Colossal 'Anomalies' in Earth's Mantle Aren't What We Thought

Deep beneath Earth’s crust sit two enormous formations whose beginnings remain unclear. Seismologists have now uncovered fresh evidence about what they are made of - findings that could force a rethink of how our planet’s interior behaves.

Back in the 1980s, seismic observations first pointed to two vast, continent-scale blobs embedded in Earth’s mantle, thousands of kilometres below the Pacific Ocean and beneath Africa.

Revisiting the Large Low Shear Velocity Provinces (LLSVPs)

Those strange regions were detected because seismic waves passing through them slow down dramatically. That behaviour led to their unwieldy label: Large Low Shear Velocity Provinces (LLSVPs), a signal that these zones are considerably hotter than the mantle around them.

"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.

Yet wave speed on its own cannot fully distinguish what these blobs are. They might simply be temporary patches of unusually hot material, or instead be longer-lived features with a distinct composition.

How scientists “listen” to Earth’s interior

Although thousands of kilometres of rock block any direct view, researchers can probe the planet’s inside using sound. Major earthquakes make Earth ring like a giant bell, sending vibrations through the globe. Instruments worldwide record these signals, allowing scientists to infer structures hidden deep below the surface.

Because seismic waves move faster or slower depending on what they pass through, tracking those changes reveals clues about the make-up of different layers and regions.

A new clue: how much energy the waves lose

A new study led by scientists in the Netherlands and the US analysed these mantle structures with a more comprehensive approach. Alongside the familiar measurements of seismic-wave speed, the team also examined damping - the amount of energy seismic waves lose while travelling through the blobs.

To do this, they used whole-Earth oscillation records from 104 previous earthquakes, building a detailed three-dimensional model spanning the upper and lower mantle. Crucially, the model incorporated damping measurements to compare how strongly different regions sap energy from passing waves.

Unexpectedly weak damping and what it implies

The team found that seismic waves lose very little energy when moving through the LLSVPs. Compared with the nearby “slab graveyard” of subducted plates, damping in the LLSVPs is notably weak.

That contrast suggests the LLSVPs are not merely hotter-than-average patches. Instead, the results indicate a compositional difference as well - potentially linked to the size of mineral grains within 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."

If the LLSVPs are made up of larger grains, that points to material that is old and stable. In turn, it implies the mantle may not circulate and remix itself as vigorously as many geology textbooks suggest.

Competing ideas about the blobs’ origin

One prominent proposal has been that the LLSVPs are themselves piles of ancient tectonic plates, given their location beside the slab graveyard. However, the apparent differences in grain size - alongside temperature contrasts - argue against the two being the same kind of material.

That instead could strengthen a different hypothesis: that the LLSVPs are leftover fragments of the ancient protoplanet that struck early Earth around 4.5 billion years ago, an impact thought to have formed the Moon.

Whatever their true origin, their apparent stiffness and durability imply that Earth’s mantle is less thoroughly blended than previously assumed.

"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.