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The mountainous, rugged island of Rum is the eroded remains of an early Palaeogene volcanic centre that existed at the time of the opening of the North Atlantic Ocean. Many of the rocks found on Rum consist of layered ultramafic rocks – remnants of the base of the volcano’s magma chamber. It is not the only volcanic centre on the west coast of Scotland to exhibit such layering, but the examples seen on Rum are well exposed and in an excellent state of preservation. They have become internationally important in their contribution to the understanding of the origins of layering igneous intrusions. As with the other Scottish Palaeogene volcanic centres, Rum has been studied by many esteemed geologists such as John MacCulloch (1773 – 1835), John Wesley Judd (1879 – 1914) and Sir Archibald Geikie, before being followed by more recent researchers such as Dr Alfred Harker (1859 – 1939) and Sir Edward Bailey (1881 – 1965).

Slump folding in allivalite near Askival summit. © Colin MacFadyen/Scottish Natural Heritage.

The Rum Complex was intruded into Lewisian gneiss basement, Torridonian sandstones (Precambrian), Mesozoic sedimentary rocks and early Palaeogene lavas. The complex itself evolved in two main stages. Initially, intrusive and extrusive activity was charged by acid magmatism and associated with ring-faulting. It is thought that the complex underwent repeated doming and subsidence (within the cauldron) at this time. Rocks associated with this first phase include porphyritic felsite, volcanic breccias and tuffs.

Askival and Hallival, Rum – this site is a unique internationally significant location for large-scale, cyclic layering in igneous rocks. Cuillin. © Colin MacFadyen/Scottish Natural Heritage.

The layered ultrabasic rocks were intruded during the second stage. The parent magma is considered to be either high-temperature picritic basalt or feldspathic peridotite that was subject to the right conditions that enabled it to rise to within a short distance of the Earth’s surface before crystallising. The intrusions consist of sixteen layers of alternating feldspathic peridotite, troctolite, troctolitic gabbro, feldspathic gabbro and anorthosite.

Note: The Rum volcanic centre forms part of the North Atlantic Palaeogene Igneous Province, along with the other centres of Skye, Arran, Ardnamurchan, Mull and St. Kilda.

Further reading:

Emeleus, C.H. & Gyopari, M.C. 1992. British Tertiary Volcanic Province, Geological Conservation Review, Series No. 4. Joint Nature Conservation Committee, Peterborough, 259 pp.

Emeleus, C.H & Bell, B.R. 2005. British regional geology: the Palaeogene volcanic districts of Scotland (Fourth edition). (British Geological Survey, Nottingham.

Goodenough, K. & Bradwell, T. 2004. Rum and the Small Isles – A Landscape Fashioned by Geology. Produced by: Scottish Natural Heritage & British Geological Survey.

Trewin, N. H. (ed.) 2002. The Geology of Scotland. The Geological Society, London.

RUM NNR Website

The Isle of Rùm, Scotland: The beauty of ancient volcanism: article by science writer Lara Reid

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