The boundary between the Earth's liquid outer core and its solid inner core is called the:

The Lehmann discontinuity is the boundary between the liquid outer core and the solid inner core of the Earth, at a depth of about 5,150 km below the surface.

Discovery:

• It is named after the Danish seismologist Inge Lehmann, who discovered the solid inner core in 1936.
• She studied seismic waves from earthquakes in New Zealand recorded in Europe and noticed that some P-waves appeared in the "shadow zone", where they should not have been seen if the core were entirely liquid.
• She concluded that these waves had been reflected by a distinct inner core, with properties different from the outer core.
• Inge Lehmann was one of the first women to make a major discovery in geophysics.

Note on naming:

The term "Lehmann discontinuity" is also used for a different boundary within the upper mantle (at about 220 km), which she also identified. In many Indian textbooks and exams, however, the outer core-inner core boundary is called the Lehmann discontinuity. Strictly, this boundary is also called the inner core boundary.

The inner core:

• It is a solid ball with a radius of about 1,220 km, made mainly of iron and nickel.
• Its temperature is about 5,000 to 6,000 degrees Celsius, similar to the surface of the Sun.
• It remains solid despite the high temperature because of the enormous pressure, more than 3 million times atmospheric pressure.
• It is slowly growing as the outer core cools and iron solidifies onto it.

Summary of major boundaries inside the Earth:

• Crust to mantle: Mohorovicic discontinuity (Moho), about 35 km under continents.
• Within the crust: Conrad discontinuity.
• Upper to lower mantle: Repetti discontinuity.
• Mantle to outer core: Gutenberg discontinuity, about 2,900 km.
• Outer core to inner core: Lehmann discontinuity, about 5,150 km.

Why not the other options?

• ✗The Gutenberg discontinuity separates the mantle and the outer core.
• ✗The Mohorovicic discontinuity separates the crust and the mantle.
• ✗The Conrad discontinuity lies within the continental crust.