Weathering

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Undergraduate Geology - Part 2 (Sedimentary Processes and Products) Note on Weathering, created by siobhan.quirk on 05/18/2013.

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When rocks are uplifted near to the surface, they are exposed at the surface to temperature, pressure and chemical conditions are different from those where they were formed. Weathering is the in situ chemical alteration and mechanical breakdown of rocks by exposure to the atmosphere, water and organic matter,Chemical WeatheringRocks decompose when the chemical structure of their minerals breaks down. Chemical weathering reactions, all of which involve water, produce ions that are removed in solution leaving an insoluble residue, usually clay minerals. Carbonation and hydrolysis are important reactions because they affect two common rock forming minerals, calcite and feldspara.CarbonationCarbon dioxide gas in the atmosphere reacts with rainwater and pore water in the soil to form carbonic acid. The air in soil pore spaces is rich in carbon dioxide due to decomposing plant litter. This makes the groundwater more acidic than rainwater. Carbonation is the most important in the weathering of limestone:CaCO3 + H2CO3 -> Ca 2+ + 2HCO3-calcite + carbonic acid -> calcium + hydrogen carbonate ions in solutionLimestones generally contains insoluble impurities. They are left behind as clay residues.HydrolysisThe reaction between water and silicate minerals, especially feldspars, is important because they are the most common rock forming minerals. Hydrolysis is speeded up if the water contains carbonic acid. H+ ions from water and carbonic acid react with the minerals' ions. The products include a residual clay mineral, silica, carbonate or bicarbonate in solution.Mechanical WeatheringExfoliationExfoliation is sometimes known as 'onion-skin weathering', because the curved sheets peel off from rocks affected by it. In hot desserts there is a marked difference between hot daytime and cold nighttime temperatures. Different minerals expand and contract by different amounts during heating and cooling, causing the rock to disintegrate.Frost ShatteringWater enters cracks, joints and bedding planes. In climates where daily temperatures fluctuate above and below 0 degrees, water freezes and expands by 9%. This exerts pressure on rocks, leading to eventual failure. It produces a residue of angular fragments called scree.Pressure ReleaseAtmospheric pressure at the Earth's surface is much lower than pressures within the Earth, even at quite shallow depths. When this pressure is released from rocks due to the erosion of rocks above them, they expand producing fractures, which are more widely spaced the further they are from the surface. Rocks that are well jointed or have many bedding planes are less likely to be affected by this form of weathering.Biological WeatheringRoot ActionTree roots can grow along bedding planes and joints and force them apart mechanically. They keep surfaces open so that water can penetrate and so make chemical weathering easier. When trees sway in the wind, their roots can prise open fractures in rocks. BurrowingBurrowing animals include worms, reptiles and mammals. Their activity brings rock material from shallow depths up to the surface, where more weathering can affect it. The burrows allow atmospheric gases and water to penetrate, making chemical weathering more likely. Fragments that may have been produced by other forms of weathering are usually reduced in size by burrowing animals. Artic (mean temp 2 degrees, mean annual rainfall 10cm) - mainly mechanical weathering (frost shattering) Temperate (mean temp 10 degrees, mean annual rainfall 50cm) - Mechanical, chemical and biological (frost shattering, carbonation, hydrolysis, root action) Warm arid (mean temp 20 degrees, mean annual rainfall 10cm) - mainly mechanical (exfoliation) Humid tropical (mean temp 20 degrees, mean annual rainfall 200cm) - intense chemical, some biological with greatest amount of residue (carbonation, hydrolysis, root action)

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