A few humans have been from the Earth to the Moon, and many more have been around the world in 80 days or fewer. But no one has yet made a journey to the center of the Earth. Our experience with the planet on which we live is almost entirely confined to the material and information that makes its way to the surface.
For that reason and others, precise measurements of Earth’s overall composition are difficult or impossible. The continental crust, the most familiar part of Earth for most of us, is not a representative sample: Its composition is not even the same as that of the crust beneath the oceans. The difference is attributed to the effects of partial melting of the silicate mantle that lies beneath the crust. Certain incompatible elements—so called because they strain the crystal lattices of the solid mantle—were preferentially pushed out of the solid...
