What do the rumblings of Iceland's volcanoes have in common with the now peaceful volcanic islands off Scotland's western coast and the spectacular basalt columns of the Giant's Causeway in Northern Ireland?
About sixty million years ago, the Icelandic mantle plume—a fountain of hot rock that rises from Earth's core-mantle boundary—unleashed volcanic activity across a vast area of the North Atlantic, extending from Scotland and Ireland to Greenland.
For decades, scientists have puzzled over why this burst of volcanism was so extensive. Now, research led by the University of Cambridge has found that differences in the thickness of tectonic plates around the North Atlantic might explain the widespread volcanism.
The researchers compiled seismic and temperature maps of Earth's interior, finding that patches of thinner tectonic plate acted like conduits, funnelling the plume's molten rock over a wide area.
Iceland , which is one of the most volcanically active places on Earth, owes its origin largely to the mantle plume. Beyond volcanism, the Iceland Plume's influence even extends to shaping the seafloor and ocean circulation in the North Atlantic and, in turn, climate through time. Despite its global significance, many aspects of the plume's behaviour and history remain elusive.
"Scientists have a lot of unanswered questions about the Iceland plume," said Raffaele Bonadio , a geophysicist at Cambridge's Department of Earth Sciences and lead author of the study.
Bonadio set out to explain why the plume's volcanic imprint was much more widespread sixty million years ago—before the Atlantic opened—forming volcanoes and lava outpourings stretching over thousands of kilometres. The pattern could be explained by the mantle plume spreading outward in a branched, flowing formation, Bonadio explained, "but evidence for such flow has been scarce."
In search of answers, Bonadio focussed on a segment of the North Atlantic Igneous Province to better understand the complex distribution of volcanoes in Scotland and Ireland. He wanted to know if the structure of Earth's tectonic plates played a role in the surface expression of volcanism.
Using seismic data extracted from earthquakes, Bonadio created a computer-generated image of Earth's interior beneath Britain and Ireland. This method, known as seismic tomography, works similarly to a medical CT scan, revealing hidden structures deep within the planet. Bonadio coupled this with seismic thermography measurements—a new method developed by the team—which reveal variations in the temperature and thickness of the tectonic plate.
He found that northwest Scotland and Ireland's volcanoes formed in areas where the lithosphere (Earth's rigid outer layer that makes up the tectonic plates) is thinner and weaker.
"We see ancient volcanoes concentrated within this corridor of thin lithosphere beneath the Irish Sea and surrounding areas," said Bonadio. He thinks the hot plume material was preferentially funnelled along this corridor, ponding in the thin plate areas due to its buoyancy.
Previously, some scientists had put forward alternative, non-mantle plume origins for the volcanic activity, said Bonadio. But his new research shows the scattering could be explained by the magma being diverted and re-routed to areas of thinner lithosphere.
Sergei Lebedev, from the University of Cambridge said, "this striking correlation suggests that hot plume material eroded the lithosphere in this region. This resulting combination of thin lithosphere, hot asthenosphere and decompression melting likely caused the uplift and volcanic activity."
Previously, the authors have found a close link between the uneven distribution of earthquakes in Britain and Ireland and the thickness of the lithosphere, showing how the scars left by the mantle plume influence seismic hazards today.
Bonadio and Lebedev are also using their methods to map geothermal energy resource potential. "In Britain and Ireland, the greatest supply of heat from the Earth's mantle is in the same places where volcanoes erupted sixty million years ago, and where the lithosphere is thinner," said Lebedev. He and Bonadio are working with international colleagues to apply their new seismic thermography methods to global geothermal assessment.
