What is a metamorphic rock?

Mica Schist

Mica Schist from Ireland

A metamorphic rock is a rock that has been altered. The word metamorphic comes from the Greek terms meta which means change and morphe which means form, so to change form. The process can alter metamorphic rocks, igneous rocks and sedimentary rocks. It is important to note that the temperatures at which metamorphism occurs is lower than that of which a rock would melt. Usually it ranges between 250°C and 700°C.

Metamorphism causes the parent rocks to alter and re-crystallise. Some metamorphic rocks such as gneiss can be identified as being metamorphosed because the minerals within the rock have realigned. This gives the rock a layered appearance, the best example of this is a schist or gneiss.

There are two types of metamorphic rock; regional metamorphism and contact metamorphism.

Slate

Slate

Regional metamorphism occurs, as its name suggests over a large area due to high temperatures and pressure. This commonly happens during mountain building when two plates are colliding. Examples of this today are the Himalayas, one of the youngest mountain chains in the world, which have formed through the collision of the Indo-Australian Plate and the Eurasian Plate. Contact metamorphism occurs through high temperatures within a smaller area, around an igneous intrusion (magma chamber).

A common example of metamorphic rocks in the UK is slate which is common in Wales but metamorphic rocks are found across the UK especially in the Lake District.

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