Earth’s Mantle Reveals Two Huge Subterranean ‘Islands’ – With The Size Of A Continent

This allows seismologists to image Earth’s interior, similar to how doctors use X-rays. Analysis of these oscillations revealed two subsurface ‘super-continents’: one under Africa and another under the Pacific Ocean, both over two thousand kilometers deep.

“Nobody knew what they are, if they are temporary or have been there for millions or billions of years,” says Arwen Deuss, a seismologist and professor. “These two large islands are surrounded by a graveyard of tectonic plates transported by ‘subduction,’ where one plate dives below another and sinks nearly three thousand kilometers deep.”

“We have known for years that these islands are located at the boundary between the Earth’s core and mantle. And we see that seismic waves slow down there.” Earth scientists therefore call these regions ‘Large Low Seismic Velocity Provinces’ or LLSVPs. “The waves slow down because the LLSVPs are hot, just like you can’t run as fast in hot weather as you can when it’s colder.”

Deuss and her colleague Sujania Talavera-Soza wanted to learn more about these regions. “We added information on the ‘damping’ of seismic waves, which is the energy lost as waves travel through Earth. We examined both how out of tune the tones were and their sound volume.”

Schematic representation of the process of subduction of tectonic plates and of a mantle plume rising from an LLSVP. In the latter, the mineral grains are larger than those in the subducted plates. Credit: Utrecht University

Talavera-Soza notes: “Contrary to our expectations, there was little damping in the LLSVP, making tones sound loud. However, we found significant damping in the cold slab graveyard, where tones were soft. In the upper mantle, it matched our expectations: it’s hot, and waves are damped—similar to feeling more tired when running in hot weather compared to cold.”

According to Deuss, grain size is crucial. Subducting tectonic plates have small grains due to recrystallization, increasing damping as waves lose energy at each boundary. LLSVPs show little damping, indicating they consist of larger grains.

Those mineral grains do not grow overnight, which can only mean one thing: LLSVPs are lots and lots older than the surrounding slab graveyards. Even more so: the LLSVPs, with their much larger building blocks, are very rigid. Therefore, they do not take part in mantle convection (the flow in the Earth’s mantle).

Thus, contrary to what the geography books teach us, the mantle cannot be well-mixed either. Talavera-Soza clarifies: “After all, the LLSVPs must be able to survive mantle convection one way or another.”

Earth’s mantle is key to studying the planet’s evolution and phenomena like volcanism and mountain building. Mantle plumes, hot material rising from deep within, cause surface volcanism in places like Hawaii, likely originating at LLSVP edges.