At school, we are thought that Earth is divided into three layers, which are the cortex, the mantle and the nucleus, which in turn is divided into an internal and external nucleus.
A basic and accurate scheme, but that nevertheless leaves out other subtler layers that scientists are now beginning to identify in the deep interior of our planet.
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.
And to be able to peer deep inside the planet, researchers had to make use of the most powerful waves that exist on our planet, the seismic waves generated by massive earthquakes.
“You want a big, deep earthquake to get the whole planet to shake,” said Irving, an assistant professor of geosciences.
For this particular study, the key data were obtained from seismic waves captured after an 8.2 magnitude earthquake, the second most powerful ever recorded, which shook Bolivia in 1994.
“Earthquakes this big don’t come along very often,” Irving said.
“We’re lucky now that we have so many more seismometers than we did even 20 years ago. Seismology is a different field than it was 20 years ago, between instruments and computational resources.”
But data alone is nothing if you don’t know how to make use of it.
That’s why scientists used the group of Tiger supercomputers from Princeton University to simulate the complex behavior of scattered seismic waves in the depths of the Earth.
The technology applied for this analysis depends almost entirely on a single property of the waves: its ability to bend and bounce.
Thus, in the same way, that light waves can bounce (reflect) in a mirror or bend (refract) when they pass through a prism, seismic waves travel directly through homogeneous rocks but are reflected or refracted when finding limits 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.”
Scientists were left stunned by the roughness of the boundary. As they explain, its rougher than the surface layer we live on.
“In other words, stronger topography than the Rocky Mountains or the Appalachians exist at the 660-km boundary, explained Wu.
While the new study undoubtedly describes one of the most sensational discoveries made beneath our feet, their statistical model doesn’t offer much insight that could allow precise height determinations.
Nonetheless, scientists say that there’s a chance that some of these underground mountains are larger than anything we’ve seen on the surface of the planet.
They say that the roughness wasn’t equally distributed, either. According to scientists, just as the crust’s surface has smooth ocean floors and massive mountains, the 660-km boundary beneath our feet has rough areas and smooth patches.