Weathering and Soil

CHAPTER 1 Weathering and Soil Weathering: It is the disintegration and decomposition of rock at or near the earth's surface. That is divided into mechanical and chemical weathering. Erosion: It is the incorporation and transportation of material by mobile agents such as water, wind or ice. Mass Wasting: It is the downslope movement of rock, regolith, and soil under the direct influence of gravity. I- Mechanical weathering: It is the break down rocks into smaller and smaller pieces by physical forces without changing their composition. Effect of physical weathering on rate of chemical weathering: Physical weathering breaks rocks down into smaller pieces thus increasing the surface area over which chemical weathering can occur (see diagram). Types of mechanical weathering: 1- Frost wedging Frost wedging: When water change from liquid state to solid state by freezing (water molecules arrange themselves into a very open crystalline structures), expands because volume increase by 9%. Thus, freezing of 1 Talus slope water filling the fractures, cracks and joints, exerting pressure and expanding them. This type of weathering is common in mountain areas in the middle latitudes where daily freeze-thaw cycles often exist. Rocks are wedged loose and may tumble into large piles called talus slopes that often form at the base of steep rock outcrops. 2- Unloading (Pressure release) or Sheeting: Intrusive igneous rocks (granite) are formed deep beneath the Earth's surface. They are under tremendous pressure because overlying rock of the material. When erosion removes the overlying rock material, these intrusive rocks are exposed and the pressure on them is released. The outer parts of the rocks then tend to expand more than the rock below, and thus separate from the rock body. Outwards expansion results in the formation of loose concentric slabs (onion-like layers). The process that generates these onion-like layers is called sheeting. Large structure result from sheeting is termed exfoliation dome. Thermal Expansion It occurs in hot desert areas where daily variations in temperature may exceed 30°C. Heating rocks causes expansion and cooling causes contraction. Repeated swelling and shrinking of minerals with different expansion rates should logically exert some stress on the rock’s outer shell. 3- Biological Activity Activities of organisms including plants, burrowing animals and human beings. Plant roots grow into fractures in their search of nutrients and water, as the root grows, they wedge the rock apart. Burrowing animals (earthworms) further break down rock by moving fresh material to the surface, where physical and chemical processes can more effectively attack it. Decaying organisms produce acids that contribute to chemical weathering. Road cuts, quarries and other human constructions play a Tarbuck, E.J. and Lutgens, F.K., 1993: role. II- Chemical Weathering: Chemical weathering involves a chemical transformation of rock into one or more new compounds. It includes dissolution, oxidation and hydrolysis. a) Dissolution: Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. CO2 + H2O => H2CO3 Carbon dioxide + water => carbonic acid Also organic acids are released as organisms decay. Sulfuric acid is produced by the weathering of pyrite and other sulfide minerals. The most soluble mineral in water is halite. Water molecules are polar, oxygen has a negative charge and hydrogen has a positive charge. As the water molecules collide with a halite crystal, oxygen contact and disrupt Na +. The attractive force of water pulls the sodium ions from the crystalline structures. The Cl - is similarly removed by hydrogen. Minerals calcite (CaCO3) which composes the common building stones marble and limestone is easily attacked by weak acidic solution. During this process, the insoluble calcium carbonate is transformed into soluble products. b) Oxidation: Rust occurs when oxygen combines with iron to form iron oxides (e.g. hematite, limonite). 4Fe + 3O2 → 2Fe2O3 Iron was oxides because it lost electrons to oxygen. It occurs in dry environments, but water greatly speeds the reaction. Oxidation decomposes such ferromagnesian minerals as olivine, pyroxene, and hornblende. Oxygen readily combines with the iron in these minerals to form reddish-brown iron oxides called hematite (Fe2O3), or in more extreme cases a yellowish-colored rust called limonite [FeO(OH)]. These products are responsible for the rusty color on the surface of dark igneous rocks, such as basalt. c) Hydrolysis Carbonic acid ionizes (breaks down) into two ions, hydrogen (H+) and bicarbonate (HCO3) -1. H2CO3 → H+ + HCO3- The free hydrogen ions may alter mineral composition by replacing other ions in a mineral’s atomic structure; this reaction is termed hydrolysis. Hydrolysis occurs when minerals react with water to form other products. Potassium feldspar, the most common mineral in rocks on the earth's surface (granite), reacts with water to form a secondary mineral such as kaolinite (a type of clay) and additional ions that are