Hot rocks Giant’s Cause­way rid­dle solved with a lit­tle Ice­landic help

The Guardian Weekly - - Uk News - Han­nah Devlin Pho­to­graph: Marco Bot­tigelli/Getty

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 of the study, said: “[This] is a ques­tion that has fas­ci­nated the world of ge­ol­ogy for a very long time. We have been want­ing 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 840C-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.” He hopes to ex­tend his in­quiry by us­ing a large pool of magma to re­pro­duce the geo­met­ric frac­tur­ing as rock is cooled – al­though he said this would have to be under con­trolled con­di­tions.

The Giant’s Cause­way formed be­tween 50m and 60m 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, in­clud­ing Devils Post­pile in Cal­i­for­nia, and the pat­tern oc­curs on many scales as faster cool­ing pro­duces smaller col­umns.

The find­ings are pub­lished in the jour­nal Na­ture Com­mu­ni­ca­tions.

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