The great di­vide

Corn­wall is French relic of ge­o­log­i­cal rift

The Guardian - - NATIONAL - Ian Sam­ple Science ed­i­tor

With what can only be de­scribed as un­for­tu­nate tim­ing, re­searchers have dis­cov­ered that there is a cor­ner of Bri­tain that will for­ever be­long to Europe.

Anal­y­sis of rock from deep be­neath the ground re­veals that Bri­tain only ac­quired Corn­wall and parts of south Devon when it was struck by the land­mass bear­ing what is now France some hun­dreds of mil­lions of years ago.

Ge­ol­o­gists at Ply­mouth Uni­ver­sity ran chem­i­cal tests on so­lid­i­fied magma that welled up long ago from a depth of 60 miles, to un­der­stand the makeup of the rock that un­der­pins the south­west of the coun­try. They found that a dis­tinct com­po­si­tional bor­der di­vides Corn­wall and south Devon from the rest of the na­tion.

The tests show the re­gion of Bri­tain south of Camelford and the Exe es­tu­ary lies on the same an­cient land­mass that sits un­der France and much of the rest of Europe, ty­ing Bri­tain’s south­west­ern tip to the con­ti­nent for­ever.

“When we started look­ing at the rocks it was very clear there were two groups,” said Dr Ar­jan Di­jk­stra, a lec­turer at Ply­mouth. “Only the rocks to the north were what we ex­pected for Bri­tain. To the south, they were iden­ti­cal to rocks found in France … French to the mi­nut­est de­tail.”

Di­jk­stra said the dis­cov­ery means we have to re­think how the Bri­tish Isles formed. Un­til now, ge­ol­o­gists be­lieved the isles came to be when two land­masses crashed to­gether 400m years ago. The north­ern­most land­mass, called Lau­ren­tia, bore what is now Scot­land and the south­ern­most, Aval­o­nia, bore Eng­land and Wales.

Re­searchers knew a third land­mass, Ar­mor­ica, was also in­volved about 100m years later, but this slab of Euro­pean rock was thought to have crunched into Aval­o­nia be­neath the Chan­nel. The lat­est tests show the col­li­sion ac­tu­ally hap­pened far­ther north, with parts of Ar­mor­ica pro­vid­ing the ge­o­log­i­cal foun­da­tions from Tav­i­s­tock down to Pen­zance.

Di­jk­stra now likens the for­ma­tion of Bri­tain to a messy three-way car crash. The first to col­lide were Aval­o­nia and Lau­ren­tia, giv­ing rise to what is now most of Great Bri­tain. Ar­mor­ica may then have crashed into Aval­o­nia from the south, only to back away and leave the ge­o­log­i­cal equiv­a­lent of its bumper be­hind. Later, it ad­vanced again and crunched into Aval­o­nia once more.

“This is a com­pletely new way of think­ing about how Bri­tain was formed,” said Di­jk­stra. “It has al­ways been pre­sumed that the bor­der of Aval­o­nia and Ar­mor­ica was be­neath what would seem to be the nat­u­ral bound­ary of the English Chan­nel. But our find­ings sug­gest that, al­though there is no phys­i­cal line on the sur­face, there is a clear ge­o­log­i­cal bound­ary that sep­a­rates Corn­wall and south Devon from the rest of the UK.”

Work­ing with an MA stu­dent, Callum Hatch, Di­jk­stra vis­ited 22 sites in Devon and Corn­wall where an­cient vol­canic erup­tions brought molten man­tle to the sur­face. The pair took rock sam­ples from each of the sites and com­pared the chem­i­cal makeup with mea­sure­ments taken from sim­i­lar ma­te­rial across the UK and Europe.

The find­ings, re­ported in Na­ture Com­mu­ni­ca­tions, ex­plain why Corn­wall and Devon have an abun­dance of tin and tung­sten, met­als found in Brit­tany and else­where in main­land Europe. “These min­er­als come from deep in the crust,” Di­jk­stra said.

“We al­ways knew that around 10,000 years ago you would have been able to walk from Eng­land to France. But our find­ings show that mil­lions of years be­fore that, the bonds be­tween the two coun­tries would have been even stronger. It ex­plains the im­mense min­eral wealth of south-west Eng­land, which had pre­vi­ously been some­thing of a mys­tery.”

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