Rum and the Small Islands

Landscape Fashioned by Geology

• Introduction
• Setting the Scene
• Ancient Rivers on Rum
• The Rum Volcano
• Explosive Eruptions
• The Shaping
• From Molten Rock
• Flowing Lavas
• Eigg
• Canna and Sanday
• The Ice Age
• Glacial Clues
• High and Dry
• Early People on Rum
• The Bulloughs and SNH
• The Landscape Today

The Shaping of Western Rum

Not all the silica-rich magma in the magma chamber was erupted, and some of it crystallised below the Earth’s surface. The rate at which the magma cooled below the Earth’s surface was slow, allowing large crystals to form, and this produced coarse-grained rocks called granites. The best examples of granites on Rum are in the western hills, particularly Orval and Ard Nev.

As the silica-rich magma was being erupted onto the surface, a slightly different type of magma was rising up into the magma chamber below. This magma was richer in iron and magnesium than the magma which formed the granites. It crystallised below the surface to form the gabbros and associated rocks that make up the more jagged peaks of the Rum Cuillin.

The contact between the granitic rocks of western Rum, and the gabbros of the Cuillin, can be seen on the southeast side of Harris Bay. Here, the darker gabbros and the pale pink granites became mixed up before they cooled and solidified, so that blocks of darker gabbro are surrounded by areas of pink granite.

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