A team of geologists has detected a previously unknown layer in the middle of the Earth’s mantle, whose characteristics are reminiscent of those of the planet’s surface.
A new study published in the Journal of Science, authored by geophysicists Jessica Irving and Wenbo Wu of Princeton University, in collaboration with Sidao Ni of the Institute of Geodesy and Geophysics of China, describes how researchers used data from the seismic waves of a large earthquake in Bolivia to locate, at a depth of 660 kilometers, a new region inside the Earth, one that has left them speechless: it features a mountain range, and plains, very similar to those on the surface of our planet.
The technology depends on a fundamental property of waves: their ability to bend and bounce. Just as light waves can bounce (reflect) off a mirror or bend (refract) when passing through a prism, earthquake waves travel straight through homogenous rocks but reflect or refract when they encounter any boundary or roughness.
“We know that almost all objects have surface roughness and therefore scatter light,” said Wu, the lead author on the new paper, who just completed his geosciences Ph.D. and is now a postdoctoral researcher at the California Institute of Technology.
“That’s why we can see these objects — the scattering waves carry the information about the surface’s roughness. In this study, we investigated scattered seismic waves traveling inside the Earth to constrain the roughness of the Earth’s 660-km boundary.”
The researchers were surprised by just how rough that boundary is — rougher than the surface layer that we all live on.
As well as determining what is happening at the boundary, Irving adds: “What’s exciting about these results is that they give us new information to understand the fate of ancient tectonic plates which have descended into the mantle, and where ancient mantle material might still reside.”
By imaging regions below tectonically active areas such as Japan as well as more seismically benign areas including Antarctica, the research indicates that these slabs may eventually penetrate into the lower mantle.
What could cause significant chemical differences? The introduction of rocks that used to belong to the crust, now resting quietly in the mantle. Scientists have long debated the fate of the slabs of seafloor that get pushed into the mantle at subduction zones, the collisions happening found all around the Pacific Ocean and elsewhere around the world. Wu and Irving suggest that remnants of these slabs may now be just above or just below the 660-km boundary.
“It’s easy to assume, given we can only detect seismic waves traveling through the Earth in its current state, that seismologists can’t help understand how Earth’s interior has changed over the past 4.5 billion years,” said Irving.
“What’s exciting about these results is that they give us new information to understand the fate of ancient tectonic plates which have descended into the mantle, and where ancient mantle material might still reside.”
She added: “Seismology is most exciting when it lets us better understand our planet’s interior in both space and time.”