Question

What caused Earth to develop layers as it cooled?

Answer 1

the difference in density is your answer

Answer 2

The answer is difference in density. We know that the high central density suggests the earth must be rich in nickel and iron and that much of the core must be liquid. This lower-density air then rises and is replaced by cooler, higher-density air. The result is atmospheric circulation that drives the weather and climate through redistribution of thermal energy.