Chemical Weathering in Engineering Geology: Types, Processes, and Soil Formation, Lecture notes of Engineering Geology

Download Chemical Weathering in Engineering Geology: Types, Processes, and Soil Formation and more Lecture notes Engineering Geology in PDF only on Docsity! ENGINEERING GEOLOGY CE-226 Lecture 04: Chemical Weathering and its types CHEMICAL WEATHERING • When rock comes in contact with components of the surface and atmosphere (water, oxygen, carbon dioxide), chemical reactions occur that alter and destroy minerals of the rock. Water is the most important agent of chemical weathering. 1. DISSOLUTION • Illustration of halite dissolving in water. A) Sodium and chloride ions are attached by the polar water molecules. B) Once removed, these ions are surrounded and held by a number of water molecules. Dissolution enlarges joints of a limestone and dissolves away sharp edges (Irland). (W.W. Norton) 2. HYDROLYSIS • Hydrolysis is the reaction of acidic solutions with silicates (the most common mineral group). For example, the weathering of K-feldspar of granite is as follows. 2KAlSi3O8+ 2(H+ + HCO3 -) + H2O -> K-feldspar carbonic acid Al2Si2O5(OH)4 + 2K+ + 2HCO3 - + 4SiO2 kaolinite in solution silica • An product of the chemical breakdown of K-feldspar is clay mineral, kaolinite, which is very stable at the surface. Consequently, clay minerals make up high percentage of soils. Weathering attacks more vigorously at edges and most vigorously at corners, resulting in a rounded block. RESISTANCE TO WEATHERING • Rock characteristics Some minerals are more susceptible to chemical weathering than others. For silicates, the order of weathering (Goldrich's mineral stability series) is the same as the order of crystallization (so called Bowen's reaction series). • Climate Climate is perhaps the single most important factor influencing weathering. Temperature and moisture have strong influences on both mechanical weathering (e.g. frost wedging) and chemical weathering. Thus chemical weathering is ineffective in polar regions or arid regions because of the lack of free water. • Some rocks are more susceptible to chemical weathering than others. The granite headstone (left) was erected in 1888, a few years after the marble headstone in 1885 (right). (Tarbuck and Lutgens). SOIL PROFILES • One consequence of weathering is the formation of the soil profile, a vertical cross section from surface down to the parent materials. A well-developed soil profile shows distinct horizons. The major horizons are A, B, and C horizons. • The A horizon is the top soil. It is a zone where downward percolating water removes soluble soil components into deeper zones (called leaching). It is also commonly rich in decomposed organic mater (humus). • The B horizon is the sub soil or the zone accumulation where the material removed from above accumulates. The accumulation of fine clay particles enhances water retention in the subsoil. Organic matter is less abundant in the B horizon. • The C horizon marks the transition from the soil profile to the unweathered parent material below. • A soil profile is a vertical cross-section from the surface down to the parent material. Well-developed soils show distinct layers (called horizons). (E.J. Tarbuck). • Idealized soil profile from a humid climate at mid latitudes. The major horizons are A, B, and C. (Tarbuck and Lugens). • The parent material for residual soils is the underlying bedrock, whereas transported soils form on unconsolidated deposits. Note as slopes become steeper, soil becomes thinner. • Soil formation: climate • Climate is perhaps the most important in soil profile development. (1) As we pointed out above, temperature and precipitation have great influence on weathering. (2) The amount of precipitation affects how much various materials are leached from the soil, thereby affecting soil fertility. (3) Climate affects the type of plant and animal life present. • Climate controls on Soil types: • The prevailing climate has controlling influences on soil types. The soil types in the U.S. can be roughly described as pedalfers in the eastern half of the U.S. and pedocals in the western half. • Pedalfer = pedon(soil)+Al(aluminum)+Fe(iron) in Greek. Pedalfers are found in the eastern U.S. with high precipitation. In pedalfers the soluble carbonates are removed and Al-rich clays and Fe oxides are carried downward to the B horizon. • Pedocal = pedon(soil)+CALcite. Pedalcals contain an accumulation of calcium carbonate in the B horizon. Pedalcals are found in the drier western U.S. with grassland and brush vegetation.