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Sites:
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.

An aerial view of southern Mull from Above Loch Buie looking
north eastwards toward Loch Spelve, at the edge of the Mull volcanic complex.
Image
provided by SNH.
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.
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