Mull – Loch Ba ring dyke

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The Loch Bà Ring Dyke was first described in 1924. Since then it has become internationally significant as the first recognised and most perfectly preserved ring dyke, forming part of a much more extensive volcanic complex. It is Palaeogene in age and is 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 Loch Bà Ring Dyke was intruded late into Centre 3 and is therefore one of the youngest intrusions of the Mull complex. It is felsitic in composition although it contains inclusions of basaltic andesite and andesite, representing a mixed-magma intrusion.

Mull from above - an aerial view of southern Mull looking north eastwards toward Loch Spelve from Loch Buie at the edge of the Mull volcanic complex. © Scottish Natural Heritage.
Mull from above – an aerial view of southern Mull looking north eastwards toward Loch Spelve from Loch Buie at the edge of the Mull volcanic complex. © Scottish Natural Heritage.

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 dyke. 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 Palaeogene Igneous Province, along with the other centres of Skye, Arran, Ardnamurchan, Rum 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.

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

Stephenson, D. 2005. Mull and Iona – A Landscape Fashioned by Geology. Produced by: Scottish Natural Heritage & British Geological Survey.