chemical weathering *AS edition*

Chemical Weathering - AND yes you should attempt to know the formulas.

Chemical Weathering: The decomposition of rock caused by a chemical change in the rock. It produces changed substances and soluble, and usually forms clay. It is more likely to occur in areas in warm moist climates where there is associated vegetation on rocks. It tends to attack certain minerals selectively and occur in zones of alternate wetting and drying (where the level of the water table fluctuates). It tends to occur mostly on the base of the slope where there tends to be wetter and warmer. These processes are more likely to occur in conjunction with another.

Hydrolysis: Hydrogen in water reacts with minerals in the rock; there is a combination of H+ and OH- ions in the water and ions of mineral (combines rather than dissolves the mineral).  It affects mostly granite (igneous rock – crystallised magma underground), which is composed of Feldspars (aluminium and potassium silicates). Feldspars (pink-grey rock forming mineral) + water à kaolinites (soft clay that is the residual weathering products) + potassium + silica oxide (Potassium and silica oxide are soluble and are washed away). The kaolinites represents the decomposition of feldspar, and the chemical weathering of granite by hydrolosis produces a chemical change in the rock. It occurs mostly in the tropics. The rate of hydrolysis depends on the amount of H+ ions, which in turn depends on the composition of the air and water in the soil, the activity of organisms, the presences of organic acids and the cat ion exchange.

Carbonation - solution: Rainwater contains carbon dioxide in solution, which produces carbonic acid (H₂CO₃). The weak acid reacts with rocks that are composed of calcium carbonate, such as limestone/ chalk and rocks that have calcareous rock. The limestone dissolves and is removed in solution by running water. Carboniferous limestone is well jointed and bedded, which results in the development of a distinctive group of landforms. Carbonation = CaCo₃ + H₂Co₃ (rainwater) à Ca (HCo₃)₂. The calcium bicarbonate is the weathered product, and is soluble (thus washed away).

Oxidation: This occurs when rocks are exposed to oxygen in the air or water. An example of this is when iron rusts. The rock or soil, which may have been blue or grey, is discoloured into a reddish-brown colour – in a process called rusting. Oxidation causes rocks to crumble more easily and occurs in iron rich rocks. In water logged areas oxidation operates in the reverse and the amount of oxygen in the soil is reduced in a process called reduction. Ferrous oxide + water à Ferric oxide. FeO + H₂O à Fe₂O₃. Sandstone is most affected by oxidation.

Hydration: Certain rocks, especially those containing salt minerals, are capable of absorbing water into their structure, causing them to swell (about 0.5%) and to become vulnerable to future breakdown. This process is most active following successive periods of wet and dry weather and is important in forming clay particles. Anhydrite + water à Gypsum. CaSo₄ + H₂O à (CaSo₄ 2H₂O) powder form. Hydration is in fact a physio-chemical process as the rocks may exert pressure as well as changing their chemical structure.

Solution: Some minerals are soluble in water and simply dissolve in situ. The rate of solution can be affected by acidity since many minerals can become more suitable as the pH of the solvent increases.

Organic Weathering/ Chelation: It requires a bio agent e.g. plants (chelates/ organic acid) and animal excretion. The decomposition of minerals in the rock leads to the crumbling of rock. Humic acid, derived from the decomposition of vegetation (humus), contains important elements such as calcium, magnesium and iron. The action of bacteria and the respiration of plant roots tend to increase carbon dioxide levels which helps accelerate solution processes, especially carbonation. Lichen can also extract iron from certain rocks through the process of reduction. High lichen and algae help in the development of the lithosphere.