Now, a study published in Nature Geoscience by Rutgers geodynamicist Yoshinori Miyazaki in combination with collaborators offers a striking new explanation for these anomalies and their role in shaping Earth's ability to support life.
The structures, known as large low-shear-velocity provinces and ultra-low-velocity zones, sit at the boundary between Earth's mantle and its core, nearly 1,800 miles beneath the surface. Large low-shear-velocity provinces are continent-sized blobs of dense, hot rock. One sits beneath Africa; the other is perched under the Pacific Ocean. Ultra-low velocity zones are thin, molten patches clinging to the core like lava puddles. Both types of structures slow seismic waves dramatically, signaling unusual composition.
"These are not random oddities," said Miyazaki, an assistant professor in the Department of Earth and Planetary Sciences in the Rutgers School of Arts and Sciences. "They are fingerprints of Earth's earliest history. If we can understand why they exist, we can understand how our planet formed and why it became habitable."
Billions of years ago, Earth was covered by a global ocean of magma, Miyazaki said. As it cooled, scientists expected the mantle to form distinct chemical layers, similar to frozen juice separating into sugary concentrate and watery ice. But seismic studies show no such strong layering. Instead, large-low shear velocity provinces and ultra-low velocity zones form irregular piles at the planet's base.
"That contradiction was the starting point," Miyazaki said. "If we start from the magma ocean and do the calculations, we don't get what we see in Earth's mantle today. Something was missing."
His collaborators concluded the missing piece is the core itself. Their model suggests that over billions of years, elements such as silicon and magnesium leaked from the core into the mantle, mixing with it and preventing strong chemical layering. This infusion could explain the strange composition of large low-shear-velocity provinces and ultra-low-velocity zones, which can be seen as solidified remnants of what the scientists termed a "basal magma ocean" contaminated by core material.
"What we proposed was that it might be coming from material leaking out from the core," Miyazaki said. "If you add the core component, it could explain what we see right now."
The discovery is about more than deep-Earth chemistry, Miyazaki said. Core-mantle interactions may have influenced how Earth cooled, how volcanic activity unfolded and even how the atmosphere evolved. That could help explain why Earth has oceans and life, while Venus is a scorching greenhouse and Mars is a frozen desert.
"Earth has water, life and a relatively stable atmosphere," Miyazaki said. "Venus' atmosphere is 100 times thicker than Earth's and is mostly carbon dioxide, and Mars has a very thin atmosphere. We don't fully understand why that is. But what happens inside a planet, that is, how it cools, how its layers evolve, could be a big part of the answer."
By integrating seismic data, mineral physics and geodynamic modeling, the study reconceived large low-shear velocity provinces and ultra-low-velocity zones as vital clues to Earth's formative processes. The structures may even feed volcanic hotspots such as Hawaii and Iceland, linking the deep Earth to its surface.
"This work is a great example of how combining planetary science, geodynamics and mineral physics can help us solve some of Earth's oldest mysteries," said Jie Deng of Princeton University, a co-author of the study. "The idea that the deep mantle could still carry the chemical memory of early core - mantle interactions opens up new ways to understand Earth's unique evolution."
Building on that idea, the researchers say each new piece of evidence helps fill in gaps in Earth's early history, turning scattered clues into a clearer picture of its evolution.
"Even with very few clues, we're starting to build a story that makes sense," Miyazaki said. "This study gives us a little more certainty about how Earth evolved, and why it's so special."
Research Report:Deep mantle heterogeneities formed through a basal magma ocean contaminated by core exsolution
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