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Mull - Loch Bà Ring Dykes

(Other Site at Mull include Ardtun Leaf Beds, Macculloch's Tree and Staffa)

The Loch Bà Ring Dykes were first described by Sir Archibald Geikie in 1888. However, it was not until 1910 that they were proposed as ring-structures, whereupon they have become internationally recognised as the first and most perfectly preserved ring dykes - forming only a part of a much more extensive volcanic complex. They are Tertiary in age and are associated with the Mull volcanic centre, which existed at the time when the North Atlantic Ocean was beginning to open.

Three centres of igneous activity have been identified on Mull. The dykes were intruded late into Centre 3 and are therefore some of the youngest intrusions of the Mull complex. They are felsitic in composition, though contain inclusions of basaltic andesite and andesite, representing a mixed-magma intrusion.

The ring-fractures formed during subsidence of the overlying crust, after the near-emptying of the magma chamber below. Following the subsidence, magma was then forced into the ring fractures, thus forming the dykes. This process of subsidence and intrusion happened many times over the lifetime of the complex. It is estimated that the overlying crust subsided for a total of about 1km.

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

Further reading:

Craig, G.Y. 1991. Geology of Scotland. 3rd edn. The Geological Society, London.

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