How does pressure lower the melting point of ice?

The melting point of ice decreases as pressure increases because of the unique way water behaves during a phase change. Unlike most substances, water is less dense in its solid state than in its liquid state due to the open, rigid lattice structure formed by hydrogen bonds.12

When pressure is applied to ice, it favors the phase that occupies less volume—in this case, liquid water. According to the Clapeyron equation, which relates the pressure and temperature of phase equilibrium, the rate of change in the melting point is determined by the heat of transition and the difference in volume between the two phases. Because ice contracts upon melting, increasing the hydrostatic pressure effectively lowers the temperature required for the solid to transition into a liquid. This phenomenon is significant in glaciology, where the weight of overlying ice can cause the ice at the base of a glacier to melt at temperatures below 0 °C.1234