| Term: | Regelation |
| Definition: |
Motion of an object through ice by melting and freezing that is caused by pressure differences; this process allows a glacier to slide past small obstacles on its bed. NSIDCCryosphere
Process by which ice melts when subjected to pressure and refreezes when pressure is removed. IACSSnow The freezing of meltwater due to a change in pressure alone. The term is often used for 'pressure melting and regelation'. Regelation happens because the pressure-melting point of ice varies inversely with pressure. Water in equilibrium with ice will freeze, releasing the latent heat of fusion, 333.5 k: J kg-1, if there is a decrease of pressure, as on the downglacier face of a bump in the bed of a temperate glacier. Ice in equilibrium with water will melt if there is an increase of pressure, as on the upglacier face of the bump. However, the latent heat of fusion must be supplied for this change of phase. A natural source is the latent heat released by regelation on the downglacier face. If pressure melting and regelation are unequal, there will be a contribution to basal ablation or basal accumulation. Smaller bumps are more favourable to pressure melting and regelation than larger ones. The Russian word 'regelatsiya' refers to refreezing rather than to regelation. IHPGlacierMassBalance Regelation is the phenomenon of melting under pressure and freezing again when the pressure is reduced. 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. TrombottoGeocryology A twofold process in which a localized region on the surface of a piece of ice melts when pressure is applied to that region (pressure melting) and then refreezes when pressure is reduced. Regelation was discovered by Faraday, who found that two pieces of ice at 0C would freeze together if pressed against each other and then released. 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 pure ice decreases with pressure at the rate of 0.0072C per atmosphere. Since this rate is very small, regelation only occurs at ice temperatures of 0C or very slightly less. The fact that snowballs can be packed well at near 0C but not at much colder temperatures is a consequence of regelation. AMSglossary GCW |