There are several reasons why the earth's interior is so much hotter than its surface. First, the earth was formed through gravitational compression of many particles; a portion of that original heat remains, trapped beneath the crust. Second, some of the heat in the middle layers of the interior are hot because the deeper core is cooling and releasing heat. Third, a major portion of the heat comes from radioactive decay. As unstable elements like uranium break down, they release heat. All of this combines, and, since all is trapped beneath a rocky mantle, much of the heat is simply contained there.

As we dig deeper into the earth, we notice that temperatures rise approximately by 30 ºC for every kilometer that we descend. Temperatures in the Upper Mantle of the earth can reach up to 1,400 ºC. The Core of the earth is very hot and temperatures here are estimated to be around 7,000 ºC.

Scientists aver that the temperatures in the interior of the earth are high because of three reasons. The first reason is that the earth is still cooling down from the early days of its formation nearly 4.5 billion years ago. The second reason is that radioactive isotopes are continuously being formed by the decay of radioactive elements, and this process releases heat. Third, there is a continuous churn happening inside the earth wherein the heavier iron-rich metals are sinking into the core and lighter liquids are rising up to the mantle. The friction caused by this event is estimated to be releasing up to 3,000 ºC of heat.

Heat trapped within the earth has remained so because the spherical shape of the earth is an ideal one for heat insulation. This has slowed down the conductive and convective processes of heat loss.