What are the natural sources of the earth's thermal energy?

Question

What are the natural sources of the earth's thermal energy?

Answer 1

According to my physics prof.

The earths thermal energy come from 4 sources, Left over energy from the big bang creation (external). The sun (external), Gravity pulling inward on the weight of the earths mantel (Internal), and Rotation, the spinning of the earths molten core under the mantel (Internal).

If one wished to call the sun Radio Active Decay, it would be accurate since the sun is a giant atomic bomb going off in space.

However

Natural Radio Active Decay on the earth gives off less energy than concrete used in buildings and other construction.

Answer 2

The earth's core is hot.

Answer 3

The initial heat of accretion.

Radioactive decay.

Gravitational contraction.

Answer 4

Radioactive decay in Earth's Mantle

Heat left over from accretionary formation

Gravitational contraction energy