Mysterious structures in Earth's core hold clues to why and how life began

About 1,800 miles (2,900 kilometers) below our feet, two enormous patches of strange rock sit above Earth’s core. New computer models now tie these deep structures to slow chemical leaks between the core and mantle that may have helped Earth stay habitable.

In a new study, researchers looked at how material could move between Earth’s metal core and rocky mantle over more than four billion years of cooling.

They used seismic maps and supercomputer simulations to see how those exchanges could build deep blobs and blur chemical boundaries.

Inside Earth’s core leaks

The work was led by Jie Deng, a geoscientist at Princeton University. His research focuses on how the deep core and mantle have traded heat and chemical elements over Earth’s long history.

Earth’s mantle, the thick rocky shell between crust and core, holds most of the planet’s solid volume and mass. We learn about it by tracking earthquake waves that curve and bounce as they travel through different layers.

Seismic tomography is a method that uses earthquake records to map Earth’s interior. Data from this technology reveals two giant, slow regions near the mantle’s base. 

Scientists call them large, low shear velocity provinces. They are vast regions where seismic waves slow down beneath Africa and beneath the Pacific Ocean.

Clinging even closer to the core are ultra-low velocity zones (ULVZ). These are thin patches of partly molten material that slow down seismic waves far more strongly.

These strange features look nothing like the smooth, layered mantle that older textbook pictures suggested.

Gaps in old models

Early in Earth’s history, giant impacts and trapped heat probably melted the mantle into a global magma ocean, a deep layer of molten rock. 

Earlier work found that melt could cool into stacked layers. This could have resulted in the densest part forming a long-lived basal magma ocean near the core.

If that picture were correct, we would expect a smooth, deep layer spread over the core, not two isolated piles and scattered patches.

Seismic images and volcanic chemistry instead point to a messy, irregular, deep mantle that older models struggled to match.

This mismatch was what pushed Deng and his colleagues to rethink the processes happening at the base of the mantle.

They wanted to know why calculations assuming a magma ocean did not produce the patchy structures that seismologists actually see.

The answer they arrived at was simple in its outline but hard to test without powerful computers. Something essential was missing from those early magma ocean calculations, and the team suspected it lay inside the core itself.

How the core leaks matter

As Earth’s core cools, some light elements can separate out in a process called exsolution. This is where a solid or liquid unmixes into distinct parts.

Recent research shows that grains of magnesium oxide and silicon dioxide can rise from the core into the mantle, carrying heat and distinct chemistry.

Deng’s group built on that idea and imagined those grains dissolving into deep melt. This would enrich a basal mantle region in silicon and magnesium.

BECMO models and simulations

In their models this region becomes a basal, exsolution-contaminated magma ocean, called BECMO. In this scenario, the solid remains of the BECMO are slightly denser than the surrounding mantle.

“What we proposed was that it might be coming from material leaking out from the core,” said Yoshinori Miyazaki, a professor at Rutgers University (RU). 

With that slow core leak included, the simulations produced irregular, deep piles and patches that resemble the observed provinces and ultra-low velocity zones.

BECMO stays rich in silicon as it crystallizes. This means the models avoid forming an iron-rich layer that basal magma ocean ideas had predicted.

Instead they produce moderately dense piles and thin zones that resemble the provinces and ultra-low velocity zones at the mantle’s base.

Core leaks shape hotspots

To see how BECMO leftovers might move, the team used geodynamic modeling. These computer calculations tracked how hot rock flows and piles up.

Over billions of years of simulation, those denser patches shifted, merged, and thinned, creating clusters instead of a continuous blanket at the base.

Furthermore, the basal clusters lined up with places where mantle plumes are thought to rise. This included regions under Hawaii and Iceland that feed volcanic hotspots.

Independent summaries have hinted that edges of the provinces sit beneath hotspots, and the modeling offers a way to link them.

“This work is a great example of how combining planetary science, geodynamics and mineral physics can help us,” said Deng. He studies how deep processes shape the evolution of Earth’s interior.

The team’s models also follow how isotopes could become concentrated inside BECMO material at depth. Those trapped signatures match patterns seen in some ocean island basalts, suggesting that plumes may tap pieces of this ancient reservoir.

Why deep structures matter for life

Deep structures like BECMO piles affect how heat leaves Earth’s interior. In turn, this shapes fast core cooling and the lifetime of the field.

Changes in that cooling history can alter how strongly the field shields the surface from charged particles that would otherwise erode the atmosphere.

“Earth has water, life and a relatively stable atmosphere,” said Yoshinori Miyazaki, noting how different that is from our neighboring planets.

He and his colleagues argue that what happens inside a planet, especially how its deep layers cool and mix, could help explain those differences.

“These are not random oddities,” said Miyazaki, referring to the deep provinces and thin zones near the core. “They are fingerprints of Earth’s earliest history.”

As more evidence emerges, the view of Earth’s deep interior is shifting from a confusing set of hints to a coherent story of change. “Even with very few clues, we’re starting to build a story that makes sense,” said Miyazaki.

The study is published in Nature Geoscience.

Image: Interior of early Earth with a hot, melted layer above the boundary between the core and mantle. Illustration by Yoshinori Miyazaki/Rutgers University

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