The Earth did not have the interior structure described in the preceding section when it was formed. The geological process by which the Earth came to have its present interior structure is called differentiation, and is illustrated in the following figure.

The process of geological differentiation

Within about 1 billion years of its formation the Earth was melted by heat arising from a combination of sources:

  1. Gravitational energy left from the formation of the planet,

  2. Meteor bombardment

  3. Decay of radioactive material trapped in the body of the Earth.
While the Earth was molten, gravity acted to concentrate more dense material near the center and less dense material nearer the surface. When the Earth solidified again (except for the liquid outer core) it was left with a layered structure with more dense material like iron and nickel near the center and less dense rocks nearer the surface. As the outer layers cooled and solidified, large cracks developed because of thermal stress, leaving the lithosphere broken up into large blocks or plates.

As we shall see, this has enormous implications for the subsequent geological history of the Earth because it produces conditions favorable for plate tectonics. One of the crucial questions that we will have of all solid bodies in the Solar System is whether they have ever been differentiated.