Geothermics - Gradient, flow and propagation

Geothermal gradient

The temperature measured through the first meters of the Earth's crust increases with deep by an average progression of 3 ºC per 100m. The connection between temperature variation and depth is named geothermal gradient.

Apart from natural background heat caused by different physical and chemical processes, there are other elements involved in the thermal equation:

Regional elements: The geological and structural contexts determine the distribution of temperatures; then the geothermal gradient can be higher than the average in places without volcanic activity or with a thick lithosphere.

Local elements: The differences between thermal properties of rocks, like thermal conductivity, produce lateral and vertical variations of geothermal gradient. The element that has more influence on geothermal gradient value is the groundwater circulation, due to its capacity to spread the heat. Then, the average geothermal gradient decreases because of descendant circulation of cold groundwater in refilling areas, whereas in discharge areas, the ascent of deep hot groundwater produces the opposite result.

Some geological structures can influence the average geothermal gradient values in localized areas, because the hot groundwater can ascent from deep areas through fault surfaces, producing local, very high temperature thermal anomalies.

Thus, the values of the geothermal gradient depend on depth, geological and structural context, on differences of thermal properties of rocks, and groundwater circulation.

Flow and propagation of Earth's heat

The Earth's heat is detected by means of shallow heat flow measurements. The heat flow is the amount of heat transferred across a given section of an isothermal surface per time unit. The measurement of shallow heat flow is done by the product of geothermal gradient and thermal conductivity. 
The Geothermal gradient is the temperature variation with depth, and thermal conductivity is the ability of a material's to conduct heat. An average value of the heat flow inside a continent is 60mW/m2. This value can decrease until 30mW/m2 in ancient continental areas, with high lithospheric thicknesses, and can increase until 120mW/m2, in areas with a thin lithosphere.