Metamorphic Rocks

Defining Metamorphic Rocks

  • Metamorphic rocks are formed from the transformation of existing rock types through heat, pressure, or chemical reactions.
  • The original rock, or ‘parent rock’, can be of igneous, sedimentary, or even another metamorphic rock type.
  • This process of formation, ‘metamorphism’, involves solid-state transformation, meaning it occurs while the rock is solid, rather than through the melting and cooling process of igneous rocks.

Types of Metamorphism

  • Regional metamorphism occurs over a larger area and is often associated with mountain building where large pieces of the Earth’s crust are subjected to high pressures and temperatures.
  • Contact metamorphism takes place locally, adjacent to a heat source such as a magma body, where the surrounding rocks are ‘baked’ or thermally altered.

Classification of Metamorphic Rocks

  • Foliated metamorphic rocks such as gneiss, slate, and schist have aligned minerals, often in bands, due to differential stress.
  • Non-foliated metamorphic rocks like marble and quartzite have mineral grains that grow and arrange randomly or are equidistant but not aligned.

Key Metamorphic Processes

  • Recrystallization - old minerals adjust to the new equilibrium conditions and grow larger.
  • Phase change - a mineral changes to another of the same composition but with a different crystal structure.
  • Neoformation - new minerals form from the elements in the parent rock.
  • Pressure solution and plastic deformation - feature in the formation of metamorphic rocks, facilitated by the high temperatures and pressures involved.

Economic Importance of Metamorphic Rocks

  • Metamorphic rocks have a variety of uses due to their durability and mineral content.
  • Many precious stones, like ruby and sapphire, are metamorphic in origin.
  • Metamorphic rocks such as slate and marble are used in construction, and graphite (a metamorphosed form of coal) is used in pencils or as a lubricant.

Metamorphic Rocks and the Rock Cycle

  • As part of the larger rock cycle, the process of metamorphism can form rocks of increased hardness and density, reintroducing them into the cycle as potential ‘parent rocks’ for further transformation.
  • Understanding the transformation of metamorphic rocks offers insight into the thermal, tectonic, and fluid processes that shape the Earth’s crust.