Microgelivation versus macrogelivation: Towards bridging the gap between laboratory and field frost weathering

The application of laboratory criteria for frost weathering to field problems needs caution, because a number of discrepancies lie between the laboratory and field conditions. This paper reviews thresholds for microgelivation of soft, intact rocks and macrogelivation of hard, jointed rocks, aiming at proposing better criteria in accordance with field conditions. The temperature at which ice segregation induces microgelivation varies significantly with lithology, ranging from about -1 degreesC in high porosity rocks to below -4 degreesC in low porosity rocks. Microgelivation can occur in initially unsaturated rocks when slow (seasonal) freezing drives prolonged water migration from surrounding rock or an external moisture source, while the occurrence requires a high degree of saturation (>80%) or a nearby moisture source when a rock undergoes rapid (diurnal) freezing. Rocks with a high internal surface area and low tensile strength favour microgelivation. These criteria are invalid for macrogelivation that tends to take place just below 0 degreesC in water-filled joints. In addition, because the depth reached by cracking varies with the type of freeze-thaw action, the analysis of thermal regimes should be based on data at the depth of actual cracking. Future targets for macrogelivation studies include the formation of new cracks in hard; intact rocks, as indicated by in situ shattering of clasts or bedrock typically observed in optimal moisture environments. Copyright (C) 2001 John Wiley & Sons, Ltd.