In the Earth's crust, temperature usually increases with depth because of the geothermal gradient. The geothermal gradient is the rate of temperature increase with depth in the Earth's interior. It is caused by the heat produced by the decay of radioactive elements inside the Earth and the heat from the Earth's core.
However, there are exceptions to the general rule of increasing temperature with depth. In some areas, the temperature may decrease with depth, which is known as a negative geothermal gradient. This can happen when there is a large amount of water in the crust, which can act as a heat sink and absorb heat from the surrounding rocks.
