Weathering of granitic rocks by chlorides: effect of the nature of the solution on weathering morphology

Five granites from NW Spain were subjected to two salt‐spray weathering tests under controlled atmospheric conditions. Granite samples were exposed to a sodium chloride solution in the first test and to a complex solution (sea water) in the second. Subsequent examination of the rocks by light and petrographic microscopy clearly demonstrated the development of different weathering morphologies in each test. The distribution of dissolved ions in samples taken at different depths from the weathered surfaces at the end of the experiments, and the changes in weight of the samples during the tests, also differed. In our opinion, these results were determined by the nature of the solution used; although sodium chloride is the predominant salt in sea water, the presence of other ions modifies its solubility, thereby varying the dynamics of mobility and precipitation and thus the weathering morphology that develops. Copyright © 2003 John Wiley & Sons, Ltd.     

Patterns of damage in igneous and sedimentary rocks under conditions simulating sea‐salt weathering

A saline‐spray artificial ageing test was used to simulate the effects produced in granites and sedimentary rocks (calcarenites, micrites and breccia) under conditions in coastal environments. Three main points were addressed in this study: the durability of the different kinds of rock to salt decay, the resulting weathering forms and the rock properties involved in the weathering processes. For this, mineralogical and textural characterization of each of the different rocks was carried out before and after the test. The soluble salt content at different depths from the exposed surfaces was also determined. Two different weathering mechanisms were observed in the granite and calcareous rocks. Physical processes were involved in the weathering of granite samples, whereas dissolution of calcite was also involved in the deterioration of the calcareous rocks. We also showed that microstructural characteristics (e.g. pore size distribution), play a key role in salt damage, because of their influence on saline solution transport and on the pressures developed within rocks during crystallization. Copyright © 2003 John Wiley & Sons, Ltd.     

Experimental frost and salt weathering of chalk—II

Experiments are described in which chalk cubes were soaked in solutions of either sodium chloride, sodium sulphate, or magnesium sulphate at concentrations of 5·5 per cent and 12·5 per cent, or in a mixed solution of sodium chloride and sodium sulphate or in distilled water. After removal of excess liquid, the cubes were subjected to six freeze–thaw cycles with temperatures ranging from either +15 to ?10°C or +15 to ?30°C. The results confirm that frost weathering can be enhanced by the presence of certain salts, but the extent of weathering was much less than that previously reported for samples frozen totally immersed in the same liquids. Evidence is presented which suggests that salt crystallization is the major weathering process operative when non–immersed samples are frozen but a combination of frost and salt weathering operates when fully immersed samples are frozen.     

Evidence for freeze–thaw events and their implications for rock weathering in northern Canada

Discussions regarding weathering in cold environments generally centre on mechanical processes and on the freeze–thaw mechanism in particular. Despite the almost ubiquitous assumption of freeze–thaw weathering, unequivocal proof of interstitial rock water actually freezing and thawing is singularly lacking. Equally, many studies have used the crossing of 0 °C, or values close to that, as the basis for determining the number of ‘freeze–thaw events’. In order to assess the weathering regime at a site in northern Canada, temperatures were collected at the surface, 1 cm and 3 cm depth for sets of paving bricks, with exposures both vertical and at 45°, orientated to the four cardinal directions. Temperature data were collected at 1 min intervals for 1 year. These data provide unequivocal proof for the occurrence of the freezing and thawing of water on and within the rock (freeze–thaw events). The freeze event is evidenced by the exotherm associated with the release of latent heat as the water actually freezes. This is thought to be the ?rst record of such events from a ?eld situation. More signi?cantly, it was found that the temperature at which freezing occurred varied signi?cantly through the year and that on occasion the 1 cm depth froze prior to the rock surface. The change in freeze temperature is thought to be due to the chemical weathering of the material (coupled with on‐going salt inputs via the melting of snowfall), which, it is shown, could occur throughout the winter despite air temperatures down to ?30 °C. This ?nding regarding chemical weathering is also considered to be highly signi?cant. A number of thermal stress events were also recorded, suggesting that rock weathering in cold regions is a synergistic combination of various chemical and mechanical weathering mechanisms. Copyright © 2003 John Wiley & Sons, Ltd.     

Incipient weathering rind development on introduced machine‐polished granite discs in an Arctic alpine environment,northern Scandinavia

Weathering rinds, zones of alteration on the exterior surfaces of rock outcrops and coarse unconsolidated surficial debris are widely used by geomorphologists and Quaternary geologists as indicators of the relative age of landforms and landscapes. Additionally they provide unique insights into the earliest stages of rock and mineral weathering, yet the origin of these alteration zones is relatively poorly understood. This lack of understanding applies especially to the initial stages of rind formation. The study reported in this paper has two principal objectives. The first is to use lightly polished granite discs inserted in soil profiles under several different plant communities in an Arctic alpine environment for a period of four or five years to investigate the nature of incipient weathering rind development. The second is to investigate the factors responsible for spatial variability in the nature and rates of rind formation. Incipient weathering rind development on the outer edges of the granite discs is observable and measurable over a period of time as short as four years in the mild Arctic alpine environment of Swedish Lapland. The earliest stages of rind development involve the development of a porous structure consisting of a combination of pits and fractures which have been solutionally enlarged and modified. Solution appears to be preferentially concentrated on the surfaces of feldspars and, to a lesser extent, quartz. In addition, iron oxides are present along grain boundaries and in grain interiors and are interpreted to have been derived from the oxidation of ferromagnesian minerals. Spatial variability in weathering rind development appears to be particularly driven by differences in moisture but is not related to soil pH. Copyright © 2005 John Wiley & Sons, Ltd.     

A Morphological Study of Greenschist Weathering on Dated Coastal Structures,South Devon,UK

Three dated structures up to 450 years in age display the effects of coastal weathering of the greenschist of which they are constructed. A variety of weathering forms is present. The various topographic surfaces of the structures create variation in weathering environments and consequent weathering processes and rates. Weathering is enhanced by direct exposure to salt-bearing spray and by humid conditions, and apparently limited by direct exposure to solar radiation. The maximum rates of weathering on the three surfaces approximate to 0·6 mm a⁻¹ over this period, consistent with measured contemporary weathering rates for a natural surface formed by this rock type in a nearby coastal location. © 1997 by John Wiley & Sons, Ltd.     

Microvariability in the early stages of cobble weathering by microenvironment on a glacier foreland,Storbreen, Jotunheimen,Norway

The initial stages of cobble weathering, measured as increasing percentage porosity, were calculated for sets of cobbles taken from the foreland of Storbreen, a cirque glacier in the Jotunheimen, Norway. Cobbles were taken from in front of the 1998 glacier snout, from the 1928, 1870, 1810 and 1750 moraine crests and from the ～10 000‐year‐old land surface beyond the Neoglacial foreland limit. Three microenvironments were examined within each site: (1) lichen‐free surfaces from exposed cobbles, (2) lichen‐covered surfaces from exposed cobbles and (3) buried cobbles taken from a soil depth of ～40 cm. Percentage porosity within plagioclase minerals was calculated using backscatter electron images of prepared thin sections. Porosity was calculated from five depth profiles into each cobble. Depth profiles were subdivided into a sequence of 50 µm × 50 µm rectangles running to at least a nominal depth of 500 µm within each cobble. Three cobbles from each dated land surface were sampled, except for the 1750 surface, which had five cobbles. Statistical analysis was by analysis of variance of rank‐order transformed data. Findings indicate that cobbles close to the glacier snout are largely unweathered; also, weathering is generally weak in the 1928, 1870 and 1810 positions, but statistically significantly higher in the 1750‐ and 10 000‐year‐old positions. Weathering of buried cobbles always exceeded weathering of exposed cobbles and may possibly reach a value beyond which it cannot progress while retaining surface cohesion. The degree of weathering on lichen‐free and lichen‐covered cobble surfaces is not initially distinguishable, but diverges sharply after ～250 years, when lichen‐covered surfaces experience significantly higher totals. Overall, the weathering trend in cobbles matches that found in soils at the same sites. Copyright © 2007 John Wiley & Sons, Ltd.     

Measurements and calculations of seasonal evaporation rate from bare sandstone surfaces: Implications for rock weathering

Evaporation from porous rock plays an important role in weathering processes. In the case of salt weathering, the evaporation rate controls supersaturation of salt solutions within pores and the amount of precipitated aggressive salts, therefore weathering occurs mostly in places with intense evaporation. Evaporation also strongly affects frost, hydric and biogenic weathering, as these are influenced by water content and its temporal changes. Despite its importance, evaporation from porous rocks has seen little scientific focus. We present a study on evaporation from bare sandstone, one of the most common rocks affected by weathering. A new method that measures the evaporation rate from the surfaces of sandstone samples under field microclimate was developed and tested. Also, a simple calculation of 1D evaporation rate from bare sandstone surfaces based on Fick's law of diffusion is presented. The measurement was performed using sandstone cores (with a set depth of the vaporization plane) in a humid continental climate and measured on a roughly monthly interval for about 1 year. For the calculations, a laboratory-measured water-vapour diffusion coefficient of the sandstone, in-situ seasonally measured vaporization plane depth, and values of air humidity and temperature were used. The sensitivity analyses showed that the most important factor controlling the evaporation rate was the vaporization plane depth, while seasonal and spatial changes of air humidity and temperature were of lesser importance. The calculated evaporation rate reasonably follows measured values. For its simplicity and the small number of parameters required, the proposed method has the potential to improve knowledge of weathering and living conditions of endolithic and epilithic organisms. Further research should focus on factors affecting the evaporation rate (wind, hygroscopicity, hydrophobicity, etc.) to improve the accuracy of the calculations, as well as to test the applicability of the method for other lithologies and climates. © 2020 John Wiley & Sons, Ltd.     

Geochronological evidence for pervasive Miocene weathering,Minas Gerais,Brazil

⁴⁰Ar/³⁹Ar laser incremental‐heating analyses of 22 individual grains of supergene cryptomelane from three weathering pro?les, up to 400 km apart, in the Rio Doce valley and Barbacena regions at Minas Gerais, Brazil, show that the formation of weathering pro?les in these regions is contemporaneous, suggesting a strong weathering event in the Middle to Late Miocene (10–8 Ma). The preservation of these Miocene samples at or near the present surface suggests that either erosion rates have been very low in the region since the Miocene or that a much thicker weathering mantle was present in the region originally. Assuming a constant thickness of weathering pro?les in the region throughout the Tertiary, we may calculate weathering front propagation rates of 4–8 m Myr^?1 during the past 10 Ma. Copyright © 2004 John Wiley & Sons, Ltd.     

Towards a protocol for laser scanning of rock surfaces

The precision of height measurements derived from laser scanning a weathered rock surface was analysed. Different registration methods for comparing surfaces to deduce weathering were assessed and the most precise was found to be the method that used registration shapes as control, located in different planes relative to the scanned surface. In addition, the different sources of error in scanning precision were assessed by varying factors such as scan distance, lens configuration, scan angle and the nature of the topography being scanned. From this analysis it was possible to suggest what the optimal scanning conditions were for this particular experimental set‐up. The procedures outlined for assessing errors in the precision of height measurements are transferable to other scanning studies. Copyright © 2010 John Wiley & Sons, Ltd.     

Measuring ignitability for in situ burning of oil spills weathered under Arctic conditions: from laboratory studies to large-scale field experiments

This paper compares the ignitability of Troll B crude oil weathered under simulated Arctic conditions (0%, 50% and 90% ice cover). The experiments were performed in different scales at SINTEF’s laboratories in Trondheim, field research station on Svalbard and in broken ice (70-90% ice cover) in the Barents Sea. Samples from the weathering experiments were tested for ignitability using the same laboratory burning cell. The measured ignitability from the experiments in these different scales showed a good agreement for samples with similar weathering. The ice conditions clearly affected the weathering process, and 70% ice or more reduces the weathering and allows a longer time window for in situ burning. The results from the Barents Sea revealed that weathering and ignitability can vary within an oil slick. This field use of the burning cell demonstrated that it can be used as an operational tool to monitor the ignitability of oil spills.     

Biogenic origin of coastal honeycomb weathering

Honeycomb weathering occurs in two environments in Late Cretaceous and Eocene sandstone outcrops along the coastlines of south‐west Oregon and north‐west Washington, USA, and south‐west British Columbia, Canada. At these sites honeycomb weathering is found on subhorizontal rock surfaces in the intertidal zone, and on steep faces in the salt spray zone above the mean high tide level. In both environments, cavity development is initiated by salt weathering. In the intertidal zone, cavity shapes and sizes are primarily controlled by wetting/drying cycles, and the rate of development greatly diminishes when cavities reach a critical size where the amount of seawater left by receding tides is so great that evaporation no longer produces saturated solutions. Encrustations of algae or barnacles may also inhibit cavity enlargement. In the supratidal spray zone, honeycomb weathering results from a dynamic balance between the corrosive action of salt and the protective effects of endolithic microbes. Subtle environmental shifts may cause honeycomb cavity patterns to continue to develop, to become stable, or to coalesce to produce a barren surface. Cavity patterns produced by complex interactions between inorganic processes and biologic activity provide a geological model of ‘self‐organization’. Surface hardening is not a factor in honeycomb formation at these study sites. Salt weathering in coastal environments is an intermittently active process that requires particular wind and tidal conditions to provide a supply of salt water, and temperature and humidity conditions that cause evaporation. Under these conditions, salt residues may be detectable in honeycomb‐weathered rock, but absent at other times. Honeycomb weathering can form in only a few decades, but erosion rates are retarded in areas of the rock that contain cavity patterns relative to adjacent non‐honeycombed surfaces. Copyright © 2010 John Wiley & Sons, Ltd.     

The covered karst, weathering crust and karst (double-level) planation surface

The thick-bedded and continuous karst crust only formed in the old stage of geomorphic development. The corresponding landscape is the karst planation surface. The karst planation surface consists of the loose weathering crust and the base weathering front below the crust. Its profile structure is similar to “double surface of leveling” model built by Budel. In the limestone area, the base weathering front is the covered karst. From the Tibet Plateau to Yun-Gui (Yunnan-Guizhou) plateau and Xiang-Gui (Hunan-Guangxi) hills, the covered karst is concomitant with the red weathering crust; all of them are the component of the double-level surfaces of karst planation. But, they belong to the different disintegration stages of planation surface. The different subtypes of the covered karst and the red weathering crust indicate the existence of karst planation surface. Thus, they can be made as a reference system when the rising degree and the rising rate of the Tibet Plateau are discussed.     

Role of biological soil crust cover in bioweathering and protection of sandstones in a semi‐arid landscape (Torrollones de Gabarda,Huesca, Spain)

Sandstone structural landscapes in the semi‐arid Torrollones de Gabarda area (Province of Huesca, NE Spain) are often covered by a well developed biological soil crust of lichens, mosses and cyanobacteria and black coatings on vertical surfaces. By using scanning electron microscopy with backscattered detector imaging, the biological soil crust studied evidenced high activity in the sandstone–crust interface. Processes such as physical disintegration, etching and dwelling as well as biomineralization by calcium oxalate and ?xation of mineral particles by extracellular polymeric substances were observed. On the horizontal sandstone surfaces these processes may cause the occurrence of gnammas and the development of a protective coating that favours intense ?aking when the crust is disturbed. On the sandstone cliffs, columnar and tafoni weathering development is clearly guided by the protective action of the biological soil crust. These qualitative observations are important to develop methodologies to address their quantitative importance in geomorphological processes in semi‐arid landscapes. Copyright © 2004 John Wiley & Sons, Ltd.     

Simulation of moisture content in alpine rockwalls during freeze–thaw events

Rock moisture during freeze–thaw events is a key factor for frost weathering. Data on moisture levels of natural rockwalls are scarce and difficult to obtain. To close this gap, we can benefit from the extensive knowledge of moisture‐related phenomena in building materials, which is incorporated into simulation software, for example the WUFI® package of the Fraunhofer Institute of Building Physics. In this paper we applied and adapted this type of simulation to natural rockwalls to gain new insights on which moisture‐related weathering mechanisms may be important under which conditions. We collected the required input data on physical rock properties and local climate for two study areas in the eastern European Alps with different elevation [Sonnblick, 3106 m above sea level (a.s.l.) and Johnsbach, 700 m a.s.l.] and different lithologies (gneiss and dolomite, respectively). From this data, moisture profiles with depth and fluctuations in the course of a typical year were calculated. The results were cross‐checked with different thermal conditions for frost weathering reported in the literature (volumetric expansion and ice segregation theories). The analyses show that in both study areas the thresholds for frost cracking by volumetric expansion of ice (90% pore saturation, temperature < ?1 °C) are hardly ever reached (in one year only 0.07% of the time in Johnsbach and 0.4% at Sonnblick, mostly in north‐exposed walls). The preconditions for weathering by ice segregation (?3 to ?8 °C, > 60% saturation) prevail over much longer periods; the time spent within this ‘frost cracking window‘ is also higher for north‐facing sites. The influence of current climate warming will reduce effective frost events towards 2100; however the increase of liquid precipitation and rock moisture will promote weathering processes like ice segregation at least at the Sonnblick site. Copyright © 2016 John Wiley & Sons, Ltd.     

Tafoni in the El Chorro area,Andalucia, southern Spain

Using a combination of field, laboratory and micromorphological evidence, this study examines tafoni (singular, tafone) in the El Chorro area of Andalucia, southern Spain, and makes inferences concerning the processes responsible for their formation. Twenty-five tafoni were randomly selected for field examination. The morphology of these cavernous rock domes is characterized by a helmet-shaped outer roof and an arched-shaped cavern, often with a partially overhanging visor; measurements of height, width and depth of the caverns revealed marked variations in size. The presence or absence of lichen cover, surface varnish, overhanging visor, cavern backwall stripes, rock flaking, weathering pits and cavern floor sediments was also noted. Surface hardness values, obtained using a Schmidt hammer, are relatively low but significantly higher on the outer roof of the tafoni than on the inner cavern walls. Analysis of sediment samples collected from the cavern backwalls and floors indicates predominantly sandy textures, alkaline pH values and some base cation enrichment. Micromorphological analysis of thin sections, prepared from undisturbed blocks, reveals large quantities of pore-filling cement, consisting mainly of calcite, mineral grains affected by weathering and pseudomorphic replacement, and dark, rounded nodules with a metallic appearance. In terms of their formation, different processes appear to act on different parts of the landform. On the outer roof surfaces, case hardening, resulting from near-surface cementation and surface varnish development, is dominant. On the inner cavern surfaces, however, core softening, resulting from granular disintegration and flaking, dominates. Exfoliation weathering, running water and wind deflation also appear to play an important role in tafone formation. A phased model of tafone evolution is proposed whereby the features pass through four phases of development–initiation, enlargement, amalgamation and degradation; in the study area there are examples of tafoni in each of these phases. Much of the evidence suggests that the tafoni are actively developing under current environmental conditions. © 1997 by John Wiley & Sons, Ltd.     

Cavernous weathering,dynamical instability and self‐organization

Cavernous weathering may be conceptualized as a self‐reinforcing process, characterized by positive feedback within the weathering system. A morphometric study of caverns in the Valley of Fire State Park, Nevada, USA, demonstrates the applicability of a dynamically unstable, or conditionally unstable, model of cavernous weathering systems. Outcrop surfaces displaying caverns tend to show increasing fragmentation of the surface in the early stages of cavernous weathering, succeeded by convergent evolution of the surface in which caverns tend to grow and coalesce. A paradoxical relationship exists between the weathering system output at the scale of individual forms and the outcrop scale: caverns tend toward minimum interior surface area by developing a spheroidal form, yet the outcrop surface tends toward maximum exposed surface area by increasing the degree of fragmentation of the surface. Copyright © 2004 John Wiley & Sons, Ltd.     

Surface and sub‐surface Schmidt hammer rebound value variation for a granite outcrop

This study presents rock strength variations at granite outcrops and in subsurface vertical profiles in the Jizerské hory Mountains, Czech Republic. Schmidt hammer rebound values in subsurface profiles change gradually from the bedrock surface downward. An exponential relation has been observed between the R‐values and depth in rock outcrops to a depth of around 4·5 m. The exponential nature of the curve indicates that rock hardness increases more rapidly with depth in the uppermost 1?m section of the rock profile. A detailed study of rebound values obtained from both intact and polished rock exposures reveal effects of surface grinding on results of the Schmidt hammer method. The range of data collected increases after grinding, allowing more precise discrimination of rock surfaces in respect of age and weathering. The Schmidt hammer method may be used effectively as a relative‐age dating tool for rock surfaces that originated during the Late Pleistocene. It is concluded that this time limitation can be significantly mitigated by surface grinding before measurement. Copyright © 2010 John Wiley & Sons, Ltd.     

Annual fossil organic carbon delivery due to mechanical and chemical weathering of marly badlands areas

A key issue in the study of the carbon cycle is constraining the stocks and fluxes in and between C‐reservoirs. Among these, the role and importance of fossil organic carbon (FOC) release by weathering of outcropping sedimentary rocks on continental surfaces is still debated and remains poorly constrained. Our work focuses on FOC fluxes due to chemical and mechanical weathering of marls in two experimental watersheds with typical badlands geomorphology (Draix watersheds, Laval and Moulin, Alpes de Haute Provence, France). Organic matter from bedrock, soil litter and riverine particles are characterized by Rock‐Eval 6 pyrolysis. FOC fluxes due to mechanical weathering are then estimated by monitoring the annual particulate solid exports at the outlets of the watersheds (1985–2005 period). FOC fluxes from chemical weathering were calculated using Ca²⁺ concentrations in dissolved loads (year 2002) to assess the amount of FOC released by the dissolution of the carbonate matrix. Results show that FOC delivery is mainly driven by mechanical weathering, with a yield ranging from 30 to 59 t km^‐2 yr^‐1 in the Moulin (0.08 km²) and Laval (0.86 km²) catchments, respectively, (1985–2005 average). The release of FOC attributed to chemical weathering was 2.2 to 4.2 t km^‐2 for the year 2002. These high FOC fluxes from badlands are similar to those observed in tectonically active mountain catchments. At a regional scale, badland outcropping within the Durance watershed does not exceed 0.25% in area of the Rhône catchment, but could annually deliver 12 000 t yr^‐1 of FOC. This flux could correspond to 27% of the total particulate organic carbon (POC) load exported by the Rhône River to the Mediterranean Sea. At a global scale, our findings suggest that erosion of badlands may contribute significantly to the transfer of FOC from continental surfaces to depositional environments. Copyright © 2012 John Wiley & Sons, Ltd.     

Tafoni on coastal slopes,South Devon,U.K.

Cavernous weathering (tafoni development) occurs on coastal slopes in greenschist bedrock at elevations up to 40 m above sea level. The freshly weathered surfaces of the cavern interiors are irregular in morphology, discordant to major rock structure, formed by substantially weakened rock and associated with granular weathering debris. The weathering debris contains soluble elements in proportions similar to those present in seawater, and the penetration of elements associated with sea salts into the weathering surface to estimated depths of at least 0·1–0·2 m is indicated by the presence of chlorine. Scanning electron microscopy and microprobe analyses suggest that rock breakdown occurs principally through limited chemical weathering at grain boundaries. The mechanism for the emplacement of marine salts within sheltered rock surfaces in the tafoni is postulated to be a combination of dry deposition under turbulent atmospheric conditions and wetting by coastal fog.