Ancient Earth's hot interior caused tectonic plates to sink, scientists say

By Amy Wallace  |  Aug. 22, 2017 at 4:37 PM
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Aug. 22 (UPI) -- Scientists at MIT have found that ancient Earth had a viscous mantle that was 200 degrees Celsius hotter than present day.

The study, published today in Earth and Planetary Science Letters, found that the Earth's ancient crust was made up of a much denser, iron- and magnesium-enriched material than today's rocky mantle.

"We find that around 3 billion years ago, subducted slabs would have remained more dense than the surrounding mantle, even in the transition zone, and there's no reason from a buoyancy standpoint why slabs should get stuck there. Instead, they should always sink through, which is a much less common case today," Benjamin Klein, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences, said in a press release.

"This seems to suggest there was a big change going back in Earth's history in terms of how mantle convection and plate tectonic processes would have happened."

Subduction is the process where two of Earth's massive tectonic plates collide, causing one to slide under the other. A hotter and denser crust caused subducting plates to sink all the way to the bottom of the mantle, 2,800 kilometers below the surface, forming a "graveyard" of slabs on top of the Earth's core, researchers say.

To make the finding, the researchers compiled a large dataset of more than 1,400 previously analyzed samples of both modern rocks and komatiites, rock types that existed 3 billion years ago but no longer exist today.

The team then used the composition of each rock sample to calculate the density of a typical subducting slab for modern day and 3 billion years ago. They used a thermodynamic model to determine the density profile of each subducting slab.

"Today, when slabs enter the mantle, they are denser than the ambient mantle in the upper and lower mantle, but in this transition zone, the densities flip," Klein said. "So within this small layer, the slabs are less dense than the mantle, and are happy to stay there, almost floating and stagnant."

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