Thermal diffusivity reaches a maximum at approximately 0.8 GPa

Scientific Explanation

Thermal diffusivity describes how quickly temperature differences equilibrate within a material. It is defined as the ratio of thermal conductivity to the product of density and specific heat capacity. For water, this quantity shows an unusual maximum at a pressure of approximately 0.8 gigapascals.

This maximum arises from the interplay of pressure-dependent changes in several material properties. The thermal conductivity of water initially rises with pressure as molecules move closer together and transfer energy more efficiently. At the same time, however, both density and heat capacity also change under pressure. At about 0.8 GPa, the ratio of these quantities reaches its optimum — the thermal conductivity is high enough, but the rising values of density and heat capacity have not yet overtaken the numerator.

At still higher pressures, the thermal diffusivity decreases again because further densification and the increase in heat capacity exceed the improvement in thermal conductivity.

Everyday Relevance

This maximum is important for geoscience modeling. Water in the Earth’s crust and upper mantle exists at gigapascal pressures. The speed at which heat equilibrates through water-bearing rock layers affects thermal models of the crust, volcanic activity, and the dynamics of hydrothermal systems. At 0.8 GPa — a pressure reached at roughly 25 kilometers depth — temperature differences in water equalize particularly quickly.