what is heat conduction?
What is mantle?
How is heat transferred?
The origin of a geothermal resource relates to the following equation for conductive heat flow:
where q is heat flow (in W m-2); k is thermal conductivity (in W m-1 K-1 ); and ΔT is the temperature difference (in kelvin or K = °C + 273 °C) through a depth z.
The geothermal gradient (Δ T/z ) has units of K km-1. Thermal conductivity expresses the ease with which a material transmits heat. Thus a metal pan has a high thermal conductivity whereas an oven glove is a poor conductor of heat.
All rocks are poor conductors of heat in the everyday sense, but some rocks, such as sandstones and granites, are better conductors of heat than others, such as shales and many metamorphic rocks.
If heat flow (q) is 100 mW m-2 (100 × 10-3 W m -2 ), as it could be in some volcanically active areas, and the geothermal gradient is 100 K km-1 , i.e. the temperature difference ΔT between the surface and at 1 km depth is 100 K, then thermal conductivity k can be calculated:
If the geothermal gradient were less, say 50 K km-1 , then the same heat flow could only be produced if it passed through rocks with a thermal conductivity given by:
So, if the geothermal gradient is halved, for the same heat flow the thermal conductivity must be doubled.