A Coast Paved With 40,000 Stone Columns, and No Giant Needed

On the north coast of Northern Ireland, at the foot of green cliffs, the land runs into the sea as a staircase of grey stepping stones. Each one is the flat top of a stone column, most of them with six neat sides, packed together so tightly that they look as if a very patient tiler laid them. Some are low enough to sit on, others are stacked high in the cliffs, and the rows march straight down into the waves as if the path carries on under the water.

This is the Giant's Causeway, and it is made of more than 40,000 columns of basalt. The tops are mostly about 30 to 40 centimetres across, a comfortable size for hopping from one to the next, which is exactly what visitors do on a sunny afternoon.

What people often get wrong

The old story says a giant built it. In the local legend, Fionn mac Cumhaill, better known as Finn McCool, laid the stones as a road across the sea. It is a wonderful tale, and the British Geological Survey mentions it with a wink before giving the real answer: the causeway was never built at all. Nobody cut those edges and nobody fitted the stones together. The rock cracked into this shape all by itself.

How the columns formed

About 60 million years ago, at the start of the Palaeogene period, this part of the world was volcanic. Runny lava spread over the land, and at the Causeway it gathered in a large lava lake some 90 metres deep, according to the British Geological Survey. A body of lava that size cools slowly and evenly. As rock cools it shrinks, and as it shrinks it pulls itself apart, much like mud cracking in a drying puddle.

Because the cooling was so even, the stresses were even too, and the cracks settled into a regular network that split the rock into mostly six-sided pieces. As the cold crept deeper into the lava, the cracks followed it downwards, slicing the whole mass into tall columns, rather like a cake cutter pressed slowly through a very thick cake.

• 40,000+ basalt columns
• 60 million years since the lava lake cooled
• 90 m depth of that lava lake
• 30-40 cm width of a typical column top

How we know, from a bowl of cornflour

Nobody can watch a lava lake cool for years, but anyone can watch a bowl of cornflour dry. In a study published in the Proceedings of the National Academy of Sciences in 2009, the physicists Lucas Goehring, L. Mahadevan and Stephen Morris mixed equal weights of cornstarch and water, dried it under heat lamps, and watched it crack into small, roughly hexagonal columns, a causeway that fits on a table. They found that speed sets the size: faster drying makes thinner columns, and in the same way, slower cooling makes wider ones in lava. The rule they worked out also fitted the measurements they took from real columns of cooled lava.

So the stepping stones on that shore are a record of how slowly a lake of molten rock once cooled. The giant makes a better bedtime story, but the real builder was patience, and the same rule is at work in a bowl of cornflour and water drying quietly under a lamp.