Asked by: Josh Philips, London
The process is called columnar jointing. It happens when a layer of molten lava gradually cools by a loss of heat through its surface. Stresses build up in this surface as it shrinks, eventually leading to a network of shrinkage cracks. The cracks tend to run in straight lines, forming a polygonal pattern as they cross over. It is not clear why most of those polygons turn out to be hexagons. What is clear is that the pattern propagates downwards into the material as it cools and solidifies, eventually producing polygonal columns which, in the Giant’s Causeway, have become exposed through weathering. The same pattern arises when starch gels dry out. In fact, this drying has been used as a model to study the way the Giant’s Causeway may have formed.