What are Metamorphic Rocks?
Metamorphic rocks are formed when existing rocks transform due to intense heat and pressure deep within the Earth. This geological process, called metamorphism, changes the rock’s texture, structure and mineral composition without melting it.
Metamorphic rocks start either as igneous or sedimentary rocks. Over time, exposure to extreme heat and pressure forces them to recrystallise, becoming denser and more compact. If the rock were to melt completely, it would be an igneous rock once it had cooled and solidified.
Metamorphism occurs in two main ways:
- Regional metamorphism happens over large areas due to tectonic movement, where rocks are buried and compressed over millions of years
- Contact metamorphism occurs when rocks are heated by nearby magma, altering their structure without melting them
The word 'metamorphic' comes from the Greek word metamorphōsis, with meta meaning 'change' and morphē meaning 'form'. It later passed into Latin as metamorphosis.
How Metamorphism Changes Rocks
During metamorphism, minerals within a rock can rearrange or form new minerals due to chemical reactions. This is why metamorphic rocks often have a distinct, banded, or crystalline appearance.Marble is a metamorphic rock that forms when limestone is exposed to intense heat and pressure. The calcite in the limestone recrystallises, creating a harder, more crystalline rock with a smooth texture. The different colours in marble come from natural impurities present during the transformation.
Marble is typically monomineralic, meaning it's composed primarily of just one mineral. Although mostly calcite, it can also contain impurities such as quartz, mica or graphite.
Lapis lazuli, which is often referred to incorrectly as a mineral, is another metamorphic rock. It forms when limestone undergoes metamorphism, but unlike marble, lapis lazuli contains several minerals, including lazurite, calcite, and pyrite.
Metamorphic rocks start as igneous or sedimentary rocks and transform into something entirely new, often with unique textures and mineral compositions.
Whether it’s the smooth, polished finish of marble or the striking blue colour of lapis lazuli, metamorphic rocks tell a story of Earth’s powerful geological forces.
Article Photo
The metamorphic rock in our first photo, courtesy of Stan Celestian, is marble. It's clickable and redirects to the original photo.
The lapis lazuli is a stock image purchased on licence.
Pop-up photos: Marble and Calcite - Courtesy of Stan Celestian. The lepidolite mica is from our collection.
Graphite - Courtesy of James St. John. Lazurite - Courtesy of Géry Parent.
