Earth CT scan links deep mantle plumes with volcanic hotspots

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Published: September 9, 2015 3:30:17 PM

Scientists have for the first time produced sharp, 3D scan of Earth's interior that conclusively connects plumes of hot rock rising through the mantle with surface hotspots that generate volcanic island chains like Hawaii, Samoa and Iceland.

Scientists have for the first time produced a sharp, 3D scan of Earth’s interior that conclusively connects plumes of hot rock rising through the mantle with surface hotspots that generate volcanic island chains like Hawaii, Samoa and Iceland.

While medical computed tomography, or CTs, employ X-rays to probe the body, the scientists mapped mantle plumes by analysing the paths of seismic waves bouncing around Earth’s interior after 273 strong earthquakes that shook the globe over the past 20 years.

Previous attempts to image mantle plumes have detected pockets of hot rock rising in areas where plumes have been proposed, but it was unclear whether they were connected to volcanic hotspots at the surface or the roots of the plumes at the core mantle boundary 2,900 kilometres below the surface.

The new, high-resolution map of the mantle not only shows these connections for many hotspots on the planet, but shows that below about 1,000 kilometres the plumes are between 600 and 1,000 kilometres across, up to five times wider than geophysicists thought.

The plumes are likely at least 400 degrees Celsius hotter than surrounding rock.

“No one has seen before these stark columnar objects that are contiguous all the way from the bottom of the mantle to the upper part of the mantle,” said first author Scott French, scientist at the US National Energy Research Scientific Computing Center (NERSC) who recently received his PhD from University of Calfornia, Berkeley.

Senior author Barbara Romanowicz, a UC Berkeley professor, noted that the connections between the lower-mantle plumes and the volcanic hotspots are not direct because the tops of the plumes spread out like the delta of a river as they merge with the less viscous upper mantle rock.

“These columns are clearly separated in the lower mantle and they go all the way up to about 1,000 kilometres below the surface, but then they start to thin out in the upper part of the mantle, and they meander and deflect,” she said.

“So while the tops of the plumes are associated with hotspot volcanoes, they are not always vertically under them, she said.

The new picture also shows that the bases of these plumes are anchored at the core-mantle boundary in two huge blobs of hot rock, each about 5,000 kilometres in diameter, that are likely denser than surrounding rock.

Romanowicz estimates that those two anchors – directly opposite one another under Africa and the Pacific Ocean – have been in the same spots for 250 million years.

Seismologists proposed some 30 years ago that stationary plumes of hot rock in the mantle occasionally punched through the crust to produce volcanoes, which, as the crust moved, generated island chains such as the Galapagos, Cape Verde and Canary islands.

Until now, evidence for this theory had been circumstantial, and some had argued that hotspots are shallow pools of hot rock feeding magma chambers under volcanoes.

The study was published in the journal Nature.

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