Geothermal Gradient

Geothermal gradient is the rate at which the Earth's temperature increases with depth below its surface.  On average, the temperature increases by about 25°C for every kilometer of depth.

Key Points about Geothermal Gradient

Heat Sources

• Radioactive Decay  -  The primary source of geothermal heat is the decay of radioactive isotopes in the Earth's mantle and crust.
• Residual Heat  -  Heat remaining from the Earth's formation.
• Mantle Convection  -  Heat transferred from the deeper mantle.

Variation with Depth

• Near the surface, the geothermal gradient is relatively high because heat from deeper within the Earth has to pass through the insulating crust.
• At greater depths, especially in the mantle, the gradient tends to be lower because the material is more conductive and convective heat transfer becomes more significant.

Geographical Variation

• Stable Cratons  -  Regions with old, stable crust typically have lower geothermal gradients.
• Tectonically Active Areas  -  Regions with volcanic activity or active tectonics often have higher gradients due to the proximity to magma and increased crustal thinning.

Applications

• Geothermal Energy  -  Understanding where heat from the Earth is used to generate electricity or provide direct heating.
• Oil and Gas Exploration  -  The gradient affects the thermal maturity of organic materials, influencing hydrocarbon formation and the location of oil and gas reserves.

Measurement  -  The geothermal gradient is measured by drilling boreholes and recording temperature at various depths.  These measurements are then used to estimate the temperature increase with depth.

 

Geothermal Gradient formula
\( g_g \;=\;  (T_{bh} - T_s \;/\; D_{bh} ) \; 100 \)     (Geothermal Gradient) \( T_{bh} \;=\;  (g_g \; D_{bh} \;/\; 100 )  + T_s   \) \( T_s \;=\;  T_{bh} -  (g_g \; D_{bh} \;/\; 100 ) \) \( D_{bh} \;=\;  100 \; (T_{bh} - T_s ) \;/\; g_g \)
Symbol	English	Metric
\( g_g \) = Geothermal Gradient	\(F \;/\; 100\;ft\)	\(C \;/\; 100\;m\)
\( T_{bh} \) (Greek symbol sigma) = Maximum Recorded Temperature	\(F\)	\(C\)
\( T_s \) = Temperature Near Surface	\(F\)	\(C\)
\( D_{bh} \) = Total Depth of Logged Borehole	\(ft\)	\(m\)