Mull and Ardnamurchan part 5 – The Geology of Staffa and Fingal’s Cave

Things have been very bird/animal heavy on my blog to-date, and I’ve always been keen to add a drop of geology to the mix. Fortunately my trip to Scotland has presented the perfect opportunity with the geological gem that is Fingal’s Cave on Staffa.

Staffa

Staffa

Staffa is a dramatic looking island, with huge columns of hexagonal columnar basalt topped with the compressed ash of volcanic explosions (called tuff). Basalt is an igneous rock, meaning it was formed from lava (or magma). Specifically, it is an extrusive igneous rock, meaning it formed as a lava flow at the surface. It also has a specific chemical composition, and this is reflected in the minerals present in the rock, and how big the crystals are (there’s a bit of a guide HERE).

As you approach by boat you see this green-topped shape looming out of the sea, and as you come closer the structure becomes apparent:

MacKinnon's Cave, Staffa

MacKinnon’s Cave, Staffa

Here we can see the less well known MacKinnon’s Cave within the columnar basalt, topped by that volcanic tuff. The smooth rock under the columns is another form of basalt.

As you come past MacKinnon’s, you come upon the famed Fingal’s Cave. Personally I much prefer the Gaelic name An Uamh Bhin, which means ‘the melodious cave’. That would in fact have lent itself better to Felix Mendelssohn for his Hebrides Overture! Instead, it was renamed after the Irish hero Fionn MacCool, hence Fingal’s Cave.

An Uamh Bhin, Staffa (Fingal's Cave if you must)

An Uamh Bhin, Staffa
(Fingal’s Cave if you must)

Handily we have some people here for scale!

As you approach, the boat crew fire up the PA system and pipe out the Mendelssohn Overture that draws the non-geologist to the Staffa. You can hear it here: MIT Symphony Orchestra (I’m actually playing this recording while writing this post).

The on-foot approach to the cave is reasonably safe these days, with non-slip surfaces and a handrail to guide you, but there are still accidents, so if you ever visit it’s best not to go round unless you are sure on your feet. You get a better view from the boat anyway!

The approach to the cave

The approach to the cave

Once you are round and on land you can get a much better look at these basalt columns:

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The first thing you’ll notice is that while I referred to the columns as ‘hexagonal’, they don’t all have six sides. This is just one of those things. Geologists defaulted to calling them hexagonal long before they were properly studied, but they can have any number of sides (usually between 3 and 7). Best not to worry about that one, or just stick to calling it ‘columnar basalt’.

The rocks formed around 60 million years ago when massive volcanoes opened up and the lava flooded out. As it cooled the lava contracted causing cracks to form, and those cracks keep growing down through the solidifying lava flow. It’s a similar principle to the way a dried-out river bed has cracks in the mud. This forms the columns. You can see the same thing from the same time in Ireland at the famous Giant’s Causeway.

The columns don’t just go straight up though, and as the land has been squished and stretched and moved about over millions of years, the columns have ended up bent in places till they sweep out in the horizontal, almost like a stack of logs:????????

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Personally, I couldn’t resist the walk round to the cave. Years of studying geology have made me pretty confident in my footing, and my balance, so I was quite happy with it. At one point when the designated path narrowed and a bottleneck developed I happily stepped down to allow others to pass. But I recommend you stick to the path. However this lead to my being trapped by my own good manners, so you should just be selfish!

???????? ????????When insiode the cave, the sound of the waves breaking and echoing makes you understand why it was called ‘the melodious cave’ and why it so inspired Mendelssohn.

You can even look up at the ceiling of this natural chapel and see the eroded and moss-covered columns:

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I got quite carried away round Staffa, and soon racked up over 100 photos of the rocks. I’ve tried to share the best of them here to illustrate the geology, and the feel of the visit. I figure piling all of them on you may be a step too far!

While on the mainland I was told quite a good tale about Fingal’s Cave that I’m assured is a true story. A visiting geologist took a boat trip out to the cave, hiring a small boat to himself. While out there he asked the pilot “Do you know how old those rocks are?” When the captain said no, the geologist told him “60 million years”. A few years later, a party of geology students were visiting Mull and happened to hire the same boat to take them out. At the cave, one of the students asked the pilot if he knew how old the rocks were. “Yes,” came the reply “60 million and three”!

I don’t know if it’s really true, it sounds apocryphal, but it’s a good story anyway.

Tomorrow will be the final post on Scotland and will just cover some recommendations and credits on staying, eating, trips out etc. I’ll throw a bunch of pics of the fabulous Scottish scenery in for good measure.

David

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One Response to Mull and Ardnamurchan part 5 – The Geology of Staffa and Fingal’s Cave

  1. Pingback: Mull and Ardnamurchan Part 6 – Visiting Scotland | Why watch wildlife?

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