Why is Middle-earth so hot?

Answer 1

Much of the heat in the Earth's core is left over from the formation of the planet, when planetesimals collided releasing large amounts of energy. Much of that leftover energy is what we see today as heat in the center of the Earth. Smaller planetary bodies cool more quickly and have lost most of their heat (moon, mars) and have cooled to the point of being solid. The center of the Earth is also hot due to the gravitational pressure of the Earth above. A simple experiment to show how pressure can increase temperature is to squeeze an ice cube and see it melt, or pump up a bicycle tube and feel the pump. Although not quite correct for solids, the ideal gas law of T = PV/nK gives a relationship between temperature (T), pressure (P), volume (V), the number of atoms (n), and a constant (K). Thus you can see how temperature can increase as pressure increases. However, in spite of the high temperatures and pressures, the Earth's inner core is solid. The insides of stars have extreme pressures due to the gravitational attraction of so much material. Like in the earth, gravity in turn, creates high pressures and high temperatures. However, in stars, the high-pressure, high-temperature condition allows for fusion of Hydrogen to Helium (etc.) which releases much energy and heat and which we see as light from the stars. This can not occur in the Earth (or even Jupiter) because there is not enough gravitational pressure (not enough mass), and (for Earth anyways) not enough Hydrogen. Some energy is transfered to the Earth through gravitational forces from the Moon's orbit. As the Earth spins (once per day) there are gravitational stresses on the surface due to the gravitational field gradient from the Moon. This causes surface effects such as tides and flexes the crust which provides some heat (through friction), but this is not enough to account for the high temperatures seen in the core, nor for the increasing temperatures seen the closer to the Earth's core you get. The earth's core is not hot due to friction from the continents moving, since the Earth's temperature increases steadily the farther down from the surface you go. Again, while friction will cause some heating, and it does contribute to some of the heat seen, it does so in specific areas (convection layers) and does not account for the high temperatures found at the core, nor does friction account for the steadily increasing temperatures seen. The earth's core is not hot due to radioactivity since the core of the Earth is largely iron and nickel. Many of the radioactive elements are found mostly near the surface. Besides, the amount of radioactive material needed to generate the temperatures seen in the Earth's core would make the planet a radioactive wasteland.

Answer 2

because there's fire there