A Pavement Forged by Fire
Around 60 million years ago, during the Paleocene Epoch, the land that is now County Antrim was a scene of intense volcanic activity. This was part of a larger event tied to the opening of the North Atlantic Ocean, which created a vast volcanic region called the Thulean Plateau. Highly fluid, iron- and magnesium-rich basaltic lava flowed across the landscape, erupting through chalk beds and filling ancient valleys.
The Giant's Causeway formed of one of these lava flows ponding in a valley and cooling slowly and evenly. As the molten rock lost heat to the atmosphere, its surface cooled faster than its interior, causing it to contract and develop tensile stress. This stress was relieved by cracking. The most efficient pattern for relieving this stress across a flat surface is a series of fractures at 120-degree angles, which forms hexagons. These cracks propagated downwards as the lava continued to cool from the top, creating an immense field of approximately 40,000 interlocking columns. Recent laboratory experiments, using basalt cores heated to over 1,800 degrees Fahrenheit, suggest the fracturing process begins when the lava cools to between 1,544 and 1,634 degrees Fahrenheit.
A Landscape of Geometric Giants
The columns are the best-known feature, but the geology of the Causeway Coast shows a layered history. The visible columns belong to the Middle Basalts. These are sandwiched between the Lower and Upper Basalts, which are separated by a reddish layer called the Interbasaltic Formation. This red band is a fossil soil, high in iron and aluminum, formed during a long dormant period between eruptions when the land was weathered by the elements.
While most of the columns are hexagonal, pillars with four, five, seven, and even eight sides can also be found. The tallest columns stand about 12 meters (39 feet) high, and in some places, the total thickness of the solidified lava in the cliffs reaches 28 meters (92 feet). Many columns exhibit horizontal fractures that create convex and concave surfaces, fitting together like a ball-and-socket joint. Over millennia, weathering has carved the columns into shapes. Visitors can spot formations like the "Giant's Boot," a basalt block shaped like a massive shoe, and the "Chimney Stacks," which are tall columns standing isolated from the main cliffs.