The boundary between the Earth's crust and the mantle is known as the:

The Mohorovicic discontinuity, usually called the Moho, is the boundary that separates the Earth's crust from the mantle below it.

Discovery:

• It was discovered in 1909 by the Croatian seismologist Andrija Mohorovicic.
• While studying seismic waves from an earthquake near Zagreb, he noticed that some waves arrived at distant stations faster than expected.
• He concluded that the waves had travelled through a deeper layer where they moved faster, showing a sharp change in rock properties at a certain depth.

Depth of the Moho:

• Under the oceans, it lies about 5 to 10 km below the sea floor, because oceanic crust is thin.
• Under the continents, it lies on average about 35 km deep.
• Under high mountain ranges, such as the Himalayas and the Andes, it can be 60 to 70 km deep, because the crust is thicker there.

What changes at the Moho:

• The speed of seismic waves increases sharply, because the rock below (mantle) is denser.
• The crust above is made of lighter rocks, such as granite (continents) and basalt (oceans).
• The mantle below is made of denser rocks rich in iron and magnesium, such as peridotite.
• Attempts to reach the Moho: no drilling project has yet reached it; the Kola Superdeep Borehole in Russia reached about 12.2 km.

Other discontinuities inside the Earth:

• Conrad discontinuity: within the continental crust, between the upper (granitic) and lower (basaltic) crust; it is not found everywhere.
• Repetti discontinuity: within the mantle, between the upper and lower mantle.
• The boundary between the mantle and the core and the boundary between the outer and inner core are also marked by discontinuities, covered in separate questions.

How we know about the interior:

Most knowledge of the Earth's interior comes from the study of seismic waves (P-waves and S-waves), which change speed and direction when they pass through layers of different materials.

Why not the other options?

• ✗The Conrad and Repetti discontinuities lie within the crust and mantle respectively.
• ✗The Byerly discontinuity lies deep within the upper mantle, about 400 km down.