LAND EVOLUTION | Path of Fire:

Formation of the Giant's Causeway

The iconic hexagonal columns of the Giant's Causeway

Many of the posts over recent months have detailed the origins of Ireland's modern mammals, with the early members of the modern mammal orders evolving in a world with a sweltering greenhouse climate in the early Eocene, around 55 million years ago. However, we have not yet seen just why the world was so hot at this time, and in the telling of this tale we can tell another: a story of fleeing giants and oceans of fire, of an ancient schism in the lands of the north which gave birth to one of the most iconic and singular features of the Irish landscape – the Giant's Causeway.


In Irish mythology, one of the most famous stories involving the great giant Fionn MacCumhaill is that of his building the Giant's Causeway so that he can fight a Scottish giant he has been trading insults with. In one version of the myth, as Fionn nears his Scottish foe, Benandonner, he realises just how large and powerful he is and so decides to hightail it back to Antrim. Benandonner pursues Fionn across the causeway to his home where he encounters Fionn's wife and what he takes to be Fionn's infant son in a cradle, although it is Fionn himself in disguise. Benandonner, convinced that the father of such a monstrous baby must be a gigantic and unassailable adversary, swiftly flees back across the causeway, destroying much of it as he goes to prevent any reprisals.


We may find this all highly colourful and be certain that it is utterly divorced from reality, but within this tale there is the kernel of truth regarding the formation of the Giant's Causeway. The creators of this origin story managed to capture something of the epic scale of the forces which created this spectacular feature, and one element of the story they got exactly right – the formation of the Giant's Causeway was related to a great struggle where one giant eventually fled from another. In reality, though, this struggle was not between man-shaped titans but rather the continental behemoths of Greenland and Europe.


The origins of this story can be traced back to the final days of the last supercontinent Pangaea around 200 million years ago, when the great heat building up under its surface caused it to rip in two, creating two massive continents in the process: Laurasia in the north and Gondwana in the south. The rifting of these two massive continents marked the birth of what would become the Atlantic Ocean, which expanded from an early narrow seaway to a great sea over millions of years as Laurasia and Gondwana continued to move away from each other and the South and Central Atlantic grew.

Map of Laurasia and Gondwana at the time they began to split from each other c. 200 million years ago

By 100 million years ago, the North Atlantic had also begun to open, and, as rifting extended ever further northwards, the long-time bond between Greenland and Europe was destined to be broken as Laurasia was ripped apart.


It was just over 60 million years ago when the first rumblings of Greenland's imminent departure began to be felt across nearby lands. A hellish hot-spot in the mantle under Greenland's east coast began to boil and writhe, stretching and fracturing the land to form a great province of fire stretching as far south as the Faroe Islands, Scotland, and the northeast of Ireland.


In two terrible volcanic pulses, beginning around 59 and 56 million years ago, and a few smaller ones in-between, great falls of ash blanketed large parts of Ireland and northwest Europe, while jets of molten rock surged into the crust and seas of lava streamed across Ireland’s northeast, these lava flows cooling to form the basalt rock which underlies Antrim and parts of surrounding counties.


However, it was during one of the smaller volcanic episodes which occurred between these two major pulses that the causeway itself was formed. Some of the lavas which blazed a path of fire across Antrim at this time found their way into an ancient river valley to form a fiery pond which then cooled in a slow and uniform manner to eventually become the great hexagonal columns of the Giant's Causeway, which now descend proudly into the wild sea towards Scotland.


But it was not only in Ireland that these ancient lavas created such great fields of hexagonal columns, though. In parts of the Inner Hebrides, those wind-blown isles off Scotland’s west coast, similar basalt columns rise from the earth, paving the land with equally impressive structures. One of these islands, the little Isle of Staffa, even derives its name directly from its great volcanic pillars, as the Vikings named the island in the Middle Ages due to these columns’ resemblance to the large wooden beams they used to form the vertical supports of their house frames, stafa in old Norse meaning ‘stave’ or ‘wooden pillar’.

Location map of the Isle of Staffa and the Giant's Causeway

Indeed, it may have been the similarity of these mighty columns on this Scottish isle to those on the north Antrim coast which ultimately gave rise to the legend of the Giant’s Causeway. It is surely no coincidence that, in the myth, it is from the Isle of Staffa that the gigantic Scot Benandonner hurls his insults, and to which he eventually returns following his abortive pursuit of Fionn. But if this chastened colossus could have expected this to be the last indignity he would suffer at the hands of Fionn MacCumhaill, he would be wrong. For on the Isle of Staffa there stands a remarkable cave walled with mighty hexagonal columns which bears not his own name but that of his Irish foe – the lava-built fortress known as Fingal's Cave.

Like the Giant's Causeway, Fingal's Cave is an amazing structure with basaltic columns

However, as undoubtedly spectacular were the effects of this age of fire and continental rupture on parts of Ireland and Scotland, they were not only to be felt in the North Atlantic. This period of volcanic fury has been implicated in establishing the hothouse climate of the early Eocene as it released untold quantities of greenhouse gases into the atmosphere, through both the volcanic eruptions themselves and the triggering of methane release from the seafloor.


So, the same process which formed the Giant's Causeway may have been responsible for the subtropical forests and strange mammals which inhabited Ireland in the early Eocene. But though this age of warmth would not last and Ireland would change in many different ways, the steps of the Giant's Causeway would endure. And, of course, in Antrim they still stand – a monument to mark an epic continental struggle, and truly the work of giants.

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