It is not known why the earth's core is hot, but there are lines of evidence. The changing thickness of the earth's crust indicates a constant cooling, which indicates that the heat was created at the beginning of the earth's formation.

About a billion years ago, all of the continents had come together forming a large super continent called Pangea. There were no mountains at that time. An "ice age" caused the whole land mass to be covered with ice, which destroyed all terrestrial life. After the ice melted, terrestrial life reevolved from the sea creatures.

The absence of mountains means the tectonic plates were very thin and light. If they were as thick and heavy as modern plates, they would have buckled and slid over and under each other as they collided to form Pangea, and mountains would have been the result. Since the plates were thin and light, they just welded together as they collided.

For example, a significant earthquake occurred in central South Dakota in 1983. Earthquakes were supposed to be impossible in the area, because there is supposedly one continuous sheet of granite under the entire state. Earth quakes are caused by two plates sliding past each other. It means there are two plates which fit together so precisely that they look like one. This conclusion is reinforced by the fact that there are two very distinct soil types above each plate. Loam sits over the east river plate, and a heavy gumbo sits over the west river plate.

So the tectonic plates were thin and light in earlier times, and now they are getting thick and heavy. This means the earth is losing heat, and the heat in the earth's core must have been there at the beginning of the earth's creation. The heat apparently resulted from small particles gravitating toward a center, and the process of colliding and compressing created heat through friction.

Additional evidence is in the observation that planets which are farther away from the sun are losing heat faster than they are acquiring it from the sun. They apparently acquired their heat during their creation.

Heat in the core of planets cannot be entirely due to nuclear reactions, because that source would require a gradual build-up rather than a gradual loss of heat. However, nuclear reactions could be occurring in the core, because heat and pressure should promote them. But then they must be producing heat at a lower rate than it is being lost through radiation into space. Otherwise, there would not be the observed cool-down.

It seems likely that ice ages on earth are caused by a nuclear hot spot in the core rotating toward the surface and heating the Pacific Ocean. The primary evidence for this is that the past ten ice ages have been cycling at 100 thousand year intervals. Environmental changes are not apt to be so cyclic, but a convectional oscillation in the earth's core could be.

It's quite significant that a large number of coral reefs are dying from over-heating. Humans are not causing the oceans to over-heat; it appears to be caused by heat from the earth's core.

Another Link: Earth's Interior