The other day I got a message asking about where the earth gets its heat. It brings up a number of misconceptions that I thought would be worth spending a post discussing, so here goes:Many people assume the earth to be millions if not billions of years old. Lava is molten, but the earth being only 8,000 miles in diameter has no internal heat source. It is almost like a thermos bottle that will lose heat over time. Many suppose that extreme pressure causes heat, but at the deepest depths of the ocean where the pressure is very high, it is also very cold.Image source: www.kidsgeo.comThis bit about pressure is correct: pressure does not cause heat if the thing you’re pressing on doesn’t change volume. However, compressing a gas can cause it to heat up. It’s called the “adiabatic lapse rate” and it explains why higher elevations see colder temperatures, and lower elevations are warmer. As air rises, it expands and cools so mountaintops are chilly. As air moves downward, it is compressed and warms up, which is why descending into the grand canyon or death valley in summer is a bad idea. But this effect can’t explain why the interior of the earth is hot, so let’s press on.Others assume that radioactive decay causes the heat, but with the advent of nuclear plants and control rods, it would take some very precise levels to control the heating process. Also, with radioactive decay, there would naturally be some radiation that would be present in many different ways and in all probability, would be exposed with any volcanic eruption.