Gneiss

I thought I’d start off this series by talking about one of my absolute favourite rocks that you can find in Scotland which the Lewisian Gneiss Complex – the oldest rocks in Britain.

First of all, Let’s talk about metamorphic rocks:

Metamorphism in geology is when rocks ‘change’ into something else due to a combination of heat and/or pressure. The heat and pressure surrounding rocks can change them over geological time and cause different chemical reactions which can form new minerals and cause minerals to recrystallisation over geological time. This transforms the rock deep within the Earth’s crust and changes it into a metamorphic rock. They can end up squeezed, foliated and altered until they turn into something entirely new.

What is Gneiss?

Gneiss - pronounced “nice,” is a high-grade metamorphic rock, meaning it’s sat at very high pressures and temperatures within the Earth’s crust for a long period of time – Where do we see this happening? Usually in the middle of big tectonic events at convergent plate boundaries (where two plates meet and one subducts under the other) that are associated with mountain building and crustal thickening.

How Do You Recognize Gneiss?

Gneiss is known for its banded appearance—layers of light and dark minerals folded and swirled together like ribbons. These bands form because the intense pressure rearranges minerals into new structures. The lighter bands are often made of minerals like quartz and feldspar, while the darker layers might contain biotite, amphibole, or other rich minerals.

Banded Gneiss, Isle of Lewis, Outer Hebrides

The Lewisian Gneiss Complex

The Lewisian Gneiss was originally a mix of granite, granodiorite with some sedimentary rocks in there too. The cool thing about these rocks is they’ve been buried to depths over and over again that they just about melted forming the beautiful bands and foliations that we can see today. They are the oldest rocks in Britain, dating back to an astonishing 3 billion years old which is madness because the earth is 4.6 billion years old so these rocks are very old. What’s really special about these rocks is they tell a story of what ancient time was like on Earth. It’s been through several tectonic cycles, heated, squished, and shoved around by colliding continents. But it still stands proud today, as a testament to the power and resilience of the Earth’s crust.

Lewiston Gneiss, Ceannabeinne Beach, Durness

Where Can You See Gneiss in Scotland?

If you’re in Scotland, you can find the Lewisian Gneiss Complex in the Outer Hebrides and the Northwest Highlands in the Hebridean Terrane.

Watch the video on Gneiss - https://www.youtube.com/watch?v=6HYzSMUk_oA