Geothermal gradient is the rate at which the Earth's temperature increases with increasing depth below the surface. The geothermal gradient is a basic concept in geology, geophysics, geothermal engineering, petroleum engineering, mining, and hydrogeology because it describes how subsurface temperatures change with depth. The increase in temperature occurs primarily because heat continuously flows outward from the Earth's interior, where it is generated by residual heat from planetary formation, radioactive decay of naturally occurring isotopes (primarily uranium, thorium, and potassium), and heat released from the Earth's core.
Geothermal Gradient Formula |
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\( g_g \;=\; \dfrac{ T_{bh} - T_s }{ D_{bh} } \cdot 100 \) (Geothermal Gradient) \( T_{bh} \;=\; \dfrac{ g_g \cdot D_{bh} }{ 100 } + T_s \) \( T_s \;=\; T_{bh} - \dfrac{ g_g \cdot D_{bh} }{ 100 } \) \( D_{bh} \;=\; \dfrac{ 100 \cdot (T_{bh} - T_s ) }{ g_g }\) |
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| Symbol | English | Metric |
| \( g_g \) = Geothermal Gradient | \(F \;/\; 100\;ft\) | \(C \;/\; 100\;m\) |
| \( T_{bh} \) (Greek symbol sigma) = Maximum Recorded Temperature | \(^\circ F\) | \(^\circ C\) |
| \( T_s \) = Temperature Near Surface | \(^\circ F\) | \(^\circ C\) |
| \( D_{bh} \) = Total Depth of Logged Borehole | \(ft\) | \(m\) |
The average continental geothermal gradient is approximately 25 to 30 C\(^\circ\)/km (about 1.4 to 1.6 F\(^\circ\) per 100 ft) within the upper continental crust. However, this value is only a global average. Actual geothermal gradients vary considerably depending on local geology, tectonic setting, rock type, groundwater movement, and regional heat flow. Stable continental interiors often exhibit gradients between 20 and 30 C\(^\circ\)/km, whereas tectonically active regions, volcanic areas, rift zones, and geothermal fields may exceed 40 to 100 C\(^\circ\)/km. Conversely, areas with significant groundwater circulation or unusually low heat flow may have gradients below 15 C\(^\circ\)/km.
