Upper left and right: White weathering rinds formed on red granite due to the process of calcification. The rocks were collected from Mt. Monster, southeastern South Australia, by Professor Charles Rowland Twidale of the University of Adelaide. Bottom left: Weathering rind with a thick transitional layer (referred to as a leached layer or a dissolution layer) formed on andesite gravel. Bottom right: Weathering rind on Sanukite (high-magnesian andesite). Thickness of the white layer affected by leaching, which exists just inside the brown layer affected by oxidation of iron, tends to decrease in rocks with low porosity.
(Photograph & Explanation: Chiaki T. OGUCHI)
A brief review of our weathering studies is presented, specifically from a geomorphic perspective. The review suggests that experimental work and microscale investigations using new analytical methods have advanced studies on weathering processes, while steady advances have been made in research on changes in properties and rates of weathering. Assignments and priorities are presented arising from our studies, including the need to: (1) accumulate data on rates of change of rock properties due to weathering (especially strength reduction); and, (2) construct a geomorphological equation of weathering rates based on the relationship between weathering processes and rock properties.
Water–rock interactions proceed as a result of reaction and transport occurring at an outcrop scale and in pores at a millimeter to nanometer scale. To quantitatively evaluate these processes, various parameters including porosity, dissolution rate, fluid flow rate, effective diffusion coefficient, and surface area are required. Focusing on rock weathering, some examples are presented of how these parameters are determined, and a brief review is provided of the nature of the parameters. Hydraulic conductivities, a measure of fluid flow rate, ranging from < 10−13 to 1 m s−1, largely depend on pore radius or fracture opening size and porosity. Effective diffusion coefficients, ranging from ∼10−15 to 10−9 m2 s−1, are known to be a power law function of porosity. Dissolution rate constants of silicate minerals, ranging from ∼10−14 to 10−9 mol m−2 s−1 under ambient conditions, are affected by the connectedness of the SiO4 tetrahedra. Using those values, some simple examples are presented of calculations of the profiles of concentration and reaction rate at an outcrop scale and at a pore scale, as well as some recent advances in pore scale geochemical processes.
Rock weathering is driven by reactions and transport mediated by water, which exists on the surface and the interior of a rock. Water saturation in a rock varies depending on the conditions of rainfall and drying, resulting in complex distributions of water and air. As the rock matrix proceeds to dry, pore water is lost first from larger pores and then from smaller ones, although evaporation occurs at any pore size. This pore-size dependence of water loss occurs because water in larger pores migrates into smaller pores due to the difference in capillary pressure between the pores. When water infiltrates into dry pores, water preferentially advances into pores having smaller apertures. In addition, air is preferentially entrapped in pores of a specific size. This entrapped air blocks the water flow and significantly decreases the hydraulic conductivity of the rock. The pore wall of entrapped air is covered with a water film having a thickness on a nanometer scale. The thickness of the water film is mainly controlled by pore size and mineral composition if pore water is dilute. From the perspective of the role of the distribution of water on rock weathering, it is important to advance knowledge of the characteristics of air entrapment and water film.
Clay minerals, metal oxyhydroxides, and carbonates are important earth surface materials, which are secondary minerals formed from primary minerals with water and atmosphere under earth surface conditions. These minerals usually possess a high surface area and are highly reactive to foreign dissolved species; therefore, they play important roles as sorbents for trace elements in natural water. Predicting distributions of trace elements between solution and mineral phases is important for understanding migration and accumulation behaviors of toxic and useful elements on the earth surface. To predict distributions, it is required to elucidate the chemical processes governing the sorption of trace elements on or in minerals, and to model chemical processes quantitatively. The important chemical processes governing the sorption of trace elements by earth surface materials are adsorption, surface precipitation, ion exchange, mineralization, and coprecipitation. The current understanding of these chemical processes as revealed from laboratory experiments is reviewed, as well as current approaches to modeling.
This study aims to clarify the solution characteristics of carbonate rock tablets in a tropical savannah climate. The study site is located within the ruins of the Angkor Wat Temple in north western Cambodia. Two kinds of carbonate rock from Japan were used: middle Carboniferous limestone from the Akiyoshi-dai Plateau and marble from the Abukuma Mountains. The tablets used in the experiment were fashioned into discs 40 mm in diameter and 4 mm in thickness. They were then placed horizontally in a secure rack 120 cm above the ground, next to a weather station, and remained exposed from August 24, 2011 to March 15, 2014 (a duration of 934 days). The tablets were weighed before, during, and after the experiment in order to measure changes due to dissolution effects. The initial weights of the tablets were 13.624 g for limestone and 13.700 g for marble; on day 934, their dry weights were 13.523 g for limestone (99.26% of initial weight) and 13.630 g for marble (99.49% of initial weight). The experiment produced three major results: (1) limestone dissolves faster than marble; (2) solution occurred most rapidly during the initial test period, and thereafter progressed constantly but at a lower rate; (3) the observed solution rate under a tropical savannah climate was lower than previously reported values from a humid temperate climate derived from equivalent methods. The faster solution of limestone is thought to occur because limestone has a higher composition of fine-grained calcite than marble; therefore, it has a larger surface area, which is more conducive to solution reactions. In tandem with surface solution of post-test tablets, calcite surfaces are covered with recrystallized products, such as speleothems. Thus, under a tropical savannah climate with a clearly defined dry season, recrystallization associated with moisture loss is likely to reduce the solution rate. For solution processes to progress most effectively within carbonate rock, appropriate temperature conditions are not sufficient; continuous contact with water is also essential.
The weathering of carbonate rocks in soil is an essential process in the evolution of karst landforms as well as in the global carbon cycle. Among the various methods for measuring the weathering rates of carbonate rocks, a field weathering experiment using rock tablets is a simple and relatively easy method to estimate the spatial distribution of weathering rates in karst terrains. This paper introduces some results of field experiments on the weathering of carbonate rocks. The results are summarized based on the following three factors: global climatic condition (annual temperature and precipitation), local environmental conditions (soil moisture), and duration of experiment. Annual precipitation is one of the major factors controlling the weathering rates of carbonate rocks. At a local scale, weathering rates are controlled by the duration of water saturation in soil, although some experiments also indicate the effects of soil water chemistry on weathering rates. Changes in the surface condition of tablets, as well as inter-annual climatic variations also affect weathering rates. Duration of experiment should be set carefully to reduce influences from the duration of the experiment.
Physical rock weathering has been studied through laboratory experiments, field observations, and numerical modeling, but linking these approaches and applying the results to weathering features in the field are often problematic. We review recent progress in three weathering processes—frost shattering, thermal fracturing, and lightning strikes—and explore better approaches to linking weathering processes and products. New visual and sensor technologies have led to great advances in field monitoring of weathering of fractured bedrock and resulting rockfalls in cold mountains. Laboratory simulations successfully produce fractures resulting from segregational freezing in various intact rocks. Modelling approaches illustrate the long-term evolution of periglacial slopes well, but improvements are required to apply laboratory-derived criteria to frost weathering. The efficacy of thermal weathering, which has long been under debate, is now partly supported by laboratory and field evidence that cracking takes place when wild fires or artificial explosions lead to thermal shock. Rock fracturing due to strong radiation is also reevaluated from the presence of large cooling/warming rates and meridian cracks in rocks exposed to arid environments. Linking laboratory simulations and natural features, however, needs further field-based observations of thermal fracturing. Irregular fractures formed in boulders are often attributed to lightning strikes, despite rarely being witnessed. Artificial lightning in the laboratory produces radial cracks, marking the first step toward interpreting irregular fractures in the bedrock that are unlikely to originate from other weathering processes. Identifying the origins of fractured rocks in the field requires distinguishing between fracture patterns derived from these weathering processes.