6_Sedimentary Rocks

6_Sedimentary Rocks - LECTURE 6 SEDIMENTARY ROCKS Steps in...

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LECTURE 6 SEDIMENTARY ROCKS Steps in forming Sedimentary Rocks: 1. WEATHERING the chemical alteration, physical , and biological breakdown of rocks during exposure to the atmosphere, hydrosphere, and biosphere -two types of weathering: (1) mechanical/physical -requires the application of some physical force or stress to be applied to the rock - - no accompanying changes to the composition of rocks mechanical breakup increases the rock’s surface area and the surface-to-volume ratio some mechanical weathering processes 1) Freezing and thawing - The expansion force of water as it freezes is sufficient to split any mineral or rock. 2) Heating and cooling - Differences in temperature in a rock give rise to differential expansion and contraction. 3) Wetting and drying - The disruption of soil results in the swelling and contracting of soil peds and particles. 4) Grinding or rubbing - Grinding action, or the rubbing of moving rock against each other. 5) Organisms - Action of organisms, including animals and plants reduces the size of rocks and minerals. 6) Unloading - the removal sediments overlying deeply buried rocks by erosion or uplift, results in the exfoliation of rocks (2) chemical - breakdown of minerals by chemical reactions with water, with chemicals dissolved in water, or with gases in the air - progression from less stable minerals to more stable minerals Some chemical weathering processes 1) Dissolution - the dissolving of a solid in a liquid 2) Hydrolysis - process of minerals reacting with water to form hydroxides, which usually are more soluble than the original mineral. example - pyroxene to Fe oxide 4FeSiO3 + H2O + O2 -->4FeO(OH) + 4SiO2 3) Acidification - Weathering is accelerated by the presence of the hydrogen ion in water, such as that provided by carbonic and organic acids. 4) Hydration - combination of a solid mineral or element with water. 5) Oxidation and Reduction - used in mineral weathering, is both the chemical combination of oxygen with a compound and the change in oxidation number of some chemical element (Reduction is the chemical process in which electrons are gained.) 6 ) Ion-exchange - involves the transfer of charged atoms (ions) of calcium, magnesium, sodium, and potassium between waters rich in one of the ions and a mineral rich in another (Most effective in clays.) Examples Some chemical weathering reactions Solution of calcite (no solid residue) Susceptibility of Minerals to Chemical Weathering: recall Bowen’s Reaction series
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6_Sedimentary Rocks - LECTURE 6 SEDIMENTARY ROCKS Steps in...

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