Regelation

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Regelation is the phenomenon of melting under pressure and freezing again when the pressure is reduced. Many textbooks and websites claim that regelation can be demonstrated by looping a fine wire around a block of ice and attaching a heavy weight to it. The pressure exerted on the ice slowly melts it locally, permitting the wire to pass through the entire block. The wire's track will refill as soon as pressure is relieved, so the ice block will remain solid even after wire passes completely through. This experiment is possible for ice at –10 °C or cooler. It has also been suggested that the heating of the wire under pressure and tension may also play a role.^[1]

If 1 mm diameter wire is used, over an icecube 50 mm wide, the area the force is exerted on is 50 mm². This is 50 x 10^-6 m².
Force (in newtons) equals pressure (in pascals) multiplied by area (in square metres).
If at least 500 atm (50 MPa) is required to melt the ice, a force of (50×10⁶ Pa)(50×10^-6 m²) = 2500 N is required, a force roughly equal to the weight of 250 kg on Earth.

Regelation was discovered by Michael Faraday. Regelation occurs only for substances, such as ice, that have the property of expanding upon freezing, for the melting points of those substances decrease with increasing external pressure. The melting point of ice falls by 0.0072 °C for each additional atm of pressure applied. For example, a pressure of 500 atmospheres is needed for ice to melt at –4 °C. ^[2]

[edit] Examples of Regelation

A glacier can exert a sufficient amount of pressure on its lower surface to lower the melting point of its ice. The melting of the ice at the glacier's base allows it to move from a higher elevation to a lower elevation. Liquid water may flow from the base of a glacier at lower elevations when the temperature of the air is above the freezing point of water.