Weathering bedrock

For a given set of conditions (lithology, climate, slope, etc.), there is presumably an optimum soil thickness that maximizes the rate of bedrock weathering (Fig. 9-3) (Carson and Kirkby, 1972 Stallard, 1985). For less than optimum soil thicknesses, there is insufficient pore volume in the soil to accept all the water supplied by precipitation and downhill flow. Excess water runs off and does not interact with the subsurface soil and bedrock. In contrast, water infiltrates and circulates slowly through thick soils (especially where forested If profile thicknesses greatly... 

On a larger scale, landscape development reflects those mechanisms that expose bedrock, weather it, and transport the weathering products away. Present and past tectonism, geology, climate, soils, and vegetation are all important to landscape evolution. These factors often operate in tandem to produce characteristic landforms that presumably integrate the effects of both episodic and continuous processes over considerable periods of time. 

On a larger scale, landscape development reflects those mechanisms that expose bedrock, weather it. 

The final composition of stream water is the product of the weathering reactions and related processes outlined above. However, the chemical processes are influenced and controlled by an intricate combination of environmental factors that are characteristic for each drainage system. Therefore, the composition of the bedrock in an area and the residual material left at the surface as soil and subsoil exert a strong influence on the chemical composition of mnoff from the area. The reactions of water with this material are the ultimate geological control and are the source of soluble weathering products. 

Weathered rock is bedrock that is deteriorating due to the weathering process. Usually this is confined to the upper layers of the bedrock. 

Plutonium is transported by the groundwater in fractures in the rock (usually <1 mm wide). A typical groundwater velocity (vw) at >100 m depth in Swedish bedrock is 0.1 tn/y. The fractures are filled with crushed, weathered, clayish minerals, which have a high capacity to sorb the plutonium. Assuming instantaneous and reversible reactions, the sorption will cause the plutonium to move considerably slower (with velocity vn) than the groundwater. The ratio between these two velocities is referred to as the retention factor (RF), defined by... 

Fig. 9-3 Conceptual model to describe the interaction between chemical weathering of bedrock and down-slope transport of solid erosion products. It is assumed that chemical weathering is required to generate loose solid erosion products of the bedrock. Solid curve portrays a hypothetical relationship between soil thickness and rate of chemical weathering of bedrock. Dotted lines correspond to different potential transport capacities. Low potential transport capacity is expected on a flat terrain, whereas high transport is expected on steep terrain. For moderate capacity, C and F are equilibrium points. (Modified with permission from R. F. Stallard, River chemistry, geology, geomorphology, and soils in the Amazon and Orinoco basins. In J. I. Drever, ed. (1985), "The Chemistry of Weathering," D. Reidel Publishing Co., Dordrecht, The Netherlands.)...

Figure 9-3 portrays a hypothetical model of how chemical weathering and transport processes interact to control soil thicknesses. The relationship between soil thickness and rate at which chemical weathering can generate loose solid material is indicated by the solid curve. The rate at which transport processes can potentially remove loose solid weathering products is indicated by horizontal dotted lines. The rate of generation by chemical weathering initially increases as more water has the opporhmity to interact with bedrock in the soil. As soil thick-... 

Fig. 9-8 Histogram of dissolved solids of samples from the Orinoco and Amazon River basins and corresponding denudation rates for morpho-tectonic regions in the humid tropics of South America (Stal-lard, 1985). The approximate denudation scale is calculated as the product of dissolved solids concentrations, mean armual runoff (1 m/yr), and a correction factor to account for large ratios of suspended load in rivers that drain mountain belts and for the greater than average annual precipitation in the lowlands close to the equator. The correction factor was treated as a linear function of dissolved solids and ranged from 2 for the most dilute rivers (dissolved solids less than lOmg/L) to 4 for the most concentrated rivers (dissolved solids more than 1000 mg/L). Bedrock density is assumed to be 2.65 g/cm. (Reproduced with permission from R. F. Stallard (1988). Weathering and erosion in the humid tropics. In A. Lerman and M. Meybeck, Physical and Chemical Weathering in Geochemical Cycles," pp. 225-246, Kluwer Academic Publishers, Dordrecht, The Netherlands.)...

In regions where the erosion regime is weathering limited, susceptibility of the bedrock to chemical and physical weathering controls erosion rates. This susceptibility relates directly to the chemical and physical properties of the rock. Susceptibility also depends on local climate. Moreover, weathering rates are affected by the... 

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