The Earth has vast mountain ranges beneath the surface, with peaks up to four times the height of Everest. But no one knows why, the BBC reports.
A team of researchers installed 15 seismological stations in Antarctica in 2015, half buried in snow, to study the interior of the planet.
The mountain-like structures they discovered using these stations are mysterious. But the research team found that these “ultra-low-velocity zones,” or ULVZs, as they’re known, are likely ubiquitous — wherever you are in the world, they’re deep beneath your feet.
“We found evidence for ULVZs just about everywhere we looked,” says researcher Samantha Hansen.
The question is: what are they and what is the interior of our planet looking for?
Earth’s mysterious inner mountains appear at a critical threshold: that between the planet’s metallic core and the rocky mantle surrounding it. This sudden transition is, as Hansen’s team points out, even more drastic than the change in physical properties between solid rock and air. For decades they have attracted experts, because they are enigmatic but also important for the geology of the planet.
Although the core-mantle boundary is thousands of kilometers from the Earth’s surface, it has effects all the way to the surface. It could be the source of the existence of volcanoes in unexpected locations, for example.
The story of the mountains in Earth’s interior began in 1996, when scientists explored the core-mantle boundary, far beneath the central Pacific Ocean. They did this by studying the seismic waves created by massive earth-shaking events – usually earthquakes, although nuclear bombs can have the same effect. These waves pass right through the Earth and can be picked up by seismic stations at other locations on its surface, sometimes more than 12,742 km away from where they started.
By examining the paths the waves take as they travel—such as how they are refracted by different materials—scientists can create an X-ray-like picture of the planet’s interior.
Meanwhile, similar mountains have been found in several places around the core. Some are particularly large: one of them occupies an area of 910 km below Hawaii.
However, to this day, no one knows how they got there or what they are made of.
One possibility is that the mountains are parts of the lower mantle that have been superheated due to their proximity to Earth’s incandescent core. While the mantle can reach 3,700C, the core can reach 5,500C – not far from the surface temperature of the Sun. It is suggested that the hottest parts of the core-mantle boundary may partially melt – and this is what geologists see as the ULVZ.
Another hypothesis is that the mountains deep in the Earth could be made of a different material than the surrounding mantle. Although it seems incredible, it is believed that it could be the remains of an ancient oceanic crust that disappeared into the depths, sinking over hundreds of millions of years to settle just above the core.
In the past, geologists have looked to a second puzzle for clues. Deep-Earth mountains tend to be found near other mysterious structures: enormous bubbles – one called “Tuzo” under Africa, and another known as “Jason” under the Pacific. They are believed to be primitive formations, possibly billions of years old. Again, no one knows what they are or how they got there. But their proximity to the mountains has led to the hypothesis that there is a connection between the two bizarre formations.
One way to explain this association is that it really all started when tectonic plates slid down into Earth’s mantle and sank down to the core-mantle boundary. This would mean that both types of formations are made of ancient oceanic crust: a combination of basalt rock and ocean floor sediments, transformed by intense heat and pressure.
But the existence of interior mountains under Antarctica could contradict this, suggests Hansen.
“Most of our study region, the Southern Hemisphere, is quite far from those larger structures,” she explains.
Editor : M.B.