The Giant’s Causeway

I began my research at the Giant’s Causeway, the most popular example of geology in Irish Folklore today. It is a strange rock formation that has a spellbinding effect on the multitude of tourists that arrive by the busload. The striking and at times, like the day of our visit when the mist hung low over the shore, eerie perfection of the precise geometric columns has led many a traveler to believe that it must have been created by human or supernatural forces. There are many myths and legends surrounding the origin of such a mysterious feature, the most prominent of them being the Legend of Finn McCool.

The Giant’s Causeway, June 2012

The Legend of Finn McCool

“Finn McCool (Fionn mac Cumhail) an Irish Giant lived on an Antrim headland and one day when going about his daily business a Scottish Giant named Benandonnar began to shout insults and hurl abuse from across the channel. In anger Finn lifted a clod of earth and threw it at Benandonnar as a challenge, the earth landed in the sea.

Benandonnar retaliated with a rock thrown back at Finn and shouted that Finn was lucky that he wasn’t a strong swimmer or he would have made sure he could never fight again.

Finn was enraged and began lifting huge clumps of earth from the shore, throwing them so as to make a pathway for the Benandonnar, Scottish giant to come and face him. However by the time he finished making the crossing he had not slept for a week and so instead devised a cunning plan to fool the Scot.

Finn disguised himself as a baby in a cot and when Benandonnar came to face him Finn’s wife Una told the Giant Benandonnar, that Finn was away but showed him his son sleeping in the cradle. The Scottish giant became apprehensive, for if the son was so huge, what size would the father be?

In his haste to escape Benandonnar sped back along the causeway Finn had built, tearing it up as he went. He is said to have fled to a cave on Staffa which is to this day named ‘Fingal’s Cave’.

Other versions of the legend include Finn throwing a huge piece of earth which then became the Isle Of Man and the hole which it left behind became Lough Neagh.

It is told that Finn was building a Causeway pathway to Scotland using the hexagonal rocks, much to the upset of Benandonnar. A battle ensued. To this day, there remain hexagonal rocks for all to see at the Isle of Staffa, however, the main breathtaking formations are situated at the Giant’s Causeway, World Heritage Site, with over 38,000 basalt columns on display.”3

The National Trust, UK

The Geology

Was The Giant’s Causeway actually formed by a giant trying to cross the Irish Sea? Unfortunately, (or maybe fortunately because just imagine a world with angry giant’s running amuck) no. The Giant’s Causeway is actually a volcanic feature known as columnar basalt.

The columns at the Causeway are a type of rock called basalt, a mafic igneous rock that is made up of the minerals clinopyroxene and plagioclase. The term mafic describes its chemical composition and means that it is rich in iron and magnesium. Igneous simply means that it was formed from molten rock: magma. It is extrusive – it cools at the surface unlike granite, an intrusive igneous rock, which cools in the crust below the surface of the earth– and forms when magma rises from the mantle, erupts onto the surface, cools and becomes solid rock.

Magma can be formed by three main processes: decreasing pressure, increasing temperature, or changing chemical composition. Decompression melting is a geologic phenomenon that occurs when the pressure of overlying material is reduced. Buried mantle rocks, under less pressure, will melt and rise to the surface where they either erupt or cool within the crust. In the case of the Giant’s Causeway basalts, the magma was formed by decompression melting of the mantle beneath the Rathlin trough, a sedimentary basin that extends from the Antrim to Islay, an island off the south coast of Scotland (McCann).

Columnar Jointing

The physical properties of columns are due to the manner in which cooling occurred. After magma erupts to the surface it is known as lava. As lava cools, it expels dissolved gases, contracts, and solidifies, all of which decrease its overall volume.  In order to accommodate the volume loss, the rock begins to crack and form what are known as joints. Under ideal conditions, joints will form at 120 degrees to one another in basalt, creating the hexagonal pattern you can see from the top of the columns. However, when walking along the top of the columns you may notice that some can be five, seven, eight, or even nine sided. This is just an example of the imperfections in nature. As further cooling ensues, the cracks will propagate downward creating vertical columns known as columnar jointing. When this pattern forms in basalt, it is known as columnar basalt.


Tomkeieff was the first to describe the Giant’s Causeway basalts as tholeiitic, a term used to describe the composition of the magma. A tholeiitic magma is enriched in iron and depleted of magnesium which preferentially crystallizes out of the cooling magma. Geochemical and petrographic analysis of the basalts led researches to conclude that they were formed by sheet flows contained within the margins of a fault basin (Lyle and Preston).


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