Loch Ba Ring Dyke

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 

 

Other photos of the location:

Click on an image to open a larger picture.

Summit of Beinn a' Ghraig from the LBRD

Summit of Beinn a' Graig from
LBRD
Eroded out dyke in the LBRD

Eroded out dyke in the LBRD
Eroded out dyke in LBRD

Eroded out dyke in LBRD
LBRD from Loch Ba track

LBRD from Loch Ba track
Mull Swarm dykes cutting LBRD

Mull Swarm dykes cutting LBRD
LBRD Felsite with mafic inclusions

LBRD Felsite with mafic
 inclusions
Mafic inclusions in the felsite

Mafic inclusions in the felsite
Mafic inclusions in the felsite

Mafic inclusions in the felsite
Looking down the LBRD

Looking down the LBRD
Near summit of LBRD

Near summit of LBRD
LBRD at summit of ridge
 
LBRD at summit of ridge
LBRD at summit ridge
 
LBRD at summit ridge
View from LBRD to Loch Ba

View from LBRD to Loch Ba
Looking across Loch Ba to the LBRD on the other side of the loch

Looking across Loch Ba to the
 LBRD on  the other side of the
loch.

Similar or related Geological Sites:

  • Staffa
  • Carsaig Arches
  • Ardtun
  • East of Carsaig
  • Glac Mhor
  • Ardmeanach - The Fossil Tree
  •  

References:

Bailey, E.B., Clough, C.T., Wright, W.B. et al. (1924) Tertiary and Post-Tertiary Geology of Mull, Loch Aline and Oban. Memoir of the Geological Survey of Great Britain, HMSO, Edinburgh.

GCR Report: Loch Ba to Ben More This is very detailed and covers a lot of area, not just the LBRD but other intrusions and lavas in the area as well.