Giant’s Causeway Geology: Basalt Columns and Northern Ireland’s Coast

Few landscapes look as though they were made on purpose. The Giant’s Causeway does: some 40,000 dark basalt columns, most of them six-sided, packed together like the cells of a honeycomb and stepping down from the foot of the Antrim cliffs into the sea. Giant’s Causeway geology has puzzled and provoked observers for more than three hundred years, and the answer they eventually reached helped settle what the science of geology would become. On the sixteenth day of Odyssey’s Treasures of Ireland small-group tour, we follow the North Coast from Bushmills to the Causeway, and stand on the stones that started the argument.

A coast built from basalt

The rocks of the Causeway Coast belong to one of the great volcanic episodes of the last hundred million years. Around 60 million years ago, early in the Paleogene, the crust between what would become Greenland and north-western Europe began to pull apart as the North Atlantic opened. Molten rock rose through the stretching crust across a vast region now called the North Atlantic Igneous Province, whose lavas and intrusions can be traced from east Greenland and the Faroe Islands to Skye, Mull, Staffa and the north-east of Ireland.

In Antrim, the lava came up through long fissures rather than from a single towering volcano, flooding the landscape in sheet after sheet. The result is the Antrim Plateau, which still covers some 3,800 square kilometres and is the largest surviving lava plateau in Europe. Beneath it lies white Cretaceous chalk, the floor the first lavas poured across, so that along parts of this coast black basalt sits directly on white limestone.

Geologists divide the lavas into three parts. The Lower Basalt Formation, built from many successive flows, is up to 531 metres thick. Above it lies the Interbasaltic Formation, a band of red, iron-rich laterite about 30 metres thick, formed when volcanic activity paused for long enough for the land surface to weather into deep soils. The Upper Basalt Formation caps the sequence. Dating published by Ganerød and colleagues in 2010 placed the Lower Basalt at about 62.6 million years and the Upper at about 59.6 million.

The Causeway itself belongs to neither. During the long lull represented by the red laterite, a separate outpouring of lava, the Causeway Tholeiite, erupted locally in the north. It arrived as nine separate flows, each of which ponded in a depression and grew as much as 30 metres thick. That thickness is the key to everything visitors come to see. In June 2026, the Geological Survey of Northern Ireland and the British Geological Survey published new work showing that the region’s volcanic rocks, the Causeway included, formed within about 5.5 million years, some eight million years less than earlier estimates had allowed.

It is worth setting this against the other great cliff landscape on the tour. The layered sandstones and shales of the Cliffs of Moher, which we see in County Clare on Day 8, were laid down by rivers and deltas some 320 million years ago. By comparison, the Antrim basalts are young rock, and fire-made rather than water-made, which is exactly the distinction early geologists struggled to draw.

How old is the Giant’s Causeway?

The columns are roughly 60 million years old. The Causeway Tholeiite has not been dated directly, but it sits on laterite containing crystals eroded from the Tardree Rhyolite, dated to about 61.3 million years, so it must be younger than that and older than the Upper Basalts above it at about 59.6 million. The shape of the coast is far more recent: the sea and the ice of the last glaciations exposed the columns only in the geologically recent past.

Giant’s Causeway geology: how the columns form

Lava shrinks as it cools. A thin sheet of lava cools quickly and cracks irregularly, much like a puddle of mud drying in the sun. A thick, ponded flow behaves differently. It loses heat slowly from its upper and lower surfaces, and as it hardens it pulls itself apart into a network of cracks that grow inward from those cooling surfaces, at right angles to them.

Where three cracks meet, the most efficient way to relieve the strain is for them to meet at roughly equal angles, around 120 degrees. Repeated across a whole flow, that geometry produces a pattern dominated by hexagons, although the Causeway also has plenty of four-, five-, seven- and eight-sided columns. The pattern is not perfect, and recent laboratory work suggests it never quite settles into perfection: a mature column network keeps a measure of residual disorder, which is part of why the Causeway looks natural rather than carved.

Look closely at the side of a column and you will often see faint horizontal bands a few centimetres apart. These record the crack growing in small increments as the cooling front advanced down through the lava, a step at a time. Many columns are also broken across into stacked segments, one cupped into the next, which is why the pavement in places resembles a field of stepping stones rather than a single sheet.

Why are the Giant’s Causeway columns hexagonal?

Because six-sided cracking is the most efficient way for a slowly cooling, contracting layer of rock to relieve its internal stress. Cracks that meet at about 120 degrees release strain evenly, and as the cooling front moves through the flow the network gradually adjusts towards that arrangement. Not every column has six sides, but hexagons are the shape the process tends towards.

Why columns here are so much more regular than in most basalt was a question that physicists answered only in 2009. Lucas Goehring, L. Mahadevan and Stephen Morris reproduced columnar jointing in trays of drying cornstarch, and showed that the same relationship between crack speed, column size and the rate at which heat or moisture can escape governs both. The slower the cooling, the larger the columns. The deep, ponded Causeway flows cooled slowly and evenly, and the result was some of the most orderly columns anywhere on Earth.

The Causeway also gave geology two of its standard words. In 1940 the geologist S.I. Tomkeieff, describing these cliffs, borrowed the terms colonnade and entablature from classical architecture: the colonnade for the regular columns at the base of a thick flow, the entablature for the more chaotic, finer jointing above it. Both terms are now used for thick basalt flows the world over.

Science and story

Long before anyone measured a column, people explained them. The best-known Irish story credits the giant Finn McCool, Fionn mac Cumhaill, with building a causeway across the sea to Scotland to fight the Scottish giant Benandonner. In the version most often told today, Finn’s rival tore up the road behind him as he fled, leaving only its ends: the stones in Antrim, and the columns on the Scottish island of Staffa. A gentler version, in which Finn built the causeway to reach a woman he loved in Scotland, is little heard now, but it was the one guides commonly told visitors in the eighteenth and nineteenth centuries.

The scientific story begins in the 1690s, when the Causeway was described to the Royal Society in London and the physician Thomas Molyneux published the first serious study of its stone, judging it one of the most remarkable natural curiosities in Europe. Its international fame came half a century later from art. Around 1740 the Dublin painter Susanna Drury made two careful views of the Causeway, which were engraved and circulated across Europe. In 1771 the French geologist Nicolas Desmarest, working only from those images, concluded that the columns had formed from molten lava.

That put the Causeway at the centre of one of the defining arguments of early geology. The Neptunists held that rocks such as basalt had precipitated from water; the Vulcanists held that they had solidified from melt. The Reverend Richard Pococke argued the Neptunist case from the Causeway from 1748, and the Reverend William Richardson took it up again in 1801. Against them, William Hamilton, in his letters on the Antrim coast published in 1786, read the same cliffs as volcanic. By the 1850s Charles Lyell could look back and wonder that the matter had ever been in doubt. The Vulcanists had won, and with that victory geology became a science of processes that can still be watched at work today.

The two kinds of explanation are less opposed than they first appear. Both begin from the same observation, that this order is too striking to be accidental, and both reach for a maker. Science simply found that the maker was heat, time and the physics of cracking stone.

Is the Giant’s Causeway connected to Fingal’s Cave in Scotland?

Geologically, yes, though not as a single continuous road. The columns of Fingal’s Cave on Staffa formed in the same way and in the same volcanic episode, as part of the North Atlantic Igneous Province around 60 million years ago. The 2026 survey work links the Antrim lavas even more firmly to Staffa and the rest of that province. The legend, in other words, joined two places that the rocks themselves also connect.

The Causeway in Northern Ireland’s coastal landscape

The famous stones are a small part of a much larger site. The UNESCO World Heritage property, inscribed in 1986 for both its natural beauty and its geological significance, takes in about six kilometres of coast between Causeway Head and Benbane Head. It was Northern Ireland’s first World Heritage Site, and most of it is owned and cared for by the National Trust. It belongs in any list of the island’s great natural places, alongside the sites gathered in our guide to Ireland’s gems.

At the foot of the cliffs, the Causeway divides into three low promontories: the Grand Causeway, the Middle Causeway and the Little Causeway. Around them the coast has its own named formations, among them the Wishing Chair, worn smooth by visitors, the tall columns of the Organ, and the Chimney Stacks on the headland beyond. Above, the cliffs read like a cross-section through the whole volcanic story, and the most useful thing to look for is colour. The rusty red band of the Interbasaltic laterite runs through the cliff faces, marking the long pause between eruptions.

That red band has its own history. It contains iron and aluminium ores that were mined along this coast, the aluminium ore notably during the First World War. For generations geologists also read the laterite as evidence of a hot, wet climate. A 2024 study of its sediments and fossil pollen proposed a different picture: a temperate land of estuaries, swamps, lakes and bogs not unlike the Ireland of today. The question remains open, which is a useful reminder that even a site studied for three centuries still has things to teach.

On the Treasures of Ireland itinerary, the Causeway comes on the last travelling day of the tour, after a visit to the Bushmills distillery, which sits about two miles inland. From here the coach turns towards Belfast, and the next day the subject changes from deep time to recent history, with a black taxi tour of the political murals of West Belfast and a visit to the Titanic Quarter, built on the ground of the Harland & Wolff shipyard. To see how this day sits within the whole clockwise route from Dublin, read our 18-day Ireland and Northern Ireland itinerary.

How much walking does a visit to the Giant’s Causeway involve?

Expect about 1.3 kilometres each way between the visitor centre and the stones on the National Trust’s Blue Trail, downhill on the way out and uphill on the return. The National Trust runs electric shuttle minibuses between the visitor centre and the stones for those who would rather not walk one or both directions. The stones themselves are uneven and can be slippery when wet, so sturdy shoes with good grip matter more here than almost anywhere on the tour. Entry is covered by the tour’s included entrance fees, and the day sits comfortably within its Level 2 activity rating.

Reading geological heritage respectfully

Visitors have been coming here for at least three hundred years, and the Causeway carries marks of their affection. In the past, columns and fragments were carried away as souvenirs, a practice long since stopped. The newer problem is coins. Visitors have taken to wedging coins into the cracks between columns, and a British Geological Survey report found that, soaked in salt spray, the coins corrode and swell to around three times their original thickness, forcing flakes off the surrounding basalt and leaving copper, nickel and iron stains on the stone. The National Trust has estimated that removing them will cost more than £30,000, and asks every visitor to leave no trace.

The other courtesy is simply to stay on the paths that are open. The cliffs here are still eroding, and sections of the lower path have been closed for years because of rockfall. The site’s management plan accepts that erosion, storms and rising seas will continue to change what can be reached and seen. That is not a threat to the Causeway’s meaning so much as a continuation of it: the coast that exposed the columns is the same coast that will one day wear them away.

There is respect owed to the story, too. Finn McCool is not a mistake that science corrected, but part of how people on both sides of the North Channel have made sense of the same stones for centuries, and the National Trust’s own guides tell both stories side by side. Standing on the columns, you can hold both at once: an Irish giant’s road to Scotland, and a slowly cooling lava lake on a coast that was being torn open by the birth of an ocean.

The Giant’s Causeway rewards a visitor who arrives knowing what to look for, which is why we place it near the end of a journey that has already crossed the limestone of the Burren and stood above the Atlantic at the Cliffs of Moher. The Treasures of Ireland tour travels in a small group of 6 to 14, solo travellers welcome, with local guides along the way and time to look properly at a coast that took sixty million years to make.