The Earth emits, on its surface, thermal energy that humans use to satisfy a fraction of its energy needs. Geothermal energy (exploitation of heat from the basement) inexhaustible to a humanity scale is one of the possible alternatives to fossil fuels.
Origin of geothermal flux:
Geothermal energy comes mainly from the decay of radioactive elements such as uranium (238U and 235U), thorium (232Th) and potassium (40K) contained in rocks: the Earth's mantle is the main producer of this heat which heat rocks and fluids in depth.
Geothermal flow géothermique corresponds to the amount of heat dissipated by the globe (87 mW.m-2 on average).
The geothermal gradient denotes the value of the temperature's increasing depending on the (expressed in °C.km-1)
By observing the evolution of the geothermal gradient in terrestrial envelopes, we can notice the existence of two geothermal gradients and thus two different heat transfer to the surface of the globe:
- In the lithosphere (from 0 to about -110 km deep), the gradient is important: the heat is transferred by conduction, ie slowly step by step without global movement of materials.
- in the lower mantle and the asthenosphere where the gradient is low, the heat transfer is more efficient because it is carried out by convection: the hot material less dense than cold material is animated with upward movements. This transfer by convection is just possible because the coat rocks are deformable at this depth, and this allows a displacement of a few millimeters per year.
We observed a significant flow to the back where the ascent of hot material and the production of new oceanic lithosphere occurs.
At subduction zones, there is a low flow area corresponding to the embedding of the cold lithospheric plate and more locally, a geothermal area of high flows caused by the magmatism related to the volcanic arc. So the Earth is a thermal machine.
The most favorable areas for geothermal electricity are the high flow and geothermal gradient zones: these are the areas associated to subduction magmatism, hot spot and rifting where electricity production is possible, we talk about high geothermal energy. When the flow and the gradient is low, we speak of low geothermal energy used mainly for collective or individual heating.
The levy of any energy by humans is only a small part of what is dissipated.
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To validate your visit:
1-. What are the temperatures of hot springs of low and medium energy? And of high energy?
2-. What is, in general, the value of the geothermal gradient? (text on red background)
3-. At what depths is found a geothermal energy interesting for exploitation in Aquitaine and in the Paris basin?
4-. What is the volcano, named in the text, which has a very important activity of geothermal energy next to it?
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