.Metamorphic Rocks = Pre-existing rocks are transformed
    by high P and/or T
    - the result of internal processes
- Pressure and Heat
changes composition and/or texture of pre-existing rocks

=> metamorphism = a change occurring in the solid state
Agents of Metamorphism.....
==> a) heat, b) pressure, and c) chemically active fluids

 A.  Heat (T) = most important agent of metamorphism...

1.  Sources of heat......

 a) contact with a body of magma
 => cooler rocks adjacent to magma altered =>
 contact metamorphism=> contact metamorphic rocks

b) deep burial => geothermal gradient (30°C/km)
-Heat = change from equilibrium conditions (stability) = changes in mineral content and mineral growth
 

Mineral Stability
• Minerals most stable near the conditions at which they form
• Stability Fields

B.  Pressure (P)

 1.  Pressure increases with depth
 
 - increasing P = deformation=> textural changes
 
 - Tectonics/plate movement => deform rocks

C. Chemically Active Fluids
 
=> water containing ions in solution (dissolved minerals)

 1. enhances metamorphism => ion exchange
 => new minerals and mineral growth........
D.Types of Metamorphism and Common Metamorphic Rocks
 Two Types => 1) Regional and 2) Contact

         1. Regional Metamorphism
 -> takes place at great depth over an extensive area....
 =>> associated with mountain building..........
Regional Metamorphic Rx.

a) two groups of rocks => 1) foliated &  2)nonfoliated

1) Foliated Metamorphic Rocks

=> foliation = alignment of mineral from directed stress

Foliated - regional metamorphic rocks

- slate (protolith = shale)
 - foliation = slatey cleavage

- phyllite (protolith = slate)
 - sheen
- schist (protolith = phyllite or igneous rock)
 - foliation = schistose / schistosity

- gneiss (protolith) = schist or igneous rock
 - foliation = gneissic banding

2) Nonfoliated Metamorphic Rocks - protolith = 1 mineral
   => mineral growth

protolith = Fossiliferous Limestone    Nonfoliated Rock = marble

protolith = quartz sandstone        Nonfoliated Rock = quartzite

Clues about internal processes from metamorphic Rocks

=> Mineral assemblages can act as thermometers and barometers

• texture and mineralogy of met. rx. reflects degree of metamorphism

= metamorphic grade

Metamorphic Grade
• Metamorphic grade - a geologic tool
• Isograds