Discovery of the Earth’s core

The discovery of the Earth’s core was a series of findings made with earthquake waves in the late nineteenth and early twentieth centuries. Richard Oldham established in 1906 that the planet has a distinct central core, Beno Gutenberg located its boundary in 1912, Harold Jeffreys showed in 1926 that the outer core is liquid, and Inge Lehmann identified the inner core in the 1930s.

Background

Early evidence about the planet’s interior came from magnetism. In 1600 William Gilbert used a magnetized iron sphere as a model of the Earth and found that the magnetic lines around it matched what a compass needle did across the globe. This suggested that the Earth holds a large amount of magnetized iron.

Compass records also showed that the magnetic field was slowly drifting westward, which seemed impossible if the interior were solid iron. In 1692 Edmond Halley proposed a model with a solid iron core, a fluid layer around it and an outer shell, with the core turning slightly slower than the crust.

Afterward many scientists favored a thin crust over a molten interior, because volcanoes erupted hot material and temperatures rose with depth. That view was challenged from the 1830s. André-Marie Ampère argued in 1833 that lunar tides acting on so much liquid would make the surface unstable, and William Hopkins noted in 1842 that pressure raises melting temperatures. Lord Kelvin’s measurements supported his claim that the Earth was as rigid as steel, and by the end of the century most geologists regarded the whole planet as solid.

The discovery

A new tool arrived with the seismograph. In 1883 John Milne suggested that sensitive instruments could detect large earthquakes anywhere in the world. In 1889 Ernst von Rebeur-Paschwitz showed that instruments in Germany had recorded vibrations from an earthquake in Tokyo that had traveled through the Earth’s interior.

Seismic waves come in several kinds. Primary waves, or P-waves, travel through solids, liquids and gases. Secondary waves, or S-waves, pass only through solids, and surface waves travel along the ground. In his report on the 1897 Assam earthquake, published in 1899, the British seismologist Richard Dixon Oldham gave the first clear evidence for all three types.

Oldham noticed that P-waves reached instruments on the far side of the planet later than expected. He concluded that they had crossed a central core where their speed was substantially lower. Since the Earth was then assumed to be entirely solid, he reasoned that the core must be much denser than its surroundings, probably iron. His first arguments were not convincing. After gathering more data he published a paper in 1906, titled The Constitution of the Interior of the Earth as Revealed by Earthquakes, that firmly established the existence of the core.

How it was confirmed

Other seismologists refined the picture. In 1910 Andrija Mohorovičić identified the boundary between the crust and the mantle. In 1912 Beno Gutenberg, a student of Emil Wiechert at Göttingen, used methods developed there to calculate wave speeds at different depths and found a major discontinuity at about 2,900 kilometers. It is now called the Gutenberg discontinuity and marks the boundary between the mantle and the core.

Whether the core was liquid took longer to settle. By 1913 Oldham was becoming convinced that no S-waves passed through it. Leonid Leybenzon had reached a similar idea in 1911 in a Russian publication that attracted little notice, and Wiechert suggested in 1924 that a sudden drop in velocity below the boundary was indirect evidence of a fluid core. Gutenberg stayed convinced that the core was solid.

In 1926 Harold Jeffreys showed that the evidence Gutenberg cited could be read in support of a liquid core. His most conclusive argument was that the average rigidity of the mantle was much greater than that of the whole Earth, which required a region of low rigidity inside the core. That result also explained why S-waves do not cross it.

The Danish seismologist Inge Lehmann then found a further boundary within the core. Analyzing data from the 1929 earthquake at Buller, New Zealand, she showed that some P-waves were reflecting from a sharp surface inside it, which she calculated at a radius of about 1,400 kilometers. This is now called the Lehmann discontinuity. Her argument that the inner core is solid was not conclusively established until the early 1970s.

Disputes over credit

Credit for the core is shared among several people, and accounts differ on details. A reference article from Gale states that Oldham, contrary to a widespread belief, did not discover that the core is liquid. He maintained that it was extremely dense and probably iron, and moved toward a fluid core only after 1913. The liquid core is generally credited to Jeffreys in 1926, with Leybenzon and Wiechert as earlier voices.

The date of the inner core also varies by source. The U.S. Geological Survey earthquake science timeline places Lehmann’s discovery of the inner core in 1935, while the Gale article dates it to 1936, the year of her published analysis. Gutenberg located the core boundary accurately but held to a solid core.

Impact and legacy

In 1939 Harold Jeffreys and Keith Bullen created a model of the Earth’s interior from seismic waves recorded around the world. Their travel-time tables were used to locate earthquakes for the next fifty years. In 1940 Gutenberg and Charles Richter published Seismicity of the Earth, which showed that earthquakes concentrate in narrow zones that were later recognized as the boundaries of tectonic plates.

The core also changed ideas about magnetism. Iron loses its magnetism at the extreme temperatures believed to exist in the inner core, so a core of iron cannot itself be a permanent magnet. Scientists turned instead to the idea that electric currents flow inside the planet. In 1946 Walter Elsasser proposed a geomagnetic dynamo in which currents are generated in the outer core by the motion of the fluid, and in 1948 Edward Ballard developed a similar and more detailed theory. Only in the last years of the twentieth century did computer models accurately simulate the westward magnetic drift and spontaneous pole reversals.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 5, 2026Researched and drafted with AI assistanceReport an issue