Kevin Hall

The role of thermal stress fatigue in the breakdown of rock in cold regions

Abstract Many geomorphological studies have ignored the role of thermal stress fatigue. Engineers have long maintained that thermal stress is an effective process. Physicists and the ceramics industry have provided a theoretical foundation to explain the nature and mode of operation of thermal breakdown. Recent geomorphological studies have shown the importance of thermal stress in cold regions by means of high-frequency rock temperature data which can identify rates of temperature change in excess of ≥2°C min −1 . The occurrence of weathered material in cold but dry regions can be explained by thermal stress/shock. Differential radiation receipts on slopes of varying orientation and gradient can exert a very strong effect.

Weathering in cold regions: some thoughts and perspectives:

Weathering in cold regions has primarily focused on the notion of ‘cold’, such that process and landform theory have generally used this both as the developmental criterion and as the outcome of palaeoenvironmental reconstructions based on landforms or sediments. As a result of this approach, the process focus in terms of weathering has been that mechanical processes predominate, with freeze-thaw weathering as the prime agent, and that chemical processes are temperature-inhibited, often to the point of nonoccurrence. Here a reconsideration of the whole conceptual framework of weathering in cold environments is undertaken. It is shown that, contrary to popular presentations, weathering, including chemical weathering, is not temperature-limited but rather is limited by moisture availability. Indeed, summer, and oft-times even winter, rock temperatures are more than adequate to support mechanical and chemical weathering if water is present. Where water is available it is clearly shown that chemical weathering can be a major component of the weathering regime. The argument is made that there is no zonality to cold environment weathering as none of the processes or process associations are unique to cold regions; indeed, many cold regions show similar weathering assemblages to those in hot arid regions. Process-form relationships are also questioned. The assumption of angularity with weathering in cold regions is questioned, all the more so as hot arid studies identify exactly the same angularity of debris form. Further, that all forms have to be angular is shown by field examples to be no more than an artefact of original unquestioning and oft-repeated assumptions, now over a century or more old. The argument is made that there is a strong need for the reconsideration of the nature of weathering in cold environments, that current theory should be questioned and challenged, and field observation undertaken within this revised frame of reference.

New insights into rock weathering from high-frequency rock temperature data: an Antarctic study of weathering by thermal stress

A major limitation of many weathering studies has been the acquisition of rock temperature data at insufficiently frequent intervals for the meaningful determination of the rate of change of temperature (ΔT/t). Equipment and/or logistical constraints frequently facilitate temperature measurement at only hourly intervals or, at best, every 10 min. Such data are not adequate for the determination of ΔT/t required for the evaluation of the freeze–thaw mechanism or thermal stress fatigue. Recent undertakings at different sites in Antarctica (and at other cold-region locations) provide rock temperature measurements at 1-min intervals, which indicate that the perception of the weathering regime would be very different from that generally assumed from longer-interval geomorphological data. These data clearly show that thermal stress fatigue and thermal shock may be more active components of the Antarctic weathering regime than have generally been recognised; the aridity of the study area limits the role of freeze–thaw weathering. Values of ΔT/t of ≥2 °C min−1 that suggest thermal stress fatigue/shock is operative were recorded; observations of rock flaking are thought to reflect the impact of thermal stress. Further, the data show that contrary to general perceptions, the southern aspect can, in summer, experience higher rock surface temperatures than the north-facing exposure. An examination of rock fracture patterns found in the field shows great similarity to fracture patterns developed in the laboratory as a direct result of thermal shock. The argument is made that greater cognizance should be given to thermal effects.

Zoogeomorphology in the Alpine: some observations on abiotic¿biotic interactions

Abstract Cryonival processes typify the alpine environment. These cold-based processes operate synergistically with wind, rainsplash, surface wash, and mass movement to create the local morphology. These traditionally accepted abiotic processes are rarely, if ever, identified as operating in conjunction with biotic processes. Paradoxically, the existence and activity of animals within the alpine zone is widely reported. Here the argument will be made for the interoperation of the biotic with the abiotic in the development of the alpine terrain. Animals frequently exert a strong geomorphic influence in alpine environments through such activities as grazing, trampling, digging, burrowing, and direct erosion of even bedrock. The impact of the animals, in a geomorphic context, can be both direct and indirect. Direct effects are soil compaction, removal of sediments, loading causing slope failure, and the introduction or removal of chemicals (nutrients). As significant as these impacts can be, so too are the indirect effects: preparation of sediments for removal by abiotic processes, decreasing of slope stability by burrowing, increasing surface wash and/or concentration of overland flow as a result of compaction, pedoturbation, and changing of soil-water chemistry because of facilitating water penetration as a result of burrowing. The abiotic and biotic impacts occur both in parallel (i.e., at the same time) and/or in series (i.e., one subsequent to another), frequently as a function of seasonal climatic influences on both forces. Climate exerts a significant impact on both biotic and abiotic processes, and aspect can greatly influence these. Solar radiation, ground temperature, wind, and precipitation affect biotic and abiotic attributes and their interaction. Slope angle and parent material(s) will also affect the degree of influence. Biotic–abiotic interactions are complex outcomes of parent material, aspect, slope, and season, and all of the factors that these influence (i.e., vegetation). Here a series of flow diagrams are used to identify both abiotic and biotic elements and to show the manner in which they interoperate to create the local alpine terrain. Examples from a variety of alpine locations (Canada and China) will be used to exemplify geomorphic outcomes.

WEATHERING BY WETTING AND DRYING: SOME EXPERIMENTAL RESULTS

A series of experiments on sandstone and dolerite was undertaken in an attempt to better understand the wetting and drying weathering process. As rock samples are frequently subjected to wet–dry cycles within the simulation of other weathering mechanisms (e.g. freeze–thaw), three common methods of moisture application were used and the influences of these evaluated. It was found that the method of moisture application could affect the nature of the weathering products resulting from wetting and drying. It was also observed that there were changes in the internal properties of the rock (e.g. porosity/microporosity) and that these could influence the synergistic operation of other weathering processes. Although not all of the observations could be explained, it is apparent that wetting and drying has both a direct and an indirect effect on the weathering of rock that has not been taken into account in simulations. Greater cognizance needs to be given to the role of this process both in the field and in laboratory simulations.

Nivation and cryoplanation: the case for scrutiny and integration

Nivation and cryoplanation are deeply entrenched and widely invoked morphogenetic concepts within periglacial geomorphology. However, given their complexity and purported significance in landform development, neither has received the scientific scrutiny merited. Nivation is generally associated with the weathering and mass movement processes stemming from late-lying seasonal snow and the ensuing landforms. As a process suite it invokes mechanisms such as freeze-thaw weathering and solifluction. Cryoplanation also invokes a process suite, but one that also embraces nivation. Research in the last two or three decades has begun to provide a much sharper definition of nivation, but the available information is still inadequate for a central mechanism in periglacial landscapes. Cryoplanation is used to explain some benches, terraces and pediments that appear to be widespread in some periglacial environments: however, substantive field research on the mechanisms involved is all but absent. A case is presented for viewing nivation and cryoplanation as a single process spectrum much in need of state-of-the-art research.

Rock temperatures and implications for cold region weathering. II: New data from Rothera, Adelaide Island, Antarctica

Rock temperature data collected at one-minute intervals from both the horizontal surface and the four cardinal directions of a rock outcrop show the influence of record interval and aspect on the thermal regime of bedrock as it applies to cryogenic weathering. High frequency data are necessary to identify components of thermal stress fatigue and thermal shock events that play a significant role in rock breakdown. The northern aspect exhibits the lowest temperatures despite its apparent preferential orientation. At the 2 cm depth, temperatures on the northern and horizontal surfaces sometimes stayed above those for the rock surface despite the daytime energy input from solar radiation. Short-term wind fluctuations are considered as a possible explanation. Because the rock temperatures are quite different from those of the air the latter can, in no way, be used as a surrogate for rock thermal conditions. The argument is made that one-minute record intervals are required for thermal data if use is to be made of this information to help explain and understand the weathering regime.

Freeze‐thaw weathering: The cold region “Panacea” 1

Abstract Freeze‐thaw weathering is commonly cited as a major agency of landform development in high latitudes and at high altitudes. This is, however, largely an unsubstantiated qualitative judgment. The reality is a paucity of data regarding key factors, such as rock temperature and interstitial rock moisture content data, necessary for the correct assumption of freeze‐thaw rock weathering. The problem is compounded when it is presumed that angular clasts in cold regions are the result of freeze‐thaw weathering and then this argument is used as a basis for the interpretation, and possible paleoclimatic reconstruction, of Quaternary environments. In reality, angular clasts can be produced by a variety of weathering processes and there are no criteria that can identify a clast as being the product of freeze‐thaw weathering. Even the term “freeze‐thaw”; is really a collective noun, for it encompasses a range of individual mechanisms, each requiring different controlling conditions and, potentially, generati...

Freeze-Thaw Activity at a Nivation Site in Northern Norway

Monitoring of rock-face temperatures at various points within an arctic nivation site indicate that there is great variation in number, duration, and amplitude of freeze-thaw cycles experienced. Th...

Late glacial ice cover and palaeotemperatures on sub-Antarctic Marion Island

Abstract By means of data from striation observations, till fabric analysis, moraine recognition, surface megaclast fabrics and till stratigraphy, a reconstruction of the ice cover for part of sub-Antarctic Marion Island is attempted. The presence of ice at a number of other locations on the island is noted but former glaciers cannot be recognised. By means of the altitude of lateral moraines a range of temperatures for glacial-maximum conditions are reconstructed. The estimated drop in temperature falls within the ranges suggested by palynological and ocean core investigations.