Water Under Pressure: Why It Flows Faster express
Water deep within the Earth is compressed more than a thousandfold compared to the surface. Even there, it continues to flow, but its internal structure changes. Researchers measured how water resists flow and compression as pressure rises from zero to 1.6 gigapascals — equivalent to placing a 16-ton weight on every square centimeter.
They found that resistance to flow increases faster than resistance to compression, with their ratio halving. Moreover, unlike most liquids, water under pressure initially rearranges faster: molecules manage to swap neighbors in about a picosecond (one trillionth of a second) at pressures around 0.5 gigapascals. Beyond that, rearrangement slows down again.
This behavior is explained by the fact that hydrogen bonds between water molecules constantly break and reform. The time it takes for a molecule to find a new position is the rearrangement time. It is minimal precisely at 0.5 GPa. This means that at such compression, water most rapidly changes its local structure, transitioning from one molecular arrangement to another. This peculiarity gives rise to a cascade of unusual properties of water, from thermal conductivity to compressibility.
A pressure of 1.6 GPa corresponds to a depth of about 50 kilometers below the surface — already in the Earth's upper mantle. There, water exists in microscopic cavities within minerals and affects their plasticity.