Metamorphism

Metamorphic rocks have had their appearance (texture and mineral composition) changed because of intense heat and/or pressure.

Here is an example:

Metamorphic changes do not involve melting (higher temperatures are needed for that), nor any change to the chemical composition of the rock. However, mineral grains (like quartz and clay) react chemically with each other to form new minerals (like mica and garnet) and new rock textures.

What causes metamorphism of rocks?

Imagine going down a very deep mine-shaft. The deeper you go, the greater the pressure on the tunnels because of the weight of rock above. Also, it gets hotter as you go down, because the Earth’s interior is very hot. At a depth of 3 km, the rock may be hot enough to boil a kettle!

Metamorphic rocks are mostly formed beneath mountain ranges where rocks are deeply buried (10 km or more) and squashed by movements of the Earth’s crust. This is called Regional Metamorphism because large regions of rock are affected.

However, rocks can also be metamorphosed at shallower depths if they are heated up by nearby intrusions of hot magma – this is called Contact Metamorphism.