Petrological influence on differential weathering and inselberg development in the Kora area of central Kenya

In the Kora area of central Kenya domed inselbergs are well developed on outcrops of granitoid migmatite, while positive relief features are rare on the surrounding gneiss. Block-strewn, vegetated hills occur on restricted areas of granoblastite, gabbro, and metagabbro. Schmidt Hammer measurements have shown that the apparent differences in resistance to weathering and erosion are not due to variations in rock hardness, since all the rock types have similar ‘R’ values. The results of geochemical analyses have shown that the migmatites are significantly more potassic than the surrounding gneiss. Samples of migmatite from the inselbergs were also found to be slightly richer in potassium than migmatite samples from the inter-inselberg areas. The variations in potassium content probably reflect differences in protolith composition, chemical fractionation during partial melting, and the effects of metasomatism. These findings support earlier suggestions that, other things being equal, potassium-rich granitoid rocks weather more slowly than less potassic rocks.     

The weathering effects of the lichen Lecidea aff. Sarcogynoides (Koerb.) on magaliesberg quartzite

The effect of the weathering processes generated by Lecidea aff. Sarcogynoides (Koerb.) on the substrate was studied by means of a scanning electron microscope. The elements present in the substrate (Magaliesberg quartzite) and in the lichen thallus were determined by X-ray fluorescence spectrometry for the purpose of comparison. The elements present were mostly similar although a few were present in the thallus which were not observed in the quartzite. It is possible that those elements present in the lichen thallus which were not present in the substrate may have been extracted from the atmosphere. The occurrence of small hollows (weathering pits) in which the early stages of plant development occurs, and the disintegration of the rock indicate that Lecidea aff. sarcogynoides (Koerb.) contributes to the chemical weathering processes by chelation and mechanically by the penetration and expansion of hyphae. A model is proposed in which a possible mechanism for these weathering processes is suggested.     

Rock moisture measurements: techniques,results, and implications for weathering

Rock moisture is an important factor for the intensity and distribution of frost weathering processes. However, quantitative measurements are scarce, which is partly due to the lack of reliable measurement techniques. This paper presents five different techniques for obtaining rock moisture data. While collecting rock pieces and two‐dimensional geoelectric measurements allow determination of the spatial moisture distribution, the temporal variability can be derived from conductivity and time domain reflectometry records. Computer simulations, using rock properties and climatic records as input data, render it possible to clarify the important aspects that are responsible for the moisture distribution. It proved to be advisable to use several methods to check and validate the results. The results, obtained in study areas in the Bavarian Alps, make it clear that direct rainfall is the main source of rock moisture. The influence of snow is limited to the immediate vicinity of the snow fields and is not equally pronounced at different times and positions. Rock moisture levels are higher in summer than they are in winter, since in winter less water is supplied in liquid form. Northerly exposed rockwalls are generally more moist than those exposed in a southerly direction, which is due to the different insolation as well as to the wind direction during rainfall. In every position the rock is, on average, wetter on the inside than it is on the surface. This means that shallow frost cycles, as typical for south‐exposed sites, are not affecting weathering, since they take place at a depth level that is mostly dried out. Numerous spatial and temporal patterns of rockfall found in the same study areas can be explained through variations in rock moisture. Thus, the moisture content of the rock is considered to be one of the major controlling factors of the frost‐shattering rate. Copyright © 2005 John Wiley & Sons, Ltd.     

Snow cornice dynamics as a control on plateau edge erosion in central Svalbard

Snow cornices grow extensively on leeward edges of plateau mountains in central Svalbard. A dominant wind direction, a snowdrift source area and a sharp slope transition largely control the formation of snow cornices in a barren peri‐glacial landscape. Seasonal snow cornice dynamics control bedrock weathering and erosion in sedimentary bedrock on the Gruvefjellet plateau edge in the valley Longyeardalen. Air, snow and ground temperature sensors, as well as automatic time‐lapse cameras on a leeward facing plateau edge were used to study seasonal cornice dynamics. These techniques allowed for monitoring of cornice accretion, deformation and collapse/melting in great detail. The active layer of the top plateau edge is characterized by high moisture content due to rain before freeze‐up in autumn and cornice meltdown during spring thaw. Thus frost weathering there can be very efficient in this otherwise cold and dry environment. Within the first autumn snowstorms, a vertical fully developed cornice was in place (190 cm thick). The backwall surface beneath the thickest part of the cornice remained in the ice segregation ‘frost cracking window’ for almost nine months. Highly weathered rock material from the plateau edge is thus incorporated into the cornice during cornice accretion. Brittle snow deformation leads to the opening of cornice tension cracks between the cornice mass and the snowpack on the plateau. These cracks are a prerequisite for cornice collapses, and often trigger cornice fall avalanches on the slope beneath. In these open cornice tension cracks, weathered rock debris, plucked from the plateau edge, can be visible, demonstrating the erosional property of the cornices. The cornice will either collapse or melt, resulting in suspended sediment transport downslope by cornice fall avalanche or release as rock fall respectively. Therefore, cornices both promote and trigger high weathering rates on Gruvefjellet, and thus control presently the development of the rockwall free faces and the talus cones. Copyright © 2012 John Wiley & Sons, Ltd.     

Bedrock freeze-thaw weathering regime in an alpine environment,colorado front range

Reduced major axis analysis is used to describe monthly temperature averages for daily maxima, minima, means and ranges at a sequence of bedrock microenvironments in the alpine zone of the Colorado Front Range. Seven thermistors buried at 1 cm in bedrock provide comparative data on easterly, southerly and westerly aspects, and also upon the impact of snow accumulation (?0.5m to ≥4.0m deep) against an east-facing rock wall. Intersite temperatures commonly differ by less than 5°C and, rarely, by more than 10°C. The freezing intensity of freeze-thaw cycles occurring within the confines of a seasonal snow patch rarely dropped to ?5°C, while at snowfree, vertical faces freezing dropped to ?5°C quite commonly. Comparison with laboratory established criteria for effective freeze-thaw weathering (abundant moisture and freezing to at least ?5°C) suggests that moisture rich microsites lack adequate freezing intensity, while adequately frozen sites lack moisture. Available data suggest that the overlap between freeze-thaw and hydration weathering requires careful re-evaluation.     

The Piedmont landscape of Maryland: A new look at an old problem

The Piedmont upland of Maryland has been variously interpreted as a peneplain, a series of peneplains, a surface of marine planation, and a landscape in dynamic equilibrium. These different perspectives of landform evolution are related to different scales of time and space. Both equilibrium and episodic erosion features can be recognized in the modern landscape. An equilibrium condition is suggested by adjustment of first and second order streams to rock structure and lithology, entrenchment of some streams against gneiss domes, altitudinal zonation of rock types around gneiss domes, correlation of lithology with overburden thickness on uplands, decreasing overburden thickness on uplands related to decreasing degree of metamorphism of crystalline rocks, and correlation of secondary mineral assemblages with subsurface drainage and slope. The long-term episodic character of erosion is suggested by clastic wedges on the adjacent Coastal Plain, an upland of low relief that truncates non-carbonate rocks of different lithologies, isovolumetric chemical weathering of alumino-silicate rocks, clastic deposition in marble valleys, and weathering profile truncation by modern drainage. The Maryland Piedmont may have been an area of positive relief subject to subaerial erosion since Triassic and possibly Permian time. The upland surface preserved in the eastern Piedmont developed by the Late Cretaceous. In the interval from the Late Cretaceous to the Late Miocene, low input of terrigenous sediments to the Coastal Plain, dominance of marine sedimentation, and spotty evidence of saprolite formation on crystalline rocks, suggest that the Maryland Piedmont was an area of low relief undergoing intense weathering. Incised valleys were formed during a cycle of erosion probably initiated in the Late Miocene and extensive colluvial sediments were deposited on hillslopes by periglacial processes during the Pleistocene.     

The physical properties of quartz-micaschist and their application to freeze-thaw weathering studies in the maritime antarctic

As part of a study on freeze-thaw weathering in the maritime Antarctic an investigation was made of the physical properties of the local rock. Tests were made of point-load compressive strength, Schmidt hammer in situ rock strength, moisture content, indentor resistance and the size range of weathering products. The resulting data were used to consider the form of freeze-thaw weathering operative in the field and its relationship to laboratory simulations. A distinct difference between ‘massive rock’ and ‘intact rock’ is observed in the field. It is suggested that future studies should generate a greater database pertaining to rock properties as it is an invaluable aid in the study of mechanical weathering.     

FIELD EXPERIMENTS ON PHYSICAL WEATHERING AND WIND EROSION IN AN ANTARCTIC COLD DESERT

Field experiments were carried out over a five year period with the aim of understanding contemporary weathering and erosional environments in the Sør Rondane Mountains, an Antarctic cold desert region. These include observations of (1) scaling from rockwalls, (2) disintegration of tuff blocks with or without saline solutions, and (3) abrasion of artificial walls by wind. Monitoring was also made of rock surface temperature and wind speed. Despite frequent temperature oscillations across 0°C, rock scaling due to frost action was generally very slow because of low moisture content in the rockwalls. Exposure to the cold, dry climate led to the rapid disintegration of porous tuff blocks including soluble salts like halite and thenardite. This indicates that rates of weathering are increased greatly with the accumulation of such salts in the bedrock. Although gypsum did not cause any visible damage over four years, its widespread occurrence in heavily damaged rocks demonstrates that increasing gypsum contents may also intensify rock breakdown. The snow-laden katabatic wind resulted in rapid wearing of the windward face of an asbestos board with the peak erosion at 30–40 cm above the ground. Nonetheless, the landforms expected from the unidirectional wind characteristics are by no means common features because of lack of abrasive materials, such as snow and sand particles. These experiments suggest that frost weathering and wind erosion are only locally effective where plenty of moisture or an abrasive material is available, whilst salt weathering and removal of the waste by wind play a major role in constructing erosional landforms over the mountains.     

Classification of rock weathering at writing-on-stone Provincial Park,Alberta, Canada: A study in applied geomorphology

At Writing-On-Stone Provincial Park in southern Alberta, Canada, weathering is causing deterioration and loss of archaeologically important Indian rock art. A procedure devised for the use of park personnel identified four classes of weathering ranging from largely unweathered rock to severely weathered. The technique employed simple visual, qualitative assessment and photo interpretation of 50 sample sections of sandstone cliff face covering a total area of 354 m². Schmidt hammer tests indicated large variations in rock strength and provided a numerical basis for the visual assessment. About 43 per cent of the cliffs are severely to completely weathered, 41 per cent show moderate weathering.     

Coeval biochemical and biophysical weathering processes on Quaternary sandstone terraces south of Rabat (Temara), northwest Morocco

Despite numerous investigations on substrate‐inhabiting microflora, especially lichens, very little is known about the colonization of coastal escarpments by lithobiontic micro‐organisms, inland of a retreating coastline in Africa. Reported herein are the results of a combined field observation and microscopy study focusing on the connection between microrelief of the substrate, colonies of lithobiontic micro‐organisms (in particular the lichen Xanthoria parietina) and microstructures of putative bacterial origin. The occurrence of weathering pits in which the early stages of the biotic development occurs, and the subsequent disintegration of the rock indicate that lichens, mosses and fungi act synergistically by alternating chemical and mechanical weathering. Penetration of grains by expansion and contraction of the hyphae depletes the rock matrix and contributes to the mechanical breakdown of the rock. Calcite rhombs on the weathered surfaces of the calcite‐cemented sandstones are severely etched with well‐developed rhomb‐shaped etch pits (‘spiky calcite’), holes, or has one or more of the faces removed, and their cores exposed and leached. Nanofilaments (c. 100–700 nm) and ‘nanomicrobial’ fruiting bodies (c. 250 nm) emanating from micropores appear to be common on affected crystalline structures. Weddellite present immediately below the thallus is a strong indicator of biomineralization. Quartz responds differently to chemical weathering by producing peeling structures and microbrecciation features. The dissolution of these crystals appears to be a surface reaction‐controlled process mediated by microbial microfilaments and nanofilaments. A model is proposed, firstly indicating early‐stage biochemical weathering, followed by biophysical weathering. Disintegration of the rock outcrops in due to a complex interplay of several events, probably beginning at the nanoscale with penetration of sites on crystal faces. Copyright © 2006 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.     

Differential weathering of feldspar and pyroxene in an arctic-alpine environment

The relative relief of adjacent plagioclase and pyroxene minerals was accurately measured on a recently-exhumed outcrop of gabbroic rocks in Leirdalen, southern Norway. Above the level of the former soil surface the feldspars protrude above the pyroxenes whereas below that level the pyroxenes are higher. Differential relief declines with increasing depth of burial down to 80 cm. On exposed surfaces the mean loss of pyroxene relative to feldspar is 103 cm³ m^?2 of rock surface. With burial down to 50 cm in an arctic-alpine Brown Soil the mean loss of feldspar relative to pyroxene is 179 cm³ m^?2. These figures represent minimum amounts of weathering over about 9000 years. The results confirm the importance of chemical weathering in arctic-alpine environments and the techniques may provide useful physical measures of degree of weathering for use in rate studies and relative-dating.     

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 relations between modulus of elasticity and temperature in the context of the experimental simulation of rock weathering by fire

Fires occur frequently in many biomes and generate high temperatures on the ground surface. There are many field examples of fire causing rock disintegration. The simulation of fire in the laboratory (using a furnace) and the monitoring of changes in rock modulus of elasticity (with a Grindosonic apparatus), reveal that different rocks respond differently to heating. Significant decreases in elasticity occur at temperatures as low as 200°C and granites display particularly marked reductions. Extended periods of heating are not required for significant reductions to occur. It is postulated that the degree of change in elasticity as a result of simulated fire is such that rock outcrops subjected to real fires are likely to be sufficiently modified as to increase their susceptibility to erosion and weathering processes.     

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.     

Enhanced boulder weathering under late-lying snowpatches

A Schmidt test hammer was employed to assess the aggregate surface hardness of samples of boulders dug out from under late-lying snowpatches at sites in Switzerland, Scotland and Norway. The results were compared with an equivalent set of readings made on boulders at nearby snow-free control sites. The results in every case reveal that the aggregate surface hardness of boulders buried by late-lying snow is significantly less (at the 0·001 level) than that of boulders at the corresponding control sites. This result indicates that late-lying snow significantly enhances rock weathering, probably because boulder and rock surfaces are exposed to prolonged wetting by percolating meltwater under late-lying snowbeds.     

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.     

Weathering of coastal defensive structures in southwest England: a 500 year stone durability trial

Historic structures can be viewed as exposure trials of the stone of which they are constructed. As such, they represent a geomorphological weathering experiment. Several structures of Henrician (sixteenth century) and greater age on the coast of southwest England have been exposed to coastal salt weathering for 500–600 years. Long‐term weathering rates on five different rock groups are derived from careful study of weathering depths and forms. There is significant variation in weathering rate between five major rock groups. Rank ordering of weathering rate values reveals a durability order of these rock groups, which is confirmed by local juxtapositions. Controls on rock durability in the coastal weathering environment include both mechanical and mineralogical characteristics. Specific density, and combined quartz and muscovite content, are positively related to durability; high feldspar and chlorite content are associated with low durability. Copyright © 2000 John Wiley & Sons, Ltd.     

Snowmelt runoff and soil moisture dynamics on steep subalpine hillslopes

Snowmelt water supplies streamflow and growing season soil moisture in mountain regions, yet pathways of snowmelt water and their effects on moisture patterns are still largely unknown. This study examined how flow processes during snowmelt runoff affected spatial patterns of soil moisture on two steep sub‐alpine hillslope transects in Rocky Mountain National Park, CO, USA. The transects have northeast‐facing and east‐facing aspects, and both extend from high‐elevation bedrock outcrops down to streams in valley bottoms. Spatial patterns of both snow depth and near‐surface soil moisture were surveyed along these transects in the snowmelt and summer seasons of 2008–2010. To link these patterns to flow processes, soil moisture was measured continuously on both transects and compared with the timing of discharge in nearby streams. Results indicate that both slopes generated shallow lateral subsurface flow during snowmelt through near‐surface soil, colluvium and bedrock fractures. On the northeast‐facing transect, this shallow subsurface flow emerged through mid‐slope seepage zones, in some cases producing saturation overland flow, whereas the east‐facing slope had no seepage zones or overland flow. At the hillslope scale, earlier snowmelt timing on the east‐facing slope led to drier average soil moisture conditions than on the northeast‐facing slope, but within hillslopes, snow patterns had little relation to soil moisture patterns except in areas with persistent snow drifts. Results suggest that lateral flow and exfiltration processes are key controls on soil moisture spatial patterns in this steep sub‐alpine location. Copyright © 2014 John Wiley & Sons, Ltd.     

Thermal properties as controls on rock surface temperature maxima,and possible implications for rock weathering

Four rock types (basalt, sandstone, granite, and chalk) are examined with respect to the maximum surface temperatures which they experience when subjected to similar conditions of exposure. Rock temperature measurements are reported for an urban environment and for two experimental situations in which an infrared lamp is used to simulate heating under cold and hot conditions. Differences in rock temperatures are discussed with reference to thermal rock properties (albedo, specific heat capacity, and thermal conductivity). Some natural situations are suggested in which thermal rock properties could conceivably play a role in determining the extent to which rocks would be affected by particular weathering processes.