Metamorphism
Metamorphism - the altering of existing rocks to form new rocks with new compositional and textural characteristics
- Occurs when a rock is subjected to high pressure, high temperature, or both.
Level of Metamorphism Depends on Three factors:
1) The nature of the original material
- some minerals change at lower P-T than others
- some minerals are physically stronger than others, resist metamorphism
2)The type of modifying process
- Pressure alone, temperature alone, or both high P and T
3) The Intensity with which the process occurs
- the greater the intensity of the P-T, the more metamorphism
Three Types of Changes May Occur during Metamorphic Events:
1) New minerals may be formed
clay minerals in shale are altered to muscovite and biotite with high P-T.
2) Rock textures change
- coarser textures may develop as metamorphism progresses, and crystal sizes tend to increase at higher levels of metamorphism.
3) New structures within the rock form
- Foliation, or planar arrangement of minerals within the rock
Two Primary Settings for Metamorphism
1) Contact Metamorphism - occurs over limited area (10’s of km’s)
- typically involves very high temperatures. Pressures may vary.
- heat and fluids alter the rock in the zone directly around the hot magma body
- The area of rock around a magma body affected by contact metamorphism is called the contact AUREOLE.
- Recrystallization is the dominant process. If a magma is “wet” and contains fluids, recrystallization takes place more intensely than if a magma is “dry”.
- usually associated with igneous activity
SHALE --> HORNFELS (not in lab)
LIMESTONE--> MARBLE
SANDSTONE--> QUARTZITE
2) Regional Metamorphism - occurs over very large areas (100’s or 1000’s of Km)
- invloves both very high Temperature and very high Pressure
- often occurs in long, linear belts associated with modern or ancient mountain ranges
- Directed pressure invloved in folding of mountain belts often forms FOLIATION.
FOLIATION is the planar arrangement of minerals within a rock perpendicular to the direction of pressure or stress on the rock.
SHALE--> SLATE --> PHYLLITE --> SCHIST --> GARNET MICA SHIST --> GNEISS
GRANITE ---> GNEISS
OLIVINE--> SERPENTINE
METAMORPHIC TEXTURES
1) FOLIATE
- contains planar alignment of minerals. Typical of REGIONAL metemorphism.
2)NONFOLIATE
- does not contain planar alignment of minerals. Typical of CONTACT metamorphism.
The two exceptions are quartzite and marble, which can form in either setting.