We know about the crust under the oceans and continents, the mantle that goes on for 2,900 kilometers (1,800 miles), and the iron core. Now, Earth’s got a new layer. By crushing minerals between anvils made of diamonds, researchers have discovered that the top part of the lower mantle contains an incredibly stiff layer of rock. The findings were published in Nature Geoscience this week.
The crust and some of the upper mantle form tectonic plates: Oceanic plates smash up against continental plates offshore from places like Chile, the Pacific Northwest, Kamchatka, and Japan. There, one edge of the oceanic plate bends into a slab that dives under the continent, descending into the mantle and triggering earthquakes and volcanic activity. This subduction process takes hundreds of millions of years.
Geologists have long puzzled over why slabs of tectonic plates sometimes stall as they subduct—thickening and even pooling as they reach about 1,500 kilometers (930 miles) underground. To investigate this “slab stagnation,” Hauke Marquardt of GFZ German Research Centre for Geosciences and University of Utah’s Lowell Miyagi squeezed the mineral ferropericlase (common in the mantle) between diamond anvils to pressures like those in Earth’s lower mantle. That’s about 640,000 times the air pressure on the surface (not to mention about 2,150 degrees Celsius).
The strength of ferropericlase starts to increase at pressures like those at the boundary of the upper and lower mantle about 660 kilometers (410 miles) deep. And its strength increases threefold by the time it peaks at pressures found at depths of 1,500 kilometers in the lower mantle.
To the right, Miyagi holds up a press that houses a diamond anvil. X-rays from an accelerator at Lawrence Berkeley National Laboratory measured the strength of the mineral at various pressures.
Then they simulated the behavior of ferropericlase as it mixes with another mantle mineral, called bridgmanite, in the lower mantle. The stiffness (or viscosity) of the mantle rock at that depth is 300 times greater than at the upper-lower mantle boundary. This increase in viscosity likely causes subducting slabs to become temporarily stuck around 1,500 kilometers belowground.
“The Earth has many layers, like an onion,” Miyagi says in a news release. “Most layers are defined by the minerals that are present. Essentially, we have discovered a new layer in the Earth. This layer isn’t defined by the minerals present, but by the strength of these minerals.”
For comparison, on the pascal-second scale, the viscosity of water is 0.001, peanut butter is 200, and the stiff mantle layer is 1,000 billion billion (or 10 to the 21st power), Miyagi says. Their findings indicate that the slabs sink more easily again into the core-mantle boundary below that 1,500-kilometer-deep zone of highest viscosity.
A stiff mantle layer helps explain some deep earthquakes high up in the mantle, suggests that Earth’s interior is hotter than we believed, and accounts for why different seafloor volcanoes have different magmas.
Images: Lowell Miyagi, University of Utah (top), Lee J. Siegel, University of Utah (middle)