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.