Chiaki T. Oguchi

Effect of porosity on the increase in weathering-rind thicknesses of andesite gravel

Abstract Factors affecting the increase in weathering-rind thickness, i.e. weathering rates, were investigated using andesite gravel in fluvial terrace deposits with a known emergence time. The time between the age of each terrace formation (20, 130, 290 and 660 ka) and the present was assumed to be the weathering period. Three zones were classified from measurements of the Vickers hardness number (VHN) across weathering rinds from the rock surface to the inner fresh part: (1) Zone I, the outermost brown layer, which has low VHN values of 10 to 80 gf/μm2; (2) Zone II, the inner gray layer which has increasing VHN values (100 to 500 gf/μm2) with increasing depth; (3) the innermost fresh part, which has large VHN values of 500 gf/μm2. In this paper, Zones I and I+II are defined as the brown layer and the weathering rind, respectively. Diffusion theory was applied to the relationship, between the thickness of the brown layer or the weathering rind and time (weathering periods). The results show that the thickness of the brown layer depends mainly on time, whereas the thickness of the weathering rind is controlled by the porosity of the host rock as well as time. Consequently, rocks with higher porosity have thicker weathering rinds than brown layers.

Weathering rates over 40,000 years based on changes in rock properties of porous rhyolite

Abstract Weathering rates based on temporal changes in a variety of rock properties were examined for four dated lava domes made of porous rhyolite. The lava domes were erupted at 1.1, 2.6, 20 and 40 ka BP. The time from each eruptive event to the present provide elapsed weathering times. The lava domes have many cooling joints formed immediately after eruption. Joint-bounded blocks have a high porosity, resulting in deep weathering. The blocks have almost uniform characteristics from top to bottom in one outcrop. The mineralogical, chemical, physical and mechanical properties of the four rhyolites were analysed. These results show that the chemical properties and physical properties, including specific surface area, change slowly in the early stage of weathering (0–20,000 years) and quickly in the later stage (20,000–40,000 years), while mechanical strength and bulk density or porosity of the rhyolites change rapidly in the early stage and slowly in the later stage. Mechanical strength reduces by 70–90% relative to fresh rock in 40,000 years, although chemical and physical properties change by only 10230 %. It is concluded that different weathering properties exhibit different rates and patterns of change over 40,000 years, and that mechanical strength is the property most susceptible to weathering.

Effect of microstructure and weathering on the strength anisotropy of porous rhyolite

Abstract To study the effect of microstructure and weathering on the strength anisotropy of rock, unconfined compressive strength (UCS) tests were carried out on three porous rhyolites having the same original lithology, but different weathering periods of 2600, 20,000 and 40,000 years. The rock is mainly composed of glassy groundmass, with flow structure. UCS tests were undertaken on a series of samples at 15° intervals, from right angles to the flow structure (β=90°) to parallel (β=0°), where β is the angle between the direction of the applied load and the direction of the flow structure. The test results show that UCS-values are maximum when β=0–30° and minimum when β=60–90°. This differs from previous reports for layered anisotropic rocks such as sandstone, sandy shale, schist etc., for which, UCS-values are maximum at β=0 or 90°. It is also found that UCS-values for β=60–90° reduce rapidly in the initial stage of weathering (the first 20,000 years), while for β=0–30°, the reduction rate increases after 20,000 years. This shows that the effect of weathering on strength anisotropy is not uniform, but depends on the weathering processes of the microstructures of the rock.

Late Quaternary rapid talus dissection and debris flow deposition on an alluvial fan in Syria

Abstract Landform development in an arid alluvial fan/source basin system in northwest Syria was reconstructed based on field surveys and correlated with dated stratigraphy in the nearby Dederiyeh Cave. During the Last Glacial Maximum, talus deposits were stored in the V-shaped valley of the source area and associated fan gravels became firmly cemented by carbonate, reflecting a slow deposition rate. In the Late Glacial, the lower parts of talus slopes were quickly dissected due to increased storm activity, and boulder debris flows were deposited on the fan surface. Subsequently, water flow removed the sandy matrix of the debris flow deposits on the northern half of the fan surface and that of talus deposits along the trunk stream of the upstream area. This was followed by a period of stability without significant geomorphic change during the Holocene. The inferred rapid geomorphic response to climatic change in the form of talus dissection, debris flows, and fan deposition contrasts with the notion that geomorphic systems in arid regions respond slowly to change in external variables.

Rates of chemical weathering of porous rhyolites: 5-year measurements using the weight-loss method

Abstract To examine the rate of chemical weathering, field experiments using microweight-loss techniques were carried out. Two kinds of porous rhyolite forming lava domes in Kozu-shima Island were selected: (1) a younger rock (weathering period is 2.6 ka) and (2) an older rock (20 ka). Both rocks seem to have been very similar in their initial composition and structure. They have, however, different properties in the present depending on the difference in weathering period. Rock tablets of these rhyolites with a diameter of 3.5 cm and a thickness of about 1 cm were enclosed in a nylon mesh bag and placed in a soil–bedrock interface on a hillslope of central Japan for over 5 years. The younger rock has a lower weight loss by about 0.5%, and the older rock has a higher weight loss of 3% to 5%. These results show that older rock has a higher rate of chemical weathering than in younger rock. This supports Oguchi et al.s (Oguchi, T.C., Hatta, T., Matsukura, Y., 1994. Changes in rock properties of porous rhyolite through 40,000 years in Kozu-shima Island, Japan. Geogr. Rev. Jpn. 67A, 775–793 (in Japanese, with English Abstr.); Oguchi, T.C., Hatta, T., Matsukura, Y., 1999. Weathering rates over 40,000 years based on changes in rock properties of porous rhyolite. Phys. Chem. Earth (A) 24, 861–870.) finding that the rate of change in chemical properties appears to accelerate with weathering time.

Simultaneous wetting/drying, freeze/thaw and salt crystallization experiments of three types of Oya tuff

Abstract An abandoned subsurface Oya tuff quarry in Japan had abundant salt efflorescence in winter. Besides salt weathering, freeze-thaw weathering and slaking were likely to occur because of winter temperatures below 0 °C and the presence of swelling clays in the rock. Field surveys were performed to collect salts. Thenardite and gypsum were detected by X-ray diffraction (XRD) as the main salts, along with zeolites as secondary minerals. Oya tuff is categorized into three types for practical usage. To investigate petrophysical differences among the three types of Oya tuff, mercury intrusion porosimetry and tensile strength tests were performed. To determine the influence of petrophysical properties on salt weathering, freeze-thaw weathering and slaking (wet-dry weathering), all three types of Oya tuff were used for experiments. Prismatic specimens, the bases of which were sunk into distilled water, were used for the freeze-thaw and slaking experiments and Na2SO4 saturated solution was used for the salt-weathering experiment. The results show that the specimens subjected to salt weathering were the most severely damaged. The coarse-type Oya tuff sustained the most severe damage, whereas the fine type received the least. There was a large amount of debris in the coarse type, but less in the fine type. The weathering susceptibility index WSI was also calculated from the results of the pore size analyses and tensile strength. The index decreases with increasing weathering cycles representing resistant rocks. The phenomena of weathering of Oya tuff were explained by three weathering experiments on three kinds of tuff. The WSI may be useful as a practical indicator of rock weathering.

Combination of chemical indices and physical properties in the assessment of weathering grades of sillimanite-garnet gneiss in tropical environment

Changes in mineralogical, chemical, and mechanical properties of a rock mass during the weathering process allow calibrating the degree of weathering. Based on those properties, various weathering indices can be used as tools for this purpose. This study focuses on using chemical weathering indices that represent the chemical behaviour of a rock mass and their relationship with other properties for better evaluation of weathering grades. Metamorphic rocks that undergo weathering due to both chemical and operational means at Smanalawewa hydropower station in Sri Lanka, where tropical climate exists, were selected for this study. Among the chemical weathering indices calculated, the product index, Ruxton ratio, silica-titania index, alumina-titania index, chemical weathering index by Sueoka, and S/SAF index change consistently with progressive weathering. They correlate well with the weathering potential index, Miura index, bulk density, and point load strength. Among the six indices mentioned, the product index, Ruxton ratio, silica-titania, and alumina-titania indices have been recognised as better measures to classify metamorphic rocks in Sri Lanka for chemical means. Since there is a good correlation with physical and mechanical properties, the chemical weathering index and S/SAF index also appear to be better indices in the same context. Weathering potential, Parker, and modified weathering potential indices, which are widely used, show minor fluctuations during weathering. On the contrary, the chemical index of weathering and chemical index of alteration fluctuate with progressive weathering. These changes exhibit the influence of bulk mineral composition of the rock, where aluminium rich minerals are present. This study clearly shows that chemical weathering indices of a particular rock mass are more reliable when they correlate well with physical and mechanical properties.

Is sodium sulphate invariably effective in destroying any type of rock

Abstract Sodium sulphate has been implicated as one of the most destructive weathering agents in many field observations and numerous laboratory studies. We hypothesize however, that sodium sulphate would not be invariably effective on any type of rock. To verify the supposition, a laboratory cyclic impregnation–drying experiment was undertaken. In addition to sodium sulphate, two other destructive hydratable salts, magnesium sulphate and sodium carbonate, were used to attack eight types of rock. In all three salt attacks, rock breakdown occurred only during immersion due to the exertion of higher crystallization pressure driven by the greater supersaturation reached after dissolution of the crystals precipitated during drying. Sodium sulphate was the most destructive salt in six out of the eight rocks tested, and even granite was substantially disintegrated. However, although probability is small, sodium sulphate indeed manifested its impotency against a relatively weak rock (Tago Sandstone). Contrary to its modest damaging power on other rocks, magnesium sulphate destroyed Tago Sandstone which could resist sodium sulphate attack. Sodium carbonate was the least destructive of the three hydratable salts. The general damage mechanism of hydratable salts, the process of damage of Tago Sandstone by magnesium sulphate and the possible reasons behind the impotency of sodium sulphate against Tago Sandstone are all investigated.

Evaluation of geomechanical and geochemical properties in weathered metamorphic rocks in tropical environment: a case study from Samanalawewa hydropower project, Sri Lanka

The effect of weathering on changes of physical and mechanical properties of rocks is a prime concern in the perspectives of geology and engineering. These properties have been studied mostly on weathered igneous and sedimentary rocks under humid climates. Studies on weathering of metamorphic rocks, especially under a tropical climate, are rare. This study evaluates change of physical, mechanical, chemical, and mineralogical properties of metamorphic rocks that weather under tropical climatic conditions. Samanalawewa hydropower project area was selected for this study, because rapid weathering of a metamorphic rock (sillimanite garnet gneiss) was observed in the project site. Fresh rocks that are subjected to weathering have reached to completely weathered condition in a time span of less than 25 years in this area. Visually assessed weathering grades were physically and mechanically evaluated using bulk density, equotip hardness, porosity, specific gravity, point load strength, and slake durability tests. Mechanical properties, especially point load strength, change rapidly at the onset of weathering, while chemical properties show significant changes at later stages of weathering. Mineralogical changes such as appearance of secondary minerals are at the latter part of weathering. Physical properties gradually change during weathering. The observed changes in physical, mechanical and chemical properties indicate that their variations during weathering are independent of lithology and climatic conditions.

Adsorption of 133Cs and 87Sr on pumice tuff: A comparative study between powder and intact solid phase

This study examines the use of intact samples as an alternative to powder in conventional batch sorption studies to determine the distribution coefficient (Kd). Stable cesium (133Cs) and strontium (87Sr) were used under specified geochemical conditions to compare the Kd values of powder and block pumice tuff samples. The aim of the study was to infer any Kd difference under laboratory and field conditions. Kd values for block samples were found to be less than one order of magnitude lower than powder materials for both Cs and Sr on fresh tuff, and more than one order of magnitude lower in oxidized tuff. Destruction of micropores in oxidized tuff was estimated to be mainly responsible for reducing Kd values in oxidized tuff. However, approximately one order of magnitude difference in Kd values indicates that homogenously prepared intact samples can be used for sorption coefficient measurement at closer to in situ conditions. Pore size distribution analysis using mercury intrusion porosimetry revealed that lower Kd values on block samples result from lower surface area available as sorption sites due to inaccessible closed pores in the intact solid.