Why does Earth’s temperature increase with depth below the surface?

Earth’s temperature increases with depth below the surface due to the planet's internal heat, a phenomenon quantified by the geothermal gradient. This temperature rise is primarily driven by the radioactive decay of elements such as potassium, thorium, and uranium within the crust and mantle.21

Additionally, heat is generated through friction along the margins of continental plates. The local temperature at any given depth is further influenced by the regional heat-flow regime, which includes heat transported by silicate melts and the tectonic movement of rocks.12

While the average increase is approximately 30 °C per kilometer (roughly 90 °F per mile) in the first 10 kilometers of the crust, this rate varies significantly based on location. For example, regions with active volcanism or mantle upwelling exhibit much higher gradients, whereas stable continental interiors or subduction zones show different thermal profiles.12