The Loch Ba Ring Dyke (LBRD) is one of the classic geological features in Mull. It has been described as "the finest ring dyke known to science" and is of international significance. It was formed as the result of molten magma filling an arcuate fracture when the central block subsided, forming a caldera. This occurred towards the end of the igneous activity in Mull and represents the last major igneous event.
The map below, reproduced from the Mull Memoir shows the track of the LBRD.
The dyke is not continuous it stands out well on the NW side of Loch Ba and also on the SE side of the same loch, but it is not so obvious further round the ring fault. In most places it is only visible in stream sections and is not seen on the open hillside.
The Loch Ba Ring Dyke forms the rocky hill in the above picture.
Near the top of the Dyke
Looking down the Dyke towards Loch Ba and Loch na Keal
One of the distinctive features of the LBRD is the presence of more basic material in amongst the acidic felsite. The basic material is of basaltic and andesitic composition and forms swirls and knots of dark rock in contrast to the light coloured felsite. This mixed intrusion shows that acidic and basic magmas were present in the same magma chamber.
Swirls of basic material are found amongst the felsite.
The downthrown block which the ring fault encircles may have been displaced by as much as 1000 metres according to research.
The LBRD, although late in the Mull igneous activity, was not the last intrusive event however. The LBRD itself is cut by several dykes of the Mull Swarm. Some of these have been eroded out to form a deep chasm cutting across the ridge as can be seen in the picture below.
One of the Mull Swarm dykes (basaltic) cutting across the Loch Ba Ring Dyke