Giant's Causeway

Long, long ago, hot melted rock called lava poured out of the ground here.

The lava filled a deep hollow, like a huge pool. It cooled very slowly. As it cooled, it shrank and cracked into columns. Much later, ice and waves uncovered them.

About 60 million years ago, the land that is now Ireland was still joined to North America. As Europe began to pull away, huge cracks opened in the Earth's surface, and lava poured out and covered the land in layers of basalt.

After a long quiet time, the volcanoes erupted again. At the Causeway, lava collected in a hollow, making a lava lake about 90 meters deep. A deep pool of lava cools slowly and evenly. As it cooled, it shrank, and evenly spaced cracks spread through it, splitting it into thousands of columns, mostly six-sided and about 30 to 40 centimeters across. During the Ice Age, glaciers scraped away the rock on top, and later waves wore back the coast to uncover the columns.

National Museums NI's Earth Science Conservation Review (ESCR) traces the rocks to the break-up of Laurasia as the North Atlantic opened: Northern Ireland's environment department, DAERA, relates the Causeway Coast's Lower, Interbasaltic and Upper Basalt Formations to the three main eruption series in the Atlantic between 53 and 65 million years ago. At least six Lower Basalt flows were followed by a long pause, tens or perhaps hundreds of thousands of years, in which weathering turned the top flow into red and gray laterite and lithomarge, the Interbasaltic Formation.

Eruptions then resumed. At the Causeway, ESCR describes lava accumulating as a lava lake about 90 m deep. Its chilled, slaggy base insulated the mass, which cooled slowly and evenly, developing regular stress patterns until evenly spaced contraction cracks created mainly six-sided columns 30 to 40 cm across. University of Liverpool experiments (Lamur and colleagues, Nature Communications, 2018) found that basalt fractures about 90 to 140 °C below the roughly 980 °C at which it crystallizes, so such columns form at about 840 to 890 °C: hot, but already solid. The National Trust adds that Ice Age glaciers stripped the upper layers and rising seas then exposed the columns. Sources describe the setting differently: ESCR places the lava lake in the bend of a river, the IUGS site listing in a paleovalley, and a 2021 paper by M. J. Simms, listed by IUGS, revisits the emplacement environment under the title 'Subsidence, not erosion'.

Long ago, scientists argued about this rock. Where did it come from?

Some said it came from the sea. Others said it came from volcanoes. The columns helped show that the volcano scientists were right.

More than 200 years ago, scientists argued about a rock called basalt. One group, the Neptunists, thought basalt had formed from minerals settling out of seawater. Another group, the Vulcanists, thought it came from volcanoes. The columns of the Giant's Causeway gave evidence that the Vulcanists were right: basalt is cooled lava.

People have been studying and drawing the Causeway for more than 300 years. In 1740 an artist from Dublin, Susanna Drury, painted it so well that engravings of her pictures, made in 1743, spread its fame across Europe, North America and East Asia.

The IUGS geoheritage listing explains that from the late 18th century the Causeway was at the center of a dispute over the origin of basalt. The Neptunists, led by Abraham Werner and locally by Rev. William Richardson, held that basalt precipitated from seawater (Richardson published in 1805); the Vulcanists, including Rev. William Hamilton (1786) and John Playfair, argued it was volcanic, an idea first proposed by Nicholas Desmarest. The Causeway's columns provided evidence for the Vulcanist view.

UNESCO says geological studies over 300 years have greatly contributed to the earth sciences, and that the succession of lavas in the cliffs has allowed a detailed analysis of Tertiary (now called Paleogene) events in the North Atlantic. ESCR records the cultural side: after many unsuccessful attempts, Susanna Drury convincingly drew the Causeway in a series of gouaches in 1740, and the Vivares engravings of 1743 spread them across Europe, North America and the Far East. The IUGS listing also notes a diverse assemblage of zeolite minerals distributed through the basalts.

The stones can be very slippery when they are wet.

Walk carefully with a grown-up and listen to the rangers. Stay back from the cliff edges, especially on windy days. Leave every stone where it is.

The National Trust warns that the causeway stones can be slippery and dangerous when wet, and asks visitors always to follow its rangers' advice. On the clifftop Red Trail, watch out for unfenced cliff edges, high winds, steep steps and falling rocks; the Shepherd's Steps are 162 steep stone steps with a handrail. The Trust calls the Grand Causeway outcrop extremely dangerous, particularly in high winds. A bus also uses the path down to the stones, so walkers must stay on the path when it runs.

In the past, visitors took stones home as souvenirs. UNESCO notes that this stopped long ago, and the site is now protected by law. Look, photograph and leave every stone in place.

The National Trust's trail pages warn that the causeway stones can be slippery and dangerous when wet, that occasional rock falls happen, and that visitors should always heed its rangers. The clifftop Red Trail has unfenced cliff edges, high winds and the 162 steep Shepherd's Steps; the Trust calls the Grand Causeway outcrop and the unfenced Aird headland extremely dangerous, particularly in high winds, and urges due caution. Pedestrians must keep to the path when the shuttle bus runs.

UNESCO notes that the removal of souvenir stones, which happened before the area was protected, has long since ceased, and that damage from human impact is controlled by law and National Trust management. ESCR adds that geological sampling by hammering is completely forbidden along this coast. UNESCO's management plan expects natural erosion to keep altering the cliffs, so paths may need to move, and says rising sea levels or more frequent storms may in future affect how accessible or visible the Causeway is.