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

Analysis of rock from deep beneath the ground reveals that the UK 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 100km, to understand the makeup of the rock that underpins the south-west of the country. They found that a distinct compositional border divides Cornwall and south Devon from the rest of the nation.

The tests show that the region of Britain to the 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 the southwestern tip of Britain to the continent for evermore.

“When we started looking at the rocks it was very clear that there were two groups,” said Arjan Dijkstra, a lecturer in igneous petrology 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. The roots of these rocks are 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 more than 400m years ago. The northernmost landmass, called Laurentia, bore what is modern-day Scotland, while the southernmost landmass, 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 somewhere beneath the Channel. The latest tests show that the collision actually happened farther north, with parts of Armorica providing the geological foundations from Tavistock down to Penzance.

Dijkstra now compares the formation of Britain to a rather messy three-way car crash. The first landmasses to collide were Avalonia and Laurentia, giving rise to what is now most of 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 on, it advanced again, and crunched into the landmass 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 his master’s student Callum Hatch, Dijkstra visited 22 sites across Devon and Cornwall where ancient volcanic eruptions had brought molten mantle from beneath Earth’s crust 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 the journal Nature Communications, go some way to explain why Cornwall and Devon have an abundance of tin and tungsten, metals found in Brittany and other areas of 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, and provides a fascinating new insight into the geological history of the UK.”