Compostion

12. Development of foliation due to directed pressure

13. Metamor-phism occurs between about 10 and 50 km of depth
Sedimentary
0 km
rock
Metamorphic
rock
Igneous
Sediment
rock
10 km
Metamor-phism occurs between about 10 and 50 km of depth
~200ºC
Sedimentary
rock
Metamorphism
Increasing depth
and temperature
The rocks don’t melt
50 km
Melting
~800ºC

14. Originally buried deep, metamorphic rocks are seen when erosion
Glaciers exposed the
Rocky
Mountains
Canadian
Shield
North
Cascades
Black
Hills
Appalachian
Mountains
Best US exposures
in New England
and the South
Grand
Canyon
Llano
Uplift
Originally buried deep,
metamorphic rocks are seen when erosion
removes covering rocks, and in the cores
of mountains

15. METAMORPHIC PROCESS

16. Metamorphism Metamorphism progresses from low to high grades
Rocks remain solid during metamorphism
Metamorphism occurs above 50km melting depth for felsic minerals

17. What causes metamorphism?
1. Heat
Most important agent
Heat drives recrystallization - creates new, stable minerals
Increasing Heat with Depth

18. Temperature Increase with Depth
“Geothermal Gradient” due to:
Radioactive Isotopes
Intruding Magma
Friction Between Moving Bodies of Rock

19. What causes metamorphism?
2. Pressure (stress)
Increases with depth
Pressure can be applied equally in all directions or differentially
All Directions = “Confining Pressure”
also called “lithostatic pressure”
Differential = “Directed Pressure”

20. Origin of pressure in metamorphism
Confining pressure aka “lithostatic”
(due to burial)
(Convergent Margin)

21. Confining Pressure

22. Directed Pressure

23. Source: Kenneth Murray/Photo Researchers Inc.
Directed Pressure causes rocks to become folded, and minerals to reorient perpendicular to the stress: “foliation”
Source: Kenneth Murray/Photo Researchers Inc.

24. Foliation
Minerals Recrystallize Perpendicular to the Directed Pressure
If the minerals are flat, such as sheet-like Micas, their parallel orientation gives a layered look; layering unrelated to the original bedding in the parent rock.

25. Role of Parent Rock
Metamorphic rocks typically have the same chemical composition as the rock they were formed from.
New minerals are formed in the rock due to the change in heat and pressure with the same composition.

26. METAMORPHIC SETTINGS

27. Metamorphic Settings Five types of metamorphic settings:
Contact metamorphism – due heat from adjacent rocks
Hydrothermal metamorphism – chemical alterations from hot, ion-rich water
Regional metamorphism -- Occurs in the cores of mountain belts and converging margins. Includes:
Burial Metamorphism – e.g. Burial of sediments deeper than 10 km – non-foliated
Dynamothermal Metamorphism – Directed pressure in Plate Tectonic Processes – foliated
Cataclasis metamorphism – Occurs at shearing Plate Boundaries.
Shock metamorphism – Meteor/asteroid collision.

28. 1. Contact Metamorphism

29. Contact Metamorphism Baking due to nearby Magma
Effect strongest in rocks in immediate contact

30. Contact Metamorphism
Metamorphic
Aureole

31. 2. Hydrothermal Metamorphism

32. Hydrothermal Metamorphism
Due to circulation of water near Magma
Important at mid-ocean ridge

33. Hydrothermal Metamorphism

34. 3. Regional Metamorphism

35. Regional Metamorphism – “Burial”
Variable T, mod-high differential P; characteristic of over-thickened plates (i.e., mountain belts) above convergent boundaries; affects large areas

36. Regional Metamorphism “Dynamothermal”
Most Dynamothermal metamorphism occurs along convergent plate boundaries

37. Dynamothermal Metamorphism,
Felsic continental materials and sediments are buoyant, they have low density
They float, cannot be subducted, so they get squashed.

38. 4. Cataclasis Metamorphism

39. Cataclasis Metamorphism
Variable T, very high directed P; typically localized
to narrow zones of intense mechanical deformation
(shear zones).

40. 5. Shock Metamorphism

41. Shock Metamorphism
Meteor/Asteroid Collision

42. THE END