By John Q. Hosedrinker 
Two massive “supercontinents” have been discovered deep within Earth’s mantle, residing beneath Africa and the Pacific Ocean. Analysis of earthquake data, focusing on seismic wave attenuation, reveals these structures are significantly older and less affected by mantle convection than previously believed, suggesting a less well-mixed mantle than was assumed. The supercontinents’ large mineral grain size and minimal energy loss from seismic waves indicate an age of at least half a billion years, possibly much older. This discovery implies these regions act as anchors, influencing plate tectonics and potentially serving as reservoirs for ancient geochemical elements.
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Earth’s mantle, that vast expanse of rock between the crust and core, holds far more secrets than we once imagined. For years, we envisioned it as a uniformly blended mass, but recent discoveries paint a much more complex picture. Instead of a smooth consistency, we’re finding massive, unmixed regions – enormous “supercontinents” buried deep within.
These supercontinents, one nestled beneath Africa and the other under the Pacific Ocean, are colossal structures spanning hundreds of thousands of square miles and towering hundreds of miles high. They represent a significant portion of the mantle-core boundary, about 20% in fact, and their discovery has been gradual, emerging initially as anomalies in seismic data.
Initially detected through seismic wave analysis, these formations exhibited a noticeable slowing of wave speed, hinting at regions hotter than their surroundings. This led to the designation of Large Low Shear Velocity Provinces, or LLSVPs. However, the exact composition, age, and role in mantle activity remained mysterious. Were they merely static piles of dense material, or did they actively participate in the planet’s convection currents?
A new approach to analyzing seismic data has provided more answers. Instead of solely focusing on wave speed, researchers also examined wave attenuation— the loss of energy as waves travel. This approach, combined with a model linking seismology and mineral physics, offers a more nuanced understanding.
The findings show that the LLSVPs exhibit surprisingly little energy loss compared to surrounding areas, suggesting a key difference in their composition. It turns out that wave damping is significantly influenced by the size of mineral grains. Smaller grains lead to greater energy loss. The minimal energy loss in the supercontinents points to the presence of much larger grains than in the surrounding mantle.
This difference in grain size strongly suggests that these subterranean supercontinents are significantly older than previously believed. The larger grains imply a much longer period of crystal growth, potentially spanning hundreds of millions, or even billions of years. This would make them considerably older than the “slab graveyards” – accumulations of sunken tectonic plates – found nearby.
These ancient, large-grained supercontinents also appear remarkably rigid, resisting the effects of mantle convection, further supporting the idea of a less thoroughly mixed mantle than previously assumed. Their stability and age suggest they act as anchors, influencing plate tectonics in ways still being explored.
The implications are far-reaching. These buried supercontinents could significantly impact our understanding of plate tectonics and the movements that create earthquakes and volcanoes. Their age and stability challenge our models of Earth’s internal dynamics and raise questions about the extent of mantle mixing. They could even represent reservoirs for geochemically primordial elements, ancient materials dating back nearly to Earth’s formation. These findings represent a major step forward, sparking new lines of inquiry into the Earth’s deep interior and the processes that have shaped our planet for billions of years. The mystery continues to unfold, promising even more intriguing discoveries in the years to come.