Abstract
Recent studies oh periglacial processes and environments are reviewed on the basis of the mechanisms of frost action in rocks and soils. Frost action modifies the landform when the ground thaws after having been subject to frost heave or shattering. Freezing expansion, the cause of frost heave or shattering, begins just after the surface temperature has fallen below 0°C, proceeds rapidly with cooling until-2 or-3°C, and eventually attains a peak value usually at-5 to-7°C. This indicates that frost heave or shattering occurs just above the descending freezing front, and that lower temperature (e.g. less than-10°C) does not increase the power of frost action. Frozen soils are deformed as a result of frost creep, solifluction or active layer glide; the type of process is dependent on their grain size and moisture condition.
Diurnal and annual freeze-thaw cycles have different effects on periglacial landforms depending on their frequency and penetration depths. Diurnal freeze-thaw penetrates no more than 20 cm in soils and 50 cm in rocks, thus causing shallow soil movement of usually less than 10 cm and producing small debris less than 20 cm in diameter. Such shallow ground activities are believed to be predominant in most mid-latitude alpine environments where diurnal cycles occur frequently. The annual cycle controls the maximum depth of soil movement and the maximum size of fallen debris, which rarely exceed 200 cm and 500 cm, respectively, in Japanese alpine environments, as indicated by the records of ground temper.ature and theoretical considerations.
Recent laboratory and theoretical works, combined with field measurements, have enabled us to construct quantitative models of frost action as a function of environmental and geological factors. For example, the rate of bedrock frost shattering was expressed as a function of freeze-thaw frequency, degree of saturation and tensile strength by a simple model which agreed well with field data from several periglacial environments.
There are many problems to be solved on frost action environments. The most important ones are the influence of permafrost on frost action, the origin of block slopes and the sensitivity of frost action to climatic change. Permafrost may intensify frost action in overlying active layers: by acting as an impermeable layer, by producing cryostatic pressure, or by causing the two-sided freezing. Field measurement data, however, do not necessarily indicate high magnitudes of frost action in permafrost regions. The influence of climatic change on frost action cannot be discussed unless this problem is solved. Block fields and slopes, usually regarded as periglacial landforms, have not yet been explained in terms of the mechanisms of frost action. We should carefully evaluate each factor controlling frost action to solve these problems.
