Gutenberg discontinuity

The Gutenberg discontinuity, also called the core–mantle boundary, marks the transition from the rocky lower mantle to Earth's metallic outer core. It lies roughly 2,900 kilometres below the surface, though sources sometimes give a figure nearer 2,800 kilometres depending on rounding and convention. It is named for seismologist Beno Gutenberg, whose work on earthquake records helped establish the boundary's position and properties. The boundary is not directly accessible to drilling; its structure is inferred mainly from how seismic waves travel through the planet.

Earthquakes produce several kinds of seismic waves. Primary waves, or P waves, are compressional and can travel through solids and liquids. Secondary waves, or S waves, are shear waves and cannot propagate through liquids. Seismologists observed that S waves do not travel through the outer core, while P waves change speed and bend as they pass the core boundary. This pattern produces seismic shadow zones and is evidence that the outer core is liquid. The Gutenberg discontinuity therefore represents a sharp contrast in material properties as well as a compositional transition between mantle rock and the core.

The core–mantle boundary is a key interface in Earth's internal structure. Changes in seismic-wave speed and direction across it let researchers estimate the density and physical properties of deep materials that cannot be sampled directly. The liquid outer core also participates in the processes that generate Earth's magnetic field, although the boundary itself is not the magnetic field's source. Modern seismic methods continue to examine variation in the boundary and the complex structures on either side of it.