Lehmanndiscontinuiteten

Lehmanndiscontinuiteten, also known as the Lehmann discontinuity in English, is the seismic boundary between Earth's solid inner core and its liquid outer core. In some Nordic or related language traditions the term Lehmanndiscontinuiteten is used to refer to the same feature. The discontinuity marks a change in the physical state of the core materials and a sharp change in seismic wave propagation.

The concept originates with Inge Lehmann, a Danish seismologist who, in 1936, inferred the existence of a solid inner core from analyses of seismic P waves. Her work showed that some seismic waves are reflected at a boundary within the deep Earth, indicating a distinct transition from a liquid to a solid phase. The Lehmann discontinuity is therefore the boundary of the inner core, located at approximately 5150 kilometers below the Earth's surface (about 1220 kilometers from the center).

Physically, the boundary is characterized by a marked contrast in seismic wave behavior: P waves speed up and refract when crossing into the inner core, while S waves cannot propagate through the liquid outer core and produce characteristic diffraction patterns. Modern seismology also notes that the boundary is not perfectly smooth or uniform; there is regional topography and anisotropy within the inner core that reflect complex growth processes and composition.

Today, the Lehmann discontinuity is a fundamental element of the planetary model of Earth’s interior. It supports the concept of a differentiated core and informs theories about the geodynamo, core solidification, and mantle-core interactions.