Geothermal Gradient
The geothermal gradient is the rate at which the temperature of the Earth's interior increases with increasing depth.
Average geothermal gradient:
Continental areas away from tectonically active zones have average geothermal gradients. The geotherm never reaches the solidus (melting point) of the rock.
High geothermal gradient:
Temperature increases quickly with depth at mid-ocean ridges. This is due to rising hot magma beneath a thinner lithosphere. Note that there is a zone in which the geotherm exceeds the solidus, leading to melting and magma formation.
Low geothermal gradient:
Temperature increases slowly with depth at subduction zones. This is due to the relatively cool sediments and fluids (i.e.: seawater) being subducted along with old, cold ocean lithosphere that penetrates to great depths. Even though the rock is relatively cool, the solidus is depressed by water-bearing rock and sediment, leading to melting and convergent plate boundary volcanism.