Earth’s inner core—a solid, Moon-sized sphere of iron and nickel—is expanding at a rate of approximately one millimetre per year, powered by the crystallization of molten iron. Recent research published in September 2025 by teams from the University of Oxford, the University of Leeds, and University College London suggests that carbon plays a critical role in this process, lowering the energy barrier required for iron to freeze under extreme planetary pressure. This ongoing phase change is the primary engine behind Earth’s magnetic field, which protects the atmosphere from solar wind.
The Mechanics of Core Growth
Located 5,150 kilometres beneath the surface, the inner core exists in an environment where pressure reaches 330 gigapascals and temperatures mirror the Sun’s surface. According to research from the University of Oxford, the University of Leeds, and University College London, pure iron would struggle to crystallize under these specific conditions. By running quantum-mechanical simulations, the researchers identified that dissolved carbon acts as a catalyst. This allows iron to lock into a solid lattice despite the intense heat, effectively driving the growth of the inner core.
Did you know?
The inner core was first identified in 1936 by Danish seismologist Inge Lehmann. She concluded that a solid centre must exist after observing that earthquake waves bent in ways inconsistent with a purely liquid core.
How the Geodynamo Sustains Life
The freezing of the inner core does more than just increase the size of the planet’s centre; it creates a chemical engine. As iron crystallizes, it excludes lighter elements like silicon, oxygen, and sulphur. These elements accumulate at the boundary, creating a buoyant fluid that rises through the liquid outer core. This movement, combined with heat-driven convection, stirs the electrically conductive outer core, generating the geodynamo that sustains Earth’s magnetic field.
Studies from ETH Zurich and the Southern University of Science and Technology, published in 2025, indicate that while a fully liquid core could theoretically support a magnetic field, the presence of a solid inner core provides the stability and longevity necessary for a planetary shield. Without this process, Earth could face a fate similar to Mars, which lost its magnetic protection billions of years ago as its core cooled, eventually leading to the depletion of its atmosphere.
Structural Complexity of the Inner Core
The inner core is not a uniform, smooth sphere. Seismological data reveals an “onion-like” structure, with layers formed over aeons as the chemical composition of the outer core shifted. Research reported in Nature Geoscience in February 2025 by USC geophysicists suggests the surface of the inner core is actively deforming. By analyzing decades of repeating earthquake data, scientists have detected structural changes and topography on the boundary, indicating that the core is a dynamic, evolving feature of the planet.
Recent analyses of seismic wave scattering are currently providing the clearest maps of the inner core’s topography to date.
Future Trends and Planetary Longevity
The growth of the inner core is a finite process. Geologists estimate that in a few billion years, the outer core will crystallize faster than the planet can dissipate heat, eventually causing the geodynamo to falter. However, this remains a long-term geological prospect. Current research continues to focus on how the inner core’s rotation—which may differ from the mantle—and its complex, layered chemistry influence surface phenomena, including the auroras and the stability of the global magnetic field.
Frequently Asked Questions
Why is the inner core still growing?
Earth is slowly losing heat to space. As the planet cools, the pressure-temperature balance at the core-mantle boundary shifts, allowing iron to crystallize and solidify onto the existing inner core.
What would happen if the inner core stopped growing?
If the freezing process ceased, the convection currents in the outer core would eventually slow down. This would weaken the magnetic field, potentially leaving the atmosphere vulnerable to solar wind erosion over tens of millions of years.
Is the inner core solid throughout?
Seismic evidence suggests the inner core is solid, though it contains complex, layered structures and variations in density, likely caused by the exclusion of light elements during the crystallization process.
What are your thoughts on the latest findings regarding Earth’s deep-interior? Share your questions in the comments below or subscribe to our newsletter for more updates on planetary science.
Related reading
