Yukinori Matsukura

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.

Effect of Rock Hardness and Moisture Content on Tafoni Weathering in the Granite of Mount Doeg‐sung, Korea

Many tafoni occur on the granite tors of Mount Doeg‐sung, Korea. Relationships are investigated between growth processes of granite tafoni and rock properties such as rock hardness and moisture content. Field evidence shows that the rock surface of the inner backwalls and ceiling of tafoni have reduced hardness and higher moisture content compared to the outer visor of tafoni and to vertical cliff walls with no tafoni. This suggests that tafoni arise in weathered rock where conditions of low strength and high moisture content are both satisfied.

Five year measurements of rock tablet weathering on a forested hillslope in a humid temperate region

In order to elucidate the effects of rock type and geomorphic setting on weathering rates, field experiments using microweight-loss techniques were carried out. Rock tablets for eight kinds of rock types: granite; granodiorite; gabbro; limestone; andesite; rhyolite; crystalline schist; and tuff, with a diameter of 3.5 cm and a thickness of about 1 cm, were enclosed in a nylon mesh bag and placed in the four corners of a soil-bedrock interface on a granodiorite hillslope of central Japan for over 5 years. The results show that: (1) the saturated grus layer is the main locus of solutional denudation; (2) the 18 and 11% weight losses of limestone and tuff under a saturated soil layer were clearly the largest of all the rock types examined; and (3) the weight loss is smaller for granite, gabbro, crystalline schist, and andesite, while it is larger for tuff and rhyolite at every location. These results suggest that weathering rates are influenced by both lithology and environmental conditions.

Field and laboratory experiments on weathering rates of granodiorite: Separation of chemical and physical processes

To study the rate and mechanism of granitic rock weathering, field weathering experiments using granodiorite tablets (diameter 3.5 cm, height 1.1 cm, weight 30 g) were conducted at a catchment over 10 yr. The tablets were exposed at three positions having different weathering conditions: ground surface, above aquifer, and in aquifer. The weight of the tablets decreased linearly with time: 0.022 wt% yr−1 for the ground surface, 0.013 wt% yr−1 above the aquifer, and 0.42 wt% yr−1 in the aquifer. Only minor changes of the tablet surface were observed after 10 yr of weathering at the ground surface and above the aquifer. In contrast, the tablets in the aquifer were remarkably weathered, and weathering of biotite was particularly noticeable, whereas that of plagioclase, K-feldspar, quartz, and hornblende was minor or moderate. The weathering of granodiorite is inferred to proceed by initial dissolution of mineral grain boundary (chemical process) and subsequent detachment of the mineral grain (physical process). To evaluate the amount of weathering caused only by the chemical process, a laboratory dissolution experiment was conducted on the granodiorite tablet using a flow-through reactor at 20 °C and pH 6–7. The obtained rates, 0.0078–0.010 wt% yr−1, were ∼50 times slower than the field rate in the aquifer. This result shows that the contribution of the physical process to granodiorite weathering in the aquifer is very large compared to that of the chemical process, and the relative intensities of the two processes during the 10 yr weathering are estimated to be chemical:physical = 1:∼50.

A new technique for rapid and non-destructive measurement of rock-surface moisture content; preliminary application to weathering studies of sandstone blocks

The possibility of in situ rapid, non-destructive measuring of rock moisture content using an infrared optical moisture meter is examined. A laboratory test indicates that a linear relation is found between absorbance intensity of infrared rays and the moisture content of the surface of rock samples. The instrument was applied in the field in order to understand the weathering processes in sandstone blocks used for a masonry bridge pier better. The results show that the spatial and temporal changes in rock-surface moisture content are accurately measured, and that the infrared moisture meter is useful for weathering studies.

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.

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.

Evolution of solution dolines inferred from cosmogenic 36Cl in calcite

Quantification of the development of solution dolines provides important information for understanding the long-term evolution of karst landscapes. This study reports the initial results of an investigation of the long-term denudation rates along the side slopes of a solution doline based on analyses of cosmogenic 36 Cl in calcite. The denudation rates increase in proportion with increasing size of the topographic contributing area, thereby supporting the hypothesis that the rate of surface lowering in carbonate terrains is controlled by water convergence in the epikarst. A simple model based on the empirical correlation between denudation rate and contributing area is successful in explaining the form of several solution dolines located close to the analyzed doline. The model reveals that these solution dolines, which have varying diameters, developed over similar time scales of the order of 10 5 yr.

POST-VOLCANIC EROSION RATES OF SHOMYO FALLS IN TATEYAMA, CENTRAL JAPAN

Abstract. Post‐eruptive fluvial erosion of welded pyroclastic flow deposits often depends on the recession of waterfalls because of their rapid erosion involved. We examine the recession rate of Shomyo Falls, which consists of Pleistocene welded pyroclastic flow deposits in Tateyama, north‐central Japan. The mean recession rate of the waterfall obtained from lithological and topographical evidences is 0.08‐0.15 m/a for 100000 a. However, the recession rate estimated by means of an empirical equation comprising physical parameters of erosive force and bedrock resistance is 0.006‐0.011 m/a with small uncertainties. The discrepancy between the geology‐based and equation‐based recession rates indicates that some factors, not taken account of in the equation, significantly influence the recession rate. We suggest that a factor in the rapid erosion of the waterfall is a large amount of transported sediment acting as abrasive material, which is supplied from high mountains in the watershed above the waterfall.