Hidden mountains: The mysterious structures deep inside Earth's mantle

Scientists have been investigating colossal structures that exist deep inside of Earth’s mantle. Located beneath Africa and the Pacific Ocean, the underground “islands” were first discovered in the late 20th century.

These vast areas, positioned more than 1,242 miles (2,000 kilometers) below the surface, are about the size of a continent and are estimated to be at least half a billion years old.

The location and staggering age of these subsurface features has drawn attention from geophysicists.

A recent study led by Professor Arwen Deuss from Utrecht University suggests that the massive structures may be more stable than previously thought.

The research challenges the long-held idea that Earth’s mantle is a rapidly mixing environment. Instead, the results suggest that there is much less flow within the mantle than previously believed.

Mysterious mantle structures

Earth’s mantle is the thick layer between the crust and the core. It includes a graveyard of subducted tectonic plates that have been pushed downward over millions of years by plate collisions. 

Experts have been using seismic signals from large earthquakes to examine the hidden structures. Deuss and her colleague at Utrecht University, Dr. Sujania Talavera-Soza, incorporated a novel approach to studying seismic waves. 

Beyond analyzing how much waves slow down, they also measured “damping,” or how much energy the waves lose as they travel through the Earth.

“Nobody knew what they are, and whether they are only a temporary phenomenon, or if they have been sitting there for millions or perhaps even billions of years,” said Professor Deuss.

Hidden zones inside Earth’s mantle

Researchers call these deep regions Large Low Seismic Velocity Provinces (LLSVP) because seismic waves lose speed as they pass through them. However, the new study has introduced an extra piece of information about just how much energy the waves lose.

“Against our expectations, we found little damping in the LLSVP, which made the tones sound very loud there. But we did find a lot of damping in the cold slab graveyard, where the tones sounded very soft,” explained Dr. Talavera-Soza.

This difference could be explained by larger mineral grains in the LLSVPs compared to the smaller grains present in old slabs.

New clues about Earth’s mantle

The presence of large, ancient features suggests the mantle is not fully churning everything into a uniform mixture.

Slower convection in these deep sections challenges assumptions found in standard textbooks, and offers a new perspective on Earth’s dynamics.

Mantle motions feed processes such as vulcanism and mountain formation. Hot blobs, known as plumes, may start at the edges of these big structures and eventually cause volcanic activity on the surface, as observed in spots like Hawaii.

Peering into the past with earthquakes

Ultra-strong earthquakes have been vital in probing our planet’s deep interior. One classic case is the 1994 Bolivia quake, which struck at a depth of around 403 miles (650 kilometers).

Seismometers collect data on how Earth “rings” following such events. Researchers analyze this ringing to detect wave speed changes and energy losses, which helps form a detailed picture of what lurks far beneath our feet.

Large grains, ancient foundations

Older slabs sink, recrystallize, and become full of tiny grains. These small particles cause waves to lose more energy because the wave boundaries are more frequent.

Meanwhile, the huge underground “islands” appear to have larger grains that likely took a very long time to develop.

The discrepancy in grain size implies that these deep features have been around for many millions of years, adding weight to the theory that they predate much of Earth’s modern surface geography.

Future research directions

New observations may reveal how these hidden structures influence surface geology. Shifts in Earth’s mantle flow might lead to changes in volcanic patterns over millennia, and the alteration of landscapes and ecosystems.

Scientists also aim to investigate whether similar deep pockets exist elsewhere. If further research uncovers more ancient zones, it could lead geologists to refine their understanding of mantle convection and the formation of continents.

The study was published in the journal Nature.

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