Solifluction rates, processes and landforms: a global review
Introduction
Freeze–thaw action induces downslope displacement of soils in cold, non-glacial environments, where vegetation is lacking or sparse (e.g., Washburn, 1979, Ballantyne and Harris, 1994, French, 1996). This process, broadly referred to as solifluction, operates slowly, in general at a rate of at most 1 m year−1. In terms of geomorphic work, solifluction may influence the denudation of mountains much less than rapid processes and geochemical transfers (e.g., Rapp, 1960, Smith, 1992). However, its widespread distribution on mountain slopes means that solifluction contributes greatly to the evolution of mountain landscapes. Moreover, landforms and subsurface structures resulting from solifluction are strongly dependent on climatic conditions. This implies that solifluction features can be used as an indicator of the climate that has affected the slope in the past (e.g., Benedict, 1976, Bertran et al., 1995).
Rates and processes of solifluction depend, in collective terms, on climate, hydrology, geology and topography. Prediction of landscape evolution in periglacial mountains requires quantitative relationships between the rate of solifluction and these variables Lewkowicz, 1988, Kirkby, 1995. For this purpose, field measurements have been undertaken in a variety of geographical situations ranging from polar hillslopes (e.g., Washburn, 1967) to tropical high mountains (e.g., Francou and Bertran, 1997) and, as a result, a large number of data-nets on solifluction rates and associated parameters have been obtained.
Recent progress in field methodology has allowed the evaluation of the timing and environmental conditions at which soil movement occurs (e.g., Matsuoka et al., 1997, Lewkowicz and Clarke, 1998). Laboratory simulations have also added knowledge of the mechanisms involved in solifluction processes Harris et al., 1993, Harris et al., 1997. These advances will enable us to link solifluction rates with their controlling factors, eventually permitting the construction of a physically based predictive model of solifluction.
This paper compiles the extant field data from a wide range of periglacial environments and presents quantitative relationships between the rates and landforms of solifluction and a variety of parameters. Analysis of these relationships permits understanding of factors affecting the spatial variability in solifluction processes and landforms. A particular focus is on climate–solifluction linkages. Finally, the possible effect of climatic change on solifluction is discussed.
Section snippets
Terminology
The term ‘solifluction’ has not yet been defined unequivocally. The original meaning was the slow downslope movement of saturated soil occurring in cold regions (Andersson, 1906). Later studies have revealed that slow mass movements in cold regions include several processes and do not necessarily require saturation. In modern usage, solifluction represents collectively slow mass wasting associated with freeze–thaw action Ballantyne and Harris, 1994, French, 1996, and the saturated soil movement
Data collection
Data for analysis are compiled from the literature, mostly written in English or Japanese. The compilation basically excluded data derived from screes and slopes subjected mainly to rapid failures. Language barriers may also have induced omission of some useful data. In total, data from 46 sites were analyzed. Available data are biased to the northern hemisphere, in particular, from the Arctic regions to mid-latitude high mountains, while only a few data are available from the Antarctic. The
The effect of the mean annual air temperature
The three movement parameters (VS, VVL and DM) vary significantly with MAAT (Fig. 4). A data point in Fig. 4 basically represents the mean value for a study site, except where data are summarized for each landform type. The surface velocity VS is very low, rarely in excess of 5 cm year−1 on slopes with MAAT<−6°C, where permafrost is largely cold and continuous. VS rises with increasing MAAT, reaching a maximum in a zone with MAAT between −3°C and −5°C (Fig. 4A). Within this optimum zone, VS
The effect of inclination
The dependence of movement on inclination has been described in several mathematical models. The simplest model gives the surface velocity VS by:where HF is the heave amount perpendicular to the slope surface and θ is the slope angle (e.g., Williams and Smith, 1989). For slopes subject to multiple frost heave cycles, Eq. (1) is rewritten as:where VS gives the annual surface velocity and ∑HF the annual total heave amount. VS in , is also called the potential frost creep. In
Movement of solifluction lobes
Lobes and sheets are surface expressions directly reflecting solifluction. They originate from overturning of the superficial soil due to reduced velocity and thus develop most extensively where gradient decreases downslope (Fig. 12) or where a fine soil layer overrides a coarse sediment (Fig. 2C). Typically, lobes consist of a riser 0.2–2 m in height and a tread 2–50 m in both width (measured along contours) and length (measured downslope) (e.g., Harris, 1981). Sheets have a similar height but
Contemporary solifluction
The freeze–thaw regime, soil characteristics, moisture status and topography are the major controls on solifluction features in modern periglacial environments. Fig. 14 illustrates how the first two factors contribute to the spatial variation in solifluction processes and landforms. Solifluction can operate where the uppermost soil encounters at least a diurnal freeze–thaw cycle. Thus, in term of the global thermal regime, the solifluction-affected environment is delimited by both warm and cold
Conclusions and prospects
Solifluction occurs widely from Antarctic nunataks to tropical mountains with large spatial variation in its nature. Solifluction involves several components: (1) needle ice creep and diurnal frost creep originating from diurnal freeze–thaw action; (2) annual frost creep, gelifluction and plug-like flow originating from annual freeze–thaw action; and (3) retrograde movement caused by soil cohesion. Long-term soil movement eventually leads to the development of features characterizing
Acknowledgements
This study was financially supported by the Sumitomo Foundation and the Tokyo Geographical Society. I would like to thank Charles Harris and Antoni Lewkowicz for their comments that help improve the manuscript.
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