The whole thickness of rock down to the Moho is called the Earth’s crust. Scientists have a fair idea of its structure.

On the continents, the outermost material is a skin mostly of sedimentary rocks, worn through here and there.

Under this skin lie rocks of the granite family. Beneath the granite the lie rocks that behave lay basalt.

But under the oceans, the crust has a different structure. There is no granite-only basalt and rocks resembling it, which extend down to the Moho.

Beneath the Moho, wave speeds increase gradually without another jump for 1,800 miles.

This whole thick region is called the Earth’s Mantle.

Then, at the 1,800-miles level, there is a sudden slow-down. This point marks the boundary f the Earth’s core, which evidently is made of different material from the mantle.

Waves from a shock spread through the Earth and are picked up by stations at various distances.

But in a board zone beginning about 7,300 miles from the Earthquake, the waves fade out. Waves miss it because they bend inward when crossing the boundary of the core, and therefore come out farther around towards the opposite side of the Earth.

No S-waves are found to pass through the core-only P-waves. This is a clue to the state of the core. P-waves go through a liquid, since it is elastic and jumps back from compression.

Another clue about the core is the drop in speed when a wave enters it. This is almost certainly caused by a sudden increase in density, as though the core were made of denser material than the mantle-possibly iron and other heavy metals.

But the core is not the same al the way through. Very faint seismic waves have been picked up in the shadow zone.

These probably are echoes from a boundary inside the core. Scientists think it is made up of metals kept solid by enormous pressure. The seismic messages seem to say: the Earth has a heart of iron.

Crust, Mantle and Core