After a substantial break from blogging due to a combination of laziness and being away on field trips I have something of a backlog of posts and photos to put up. The first batch of which are from our 3rd year field trip to Ardnamurchan.
The field trip consisted of 5 days of mapping the slopes of Ben Hiant as well as a day doing something of a guided tour of the peninsula as a whole. What follows are photos of some of the more interesting features we observed, as well as brief explanations of their geological context.
Radially Jointed Vitrophyre
Here we can see a lovely radially jointed vitrophyre outcrop on the side of a steep gulley. The jointing nicely illustrates the intrusive relationship with the surrounding Breccia.
Annotated Vitrophyre
The vitrophyre outcrops in this area are part of a group of inclined sheet intrusions which are part of the British-Irish Paleogene Igneous Province (or whatever the current politically correct name is, I favour the North-Western European Archipelago Paleogene Igneous Province personally). Other inclined sheets of varying crystal size from glass up to Dolerite abound in the area, as cone sheets emanating out from the intrusive centres.
Dolerite Sheet on Ben Hiant
Cone Sheets on the Shore at Kilchoan
Xenoliths in Dolerite Sheet
Closer to the middle of one of the intrusive centres (Centre 3) is The Great Eucrite – a striking ring of Gabbroic ridges surrounding the inner parts of the complex.
View Northwards from the heart of Centre 3 - The Ridge is the Great Eucrite
The rocks towards the centre get steadily more silicic in composition, with Tonalites and then a Quartz Monzonite outcrop in the very centre. The current understanding of this structure is that it represents an uplifted and eroded Lopolith (not, as originally thought, a ring dyke).
Another location we visited was the Gabbros of Sanna Bay. These are a sequence of coarse grained basic rocks which show distinct mineralogical layering, both modal and phase layering.
Layered Igneous Intrusion
Layered Gabbros
Interestingly some of these units also show evidence of sedimentary structures such as normally graded bedding:
Normally Graded Bedding
Lens-Like Structures and Undulose Bedding (With my Cousin Karen for scale)
These are a classic layered intrusion similar to the grander examples found elsewhere such as Skaergaard or the Bushveld. We spent a fair bit of time studying these in the first semester of third year, so it was nice to see them in the field.
Another location visited was a delightful example of magma mixing at Eilean Carrach, opposite the Lighthouse on the point of Ardnamurchan.
Magma Mixing
For those of you who have this trip to look forward to (i.e. potential/current Glasgow Uni Earth Science Undergraduates) then expect to spend a significant amount of your evenings doing this:
Working on a Report at 3am
But also some of this as well:
Work Hard, Play Hard
The trip also provides no end of opportunities for posing dramatically on hillsides:
Ben Hiant
(and beaches)
Beach at Eilean Carrach
Further Reading
Gribble, CD (Ed). Ardnamurchan: a guide to geological excursions. 1976. Edinburgh Geological Society. – Nice book, good descriptions and a fold out map. Interpretations are a bit out of date however.
McBirney, A. Igneous Petrology. Third Edition. 2007. Jones and Bartlett. – Brilliant Section on Layered Igneous Intrusions, I highly recommend it.
Tags: Ardnamurchan, Geology, Paleogene
What do you know about the Solway fault? It is suppossed to be a continuation of Cameron’s line/Baie Verte Brampton line in the US and Canada. I am working on the part that passes through western Connecticut. It is our understanding that it truncates in the Baie Verte Peninsula of Newfoundland popping back up in Ireland where it is apparently called the Iapetus Ocean Suture Zone.
from Lindisfrane I understand it comes back to land in western Norway. What we have found in Connecticut we intrepret as a fossilized subduction zone. I’d send some pictures but need your Email address to upload them as attachments.
Regards,
John Carter
Geologist
Not really my area of expertise I’m afraid. The Iapetus Suture isn’t fantastically well exposed in the UK, and I know very little about where it continues laterally outside mainland Britain. It’s called different things in different regions, due to the numerous terranes and microterranes/island arcs involved.
In the UK it’s generally considered to be the result of subduction within the Iapetus ocean during the collision between Laurentia and Avalonia & Baltica. The evidence for this is mostly based on Faunal differences across the suture line.
For a decent summary/jumping off point for Terrane boundaries in the UK I would recommend reading:
Bluck, B.J., Gibbons, W. & Ingham, J.K. (1992) Terranes. Geological Society, London, Memoirs, 13, pp. 1-4. Available at: http://mem.lyellcollection.org.....act/13/1/1.