Cold Climate Weathering

Norikazu Matsuoka, Institute of Geoscience, University of Tsukuba, Ibaraki 305-8571, Japan

Processes

Cold climate weathering has been studied in both the field and laboratory. Early studies emphasized the predominance of frost weathering and production of angular blocks. The pioneering laboratory work by Tricart (1956) has been followed by a number of experiments exploring the contribution of environmental and geological factors to frost weathering (e.g. Lautridou and Ozouf, 1982). Whereas early researchers regarded the 9 % volumetric expansion upon phase change as the major cause of rock breakdown, recent theoretical and experimental studies have highlighted ice segregation in rocks, similar to frost heave in soils (e.g. Walder and Hallet, 1985; Akagawa and Fukuda, 1991). Long-term ice segregation may produce brecciated bedrock near the top of permafrost (Murton, 1996; Murton et al., 2000). Laboratory results have often been used to analyze field data (e.g. Thorn, 1979). Field data on bedrock shattering, temperature, moisture and rock mass strength have been integrated to construct an empirical model of the rate of frost weathering (Matsuoka, 1991).

Whereas numerous field observations suggest that frost weathering prevails on humid rocks subjected to meltwater from nearby snowpatches (e.g. Berrisford, 1991) or located close to water table (e.g. Matthews et al., 1986), recent studies demonstrate that other weathering processes dominate depending on the environmental and geological conditions. In the Antarctic cold desert, for instance, the primary weathering process varies with aspect (intensity of insolation or exposure to the prevailing wind), moisture availability and lithology (e.g. Hall, 1992; Matsuoka et al., 1996). Hydration weathering is operative even far below 0 degree C preferably in argillaceous rocks, possibly controlled by the adsorptive clay composition (Dunn and Hudec, 1972). Water chemistry indicates that chemical weathering is also significant even in non-karstic cold regions (e.g. Caine, 1992; Darmody et al., 2000).

Techniques

Review

Field measurements on cold climate weathering should target both process itself and controlling factors. Field data on the former have been only sporadic because weathering progresses so slowly that long-term monitoring is necessary. The most popular method for estimating the rate of weathering is to measure the volume of rockfalls from a rockwall using a trap (e.g. Rapp, 1960; Church et al., 1979; Fahey and Lefebure, 1988). Another method is to investigate the fragmentation of a painted rock (Matsuoka, 1991; Mackay, 1999). Despite permitting direct determination of the rate of rockwall retreat, these manual methods require frequent field visits when seasonal variation in the rate is determined. Indirect measurements include the exposure of rock tablets to a variety of field situations (e.g. Matsuoka et al., 1996). Removing the influence of lithological variation, this method highlights environmental controls on the rate of weathering.

Recent advances in data logging technology have enabled automated, continuous recording of weathering processes. A relevant contribution has been provided by monitoring of strain in alpine rockwalls, where significant damage of rocks and frequent rockfalls are actually observed. Recorded movements include widening of rock joints on a rock face (e.g. Matsuoka et al., 1997) and dilatation in bedrock permafrost (e.g. Wegmann and Keusen, 1998), both indicating expansion of rock joints associated diurnal and/or seasonal freezing.

Parameters controlling cold climate weathering have often been measured separately. Rock temperature has most widely been measured by installing thermal probes (thermistor, thermocouple, etc.) in the bedrock (e.g. Thorn, 1979; Coutard and Francou, 1989; Gardner, 1992). The probes are connected to a data logger that can store 10³-10⁵ data. Sampling intervals are usually set at 1-3 hours when frost weathering is evaluated, while 1-minute intervals are preferred for the analysis of insolation weathering that is most effective when the rock surface is warmed or cooled at >2 degrees C min^-1 (Hall, 1997). Monitoring of near-surface thermal profiles is essential to determine the depth reached by fracturing (e.g. Matsuoka, 1994; Anderson, 1998).

The second parameter is the moisture content of rocks. Where the rock surface is distant from the water table, water migration and resulting ice segregation are limited and a high initial saturation level (>80 %) is required for significant frost weathering to occur (e.g. Prick, 1997). Where moisture is restricted during the freeze-thaw period, other weathering processes would be more effective, although slow seasonal freezing may allow slow but long-lasting water migration that eventually produces ice lenses at a certain depth in the bedrock. The rock moisture has been determined manually by weighing a rock tablet placed at a field situation (e.g. Hall, 1988; Humlum, 1992). This technique requires an observer staying in the field over the expected weathering period. Other manual measurements include the collection of seepage water from rock joints (Fahey and Lefebure, 1988). Automated measurements (e.g. with TDR sensors) are preferable but under development (Schneebeli et al., 1995).

Rock properties are another parameter governing cold climate weathering. As regards frost weathering, physical properties such as porosity, permeability and specific surface area contribute to the ice pressure, while tensile strength determines the resistance against the ice pressure (Matsuoka, 1990). In the field, a crucial rock property is the degree of fractures, because the most effective damage occurs in the pre-existing fractures in the bedrock (e.g. McGreevy and Whalley, 1982). The seismic measurement enables us to estimate the strength of fractured bedrock (Matsuoka, 1991). Point-load compressive strength and Schmidt Hammer rebound are indicative of the inter-joint strength of bedrock (e.g. Hall, 1987).

Combined measurements of several parameters provide data for understanding the timing and trigger of rock disintegration. For instance, monitoring of rock temperature, water seepage and rockfall volume highlighted the contribution of thermal and moisture regimes to the rockfall activity (Fahey and Lefebure, 1988).

Examples of Recommended Measurement Techniques

Since the rate and process of rock weathering vary seasonally, year-round automated measurements are recommended. In addition, detailed analysis of processes requires simultaneous measurements of rock disintegration and its variables. The crack extensometer (Figure 1a) is a useful tool for measuring frost wedging in a rock joint (see Appendix for details). Where winter snowcover exerts significant pressure, the sensor should be protected (Figure 1b). Thermal probes are installed in the joint or boreholes as well as attached to the sensor. The latter provides data for calibration against the thermal drift of the extensometer. Figure 2 displays data from an alpine rockwall, which demonstrate joint widening associated with cooling below 0 degree C in the joint (Matsuoka, 2001).

Another useful method is to expose rock tablets to a variety of field situations. This method is useful for exploring environmental factors affecting small-scale disintegration. The tablets are prepared in the laboratory to have the same size and lithology. In order to obtain significant results within a few years, we should choose lithology sensitive to weathering (i.e. rapidly broken in the laboratory). In addition to periodical measurements of weight loss of the tablet, automated monitoring of dilatation using a strain gauge, with simultaneous recording of temperature, allows us to detect progressive rock deterioration and the thermal regime at which the tablet cracks. A global monitoring network using the same lithology will provide data source for establishing the quantitative relationship between climate and weathering rate.

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References

Akagawa S, Fukuda M. 1991. Frost heave mechanism in welded tuff. Permafrost and Periglacial Processes 2: 301-309.

Anderson RS. 1998. Near-surface thermal profiles in alpine bedrock: implications for the frost weathering of rock. Arctic and Alpine Research 30: 362-372.

Berrisford MS. 1991. Evidence for enhanced mechanical weathering associated with seasonally late-lying and perennial snow patches, Jotunheimen, Norway. Permafrost and Periglacial Processes 2: 331-340.

Caine, N. Spatial patterns of geochemical denudation in a Colorado alpine environment. In Periglacial Geomorphology, Dixon JC, Abrahams AD (eds.), Wiley: Chichester; 63-88.

Church M, Stock RF, Ryder JM. 1979. Contemporary sedimentary environments on Baffin Island, N. W. T., Canada: debris slope accumulations. Arctic and Alpine Research 11: 371-402.

Coutard JP, Francou B. 1989. Rock temperature measurements in two alpine environments: implications for frost shattering. Arctic and Alpine Research 21: 399-416.

Darmody, RG, Thorn, CE, Harder, RL, Schlyter, JPL, Dixon, JC. 2000. Weathering implications of water chemistry in an arctic-alpine environment, northern Sweden. Geomorphology, 34, 89-100.

Dunn, JR, Hudec PP. 1972. Frost and sorption effects in argillaceous rocks. Highway Research Record 393: 65-78.

Fahey BD, Lefebure TH. 1988. The freeze-thaw weathering regime at a section of the Niagara Escarpment on the Bruce Peninsula, Southern Ontario, Canada. Earth Surface Processes and Landforms 13: 293-304.

Gardner JS. 1992. The zonation of freeze-thaw temperatures at a glacier headwall, Dome Glacier, Canadian Rockies. In Periglacial Geomorphology, Dixon JC, Abrahams AD (eds.), Wiley: Chichester; 89-102.

Hall KJ. 1987. The physical properties of quartz-micaschist and their application to freeze-thaw weathering studies in the maritime Antarctic. Earth Surface Processes and Landforms 12: 137-149.

Hall KJ. 1988. Daily monitoring of a rock tablet at a maritime Antarctic site: moisture and weathering results. British Antarctic Survey Bulletin 79: 17-25.

Hall KJ. 1992. Mechanical weathering in the Antarctic: a maritime perspective, In Periglacial Geomorphology, Dixon JC, Abrahams AD (eds.), Wiley: Chichester; 103-123.

Hall KJ. 1997. Rock temperature and implications for cold region weathering, I: new data from Viking Valley, Alexander Island, Antarctica. Permafrost and Periglacial Processes 8: 69-90.

Humlum O. 1992. Observations on rock moisture variability in gneiss and basalt under natural, Arctic conditions. Geografiska Annaler 74A: 197-205.

Lautridou JP, Ozouf JC. 1982. Experimental frost shattering: 15 years of research at the Centre de Geomorphologie du CNRS. Progress in Physical Geography 6: 215-232.

Mackay JR. 1999. Cold-climate shattering (1974 to 1993) of 200 glacial erratics on the exposed bottom of a recently drained Arctic Lake, Western Arctic Coast, Canada. Permafrost and Periglacial Processes 10: 125-136.

Matsuoka N. 1990. Mechanisms of rock breakdown by frost action: an experimental approach. Cold Regions Science and Technology 17: 253-270.

Matsuoka N. 1991. A model of the rate of frost shattering: application to field data from Japan, Svalbard and Antarctica. Permafrost and Periglacial Processes 2: 271-281.

Matsuoka N. 1994. Diurnal freeze-thaw depth in rockwalls: field measurements and theoretical considerations. Earth Surface Processes and Landforms 19: 423-435.

Matsuoka N. 2000. Direct observation of frost wedging in alpine bedrock. Earth Surface Processes and Landforms: in press.

Matsuoka N, Moriwaki K, Hirakawa K. 1996. Field experiments on physical weathering and wind erosion in an Antarctic cold desert. Earth Surface Processes and Landforms 21, 687-699.

Matsuoka N, Hirakawa K, Watanabe T, Moriwaki K. 1997. Monitoring of periglacial slope processes in the Swiss Alps: the first two years of frost shattering, heave and creep. Permafrost and Periglacial Processes 8: 155-177.

Matthews JA, Dawson AG, Shakesby RA. 1986. Lake shoreline development, frost weathering and rock platform erosion in an alpine periglacial environment, Jotunheimen, southern Norway. Boreas 15: 33-50.

McGreevy JP, Whalley WB. 1982. The geomorphic significance of rock temperature variations in cold environments: a discussion. Arctic and Alpine Research 14: 157-162.

Murton, JB. 1996. Near-surface brecciation of chalk, Isle of Thanet, south-east England: a comparison with ice-rich brecciated bedrocks in Canada and Spitsbergen. Permafrost and Periglacial Processes 7: 153-164.

Murton, JB, Coutard, JP, Lautridou, JP, Ozouf, JC, Robinson, DA, Williams, RBG, Guillemet, G, Simmons, P. 2000. Experimental design for a pilot study on bedrock weathering near the permafrost table. Earth Surface Processes and Landforms, 25: 1281-1294.

Prick A. 1997. Critical degree of saturation as a threshold moisture level in frost weathering of limestones. Permafrost and Periglacial Processes 8: 91-99.

Rapp A. 1960. Recent development of mountain slopes in Kärkevagge and surroundings, northern Scandinavia. Geografiska Annaler 42A: 65-200.

Schneebeli M, Fluhler H, Gimmi T, Wydler H, Laser HP, Baer T. 1995. Measurements of water potential and water content in unsaturated crystalline rock. Water Resources Research 31: 1837-1843.

Thorn CE. 1979. Bedrock freeze-thaw weathering regime in an alpine environment, Colorado Front Range. Earth Surface Processes 4: 211-228.

Tricart J. 1956. Etude experimentale du probleme de la gelivation. Biuletyn Peryglacjalny 4: 285-318.

Walder J, Hallet B. 1985. A theoretical model of the fracture of rock during freezing. Geological Society of America Bulletin 96: 336-346.

Wegmann M, Keusen HR. 1998. Recent geophysical investigations at a high alpine permafrost construction site in Switzerland. In Proceedings of the Seventh International Conference on Permafrost, June 23-27, 1998, Yellowknife, Canada, Lewkowicz AG, Allard M (eds.), Nordicana; 1119-1123.

Appendix

The crack extensometer

The crack extensometer measures movement of a crack with strain gauges. A commercialized example is the crack extensometer BCD-5B (Figure 3) manufactured by Kyowa Instruments, Japan: cost ca. US$ 300. Similar sensors may be purchased from other companies dealing with strain gauges. The BCD-5B sensor consists of a curved steel strip connecting two steel bars. Widening of the crack reduces the curvature of the curved strip, which is recorded as a change in electric resistivity of two strain gauges attached to the strip. Two bolts anchored in boreholes at both sides of the crack allow us to fix the steel bars to the bedrock. This sensor detects widening of a crack of up to 10 mm with a resolution of 2.5×10^-3 mm.

The extraction of frost-induced strains from the extensometer readings requires the elimination of three kinds of thermally driven strains, which originate from the expansion/contraction of (1) the strain gauge itself, (2) the steel bar of the extensometer and (3) the bedrock. However, distinguishing the strain components is usually difficult. Alternatively, the field data can be used to estimate the total thermal strain arising from both the extensometer (including the gauge and steel) and inter-joint bedrock (see Matsuoka, 2001, for more details). When highlighting insolation weathering, the above strains (1) and (2) should be eliminated.

Correlating crack widening events with thermal regimes of the bedrock requires a multi-channel data logger which permits year-round, concurrent monitoring of strain and temperature. Examples of the commercialized loggers are the CR10X logger (Campbell Scientific Inc., USA) and the B5-Strain-8A logger (Log Electronics, Japan): cost from US$ 2500, changeable with options.

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Latest update: 22. November 2001.