Location of the LLSVPs and a schematic representation of the Earth’s cross-section for speed and damping of the seismic waves. Credit: Utrecht University

Hidden deeply in the Earth’s mantle there are two huge ‘islands’ the size of a continent, and new research suggests they must be ancient.

The research from Utrecht University shows that these regions are not only hotter than the surrounding graveyard of cold sunken tectonic plates, but also that they must also be at least half a billion years old, perhaps even older.

These observations contradict the idea of a well-mixed and fast flowing Earth’s mantle, a theory that is becoming more and more questioned.

Large earthquakes make the whole Earth ring like a bell with different tones, just like a musical instrument.

Seismologists study Earth’s deep interior by investigating how much these tones are ‘out of tune’, because whole Earth oscillations will sound out of tune or less loud when they encounter anomalies.

This way seismologists will be able to make images of the interior of our planet, just like a hospital doctor can ‘see’ through your body with X-rays.

At the end of the last century, an analysis of these oscillations showed the existence of two subsurface ‘super-continents’: one under Africa and the other one under the Pacific Ocean, both hidden more than two thousand kilometres below the Earth’s surface.

Arwen Deuss, seismologist and professor of structure and composition of Earth’s deep interior at Utrecht University in the Netherlands, said: “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.

“These two large islands are surrounded by a graveyard of tectonic plates which have been transported there by a process called ‘subduction’, where one tectonic plate dives below another plate and sinks all the way from the Earth’s surface down to a depth of almost three thousand kilometres.”

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.

Slow waves

Deuss added: “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 were keen to find out if they could discover more about these regions.

Deuss said: “We added new information, the so-called ‘damping’ of seismic waves, which is the amount of energy that waves lose when they travel through the Earth.

“In order to do so, we did not only investigate how much the tones where out of tune, we also studied their sound volume.”

Talavera-Soza added: “Against our expectations, we found little damping in the LLSVPs, 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.

“Unlike the upper mantle, where we found exactly what we expected: it is hot, and the waves are damped.

“Just like when the weather is hot outside and you go for a run, you don’t only slow down but you also get more tired than when it is cold outside.”

Grain size

Their colleague Laura Cobden, who specialises in the minerals that we find deep in the Earth, suggested studying the grain size of the LLSVPs. According to their American colleague Ulrich Faul, temperature alone cannot explain the absence of high damping in the LLSVPs.

Deuss said: “Grain size is much more important. Subducting tectonic plates that end up in the slab graveyard consist of small grains because they recrystallise on their journey deep into the Earth.

“A small grain size means a larger number of grains and therefore also a larger number of boundaries between the grains.

“Due to the large number of grain boundaries between the grains in the slab graveyard, we find more damping, because waves loose energy at each boundary they cross.

“The fact that the LLSVPs show very little damping, means that they must consist of much larger grains.”

Ancient

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 explained: “After all, the LLSVPs must be able to survive mantle convection one way or another.”

Engine

Knowledge of the Earth’s mantle is essential to understand the evolution of our planet, as well as to understand other phenomena at the Earth’s surface, such as vulcanism and mountain building.

Deuss added. “The Earth’s mantle is the engine that drives all these phenomena. Take, for example, mantle plumes, which are large bubbles of hot material that rise from the Earth’s deep interior as in a lava lamp.”

Once they finally reach the surface, they cause vulcanism, like under Hawaii and Deuss and her colleagues think those mantle plumes originate at the edges of the LLSVPs.

The research is published in Nature.