1. Metamorphism and Metamorphic Rocks
French Alps, highly deformed and metamorphosed
GLY Summer Lecture 11

2. Metamorphism
The mineralogical, chemical, and structural adjustment of solid rocks to physical and chemical conditions
These conditions have generally been imposed at depth below the surface zones of weathering and cementation
They differ from the conditions under which the rocks in question originated

3. Metamorphic Rock
Any rock derived from pre-existing rocks by mineralogical, chemical, and/or structural changes, essentially in the solid state
Derivation is in response to marked changes in temperature, pressure, shearing stress, and chemical environment, generally at depth in the Earth's crust

4. Agents of Metamorphism
Temperature
Pressure
Chemically Active Fluids
Covered on following slides

5. Temperature Minerals in rocks which are heated may become unstable
Crystals may grow together to form larger crystals
New minerals may form
The usual temperature of metamorphism is 200 to 800C
Temperature increases with depth - geothermal gradient
Two sources of heat
Heat from the original accretion of the earth
Heat from radioactive decay of long-lived isotopes
235U (Uranium)
238U (Uranium)
232Th (Thorium)
40K (Potassium)

6. Pressure
Minerals in rocks which are under increased pressure may become unstable
Pressure may be directed or hydrostatic

7. Directed Pressure Pressure may be directed (along a single axis)
Causes shortening of the affected objects
Tectonic collisions and impacts produce directed pressure
Leads to foliation

8. Metamorphism from Compression

9. Effect of Compression
Compression has deformed the strata and caused metamorphism
This represents a continent-continent plate collision

10. Lithostatic Pressure Lithostatic pressure is equal in all directions
Load pressure is a type of hydrostatic pressure and increases with depth
Divers experience hydrostatic pressure
Rock or sediment are much heavier than water, the corresponding pressures under these substances are 2-3x larger than under an equal thickness of water
New, denser minerals may form
Example: Calcite  Aragonite

11. Confining Pressure
At depth, lithosphatic pressure is due to confinement

12. Chemically Active Fluids
Fluids from different sources
Fluids are capable of dissolving ions and ion-exchanging with existing minerals, to produce new minerals
Metamorphism releases fluids as temperature and pressure increase
Fluids may come from other sources, such as escaping from magma

13. Regional Metamorphism
Burial
Dynamothermal
Fault
Metasomatism
Each topic is discussed on further slides

14. Burial When sediments are deposited, they begin to change
Often associated with large sedimentary basins
First changes are to sedimentary rock
Increasing depth of burial raises the temperature and pressure, and begins to form new minerals

15. Dynamothermal Rocks caught between two colliding plates
Seen as the roots of ancient mountain regions, such as in parts of Minnesota, Wisconsin, and Michigan
Directed pressure may forces some rocks up, while others are thrust down into the earth
Rocks deep within the earth are subject to additional load pressure, and heat

16. Fault
Movement of rock along fault generates shearing stress in rock (increased pressure)
Stress can lead to smearing of rocks along the fault, and the creation of some new minerals

17. Metasomatism
The presence of interstitial, chemically active pore liquids or gases contained within the rock body or introduced from external sources are essential for the replacement process
Often, though not necessarily, this occurs at constant volume with little disturbance of textural or structural features

18. Contact Metamorphism aka Thermal
Proximity to a heat source may cause new minerals to form
Zone of contact metamorphism is known as contact aureole
Magma is usually the heat source
Grain size is usually small
Does not produce large volume of rock
Image source:

19. Contact Metamorphic Diagram
Development of contact metamorphic aureole

20. Foliation
A general term for a planar arrangement of textural or structural features in any type of rock
Involves alignment of platy or elongated minerals grains
Especially the planar structure that results from flattening of the constituent grains of a metamorphic rock

21. Foliation exposed in a mine
Björnevatn Mine, Kirkenes, Norway

22. Foliated Vs. Nonfoliated
Metamorphic rocks produced by directed pressure, such as tectonic or shearing forces, will be foliated
Rocks produced by load pressure, or not involving pressure, will not be foliated

23. Development of Foliation

24. Metamorphic Grade
Grade indicates the severity of conditions at the time of metamorphism
Grade is judged on the basis of:
Foliation
Grain size
Mineral assemblage - index minerals

25. Metamorphic Grade Indicators

26. Metamorphic Mapping
The first appearance of grade minerals can be used to produce maps of metamorphism

27. Foliated Rocks
Rocks shown following slides

28. Slate Red slate contains oxidized iron
A compact, fine-grained metamorphic rock that possesses slaty cleavage and hence can be split into slabs and thin plates
Most slate was formed from shale
Cinema Expeditions CD-ROM 5 Rocks
Upper Photo \metamorp\slate\ jpg

29. Slaty Cleavage
Pervasive, parallel foliation of fine-grained, platy minerals (mainly chlorite and sericite) in a direction perpendicular to the direction of maximum finite shortening
Developed in slate or other homogeneous sedimentary rock by deformation and low-grade metamorphism
Text photo 7-7
Gray slate showing foliation (slaty cleavage) in quarry near Alta, Norway

30. Uses of Slate Pool table at Campobello, F.D. Roosevelt's home
Slate is used as the bed of high quality pool tables
Slate may be used for roofing, and sometimes for walkways, although it is quite slippery when wet
Cinema Expeditions CD-ROM 5 Rocks
Left Photo \metamorp\slate\ jpg
Right photo:
Slate Roof

31. Phyllite
A metamorphosed rock, intermediate in grade between slate and mica schist
(Upper) Cleavage surfaces have a silky sheen
(Lower) Phyllites commonly exhibit corrugated cleavage surfaces
Cinema Expeditions CD-ROM 5 Rocks
Upper Photo \metamorp\phllite\ jpg
Lower Photo \metamorp\phllite\ jpg

32. Schist
A strongly foliated crystalline rock, formed by dynamic metamorphism
May be readily split into thin flakes or slabs due to the well developed parallelism of more than 50% of the minerals present (schistosity), particularly those of tabular or elongate prismatic habit
Garnet is usually a metamorphic mineral
Cinema Expeditions CD-ROM 5 Rocks
Upper Photo \metamorp\schist\ jpg
Garnet schist showing well-developed schistosity

33. Mica Schist (Above) Mica imparts a shiny luster to rock
(Left) Garnet mica schist
Mica is very commonly found in schist - the island of Manhatten is composed of mica schist
Cinema Expeditions CD-ROM 5 Rocks
Upper Photo \metamorp\schist\ jpg
Lower Photo: Text figure 7-9

34. Gneiss High grade regional metamorphic rock
A foliated rock formed by regional metamorphism, in which bands of granular minerals alternate with bands in which minerals having flaky or elongate prismatic habits predominate
Generally less than 50% of the minerals show preferred parallel orientation
Cinema Expeditions CD-ROM 5 Rocks
Photo \metamorp\gneiss\ jpg
High grade regional metamorphic rock

35. Gneiss Continued
Cinema Expeditions CD-ROM 5 Rocks
Upper Photo \metamorp\gneiss\ jpg
Gneissic foliation often involves felsic and mafic layers

36. Contortion in Gneiss
Gneiss rocks form under great pressure and at high temperatures
They may show contorted folding
Folding is a response to directed pressure - the rock has shortened along the horizontal direction
Cinema Expeditions CD-ROM 5 Rocks
Upper Photo \metamorp\gneiss\ jpg

37. Migmatite
Figure 7.24 in text

38. Increasing Metamorphic Grade
Figure 7-31 in text

39. Non-foliated Rocks
Each rock type is shown on following slides

40. Marble Metamorphic equivalent of limestone
Not foliated during metamorphism; may contain relict sedimentary bedding, as in the lower photo
Dark band is organic matter that was deposited with the limestone
Cinema Expeditions CD-ROM 5 Rocks
Upper Photo \metamorp\marble\ jpg
Lower Photo \metamorp\marble\ jpg

41. Quartzite A metamorphic rock consisting mainly of quartz
Formed by recrystallization of sandstone or chert by either regional or contact metamorphism
Cinema Expeditions CD-ROM 5 Rocks
Photo \metamorp\quartzite\ jpg
A metamorphic rock consisting mainly of quartz
Sioux Quartzite, South Dakota

42. Hornfels A fine-grained rock
Composed of a mosaic of equidimensional grains
No preferred orientation
Typically formed by contact metamorphism
Heat from the magma backs the rock it comes in contact with
Sometimes a piece of rock falls into the magma - if it doesn’t melt, it will be backed on all sides
Photo #d39.jpg

43. Table Mountain Capetown, South Africa
Table Mountain is a famous landmark in South Africa
The Malmesbury group is seen to the lower left of the photo, as shown in the diagram - It contains a hornfels
Upper photo #d45.jpg
Lower diagram tm1.gif

44. Malmesbury Hornfels
Close up, the Peninsula granite is a coarse-grained rock consisting of large (2-5cm) white or pink feldspar crystals, glassy brown quartz and flakes of black mica containing inclusions (xenoliths) of dark Malmesbury hornfels
The Malmesbury Group was made famous by Charles Darwin during his voyage of scientific discovery on H.M.S. Beagle in 1844

45. Anthracite Anthracite is metamorphosed coal It is almost pure carbon
Represents the highest grade of coal
May be recognized by its shiny appearance
Cinema Expeditions CD-ROM 5 Rocks
Photo \metamorp\anthrac\ jpg
Anthracite is metamorphosed coal

46. Fault Metamorphism Brittle fracture under low confining pressure
Ductile flow under high confining pressure
Figure 7-20 in text

47. Skarn
Lime-bearing silicates, of any geologic age, derived from nearly pure limestone and dolomite with the introduction of large amounts of silicon, aluminum, iron and magnesium
Skarns can form during regional or contact metamorphism
Form from a variety of metasomatic processes involving fluids of magmatic, marine and/or metamorphic origin
Mineralogy defines a rock as skarn

48. Skarns As Ores
Gold-bearing vesuvianite and grossular skarn, near Benambra, northeast Victoria, Australia
This deposit contains some exotic secondary phosphate and vanadate minerals
Skarn formation often produces metal-containing minerals enriched enough to be ores
Photo #vesu.jpg