Weathering
|
|
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.
The breakdown products, after chemical weathering of rock and sediment minerals and the leaching out of the more soluble parts, when combined with decaying organic material, is called soil. The mineral content of the soil is determined by the parent material, thus a soil derived from a single rock type can often be deficient in one or more minerals for good fertility, while a soil weather from a mix of rock types (as in glacial, eolian or alluvial sediments) often makes a richer soil.
| Contents |
Mechanical Weathering
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
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
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.
Freeze-thaw action occurs mainly in environments where there is a lot of moisture, and temperatures frequently fluctuate above and below freezing point - that is, mainly alpine and periglacial areas.
When water that has entered the joints freezes, the ice formed strains the walls of the joints and causes the joints to deepen and widen. This is because the volume of water expands by 9% when it freezes.
When the ice thaws, water can flow further into the rock. When the temperature drops below freezing point and the water freezes again, the ice enlarges the joints further.
Repeated freeze-thaw action weakens the rocks which, over time, break up along the joints into angular pieces. The angular rock fragments gather at the foot of the slope to form a talus slope (or scree slope). The spitting of rocks along the joints into blocks is called block disintegration. The blocks of rocks that are detached are of various shapes depending on their rock structure.
Ice crystals can also form in the pore spaces of rocks. They grow larger as they attract water that has not frozen from the surrounding pores. The ice crystal growth weakens the rocks which, in time, break up. An example of rocks susceptible to frost action is chalk, which has many pore spaces for the growth of ice crystals.
Laboratory tests show that frequent daily freeze-thaw cycles are more condusive than seasonal freeze-thaw cycles to frost shattering.
Pressure release
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.
Intrusive igneous rocks (e.g. granite) are formed deep beneath the earth's surface. They are under tremendous pressure because of the overlying rock material. When erosion removes the overlying rock material, these intrusive rocks are exposed and the presure on them is released. The outer parts of the rocks then tend to expand. The expansion sets up stresses which cause fractures parallel to the rock surface to form. Over time, sheets of rock break away from the exposed rocks along the fractures.
Salt-crystal growth
Salt crystallisation causes disintegration of rocks when saline (see salinity) 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.
The salts which have proved most effective in disintegrating rocks are sodium sulfate, magnesium sulfate, and calcium chloride. Some of these salts can expand up to three times or even more.
Chemical Weathering
Carbonation-solution
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
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
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
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).