Rocky coasts composed of sandstone and tuff often show a honeycomb structure, large forms of which are designated tafoni. The cavernous structure prevails in the wave-splash zone where wave erosion is weak but wave splashes frequently provide seawater. Granular disintegration by salt weathering is likely to produce the structure, although the origins of the honeycomb geometry are unclear. The sizes of holes depend on pore structure and rock strength, as well as the duration of exposure to salt weathering.
(Photograph and Explanation: Norikazu MATSUOKA Photographed on December 28, 2005)
To investigate cliff recession processes and rates for the purpose of studying the development of horizontal shore platforms, data taken from a masonry seawall at Ashikajima located on the Choshi Peninsula coast is used. The seawall is composed of artificially cut blocks of Cretaceous sandstone, which is the same rock type as that forming shore platforms in the area. The seawall with a horizontal length of 100 m was built 90 years ago to protect reclaimed land on pre-existing shore platforms. Two sites are selected for this study: Site A without a shore platform in front of the seawall and Site B with a platform. The surfaces of sandstone blocks in the supratidal zone are depressed at both sites; Site B has a more noticeable depression. The depression depth (i.e. erosion depth) after the period of 90 years is considerably larger (more than double) at Site B than at Site A, in spite of Site A suffering direct attacks from waves, irrespective of tidal stages, while Site B experiences low-energy waves only during high tides due to the presence of the horizontal platform. Granular disintegration occurs markedly on the sandstone surface at Site B, but little occurs at Site A. This strongly suggests that salt weathering is prevalent at Site B, reducing the strength of rocks. The moisture content in sandstone blocks at Site B is lower than that at Site A, which implies that Site B is more susceptible to weathering than Site A which is always exposed to waves and tides. It would be reasonable to consider that the seawall at Site A is analogous to a cliff at the initial stage of platform development, while the seawall at Site B is similar to a cliff at its middle stage. Horizontal shore platforms start to develop due to wave action alone, but as they grow wider the importance of salt weathering increases and the combined effects of waves and weathering become crucial to platform development.
Morphological features of notches and tafoni, which are well developed on volcanic breccia cliffs of Miocene Setouchi volcanic rocks in Kagawa, Shikoku, Japan, are described. The moisture content of a rock surface shows the highest value at the concave roof portion. The presence of a salt, such as gypsum, on notch walls suggests these notches are formed by salt weathering under a high humidity. Notches and tafoni develop to produce a visor overhang on a steep cliff, and such unstable blocks occasionally collapse as rock falls. In addition to the development of notches and tafoni, cracks within volcanic breccia can also cause rock falls. Consequently, the rate at which notches deepen due to salt weathering and the spacing of tension cracks may control the frequency of rock falls from these cliffs.
Environmental monitoring and rock-property investigations are performed to explain the mechanism of deterioration observed at two buildings constructed in the medieval age and the modern age at Orval Abbey (Belgium). The medieval building is composed of two limestones, while the modern building is composed of reconstituted stone agglutinated using cement containing crushed natural limestones. Deterioration due to salt efflorescence is observed only at the ground floor wall of the modern building. Four measuring sites are set up at these buildings to monitor air temperature and relative humidity. Moisture content and Equotip hardness are measured on a wall at each site. Salt at each site, soils around buildings, underground river water, and three types of stone are also sampled for further laboratory investigation. From the XRD analysis, only calcite (CaCO3) is detected from salts at sites of the medieval building, whereas calcite, thenardite (Na2SO4), mirabilite (Na2SO4·10H2O), and gypsum (CaSO4·2H2O) are detected at sites of the modern building. The source of Na and S in these salts is underground river water, not the reconstituted stone. Therefore, crystallization of sodium sulfates from constituents of the river water is considered to be the main cause of the deterioration of the modern building wall.
It is difficult to estimate weathering rates of rocks based on actual landforms. However, using stone-built architectures, artifacts, and traces of human activity on rock surfaces, weathering rates of rocks under weathering-limited conditions can be obtained easily because stone-built heritages, in general, have a geometrical shape and zero-datum levels. In addition, it is possible to estimate weathering rates of a millennium-scale and changes of rates up to a millennium scale. Many studies on weathering rates of rocks use stone-built heritages. This study reviews recent geomorphological studies that estimate weathering rates, and summarizes their trends. Most of the studies analyze gravestones and churches built since the 19th and 11th centuries, respectively. Such stone-built heritages are more commonly located in humid temperate areas. Weathering rates are estimated mainly from surface recession or surface loss of gravestones and church-building stones. The major three building stones—carbonate rocks (rate: 2-90 mm/ka), sandstone (8-100 mm/ka), and granite (5-65 mm/ka)—have different ranges of weathering rates. Among these stones, the rates for carbonate rocks are sensitive to climatic conditions and atmospheric sulfur dioxide concentrations. The results of the studies reveal that weathering rates show an obvious dependence on aspects. North-facing surfaces tend to have lower rates than surfaces facing other cardinal directions because each surface has different temperature and moisture conditions due to insolation. Moreover, the studies reveal that temporal changes in weathering rates rarely fit a simple linear model. Changes in atmospheric acidity, landform development, and vegetation cover rapidly affect the intensity of weathering processes and cause fluctuations in weathering rates.
Many researchers have studied rates of weathering layer development. Weathering layers broadly include weathering rinds, hydration layers, rock varnish, and weathering crusts, each of which has been discussed within individual research fields. To understand the development of these weathering layers, previous research on the growth rates of weathering layer thickness and formation processes is reviewed. Then, terminology relating to weathering layers is elucidated. Weathering rind development is affected by the porosity and mobility of many elements. Conversely, the obsidian hydration layer is mainly controlled by moisture content and rock varnish is influenced by microbial activities. Environmental differences in the thickness of these thin weathered or alteration layers with weathering time are also examined. Although data are limited, the growth of weathering rind thickness varies by rock types and environmental conditions. The next stage of weathering rind studies will be supported by the progress of analytical methods including isotopic elements, interpretation of rock properties, biomineralization, and introduction of environmental parameters.
This paper reviews the methodology and applications of terrestrial cosmogenic nuclides as a tool for quantifying rates of geomorphic processes. The review starts from systematics in the production of cosmogenic 10Be and 26Al in quartz, and 36Cl in calcite, and then describes the basic modeling of the accumulation of those nuclides under varying denudation rates. Procedures for sample preparation and nuclide measurement using accelerator mass spectrometry are also summarized. Recent research reveals denudation rates of bare rock surfaces for both silicates and carbonates, as well as soil production rates from saprolite beneath the soil layer on hillslopes. The empirical formulation of soil production rates as a function of soil thickness enables us to test hypothetical transport laws of soil particles through a combined analysis with topographic parameters of hillslopes. Chemical processes contributing to soil production and denudation have been quantified with a coupled approach using cosmogenic nuclide analysis and geochemical mass balance method. However, linkages across climate conditions, element leaching, and denudation rates are still debated because of timescale discrepancies between soil and saprolite formation. Climate seems to affect soil production indirectly by reducing the mechanical strength of saprolite resulting from chemical weathering of bedrock. A theoretical framework is presented for modeling saprolite weakening and denudation, which connects bedrock weathering, erodibility of uppermost saprolite, soil production and transport with steady-state topography of hill-noses.
Weathering is deeply related to global climate change. In the carbon cycle, silicate weathering, especially volcanic rock weathering, transfers carbon in the atmosphere (as CO2) to the lithosphere, and oxidative weathering of organic matter releases carbon (as CO2) from the biosphere to the atmosphere. Moreover, as an indirect effect of weathering on climate change, negative feedback in the climate system, which results from the dependence of weathering rate on temperature and evolution of terrestrial plants, is crucial. It has stabilized the long-term global climate throughout the Phanerozoic. Weathering rate is controlled by several geochemical external factors: tectonic forces such as lithology, continental uplift, and continental drift (paleogeography); climate forces such as temperature, runoff, and glaciations; and, biological forces such as terrestrial plant evolution. Regarding biological forces, accelerated weathering assisted by ectomycorrhizal fungi (EM fungi) and arbuscular mycorrhizal fungi (AM fungi), as well as vascular plants of gymnosperms and angiosperm, are emphasized. Variations of global weathering in the geological past are estimated using experimental approaches, such as isotope analysis (e.g., 87Sr/86Sr, 187Os/186Os, δ7Li), and theoretical approaches, such as numerical simulations (e.g., carbon cycle model). Each is used differently according the purpose of a study. Based on these estimates, geological past climate changes in the Phanerozoic are found to be closely related to weathering. For example, on the order of magnitude of 107 years, changes in weathering patterns due to continental drift (paleogeography) have resulted in variations of atmospheric CO2, hence climate change. On the order of magnitude of 106 years, it is suggested that a decrease in atmospheric CO2 from the mid- to late Cretaceous was caused by enhanced weathering according to terrestrial plant evolution and that variations of atmospheric CO2 in the late Cenozoic were regulated by weathering directly or indirectly influenced by continental uplift. Additionally, contributions of weathering to global climate change involved in oceanic anoxic events in the Mesozoic have been investigated.