Frost Weathering

The widespread occurrence of angular rock debris and fractured rocks in Arctic environments has led to the popular belief that repeated freezing and thawing, with accompanying expansion and contraction, of water is the dominant weathering process in these environments. The nature of the process or processes responsible for fracturing is not well understood. Traditionally, frost weathering has been viewed as resulting from the 9% volumetric increase that accompanies the water to ice phase change, and the associated production of expansive forces as high as 21,000 kg cm-1 at -22°C (French, 1996). However, this model makes several assumptions that are widely regarded as being unmet in natural settings. It assumes that water and ice are contained in hydraulically closed systems, which are cooled rapidly downward. The model ignores the fact that the presence of air bubbles and the presence of pore spaces reduces maximum attainable pressures. Thirdly, rock and soil lack the strength to sustain such theoretically high pressures and therefore are unlikely to ever develop. Recently, a more realistic model of frost weathering has been developed, which relies on a much more plausible set of natural conditions. This model views expansion as a result of liquid water migration toward a growing ice lens associated with a migrating freezing front. Frost weathering results from the progressive growth of microfractures and pore spaces wedged open by ice growth (Walder and Hallet, 1985). This model permits rapid crack growth in a hydraulically open system at temperatures ranging from -4°C to -15°C with slow cooling rates, high pore water pressure, and prolonged freezing. This largely theoretical model has the advantage that it is accompanied by a growing body of experimental support.

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