What it is

Our planet constantly emits energy in the form of heat, which propagates from deeper ground towards the surface. This is the so-called flow of heat or geothermal flow.
The Sun  heats the Earth’s surface with a flow of heat that is almost 6,000 times greater than the heat from inside the Earth, however, the constant and continuous geothermal flow is an important source of heating for our Planet. With an average of 0.06 watt per sq. m, an overall amount of heat, equal to approximately 30,000 billion watts is radiated from the entire surface of the Earth.
The Earth gets hotter as we dig deeper in the ground, and this is a phenomenon that is well known to miners. Some of the deep mines and galleries reach temperatures that are near the boundary of human survival (which is not the case in caves, where the natural circulation of air and water remarkably lower the temperature, so that an increase in temperature related to depth is practically not felt). The Earth’s heat, is mostly due to energy freed in the decay process  of the radioactive isotopes of some elements such as  potassium, thorium and uranium. Due to the different thicknesses of the Earth’s crust and the different geological situations which can cause the rise of warmer materials from deeper zones, the geothermal gradient (i.e. the increase in temperature, due to greater depth) is not equal all over the World. On average, the temperature increases 2-3°C per 100 m in depth, but the increase can vary  from 1° up to 5°C/100 m.
In order to measure the geothermal gradient, wells are dug at least 300 m deep  (so that the effect of daily and annual variations in the temperature, due to climatic influences, is not felt). In these wells, special thermometers  which record the temperature at the different depths are positioned.
The flow of heat is greater where the thickness of the lithosphere is less, as for example on the ocean ridges or in the continental rifting zones or in volcanic areas where different geological processes lead to rock melting, or in areas where there is slowly cooling magma in the subsoil.

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