According to legend, the Giant’s Causeway was built by the Irish giant, Finn MacCool, as a crossing to confront his Scottish rival. Scientists have an alternative explanation, and for the first time, they have reproduced in the laboratory the process through which the causeway’s 40,000 near-perfect hexagonal columns were formed.
Geometric columns are seen in a variety of volcanic rocks across the Earth and are known to form as the rock cools and contracts, resulting in a regular array of polygonal prisms or columns. But until now, geologists had been unsure of the threshold at which cooling magma suddenly fractures into a geometric pavement. Yan Lavallée, professor of volcanology at the University of Liverpool and lead author, said: “[This] is a question that has fascinated the world of geology for a very long time. We have wanted to know whether the temperature of the lava that causes the fractures was hot, warm or cold.”
To answer the question, Lavallée and colleagues recreated the process in the laboratory using basalt cores drilled from the Eyjafjallajökull volcano in Iceland. The 20cm-long cylinders, gripped by a clamp at each end, were heated to more than 1,000C until they began to soften into lava. The samples were fixed at each end in a mechanical grip and cooled to test at what point they snapped. The basalt magma fractured at between 840-890C, the study found, suggesting that this is the temperature at which the Giant’s Causeway would have formed. “I have spent over a decade pondering how to address this question and construct the right experiment to find the answer to this question,” said Lavallée. “Now, with this study, we have found that the answer is hot, but after it solidified.” In the future, he hopes to extend the investigation by using a large pool of magma to reproduce the geometric fracturing as the rock is cooled – although he said such an experiment would need to be performed under carefully controlled conditions.
The Giant’s Causeway formed between 50 and 60 million years ago when the region that now sits on the Antrim coastline was subject to intense volcanic activity. Molten basalt erupted through chalk beds and formed a lake of lava. As this cooled and contracted, cracks propagated across the plateau to form hexagonal stepping stones. Similar geological structures are seen elsewhere in the world, including Devils Postpile in the US, and the pattern occurs on many scales because faster cooling produces smaller columns.
The findings are published in the journal Nature Communications.
Credit: Hannah Devlin for The Guardian, 12 April 2018.