B. Chemical Weathering Definition

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Presentation transcript:

B. Chemical Weathering Definition Chemical weathering is the decomposition or decay of rocks in situ due to the chemical reactions between rock minerals and air, moisture, rainwater, or seawater.

B. Chemical Weathering Causes / conditions favouring chemical weathering 1. High relative humidity 2. Abundant rainfall rainwater absorbs carbon dioxide in the atmosphere and form a mild carbonic acid. 稀碳酸This acid rain can dissolve soluble minerals in rocks and cause chemical changes.

4. Weak / soluble minerals 3. High temperature High temperatures speed up the rate of chemical reactions. 4. Weak / soluble minerals Rocks contain minerals of different chemical compositions. The weak minerals undergo chemical changes when in contact with rainwater, acids or gases. 5. Organic acids These are released by decaying organic matter, e.g. decaying plants or animals. When in contact with rocks, these acids act on rock minerals and lead to chemical changes.

6. Bare / exposed / unvegetated surface When rock minerals come into contact with air or gases like oxygen, they are also weathered.

B. Chemical Weathering Processes in chemical weathering Water is an effective agent in chemical weathering. It works in several ways. 1. Oxidation氧化作用 Oxygen dissolved in water reacts with certain rock minerals especially iron, to form oxides and hydroxides. Fe + O2  FeO2

iron oxide + water  iron hydroxide 2. Hydration水化作用 Some minerals absorb water and expand, thereby producing stress within rocks iron oxide + water  iron hydroxide Calcium sulfate + water  gypsum

Feldspar + water  kaolin (clay) 3. Hydrolysis水解作用 Water combines with rock minerals and form new products such as clay minerals. Feldspar + water  kaolin (clay) 高嶺土

CO2 + H2O  H2CO3 carbonic acid CaCO3 + H2CO3  Ca(HCO3)2 4. Carbonation碳化作用 Rainwater combines with carbon dioxide in the air and forms a mild carbonic acid that acts on minerals in rocks. CO2 + H2O  H2CO3 carbonic acid CaCO3 + H2CO3  Ca(HCO3)2 Calcium carbonate carbonic acid calcium bicarbonate

5. Solution溶解作用 Water dissolves soluble minerals in rocks. As a result, cavities or holes are formed on the rock surface. This process is known as honeycomb weathering.