6.7 The Structure of the Earth - Indirect Evidence (Chapter 6: Earth's Structure: Direct and Indirect Evidence)

The Structure of the Earth –
Indirect Evidence

, Density of the Earth
• To have validity, the proposed model of a layered Earth with a dense
core must agree with known data such as the mean density of the
whole planet.
• Direct measurements show that the density of the rocks of the
continental crust is 2.7g/cm³ and the oceanic crust is 2.9g/cm³.
• The density of the core must be higher to balance the lighter crust.
• On the surface the density of iron is 7.9g/cm³ and nickel is 8.9g/cm³;
these values would be substantially higher, at least 12g/cm³ under
the extreme pressures within the core.
• The average density of the Earth is 5.5g/cm³.

, Density of the Earth
• By calibrating seismic wave velocities in rocks of known physical
properties, we can suggest densities for the various layers of the
Earth’s interior, taking into account the expected conditions of
temperatures and pressures.
• The graph shows the changes in density with depth, with a clear
increase in the mantle-core boundary where the composition is
believed to change from silicates to iron and nickel.