Metamorphic Environments

Metamorphism occurs in different settings. Different settings yield different effects via…

• Geothermal gradient.
• Differential stresses.
• Hydrothermal fluids.

These characteristics are governed by tectonics.

 

Thermal

Thermal metamorphism occurs when the hot molten magma comes in contact with the cold country rock. Ever burn your hand on the handle of a hot pot? Congratulations! You've experienced thermal metamorphism!

The metamorphosed area surrounding the magma chamber is called the metamorphic aureole or contact aureole. The aureole width is due to two factors: 1) The size of the plutonic intrusion and 2) The degree of metasomatism.

 

Burial

As sediments get burried by newly deposited layers of sediment, the pressure and temperature increases with depth. Pressure increases because of the weight of the overburden; Temperature increases because of the geothermal gradient.

 

Dynamic

Dreakage of rock by shearing at a fault zone causes dynamic metamorphism. In shallow crust in the upper 10-15 km. rocks behave in a brittle fashion. Mineral grains crush and pulverize forming fault breccia. Deeper parts of the crust (below 10-15 km.), rocks behave in a ductile manner. Minerals smear like taffy to form mylonite.

 

Regional

Orogenesis (the term for all mountain building events) generates a wide range of temperatures. This generates the greatest amount of metamorphism, and both contact and/or hydrothermal metamorphism can occur over wide areas.

Regional metamorphism originates from:

• Continental collisions
• Continental accretion Accreted crustal blocks = terranes

 

Hydrothermal

Alteration by hot, chemically aggressive water.

 

Subduction

Orogenesis refers to processes that collectively produce a mountain belt. The three mechanisms for mountain building include:

• Ocean-Ocean & Ocean-Continental Convergence (subduction) - A oceanic plate is subducted beneath either another Oceanic Pate or a Continental Plate. Metamorphism occurs due to 1) friction as the subducting plate is pushed through the mantle, 2) heat due to the geothermal gradient, 3) heat due to the magma generated from the melting of the surface of the subducting plate.
• Continental-Continental Convergence (Regional metamorphism) - Continental plates are too thick and light to be subducted. Therefore, when they converge, they collide together. This collision produces intense heat and pressure at the center of the collision, and less the further away from the center of impact.
• Continental Accretion (Regional metamorphism) - Sometimes there is a peice of continental crust or an island attached to a subducting oceanic plate. These features cannot be subducted. Instead, they are plastered onto the overriding plate via a process called accretion.

 

Shock

Rarely, Earth is struck by a comet or asteroid. These massive impacts generate a compressional shock wave in addition to extremely high pressures and temperatures. The heat that generated at the impact site vaporizes or melts large masses of rock. These conditions generate two high-pressure minerals: Coesite & Stishovite