The Gutenberg discontinuity separates the:

The Gutenberg discontinuity is the boundary between the Earth's lower mantle and its outer core, at a depth of about 2,900 km (about 2,890 km) below the surface.

Discovery:

• It is named after the German-American seismologist Beno Gutenberg, who determined its depth with great accuracy in 1913-1914.
• Earlier, the German geophysicist Emil Wiechert had suggested the existence of a dense core, so it is sometimes called the Wiechert-Gutenberg discontinuity.

What happens at the boundary:

• Above it, the mantle is made of solid (though slowly flowing) silicate rock.
• Below it, the outer core is made of liquid metal.
• The density increases sharply, from about 5.5 to about 10 grams per cubic centimetre.
• P-waves (primary waves) slow down suddenly and are bent (refracted), creating a P-wave shadow zone on the surface between about 103 and 142 degrees from the earthquake's epicentre.

S-waves (secondary waves) cannot pass into the liquid outer core at all, so they stop at this boundary. This creates an S-wave shadow zone beyond 103 degrees from the epicentre.

These observations proved that the outer core is liquid.

Layers of the Earth and their boundaries:

• Crust to mantle: Mohorovicic discontinuity (the Moho), about 35 km under continents.
• Upper mantle to lower mantle: Repetti discontinuity, about 660 km.
• Mantle to outer core: Gutenberg discontinuity, about 2,900 km.
• Outer core to inner core: another boundary at about 5,150 km, covered in a separate question.

Temperature:

The temperature at the core-mantle boundary is estimated to be about 4,000 degrees Celsius.

Why not the other options?

• ✗The crust and mantle are separated by the Moho.
• ✗The outer and inner core are separated by a different discontinuity.
• ✗The upper and lower crust are separated by the Conrad discontinuity.