A new deep-Earth analysis reveals colossal structures beneath Africa and the Pacific that may be older than complex life, and far more extreme than anyone imagined.(Everest Mountain )
When a magnitude 8.0 earthquake strikes, the strongest shaking may last only seconds. But the planet does not fall silent. For hours afterward, Earth continues to vibrate at frequencies too low for human ears to detect. Seismometers around the world record these faint oscillations, preserving clues about structures far beneath the surface.
For decades, seismologists have known that some seismic waves behave unusually after passing through the deep mantle beneath Africa and the central Pacific. In those regions, the waves slow dramatically, suggesting that the material there differs from the surrounding mantle. These signals have long pointed to something enormous near the boundary between Earth’s core and mantle, roughly 2,900 kilometers below the surface.
What Scientists Found Deep Inside Earth
Now, researchers at Utrecht University have used those vibrations to build a clearer picture of what lies there. Rather than looking only at wave speed, the team also measured how much energy the waves lost as they traveled through Earth, a property known as attenuation. By treating the planet as a single vibrating system, they produced a new global model of the mantle’s deep interior.
Their results, published in Nature, reveal two vast structures rising from the core-mantle boundary beneath Africa and the central Pacific Ocean. Known as Large Low Shear Velocity Provinces, or LLSVPs, these features extend roughly 1,000 kilometers upward and span as much as 5,000 kilometers across. They are not mountains in the ordinary sense, but thermochemical structures deep inside the planet.
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How Earth’s Vibrations Revealed the Structures
The research relied on a method called normal-mode seismology, which analyzes Earth’s free oscillations after major earthquakes. Unlike conventional seismic tomography, which mainly maps changes in wave speed, this approach can resolve both elastic and anelastic properties of the mantle. Using this method, the team built a 3D global model called QS4L3, constrained to spherical harmonic degree four across the whole mantle.
Lead author Sujania Talavera-Soza and her colleagues used earthquakes strong enough to excite Earth’s normal modes. That allowed them to separate the effects of temperature from the effects of composition more clearly than many earlier models could.
Why the LLSVPs May Be Compositionally Distinct
The study found a striking contrast between the upper and lower mantle. In the upper mantle, areas with high attenuation also show low seismic velocity, which is the pattern expected for hotter rock. In the lower mantle, however, the pattern reverses. The LLSVPs show low velocity but also relatively low attenuation, meaning the waves pass through them more efficiently than temperature alone would predict.
That combination suggests the structures are not simply hotter than their surroundings. Instead, they appear to be compositionally distinct. The researchers concluded that the LLSVPs likely contain larger mineral grains than the surrounding mantle and differ chemically from nearby material.
To test that interpretation, the team compared their seismic results with a laboratory-based viscoelastic model developed by Ulrich Faul of MIT and Ian Jackson. That comparison suggested that the circum-Pacific region is colder and characterized by smaller grain sizes, while the LLSVPs are warmer and contain larger grains.
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What These Structures Could Mean for Earth’s History
One leading idea is that these deep structures include ancient subducted material; oceanic crust that sank into the mantle billions of years ago and accumulated near the core-mantle boundary. Because of their unusual chemistry, they may resist mixing with the rest of the mantle during convection. In that case, they would represent long-lived domains that have persisted since early in Earth’s history.
The new model also marks an important technical advance. Previous global attenuation models mainly resolved the upper mantle, but QS4L3 extends that picture through the whole mantle in three dimensions. The researchers used splitting-function measurements from normal modes to track how Earth’s free oscillations shift as they pass through laterally varying structures. Their analysis showed the highest lower-mantle attenuation in the fast seismic “ring” around the Pacific and the lowest attenuation within the LLSVPs themselves.
The team, which also included Utrecht University researcher Laura Cobden, used those results to estimate viscosity from differences in grain size and temperature. Those calculations suggest that the LLSVPs are stable, long-lived features rather than temporary anomalies. That conclusion aligns with earlier work proposing that such structures may survive for hundreds of millions, or even billions, of years.
These deep mantle provinces may also help shape processes closer to the surface. Because they sit above the outer core and interact with mantle flow, scientists think they may influence plume formation, plate motion, and long-term volcanic activity in some regions.
No one will ever see these structures directly. Yet every major earthquake sends vibrations through the planet, and those vibrations carry information about Earth’s deepest interior. Far below the continents and oceans, beneath Africa and the Pacific, these immense mantle structures may be among the largest and longest-lived features inside our planet.
NOTE – This article was originally published in Daily Galaxy and can be viewed here
