Brainpop: earth’s structure

There are three main sources of heat in the deep earth: (1)heat from when the planet formed and accreted, which has not yet been lost (think small bodies like moon that have cooled off) (2) frictional heating, caused by denser core material sinking to the center of the planet(3) the center of the Earth is also hot due to the gravitational pressure of the Earth above (think pump)(4) heat from the decay of radioactive elements (not much)

Then, earth's plates act as a blanket on the interior.