Some of the heat in the centre of the Earth could have been left over from the formation of the planet. However, this would not be enough alone to explain certain terrestrial activities, such as techtonic plate movements. Another cause of internal heat is radioactive decay.
Radioactive decay occurs in unstable elements.
The continual compression and expansion of the planet due to the gravitational pull of the moon is also thought to contribute to the huge temperatures found at the centre of the Earth. Differentiation' is an imposing term used to describe what is, in fact, quite a simple concept. The result of this process is that the heavier elements sink towards the core of a planet, in this case Earth, to form the core, while the lighter elements 'float' above the core in layers from heaviest up to the lightest elements which make up the atmosphere. Hence, due to the effects of gravity, the Earth is differentiated. While CO 2 is transparent to visible light, the opposite is true when it comes to infra-red radiation.