Recently, I took a trip up to Edinburgh with my boyfriend for a wedding, and whilst we were there we had a day exploring the city, including a walk up Arthur's seat. The views were fantastic: a city rich with history, with a mountain in the middle and the beach at the side, is just incredible and I definitely want to go back! Whilst walking up Arthur's Seat, I made an effort to just look at (not talk about) the rocks in order to save my boyfriend form more geology chat than he already has to endure. Later on, during a tour of Holyrood Palace, Arthur's seat was mentioned many times as a focal point for Edinburgh's history, and also as being an ancient volcano. This got me thinking a lot more about the geology of the city, and Arthur's seat itself which I thought I would explore in today's post.
A view over Edinburgh taken at the top of Arthur's seat. I took very few photos while we were there (living in the moment etc) so unfortunately this is the best I have.
The majority of the rock below the city of Edinburgh is sedimentary, deposited about 350 million years ago during the Carboniferous. At this point in time, Scotland was practically on the equator, making it much warmer than Edinburgh is today. Notably, the rocks of the area show us that Edinburgh was a swampy, coastal landscape dotted with shallow lagoons and a few sandy beaches. The sedimentary rocks underlying the city, and making up the base of Arthur's seat, record wave ripples, desiccation cracks, and other identifying features of a swampy coastal area. Fossils of little sea creatures such as shrimps and mussels can also be seen in the rocks.
About 5 million years later, further into the Carboniferous, the volcanic eruptions began. It's weird to think of a volcano being in the middle of Edinburgh, but of course Edinburgh was not Edinburgh back then. Dinosaurs hadn't even began to roam around let alone humans, but indeed thats what Arthur's seat was there, as a volcano in all its glory. When climbing up it, its hard to believe it was a volcano, the classic volcano-esque shape has been lost to millions of years of erosion, and what we are left with today is only a subtle indication in the rock of it's volcanic past.
Arthur's seat erupted a number of times throughout the Carboniferous, whilst Edinburgh went through periods of sea level rise and fall, producing the coral and algal fossils. The volcano itself, as volcanoes do, produced two main types of deposits, thick dark basalts that cooled from lava flows, and thinner bands of volcanic ash that settled down out of the air after big explosive events that would have thrown up huge plumes of ash. Within the rocks around Arthur's seat you can see these distinct ash layers, which can then be aged to tell us when the more explosive eruptions took place. The basalts created from cooling lavas flows would have been produced during less violent events, when lava just flowed down the side of the mountain as opposed to ash being thrown up into the air.
An example of some of the dark basalt, that my boyfriend decided he wanted to climb. Classic.
Arthur's seat is what is known as a composite volcano, meaning it would have been tall, more triangular shaped as opposed to a flatter shield volcano. The shape of the volcano is generally determined by the viscosity of the lava it produces, and therefore by the composition of that lava. The lava at Arthur's seat would have been more viscous, meaning it was not able to flow as far before it cooled, creating the distinct cone shape of a composite volcano that has now been lost top time and erosion. Arthur's seat has five separate vents which all produced lava flows at different times, and a lava lake can even be seen at one of them.
In the modern age, there are no active volcanoes anywhere near the British Isles which lies far away from any tectonic boundary (where volcanoes usually occur) or intraplate hotspot. Back in the Carboniferous, however, this was not the case. Scotland was moving northwards, away from the equator, and as it did the continent was being stretched. This stretching motion, known as rifting, allowed for magma to rise up from under the ground and erupt as volcanoes such as Arthur's Seat. Rifting is no longer happening in the British Isles today, but it is in places such as the Basin and Range province in the southwest of the United States.
The erosion of the distinct cone shape is thought to have largely taken place by glacier action during the Quaternary Period (when Scotland experienced what is commonly known as the Ice Age). Glaciers plough through the landscape, eroding much of what is in their way and leaving behind distinct indicators of their presence. Lucky for us, this erosion has exposed the rocks that would have been inside the heart of the volcano, creating a wonderful field location to study these rocks at outcrop. This is especially significant as Edinburgh is where James Hutton (often known as the father of modern geology) did a lot of his work. Hutton is famous for realising the vastness of geological time, most notably using evidence from the relationship between rocks at Siccar Point, about 35 miles east of Edinburgh. He showcased how the Earth was much much older than the bible said it was. In actual fact it is 4.5 billion years old, and the bible claimed only 6,000 years. Hutton's work had major implications for the scientific renaissance (and moving science away from the grips of religion) notably by providing the length of time needed for Charles Darwin to suggest his theory of evolution. Darwin knew evolution was a very slow process and couldn't fit it in within just the 6,000 years that society thought the Earth had been around for. Hutton's new claim that the Earth was much, much older allowed Darwin the timescale for his theory of evolution to have occurred. Hutton also did some work on this theory around Arthur's Seat, most notably at a location called Hutton's Section by Salisbury Crags. Here you can see a dolerite sill intruded into the surrounding rock, with some classic hexagonal columns formed by the cooling of the sill. The way Hutton used this to prove his age of the Earth theory is the same way he did with the rocks at Siccar Point. Knwoing how long it took for rocks to form, based on present day observations, he reasoned that there was no way that the dolerite sill, and the surrounding sedimentary rocks could have formed at the same time. By calculating how long it would have taken for the sedimentary deposition to occur, and then the intrusion afterwards, he showed much more than 6,000 years would have been needed. At Hutton's unconformity, there is an unconformity in the rock, where they have been tilted and eroded before new ones were then deposited on top, rather than an intrusion but the same principle applies, a vast amount of time would have been needed for all the events in this sequence to occurred.
A photo of Hutton's Section at Salisbury Crags, credit to BGS.
Arthur's seat however is not just key in the history of geology as a subject, but in the history of the city of Edinburgh. There are many signs of human occupation on Arthur's seat including an Iron Aged fort, and a castle thing that my boyfriend decided to run up the hill to (and I'm not actually sure what it is). In addition, no one is really clear on were the name Arthur's Seat actually came from, there has been discussion (albeit with little proof) that Arthur's Seat was the site of Camelot, home to King Arthur and the Knights of the Round Table (a very exciting concept if you grew up watching Merlin) but many other theories, probably more plausible ones, are out there.
Photo of the random ruins my boyfriend ran up the hill to (taken from the bottom of the hill as after having just got back down from the top of Arthur's seat I did not fancy running up there myself)
The final fact I found out in my research regarding Arthur's seat is that there is apparently also an Arthur’s seat in Victoria, Australia, named because of it similarity in looks to the original one in Scotland.
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