Scientists have recently uncovered two massive regions deep within Earth’s mantle, which they call “islands.” A study from Utrecht University reveals that these areas are unusually hot and likely over half a billion years old. This challenges the old view that the mantle is a fast-flowing, mixed system.
These hidden islands lie about 1,200 miles (2,000 km) under the surface, towering at around 620 miles (1,000 km)—much taller than any mountain we know. They’re located beneath Africa and the Pacific Ocean and were first detected in the late 20th century by examining seismic waves created by large earthquakes.
When significant quakes occur, they cause the Earth to vibrate, creating waves that scientists can study to spot unusual structures underneath. Researchers identified these features as Large Low Seismic Velocity Provinces (LLSVPs). “Nobody knows if they are a temporary phenomenon or have been there for millions or billions of years,” says Arwen Deuss, a seismologist and senior author of the study.
These LLSVPs are surrounded by colder, denser remains of tectonic plates that have sunk into the mantle through a process called subduction. Unlike their colder surroundings, seismic waves slow down considerably in the LLSVPs due to their high temperatures.
Deuss and her colleague, Sujania Talavera-Soza, used new methods to study these areas further. They measured not just how much seismic waves slow down but also how much energy they lose as they travel. Surprisingly, LLSVPs showed very little energy loss, while the cold plates did. “This was unexpected,” Talavera-Soza remarked.
One key factor is grain size. According to co-author Laura Cobden, the minerals in the LLSVPs appear to be larger than those in the cold graveyard of plates. These larger grains let the seismic waves pass through with less damping. This suggests that the LLSVPs are much older and stable, contrasting sharply with the constantly changing material surrounding them.
So why is this significant? The stability and age of the LLSVPs challenge the belief that the mantle is uniformly mixed. “For them to survive mantle convection, they must have some kind of resilience,” Talavera-Soza noted. This discovery has major implications for understanding how Earth’s surface evolves. For example, volcanos like those in Hawaii may be connected to hot material from these regions.
To study these deep structures, scientists analyze seismic waves produced during strong earthquakes, like the 1994 Bolivia quake, which occurred far beneath the surface without causing damage above. Seismology data dating back to 1975 allows researchers to revisit past events for further insights.
This research reshapes our understanding of Earth’s mantle. Instead of seeing it as a simple, mixed system, we now recognize it as containing stable, ancient structures that play essential roles in surface processes. Scientists believe that mantle plumes, which cause volcanic activity, likely originate near the edges of the LLSVPs.
The study, published in the journal Nature, marks a significant advancement in our comprehension of Earth’s inner workings. Understanding these deep regions is crucial for interpreting everything from earthquakes to volcanic eruptions. It reinforces how interconnected the layers of our planet truly are.
