Cuillin Hills, Skye: Mountains of fire and ice

Explore the inside of an ancient volcanic complex and one of the best examples of glaciated hills in the UK

Jagged peaks of gabbro on the Cuillin ridge. Image: John Gordon.

The Cuillin Hills on the Isle of Skye have fascinated geologists for over 200 years, encouraging multiple field expeditions despite the steep, jagged peaks. The story of these hills begins more than 61 million years ago with volcanic activity during the initial opening stages of the North Atlantic Ocean. As North America and Europe ripped apart, large volumes of basalt lava were erupted from long narrow fissures on what is now the West coast of Scotland. As time went on, the volcanism became focused at several specific locations, creating large, central volcanoes. The 10-kilometre-wide Cuillin Centre was active around 60 million years ago.

Volcanoes are fuelled by magma – molten rock comprising crystals and gas and sourced from the Earth’s mantle, which is only called lava once it erupts at the Earth’s surface. The magma rises through the crust before pooling in chambers beneath active volcanoes, where it undergoes various transformational processes according to factors like chemical composition and temperature. During a volcanic eruption, not all the magma erupts from the magma reservoir; some remains within the crust and slowly cools and solidifies. With time, the rock overlying the interior of the ancient volcano complex on Skye has been eroded away, creating a new landscape. Erosion over the last 60 million years has removed over 2 kilometres of rock, exposing the volcanic chamber at the surface today.

The Cuillin Hills are composed of a gabbro, an iron- and magnesium-rich, slow-cooled intrusive rock with large crystals of plagioclase feldspar and pyroxene. These gabbros represent some of the earliest phases of magma generation in the active magma chamber. They display distinctive features such as large-scale layering formed by crystal settling, fine grain laminations made by the growth of new crystals (for example at Meall na Cuilce), and massive bodies of coarse crystals. The oldest gabbros are located at the margin of the volcanic centre and can be visited at the Fairy Pools. The youngest gabbros are located high in the hills around the Bloody Stone, a glacial erratic and the site of a bitter clan battle.
As the magma matured, its composition changed and became richer in silica and poorer in iron and magnesium. If a magma slowly cools at this point, granite is formed instead of gabbro – this is seen to the east in the Red Cuillin hills. Interestingly, the Red Cuillin have recently been proposed as a potential source for the pitchstone that forms the Sgurr of Eigg.

During the last glacial maximum, the Cuillin Hills were covered in ice with glaciers flowing out from a central area, scraping and eroding away the underlying rock. Evidence of this can be seen today as large parallel grooves in the rocks, which show the direction that the ice was flowing.
The Cuillin Hills represent one of the best places in the world to study the internal plumbing system of a volcano and as such will continue to attract visitors for years to come.

Text contributed by Kate Saunders and Angus Miller

Find out more
Skye: A landscape fashioned by Geology provides a great introduction to the geology of the wider area. Download from https://www.scottishgeology.com/find-out-more/publications/#skye

Spectacular exposures of ice-moulded gabbro. Image: John Gordon.

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