The mysterious mountain ranges in the depths of the Earth – 06/18/2023 – Science

It was a bright summer day in Antarctica. With frozen lashes, Samantha Hansen looked at the amorphous landscape: a white wall, where the top was the same as the bottom, and the floor blended perfectly with the sky.

Amid these bewildering conditions, with temperatures hovering around -62°C, she identified a suitable spot in the snow and grabbed a shovel.

Hansen was in the inhospitable interior of the white continent. Not in the picturesque — and slightly warmer — Antarctica that cruise excursions land in, but in an unforgiving environment where not even wildlife ventures.

As part of a team from the University of Alabama and Arizona State University, both in the United States, she was looking for hidden “mountain” ranges: peaks that no explorer has ever trod, that sunlight has never illuminated.

These mountains are located deep within the Earth.

It was 2015, and researchers were in Antarctica to set up a seismological station —equipment partially buried in the snow that makes it possible to study the interior of our planet. In all, the team installed 15 across Antarctica.

The mountain-shaped structures they revealed are completely mysterious. But Hansen’s team found that these ultra-low velocity zones, or ULVZs as they’re known, are probably pretty common too.

“We found evidence of ULVZ everywhere,” says Hansen.

The question is: what are they? And what are they doing inside our planet?

a mysterious story

The strange mountains in the Earth’s interior appear in a critical place, between the planet’s metallic core and the rocky mantle that surrounds it.

This abrupt transition is, as Hansen’s team claims, even more drastic than the change in physical properties between solid rock and air. And it has intrigued experts for decades.

Although this “boundary” between the core and the mantle is thousands of kilometers from Earth’s surface, there is a surprising amount of exchange between its depths and our own world.

It’s thought to be a sort of burial ground for ancient chunks of the ocean floor — and it may even be behind volcanoes in unexpected places like Hawaii by creating superheated pathways to the crust.

Detecting mountains deep in the Earth began in 1996, when scientists explored the boundary between the core and mantle deep beneath the central Pacific Ocean.

They did this by studying the seismic waves created by large earthquake events: usually earthquakes, although nuclear bombs can have the same effect.

These waves traverse the Earth and can be picked up by seismological stations elsewhere on the surface—sometimes more than 12,742 km away from where they originated.

By analyzing the paths the waves take as they travel, as they are refracted by different materials, scientists can piece together a picture similar to X-ray images of the planet’s interior.

When the researchers looked at the waves generated by 25 earthquakes, they found that they inexplicably slowed down when they hit a jagged patch at the boundary between the core and the mantle.

What can they be?

This vast mountain range, which seemed from another world, was very variable: some peaks reached 40 km towards the mantle, which is equivalent to 4.5 times the height of Everest; others were only 3 km high.

Since then, similar mountains have been found in scattered locations around the core. Some are particularly large—one monstrous specimen occupies a 910km-wide area beneath Hawaii.

However, to this day, no one knows how they got there or what they are made of.

One theory is that the mountains are parts of the lower mantle that have become superheated due to their proximity to the Earth’s glowing core.

The mantle can reach 3,700°C, but this temperature is relatively low: the core can reach maximums of 5,500°C, not far from the surface temperature of the Sun.

It has been suggested that the hottest parts of the core-mantle boundary may partially melt, and this is what geologists see as the ULVZ.

Another theory indicates that mountains deep within the Earth may be made of material that is subtly different from the surrounding mantle.

It is believed that they could be the remnants of ancient oceanic crust that disappeared in its depths, eventually sinking over hundreds of millions of years to settle just above the core.

In the past, geologists looked for clues in a second puzzle.

Mountains deep in the Earth tend to be found near other mysterious structures: massive blobs or Large Low Shear Velocity Provinces (LLSVP).

There are only two: an amorphous bulge called “Tuzo”, below Africa, and another known as “Jason”, below the Pacific.

They are believed to be truly primitive, possibly billions of years old. And no one knows what they are or how they got there, but their proximity to the mountains suggests they are connected in some way.

One way to explain this association is that it actually started with tectonic plates sliding into the Earth’s mantle and sinking to the boundary between the core and the mantle. Then they slowly spread out to form a variety of structures, leaving a trail of mountains and bubbles.

If so, that would mean that both are made of ancient oceanic crust: a combination of basaltic rock and sediment from the ocean floor, yet transformed by intense heat and pressure.

But the existence of deep mountains beneath Antarctica could contradict this, suggests Hansen.

“Most of our study region, the southern hemisphere, is quite far from these larger structures.”

an icy quest

To set up their seismological stations in Antarctica, Hansen and his team flew to the appropriate locations in helicopters and small planes, placing equipment in the snow, some close to shore, under the curious gaze of penguins and otters.

It only took a few days for them to get the first results.

Instruments can detect earthquakes almost anywhere on the planet. “If it’s big enough, we can see it,” says Hansen, and there are plenty of opportunities like that.

The US National Earthquake Information Center records about 55 worldwide on a daily basis.

Although the identification of mountain ranges deep in the Earth has been done before, no one has checked below Antarctica.

They are not near any of the mysterious bubbles, nor are they near where tectonic plates have recently plunged.

But to the team’s surprise, they found them everywhere they sampled.

Previously, the mountains were thought to be scattered near spots occupied by bubbles. But Hansen’s results suggest they may form a continuous “blanket” that surrounds the Earth’s core.

Testing this thesis will require much more study: before the research in Antarctica, only 20% of the boundary between the core and the mantle had been verified.

“But we hope to fill this gap”, says Hansen, who explains that this also depends on the development of new techniques to identify smaller structures.

In some regions, ULVZ structures look more like narrow plateaus than mountains, so you still can’t see the entire layer; they don’t show up on seismographs, if they are there at all.

However, if mountains really are that widespread, it would have implications both for what they are made of and how they are linked to larger bubble structures.

Could the remnants of smaller, mountain-sized tectonic plates really have spread that far from the larger blobs?

Regardless of what we discover, it’s oddly fitting that the alien, icy landscape of Antarctica has provided us with clues about the curious superheated mountains deep within the Earth.

Read the original English version of this article on the BBC Future website.

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