Metamorphic rocks and processes

Metamorphic Rocks

Metamorphic rocks are formed from the transformation of existing rock types under the influence of heat, pressure, and chemically active fluids.
These rocks may initially be sedimentary, igneous, or older metamorphic rocks.
Metamorphic changes occur primarily within the Earth’s crust and below the surface.
Common metamorphic rocks include slate, schist, gneiss, marble and quartzite.

Transformation into a metamorphic rock involves processes of recrystallisation, where the mineral grains of the original rock are reorganised into a new crystalline structure.
Contact metamorphism occurs when rocks are heated by close proximity to magma. This process often results in a coarse-grained texture known as hornfels.
Regional metamorphism is due to deep burial and elevated pressure and temperature over a large area, often associated with tectonic activity, such as subduction zones and mountain ranges.
Metasomatism involves chemical alteration of the rock, typically via hydrothermal fluids.

Metamorphic rocks often display distinctive textures, such as foliation. This is a layered or banded appearance caused by the alignment of platy or elongated minerals, often resulting from pressure.
Non-foliated metamorphic rocks, such as marble or hornfels, lack a layered or banded appearance. Instead, they often display granoblastic texture, composed of interlocking, equi-dimensional grains of minerals.

The level of metamorphism is referred to as ‘metamorphic grade’. Three broad grades are generally recognised: low-grade, moderate-grade, and high-grade.
Certain minerals, termed index minerals, are useful in identifying the metamorphic grade of a rock. For instance, chlorite is indicative of low-grade metamorphism, while kyanite and sillimanite indicate high-grade metamorphism.

Metamorphic rocks are significant for a variety of economic reasons.
Many precious gemstones, including emerald, ruby, and sapphire, are found in metamorphic rocks.
Non-foliated metamorphic rocks like marble are valued in construction and sculpture for their aesthetic appeal.
Slate, another metamorphic rock, is commonly used in roof and flooring tiles due to its weather resistance and durability.