The Historical Legacy of Spatial Scales in Freeze–Thaw Weathering: Misrepresentation and Resulting Misdirection

GEOMOR-03394; No of Pages 8 Geomorphology xxx (2010) xxx–xxx

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Geomorphology

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The historical legacy of spatial scales in freeze–thaw weathering: Misrepresentation and resulting misdirection

Kevin Hall a,⁎, Colin Thorn b a Department of Geography, Geoinformatics & Meteorology, University of Pretoria, Pretoria, 0002, South Africa b Department of Geography, University of Illinois at Urbana-Champaign, 220 Davenport Hall, 607 South Mathews, Urbana, IL 61801, USA article info abstract

Article history: Discussion of weathering in cold regions has historically been dominated by widespread acceptance of the Received 23 July 2008 significance of the freeze–thaw concept among periglacial geomorphologists, and an essentially universal Received in revised form 16 July 2009 acceptance by those geomorphologists not directly involved in researching the topic. Debris produced by Accepted 4 October 2010 freeze–thaw is frequently deemed to be angular in form and the observation of such debris has been used to Available online xxxx identify the former or present operation of this weathering mechanism. Large debris (‘blocks’) and small debris ('grains’ or ‘flakes’) have been recognised as the outcome of the two scales of freeze–thaw weathering: Keywords: Weathering macrogélivation and microgélivation respectively. However, the fusion of climatic geomorphology and Freeze–thaw process geomorphology in the ongoing development of the freeze–thaw concept has resulted in the confusion Microgélivation of product with process—whereby microgélivation (producing small products) and macrogélivation Macrogélivation (producing large products) are seen, due to the product dichotomy, as distinctly different processes. Despite Process-product scales the recent, highly sophisticated laboratory experimentation on freeze–thaw weathering, this historical- process scale-dichotomy still pervades thinking and experimental evaluation. Here we consider the historical development of microgélivation/macrogélivation and outline what are thought to be fundamental flaws with the concepts and their underpinnings. Built within the two notions are elements of rock properties (‘hard’ and ‘soft’) and scale issues regarding rock attributes (‘solid rock’ and ‘existing weaknesses’ in rock) that serve only to confuse the process and scale issues even more. The whole notion of frost-weathered debris having a specific form is also highly spurious, there being no shape attribute that is uniquely diagnostic of frost action and this, in turn, leads to the further problem of process synergy or other processes entirely being the cause of rock failure in cold regions. Ultimately we argue that while there may be a range of product sizes (and shapes) resulting from frost weathering per se the widely invoked scale concepts are fraught with problems and are best dropped—as too are any process-shape connotations. © 2010 Published by Elsevier B.V.

1. Introduction geomorphology. These considerations are pervaded with scale concepts varying from the scale of the processes and of the landforms to the use of Geomorphologists consider landforms to be the product of geomor- scale-defined terminology, and finally to the broad scientificcontextof phic processes and, in turn, influence those processes. They recognize scale concepts. Our objective is to illuminate how the legacy of that this interaction is sensitive to initial conditions (e.g., Phillips, 1988, geomorphic thinking has constrained and steered contemporary 1999), plays out at multiple spatial and temporal scales (Schumm and research into freeze–thaw weathering including the role scale has Lichty, 1965), and that the process–form interactions they choose to played and is playing in the matter. Freeze–thaw weathering, broadly invoke may change through time (e.g., Haff, 1996). Such a claim raises a the belief that the alternate freezing and thawing of hydrated materials deep philosophical issue—are geomorphologists identifying natural (especially rock) on short (e.g., diurnal or synoptic) or longer (e.g., kinds when they identify landforms, or are they carving categories from seasonal) scales breaks them up, is widely invoked by periglacial a landscape continuum (Rhoads and Thorn, 1996a)? Here we skirt such geomorphologists, and essentially universally-accepted as active in ‘deep philosophy’, preferring to focus upon the ‘shallow philosophy’ periglacial regions by geomorphologists not actively engaged in cold (often called ‘methodology’) associated with everyday terminology, the region research. As such it is an important geomorphic concept that interpretation of field evidence, and the nature of experimental needs to be well-founded and clearly conceptualized or defined. Fundamentally, a geomorphologist seeks to explain a (land)form by understanding the interaction between process and material. It seems intuitively reasonable to assign the process(es) a causal role ⁎ Corresponding author. Present address: Geography Programme, University of fl Northern British Columbia, 3333 University Way, Prince George, BC, V2N 4Z9, Canada. and the landform one of effect. However, this clearly con ates the E-mail addresses: [email protected] (K. Hall), [email protected] (C. Thorn). process and the material and it is much less clear how these two

Please cite this article as: Hall, K., Thorn, C., The historical legacy of spatial scales in freeze–thaw weathering: Misrepresentation and resulting misdirection, Geomorphology (2010), doi:10.1016/j.geomorph.2010.10.003 2 K. Hall, C. Thorn / Geomorphology xxx (2010) xxx–xxx interact. Indeed, clearly the material also has some casual, as well as “frost penetrates deeply enough to freeze water contained in joints” and effect, role. There is no a priori reason to believe that the scale of a “springs (presumably this could/should also be translated as ‘heaves’) geomorphic process, as opposed to the characteristics of the material, joint blocks of several cubic metres” (Tricart, 1970,p.115).Micro- inevitably controls the scale of the geomorphic product—the gélivation on the other hand, occurs where “night frost succeeds in landform. Furthermore, if a landform is the product of the interaction reducing to sand and gravel, extremely hard volcanic rocks” (Tricart, between a geomorphic process (the driving force) and an existing 1970, p. 115). Thus, large amplitude, long duration freezes produce material there is no inherent reason to believe that: 1) the same (only) large material while short duration freezes can only produce driving force will always produce the same resulting landform (the small sized material: a process–product scale relationship. The evidence forward-looking view); 2) any form is necessarily always the product of both large blocks and fine material in cold regions adds visible of the same process (the backward-looking view); and 3) driving credence to the distinction. At this juncture we might identify Tricart as forces might not be constrained or dictated by the initial nature of the using ‘morphogenic’ terms—such terms are not only descriptive but material. Finally, the explanation of any existent landform assemblage inherently connote the origin of the form as well. Morphogenetic (landscape) is always plagued by largely uncertain initial conditions, terminology is widespread in geomorphology and has consistently changing environment(s) or inheritance (temporal contamination), plagued the discipline as theory has progressed, thereby bypassing the and interference (spatial contamination). genetic component of a term while researchers have sought to retain the descriptive (morphological) component. This has usually resulted in the 2. Terminology messy use of an established morphogenetic term supposedly shriven of its genetic connotations. A classic example of this would be the way that Historically geomorphology was essentially an ‘eye-ball’ science the Davisian term ‘peneplain’ has often been used subsequently to mean and geomorphologists investigating the nature of freeze–thaw ‘an eroded regional surface’, but the user has not subscribed to the weathering appear to have conflated the process and the product. development components of the Davisian landscape model. As we have addressed this issue previously (e.g., Thorn, 1979; Hall et However, Tricart's (1970, p. 76) position is actually much more al., 2002) the reader is referred to these earlier publications where this complicated because he also states that “sedimentary rocks are more admittedly rather sweeping generalization is generously referenced. subject to microgélivation, the less they are compacted.” The issue Furthermore, many periglacial geomorphologists seem to have becomes even more convoluted when Tricart (1970, p. 76) states assigned an uncertain, sometimes unrecognized, or variable role to “Macrogélivation exploits existing weaknesses in the rock; micro- the material(s) involved. The result is that the nature of the process gélivation cuts into solid rock” and then proceeds to claim that (es) remains(remain) uncertain and is(are) still commonly viewed “Weathered crystals are at once less resistant to pressure and more and understood in terms of the ensuing products (landforms or permeable; it is these that microgélivation exploits”. Quite how landform elements). This conflation of ‘process’ and ‘product’ is well weathered crystals are ‘solid rock’ is not clear (see Hall, 2006a for a illustrated by the following quote: “It is difficult to understand how discussion on this attribute). So here we see an attempt to integrate items of metre-scale dimension are the product of nano-scale climate, process and product within a scale framework, and then processes” (Anon, 2006). Clearly, this remark not only reflects the extend it to field observations of variable lithological responses to confusion already suggested, but also highlights spatial scale. In many freeze–thaw weathering. Tricart's goal is far too ambitious as it ways the present problem is probably dominantly a historical legacy attempts to reduce multiple factors to two terms. His argument stemming from periglacial geomorphology's roots in climatic geo- proceeds from field observation of two categories of cold region morphology, but onto which has been grafted modern process debris, namely large and small, recognizes several important geomorphology. As climatic and process geomorphology have (conceptual) nuances and then attempts to shoehorn everything radically different starting points, conflation of the two approaches, into just two terms. and the use of terminology taken from one branch into the other, sows The seeming elegance of Tricart's simple duality continues to steer the seeds of confusion. Fruition of these seeds is all too apparent to influential and recent freeze–thaw weathering studies. For example, anyone investigating the history of freeze–thaw weathering as a Grossi, et al. (2007, p. 274–5) see microgélivation as producing flakes geomorphic topic. However, whatever the root cause, a perusal of the through ice segregation, while macrogélivation opens pre-existing literature makes it abundantly clear that it is scrutiny of the freeze– macro-fractures (but the process is not specified). However, perspec- thaw weathering process through the lens of its products that tives driven by experimental work appear to be evolving, this may be produces the shallow philosophical problems highlighted above: seen in two recent reviews of the topic (Matsuoka, 2001; Matsuoka namely, a flawed terminology, frequent misinterpretation of field and Murton, 2008). Matsuoka (2001) uses the terms microgélivation evidence, and a weak experimental approach. Consideration of the and macrogélivation liberally, and in a foundational manner in his scale issues pertaining to the freeze–thaw weathering process and its paper. While the two terms are used less liberally by Matsuoka and derivative forms can only be pursued constructively after a review of Murton (2008) they nevertheless retain them as central concepts and/ these issues. or terms. The inherent logical flaws within the definitions of Ideas concerning the reduction of bedrock by freezing and thawing microgélivation and macrogélivation at the very least hamper are quite old, reaching back at least to Hobbs (1910), and probably even research pursued within their fuzzy frameworks. What we now see earlier (see Thompson (1959) citing the work of Palissy (1563, 1580) is leading experimentalists increasingly confronting the limitations who commented on frost shattering in the 1500s). Consequently, the inherent to the inherited terminology as the experimental data idea emerged in a world of qualitative geomorphology wholly devoid of improve (e.g., Matsuoka and Murton, 2008). However, paralleling experimental underpinning as it might be pursued today. Before any this, we also see in them a great reluctance to abandon the modern-style empirical investigations of freeze–thaw weathering took terminology itself. place Tricart (1956, 1970) attempted to systematize the concept, A second terminological problem deeply embedded within the producing in the process terminology that remains very much alive in freeze–thaw weathering terminology is the use of the terms ‘soft’ and the lexicon of contemporary periglacial geomorphology. In as much as ‘hard’ rocks (e.g., Grossi et al., 2007; Matsuoka and Murton, 2008) with Tricart (1970, p. 115) coined the terms macrogélivation and micro- the suggestion that microgélivation affects ‘soft’ rocks while macro- gélivation his work must be viewed as seminal and needs to be parsed gélivation affects ‘hard’ rocks. The seemingly harmless substitution of carefully. Clearly rooted in a climatic geomorphological perspective, ‘soft’ and ‘hard’ for the terms ‘weak’ and ‘strong’ is, in fact, a substantial Tricart appears to invoke a mixture of climatically-based weathering shift. ‘Hardness’ is a property generally related to erosion; while process scales and resulting product scales. Macrogélivation is where ‘strength’ generally relates to fracture (by weathering). Strength is the

Please cite this article as: Hall, K., Thorn, C., The historical legacy of spatial scales in freeze–thaw weathering: Misrepresentation and resulting misdirection, Geomorphology (2010), doi:10.1016/j.geomorph.2010.10.003 K. Hall, C. Thorn / Geomorphology xxx (2010) xxx–xxx 3 resistance to failure induced by several possible stresses (e.g. flexure, established quantitatively, and that even this step does not axiom- shear, compressive and/or tensile: Goodman, 1989) while hardness is atically preclude other important weathering processes. a “measure of a material's resistance to localised elasto-plastic The concept that angularity bespeaks freeze–thaw weathering has deformation” (Szwedzicki, 1998, p. 825), solely a compressive stress been challenged a number of times (e.g. Hall, 1997; Hall, et al., 2002) situation. The two properties (strength/hardness) are certainly and we will not address the details further here. Philosophically, connected, especially as hardness is related to the compressive yield interpretation of rock debris angularity as being inherently the strength of the material, but in geomorphic use the term hardness is product of freeze–thaw weathering is an excellent illustration of the more commonly related to “the ability to resist scratching or abrasion” problem of abduction (Rhoads and Thorn, 1996b). This is an issue that (Hunt, 2005, p. 147). Consequently, there is nothing to prevent confronts earth scientists endlessly. Deduction, loosely meaning macrogélivation from opening pre-existing macro-fractures in soft knowing a cause and predicting the result, is logically the most (weak) rocks—indeed, as the tensile stresses induced by the powerful explanatory tool available; abduction, roughly meaning volumetric growth of ice must overcome the tensile strength of the looking at the result and retrodicting, is always open to question. rock it would seem more likely that weak (‘soft’) rocks would fail However, it is invariably the only pathway open to the earth scientist sooner than strong (‘hard’). It would seem that the introduction of rock supposing that he or she actually understands whatever is invoked as attributes (strength) as a further component, linked through process the causal agent. to scale, adds to the overall confusion and unnecessary complexity The abduction problem in the freeze–thaw and angular rock debris associated with experimental laboratory research into freeze–thaw context is essentially a small-scale illustration of equifinality or weathering. convergence (Thorn, 1988). It is well illustrated by a closely akin Furthermore, Matsuoka and Murton (2008, p. 199) employ the phenomenon, namely spalling. A spall comprises a flake of material term ‘hard’ for a rock that is not frost susceptible and ‘soft’ for a rock broken off a larger solid body by any mechanism, and is actually a considered susceptible. As Matsuoka and Murton are clearly looking ‘buckle’ that continues to propagate until it fractures the host material at freeze–thaw weathering, driven by the freezing (or not) of water, thereby resulting in the flake being released, a ‘spall’ (e.g. Wang and should not the terms simply be ‘permeable’ and ‘impermeable’ rather Evans, 1999, p. 708); the buckle itself being a manifestation of fatigue than hard and soft? ‘Hard’ rocks in this instance derive their hardness along a boundary layer (Wang and Evans, 1999, p. 705). This is a from a lack of pores/microfractures that facilitate permeability; while descriptive definition and permits both the buckle and spall to be ‘soft’ rocks with the high porosity/microfracturing are permeable. An driven by a multiplicity of processes (including thermal stresses) and additional layer of terminology seems superfluous, particularly when hence the product is not an indicator of the originating process, nor is it invokes terms with other connotations. Indeed, consideration of the buckling process itself well understood (e.g. Hettema, et al., 1998). Tricart's ideas (1970,p.76citedabove)showsthathesaw While Bland and Rolls (1998) also see the flake (spall) as solely a sedimentary rocks as being more frost susceptible “the less they are product, with a multiplicity of possible causes, Ahnert (1996) and compacted”: i.e. the more permeable they are. If this assertion is Leask and Wilson (2003) associate spalling with thermal stresses, incorrect then there is a duality that is not being considered by Ollier (1984) and Clark and Small (1982) link it to unloading, and Matsuoka and Murton (2008) and others; namely, hard vs soft and French (2007, p. 64) directly and solely equates insolation weathering permeable vs impermeable, and all the possible combinations of these with ‘spalling’. No matter how one may view any individual two groups. However, such a suggestion is hard to reconcile with the perspective, there is clearly no consensus or commonality defining information presented by Matsuoka and Murton (2008, p. 204) who the origin of the product. When multiple abductive pathways are state “moist, porous rock behaves remarkably like moist, frost- available the only surety is uncertainty and it becomes essential to susceptible soil”—the key parameter here is the permeability of the devise critical experiments designed specifically to weed out options material, so that increasing permeability (by means of inter- and/or conflicts—this is true of spalling, freeze–thaw breakdown of connected pores) is associated with decreasing strength of the rock material, and several other allied weathering issues. material. Thus, we would argue, it is not ‘hard’ and ‘soft’ rocks that are at issue, but rather, as has always been the case, the permeability/ 4. Experimentation impermeability of the rock—the wetter the rock the more frost- susceptible it would be given freezing conditions. The use of ‘hard’ and Experimentation in geomorphology may be a field undertaking in ‘soft’ simply add further obfuscation to what is otherwise already a which there is usually no experiment per se, although it is feasible, but confused concept, and thus should be avoided. In fairness, we would rather very close monitoring of natural conditions that permits point out that Matsuoka and Murton (2008) indicate that it is frost identification of a single process, comparison of two or more processes, susceptibility versus non-susceptibility that they are really trying to and/or evaluation of process frequency or rate. Such research is clearly distinguish, but this distinction doesn't map directly on to the beholden to natural conditions beyond the researcher's control. ‘shorthand’ terminology widely used. Alternatively, the researcher may undertake laboratory experimentation in the fundamental sense. The history of experimental freeze–thaw weathering studies has been reviewed elsewhere (e.g., Thorn, 1979; 3. Field (mis)interpretation Hall, 1986, 1988; Matsuoka, 2001; Matsuoka and Murton, 2008). In broad terms early laboratory work was relatively unsophisticated and in One of us (Thorn, 1992) has argued that the stranglehold that particular failed to combine moisture and temperature appropriately. freeze–thaw weathering has on many geomorphologists contemplat- Instead, it emphasized the pattern and intensity of sub-zero tempera- ing a cold landscape was built on repetition of a circular argument tures, as well as the frequency and duration of such events. Another devoid of data, or at least devoid of relevant data. Cold regions are cold important flaw was that the design of the experiments conducted failed by definition, they commonly experience atmospheric freezing and to exclude the possibility that the observed debris breakdown was due thawing of varying duration. Many of these regions also exhibit to other processes. widespread angular debris. This correlation was given unproven Today cutting-edge laboratory experimentation is considerably causality and angular debris became ‘proof’ of the effectiveness of more sophisticated. It may effectively be subdivided into two themes: freeze–thaw weathering. This mantra was established in a qualitative 1) a more careful parsing of the mechanism(s) involved in the discipline that is now determinedly quantitative. It is not that we wish classical notion of freeze–thaw weathering; 2) rigorous investigation to claim that freeze–thaw weathering is absent in cold regions, but of alternative explanations for the breakdown of material in contexts rather that we wish to point out that its presence needs to be where the traditional explanation for comminuted rock has occurred

Please cite this article as: Hall, K., Thorn, C., The historical legacy of spatial scales in freeze–thaw weathering: Misrepresentation and resulting misdirection, Geomorphology (2010), doi:10.1016/j.geomorph.2010.10.003 4 K. Hall, C. Thorn / Geomorphology xxx (2010) xxx–xxx exclusively in terms of the freeze–thaw mechanism. An important scale” influence on thermal weathering wherein the surface roughness shift in (1) has been movement away from purely empirical work at that scale creates the very same shading to incoming radiation as with only a very weak theoretical underpinning to experiments found on any large building due to facades, etc. Composition, impacted closely tied to theoretical development. However, the latter is by the resulting thermal regime, plays a role dependent on physical relatively sparse and highlighted by Walder and Hallet (1985, properties, for example the anistropic response of quartz to heating 1986), and Hallet (2006). with the thermal coefficient of expansion being 7.5 m×10−6 K−1 Matsuoka (2001) took up Tricart's microgélivation and macrogéliva- parallel to the c-axis but 13.7 m×10−6 K−1 perpendicular to the c-axis tion concepts and addressed the research upon them in both field and (see Hall, et al., 2007 and Hall et al., 2008a,b for discussion on this). laboratory contexts, noting important mismatches between the two. He Indeed, de Castro Lima and Paraguassú (2004) show that the presence assigns an emphasis relevant to microgélivation on laboratory research and amount of quartz greatly impact the degree of cracking in the and one relevant to macrogélivation to field studies. Invoking outer shell of granite. Thus, “it is the impact of thermal conditions on theoretical work he immediately subdivides ‘freeze–thaw or frost individual minerals and their association with each other that is critical weathering’ into two broad process categories, one driven by volumetric at the grain scale” (Hall et al., 2008b, p. 490). Further, as grain size expansion and the other by ice segregation. While recognizing the increases so there is a decrease in the thermal expansion coefficient feasibility of soft (permeable?) rock breakdown by multiple processes, (α) but with every 10% increase in quartz content there is a ≈20% Matsuoka (2001, p. 301) does clearly state a criterion for assigning increase in α and, at the same time, porosity greatly impacts the α laboratory experiments to volumetric expansion on freezing. In the value with an increase of porosity by 1% resulting in a decrease in α by second part of his paper Matsuoka highlights the much more uncertain 2.3×10−3 mm/m ºC (de Castro Lima and Paraguassú, 2004): see Hall empirical understanding of joint propagation by freezing and thawing in et al. (2008bp. 490–491) for a more detailed discussion of the hard (impermeable?), but jointed, rocks. interactions of porosity, α, and temperature. These thermal processes Matsuoka and Murton's (2008) review of frost weathering will produce granular disintegration indistinguishable in product from represents a summary of the present experimental understanding of ‘microgélivation’ and, in the absence of water, by the same thermal the topic. Here we only highlight only a few salient points. Laboratory variations that drive microgélivation. research is now sharply focused upon volumetric expansion and ice Despite the concerns of some researchers (e.g., French, 2007, p. 66) segregation as two separate issues. Crack or joint opening (i.e., a that actual thermal stresses have not been measured, the reality is central component of classic macrogélivation) is apparently produced that, although highly complex, and unique to every point in a rock, it is by different types of freeze–thaw cycles. Ice fracture of ‘soft’ possible to evaluate the stresses for a rock as a whole (de Castro Lima (permeable) rocks by the ice segregation mechanism is apparently and Paraguassú, 2004). As much as the foregoing was related to feasible at the base of the active layer and the top of the abutting problems associated with thermal stresses at the grain scale, the permafrost layer (Murton et al., 2006). Laboratory research has yet to reality is that ‘freeze–thaw’ within microfractures is also driven by address joint development in hard rocks effectively. thermal variations (simply, in this case, impacting water as the It is clear from Matsuoka and Murton's (2008) review that cutting- driving force). Thus, the same complexities that impact thermal stress edge laboratory research is finally escaping the intellectual con- may also affect freeze–thaw—with the addition of process synergy straints of Tricart's microgélivation and macrogélivation. However, (thermal and freeze–thaw) both operating simultaneously. Such the old terminology still emerges even here where it sits very problems are not solely the purview of cold region scientists, as uncomfortably, and it is certainly still responsible for the creation of a Viles (2005, p. 190) points out with respect to weathering in hot research mindset that suggests there are (just) two scales upon which deserts: “Uncertainty still exists, for example, as to what the dominant to focus. What Matsuoka and Murton (2008) do not do, nor do they weathering processes are and how they operate, where and why rates claim to at any point, is address the growth of studies centered on of weathering are fastest, and how weathering contributes to mechanisms that may be just as relevant as freeze–thaw to bedrock landform development.” Clearly, weathering is not as well understood and debris weathering in cold environments. as our entrenched responses would suggest. Here we ignore all weathering work in cold regions that addresses At the larger scale of “macrogélivation” there are many similar biological and chemical weathering in order to focus on mechanisms kinds of intellectual problems. Thermal stresses (the same drivers as that might actually be confused directly with classical freeze–thaw are required for the freezing and thawing of water) can produce weathering if the researcher views the topic through the lens of the “macro-cracks”, with a form that is ‘angular’ but, at the same time, product, namely angular material. What mechanical process might be very difficult to explain by frost processes. The cracks take on a operative at what scale is predicated upon the assumption that the rectilinear form, with a hierarchy of cracks normal to each other and processes themselves are well understood. Whilst this may be true for which show no relationship to pre-existing lines of weakness in the some mechanical weathering processes, especially perhaps salt rock (Hall, 1999), and which are of the same form as found in hot weathering (e.g. Houck and Scherer, 2006, although also see Smith deserts (Soleilhavoup, 1977, 1978). Examples of such fractures from et al., 2005) and dilatation (e.g. Chun'an, 1997), this is less the case for the high Andes can be seen in Hall (1999, Figs. 1–3) while Bahr et al. thermal stresses (e.g. Logan, 2004; Gómez-Heras, et al., 2006), wetting (1986, p. 2717) show comparable fractures generated in ceramic disks and drying (e.g. Sumner and Loubser, 2008), or their synergistic subject to thermal shock, and Moores et al. (2008) provide a operation (e.g. Mutlutürk et al., 2004; Weiss, et al., 2004; Smith, et al., theoretical explanation, based on field data, for the crack propagation 2005; Hall, 2006b; Ruedrich and Siegesmund, 2007) despite the often by thermal stresses. Thus, large angular rocks, in cold regions, can be simplistic statements to the contrary (e.g. Grossi et al., 2007; Tarbuck, produced solely by thermal stresses and, it is argued (e.g. Hall, 1999) et al., 2009). For example within the realm of contemporary that cold regions may be more conducive to thermal stresses than hot weathering Hall et al. (2008a) have shown that where components regions as the cooling phase is twice as stressful as the warming phase of a rock are light transmissive the surface weathering zone is (Marovelli, et al., 1966). Hence, other mechanical processes operating extended (to a depth of a centimeter or more) and may impact process alone, as well as in synergy, can produce angular rocks at a host of (es) when compared to a non-transmissive lithology. The same scales within cold regions. thermal/radiation conditions will induce quite different weathering responses dependent on the mineral composition and hence the 5. Form degree of light transmissivity; which is also impacted by mineral orientation relative to the light-receiving surface In addition, Hall et al. With respect to the commonly cited (e.g. Hanvey and Lewis, 1991; (2008b), following Gómez-Heras et al. (2006), argue for a “micro- Czudek, 1993; Heine, 1994; Coltorti and Dramis, 1995) relationship

Please cite this article as: Hall, K., Thorn, C., The historical legacy of spatial scales in freeze–thaw weathering: Misrepresentation and resulting misdirection, Geomorphology (2010), doi:10.1016/j.geomorph.2010.10.003 K. Hall, C. Thorn / Geomorphology xxx (2010) xxx–xxx 5 between clast angularity and its origin by freeze–thaw, there are two data) or convert interval data to some other form (e.g., a proportion fundamental questions. First, is there any intrinsic characteristic of the such as percentage), the latter are ratio data. angular clast that uniquely identifies it as of freeze–thaw origin? In The terms macro-, meso-, and micro- are widely used in the light of our present knowledge, the answer must be an geomorphology, but with purely relative and variable meaning. This unequivocal "no". Identical angularity can be produced by salt, means that their use is inconsistent and inconsistency is the bane of thermal, wetting and drying, or biologically-induced mechanical professional terminology whose utility is essentially that of a weathering processes. Angular clasts in hot arid or semi-arid regions definitive shorthand. In the present context it is quite apparent that (e.g. Cooke and Warren, 1973) and periglacial regions show identical microgélivation and macrogélivation are purely relative and have form. Angularity itself simply does not identify process. Second, is served to steer contemporary research. The result has been investi- there any intrinsic process-controlled reason why a mechanically- gation focused upon two discrete scales with little or no evidence to weathered clast should be angular? Again, with the possible exception establish that they are the key, let alone only, scales. Rather as Viles of those derived from thermal shock, the answer must be "no". The (2001, pp.65–66), following Phillips (1999), has pointed out “we do stresses created by the growth of ice, salt, a biological agency, etc., not have vast amounts of quantitative data with which to analyze” serve to exploit a pre-existing weakness. In many rocks, much of the and thus the “gaps found may simply be measurement gaps rather time, these weaknesses are linear—hence the preponderance of than the fundamental ‘spectral gaps’ of Blöschl and Sivipalan (1995).” angular\platy clasts in areas of mechanical weathering. Indeed, linear Such problems can only be confounded by the fact that laboratory elastic fracture mechanics (LEFM) calculations of fracture extension research is being conducted on the formative processes while the assume linearity; but this is primarily to simplify the mathematics and original focus in the field was upon the products. it is recognised they can be curvi-linear (e.g. Rossmanith, 1983). In Association of macro-, meso-, and microscales with absolute SI some lithologies internal lines of weakness may have a curvi-linear units cannot be undertaken comprehensively. The prefix ‘macro-‘ is form: cooling or micro-flow structures in some igneous rocks not an SI designator, but seems to be associated with things visible to (Romaní, et al., 2006), sedimentary structures in some sandstones the naked eye. In SI terminology this would take us from 1:1 or shales. The exploitation of these non-linear weaknesses by (something that has no SI term), through deca- and hecto-, to some mechanical stresses results in curved forms (Hall, 1997, 1998 and lower boundary. It could perhaps be designated as roughly N10−2 Hall et al., 2002 show examples of rounded mechanically-weathered (technically from the centiscale to the yottascale (1024m)). Microscale forms from the Antarctic). The simple reality is that the preponder- would be 10−6 m (strictly speaking) and nano-scale would be 10−9 ance of weakness in most rocks is linear and hence angular clasts are (Hochella, 2002). Both micro- and nanoscale would need to be viewed the most common. To belabor the point, form does not identify across at least plus and minus one order of magnitude to be truly process and thus angular clasts in cold regions are no more an useful terminology. Mesoscale simply has no well articulated range, indicator of freeze–thaw than are mechanically-weathered rounded although Hall (2006a) suggested 10−2 to 10−5 m, but even here we clasts. Arguments for any such relationships (e.g. Hanvey and Lewis, see ourselves having confused relative and absolute concepts because 1991, p. 35) merely serve to perpetuate the myth through repetition. this suggestion itself crosses the centi- and milli- Si boundaries. It should be noted that Derbyshire (1972) in a discussion regarding Discussion of relative versus absolute scale serves two potential tors in Antarctica, identified both rounded and angular weathering purposes. The simpler of the two ideas is that both permit communi- forms that may be associated, at some point, with frost action. cation, but absolute scale permits sounder scientific communication. Nevertheless, the bulk of the discussion regarding rounded versus The more penetrating idea is if scale actually serves to identify processes angular is concerned with the interaction of chemical weathering that are physically and/or chemically different from each other. This is with mechanical processes, with the former being the prime cause of apparently a central issue in freeze–thaw weathering where there is rounding and the latter of angularity. Thus, the issues associated with apparently a belief, however weakly defined, that there are two form still remain. different processes working at smaller and larger scales. Using this as a starting point it should be possible to seek the upper and lower limits of the two mechanisms in terms of an absolute scale (presumably the SI 6. Scale framework). Laboratory experiments could be directed at the two perceived processes individually; however, identification needs to be in Scale pervades science including geomorphology. Two of the terms of the process not the product. This would need to be stated in greatest problems facing geomorphologists are scale itself and scale- terms of temperature and water/ice. Identification of the process in linkage. Both issues plague freeze–thaw weathering research. Theo- terms of the product raises the importance of scale-linkage as a separate retical understanding is patchy and incomplete, being constrained to problem. the scale at which the research was undertaken. Consequently we must question the scales of established research by seeking to expand 7. Scale-linkage their applicability or they continually constrain future research. Such expansion immediately confronts the problem of scale-linkage in Scale-linkage is perhaps an even more vexing intellectual which the research problems at one scale may not pertain to other challenge than scale itself. At least a comprehensive analytical new problems (so called ‘emergent variables’) may appear. There understanding at one scale is just that—a comprehensive understand- have been several recent reviews of scale problems in geomorphology ing. However, scale-linkage involves numerous new uncertainties and (e.g., Phillips, 1988; Malanson, 1999; Viles, 2001; Inkpen, 2005). We assumptions; of course, once they are resolved the scope of our take issue with none of these reviews rather we seek to apply portions analytical understanding is expanded and axiomatically the scale- of them directly to the current understanding of freeze–thaw linkage problem eliminated. Nevertheless, the mismatch between weathering. Newtonian physics and quantum mechanics serves as a reminder of Scale reflects measurement and thus the metrics we choose, or are just how enormous scale-linkage problems can be. forced upon us by conceptual and/or technological limitations. The Is the scale of the driving force, namely freezing, an important most fundamental split is between relative and absolute measures. variable? Such a question embraces the rate, duration, intensity, and Relative scale is associated with nominal (categorical) and ordinal frequency of freezing. Can all of these variables be meaningfully linked measurement, that is variability among categories with no real metric to each other? Are any of them independently limiting and/or to what and among those with only order or rank. Absolute measures provide degree can the variability in one substitute for variation in others? exact measurement according to a standard unit or frequency (interval Does reality (natural climate) serve as a good starting point for

Please cite this article as: Hall, K., Thorn, C., The historical legacy of spatial scales in freeze–thaw weathering: Misrepresentation and resulting misdirection, Geomorphology (2010), doi:10.1016/j.geomorph.2010.10.003 6 K. Hall, C. Thorn / Geomorphology xxx (2010) xxx–xxx experimentation? We can then move on to scale constraints on the rock's origin and history (Sousa, et al., 2005; Romaní, et al., 2006). substance itself—water. What are the scale limits on water availability Thermal cracking and stress-induced cracking of the granite can cause and mobility and to what extent can shortcomings be offset by the micro-cracks that may coalesce into macroscopic fractures (David, et four temperature variables? Finally, how do all of these variables al., 1999) that can, in turn, facilitate the operation of other weathering interact with another profoundly important variable—the host rock? mechanisms (especially salt weathering: Sousa, et al., 2005). Recent Laboratory research on freeze–thaw weathering has fallen foul of studies by Moores et al (2008) have also shown that the impact of many of the issues implicit in the series of questions posed above. Hall thermal stresses on crack propagation can be impacted by whether (1998, 1999) and Hall and André (2001, 2003) have demonstrated moisture at the crack tip remains or is driven off by solar heating; that freezing rates in nature may well be adequate to produce thermal more weathering occurring when the moisture remains to facilitate fatigue rather than freeze–thaw weathering. There has been a other (i.e. not thermal stress) weathering processes. Thermal cracking prolonged engagement with the geomorphic significance believed itself can be the result of thermal stresses and/or thermal shock. Many to be implicit to diurnal, seasonal, and annual freeze–thaw cycles and authors use slow rates of change of temperatures (ΔT/Δt) in the range their varying parameters. However, we might question the validity of of 1 °C min−1 to preclude thermal shock (e.g. David, et al., 1999; Lion focusing upon any one of these scales when all are present et al., 2005; Reuschlé, et al., 2006), with values in excess of 2 °C min−1 simultaneously. Such a perspective casts a real shadow on making deemed to facilitate thermal shock (see Hall, 1999 and Hall and André, mimicry of any naturally occurring freeze–thaw category THE 2003 for a discussion). Values recently used to create thermal shock appropriate starting point of experimentation. range from 2.4 °C min−1 (Ferrero and Marini, 2001), through 10° to The jump of viewing a geomorphic process through the lens of its 15 °C min−1 (Ghaffarian, 2001), to 25° to 44 °C min−1 (Peng et al., products, one that underpins the original freeze–thaw weathering 2008). Although, theoretically, the boundary values for the thermal perspective, clearly fuses two scale issues together, namely those of shock of any specific material may be calculated (e.g. Goncharov, et al., the process(es) and the product(s). None of the very real problems 1968; Tite, et al., 2001); such factors such as anisotropy (e.g. Amadei, facing geomorphologists investigating freeze–thaw weathering seems 1996; Mahmutoglu, 1998; Yavuz, et al., 2006), crystal composition, to be as great as the problem faced by those using the product as the size, orientation, and connectivity (e.g. Sousa, et al., 2005; Hall et al., starting point. The simplest objection to this approach is perhaps the 2008a,b) will greatly influence the actual outcomes. In broad terms, most telling—if freeze–thaw weathering is viewed by examining and certainly germane to the arguments here, the same may be the weathered products the researcher is confronting the maximum case for freeze–thaw weathering when “…pressure due to water number of variables at once and making the implicit assumption that expansion (as it turns to ice) reaches the tensile strength of the rock, two very complicated sets of variables are inherently matched in a new microfractures are developed and present ones (our italics) are systematic scale-sensitive fashion. Both analytical and empirical deepened and widened. After thawing, water can migrate into the investigations normally start off in exactly the opposite fashion, by newly developed microfractures. Recurrent freeze–thaw cycles cause making everything as simple as possible, even at the risk of making enlarging of existing fractures and further weakening of the material” them unrealistic initially. (Yavuz, et al., 2006, p. 767). There is clearly a sequence of scale events, but all by the same process without any spatial scale dependency—yet 8. Discussion product scale outcomes continue to change. Many geomorphologists might want to discount, or dismiss It seems redundant to consider the spatial applicability of process entirely, the sorts of issues raised here. Such a decision is not only operation when the very process itself is so poorly understood and its foolhardy, it is not really theirs to make. The inexorable trend in interaction with other processes even less known. In some ways this is geomorphology has moved the discipline from one with an essentially an underlying tenet of the whole problem: creating a process-scale- free-standing theoretical foundation (think Davisian and/or climatic linkage where the parameters of process operation are themselves geomorphology) to one that increasingly embeds itself deeply in the unknown. For example, if one accepts the hypothesis of segregation ice theoretical and experimental structures of science at large. Conse- growing in a crack to force the rock apart by means of an excess of ice quently, the geomorphology and geomorphologists of the future will over space available (as argued by Walder and Hallet, 1985), then need be judged not by geomorphologists alone, but rather by the broader this have any spatial attributes? Presumably, following the original scientific community. Weathering studies specifically have clearly hypothesis, the mechanism can operate at a small scale (micro-?) in entered a period of reductionist science where the inspiration for spaces between grains (as suggested by Grossi, et al., 2007) or even geomorphic research appears to be the research model of materials perhaps within micro-cracks (sensu stricto) in exactly the same way as science. There is much to be gained from this, although it will certainly it is postulated (Matsuoka, 2001) to separate large joint blocks (where not be a panacea for the field, but in order to be successful in this the product is macro-scale) along wide joint lines—“frost-wedging”. milieu geomorphologists will have to shed the standards of a bygone What is absent are actual data at the appropriate range of scales. The era in geomorphology and embrace contemporary standards within limiting factor (other attributes such as chemistry held constant) will material science. Materials science research undertaken with the be the stresses induced on the water as scale diminishes such that the dated standards and terminology of field geomorphology from the water will not freeze; the corollary would be, can the ice in large cracks early to mid-1900s is a sure recipe for ongoing confusion. exert a confined stress that can cause fracture as opposed to causing solely movement of already unbound blocks? Again, it is the absence of 9. Conclusions unequivocal data that is the limitation, coupled with the assumption of these spatial scales of operation. While clearly the products of freeze–thaw weathering, as with Further, the whole notion of a spatial–process relationship is almost any weathering process, may range in size from grains to fraught with problems of temporal process synergy—the process boulders, the notion of a process dichotomy (microgélivation/ observed may be dependent upon a preceding process not, or less macrogélivation) is, at best, naïve, and is likely more confusing than easily, observed (see Smith et al., 2005 for a discussion on this very enlightening. At the root of the issue is the confusion of product with point but in respect to salt weathering in hot deserts and McCabe et process. Attempts to validate the two weathering scales have resulted al., 2007 for sandstone buildings). For example, in respect to granites, in the acceptance of many assumptions (not the least that there are as with many other lithologies, the amount and rate of subsequent two and only two weathering scales) resulting in misdirection and weathering are highly dependent upon micro-cracks (intergranular, poor field evaluative approaches (i.e. product size identifies process intragranular, and transgranular) that may be the product of the and hence climate conditions, past or present). These poor field

Please cite this article as: Hall, K., Thorn, C., The historical legacy of spatial scales in freeze–thaw weathering: Misrepresentation and resulting misdirection, Geomorphology (2010), doi:10.1016/j.geomorph.2010.10.003 K. Hall, C. Thorn / Geomorphology xxx (2010) xxx–xxx 7 evaluative tools have been further exacerbated by the totally Hall, K., 1997. Rock temperature data and implications for cold region weathering. I. fi – New data from Viking Valley, Alexander island, Antarctica. Permafrost and unproven presumption that angular debris is an identi er of freeze Periglacial Processes 8, 69–90. thaw weathering. In addition, the reality is that there are minimal data Hall, K., 1998. Rock temperatures and implications for cold region weathering. II: new data relating to smaller-scale frost weathering, and that which do exist are from Rothera, Adelaide Island, Antarctica. Permafrost and Periglacial Processes 9, 47–55. – Hall, K., 1999. The role of thermal stress fatigue in the breakdown of rock in cold not particularly supportive of freeze thaw as the sole or, in some regions. Geomorphology 31, 47–63. cases, even the most probable weathering mechanism. For larger Hall, K., 2006a. Perceptions of rock weathering: some thoughts on attributes of scale. joint-bounded blocks there remains the question of whether Géomorphologie 3, 187–196. measured crack widening is an expression of weathering enlarging Hall, K., 2006b. Monitoring of thermal conditions in building stone with particular reference to freeze–thaw events. In: Kourkoulis, S.K. (Ed.), Fracture and Failure of the crack or if it is simply measurement of highly localized transport of Natural Building Stones. Springer, Dordrecht, pp. 373–394. an already liberated (weathered-free) block. The weathering of a fresh Hall, K., André, M.-F., 2001. 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