GEOMOR-03394; No of Pages 8
Geomorphology xxx (2010) xxx–xxx
Contents lists available at ScienceDirect
Geomorphology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h
The historical legacy of spatial scales in freeze–thaw weathering: Misrepresentation
and resulting misdirection
Kevin Hall a,⁎, Colin Thorn b
a
b
Department of Geography, Geoinformatics & Meteorology, University of Pretoria, Pretoria, 0002, South Africa
Department of Geography, University of Illinois at Urbana-Champaign, 220 Davenport Hall, 607 South Mathews, Urbana, IL 61801, USA
a r t i c l e
i n f o
Article history:
Received 23 July 2008
Received in revised form 16 July 2009
Accepted 4 October 2010
Available online xxxx
Keywords:
Weathering
Freeze–thaw
Microgélivation
Macrogélivation
Process-product scales
a b s t r a c t
Discussion of weathering in cold regions has historically been dominated by widespread acceptance of the
significance of the freeze–thaw concept among periglacial geomorphologists, and an essentially universal
acceptance by those geomorphologists not directly involved in researching the topic. Debris produced by
freeze–thaw is frequently deemed to be angular in form and the observation of such debris has been used to
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:
macrogélivation and microgélivation respectively. However, the fusion of climatic geomorphology and
process geomorphology in the ongoing development of the freeze–thaw concept has resulted in the confusion
of product with process—whereby microgélivation (producing small products) and macrogélivation
(producing large products) are seen, due to the product dichotomy, as distinctly different processes. Despite
the recent, highly sophisticated laboratory experimentation on freeze–thaw weathering, this historicalprocess 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
Geomorphologists consider landforms to be the product of geomorphic processes and, in turn, influence those processes. They recognize
that this interaction is sensitive to initial conditions (e.g., Phillips, 1988,
1999), plays out at multiple spatial and temporal scales (Schumm and
Lichty, 1965), and that the process–form interactions they choose to
invoke may change through time (e.g., Haff, 1996). Such a claim raises a
deep philosophical issue—are geomorphologists identifying natural
kinds when they identify landforms, or are they carving categories from
a landscape continuum (Rhoads and Thorn, 1996a)? Here we skirt such
‘deep philosophy’, preferring to focus upon the ‘shallow philosophy’
(often called ‘methodology’) associated with everyday terminology, the
interpretation of field evidence, and the nature of experimental
⁎ Corresponding author. Present address: Geography Programme, University of
Northern British Columbia, 3333 University Way, Prince George, BC, V2N 4Z9, Canada.
E-mail addresses: [email protected] (K. Hall), [email protected] (C. Thorn).
geomorphology. These considerations are pervaded with scale concepts
varying from the scale of the processes and of the landforms to the use of
scale-defined terminology, and finally to the broad scientific context of
scale concepts. Our objective is to illuminate how the legacy of
geomorphic thinking has constrained and steered contemporary
research into freeze–thaw weathering including the role scale has
played and is playing in the matter. Freeze–thaw weathering, broadly
the belief that the alternate freezing and thawing of hydrated materials
(especially rock) on short (e.g., diurnal or synoptic) or longer (e.g.,
seasonal) scales breaks them up, is widely invoked by periglacial
geomorphologists, and essentially universally-accepted as active in
periglacial regions by geomorphologists not actively engaged in cold
region research. As such it is an important geomorphic concept that
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
and the landform one of effect. However, this clearly conflates the
process and the material and it is much less clear how these two
0169-555X/$ – see front matter © 2010 Published by Elsevier B.V.
doi:10.1016/j.geomorph.2010.10.003
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
effect, role. There is no a priori reason to believe that the scale of a
geomorphic process, as opposed to the characteristics of the material,
inevitably controls the scale of the geomorphic product—the
landform. Furthermore, if a landform is the product of the interaction
between a geomorphic process (the driving force) and an existing
material there is no inherent reason to believe that: 1) the same
driving force will always produce the same resulting landform (the
forward-looking view); 2) any form is necessarily always the product
of the same process (the backward-looking view); and 3) driving
forces might not be constrained or dictated by the initial nature of the
material. Finally, the explanation of any existent landform assemblage
(landscape) is always plagued by largely uncertain initial conditions,
changing environment(s) or inheritance (temporal contamination),
and interference (spatial contamination).
2. Terminology
Historically geomorphology was essentially an ‘eye-ball’ science
and geomorphologists investigating the nature of freeze–thaw
weathering appear to have conflated the process and the product.
As we have addressed this issue previously (e.g., Thorn, 1979; Hall et
al., 2002) the reader is referred to these earlier publications where this
admittedly rather sweeping generalization is generously referenced.
Furthermore, many periglacial geomorphologists seem to have
assigned an uncertain, sometimes unrecognized, or variable role to
the material(s) involved. The result is that the nature of the process
(es) remains(remain) uncertain and is(are) still commonly viewed
and understood in terms of the ensuing products (landforms or
landform elements). This conflation of ‘process’ and ‘product’ is well
illustrated by the following quote: “It is difficult to understand how
items of metre-scale dimension are the product of nano-scale
processes” (Anon, 2006). Clearly, this remark not only reflects the
confusion already suggested, but also highlights spatial scale. In many
ways the present problem is probably dominantly a historical legacy
stemming from periglacial geomorphology's roots in climatic geomorphology, but onto which has been grafted modern process
geomorphology. As climatic and process geomorphology have
radically different starting points, conflation of the two approaches,
and the use of terminology taken from one branch into the other, sows
the seeds of confusion. Fruition of these seeds is all too apparent to
anyone investigating the history of freeze–thaw weathering as a
geomorphic topic. However, whatever the root cause, a perusal of the
literature makes it abundantly clear that it is scrutiny of the freeze–
thaw weathering process through the lens of its products that
produces the shallow philosophical problems highlighted above:
namely, a flawed terminology, frequent misinterpretation of field
evidence, and a weak experimental approach. Consideration of the
scale issues pertaining to the freeze–thaw weathering process and its
derivative forms can only be pursued constructively after a review of
these issues.
Ideas concerning the reduction of bedrock by freezing and thawing
are quite old, reaching back at least to Hobbs (1910), and probably even
earlier (see Thompson (1959) citing the work of Palissy (1563, 1580)
who commented on frost shattering in the 1500s). Consequently, the
idea emerged in a world of qualitative geomorphology wholly devoid of
experimental underpinning as it might be pursued today. Before any
modern-style empirical investigations of freeze–thaw weathering took
place Tricart (1956, 1970) attempted to systematize the concept,
producing in the process terminology that remains very much alive in
the lexicon of contemporary periglacial geomorphology. In as much as
Tricart (1970, p. 115) coined the terms macrogélivation and microgélivation his work must be viewed as seminal and needs to be parsed
carefully. Clearly rooted in a climatic geomorphological perspective,
Tricart appears to invoke a mixture of climatically-based weathering
process scales and resulting product scales. Macrogélivation is where
“frost penetrates deeply enough to freeze water contained in joints” and
“springs (presumably this could/should also be translated as ‘heaves’)
joint blocks of several cubic metres” (Tricart, 1970, p. 115). Microgélivation on the other hand, occurs where “night frost succeeds in
reducing to sand and gravel, extremely hard volcanic rocks” (Tricart,
1970, p. 115). Thus, large amplitude, long duration freezes produce
(only) large material while short duration freezes can only produce
small sized material: a process–product scale relationship. The evidence
of both large blocks and fine material in cold regions adds visible
credence to the distinction. At this juncture we might identify Tricart as
using ‘morphogenic’ terms—such terms are not only descriptive but
inherently connote the origin of the form as well. Morphogenetic
terminology is widespread in geomorphology and has consistently
plagued the discipline as theory has progressed, thereby bypassing the
genetic component of a term while researchers have sought to retain the
descriptive (morphological) component. This has usually resulted in the
messy use of an established morphogenetic term supposedly shriven of
its genetic connotations. A classic example of this would be the way that
the Davisian term ‘peneplain’ has often been used subsequently to mean
‘an eroded regional surface’, but the user has not subscribed to the
development components of the Davisian landscape model.
However, Tricart's (1970, p. 76) position is actually much more
complicated because he also states that “sedimentary rocks are more
subject to microgélivation, the less they are compacted.” The issue
becomes even more convoluted when Tricart (1970, p. 76) states
“Macrogélivation exploits existing weaknesses in the rock; microgélivation cuts into solid rock” and then proceeds to claim that
“Weathered crystals are at once less resistant to pressure and more
permeable; it is these that microgélivation exploits”. Quite how
weathered crystals are ‘solid rock’ is not clear (see Hall, 2006a for a
discussion on this attribute). So here we see an attempt to integrate
climate, process and product within a scale framework, and then
extend it to field observations of variable lithological responses to
freeze–thaw weathering. Tricart's goal is far too ambitious as it
attempts to reduce multiple factors to two terms. His argument
proceeds from field observation of two categories of cold region
debris, namely large and small, recognizes several important
(conceptual) nuances and then attempts to shoehorn everything
into just two terms.
The seeming elegance of Tricart's simple duality continues to steer
influential and recent freeze–thaw weathering studies. For example,
Grossi, et al. (2007, p. 274–5) see microgélivation as producing flakes
through ice segregation, while macrogélivation opens pre-existing
macro-fractures (but the process is not specified). However, perspectives driven by experimental work appear to be evolving, this may be
seen in two recent reviews of the topic (Matsuoka, 2001; Matsuoka
and Murton, 2008). Matsuoka (2001) uses the terms microgélivation
and macrogélivation liberally, and in a foundational manner in his
paper. While the two terms are used less liberally by Matsuoka and
Murton (2008) they nevertheless retain them as central concepts and/
or terms. The inherent logical flaws within the definitions of
microgélivation and macrogélivation at the very least hamper
research pursued within their fuzzy frameworks. What we now see
is leading experimentalists increasingly confronting the limitations
inherent to the inherited terminology as the experimental data
improve (e.g., Matsuoka and Murton, 2008). However, paralleling
this, we also see in them a great reluctance to abandon the
terminology itself.
A second terminological problem deeply embedded within the
freeze–thaw weathering terminology is the use of the terms ‘soft’ and
‘hard’ rocks (e.g., Grossi et al., 2007; Matsuoka and Murton, 2008) with
the suggestion that microgélivation affects ‘soft’ rocks while macrogélivation affects ‘hard’ rocks. The seemingly harmless substitution of
‘soft’ and ‘hard’ for the terms ‘weak’ and ‘strong’ is, in fact, a substantial
shift. ‘Hardness’ is a property generally related to erosion; while
‘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
resistance to failure induced by several possible stresses (e.g. flexure,
shear, compressive and/or tensile: Goodman, 1989) while hardness is
a “measure of a material's resistance to localised elasto-plastic
deformation” (Szwedzicki, 1998, p. 825), solely a compressive stress
situation. The two properties (strength/hardness) are certainly
connected, especially as hardness is related to the compressive yield
strength of the material, but in geomorphic use the term hardness is
more commonly related to “the ability to resist scratching or abrasion”
(Hunt, 2005, p. 147). Consequently, there is nothing to prevent
macrogélivation from opening pre-existing macro-fractures in soft
(weak) rocks—indeed, as the tensile stresses induced by the
volumetric growth of ice must overcome the tensile strength of the
rock it would seem more likely that weak (‘soft’) rocks would fail
sooner than strong (‘hard’). It would seem that the introduction of rock
attributes (strength) as a further component, linked through process
to scale, adds to the overall confusion and unnecessary complexity
associated with experimental laboratory research into freeze–thaw
weathering.
Furthermore, Matsuoka and Murton (2008, p. 199) employ the
term ‘hard’ for a rock that is not frost susceptible and ‘soft’ for a rock
considered susceptible. As Matsuoka and Murton are clearly looking
at freeze–thaw weathering, driven by the freezing (or not) of water,
should not the terms simply be ‘permeable’ and ‘impermeable’ rather
than hard and soft? ‘Hard’ rocks in this instance derive their hardness
from a lack of pores/microfractures that facilitate permeability; while
‘soft’ rocks with the high porosity/microfracturing are permeable. An
additional layer of terminology seems superfluous, particularly when
it invokes terms with other connotations. Indeed, consideration of
Tricart's ideas (1970, p. 76 cited above) shows that he saw
sedimentary rocks as being more frost susceptible “the less they are
compacted”: i.e. the more permeable they are. If this assertion is
incorrect then there is a duality that is not being considered by
Matsuoka and Murton (2008) and others; namely, hard vs soft and
permeable vs impermeable, and all the possible combinations of these
two groups. However, such a suggestion is hard to reconcile with the
information presented by Matsuoka and Murton (2008, p. 204) who
state “moist, porous rock behaves remarkably like moist, frostsusceptible soil”—the key parameter here is the permeability of the
material, so that increasing permeability (by means of interconnected pores) is associated with decreasing strength of the
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/
impermeability of the rock—the wetter the rock the more frostsusceptible it would be given freezing conditions. The use of ‘hard’ and
‘soft’ simply add further obfuscation to what is otherwise already a
confused concept, and thus should be avoided. In fairness, we would
point out that Matsuoka and Murton (2008) indicate that it is frost
susceptibility versus non-susceptibility that they are really trying to
distinguish, but this distinction doesn't map directly on to the
‘shorthand’ terminology widely used.
3. Field (mis)interpretation
One of us (Thorn, 1992) has argued that the stranglehold that
freeze–thaw weathering has on many geomorphologists contemplating a cold landscape was built on repetition of a circular argument
devoid of data, or at least devoid of relevant data. Cold regions are cold
by definition, they commonly experience atmospheric freezing and
thawing of varying duration. Many of these regions also exhibit
widespread angular debris. This correlation was given unproven
causality and angular debris became ‘proof’ of the effectiveness of
freeze–thaw weathering. This mantra was established in a qualitative
discipline that is now determinedly quantitative. It is not that we wish
to claim that freeze–thaw weathering is absent in cold regions, but
rather that we wish to point out that its presence needs to be
3
established quantitatively, and that even this step does not axiomatically preclude other important weathering processes.
The concept that angularity bespeaks freeze–thaw weathering has
been challenged a number of times (e.g. Hall, 1997; Hall, et al., 2002)
and we will not address the details further here. Philosophically,
interpretation of rock debris angularity as being inherently the
product of freeze–thaw weathering is an excellent illustration of the
problem of abduction (Rhoads and Thorn, 1996b). This is an issue that
confronts earth scientists endlessly. Deduction, loosely meaning
knowing a cause and predicting the result, is logically the most
powerful explanatory tool available; abduction, roughly meaning
looking at the result and retrodicting, is always open to question.
However, it is invariably the only pathway open to the earth scientist
supposing that he or she actually understands whatever is invoked as
the causal agent.
The abduction problem in the freeze–thaw and angular rock debris
context is essentially a small-scale illustration of equifinality or
convergence (Thorn, 1988). It is well illustrated by a closely akin
phenomenon, namely spalling. A spall comprises a flake of material
broken off a larger solid body by any mechanism, and is actually a
‘buckle’ that continues to propagate until it fractures the host material
thereby resulting in the flake being released, a ‘spall’ (e.g. Wang and
Evans, 1999, p. 708); the buckle itself being a manifestation of fatigue
along a boundary layer (Wang and Evans, 1999, p. 705). This is a
descriptive definition and permits both the buckle and spall to be
driven by a multiplicity of processes (including thermal stresses) and
hence the product is not an indicator of the originating process, nor is
the buckling process itself well understood (e.g. Hettema, et al., 1998).
While Bland and Rolls (1998) also see the flake (spall) as solely a
product, with a multiplicity of possible causes, Ahnert (1996) and
Leask and Wilson (2003) associate spalling with thermal stresses,
Ollier (1984) and Clark and Small (1982) link it to unloading, and
French (2007, p. 64) directly and solely equates insolation weathering
with ‘spalling’. No matter how one may view any individual
perspective, there is clearly no consensus or commonality defining
the origin of the product. When multiple abductive pathways are
available the only surety is uncertainty and it becomes essential to
devise critical experiments designed specifically to weed out options
and/or conflicts—this is true of spalling, freeze–thaw breakdown of
rock material, and several other allied weathering issues.
4. Experimentation
Experimentation in geomorphology may be a field undertaking in
which there is usually no experiment per se, although it is feasible, but
rather very close monitoring of natural conditions that permits
identification of a single process, comparison of two or more processes,
and/or evaluation of process frequency or rate. Such research is clearly
beholden to natural conditions beyond the researcher's control.
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;
Hall, 1986, 1988; Matsuoka, 2001; Matsuoka and Murton, 2008). In
broad terms early laboratory work was relatively unsophisticated and in
particular failed to combine moisture and temperature appropriately.
Instead, it emphasized the pattern and intensity of sub-zero temperatures, as well as the frequency and duration of such events. Another
important flaw was that the design of the experiments conducted failed
to exclude the possibility that the observed debris breakdown was due
to other processes.
Today cutting-edge laboratory experimentation is considerably
more sophisticated. It may effectively be subdivided into two themes:
1) a more careful parsing of the mechanism(s) involved in the
classical notion of freeze–thaw weathering; 2) rigorous investigation
of alternative explanations for the breakdown of material in contexts
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
shift in (1) has been movement away from purely empirical work
with only a very weak theoretical underpinning to experiments
closely tied to theoretical development. However, the latter is
relatively sparse and highlighted by Walder and Hallet (1985,
1986), and Hallet (2006).
Matsuoka (2001) took up Tricart's microgélivation and macrogélivation concepts and addressed the research upon them in both field and
laboratory contexts, noting important mismatches between the two. He
assigns an emphasis relevant to microgélivation on laboratory research
and one relevant to macrogélivation to field studies. Invoking
theoretical work he immediately subdivides ‘freeze–thaw or frost
weathering’ into two broad process categories, one driven by volumetric
expansion and the other by ice segregation. While recognizing the
feasibility of soft (permeable?) rock breakdown by multiple processes,
Matsuoka (2001, p. 301) does clearly state a criterion for assigning
laboratory experiments to volumetric expansion on freezing. In the
second part of his paper Matsuoka highlights the much more uncertain
empirical understanding of joint propagation by freezing and thawing in
hard (impermeable?), but jointed, rocks.
Matsuoka and Murton's (2008) review of frost weathering
represents a summary of the present experimental understanding of
the topic. Here we only highlight only a few salient points. Laboratory
research is now sharply focused upon volumetric expansion and ice
segregation as two separate issues. Crack or joint opening (i.e., a
central component of classic macrogélivation) is apparently produced
by different types of freeze–thaw cycles. Ice fracture of ‘soft’
(permeable) rocks by the ice segregation mechanism is apparently
feasible at the base of the active layer and the top of the abutting
permafrost layer (Murton et al., 2006). Laboratory research has yet to
address joint development in hard rocks effectively.
It is clear from Matsuoka and Murton's (2008) review that cuttingedge laboratory research is finally escaping the intellectual constraints of Tricart's microgélivation and macrogélivation. However,
the old terminology still emerges even here where it sits very
uncomfortably, and it is certainly still responsible for the creation of a
research mindset that suggests there are (just) two scales upon which
to focus. What Matsuoka and Murton (2008) do not do, nor do they
claim to at any point, is address the growth of studies centered on
mechanisms that may be just as relevant as freeze–thaw to bedrock
and debris weathering in cold environments.
Here we ignore all weathering work in cold regions that addresses
biological and chemical weathering in order to focus on mechanisms
that might actually be confused directly with classical freeze–thaw
weathering if the researcher views the topic through the lens of the
product, namely angular material. What mechanical process might be
operative at what scale is predicated upon the assumption that the
processes themselves are well understood. Whilst this may be true for
some mechanical weathering processes, especially perhaps salt
weathering (e.g. Houck and Scherer, 2006, although also see Smith
et al., 2005) and dilatation (e.g. Chun'an, 1997), this is less the case for
thermal stresses (e.g. Logan, 2004; Gómez-Heras, et al., 2006), wetting
and drying (e.g. Sumner and Loubser, 2008), or their synergistic
operation (e.g. Mutlutürk et al., 2004; Weiss, et al., 2004; Smith, et al.,
2005; Hall, 2006b; Ruedrich and Siegesmund, 2007) despite the often
simplistic statements to the contrary (e.g. Grossi et al., 2007; Tarbuck,
et al., 2009). For example within the realm of contemporary
weathering Hall et al. (2008a) have shown that where components
of a rock are light transmissive the surface weathering zone is
extended (to a depth of a centimeter or more) and may impact process
(es) when compared to a non-transmissive lithology. The same
thermal/radiation conditions will induce quite different weathering
responses dependent on the mineral composition and hence the
degree of light transmissivity; which is also impacted by mineral
orientation relative to the light-receiving surface In addition, Hall et al.
(2008b), following Gómez-Heras et al. (2006), argue for a “micro-
scale” influence on thermal weathering wherein the surface roughness
at that scale creates the very same shading to incoming radiation as
found on any large building due to facades, etc. Composition, impacted
by the resulting thermal regime, plays a role dependent on physical
properties, for example the anistropic response of quartz to heating
with the thermal coefficient of expansion being 7.5 m × 10−6 K−1
parallel to the c-axis but 13.7 m × 10−6 K−1 perpendicular to the c-axis
(see Hall, et al., 2007 and Hall et al., 2008a,b for discussion on this).
Indeed, de Castro Lima and Paraguassú (2004) show that the presence
and amount of quartz greatly impact the degree of cracking in the
outer shell of granite. Thus, “it is the impact of thermal conditions on
individual minerals and their association with each other that is critical
at the grain scale” (Hall et al., 2008b, p. 490). Further, as grain size
increases so there is a decrease in the thermal expansion coefficient
(α) but with every 10% increase in quartz content there is a ≈20%
increase in α and, at the same time, porosity greatly impacts the α
value with an increase of porosity by 1% resulting in a decrease in α by
2.3 × 10−3 mm/m ºC (de Castro Lima and Paraguassú, 2004): see Hall
et al. (2008bp. 490–491) for a more detailed discussion of the
interactions of porosity, α, and temperature. These thermal processes
will produce granular disintegration indistinguishable in product from
‘microgélivation’ and, in the absence of water, by the same thermal
variations that drive microgélivation.
Despite the concerns of some researchers (e.g., French, 2007, p. 66)
that actual thermal stresses have not been measured, the reality is
that, although highly complex, and unique to every point in a rock, it is
possible to evaluate the stresses for a rock as a whole (de Castro Lima
and Paraguassú, 2004). As much as the foregoing was related to
problems associated with thermal stresses at the grain scale, the
reality is that ‘freeze–thaw’ within microfractures is also driven by
thermal variations (simply, in this case, impacting water as the
driving force). Thus, the same complexities that impact thermal stress
may also affect freeze–thaw—with the addition of process synergy
(thermal and freeze–thaw) both operating simultaneously. Such
problems are not solely the purview of cold region scientists, as
Viles (2005, p. 190) points out with respect to weathering in hot
deserts: “Uncertainty still exists, for example, as to what the dominant
weathering processes are and how they operate, where and why rates
of weathering are fastest, and how weathering contributes to
landform development.” Clearly, weathering is not as well understood
as our entrenched responses would suggest.
At the larger scale of “macrogélivation” there are many similar
kinds of intellectual problems. Thermal stresses (the same drivers as
are required for the freezing and thawing of water) can produce
“macro-cracks”, with a form that is ‘angular’ but, at the same time,
very difficult to explain by frost processes. The cracks take on a
rectilinear form, with a hierarchy of cracks normal to each other and
which show no relationship to pre-existing lines of weakness in the
rock (Hall, 1999), and which are of the same form as found in hot
deserts (Soleilhavoup, 1977, 1978). Examples of such fractures from
the high Andes can be seen in Hall (1999, Figs. 1–3) while Bahr et al.
(1986, p. 2717) show comparable fractures generated in ceramic disks
subject to thermal shock, and Moores et al. (2008) provide a
theoretical explanation, based on field data, for the crack propagation
by thermal stresses. Thus, large angular rocks, in cold regions, can be
produced solely by thermal stresses and, it is argued (e.g. Hall, 1999)
that cold regions may be more conducive to thermal stresses than hot
regions as the cooling phase is twice as stressful as the warming phase
(Marovelli, et al., 1966). Hence, other mechanical processes operating
alone, as well as in synergy, can produce angular rocks at a host of
scales within cold regions.
5. Form
With respect to the commonly cited (e.g. Hanvey and Lewis, 1991;
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
between clast angularity and its origin by freeze–thaw, there are two
fundamental questions. First, is there any intrinsic characteristic of the
angular clast that uniquely identifies it as of freeze–thaw origin? In
the light of our present knowledge, the answer must be an
unequivocal "no". Identical angularity can be produced by salt,
thermal, wetting and drying, or biologically-induced mechanical
weathering processes. Angular clasts in hot arid or semi-arid regions
(e.g. Cooke and Warren, 1973) and periglacial regions show identical
form. Angularity itself simply does not identify process. Second, is
there any intrinsic process-controlled reason why a mechanicallyweathered clast should be angular? Again, with the possible exception
of those derived from thermal shock, the answer must be "no". The
stresses created by the growth of ice, salt, a biological agency, etc.,
serve to exploit a pre-existing weakness. In many rocks, much of the
time, these weaknesses are linear—hence the preponderance of
angular\platy clasts in areas of mechanical weathering. Indeed, linear
elastic fracture mechanics (LEFM) calculations of fracture extension
assume linearity; but this is primarily to simplify the mathematics and
it is recognised they can be curvi-linear (e.g. Rossmanith, 1983). In
some lithologies internal lines of weakness may have a curvi-linear
form: cooling or micro-flow structures in some igneous rocks
(Romaní, et al., 2006), sedimentary structures in some sandstones
or shales. The exploitation of these non-linear weaknesses by
mechanical stresses results in curved forms (Hall, 1997, 1998 and
Hall et al., 2002 show examples of rounded mechanically-weathered
forms from the Antarctic). The simple reality is that the preponderance of weakness in most rocks is linear and hence angular clasts are
the most common. To belabor the point, form does not identify
process and thus angular clasts in cold regions are no more an
indicator of freeze–thaw than are mechanically-weathered rounded
clasts. Arguments for any such relationships (e.g. Hanvey and Lewis,
1991, p. 35) merely serve to perpetuate the myth through repetition.
It should be noted that Derbyshire (1972) in a discussion regarding
tors in Antarctica, identified both rounded and angular weathering
forms that may be associated, at some point, with frost action.
Nevertheless, the bulk of the discussion regarding rounded versus
angular is concerned with the interaction of chemical weathering
with mechanical processes, with the former being the prime cause of
rounding and the latter of angularity. Thus, the issues associated with
form still remain.
6. Scale
Scale pervades science including geomorphology. Two of the
greatest problems facing geomorphologists are scale itself and scalelinkage. Both issues plague freeze–thaw weathering research. Theoretical understanding is patchy and incomplete, being constrained to
the scale at which the research was undertaken. Consequently we
must question the scales of established research by seeking to expand
their applicability or they continually constrain future research. Such
expansion immediately confronts the problem of scale-linkage in
which the research problems at one scale may not pertain to other
new problems (so called ‘emergent variables’) may appear. There
have been several recent reviews of scale problems in geomorphology
(e.g., Phillips, 1988; Malanson, 1999; Viles, 2001; Inkpen, 2005). We
take issue with none of these reviews rather we seek to apply portions
of them directly to the current understanding of freeze–thaw
weathering.
Scale reflects measurement and thus the metrics we choose, or are
forced upon us by conceptual and/or technological limitations. The
most fundamental split is between relative and absolute measures.
Relative scale is associated with nominal (categorical) and ordinal
measurement, that is variability among categories with no real metric
and among those with only order or rank. Absolute measures provide
exact measurement according to a standard unit or frequency (interval
5
data) or convert interval data to some other form (e.g., a proportion
such as percentage), the latter are ratio data.
The terms macro-, meso-, and micro- are widely used in
geomorphology, but with purely relative and variable meaning. This
means that their use is inconsistent and inconsistency is the bane of
professional terminology whose utility is essentially that of a
definitive shorthand. In the present context it is quite apparent that
microgélivation and macrogélivation are purely relative and have
served to steer contemporary research. The result has been investigation focused upon two discrete scales with little or no evidence to
establish that they are the key, let alone only, scales. Rather as Viles
(2001, pp.65–66), following Phillips (1999), has pointed out “we do
not have vast amounts of quantitative data with which to analyze”
and thus the “gaps found may simply be measurement gaps rather
than the fundamental ‘spectral gaps’ of Blöschl and Sivipalan (1995).”
Such problems can only be confounded by the fact that laboratory
research is being conducted on the formative processes while the
original focus in the field was upon the products.
Association of macro-, meso-, and microscales with absolute SI
units cannot be undertaken comprehensively. The prefix ‘macro-‘ is
not an SI designator, but seems to be associated with things visible to
the naked eye. In SI terminology this would take us from 1:1
(something that has no SI term), through deca- and hecto-, to some
lower boundary. It could perhaps be designated as roughly N10−2
(technically from the centiscale to the yottascale (1024m)). Microscale
would be 10−6 m (strictly speaking) and nano-scale would be 10−9
(Hochella, 2002). Both micro- and nanoscale would need to be viewed
across at least plus and minus one order of magnitude to be truly
useful terminology. Mesoscale simply has no well articulated range,
although Hall (2006a) suggested 10−2 to 10−5 m, but even here we
see ourselves having confused relative and absolute concepts because
this suggestion itself crosses the centi- and milli- Si boundaries.
Discussion of relative versus absolute scale serves two potential
purposes. The simpler of the two ideas is that both permit communication, but absolute scale permits sounder scientific communication.
The more penetrating idea is if scale actually serves to identify processes
that are physically and/or chemically different from each other. This is
apparently a central issue in freeze–thaw weathering where there is
apparently a belief, however weakly defined, that there are two
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
framework). Laboratory experiments could be directed at the two
perceived processes individually; however, identification needs to be in
terms of the process not the product. This would need to be stated in
terms of temperature and water/ice. Identification of the process in
terms of the product raises the importance of scale-linkage as a separate
problem.
7. Scale-linkage
Scale-linkage is perhaps an even more vexing intellectual
challenge than scale itself. At least a comprehensive analytical
understanding at one scale is just that—a comprehensive understanding. However, scale-linkage involves numerous new uncertainties and
assumptions; of course, once they are resolved the scope of our
analytical understanding is expanded and axiomatically the scalelinkage problem eliminated. Nevertheless, the mismatch between
Newtonian physics and quantum mechanics serves as a reminder of
just how enormous scale-linkage problems can be.
Is the scale of the driving force, namely freezing, an important
variable? Such a question embraces the rate, duration, intensity, and
frequency of freezing. Can all of these variables be meaningfully linked
to each other? Are any of them independently limiting and/or to what
degree can the variability in one substitute for variation in others?
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
substance itself—water. What are the scale limits on water availability
and mobility and to what extent can shortcomings be offset by the
four temperature variables? Finally, how do all of these variables
interact with another profoundly important variable—the host rock?
Laboratory research on freeze–thaw weathering has fallen foul of
many of the issues implicit in the series of questions posed above. Hall
(1998, 1999) and Hall and André (2001, 2003) have demonstrated
that freezing rates in nature may well be adequate to produce thermal
fatigue rather than freeze–thaw weathering. There has been a
prolonged engagement with the geomorphic significance believed
to be implicit to diurnal, seasonal, and annual freeze–thaw cycles and
their varying parameters. However, we might question the validity of
focusing upon any one of these scales when all are present
simultaneously. Such a perspective casts a real shadow on making
mimicry of any naturally occurring freeze–thaw category THE
appropriate starting point of experimentation.
The jump of viewing a geomorphic process through the lens of its
products, one that underpins the original freeze–thaw weathering
perspective, clearly fuses two scale issues together, namely those of
the process(es) and the product(s). None of the very real problems
facing geomorphologists investigating freeze–thaw weathering seems
to be as great as the problem faced by those using the product as the
starting point. The simplest objection to this approach is perhaps the
most telling—if freeze–thaw weathering is viewed by examining
weathered products the researcher is confronting the maximum
number of variables at once and making the implicit assumption that
two very complicated sets of variables are inherently matched in a
systematic scale-sensitive fashion. Both analytical and empirical
investigations normally start off in exactly the opposite fashion, by
making everything as simple as possible, even at the risk of making
them unrealistic initially.
8. Discussion
It seems redundant to consider the spatial applicability of process
operation when the very process itself is so poorly understood and its
interaction with other processes even less known. In some ways this is
an underlying tenet of the whole problem: creating a process-scalelinkage where the parameters of process operation are themselves
unknown. For example, if one accepts the hypothesis of segregation ice
growing in a crack to force the rock apart by means of an excess of ice
over space available (as argued by Walder and Hallet, 1985), then need
this have any spatial attributes? Presumably, following the original
hypothesis, the mechanism can operate at a small scale (micro-?) in
spaces between grains (as suggested by Grossi, et al., 2007) or even
perhaps within micro-cracks (sensu stricto) in exactly the same way as
it is postulated (Matsuoka, 2001) to separate large joint blocks (where
the product is macro-scale) along wide joint lines—“frost-wedging”.
What is absent are actual data at the appropriate range of scales. The
limiting factor (other attributes such as chemistry held constant) will
be the stresses induced on the water as scale diminishes such that the
water will not freeze; the corollary would be, can the ice in large cracks
exert a confined stress that can cause fracture as opposed to causing
solely movement of already unbound blocks? Again, it is the absence of
unequivocal data that is the limitation, coupled with the assumption of
these spatial scales of operation.
Further, the whole notion of a spatial–process relationship is
fraught with problems of temporal process synergy—the process
observed may be dependent upon a preceding process not, or less
easily, observed (see Smith et al., 2005 for a discussion on this very
point but in respect to salt weathering in hot deserts and McCabe et
al., 2007 for sandstone buildings). For example, in respect to granites,
as with many other lithologies, the amount and rate of subsequent
weathering are highly dependent upon micro-cracks (intergranular,
intragranular, and transgranular) that may be the product of the
rock's origin and history (Sousa, et al., 2005; Romaní, et al., 2006).
Thermal cracking and stress-induced cracking of the granite can cause
micro-cracks that may coalesce into macroscopic fractures (David, et
al., 1999) that can, in turn, facilitate the operation of other weathering
mechanisms (especially salt weathering: Sousa, et al., 2005). Recent
studies by Moores et al (2008) have also shown that the impact of
thermal stresses on crack propagation can be impacted by whether
moisture at the crack tip remains or is driven off by solar heating;
more weathering occurring when the moisture remains to facilitate
other (i.e. not thermal stress) weathering processes. Thermal cracking
itself can be the result of thermal stresses and/or thermal shock. Many
authors use slow rates of change of temperatures (ΔT/Δt) in the range
of 1 °C min−1 to preclude thermal shock (e.g. David, et al., 1999; Lion
et al., 2005; Reuschlé, et al., 2006), with values in excess of 2 °C min−1
deemed to facilitate thermal shock (see Hall, 1999 and Hall and André,
2003 for a discussion). Values recently used to create thermal shock
range from 2.4 °C min−1 (Ferrero and Marini, 2001), through 10° to
15 °C min−1 (Ghaffarian, 2001), to 25° to 44 °C min−1 (Peng et al.,
2008). Although, theoretically, the boundary values for the thermal
shock of any specific material may be calculated (e.g. Goncharov, et al.,
1968; Tite, et al., 2001); such factors such as anisotropy (e.g. Amadei,
1996; Mahmutoglu, 1998; Yavuz, et al., 2006), crystal composition,
size, orientation, and connectivity (e.g. Sousa, et al., 2005; Hall et al.,
2008a,b) will greatly influence the actual outcomes. In broad terms,
and certainly germane to the arguments here, the same may be the
case for freeze–thaw weathering when “…pressure due to water
expansion (as it turns to ice) reaches the tensile strength of the rock,
new microfractures are developed and present ones (our italics) are
deepened and widened. After thawing, water can migrate into the
newly developed microfractures. Recurrent freeze–thaw cycles cause
enlarging of existing fractures and further weakening of the material”
(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
product scale outcomes continue to change.
Many geomorphologists might want to discount, or dismiss
entirely, the sorts of issues raised here. Such a decision is not only
foolhardy, it is not really theirs to make. The inexorable trend in
geomorphology has moved the discipline from one with an essentially
free-standing theoretical foundation (think Davisian and/or climatic
geomorphology) to one that increasingly embeds itself deeply in the
theoretical and experimental structures of science at large. Consequently, the geomorphology and geomorphologists of the future will
be judged not by geomorphologists alone, but rather by the broader
scientific community. Weathering studies specifically have clearly
entered a period of reductionist science where the inspiration for
geomorphic research appears to be the research model of materials
science. There is much to be gained from this, although it will certainly
not be a panacea for the field, but in order to be successful in this
milieu geomorphologists will have to shed the standards of a bygone
era in geomorphology and embrace contemporary standards within
material science. Materials science research undertaken with the
dated standards and terminology of field geomorphology from the
early to mid-1900s is a sure recipe for ongoing confusion.
9. Conclusions
While clearly the products of freeze–thaw weathering, as with
almost any weathering process, may range in size from grains to
boulders, the notion of a process dichotomy (microgélivation/
macrogélivation) is, at best, naïve, and is likely more confusing than
enlightening. At the root of the issue is the confusion of product with
process. Attempts to validate the two weathering scales have resulted
in the acceptance of many assumptions (not the least that there are
two and only two weathering scales) resulting in misdirection and
poor field evaluative approaches (i.e. product size identifies process
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
evaluative tools have been further exacerbated by the totally
unproven presumption that angular debris is an identifier of freeze–
thaw weathering. In addition, the reality is that there are minimal data
relating to smaller-scale frost weathering, and that which do exist are
not particularly supportive of freeze–thaw as the sole or, in some
cases, even the most probable weathering mechanism. For larger
joint-bounded blocks there remains the question of whether
measured crack widening is an expression of weathering enlarging
the crack or if it is simply measurement of highly localized transport of
an already liberated (weathered-free) block. The weathering of a fresh
joint boundary must be at a “micro”-scale—the product is simply a
block (bounded by the weathered joints) and so there still remains
the question of which process exploited the joint. Thus, while scale
attributes are critical to issues within freeze–thaw weathering, their
consideration without careful reasoning and forethought may be but
to cause more confusion than enlightenment. It would seem that
geomorphologists can better go forward by forgetting the past and
adopting an approach more related to materials science coupled with
holistic field monitoring with data acquisition appropriate to a host of
scales—and processes.
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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