Temperature within the Earth's crust increases with more depth, this phenomenon is called geothermal gradient. The heat in the crust comes from the movement of continental plates, the earth's core and decay of naturally occurring substances within the crust as they slide against and underneath each other. Geothermal systems are found in regions with a geothermal gradient higher than the average value, and especially in regions around active tectonics and volcanoes where the geothermal gradient is usually significantly higher than the average value. It is estimated the thermal energy in the uppermost 6 miles of the earth's crust contains 50,000 times the energy of the entire world's gas and oil resources.
Geothermal energy is useful for a wide range of temperatures, from room temperature to over 300°C. The resources are usually divided into low and high temperature resources, according to a criterion that is generally based on the energy content of the fluids and their potential forms of utilization. Low-temperature resources are those with temperature under 150°C and high-temperature those with temperature over 150°C.
In order to have a geothermal system that can be utilized, several factors need to coincide. The heat source needs to be large, presence of water is essential and the bedrock has to be porous, since cold water must be able to enter the system, be heated up and ascend to the surface as hot water or steam. In some systems, convection is effective in bringing hot fluids up towards the surface.
