Lehmann discontinuity

The Lehmann discontinuity is an abrupt increase of P-wave and S-wave velocities at the depth of 220 km in Earth's mantle, discovered by seismologist Inge Lehmann. The thickness is 220 km. It appears beneath continents, but not usually beneath oceans, and does not readily appear in globally averaged studies. Several explanations have been proposed: a lower limit to the pliable asthenosphere, a phase transition, and most plausibly, depth variation in the shear wave anisotropy.

General references

 * – some historic background.