A recent study investigating giant “island” structures near Earth’s core has deepened the mystery surrounding them: As well as suggesting they are truly ancient structures, the research may alter what we think we know about the Earth’s mantle.
Lying beneath Africa and the Pacific in the lowermost part of the Earth’s mantle, surrounding the Earth’s core, there are two gigantic blobs that occupy around 3 to 9 percent of the volume of the Earth.
There are, of course, no direct ways of seeing the Earth’s core – at least without being burned to a crisp or fighting your way through the mole people. However, we can look under the surface pretty effectively by utilizing earthquakes in a technique known as seismic tomography.
When earthquakes occur, seismic waves are sent out in all directions. By measuring the tremors from several locations at the surface, scientists can create a map of the Earth’s interior. Since rocks and liquids within the Earth are of different densities, the waves move through them at different speeds, allowing geologists to figure out what type of material the waves are going through.
In the late 1970s, two strange large structures were found, known as large low shear velocity provinces (LLSVPs). In these areas, sometimes referred to as “blobs”, waves travel more slowly than through the surrounding lower mantle.
“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,” study co-author Arwen Deuss of Utrecht University said in a statement
“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.”
Under Africa, the area known as “Tuzo” is thought to be about 800 kilometers (497 miles) in height, or about 90 Mount Everests.
“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,” Deuss added. “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.”
So, what are they? Unfortunately, we still aren’t entirely sure, though we have a few sound ideas. Given that the objects are denser than the surrounding mantle, it’s assumed that they are made of a different material, though we cannot tell exactly what it is. A leading hypothesis is that the LLSVPs are piles of oceanic crust that have been subducted and accumulated over billions of years. Another slightly more fun theory is that the pieces are chunks of an ancient planet.
Theia is a hypothetical Mars-sized planet that hit Earth around 4.5 billion years ago, throwing off enough rock to form the Moon. It has been suggested that the blobs are in fact pieces of Theia itself: denser mantle from the proto-planet that got mixed in with Earth’s during the collision. In 2021, a team modeled simulations of the scenario, finding that Theia’s mantle could survive if it were only 1.5 to 3.5 percent denser.
In the new study, the team attempted to investigate the blobs further using new methods.
“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,” co-author Sujania Talavera-Soza of Utrecht University explained. “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. Unlike the upper mantle, where we found exactly what we expected: it is hot, and the waves are damped.”
Temperature alone cannot explain the difference in wave propagation speeds. According to the team, much more important is the size of the grains which make them up.
“Subducting tectonic plates that end up in the slab graveyard consist of small grains because they recrystallize 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,” co-author Ulrich Faul of MIT added. “Due to the large number of grain boundaries between the grains in the slab graveyard, we find more damping, because waves [lose] energy at each boundary they cross. The fact that the LLSVPs show very little damping, means that they must consist of much larger grains.”
LLSVPs were found to contain large grains.
Image credit: Utrecht University
The team believes that the large grain sizes indicate that the LLSVPs are ancient and fairly rigid structures, which do not take part in mantle convection. They add that it suggests the mantle cannot be as well mixed as we thought either, with Talavera-Soza adding “after all, the LLSVPs must be able to survive mantle convection one way or another.”
If correct, that could change our understanding of the mantle, to how volcanoes and mountains are built.
“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,” Deuss added. “And we think that those mantle plumes originate at the edges of the LLSVPs.”
Though we don’t know for certain what the blobs are – and will never see them directly – looking more at damping, and other oscillations occurring within Earth could offer more clues. Thankfully, we will not have to wait for more earthquakes for such research to take place: Data gathered by seismometers since the 1970s could be good enough for studying the Earth’s interior further.
The study is published in the journal Nature.
