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Freeze‐thaw weathering: The coldregion “Panacea”Kevin Hall aa Geography Programme , University of Northern BritishColumbia , Prince George, B.C., V2N 4Z9, CanadaPublished online: 23 Dec 2008.

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FREEZE-THAW WEATHERING:THE COLD REGION "PANACEA"1

Kevin Hall(Geography Programme, University of Northern British Columbia,

Prince George, B.C. V2N 4Z9, Canada)

Abstract: Freeze-thaw weathering is commonly cited as a major agency oflandform development in high latitudes and at high altitudes. This is, however,largely an unsubstantiated qualitative judgment. The reality is a paucity of dataregarding key factors, such as rock temperature and interstitial rock moisture contentdata, necessary for the correct assumption of freeze-thaw rock weathering. Theproblem is compounded when it is presumed that angular clasts in cold regions arethe result of freeze-thaw weathering and then this argument is used as a basis for theinterpretation, and possible paleoclimatic reconstruction, of Quaternary environ-ments. In reality, angular clasts can be produced by a variety of weathering processesand there are no criteria that can identify a clast as being the product of freeze-thawweathering. Even the term "freeze-thaw" is really a collective noun, for it encom-passes a range of individual mechanisms, each requiring different controlling condi-tions and, potentially, generating different weathering effects. This problem isdiscussed and some of the fallacies outlined.

INTRODUCTION

Almost all texts on Arctic weathering processes and/or landforms cite"freeze-thaw" weathering (or any of the many synonyms) as the major process.Yet, at the same time, we can read " . . . what periglacial geomorphologists needmore than any other single item is a way to determine in the field whether or notbedrock fragments have been frost weathered" (Thorn, 1992, p. 11). This contra-diction is not new, but it is one that has plagued periglacial geomorphology forthe better part of a century. The heart of the problem, as Thorn (1992, p. 11)explains, is that "the concept that freeze-thaw weathering dominates cold regionsgained respectability long before there was the ability to test it in the field. Asangular rock fragments are common in cold environments they were assumed tobe the product of the dominant process, namely freeze-thaw weathering. Today itis common to assume that angular rock fragments are definitive evidence of frost

'This paper was presented at the XIV International Congress of the International Union forQuaternary Research (Berlin), funding for which was kindly provided by the University of NorthernBritish Columbia. Field information was obtained as a result of work with the British AntarcticSurvey and the U.S. Foundation for Glacier and Environmental Research; the assistance of bothorganizations is gratefully acknowledged. Three referees kindly offered a range of comments,corrections, and further thoughts or guidance, and I am grateful for their help.

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Polar Geography and Geology, 1995, 19, 2, pp. 79-87.Copyright © 1995 by V. H. Winston & Son, Inc. All rights reserved.

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weathering," and that " . . . the majority of periglacial researchers believe thatfreeze-thaw weathering of bedrock is an established fact. . . ." Thus, " the storyis one of casual empiricism gathering respectability by repetition until it attainedthe stature of an article of faith" (Thorn, 1992, p.10) or, as Thorn (1988, p. 12)described it earlier, " . . . the status of a sacred cow within periglacial geomor-phology"!

First, perhaps, it is best to be clear as to what is meant by "freeze-thaw"weathering—or any of the many synonyms (e.g., frost wedging, gelifraction,frost shattering, frost riving, etc.). The term implies the mechanical disintegra-tion, splitting, or break-up of rock by the pressure of the freezing water in cracks,crevices, pores, joints, or bedding planes in that rock (van Everdingen, 1994).This definition generates a number of clear assumptions. First, that it is not achemical or biological weathering process. Second, it requires water—the pres-ence of water within the rock is necessary for it to operate, and thus without waterfreeze-thaw weathering cannot take place. Third it requires sub-zero [°C] tem-peratures of adequate duration and sufficient magnitude to make the water withinthe rock actually freeze—if this does not happen within the rock, then this processcannot be presumed to have occurred. Last, it requires that, by some mechanism,the freezing of water within the rock does actually effect some damage—i.e.,freezing and thawing of the water could occur without effecting any damage. Italso should follow that the expression of this breakdown due to freeze-thaw isangular debris and that any damage can only produce debris that is angular inform.

It also must follow from all the above that the angularity of the clasts couldnot, within the broad climatic conditions under consideration (i.e., in this instancea "cold" climate) be created by any other mechanism such that, de facto, thefinding of the angular clasts must indicate freeze-thaw weathering. A corollary ofthis is that other than angular clasts cannot be the result of freeze-thawweathering. If it were considered that "rounded" clasts could also be producedby freeze-thaw, then the situation would become sufficiently confused thatclast shape could no longer equate to a specific environment. Furthermore, itmust follow that the use of the term " freeze-thaw weathering" (or any of thesynonyms) is, of itself, unambiguous as to the exact conditions required for itsoperation. In other words, it cannot hide within itself a variety of conditionsof sufficient magnitude that would indicate other than a singular climatic infer-ence.

DISCUSSION

The assumptions and/or constraints outlined above are not new. As early as1897, Merrill noted that in the absence of water the effectiveness of freeze-thawwould be minimized. Then, in 1936 Grawe complained about the presumption offreeze-thaw and its effectiveness in the absence of confirming data. He explainedthat the frequently cited pressures that could be generated by frozen water(2115 kg cnr2) were hypothetical insofar as no rock could constrain them.Furthermore, he noted that such pressures were only possible if the rock wassaturated (i.e., no air was present) and that the water (which had to be pure)

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was held within a closed system during a period when the temperature in the rockwas -22° C and falling. Without all of these conditions, the pressures could notbe attained even if the rock could withstand them. The reality is that most rocksdo not have a tensile strength much above 250 kg cm"2—almost an order ofmagnitude lower than the theoretical maximum pressures. Despite these warn-ings, authors continued to cite freeze-thaw weathering as operative and, eventoday, texts repeat such values (e.g., Renton, 1994, p. 156)—and often inaccu-rately!

In 1973, Ives (1973, p.l) once again questioned the role of freeze-thaw when,as part of a list of four subjects most in need of research, he included "the efficacyof freeze-thaw process in the role of bedrock disintegration." White (1976)undertook a review similar to this present one and even begged the question as towhether it was actually hydration shattering that was operative rather than freeze-thaw. Again, despite White's outlining of the many problems, noting the need forempirical data, and the suggestion of an alternate mechanism, the majority ofwriters continue to assume freeze-thaw weathering as dominant in cold regions.The inadequacy of temperature and moisture data were discussed by Mcgreevyand Whalley (in 1982 and 1985, respectively), but this still seemed not toengender any greater rigor in Arctic studies. As a result of the almost totalabsence of rock temperature and moisture data, laboratory experiments arebrought into question, thereby denying that avenue of investigation. As early as1914, Warren (p. 413) warned that it would be unsound " . . . to assume that theresults of a certain experiment must also be produced by natural agencies, withoutevidence that similar conditions exist in Nature to those employed in the experi-ments." As the bulk of experiments employ samples that first are saturated, arestanding in water, or are submerged in water when subject to freezing, these are" . . . certainly unnatural conditions affecting the results" (White, 1976, p. 3).

This absence of rock temperature and moisture data continues to be high-lighted by a few authors (e.g., Thorn, 1988, 1992; Matsuoka, 1990; Humlum,1992; Hall, 1993) but freeze-thaw continues to be relied upon to explain manycold-region landforms and sediments. Even the hypothetical models of rockbreakdown by frost action (e.g., Hallet, 1983) suffer the inability to be testedbecause of this absence of data. What compounds the issue is that freeze-thaw isconsidered " . . . an acceptable premise upon which to base many secondaryconcepts (e.g., cryoplanation)" (Thorn, 1992, p. 11). Thus we see explanation ofthe existence of such controversial landforms as nivation hollows and cryoplana-tion terraces having as a central tenet the action of freeze-thaw, without recogni-tion of the inadequacy of our understanding of this process or even any proof asto whether the process actually is operative.

This is not even a new problem but, sadly, one that has been ignored for a verylong time. In 1914 Warren confronted the very same problem when he enteredinto the controversy of whether a number of flints were made by humans or werethe product of Nature. Essentially the case was the same—were they the productof weathering or of human action, and, if the former, then what form of weather-ing? The bottom line was that the clasts themselves do not offer any evidence asto their origin, there being no characteristic or diagnostic feature indicative of aparticular weathering process.

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For example, angular clasts are produced by, among other processes, saltweathering, thermal stress fatigue, wetting and drying, and biologically inducedmechanical processes (e.g., the mucilage expansion and contraction of endolithicalgae). As all of these processes can be active in cold climates, why should it bepresumed that the angular clasts are the product of " freeze-thaw" ? Furthermore,in arid cold climates, it is very likely that processes other than freeze-thaw areactive (there, by definition, being limited water), but angular clasts may still beproduced. Even allowing that water can be available for limited periods in aridpolar areas (e.g., water crystallization onto rock at dew point in the early morningand then melting from the early morning sun, etc.), it is unlikely to be eithersufficient or frequent enough to cause freeze-thaw to exert a major role, but itcould be sufficient to aid other processes (e.g., hydration of salts, chasmoendo-lithic biological activity, etc.). Thus, the presumption of frost action based uponangular clasts may incorrectly deduce that there was moisture available to freezein what may have been an arid climate. Put simply, at the present time there is noway of deducing the origin of the clasts by their form. However, authors docontinue to use the angularity of clasts as an indicator of frost action: "Becausethe dominant characteristic of the stratified deposits at Sonskyn is the angularityof the clasts, it is likely that frost action is the primary agent for their derivation,"and "the preponderance of angular clasts . . . suggests] that this unit has accretedunder alternating freeze-thaw conditions" (Hanvey and Lewis, 1991, p.35). Tocompound the problem, Hanvey and Lewis then generate a whole model for thedevelopment of these sediments based on this singular (fallacious) argument! Itshould be noted that Hanvey and Lewis are far from the only ones to stillassociate angularity with freeze-thaw (e.g., see Czudek, 1993; Heine, 1994;Coltorti and Dramis, 1995 as random examples, taken from a single journal,where freeze-thaw and angularity are presented as "facts").

To further confound this issue, recent studies (Hall, pers. obs.) in an aridregion of the Antarctic found that a dark-colored, coarse-grained sandstoneproduced angular clasts, while on exactly the same surface, with the sameexposure, etc., only 1 cm away across a lithologic junction a light-colored,coarse-grained sandstone produced rounded clasts. Chemical weathering wasabsent, and freeze-thaw could take place only where water was provided bymelting snow, which itself was limited by the aridity of the area, and yeteverywhere, on horizontal and vertical exposures, on all aspects, the light-coloredsandstone produced rounded forms. A discussion of why this occurs is notappropriate here, but the important question is how would the rounded clasts havebeen interpreted and how would the angular have been perceived if found in aQuaternary sediment? Equally, without the clear juxtaposition of the two shapesin this area, how would the angular or the rounded clasts, if found independently,have been interpreted? Considered independently, I would have to argue that twovery different scenarios, with very different climatic conditions, would have beengenerated—and yet here they are contemporaneous, adjacent to one another, andexperiencing the same basic conditions. In a recent discussion, Ballantyne andHarris (1994) indicate much the same finding when they state: "The effects ofgranular disintegration are apparent from the rounded appearance of exposedrock and clast surfaces, especially on granite and sandstone mountains. Such

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rounding contrasts strongly with the angularity of buried clasts and bedrock."They proceed to argue that it is the granular disintegration by microgelivationthat produces rounding of exposed clasts, whereas protected, buried blocksremain angular. Although I do not necessarily disagree with Ballantyne andHarris as to the explanation for the clast differences they discovered it also mustbe noted that the two different forms of weathering can occur simultaneously, asfound here, and that it need not be only frost action that operates in this lithologi-cally constrained differential fashion.

If the other possible weathering processes are considered, then it can be seenthat those factors that control freeze-thaw also control them. The major controlson weathering are rock temperatures (i.e., range, extremes, variability, and rateof change through time and with depth), interstitial rock moisture (i.e., chemistry,distribution, amount, and state, together with their spatial and temporal variabil-ity), and rock properties (i.e., permeability, porosity, tensile strength, thermalconductivity, albedo, etc.). All of the weathering processes are affecting rock,and so the properties of the rock exert an influence on what processes can occurand at what rates. Temperature and moisture thus are the dominant factors ininfluencing nearly all (dilatation being the major exception) weathering proc-esses—chemical, mechanical, and biological. There really is no one factor that isunique to freeze-thaw other than the actual freezing of water; that which con-strains and controls whether there is water to freeze and whether it does freeze(and subsequently thaw) thus also exerts an effect on the other processes. Also,it must be recognized that there is a "series" of processes occurring at any onepoint as a function of diurnal, seasonal, and/or annual changes in temperatureand/or moisture together with the synergistic effects of these combinations. Thus,there is no region where a single process operates in isolation. Therefore, evenwith the data to actually prove it, to say freeze-thaw is the main or even dominantweathering process in any given cold region is to hide and obscure the synergisticrelationships that occur and have facilitated and/or enhanced the role of thatfreeze-thaw activity. To date, however, those data are absent and so the statementcannot be made.

The two "keys" here are temperature and moisture—for any one given rocktype, the properties of that rock can be considered a constant. Only when consid-ering between rock types does the importance of individual rock properties needto be added to temperature and water. In the case of both temperature and water,this refers to rock temperatures and interstitial rock water. Air temperatures areirrelevant—they are not a surrogate for rock temperatures (e.g., Thorn, 1992).The many freeze-thaw cycles cited for so many locations based on air temperatureare of no consequence, as the rock may not experience any of them if covered bysnow. Conversely, in high polar or altitudinal locations where the air temperaturerarely rises above 0° C, the rock may experience large diurnal variations as wellas shorter-term oscillations across the freezing point as a result of radiativeheating. Thus, unless the available temperature data pertain to rock temperatures,they are meaningless. With respect to water, it needs to be known how muchwater there is, how it is distributed, what its chemistry is and, for freeze-thaw,whether it actually froze—this latter as detected either by actual monitoringor by calculation based on a knowledge of pore size, moisture distribution, rock

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temperature and duration, and the calculation of the freezing-point depression asa function of moisture chemistry (Van't Hoff Factor).

At this point it is worth briefly noting the following:

(1) the presence and variability of rock moisture, even in theabsence of any chemical weathering, will effect weathering bywetting and drying, which also can play a synergistic role;

(2) in the presence of interstitial salts, both moisture and tem-perature variability will cause salt weathering;

(3) temperature oscillations alone can produce thermal stressfatigue and even thermal shock;

(4) as a result of the presence of water and the occurrence ofhigh rock temperatures, chemical weathering could play a role,even in high latitudes and altitudes; and

(5) the presence of moisture and high rock temperatures withina porous rock could produce an ideal ecological niche for organisms(endolithic and chasmolithic colonization (Viles, 1995), which cancause both mechanical and chemical weathering).

Thus, the deduction that in cold regions it is freeze-thaw that causes rock break-down is far from a simple presumption.

All of the information on rock temperature and moisture content is necessary,not only for deduction of which processes are operative, but also because thereare a variety of freeze-thaw mechanisms and each has different constraintsregarding the amount of water required, the rate of change of temperature, andthe amplitude of freeze. Not only is there no one singular mechanism thatconstitutes "freeze-thaw," but the variety of processes that fall within thiscollective noun can have different effects in terms of their debris production. Thisalso presumes the absence of any other weathering process, acting synergisti-cally, that also could strongly influence the nature, extent, and character of theweathering.

At this point, it is useful in describing the reality of the situation to return toThorn's (1992, p. 11) observation above that "the concept that freeze-thawweathering dominates cold regions gained respectability long before there wasthe ability to test it in the field. As angular rock fragments are common in coldenvironments they were assumed to be the product of the dominant process,namely freeze-thaw weathering. Today, it is common to assume that angular rockfragments are definitive evidence of frost weathering. Nevertheless, the angular-ity of comminuted bedrock must always be strongly influenced by lithology andis certainly likely to stem from processes other than freezing and thawing in manyinstances." What we have is, largely, the enforcing of the concept by unqualifiedrepetition and a distinct absence of quantitative testing.

Three examples are presented here to illustrate the preceding statement.First, in many high-altitude and high-latitude locations, water is the limitingfactor. Rock temperature fluctuations are frequent and can be quite large. Theproblem has been the absence of data on rock temperatures (as opposed to thoseof the air) recorded at sufficient frequency to facilitate meaningful analysis.

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Recent studies (Hall, pers. obs.) with temperatures collected at 20-second to1-minute intervals have shown that rates of change of temperature are quite closeto or exceed the 2° C per minute threshold for thermal stress fatigue. The resultis fracturing of the rock, sometimes explosively (as with thermal shock), and theproduction of highly angular fragments. In some instances, as in the high Andesor parts of Antarctica, water can be considered nonexistent for all practicalpurposes. Thus, the angular debris clearly is not an indicator of freeze-thaw—which requires water—but rather of a dry environment, with sub-zero air tem-peratures but high radiative inputs that can generate rock temperatures on theorder of 30° C. Environmental ramifications with respect to climate, vegetation,processes, etc. follow and would not conform to that of a "freeze-thaw" environ-ment.

The second example is from the Juneau Icefield area of Alaska, where thedestruction of granites on nunataks has been perceived to be the result of freeze-thaw. Detailed studies, however, demonstrated that moisture availability wasminimal and only penetrated a few millimeters into the rock. During this sameperiod, there were no freeze-thaw events. Rather, chasmolithic algae were foundin the granite below an outer shell of 1 to 2 mm. The available moisture causedswelling of the algal mucilage such that the algae are the main cause of rockbreakdown and produce angular flakes to the extent of, at some sites, more than1 kg nr2 yr1. Again, angular debris is produced, but not as a result of freeze-thawand from a very different environment than in the preceding example.

The final example is from the maritime Antarctic, where freeze-thaw is sooften cited as the main process. Here there can be large freeze-thaw cycles in lateautumn through the end of spring, but there is such extensive snowfall that mostbedrock is insulated and does not experience thermal oscillations until the snowcover ablates. Through the summer the air temperatures are low (ca. 2° C), withrare, low-amplitude and often short-duration freeze-thaw events in the air, butrock temperatures remain above zero. There is, however, a great deal of precipi-tation in the form of rain, together with strong winds and intermittent sun. Theend result is extensive weathering by wetting and drying. Rock fragments areangular, and while probably not solely produced by wetting and drying, this is,nonetheless, the major process. So, again, angular debris is produced, but inanother, entirely different environment.

CONCLUSION

The argument presented in this paper is that angular clasts are not causedsolely by freeze-thaw and therefore are not an indicator of the occurrence of thisprocess. Greater care should be used in Quaternary and modern-day interpreta-tions of process and in the reconstruction of climate based on such criteria.Freeze-thaw is not as ubiquitous as we often think and many other processes maybe at least as effective. Freeze-thaw has been cited wrongly in many modernstudies, and therefore its use in any paleo-interpretation is even more doubtful.Freeze-thaw is not the panacea (universal remedy) so frequently considered. Wenow should attempt to overcome the psychological need for citing freeze-thawand find the actual cause—only then can we progress to a meaningful diagnosis.

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LITERATURE

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Czudek, T. "Pleistocene periglacial structures and landforms in westernCzechoslovakia," Permafrost and Periglacial Processes, Vol. 4, 1993,pp. 65-75.

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Hallet, B. "The breakdown of rock due to freezing: A theoretical model,"Proceedings of the 4th International Conference on Permafrost, Fairbanks,Alaska. Washington, DC: National Academy Press, 1983, pp. 433-438.

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Humlum, O. "Observations on rock moisture variability in gneiss and basaltunder natural arctic conditions," Geografiska Annaler, Vol. 74A, 1992,pp. 197-205.

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Matsuoka, N. "The rate of bedrock weathering by frost action: Field measure-ments and a predictive model," Earth Surface Processes and Landforms,Vol. 15, 1993, pp. 73-90.

Mcgreevy, J. P. and W. B. Whalley. "The geomorphic significance of rocktemperature variations in cold environments. A discussion," Arctic andAlpine Research, Vol. 14, 1982, pp. 157-162.

Mcgreevy, J. P. and W. B. Whalley. "Rock moisture content and frost weather-ing under natural and experimental conditions: A comparative discussion,"Arctic and Alpine Research, Vol. 17, 1985, pp. 337-346.

Merrill, G. P. A Treatise on Rocks, Rock Weathering and Soils. London:Macmillan, 1897, 400 pp.

Renton, J. J. Physical Geology. St. Paul, MN: West Publishing, 1994, 607 pp.Thorn, C. E. "Nivation: A geomorphic chimera, in: M. J. Clark, ed., Advances

in Periglacial Geomorphology. Chichester, UK: John Wiley, 1988, pp. 3-31.Thorn, C. E. "Periglacial geomorphology: What, Where, When?," in: J. C.

Dixon and A. D. Abrahams, eds., Periglacial Geomorphology. Chichester,UK: John Wiley, 1992, pp. 1-30.

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van Everdingen, R. O. Multilanguage Glossary of Permafrost and RelatedGround-ice Terms. Calgary: University of Calgary, 1994.

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Warren, S. H. "The experimental investigation of flint fracture and its applica-tion to problems of human implements," Journal of the Royal Anthropologi-cal Institute, Vol. 44, 1914, pp. 412-450.

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