What is the relationship between the rock temperature and the depth beneath the surface?

The temperature of rock generally increases with depth beneath the Earth's surface, a phenomenon known as the geothermal gradient. This increase is primarily due to the heat generated by the Earth's core and the decay of radioactive isotopes within the crust. On average, the temperature rises about 25 to 30 degrees Celsius for every kilometer of depth, although this can vary depending on local geological conditions. Thus, deeper layers of rock tend to be significantly hotter than those closer to the surface.