Leaking Early Core May Explain Earth's Two Giant Deep 'Blobs'

One of the most puzzling features of our planet is the presence of two enormous, unusually dense regions clustered just above Earth's core. New geodynamic models suggest a surprising origin: instead of being only remnants of sinking tectonic slabs or debris from a giant impact, some material may have seeped from a leaking early core and altered the chemistry of the deep mantle to form the so-called large low-shear-velocity provinces (LLSVPs).

The two LLSVPs were first inferred from seismic records in the 1980s. Seismic waves from earthquakes travel noticeably more slowly through two vast regions in the lowermost mantle — one beneath Africa and the other beneath the Pacific — rising from the core–mantle boundary about 2,900 kilometers beneath the surface. These anomalies imply a composition distinct from surrounding mantle rock.

Scientists have long proposed competing hypotheses for the LLSVPs' origin: accumulated remnants of old tectonic slabs, relics of a cooling global magma ocean, or fragments of a Mars-sized body (often called Theia) that struck early Earth. Each scenario implies a different chemical makeup and evolutionary history for Earth's interior.

Recent work led by geodynamicist Yoshinori Miyazaki of Rutgers University revisited the magma-ocean idea with new cooling and differentiation simulations. The team found a key ingredient that was previously overlooked: selective crystallization and expulsion of lighter oxides from a cooling core. As the core cooled and contracted, components such as magnesium oxide (MgO) and silicon dioxide (SiO2) can crystallize and migrate upward. Those lighter crystals are expelled across the core–mantle boundary and dissolve into the overlying magma ocean, altering its chemistry.

That chemical shift favors formation of silicate minerals like bridgmanite and seifertite at the base of the mantle while keeping ferropericlase concentrations low. Over geological time, mantle convection gathers these silicate-rich materials into the dense pile structures seismic observations now detect as LLSVPs. The models show these deposits can remain distinct for billions of years, consistent with Earth's ~4.5-billion-year history.

Why this matters

These deep structures are more than curiosities. The African LLSVP has been linked in some studies to changes in Earth's magnetic field above the Atlantic, and researchers have argued that LLSVPs could influence the locations where tectonic plates form and evolve. If LLSVPs originated from early core leakage, that offers new constraints on how Earth differentiated, how its deep chemistry evolved, and how those processes affected surface habitability.

"These are not random oddities," Miyazaki says. "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."

The new results revive a modified magma-ocean scenario by showing how core-derived oxides could reconcile previous contradictions between simple magma-ocean models and seismic observations. The research appears in Nature Geoscience.