1. Metamorphism and Metamorphic Rocks
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
Kyanite, Sillimanite, and Andalucite

2. 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
The rocks don’t melt
Metamorphism
Increasing depth
and temperature
50 km
Melting
~800ºC

3. 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
Usually buried deep,
metamorphic rocks are seen when erosion
removes covering rocks, and in the cores
of mountains

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

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

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

7. What causes metamorphism?
2. Pressure (stress) = Force/meters2
Increases with depth
Pressure can be applied equally in all directions or differentially
All Directions = “Confining Pressure”
also called “lithostatic pressure”
Differential = “Directed Pressure”

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

9. Confining Pressure

10. Directed Pressure

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

12. 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.

13. Main factors affecting metamorphism
3. Parent rock
Metamorphic rocks typically have the same chemical composition as the rock they were formed from.
Different minerals, but made of the same atoms.
Exception: when hot water is involved.

14. Metamorphic Settings Most is Dynamothermal
Three types of metamorphic settings:
1. Contact metamorphism – due heat from adjacent rocks
2. Hydrothermal metamorphism – chemical alterations from hot, ion-rich water
3. Regional metamorphism -- Occurs in the cores of mountain belts and subduction zones (Converging Margins) . Makes great volumes of metamorphic rock. Includes:
a. Burial Metamorphism – e.g. Burial of sediments deeper than 10 km – non-foliated
b. Dynamothermal Metamorphism – Directed pressure in Plate Tectonic Processes - foliated
Most is Dynamothermal

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

16. Contact metamorphism
Produced mostly by local heat source

17. Contact Metamorphism
Metamorphic
Aureole

18. 2. Hydrothermal Metamorphism
Due circulation of water near Magma
Important at mid-ocean ridge

19. Hydrothermal Metamorphism

20. 3. Regional Metamorhism
Most Dynamothermal metamorphism occurs along convergent plate boundaries
Example 1: Continent-Continent Collisions
Compressional stresses deforms plate edge
Continents Collide
Major Folded Mountain Belts: Alps, Himalayas, and Appalachian Mts.

21. Dynamothermal Metamorphism, Before collision
Sediments are “unconsolidated”. They will fold if pushed.

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

23. 2. Regional Metamorphism (continued)
Most Dynamothermal metamorphism occurs along convergent plate boundaries
Example 2: In Subduction Zones

24. Metamorphism in a Subduction Zone

25. Metamorphic Grade and Index Minerals
Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form

26. Index Minerals in metamorphic rocks
Note Temperature gradient
Index Minerals in metamorphic rocks
580oC
220oC
460oC
690oC
Note Quartz and Feldspar are not index minerals: Why?

27. Here is an internally heated pressure vessel at the AMNH
With these you can study, for example:
1. the temperature and pressure conditions at which polymorphs change from one form to another. (next slide)
2. the reactions of minerals with fluids (for example salty or alkaline water) at high temperatures and pressures.
3. the conditions necessary to change one assemblage of minerals to another

28. Thermometers and Pressure Gauges
Sillimanite
Kyanite
Polymorphs of Al2SiO5
Andalusite

29.           New England Dynamothermal Metamorphism
7_21
CANADA
New England
Dynamothermal
Metamorphism 
MAINE
CANADA
Augusta 
U.S.A.
Caused by C-C collision M-P
Most of Appalachians
Montpelier
NEW
HAMPSHIRE
VERMONT 
Concord
ATLANTIC
OCEAN 
Boston 
Albany
MASSACHUSETTS 
NEW YORK 
R.I. 
Hartford
Binghamton
Providence
CONNECTICUT
Unmetamorphosed y  e a l l
Low
grade
Chlorite/muscovite zone
PENNSYLVANIA  v
Biotite zone
Scranton
Long
Island
Medium
grade
Garnet zone t f i
Staurolite zone
NEW r
Newark
High grade
Sillimanite zone
JERSEY
Increasing pressure and temperature
DIAGENESIS
LOW GRADE
INTERMEDIATE GRADE
HIGH GRADE
MELTING
Chlorite and muscovite
Biotite
Garnet
Staurolite
Sillimanite

30. Metamorphic Environments
Metamorphic grade or Facies
A group of minerals that form in a particular P-T environment
Can be used to deduce T-P conditions of formation

31. Metamorphic Environments in Subduction Zones
We can look at minerals in Metamorphic Rocks and determine where they formed.
Water facilitates metamorphic reactions by allowing movement of atoms and ions

32. Greenschist Hand Sample
Greenschist Thin Section
Chl-Ep

33. Blueschist glaucophane
Amphibolite

34. Common metamorphic rocks
Nonfoliated rocks: the Field Geologist’s friend
Quartzite
Formed from a parent rock of quartz-rich sandstone
Quartz grains are fused together
Forms in intermediate T, P conditions

35. Sample of quartzite Thin section of quartzite
Field Geologists are grateful for quartzites. They don’t foliate, so you can see the folds. Mudrocks foliate; much harder to map.
Sample of
quartzite
Thin section
of quartzite

36. Flattening of quartz grains in quartzite

37. Sandstone: grains and cement
7_18
Fracture
Sandstone: grains and cement
Fracture
Quartzite: grains interlock

38. Common metamorphic rocks
Nonfoliated rocks (cont.)
Marble
Coarse, crystalline
Parent rock usually limestone
Composed of calcite crystals
Fabric can be random or oriented

39. Marble (nonfoliated)

40. Common metamorphic rocks
Foliated rocks
Type formed depends on metamorphic grade
Grade depends on depth

41. Change in metamorphic grade with depth
Mudstones are sediments, can be squashed by burial and/or in continent-continent collisions
Change in metamorphic grade with depth
Mudstone is most common sedimentary rock. When metamorphosed, rocks reveal grade:
Increasing Directed Pressure and increasing Temps =>

42. Common metamorphic rocks
Foliated rocks
Slate
Very fine-grained
Excellent rock cleavage, often perp. to original
Made by low-grade metamorphism of shale

43. Example of slate

44. Common metamorphic rocks
Foliated rocks
Phyllite
Grade of metamorphism between slate and schist
Made of small platy minerals
Glossy sheen with rock cleavage
Composed mainly of muscovite and/or chlorite

45. Phyllite (l) and Slate (r) lack visible mineral grains

46. Common metamorphic rocks
Foliated rocks
Schist
Medium- to coarse-grained
Comprised of platy minerals (micas)
The term schist describes the texture
To indicate composition, mineral names are used (such as mica schist)

47. A mica garnet schist

48. Common metamorphic rocks
Foliated rocks
Gneiss
Medium- to coarse-grained
Banded appearance
High-grade metamorphism
Composed of light-colored feldspar layers with bands of dark mafic minerals

49. Gneiss displays bands of light and dark minerals

50. 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, perpendicular to the directed pressure
Common near convergent margins

51. Outcrop of foliated gneiss

52. 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

53. Development of foliation due to directed pressure

54. Migmatites- When Partial Melting Starts
Heat the rock, when the minerals with the lowest melting points (Qtz, Feldspar) at that pressure melt then recrystallize, we get separate bands of Metamorphic and Igneous rock