Why Is The Giant's Causeway Hexagonal?

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Robin Andrews 15 Oct 2015, 16:00

Have you ever been to the Giant’s Causeway in Northern Ireland? It’s a remarkable natural wonder: columns of hexagons, all reaching up into the sky right beside the coast. Although we mostly associate hexagons with honeycomb patterns in beehives, this shape can also be found across the world in lava formations, specifically columnar basalt, a common type of lava. Although volcanologists knew how these columns formed, it remained a mystery as to why they formed in hexagons, as opposed to pentagons or triangles or any other shape. Now, thanks to a team of researchers at the Dresden University of Technology, this mystery has been solved.

Columnar basalt is unlike normal basaltic lava as it cools. Most commonly effusing from volcanic vents of various types, basalt tends to either form a’a, a rubbly kind of lava, or pahoehoe, a stringy, ropey form of lava. Either way, both types start as lava emerging from a vent and eventually cooling a little further down the volcanic slope. The lava flows don’t cool at the same rate all the way through, however; while the top surface of the lava flows cool, the molten lava beneath – being thermally insulated from the atmosphere – takes a little longer.

Columnar basalt only crystallizes within a lava flow, slowly cooling in a more unusual way. When a small segment of the lava flow begins to cool from the inside, it contracts, and it fractures as it does so. When cooling rates are fairly uniform, with the heat from the lava escaping at regular intervals, it contracts and fractures fairly evenly, leading to tall, well-developed, generally hexagonal basalt columns.

The cooling rates of developing columnar basalt are often far from uniform, though. In many cases, the lava cools too rapidly, and smaller, irregular, glassy columns crystallize out, with pentagonal shapes, hexagonal shapes, and sometimes heptagonal shapes all precipitating.

Either way, hexagonal shaped-columns mostly tend to form, and the precise reason for why this morphology was preferred has remained a mystery. A new study published in the journal Physical Review Letters has solved this conundrum by creating a computer model to work it out.

When the researchers’ virtual lava began to cool evenly, much in the way natural subsurface lava cools, it began generating cracks at 90° to each other. As cooling continued, these cracks grew larger, forcing the angles to change to 120° – the same angle found in each corner of a hexagon.

As one column began to cool down to ambient temperatures, preserving its hexagonal shape, the same cooling process occurred around it. When one well-developed, symmetrical hexagonal column forms, therefore, many more tend to form around it, producing magnificent collections of columnar basalt like that seen at the Giant’s Causeway.

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