The geothermal gradient is the rate of temperature increase with depth inside the Earth, typically expressed in °C/km or °C/100 m. The average continental geothermal gradient is ~25–30°C/km, but it varies significantly with tectonic setting: higher in volcanic/rift zones (50–100°C/km) and lower in stable cratons (15–20°C/km). It drives the maturation of organic matter in source rocks and is a critical parameter in petroleum system modelling.