How an 'ocean of fire' shaped early Earth's core, according to a new study

Earth’s deepest layers were not always solid rock blanketing a molten core. New simulations show a hidden sea of magma pooled above the core during the planet’s chaotic youth and how it still shapes the underground landscape today.

Charles‑Édouard Boukaré, a planetary physicist at York University, guided the work and argues that the ancient melt layer, or basal magma ocean, was an unavoidable consequence of early cooling. 

“Another way to say this is there is a memory,” Boukaré said, explaining that the planet’s interior still remembers its fiery beginnings.

Modeling the infant Earth

The research team combined isotope data from ancient rocks with modern seismic images to build a three‑dimensional model of a newborn planet.

Their code tracked how iron‑rich liquid separated from lighter crystal mush and trickled to the bottom of the mantle.

Even when they forced crystallization to start in mid‑mantle zones, dense melt still slid downward, proving that the deep pool forms, no matter where solidification begins.

The result overturns earlier one‑directional models that made the mantle freeze from the core upward.

The simulations reproduced present‑day mantle temperatures and predicted a lingering layer of melt up to 60 miles (96 kilometers) thick.

That prediction matches tiny signals in seismic data that hint at pockets of extraordinary heat above the core.

Heat flow kept Earth’s core liquid

Cooling crust near the surface formed the first minerals, but their extra weight made them plunge back into the mantle.

Most solidified crystals remelted on the trip down, yet some carried shallow chemical signatures that are now buried nearly 1,800 miles (2,900 kilometers) deep.

Iron oxide lowered the melting point of these sinking masses and helped them merge into the basal ocean. Heat flowing out of Earth’s core kept the iron‑rich mixture liquid long after the rest of the mantle stiffened into rock.

Because the melt is denser than the surrounding solids, it refuses to rise and cool.

That quirk sealed in a reservoir of incompatible elements such as neodymium and tungsten, explaining why modern lava sometimes carries ancient isotopic fingerprints.

Clues from mantle blobs

Seismologists have mapped two continent‑sized zones beneath Africa and the Pacific where earthquake waves slow sharply.

These large, low‑shear velocity provinces might be the frozen rims of the primeval ocean, dating back more than 4.4 billion years.

Alternative theories say the blobs are piles of sunken ocean crust recycled by plate tectonics. Yet the volume and dense, iron‑enriched makeup of the blobs fit the basal magma ocean story far better than crustal recycling alone.

If the blobs truly are relics of early melt, they could act as anchors that pin mantle plumes in place, explaining the long‑lived volcanic tracks that dot the Pacific seafloor.

Their presence also helps seismologists interpret odd, low‑velocity patches found near Earth’s core–mantle boundary worldwide.

Earth’s core drives convection

Heat leaving the core drives convection that powers the geodynamo, the engine behind Earth’s magnetic field. A thick, insulating sheet of iron‑rich melt alters that heat flow and could modulate magnetic strength over tens of millions of years.

“Continent drift might affect the location of tectonic plates,” notes Boukaré, hinting that the drift could partially reflect the deep ocean’s rhythm of currents. Changes in melt thickness also tweak how stiff slabs sink and how buoyant plumes rise. 

Isolated melt pockets may even lubricate slab edges, letting plates slide with less friction. That effect could explain why subduction zones sometimes shift along straight lines rather than wandering like rivers.

Chemical prints in ancient rocks

Isotopic ratios of samarium–neodymium and lutetium–hafnium vary subtly in rocks that are older than 3 billion years.

The variations match the signature expected when shallow crystals rain into a deep, iron‑saturated melt, then remix into later lavas.

Some Archean basalts in western Greenland preserve these signals, confirming that early differentiation products were never fully erased by later convection.

The model reconciles that evidence with the fact that most upper mantle rocks look chemically uniform today.

Geochemists once limited the amount of bridgmanite crystallization because they feared it would skew surface isotopes, but the new work shows that shallow and deep processes can cancel each other’s signals. This finding opens room for more extensive early mantle stratification than previously thought.

Echoes on other worlds

It is theorized that Mars lost its magnetic shield early, and Venus never developed plate tectonics, outcomes that may reflect how long their own basal oceans survived.

Running Boukaré’s code with Martian gravity produces a melt layer that freezes quickly, starving the core of heat and ending dynamo action.

Super‑Earth exoplanets, with stronger gravity and thicker mantles, may hold basal oceans far longer.

That persistence could dampen surface volcanism and help the planets retain atmospheres, parameters that astronomers use when ranking habitability.

Future questions about Earth’s core

Laboratory experiments that squeeze minerals to core pressures are planned to test how truly dense molten iron‑silicate mixtures become. Those results will refine melt‑mobility numbers in the next generation of mantle models.

Seismologists are also hunting for ultrasonic echoes that bounce off the melt’s upper surface, a signal that would prove the ocean still exists today. Any detection would transform theories about mantle convection and core cooling in one stroke.

Looking ahead, the team plans to fold more trace elements into the model and simulate mantle overturn events triggered by giant impacts.

Those runs could reveal whether the basal ocean ever mixed completely or still hides a liquid heart.

Success in either search would ripple into planetary science, informing models of exoplanets and guiding missions that probe the deep interiors of rocky worlds.

The next decade of experiments and observations could turn the idea of a lasting magma sea from theory into accepted geology.

The study is published in Nature.

Photo credits: @Sylvain Petitgirard/University of Bayreuth.

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