Why the Earth's Interior is so Hot
I've read a good amount of science fiction in my time, but Jules Verne's 1864 classic, "A Journey to the Center of the Earth," is one book I can honestly say I've never even considered skimming through. Written by one of history's most inspirational author's, it's undoubtedly a profoundly entertaining novel, but given modern knowledge involving subterranean temperatures, little of which was known during Verne's generation, the title alone is a clear indication it's content likely contains far more fiction than it does actual science.
With an estimated average geothermal gradient, (rate at which the temperature increases the deeper into the earth's interior), of 1°F every 70 feet beneath the Earth's surface, anyone planning a subterranean adventure under solid ground would likely survive for a greater distance without protective gear traveling in the opposite direction. Hard to believe? The fact that magma from a volcano reaches the surface as molten rock serves as proof that it has to be awfully hot down there, even at shallow depths. As for the earth's center itself, recent reliable scientific research involving crystalline solids subjected to pressures similar to expected pressures at the Earth's core has estimated a center temperature of the Earth at roughly 11,000°F. That's approximately 1,000°F hotter than the surface of the sun!
Why is it so unimaginably hot beneath the Earth's surface? A big part of the answer to this question is that it simply started out that way, and it hasn't since been able to cool down. According to reliable scientific conclusions, the Earth was created roughly 4.5 billion years ago as the result of a series of colliding meteors. Moving at extremely high velocities, these meteors released an enormous amount of energy upon collision. As such, the earth started out a much hotter planet even than it is today.
Gradual cooling, however, gave rise to a solid layer of crust, averaging roughly 50 miles in depth beneath solid surfaces, which has since insulated the much hotter layer, called the mantle, that lies beneath it, much like a thermos insulates hot coffee. The silica rich mantle has provided further insulation to the even deeper and hotter two layers, called the outer and inner cores, that lie at the Earth's center of gravity. Largely attributable to this insulation, the Earth has been cooling at an extremely slow rate. While cooling rate estimates vary, all Earth researchers would agree that the Earth should maintain heat produced during its formation for eons yet to come.
Insulating upper layers, however, aren't the only reason the Earth's interior has been cooling down so slowly. Even with the Earth's crust and mantle trapping in much of it's original heat, one would still expect 4.5 billion years to have been a more than adequate amount of time for the entire Earth to have cooled down to a stable temperature. Such may have been the case, if not for the fact that the Earth has also been creating its own heat since its formation, by way of radioactive isotope decay. This is the other big part of the answer to the above-referenced question. Radioactive isotopes that lie deep beneath the Earth's crust, primarily uranium, potassium and thorium, release energy as they decay. Based on recent scientific research, it's estimated that close to half of the 44 trillion watts of energy the Earth continually releases is coming from these isotopes, (the remainder coming from the cooling process referenced above). So long as these radioactive elements remain buried within the Earth, this planet's interior will likely never cool down to a temperature matching that of its surface. The fact that the Earth's magnetic field, which protects us from life-threatening solar wind, depends on its heated iron core, this knowledge is something we can all take comfort in knowing.
So while humans have taken trips to the bottom of the Mariana Trench in electrically powered submarines, similar to parts of the voyage described in Jules Verne's classic novel from 1870, "Twenty Thousand Leagues Under the Sea," it's fair to say a journey to the center of the Earth is one mission Verne evidently once envisioned that will likely never happen. Even if we could figure out a way to shield ourselves from the tremendous heat, the crushing gravitational forces deep within the Earth would prevent any human from ever even getting close. Considering the 2008 blockbuster film with the same title, however, the idea of such a journey apparently still never gets old.