Science Fair Project Encyclopedia
Weathering is the process of decomposition and/or disintegration of rocks in situ, that is, in place. It is not to be confused with erosion, which is the movement of rocks and/or weathering products by water, wind, ice or gravity.
Mechanical weathering is the cause of the disintegration of rocks. Most of the times it produces smaller angular fragments (like scree) as compared to chemical weathering.
Disintegration often occurs in hot areas, like deserts, where there is a large diurnal temperature range. The temperatures soar high in the day, while dip to a few degrees at night. As the rock heats up and expands by day, and cools and contracts by night, stress is often exerted on the outer layers.
This stress causes the peeling off of the outer layers of rocks in thin sheets, which is known as exfoliation. Though this exfoliation is caused mainly by temperature changes, a little moisture is also essential in this process.
Freeze-thaw action, sometimes known as ice crystal growth or frost shattering, occurs when water in cracks and joints of rocks freeze and expand. In the expansion, it can exert pressures up to 2100 kg/cm2 at -22°C. This pressure is often higher than the resistance of most rocks and causes the rock to shatter.
In pressure release, overlying materials (not necessarily rocks) are removed (by erosion, or other processes), which causes underlying rocks to expand and fracture parallel to the surface. Often the overlying material is heavy, and the underlying rocks experience high pressure under them, for example, a moving glacier. Pressure release may also cause exfoliation to occur.
Salt crystallisation causes disintegration of rocks when saline solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. These salt crystals expand as they are heated up exerting pressure on the confining rock.
Salt crystallisation may also take place when solutions decompose rocks (for example, limestone and chalk) to form salt solutions of sodium sulfate or sodium carbonate, of which the moisture evaporates to form their respective salt crystals.
Carbonation occurs on rocks which contain calcium carbonate such as limestone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid which reacts with calcium carbonate and forms calcium bicarbonate.
The reactions as follows
CO2 + H2O ⇌ H2CO3 carbon dioxide + water ⇌ carbonic acid
H2CO3 + CaCO3 → Ca(HCO3)2 carbonic acid + calcium carbonate → calcium bicarbonate
Hydration (not to be confused with hydrolysis) is the process whereby minerals in the rock absorb water and expand, and sometimes change. One example is how anhydrite changes to gypsum by absorbing water.
CaSO4 + 2H2O → CaSO4.2H2O anhydrite + water → gypsum
Though chemical, this process may contribute to mechanical weathering as well, as some materials expand upon absorption of water. These materials may expand up to sixteen times their size, especially mudstones containing montmorillonite clays or bentonite.
Hydrolysis involves the action of acidic water on rock forming minerals like pyroxenes, amphiboles, and feldspars. For example orthoclase feldspar reacts with acidic water to form kaolin, silica and potash. Only kaolin and silica remain as potash is removed in solution.
2KAlSi3O8 + 2H2O → Al2Si2O5(OH)4 + 4SiO2 + K2O orthoclase feldspar + water → kaolin + silica + potash
Oxidation is the reaction of iron compounds with oxygen and water in the air to give rocks a reddish-brown colouration on the surface. Oxidation occurs when iron (II) oxide (FeO) is oxidised into iron (III) oxide (Fe2O3).
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