Giants Causeway
The Giant's Causeway on the north Antrim coast of Northern Ireland is the most visited tourist attraction in Ireland and one of the most spectacular geological formations in the world, a UNESCO World Heritage Site where approximately forty thousand interlocking basalt columns formed by the slow cooling of volcanic lava approximately sixty million years ago create a pavement of extraordinary geometric regularity extending from the base of the sea cliffs into the Atlantic Ocean. The columns are predominantly hexagonal in cross-section, ranging from a few centimetres to over forty centimetres across, and their regular, almost architectural form seems more like deliberate construction than geological process.
The volcanic episode that created the Causeway was part of the great North Atlantic rifting event of the Eocene period, when enormous quantities of basalt lava erupted across what is now northeastern Ireland and western Scotland, creating formations that appear at the Giant's Causeway, on the island of Staffa in Scotland and at several other locations along what was once a continuous volcanic landscape. As the thick lava flows cooled slowly from the top and bottom simultaneously, contraction fractures propagated inward through the cooling rock, intersecting in the polygonal pattern that reflects the most efficient packing of cracks in a homogeneous medium. The geometry is the same reason soap bubbles pack in hexagonal arrangements.
The legend of Fionn mac Cumhaill, the Irish giant who built the Causeway to reach his Scottish rival Benandonner, is one of Ireland's most famous myths and provides an entirely satisfying alternative explanation for a formation that does appear almost impossibly constructed. The story connects the Causeway to the identical formation at Fingal's Cave on Staffa, a geological relationship that the legend understood intuitively without recourse to volcanology.
The National Trust visitor centre provides excellent interpretation and the clifftop coastal path gives access to spectacular views of the Causeway from above.