Metamorphic Rocks

Metamorphic rocks form from other rocks that have been subjected to heat and pressure.

Agents of Metamorphism

  1. Heat
    Metamorphism can take place from heat causing the recrystallization of minerals. Metamorphic temperatures are greater than seen during lithification of sediments, but less than melting to form magma.
  2. Pressure
    In addition to the confining pressure of burial, there is often a directed pressure that causes deformation and reorientation of mineral crystals.
  3. Chemically-active fluids (hydro-thermal)
    Most pore spaces in rock are filled with water, this and the water found in some minerals allows much greater mobility of ions and increased ion exchange when the fluid is super-heated.

Metamorphic Changes

  1. Recrystallization of existing minerals, usually forming larger crystals.
  2. Changes in crystal orientation.
  3. Changes in mineral complement (see index minerals).

Metamorphic Textures

  1. Foliated
    These rocks have minerals aligned in parallel planes. Found in high grade metamorphism, interlocking crystals similar to phaneritic igneous rock can be seen. Four types of foliation are described based mostly on crystal size. Examples: slate, phyllite, schists, gneiss.
  2. Nonfoliated
    Typically seen in low grade metamorphic rock or when parent rock consists of one mineral. Examples: marble, quartzite.

Types of Metamorphism

  1. Regional
    Cover areas of 100's or 1000's km2. Produces the largest quantity of metamorphic rock. Occurs at convergent plate boundaries. Uplift of rock is also accompanied with downwards thrusting. Usually foliated and exhibits a gradation from low to high grade metamorphism.
  2. Contact
    Caused by heat from an intruding body of magma. Usually nonfoliated and often assisted by the action of chemically-active fluids. When metamorphic rocks are formed deep in the crust it is hard to distinguish between contact or regional metamorphism.
  3. Although not a major source of metamorphic rock, fault zones can produce small amounts of fault breccia.
 

Index minerals

Certain minerals only form under specific heat and pressure conditions. These index minerals allows geologists to determine different zones of metamorphism. As metamorphism proceeds a particular mineral will appear then be replaced by a subsequent mineral. For example the mineral chlorite found in low grade metamorphic rock is gradually replaced by micas. If metamorphism continues the micas disappear to be replaced by garnet. Finally in high-grade metamorphic rock the mineral sillimanite appears. The minerals mentioned are all index minerals.

Plate Tectonics

  1. High pressure / low temperature environments found where crust is subducted near ocean trenches at convergent boundaries.
  2. Low pressure / high temperature zones near igneous intrusions closer to the surface at convergent boundaries.
  3. High pressure / high temperature areas near deeper igneous intrusions at convergent boundaries.
  4. High pressure / high temperature areas of mountain building at continental-continental convergent boundaries.
  5. Low pressure / high temperature zones near divergent boundaries with hydro-thermal circulation.

  metamorphic rock conditions