Geothermal Gradient vs. Tectonic Setting
The average geothermal gradient of the earth is about 25°C/km. However, in volcanically active areas like island arcs, the temperature may increase by about 30-50°C/km. In areas such as ocean trenches, the temperature may increase by as little as 5-10°C. The following diagram illustrates how the earth's geothermal gradient varies with tectonic setting. The dashed yellow line represents the 500°C isotherm.
- High Geothermal Gradient: Mid Ocean Ridges, Arc Orogens, Spreading Ridges.
Temperature increases quickly with depth, due to rising hot magma beneath.
- Low Geothermal Gradient: Subduction Zone. Temperature increases slowly with depth, ie: cool rocks are found deeper in the earth. This is due to the relatively cool sediments and fluids (ie: seawater) being subducted. [At great depth the rocks start to metamorphose and eventually melt, forming magma. The temperature gradient is then deflected back upwards due to the rising hot magma.]
- Average Geothermal Gradient: Continent. Continental areas away from tectonically active zones have average geothermal gradients.
[Modified from Raymond]
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