Deep below Earth’s surface lie two gigantic structures with mysterious origins. Now, seismologists have found new clues about their composition that could upend our understanding of the whole planet’s geology.
In the 1980s, seismic data revealed the two colossal, continent-sized blobs of material in Earth’s mantle, thousands of kilometers beneath the Pacific Ocean and the continent of Africa.
A new study by researchers from the Netherlands and the US has examined the structures in more detail. In addition to measuring changes in the speed of seismic waves, which past research has focused on, the team looked at how much energy these waves lose as they pass through the blobs.
To their surprise, they found that seismic waves lose very little energy when traveling through the blobs. That has a few major implications: first, the minerals in them are made of larger ‘grains’ than expected. That implies the structures are old and stable, which in turn suggests the mantle doesn’t churn as much as geology textbooks would have us believe.
Thousands of kilometers of rock make it difficult to see into Earth’s interior, but scientists can study it through sound instead. Large earthquakes cause the planet to resonate like a giant bell, sending seismic waves rippling through the planet. Detectors around the world can then pick up these signals and reveal hidden structures.
For example, seismic waves travel through different materials at different speeds, so measuring how they speed up and slow down tells scientists what different regions and layers are made of.
This is how the big weird blobs were first identified decades ago. Seismic waves were seen to slow down drastically in these areas, which earned them the awkward scientific name of Large Low Seismic Velocity Provinces (LLSVPs). This indicates the regions are much hotter than the surrounding mantle.
“These two large islands are surrounded by a graveyard of tectonic plates that have been transported there by a process called ‘subduction’, where one tectonic plate dives below another plate and sinks all the way from Earth’s surface down to a depth of almost 3,000 kilometers [1,864 miles],” says senior author Arwen Deuss, a seismologist at Utrecht University in the Netherlands.
But seismic wave speeds alone can only paint part of the picture. These LLSVPs could be short-lived thermal anomalies, or they could be longer-lasting lumps with a different composition.
To find out, the team used whole-Earth oscillation data from 104 past earthquakes to create a detailed 3D model of the upper and lower mantle. Specifically, they incorporated data on the damping of the waves – how much energy they lose as they pass through different regions.
To their surprise, the LLSVPs were found to have very weak damping compared to the plates in the nearby graveyard. This suggests that the LLSVPs aren’t just temperature anomalies but compositional ones too. The key could be the size of the mineral grains that make up the material.
“Subducting tectonic plates that end up in the slab graveyard consist of small grains because they recrystallize on their journey deep into Earth,” says Deuss.
“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 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.”
One of the leading theories about the origin of the LLSVPs is that they’re also chunks of old tectonic plates, given their proximity to the so-called graveyard. But the differences in grain size and temperature suggest otherwise.
That could lend weight to another theory: that the LLSVPs are remnants of the ancient protoplanet that collided with early Earth about 4.5 billion years ago, giving birth to the Moon.
Whatever they are, their rigidity suggests the mantle isn’t as well-mixed as previously thought.
“After all, the LLSVPs must be able to survive mantle convection one way or another,” says Utrecht seismologist and first author Sujania Talavera-Soza.
The research was published in the journal Nature.
