Erosion in loess is a widespread phenomenon, as loess covers about 10% of the Earth's land surface. While erosion of loess soil has been intensively studied, the mechanisms controlling erosion in ...gullies and pipes in loess are poorly understood. Cohesion plays an important role in erosion in loess. Interactions between erosional processes and stabilizing mechanisms in loess are poorly understood. This study focuses on the interaction between air slaking and stabilization by confinement to improve leaching techniques for identifying cohesion sources in undisturbed loess from four sites in Czechia. The decrease in tensile strength of samples after leaching in distilled water, dithionate, HCl, and hydrogen peroxide was used to selectively remove cohesion sources such as Fe‐Al oxides and hydroxides, carbonates, and organic matter. Experiments showed that confinement and overburden stress are important but neglected stabilizing mechanisms in loess. Leaching of unconfined loess samples gave misleading results because of ubiquitous air slaking. To reliably identify cohesion sources, samples confined in compacted sand were used. Leaching of confined samples showed that Fe‐Al oxides and hydroxides are major sources of cohesion (60–90%), while carbonates and organic matter are of minor importance (0–30%). To avoid misleading results, examination of loess structure after leaching is critical to identify samples with damaged structure due to enhanced air slaking caused by bubbles generated during leaching. Air slaking is a powerful and rapid damage mechanism, but it occurs only in dry or semi‐dry loess near the soil surface. In contrast, chemical weathering is able to remove cohesion sources in deeper parts of the loess profile.
Reduction of tensile strength after leaching used to quantify cohesion sources. Stabilization of loess by confinement needed during leaching tests. Al‐Fe oxides and hydroxides are major source of loess cohesion.
Cavernous weathering (cavernous rock decay) is a global phenomenon, which occurs in porous rocks around the world. Although honeycombs and tafoni are considered to be the most common products of this ...complex process, their origin and evolution are as yet not fully understood. The two commonly assumed formation hypotheses – hydraulic and case hardening – were tested to elucidate the origin of honeycombs on sandstone outcrops in a humid climate. Mechanical and hydraulic properties of the lips (walls between adjacent pits) and backwalls (bottoms of pits) of the honeycombs were determined via a set of established and novel approaches. While the case hardening hypothesis was not supported by the determinations of either tensile strength, drilling resistance or porosity, the hydraulic hypothesis was clearly supported by field measurements and laboratory tests. Fluorescein dye visualization of capillary zone, vapor zone, and evaporation front upon their contact, demonstrated that the evaporation front reaches the honeycomb backwalls under low water flow rate, while the honeycomb lips remain dry. During occasional excessive water flow events, however, the evaporation front may shift to the lips, while the backwalls become moist as a part of the capillary zone. As the zone of evaporation corresponds to the zone of potential salt weathering, it is the spatial distribution of the capillary and vapor zones which dictates whether honeycombs are created or the rock surface is smoothed. A hierarchical model of factors related to the hydraulic field was introduced to obtain better insights into the process of cavernous weathering.
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•Laboratory and in situ visualization of capillary moisture in honeycombed sandstone were completed.•Hydraulic hypothesis was confirmed to be a key mechanism for honeycomb origin.•Hierarchical model shows factors involved in honeycomb formation.•Initial and external factors control hydraulic field, which in turn controls salt weathering pattern.
Knowledge of the evaporation rate from rock surfaces is critical for obtaining the water flux in the rock-atmosphere interphase, for understanding moisture distribution, and for quantification of ...damage from salt crystallization within the rock. Evaporation from rocks is a poorly understood, yet important process. We present a study on evaporation from 10 lithologies, including sedimentary, igneous, and metamorphic granular rocks. The evaporation rate was measured from rock cores with a set vaporization plane depth in a humid temperate continental climate during at least eight observation periods for eight months. The measured evaporation rate varied over four orders of magnitude (0.4–2447 mm/year), being dependent on the vaporization plane depth, lithology, and climate seasonality at the site. The evaporation rate from the rock cores was calculated based on Fick's law. The calculations reasonably followed the measured values. Using contrasting, yet field-realistic values in the calculation, virtual time series of the seasonal evaporation rate from natural rock outcrops in three different climates (arid, semi-arid, humid) were constructed. This revealed possible annual evaporative losses from the rock outcrops (0.1 mm–896 mm). Within the range of observed values, the evaporation rate was mostly influenced by the vaporization plane depth (by up to 2.2 orders of magnitude), which was followed by: lithology (up to 1.1 order of magnitude), local climate (up to 1.0 order of magnitude), and climate seasonality (up to 0.8 order of magnitude). Thus, our study shows the key role of the vaporization plane depth in the evaporation rate. This approach can find employment in a large number of investigations such as in the evaporation estimates and hydrologic balance in rock landforms and rocky slopes, hydrologic processes in the shallow rock subsurface, living conditions of endolithic and epilithic organisms, weathering processes, and in the protection of carved or rock constructed cultural heritage.
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•Method to measure and calculate evaporation rate was tested in various lithologies.•Calculation of evaporation rate relies on easily obtainable parameters.•Calculations reasonably follow measured values in majority of lithologies.•Vaporization plane depth controls evaporation rate, climate is less important.•Applicable to hydrology, weathering, epi- and endolithic biology at rock surfaces
Abstract Cavernous weathering forms have long been studied and discussed as enigmas in geomorphology. Recently, their evolution has been shown to be controlled by moisture patterns, which are still ...poorly understood. For the first time, capillary water and vapor fluxes were characterized in detail at tafone in a temperate climate of central Europe using a wide range of methods adapted from soil hydrology. Time domain reflectometry showed that moisture flows from the rock interior to the evaporation front in the shallow subsurface of both — the backwalls and the outer surface. When overland flow occurs on the outer surfaces (after heavy rains), 10 mm/day can infiltrate and flow toward the backwalls. The main sources of water for tafone are the influx of water from the rock interior and the infiltration of overland flow after heavy rains, while condensation of air humidity is a minor source. Influx from the rock interior is coupled to the evaporation rate, which varies between 100 and 300 kg/m 2 /year in summer and less than 15 kg/m 2 /year in winter. More water evaporates from the backwall of the tafone than from the outer surface, and more salt is deposited in the backwalls, resulting in predominant salt weathering in the backwalls. The tafoni studied thus evolve, and the cavities deepen. Tafoni in arid and semi‐arid environments generally show a much higher contrast between evaporation rates from backwalls and outer surfaces than tafoni and honeycombs in temperate and coastal environments. Tafoni in temperate settings are therefore more susceptible to degradation when evaporation decreases or inflow to the tafone increases. This study also shows that microtensiometers can be used to determine moisture content with high spatial resolution, while time domain reflectometry allows accurate characterization of moisture patterns with depth.
Cavernous weathering (honeycombs, tafoni) is a common weathering feature of both natural and artificial exposures. Honeycombs are known from various environments but are best developed in coastal ...areas. There are several theories as to their origin, with salt weathering currently being the most favoured by the geomorphological community. To test if the drying pattern of salt‐laden moisture results in honeycombs (the theory of Huinink et al., Earth Surface Processes and Landforms, 29(10), 1225–1233, 2004), coastal honeycombs in the metasandstone of Tuscany (Italy) were studied both in the field and with a laboratory evaporation experiment. The depth of the evaporation front was measured by the ‘uranine‐probe’ method in the honeycomb pits and lips. The evaporation rate was calculated from the depth of the evaporation front as well as the climatic conditions at the study site. Lastly, the amounts of precipitated salts were estimated based on the evaporation rate of seawater. In the evaporation experiment, the evaporation front retreated faster in the lips than in the pits, and the field measured evaporation front was closer to the surface in the pits (2 mm) than in the lips (7 mm). Thus, the calculated evaporation rate was higher in the pits than in the lips (16.1 and 4.6 mm/yr, respectively). Similarly, the amount of salts precipitated was also higher in the pits (0.7 kg/m2/yr compared to 0.2 kg/m2/yr in lips). Faster salt deposition in the pits as well as the evaporation front position fits well with the theory of Huinink et al. Based on surface tensile strength measurements, case hardening is not protecting the honeycomb lips.
Moisture patterns and evaporation rate were studied in coastal honeycombs in Tuscany (Italy) metasandstone to elucidate their origin. Field measurements and a laboratory drying experiment on honeycomb‐covered rock demonstrated that the most intense drying and related salt deposition occurs in honeycomb pits, enlarging them by salt weathering, and that case hardening does not play any role. The findings demonstrated that cavernous weathering originated by wetting/drying cycles.
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•Evaporation depth in porous materials can be measured by a “uranine-probe”.•A stainless-steel rod, polyurethane adhesive, and uranine dye perform the best.•The method can be used in ...the study of evaporation, weathering, or living conditions.•The method is applicable in a range of climates from humid continental to desert.•The probe can be used for various kinds of porous materials.
The vaporization plane, a narrow zone of subsurface evaporation often present in porous rocks, separates the region where water flows due to capillary forces from the dry zone where moisture moves in gas phase only. The knowledge of its depth and geometry is critical for estimating water flux in rock-atmosphere interphase, for understanding moisture distribution and for localization of damaging salt crystallization. Yet, an easy-to-use method applicable in the exterior has been missing. This strongly limits interpretation of moisture-related measurements as moisture content differences in the above-mentioned zones are often immeasurable by currently used field techniques. We have introduced a new micro-destructive method to measure the vaporization plane depth using an instrument consisting of a rod, adhesive, and dye powder, reacting with moisture, that is inserted into porous materials in 2 mm diameter holes. We tested different rods, adhesives, and dyes, and the best combination of these has been used in >500 experiments to determine the vaporization plane depth in porous rocks and building materials. The knowledge of vaporization plane depth enables more reliably to interpret the moisture and suction data obtained from numerous existing techniques. This new uranine-probe method should be thus of interest to many scientific disciplines: evaporation, unsaturated hydrology, weathering, or geobiology.
Arcades, i.e. lenticular and other specifically shaped hollows controlled by discontinuities, have recently been recognized as a weathering form typical for sandstones, weathered quartzites, ...granites, or tuffs. They are produced by accelerated weathering and erosion in stress shadows related to the redistribution of gravity-induced stress along planar discontinuities in the rock. These forms occur worldwide in various settings (inland humid, arid, and coastal). The origin of arcades has been demonstrated via physical experiments and supported by a relatively simplistic numerical modeling. However, details on their shaping and the evolution of related forms have not been explained. We performed an advanced numerical modeling to produce various shapes of arcades and rock pillars during the erosion of rock masses dissected by discontinuities. We demonstrate that the erosion model, in which erosion takes place when the maximum principal stress is below a certain critical value, can adequately describe the formation of arcades. In the modeling, we set higher critical values for stresses at discontinuities than in a homogeneous material (representing a rock mass) to represent the higher tendency for disintegration of the discontinuity material, which was weakened by the discontinuity formation processes. By applying various discontinuity geometries and values of critical stresses, we were able to reproduce the formation of various arcade shapes and complex-three-dimensional clusters of arcade cavities with rock pillars. Discontinuities and stress-controlled erosion/weathering are the only necessary conditions for arcade formation.
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•Arcades are specific cavernous weathering forms.•Gravity-induced stress along planar discontinuities is the key factor.•Innovative numerical modeling explains the evolution of arcade-derived forms.•Rock pillars are products of the process of arcade evolution.•Modeling is a powerful tool for studying the process of stress-controlled erosion.
