Earth

Since we do not have direct access to any planet's interior, all the knowledge we have is based on indirect evidence and computer modeling. As far as the Earth is concerned, we have the additional advantage of recording and analyzing earthquakes at several locations. Since the seismic waves which cause earthquakes propagate differently through materials of different compositions, Earth's interior has been mapped more accurately than that of any other planet. We can distinguish three main regions: the crust, the mantle, and the core. Of the three, the crust is farthest from the planet's center, under the least amount of pressure, and faces the lowest temperature. The core is closest, hottest, and under the greatest pressure.

The crust is a solid layer of variable thickness, ranging from 20 to 60 kilometers, constituting the external skin of the planet. It is composed of silicates, such as X_nSiO₃(4), where X represents a metal.

The mantle rests between the core and the crust and extends to a distance of about 3,500 km from the center of the Earth. The composition of the mantle is similar to that of the crust, but the silicates contain more iron (Fe) and magnesium (Mg) than crustal rocks, and have a higher density. Though made up of denser rock than that found in the core, the mantle faces such intense pressure and heat (ranging up to 3,000 K) that it is semi-solid and viscous. As a viscous material with non-uniform heat distribution, the mantle experiences convectional movement, with the hotter magma pushing up to the surface and pushing against the crust as it diffuses its hear. These currents are similar to those found in a boiling pot, though the mantle moves on a much larger and slower scale. The convectional currents of the mantle are responsible for the plate tectonics of the crust.

The core is situated within a 3,500 km radius from the center of the Earth. It is divided into two different regions, the external and internal core. The external core is liquid while the internal is solid. The external core, comprising the region between 3,500 and 1,500 km from the center, is made of iron (Fe) and sulphur (S) and has a temperature of 3,000-4,000 K. Scientists think that the magnetic field of the Earth originates here, due to a mechanism involving currents within this liquid medium in concert with Earth's rotation around its axis. While a similar outer core is probably present on Venus as well, the absence of a magnetic field on Venus might be explained by its slow rotation (243 days). The innermost core of the Earth is a solid ball of iron (Fe) and nickel (Ni), highly compressed and at a temperature of about 4,500 K degrees.

The Earth maintains such high temperatures in its mantle and core through the radioactive decay of elements such as uranium. The nuclei of these elements are unstable and tend to transmute into other, more stable, nuclei. Every instance of decay contributes a tiny amount of energy to the atoms around; this energy ends up as heat. Though each individual event creates limited amounts of energy, the enormous number of such decays has an equally enormous cumulative effect. The heat produce in the mantle and core can only escape through the crust, and that escape is precisely what plate tectonics and volcanism provide.