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Environmental Science15th EditionBradley Smith, Eldon D. Enger 427 solutions Environmental Science15th EditionBradley Smith, Eldon D. Enger 427 solutions Since igneous rocks form at high temperatures, and under pressure conditions ranging from one to several atmospheres. However, the conditions at the Earth's surface are somewhat different than the conditions at which most rocks and minerals form. Therefore, the materials are no longer at equilibrium when they are exposed to surface conditions. Under these conditions, there is a tendency for all ordered systems to seek lower levels of energy or order. This is all done through weathering. Weathering - the disintegration and decomposition of rock at or near the surface of the earth. It affects the rocks in place and no transport is involved. This distinguishes weathering from erosion. Mechanical/physical weathering - physical disintegration of a rock into smaller fragments, each with the same properties as the original. Occurs mainly by temperature and pressure changes. Chemical weathering - process by which the internal structure of a mineral is altered by the addition or removal of elements. Change in phase (mineral type) and composition are due to the action of chemical agents. Chemical weathering is dependent on available surface for reaction temperature and presence of chemically active fluids. Smaller particle sizes weather by chemical means more rapidly than large particles due to an increase of surface area. Look at the diagram below and you will see that as the particles get smaller, the total surface area available for chemical weathering increases. Erosion - the incorporation and transportation of weathering products by a mobile agent such as wind, water, ice. All three processes may act independently, but will more often than not, occur simultaneously. Different circumstance will have one weathering process more important than another. The processes may also act in concert with one another. Types of Mechanical Weathering:
Types of Chemical Weathering: H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3-Dissolution is very common in areas that have a great deal of limestone. Acidic waters (from pollution or natural) dissolve limestone allowing for additional water to gain entrance. Can cause sinkholes and karst features as well as dissolution of statutes and grave stones. Oxidation (rust) 4Fe+2 +3O2 --> 2Fe2O3Hydrolysis 2KAlSi3O8 + 3H20 --> Al2Si2O5(OH)4 + 4SiO2 + 2K(OH)
Chemical Weathering Products Soils and Soil Formation Dependence of weathering type on the mean temperature and annual rainfall. Weathering rates depend on the composition of the rock, temperature range and rainfall amount. Weathering produces soils. Soils may or may not remain in place, and any soil may be a combination of residual and transported material. Residual soil: Remains in place; has not been transported (gruss). Transported soil: Transported by wind or water and deposited. A complete soil profile will have the following components: O horizon: Organic debris and leaf litter on the surface.
Factors in Soil Formation
Mineral stability Sediments are the by-product of weathering. Sediments are particles of minerals, some of them altered from the original rock, some simply reduced in sized, and some new minerals by reaction. The stability of minerals can be predicted using the Bowen's reaction series, however, in the case of the weathering series this is known as the Goldich Dissolution Series: Olivine Mg Pyroxene Calcic Plagioclase Mg-Ca Pyroxene Calcic-Alkalic Plagioclase Amphibole Alkalic-Calcic Plagioclase Biotite Alkalic Plagioclase Potassium Feldspar Muscovite Quartz Minerals crystallize from a melt at different temperatures during the migration and emplacement of the magma. Those minerals that crystallize at higher temperatures will be the least stable at the surface. From this it is obvious that quartz will be the most stable mineral in the weathering environment, and will be a dominant constituent of sediments and sedimentary rocks. |