Earth's inner core

Lore & Background

Adam Dziewonski and James Freeman Gilbert established that measurements of normal modes of vibration of Earth caused by large earthquakes were consistent with a liquid outer core.

Reader's Guide

The inner core is significant as the deepest layer of Earth, whose properties constrain models of planetary formation, geodynamics, and the magnetic field. The inner core's density (about 13.0 kg/L at center) and mass (about 10^23 kg) influence Earth's gravitational field and rotation. Its boundary with the outer core is sometimes called the Lehmann discontinuity. The inner core is not ferromagnetic due to being above the Curie temperature, but it affects the outer core's fluid motions that generate Earth's magnetic field.

Did You Know?

• The inner core's radius is about 70% of the Moon's radius.

The Discovery of a Solid Heart

In 1936, Danish seismologist Inge Lehmann made a discovery that reshaped our understanding of Earth's interior. By studying seismograms from earthquakes in New Zealand, recorded by sensitive instruments on the surface, she noticed that seismic waves were reflecting off an internal boundary. From this, she inferred the existence of a solid inner sphere distinct from the molten outer layer, estimating its radius at roughly 1,400 km—remarkably close to the currently accepted value of about 1,221 km. Two years later, Beno Gutenberg and Charles Richter analyzed a more extensive dataset and proposed a 300 km transition zone between the two core layers, placing the inner core's radius between 1,230 and 1,530 km. By 1940, the idea that this inner sphere was solid iron had gained traction, and in 1952 Francis Birch published a thorough analysis concluding it was probably crystalline iron. The inner core's rigidity was formally confirmed in 1971, and in 2005 the detection of shear waves traversing it—initially controversial—further solidified its status as a solid body.

Reading the Planet Through Its Vibrations

Since no drill has ever reached the inner core and no direct samples exist, scientists rely almost entirely on seismic waves to characterize it. Deep earthquakes, originating 30 km or more below the surface where the mantle is relatively uniform, generate waves that travel through the planet and are recorded by seismographs worldwide. P-waves, or compressional waves, can pass through both solid and liquid materials, while S-waves, or shear waves, propagate only through rigid solids. The most informative signals include PKiKP waves, which reflect off the inner core boundary, and PKIKP waves, which pass straight through the inner core. Crucially, when P-waves strike the inner-outer core boundary at an oblique angle, they can convert into S-waves, travel through the inner core as shear waves, and convert back to P-waves upon exit. These PKJKP waves provide direct evidence that the inner core is solid enough to support shear motion, confirming what had long been suspected.

A Forging Under Extreme Pressure

The inner core is a solid ball with a radius of approximately 1,230 km, making it about 20% of Earth's total radius and roughly 70% the size of the Moon. Its volume is around 7.6 billion cubic kilometers, accounting for less than one percent of Earth's total volume. Despite sitting at a temperature near 5,700 K—comparable to the Sun's surface—it remains solid because of the immense pressure at Earth's center, a relationship described by the Simon-Glatzel equation. Its composition is believed to be an iron-nickel alloy with trace additional elements, though it is not ferromagnetic because it sits above the Curie temperature. Seismically, P-wave velocities range from about 11.4 km/s at the center to 11.1 km/s at the surface, dropping abruptly to 10.4 km/s at the inner-outer boundary. S-wave velocities shift from roughly 3.7 km/s centrally to 3.5 km/s at the boundary—both considerably slower than in the deep mantle. Its shape is thought to be a slightly flattened oblate ellipsoid, with a flattening ratio between 1/400 and 1/410.

A Shifting Paradigm and Broader Influence

For decades the inner core was treated as a static, perfectly rigid iron-nickel sphere. Yet as recently as February 2025, new indications have emerged suggesting it is partially deformable and undergoing viscous changes—a development that challenges the long-held picture of an immutable solid ball. Beyond its own physical properties, the inner core exerts a profound influence on Earth's magnetic field. Although that field is generated primarily by fluid and electric currents in the outer core, those currents are strongly shaped by the solid inner core's presence and the heat flowing outward from it. The core's density, dimensions, and shape also affect Earth's gravitational field, angular inertia, and the natural oscillation frequencies that cause the planet to ring like a bell after large earthquakes. Because no direct samples are accessible, every understanding of the inner core remains an inference drawn from seismic wave behavior, magnetic field measurements, and the planet's bulk mechanical properties—a remarkable feat of indirect science.

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