Metals and Geochemistry

The average composition of the Earth s crust is essentially the composition of igneous rocks. 

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Natural mobilization includes chemical, mechanical, and biological weathering and volcanic activity. In chemical weathering, the elements are altered to forms that are more easily transported. For example, when basic rocks are neutralized by acidic fluids (such as rainwater acidified by absorption of CO2), the minerals contained in the rocks can dissolve, releasing metals to aqueous solution. Several examples are listed below of chemical reactions that involve atmospheric gases and that lead to the mobilization of metals  

Biological and volcanic activities also have roles in the natural mobilization of elements. Plants can play multiple roles in this process. Root growth breaks down rocks mechanically to expose new surfaces to chenaical weathering, while chemical interactions between plants and the soil solution affect solution pFF and the concentration of salts, in turn affecting the solution-mineral interactions. Plants also aid in decreasing the rate of mechanical erosion by increasing land stability. These factors are discussed more fully in Chapters 6 and 7. 

Volcanic activity has a significant effect on the mobilization of metals, particularly the more volatile ones, e.g., Pb, Cd, As, and FFg. Effects of volcanism are qualitatively different from those of the weathering and other near-surface mobilization processes mentioned above, in that volcanism transports materials from much deeper in the crust and may inject elements into the atmospheric reservoir. 

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