L16 - Weathering
1.
Disintegration implies nothing more than a physical reduction in particle size, giving rise to the terms physical or mechanical weathering.
2.
Decomposition implies a chemical change that can be total or partial. The only requirement of chemical weathering is that the products are different from the reactants.
II. What are the agents of physical weathering? A. The primary agent of physical weathering is the freezing and thawing of water, the process called frost wedging that we discussed earlier. The 10% expansion as water is converted to ice exerts enormous pressures on any containment. Should that containment be a crack in a rock, the rock will eventually fail after repeated episodes of freezing and thawing. Frost wedging takes place primarily in temperate climates. B. Growing plant roots are also effective agents of physical weathering because they grow within cracks and wedge the cracks open as they increase in diameter. In addition, plants are chemically active and, thus, are agents of chemical weathering. C. Another effective process is efflorescence. This results in the exfoliation of a rock surface through the growth of soluble crystals precipitated from groundwater just beneath the rock surface. Such a process is most effective in desert regions or in outcrops that face the Sun. As groundwater evaporation continues just below the surface and more salts precipitate, the growth pressure exerted by the minerals causes a crack to form and widen parallel to the rock surface. Eventually, portions of the rock will flake off and fall to become part of the talus below. D. Another process, exfoliation, can be seen at work in geological Features called batholiths.
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