Metamorphism
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
Metamorphism
- Changes in Rock Composition or Texture
- Due to Heat, Pressure and Action of Fluids
Where Does the Heat Come from?
- Uranium and Thorium to Other Elements + Lead + Radiation
- Potassium-40 to Calcium-40 or Argon-40 + Radiation
Where Does the Pressure Come from?
- Air Pressure = 14 P.s.i. (1 Atmosphere or 1 Bar = 100,000
pascals)
- Pressure Beneath 10 Meters (33 Ft.) Of Water = 1 Atm. = 1
Bar.
- Same Pressure Beneath 3.5 M (10 Ft.) Of Rock
- Pressure in Deepest Part of Ocean = 1000 Bar.
- Pressure under One Mile of Rock = 500 Bar.
1000 Bars (2 Mi. or 3 km Of Rock) = 1 Kilobar (Kb.)
Metamorphic Rocks Form Many Km below the Surface
How Do They Get to the Surface?
- Tectonic Uplift & Erosion
- Isostasy & Erosion
Types of Metamorphism
Contact
- Around Intrusions
- Shallow: 0-6 Km
- Low Pressure
- Local
Regional
- Wide Areas
- 5-20 Km, Sometimes 30+
- High Pressure
- Usually Accompained by Deformation
What Happens During Metamorphism
Minerals React to Form New Minerals
- 2SiO2 + CaMg(CO3)2 == CaMgSi2O6 + 2CO2
- Quartz + Dolomite == Pyroxene
Minerals Change Form
- Al2SiO5 == Al2SiO5
- Andalusite == Kyanite
New Materials Are Added (Metasomatism)
- CaMg(SiO3)2 + 2CO2 == CaMg(CO3)2 + 2SiO2
- Pyroxene + CO2 == Dolomite + Quartz
- Minerals in Solution == Ore Bodies
Recrystallization
Why Don't Rocks "De-metamorphose"?
- Reactions Can't Reverse Because Ingredients Lost
- 2AlSi2O5(OH) == Al2SiO5 + 3SiO2 + H2O
- Clay Mineral == Andalusite + Quartz + Water (Lost)
- An example of carbonate metamorphism:
- CaMg(CO3)2 + 2SiO2 == CaMgSi2O6 +2CO2
- Dolomite + Quartz == Pyroxene + CO2 (Lost)
- Reactions "Freeze"
- Sometimes it Does Happen
Grade - Degree to Which the Rock Has Changed Composition
- Can Often See Original Bedding
- Can Sometimes Even See Deformed Fossils
- At High Grades, Rocks Can Often Lose All Trace of Their
Original Appearance
Major Metamorphic Rock Types
Temp C |
Temp F |
Coal |
Limestone |
Sandstone |
Basalt |
Shale |
Index Minerals |
|
|
Lignite
Bituminous |
|
|
|
|
|
|
500 |
Anthracite |
|
|
|
|
|
300 |
600 |
Graphite |
Marble |
|
|
Slate |
Chlorite |
|
700 |
|
|
Quartzite |
|
|
|
|
800 |
|
|
|
Greenstone |
Phyllite |
Biotite |
500 |
900 |
|
|
|
|
Schist |
Garnet |
|
1000 |
|
|
|
Amphibolite |
|
Staurolite |
600 |
1100 |
|
|
|
|
Gneiss |
Kyanite |
|
1200 |
|
|
|
|
|
Sillimanite |
700 |
|
|
|
|
|
|
Melting Begins |
polymorphism
- Al2SiO5
- Kyanite
- Sillimanite
- Andalusite
- Ice - 6 high pressure forms
- Diamond - Graphite
- Calcite - Aragonite
- Quartz -
- - Tridymite - Cristobalite (increasing
temperature)
- - Coesite - Stishovite (increasing pressure)
Metamorphic Facies
Not all changes in rocks are metamorphism:
- Surface temperature and pressure --- Weathering
- Sedimentary rocks, surface to 250 C --- Diagenesis
Depth\Temp |
300 C |
400 C |
500 C |
600 C |
700 C |
800 C |
5 km |
Zeolite |
Contact Metamorphism -
Andalusite forms |
10 km - 3 kb |
Greenschist
Chlorite, Biotite form
- Slate
- Greenstone
- Quartzite
- Marble
|
Amphibolite
Garnet, Staurolite, Kyanite form
- Schist
- Amphibolite
- Quartzite
- Marble
- Gneiss
|
Granulite
Sillimanite forms
Muscovite breaks down to K-feldspar
Partial Melting
|
15 km |
Blueschist |
20 km - 6 kb |
25 km |
30 km - 9 kb |
35 km |
40 km - 12 kb |
|
|
Eclogite (Mantle) |
Return to 296-202 Visuals
Index
Return to Professor Dutch's home page
Created March 5, 1997; Last Update November 10, 1998