Within about 1 billion years of its formation the Earth was melted by heat arising from a combination of sources:
- Gravitational energy left from the formation of
the planet,
-
Meteor bombardment
-
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.
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